the prize beneath the salt

4 Oilfield Review

The Prize Beneath the Salt

John R. DribusNew Orleans, Louisiana, USA

Martin P.A. JacksonBureau of Economic GeologyThe University of Texas at AustinAustin, Texas, USA

Jerry KapoorMartiris F. SmithHouston, Texas

For help in preparation of this article, thanks to Joelle Fay and Mark Riding, Gatwick, England; and Chris Garcia, Mexico City, Mexico. Q-Marine is a mark of Schlumberger.

Exploration and production activities in the deep and ultradeep waters of the northern

Gulf of Mexico have led the way to the latest subsalt play. Lessons learned from this

play may, in turn, open the way for subsalt exploration in other basins around the world.

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highly competitive arena, where choice blocks ofoffshore real estate can lease for more thanUS $100 million for a 10-year lease. But secrecyand competition are not the only impediments toexploration in this area.

Perhaps the greatest challenge is posed bythick layers of salt in the subsurface. Severaldeepwater prospects lie beneath sheets of salt—some up to 20,000 ft [6,100 m] thick.1 Like anundulating and tattered canopy, coalesced saltsheets extend from the US Continental Shelf tothe deepwater Continental Slope off the coasts ofTexas and Louisiana. A similar subsalt provincelies well to the south, in less-explored waters offMexico’s Yucatan Peninsula.

In their pursuit of new exploration targets,many offshore operators have had to drillthrough hundreds or even thousands of feet ofsalt to discover pay sands. Their efforts have led

to notable subsalt discoveries on the Shelf, suchas Mahogany; or in deeper waters, such asGemini, Atlantis, Tahiti, Mad Dog and Pony; or in ultradeep waters, such as Thunder Horse, St. Malo, Jack and Kaskida.2

Salt is a particular challenge for drillers, whomust contend with high-pressure sediment

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Map adapted from Taylor LA, Holcombe TL and Bryant WR: Bathymetry of the Northern Gulf of Mexico and the Atlantic Ocean East of Florida. Boulder, Colorado,

USA: National Geophysical Data Center, 2000. This US government publication is

in the public domain: http://www.ngdc.noaa.gov/mgg (accessed October 23, 2008).

Bathymetry lines represent 100m [330 ft] increments.

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Successes in deep- and ultradeepwater prospectsare spurring a resurgence in exploration in theGulf of Mexico. Announcements of significantdiscoveries or record-setting achievements havepiqued interest in the activities of deepwateroperators, as evidenced by bidding competitionat recent offshore lease sales. Exploratorydrilling has also confirmed the presence ofreservoir-quality sands that are much fartherfrom shore than expected. The discovery ofhydrocarbons in several of these sands is fuelingspeculation about the tantalizing potential thatmight lie beneath deep Gulf waters.

Speculation is in large supply. Offshoreoperators diligently safeguard their expensiveand hard-won secrets of the deep and releaseonly the data required by governmental mandate.Press releases are carefully constructed to avoidrevealing the full extent of discoveries in this

1. Close F, McCavitt RD and Smith B: “Deepwater Gulf ofMexico Development Challenges Overview,” paper SPE113011, presented at the SPE North Africa Technical Conference and Exhibition, Marrakech, Morocco, March 12–14, 2008.

2. The concept of deep water has evolved considerablythroughout the years. The US Department of Interior Minerals Management Service (MMS) originally defineddeep water as 200 m [656 ft]. This mark was later eclipsedby industry drilling trends, and now the deepwater stan-dard is set at 1,000 ft [305 m]. The MMS has designateddepths greater than 5,000 ft [1,524 m] as ultradeep water.In the Gulf of Mexico, the deepest waters are found in the Sigsbee Deep, whose estimated depths range from12,303 ft [3,750 m] to 14,383 ft [4,384 m].


inclusions or rubble zones as they drill throughsalt bodies (see “Meeting the Subsalt Challenge,”page 32). It also poses substantial difficulties forgeophysicists as they attempt to image deepstructures beneath irregularly shaped saltbodies. In salt, seismic waves can reachvelocities of 14,500 to 15,100 ft/s [4,400 to 4,600m/s]—in some cases roughly twice the velocitythey would travel in surrounding sediments. Thisvelocity contrast causes geophysical imagingproblems that can mask underlying structuresand prevent geoscientists from determining thelocation or extent of potential reservoirs.3

However, advances in seismic acquisition andprocessing techniques are helping geophysicistsresolve problems that previously preventedimaging beneath the salt (see “Shooting SeismicSurveys in Circles,” page 18).

The process of drilling and producing asubsalt well requires significant planning,operational skill and investment. Given thedifficulties associated with drilling in deepwaters, the decision to take on the additionalchallenge of drilling through salt must bejustified by potential for payouts and returnsworthy of the additional expense and riskinvolved. One frequently cited article, written in1997, noted that potential subsalt reserves from25 or more significant fields located primarily on

the Continental Shelf of the northern Gulf hadbeen estimated at 1.2 billion bbl [190 million m3]of oil and 15 Tcf [435 billion m3] of gas.4 Theseestimates do not include additional reserves thathave since been discovered in the deepwaterMiocene play—including Mad Dog, Pony, Tahitiand others—nor do they include ultradeepwatersubsalt discoveries such as Jack, Kaskida,St. Malo and Thunder Horse.

This article describes the evolving subsaltplay in the northern Gulf of Mexico. We brieflyreview the geologic processes that led todeposition of the Louann Salt and the transportof reservoir-quality sands into deeper reaches ofthe basin. We examine the role of saltmobilization and its effect on overlying sedimentin the formation of traps and migration pathwaysnecessary to complete an effective petroleum

system in the Gulf of Mexico. We also discuss howsands that were originally deposited on top ofsalt have come to lie beneath it. Although thisarticle focuses primarily on the intensivelyexplored US waters of the northern Gulf ofMexico, some of the principles described are alsorelevant to other basins around the world.

Evolution of the Gulf of Mexico BasinDiscoveries in the Gulf of Mexico havechallenged previous thinking regarding theoccurrence of hydrocarbon-bearing sands that liebeneath great thicknesses of salt. This salt isactually older than the sands that lie beneath it.However, the salt has moved and in some casescreated seals capable of trapping oil and gas.Understanding the geological complexities ofthese discoveries requires a step back in time.

The evolving subsalt play in the Gulf of Mexicois inextricably tied to the geologic history of theGulf itself. This history goes back eons, before theGulf of Mexico existed, to a time after most of theworld’s continental plates had converged into asupercontinent known as Pangea (left). Thetectonic activity that followed would mold theGulf basin and influence the distribution ofsediments that subsequently filled it.

In addition to unremittingly gradual tectonicand depositional processes in this basin, otherforces were at work. The early history of the Gulfof Mexico was at times abruptly punctuated bycatastrophic events that not only influenced theformation of the Gulf, but also changed the world.These events were remarkable in their scale, butwere certainly not unique; one such cataclysmoccurred just before the Gulf began to open.

Approximately 250 million years ago, much oflife on Earth was extinguished. On land, insectbiodiversity plummeted, while plant and animallosses were even more severe, as 70% of landspecies vanished. In the seas, trilobites, tabulateand rugose corals, and nearly all crinoids becameextinct, along with roughly 90% of all marinespecies. Atmospheric oxygen levels dropped from30% to less than 15%.

6 Oilfield Review

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> Earth during the Triassic period. The eventual breakup of the Pangeansupercontinent was manifested in part by rifting between the continentalplates of Africa and the Americas, which eventually led to the formation ofthe Gulf of Mexico.

3. Farmer P, Miller D, Pieprzak A, Rutledge J and Woods R:“Exploring the Subsalt,” Oilfield Review 8, no. 1 (Spring 1996): 50–64.

4. Montgomery SL and Moore DC: “Subsalt Play, Gulf of Mexico: A Review,” AAPG Bulletin 81, no. 6 (June 1997): 871–896.

5. Salvador A: “Late Triassic-Jurassic Paleogeography andOrigin of Gulf of Mexico Basin,” AAPG Bulletin 71, no. 4(April 1987): 419–451.

6. A graben is a downthrown fault block that is bounded oneither side by opposing upthrown normal fault blocks.Grabens occur in areas of rifting or extension, where theEarth’s crust is being pulled apart.Red beds are reddish sedimentary strata, such as sand-stone, siltstone or shale, which have accumulated under

oxidizing conditions. The red color results from specks ofiron oxide minerals in the sediments. Oxidizing conditionsare common in hot, arid environments and thereforeimply that the sediments have been exposed to theseconditions through surface weathering as a result ofuplift or erosion of overlying sediment. Red beds arecommonly associated with rocks of the Permian and Triassic periods.

7. Salvador, reference 5.8. Pindell J and Kennan L: “Kinematic Evolution of the

Gulf of Mexico and Caribbean,” in Fillon RH, Rosen NC,Weimer P, Lowrie A, Pettinghill H, Phair RL, Roberts HHand van Hoorn B (eds): Proceedings, 21st Annual GulfCoast Section SEPM Foundation Bob F. Perkins ResearchConference (2001): 193–220.


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This was the great Permian-Triassic extinc -tion event (above). The cause of this massextinction is subject to debate. While recentdiscoveries point to an asteroid impact, otherevidence supports a variety of theories includingmassive volcanic activity, falling sea levelsbrought on by the formation of continental icesheets, anoxia caused by sluggish oceancirculation, a massive release of methane fromseafloor hydrates, or some combination of theseevents. Regardless of the cause, this extinctionestablished a prominent geological benchmarkin outcrops around the world, providing a gaugeby which to establish the timing of subsequentprocesses that led to the formation of featuressuch as the Gulf of Mexico.

After this extinction, Pangea began to driftapart as a precursor to the development of theGulf of Mexico basin. In the Late Triassic, as theNorth American plate pulled away from the SouthAmerican and African plates, deep rifts began toform. These rifts were associated with stretchingof the continental crust.5 Still part of the NorthAmerican plate, the area that would eventually

become the Gulf of Mexico was cut by grabens,which gradually subsided as they filled withvolcanic deposits and nonmarine red bedsderived from sediments eroded from adjacentelevated areas.6 These red beds, which providesome of the earliest records of Gulf of Mexicorifting, make up part of the Eagle Mills formation.

Subduction-related tectonism along thewestern margin of the North American platepermitted sporadic encroachment of the PacificOcean. During the Middle Jurassic, Pacific stormsurges reached eastward across Mexico to fillshallow depressions in the proto-Gulf—thesurface expressions of continually subsidinggrabens activated during the Late Triassic.Between surges, the connection with the PacificOcean would close, leaving behind isolatedbodies of salt water.

Throughout countless cycles of replenish -ment and evaporation, salinity in these bodies ofwater steadily increased. This resulted in haliteprecipitation in the center of the hypersalinebasins and anhydrite precipitation along parts ofthe periphery. Local subsidence accom modatedthe pace of halite precipitation, as evidenced byevaporite deposits that are thousands of feetthick. In the northern Gulf of Mexico basin, theseextensive salt deposits came to be called theLouann Salt, of late Middle Jurassic age.

In the Late Jurassic, during what is inferredto be the final stages of rifting, continuedstretching of the continental crust caused theYucatan platform to separate from the NorthAmerican plate, taking a portion of the salt bodyalong with it. A connection between the earlyGulf of Mexico and the Atlantic Ocean probablyopened late in the Jurassic, when a passagewaybetween the Florida and Yucatan platforms wasestablished, and the connection to the PacificOcean became restricted.7 The Yucatan rotatedcounterclockwise as it continued to driftsouthward.8 It finally came to rest on the northernedge of the South American plate during theEarly Cretaceous (below).

Tectonics beyond the basin greatly influencedthe sequence and areal extent of sedimentarydeposits that began filling the Gulf and buryingthe thick layers of Louann Salt. Some of thesesediments would later become hydrocarbonsources, while others would become possiblehydrocarbon reservoirs that today are beingtargeted for exploration.

Jurassic uplift of the Appalachian Mountainswas accompanied by erosion of graniticmountain materials. As they weathered, thefeldspar and mica minerals within these granitesbroke down to produce clastic deposits rich in

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> Geologic time scale. Convergence ofcontinental plates during Mississippian andPennsylvanian periods resulted in the formationof the supercontinent of Pangea. By the LateTriassic, upwelling mantle broke up the super -continent and opened the ancestral Gulf ofMexico and Atlantic Ocean basins.

> Yucatan microplate movement. The Louann Salt basin split apart as the Yucatan microplate (dashedyellow line) rotated and drifted southward (white arrow). (Adapted from Pindell and Kennan, reference 8.)


clay, known as feldspathic sandstone. Locally,arid winds transported some of the clasticsediments, winnowing the clays away from thequartzose fraction, resulting in eolian depositsof the Upper Jurassic Norphlet sandstone.9

Meanwhile, the Florida Shelf and Yucatan Shelfbecame starved of clastic sediments and weredominated by the deposition of massive layers ofche mical carbonates.

Following Norphlet deposition, a rise in sealevel produced a transgressive period duringwhich localized deposition of evaporites, shallowmarine clastics and organic-rich carbonatesoccurred. The organic matter, derived fromalgae, plankton and other materials in themarine environment, was mixed and buriedwithin layers of carbonates and shales. As thesediments were buried deeper over time, heatand pressure resulting from the accumulatingoverburden transformed the organic matter intoType I and Type II kerogens, essential precursorsto the generation of hydrocarbons.10

During the Cretaceous, thick deposits ofinterbedded carbonates, marls and organic-richmarine shales were laid down during anotherseries of marine transgressions. Like theirdeeper Jurassic equivalents, these organic-richdeposits would become important source rockswhen buried deep enough to generate hydro -carbons. The end of the Cretaceous—about65 million years ago—was marked by the arrivalof a large asteroid, which heralded a new era inthe geologic history of the Earth.

This asteroid, 5 to 6 mi [8 to 10 km] indiameter, impacted near the present-day town ofChicxulub Puerto, on Mexico’s YucatanPeninsula (above). Upon impact, the asteroidexcavated a crater in the Yucatan carbonateShelf that spans more than 112 mi [180 km] indiameter and melted the Earth’s crust to a depthof about 18 mi [29 km].11 The energy released onimpact exceeded that released from 100 millionmegatons of TNT.12 In the Gulf basin, massive

earthquakes induced slump ing of coastalsediments, while 330-ft [100-m] tsunamisradiated across the Gulf of Mexico and the proto-Caribbean and Atlantic basins.13

Attesting to the physical effect of the impactis a layer of melt spherules found across Haiti, aswell as in the United States, Canada, Spain andNew Zealand. These spherules were produced astons of molten ejecta blasted out of the crater,forming a red-hot plume that circled the globeand rained molten glass for days, sparking firesacross parts of North and South America, centralAfrica, India and Southeast Asia. As a conse -quence, a deposit of ash and soot is preserved inthe sediments of Europe and the western USA.During this time, greenhouse gases increasedmore than a hundredfold, atmospheric sulfurcontent increased by a factor of a thousand, andchlorine gas destroyed the ozone, creating anenvironment that ended the age of the dinosaursand extinguished 75% of the Earth’s species.14

Following the Cretaceous-Tertiary cataclysm,sediments shed from mountain-building eventson the western margin of the Gulf began to fillthe subsiding basin. At the beginning of thePaleocene, clastic detritus from the westresulted in a Lower Wilcox basin-floor fancomprising poorly sorted, gravity-driven arkosicsandstone and siltstones with abundant clays.These arkosic turbidite sandstones are reportedto have permeabilities from 1 to 10 mD andporosities between 14% and 18%.15

As basin fill continued during the EarlyEocene, amalgamated deepwater channel-leveesequences buried the basin-floor fan complex.These Upper Wilcox sediments were depositedunder higher energy conditions than those of theLower Wilcox and are better sorted than theirbasin-floor counterparts. Consequently, theseamalgamated sequences have fewer rock frag -ments, less clay and better reservoir properties,as evidenced by reported values of 50- to 200-mDpermeability and 20% to 28% porosity.

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9. Mink RM, Bearden BL and Mancini EA: “Regional Geologic Framework of the Norphlet Formation of theOnshore and Offshore Mississippi, Alabama and Florida Area,” Proceedings, Oceans ’88 MTS-OES-IEEEConference and Exposition (1988): 762–767.

10. Following burial to depths of 0.6 to 1.2 mi [1 to 2 km] andheating to temperatures of 140°F [60°C], the kerogensserve as primary feedstock for the generation of hydrocarbons.

11. Kring DA: “Composition of Earth’s Continental Crust asInferred from the Compositions of Impact Melt Sheets,”presented at the 28th Lunar and Planetary Science Conference, Houston, March 17–21, 1997, http://www.lpi.usra.edu/meetings/lpsc97/pdf/1084.PDF (accessedAugust 21, 2008).

12. NASA/University of Arizona Space Imagery Center’sImpact Cratering Series, http://www.lpl.arizona.edu/SIC/impact_cratering/Chicxulub/Discovering_crater.html(accessed August 11, 2008).

13. Kring DA: “The Chicxulub Impact Event and Its Environmental Consequences at the Cretaceous-TertiaryBoundary,” Palaeogeography, Palaeoclimatology,Palaeoecology 255 (November 2007): 4–21.

14. For more on the Chicxulub impact and its relationship tothe Cretaceous-Tertiary boundary: Pati JK andReimold WU: “Impact Cratering—Fundamental Processin Geoscience and Planetary Science,” Journal of EarthSystem Science 116, no. 2 (April 2007): 81–98.Simonson BM and Glass BP: “Spherule Layers—Records of Ancient Impacts,” Annual Review of Earthand Planetary Science 32 (May 2004): 329–361.Smit J: “The Global Stratigraphy of the Cretaceous-Tertiary Boundary Impact Ejecta,” Annual Review ofEarth and Planetary Science 27 (March 1999): 75–113.Olsson RK, Miller KG, Browning JV, Habib D and Sugarman PJ: “Ejecta Layer at the Cretaceous-Tertiary Boundary, Bass River, New Jersey (OceanDrilling Program Leg 174AX),” Geology 25, no. 8 (August 1997): 759–762.

15. Meyer D, Zarra L and Yun J: “From BAHA to Jack, Evolution of the Lower Tertiary Wilcox Trend in the Deepwater Gulf of Mexico,” The Sedimentary Record 5,no. 3 (September 2007): 4–9.

16. Some authors have proposed that a collision of theCuban Arc against the Yucatan and Florida blocks duringthe Paleocene and Eocene may have isolated the Gulf ofMexico, prompting a dramatic short-term regressive lowering of sea level through evaporation. This wouldhave allowed reworking of previously deposited Wilcoxsands. As the sea level dropped, these sands would havebeen redeposited onto the deep basin floor in a series ofproximal basin-floor fans. For more on this scenario:Rosenfeld JH and Blickwede JF: “Extreme EvaporativeDrawdown of the Gulf of Mexico at the Paleocene-Eocene Boundary,” presented at the AAPG AnnualConvention, Houston (April 9–12, 2006), http://www.searchanddiscovery.com/documents/2006/06065rosenfeld/index.htm (accessed September 30, 2008).

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> Chicxulub impact map. Spherules from the impact havebeen found at sites (green circles) along the gulf coast andnortheast coast of the USA, in boreholes drilled off the easternUS coast and in outcrops in Mexico and Haiti. During theimpact, which marked the Cretaceous-Tertiary boundary, Haitiwas located about 435 mi [700 km] south of Chicxulub. (Adaptedfrom Olsson et al, reference 14.)


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Major canyon systems cut through CenozoicShelf margins and channeled the Wilcox clasticsfar from shore.16 These clastics were carried outto the deepwater basin floor (above). Theresulting thick deposits of clastic reservoir rock

are now being targeted for exploration, andoffshore operators have announced discoveriescontaining signifi cant oil accumulations in areasof the Perdido and Mississippi Canyon fold belts,as well as beneath the salt canopy at the Jack,Kaskida and St. Malo prospects.

The Upper Wilcox deposits were buriedbeneath thick layers of deepwater marine shaleduring the Late Eocene. During the Oligocene,another influx of clastic sediment was deliveredby a series of deltas sourced from uplift of the

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> Moving sands to deeper waters. When a river meets the ocean, watervelocity dictates where it will deposit the sediments it carries in suspension.Heavier materials—typically coarse- to medium-grained sands—drop outfirst. Then, as velocity decreases with distance from shore, finer sands andsilts are deposited, followed by very fine particles that make up clays. Such adepositional progression is seen in the formation of river deltas on thecontinental shelf (map view and cross section, top left ). However, waterlevels rise and fall—the result of glacial activity and cycles of tectonic platedispersion or collision—and this variance has an impact on sedimentaryprocesses. Thus, during periods of glaciation, water becomes locked up incontinental ice sheets, which can dramatically lower sea levels. The ensuingregression draws water away from existing coastlines and deltas until itreaches a maximum fall of sea level, or lowstand. As they become exposedto weather, these coastlines and deltas erode and successively reveal

deposits of sand, silt and clay when the sea recedes. As they are eroded,these sediments are redeposited downbasin—farther from their originalsource—and some rest temporarily on the steeper continental slope, awayfrom the gentler dipping shelf that they were originally deposited on (topright ). As deposition on the slope continues, these water-laden, shelf-edgedeposits become steeper and more unstable (bottom left ). An earthquake,loop current or major hurricane may eventually trigger the release of thesesediments. When they give way, turbidity currents carry the sedimentstoward the abyssal plain, to be deposited in basin-floor fans (bottom right). Inother cases, entire fault blocks can also be transported downdip intact orwith varying degrees of mass translation. With changing glacial or tectonicconditions, sea level will eventually encroach upon the land in what istermed as a transgression. During this highstand, a new delta may formwhere the river meets the sea. (Used with permission of John R. Dribus.)


Sierra Madre range, to the west. Ensuingerosional processes led to sequences of inter -bedded deltaic arkosic clastic sediments, marineshales and volcaniclastics.

By the Early to Middle Miocene, deepwaterclastics entering the Gulf of Mexico basin wereincreasingly sourced from the northernMississippi River system as uplift and erosion ofthe Rocky Mountains continued, and sedimentinput from the Sierra Madres in the west beganto decline. As these western-sourced systemsdiminished, clastic deposits in the Gulf becamemore quartzose and less arkosic, therebycreating reservoir rock with less clay and betterreservoir potential. In the Middle Miocene, sand-rich turbidites, fed almost entirely from theMississippi River system, formed sheet depositsacross the Gulf basin floor, and by Late Miocene,contributions from western river systems becamenegligible (above).

Throughout transgressive and regressivedepositional cycles spanning the Jurassic andMiocene, the Louann Salt has been responding tobasin tectonics, as well as to clastic loadingprocesses from deltas and turbidite fans thatcontinue to this day. Depositional loading hashad profound effects on the salt and thepotential for viable prospects throughout muchof the Gulf of Mexico.

Salt TectonicsA basic knowledge of salt tectonics is helpful inunderstanding how hydrocarbon traps formedabove deposits of salt, and how these same trapslater became covered by thick layers oftectonically emplaced salt. Thick layers of salt,when buried and deformed, result in continentalmargin stratigraphy and structures that areutterly different from those in margins lackingsalt. These tectonic effects are a product of thedistinctive properties of salt.

Pure rock salt is composed of sodium andchloride, forming a mineral known as halite.Other minerals formed by evaporating seawater,such as gypsum and anhydrite, are commonlyinterlayered with halite, and the entire accumu -lation of evaporite minerals is referred to simplyas “salt.” As these minerals precipitate frombrine, they form a crystalline rock.

One of the more important properties of rocksalt is that it is much weaker than surroundingsedimentary rocks, such as sandstone or shale.Its strength diminishes with decreasing crystal -line grain size and increasing tempera ture, orwhen thin films of original seawater remainbetween salt grains.17 Failure in salt often leadsto ductile flow. Even at ambient temperaturesand pressures, salt can flow at a rate of metersper year, as has been measured in the saltglaciers of Iran (next page, top).

Salt is also distinctive for its low density.Freshly deposited mud and sand are less densethan salt. However, these sediments expel theirinterstitial fluids and compact during burial,eventually becoming denser than salt. The saltconsequently becomes more buoyant by compari -son. In the Gulf of Mexico, the average density ofa sedimentary column does not usually exceedthat of salt until the overburden thicknessreaches 1 to 2 mi [2 to 3 km].18 Another impor tantproperty of salt is its permeability. It is so low thatsalt acts as a seal to liquids and gases, and thuscan stop fluid migration and trap hydrocarbons.

Salt is mechanically stable if compressedequally from all sides during burial. However,salt’s low viscosity allows it to flow underunbalanced forces or loads, which occur innature primarily under two conditions.Gravitational loading results when overlying

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18. Hudec MR, Jackson MPA and Schultz-Ela DD: “The Paradox of Minibasin Subsidence into Salt: Clues to theEvolution of Crustal Basins,” Geological Society of Amer-ica Bulletin (in press).

19. In the US waters of the northern Gulf of Mexico, all theseforces are imposed by gravity. In the southern Gulf, bothgravitational and plate-tectonic forces are at play.

20. Much of the greatest mountain on Earth actually liesbeneath the Pacific Ocean. Mauna Kea, one of five volcanic masses that form the island of Hawaii, USA,rises from the depths of the Pacific Ocean to a height of 33,476 ft [10,203 m]. Because only about 13,796 ft[4,205 m] is above sea level, it is commonly outranked by Mt. Everest, which is 29,028 ft [8,848 m] high.

> Shifting depocenters. Major depositional centers of the northern Gulf of Mexico basin exhibitsignificant shifts over time, gradually moving west to east and extending basinward from north tosouth. [Modified from Seni SJ, Hentz TF, Kaiser WR and Wermund EG, Jr (eds): Atlas of Northern Gulfof Mexico Gas and Oil Reservoirs, vol 1: Miocene and Older Reservoirs. Austin: Bureau of EconomicGeology, The University of Texas at Austin, 1997.]


Autumn 2008 11

sediments vary laterally in thickness or density,causing the underlying salt to flow laterallytoward thinner or less-dense overburden.Displacement loading is the second form ofinstability. It is driven by tectonic forces andtypically acts horizontally.19 If the sedimentsflanking a salt body pull away laterally, the saltcan stretch and sag into the resulting gap.Conversely, if the flanking sediments presstogether, any intervening salt body will be

squeezed, tending to rise like toothpasteextruded from a tube.

However, even if unbalanced forces areimposed on weak salt, it may not deform. Twoimportant forces resist salt flow. First is thestrength of overlying sediment layers. For salt torise, it must penetrate or lift the sediments aboveit. If the overlying sediments are thick enough,they will be too heavy to be lifted and too strongto be pierced by the salt, despite its buoyancy.

Second is boundary drag, caused by frictionagainst the top and bottom of the salt layer.Where a salt layer feeds a growing salt structurenearby, it becomes exponentially more difficultfor this feeder layer to flow laterally as the saltsource depletes and the layer grows thinner.

Salt is originally deposited in flat layers. Theforces described above transform these layersinto underground mountain ranges of salt—some hundreds of miles long—that can growtaller than the greatest mountains on Earth.20

Because it was originally thought that such saltmasses push through overlying sediments, theyare called diapirs, from the Greek worddiapeirein, to pierce.

Diapirs are unquestionably the tallest andmost spectacular of the various salt bodies (left).Horizontal salt layers may transform intosubsurface mountainous diapirs in three ways.First, where a sedimentary basin is stretched,reactive diapirs can rise to create sharp-crestedridges below strata that are thinned by exten -sional faulting. Second, active diapirs can breakthrough arch-like folds whose crests have beenthinned by erosion, especially in areas wheretectonic squeezing has pressurized the salt.Third, passive diapirs can grow like “islands” ofsalt, exposed on the Earth’s land surface orseafloor, while the base of the diapir andsurrounding sediments sink as the sedimentarybasin fills.

> Allochthonous salt spreading subaerially. Kuh-e-Namak (Mountain of Salt in the Farsi language) is the most famous salt diapir in Iran. This salt glacier(light gray) emerges from an anticline, its summit towering 1,400 m [4,593 ft] above the surrounding plain. Note the vehicle (circled) in the foreground forscale. Here, the Infra-Cambrian Hormuz salt spreads over much younger Jurassic-Cretaceous strata (tan), which form the anticline. The main body of thissalt glacier advances at an average rate of about a meter a year. A collision between the Arabian and Iranian microplates created the Zagros Mountainsand enhanced the rise and extrusion of the salt. (Photograph courtesy of Martin Jackson.)

West

Dept

h, ft

0

5,000

10,000

15,000

20,000

25,000

30,000

East

2 mi

3.2 km

> Rising from the depths. This seismic display shows two salt diapirs beneath the Continental Shelf,offshore Louisiana. The diapir at left, which extends to within about 1,200 ft [366 m] of the seafloor,exhibits a vertical relief in excess of 18,000 ft [5,486 m]. Thinning and upturning of sediments againstthe salt flanks indicate different phases of salt piercement history.


Where the original source layer of salt is thickenough, the crests of the largest diapirs canbegin to spread laterally at or below the seafloor,forming a shallow sheet of salt. The source layerof salt is said to be autochthonous, or formed inplace (above). Autochthonous salt overlies olderrocks and is, in turn, overlain by younger strata.By contrast, the shallower sheets of salt thatspread out from the diapir are said to beallochthonous, or formed out of place and awayfrom their original source. Allochthonous saltsheets overlie younger strata. Thus, while theautochthonous Louann Salt dates back about160 million years to the Jurassic period, theshallow sheets of allochthonous salt derived fromthe Louann Salt may overlie strata as young as1 million years of age.

These fundamental processes of salt tec -tonics combine to create continental margins ofgreat complexity, and no divergent margin ismore complex than the Gulf of Mexico. One wayto better understand this margin is to divide theregion into provinces, each of which has adominant structure or distinctive geologichistory. This approach has become more refinedwith improvements in seismic acquisition and

processing that allow better visualization of thegeometries of the deep salt.

Geoscientists initially investigated this regionusing well data linked by a grid of 2D seismicsurveys. However, the subsalt regions tended tobe poorly imaged, so the structural provinceswere based mostly on mapping of shallow saltstructures.21 The advent of widespread 3Dseismic coverage during the 1990s enabledgeoscientists to start defining structuralprovinces on the basis of deep autochthonoussalt and related structures.22 These refinementshelped shape the following summary of keyprocesses and events, roughly in chronologicalorder, which shows how salt tectonics influencedbasin evolution of the Gulf.

Seismic data reveal that the Louann Saltvaried in thickness from almost zero to perhapsas much as 2.5 mi [4 km] as it accumulated on asurface made uneven by faulting, erosion orvolcanism. Countless cycles of seawater influxand evaporation resulted in this massivethickness of salt. Many of the crustal structurescontrolling original salt thickness are orientedalong a northwest-southeast trend. Verticallyconnected by salt, these deep structures appear

to have influenced much shallower structures,causing them to trend along a similar direction.

Crustal stretching and basin-center riftingsevered the Louann Salt basin into northern(USA) and southern (Mexico) parts. This wasfollowed by cooling of newly formed oceaniccrust and exhumed upper mantle in the center ofthe opening Gulf, which created a densityincrease that caused the basin floor to sink. Theresulting basinward tilt sent the severed saltflowing toward the center of the Gulf. At thesame time, sediments began to pile up on thenew oceanic crust, ahead of the spreading salt.This sedimentation caused the base of thespreading salt to climb over the accumulatingstrata, building a seaward-climbing wedge ofallochthonous salt during the Jurassic and EarlyCretaceous.23 The wedge formed a fringe of salt atleast 19 to 25 mi [30 to 40 km] wide beneath theSigsbee Escarpment between the MississippiCanyon and Keathley Canyon areas, marking thefirst of several salt-sheet emplacements in theGulf of Mexico.

During the Late Jurassic to Miocene periods,folding of strata above the salt began in theWalker Ridge area, along the eastern edge of theSigsbee Escarpment. This folding was partly aresult of loading caused by uneven clasticsedimentation from deltaic systems, whichforced the salt to flow away from thickdepocenters and into areas below thinner layersof sediment. In addition, lateral compressioninduced by tilted strata caused the saltoverburden to buckle.

Some areas, such as the eastern MississippiCanyon, originally contained thin layers ofautochthonous salt and yielded only a fewscattered tall diapirs. However, in other places,such as the Green Canyon and Atwater Valleyareas, thin deposits of salt were subsequentlythickened by salt inflation, beginning in the LateJurassic. Inflation occurred as sediments shedoff the North American continent were depositedinto the thick salt basin and displaced underlyingautochthonous salt ahead of the advancingsedimentary load. The displaced salt flowedhorizontally into peripheral regions, wherethinner layers of salt had been deposited. Asdisplaced salt built up these deposits on theperiphery, salt inflation enabled growth of muchlarger diapirs and folds than would otherwisehave been possible.

During the Cenozoic, folding and thrustingbegan in earnest on the lower Continental Slope.This folding was driven by gravitationallyinduced compression caused by the basinwardslope of the seafloor. The Perdido fold belt, in theAlaminos Canyon area, formed on a thick cushion

12 Oilfield Review

North South

5,000 ft

Allochthonous salt canopy

Stock

Autochthonous salt layer

40,000 ft

45,000 ft

13,715 m

> Evolving salt structures. A thick allochthonous salt canopy spreads laterally beneath theContinental Slope off Louisiana. This layer has been fed by two stocks rising from an underlying layer ofautochtho nous salt, which, in turn, has been nearly depleted as a result of salt withdrawal. The base ofone stock is clearly imaged in the center of the autochthonous layer. To the right of this structure, thesecond stock is largely pinched off.


Autumn 2008 13

of autochthonous salt in the Late Oligocene andEarly Miocene. To the east, the Mississippi Fanfold belt formed on the deep wedge ofallochthonous salt in the Atwater Valley areaduring the Late Miocene. Across a broad areabetween these fold belts extending fromKeathley Canyon to west Walker Ridge, deep foldbelts have not been recognized because they arebelow the deepest part of the basin, which hasbeen poorly imaged by seismic surveys.

Concurrent with this folding was the mostremarkable process of all. From the Miocene tothe present, vast salt sheets spread laterally likepancake batter wherever salt supplies fromdepth were sufficient to feed their expansion.These sheets then coalesced to form shallow saltcanopies. Some shallow salt sheets were fed bymassive salt diapirs sourced from the deepautochthonous layer below. Other sheets werefed by overlying, yet deeply emplacedallochthonous canopies.

This massive spreading of salt was highlyvariable. In the eastern Mississippi Canyon area,where autochthonous salt was thin, onlyscattered, small salt sheets formed. To the west,in the Green Canyon area, where deep salt wasthicker, most diapirs merged into canopies. Evenfarther west, from the Walker Ridge to AlaminosCanyon areas, where autochthonous salt wasthickest, massive diapiric walls of salt fed asingle giant canopy that spread southward formany tens of kilometers.24

Allochthonous salt spread throughout theCenozoic, and its lateral extent increased overtime. The main driver for the Neogene spreadingof canopies was a mid-Miocene switch incontinental sediment sources, which moved fromthe west and northwest to the northern rim ofthe Gulf.25 This switch increased the sedimentaryload in areas where autochthonous salt was stillthick. As the shallow salt sheets surgedsouthward, they either pushed and smearedsmaller salt sheets ahead of them or overrodesmall diapirs that were locally sourced from thin,autochthonous salt.

Clues to how the salt canopies spread arefound along the Sigsbee Escarpment, where aveneer of Pleistocene sediment covers theleading edge of the shallow spreading salt(above). The Sigsbee Escarpment is the largestdeformation structure affecting the seafloor inthe Gulf of Mexico and is the largest exposed saltstructure in the world. The escarpment reaches aheight of approximately 4,100 ft [1,250 m]. It hasa great-circle length of about 350 mi [560 km]and a sinuous and buried length of more than620 mi [1,000 km]. The salt canopies continue to

advance today over about 60% of the escarpment,gradually obscuring much of the subsalt geology.

Initially, salt sheets extrude across theseafloor as salt glaciers, and much of the solublesalt dissolves into the seawater. However, thespreading salt is partly protected by deep-marineclay that has settled as a muddy veneer.Moreover, as the most-soluble salt mineralsdissolve, a mushy layer of less-soluble mineralsremains as a thickening protective blanket.Today, sediments bury almost all of the SigsbeeEscarpment, hindering salt extrusion. Thus, saltand its roof must advance together over the

abyssal plain, causing thrusting along the base ofthe escarpment. Either the compressed sedi -ments ahead of the advancing salt break cleanlyas a single thrust fault, or they are bulldozed intoa tapering prism of thrust slices.

The youngest salt sheets are now found inpristine form along the Sigsbee Escarpment, atthe foot of the Continental Slope. Progressinglandward up the Slope, the covering sedimentsthicken. This increasing sedimentary load on thesheets causes the salt within them to be expelledinexorably seaward.

Sedimentation is highly irregular andtypically creates minibasins in the top of thesalt’s surface. Some minibasins begin as meredimples in the top of the salt sheets, then deepeninto sediment-filled dishes 6 to 25 mi [10 to40 km] wide. Once the minibasins becomegreater than 1 or 2 mi thick, their compacteddensity is enough to make them sink, causingdiapiric salt to well up around them. Otherminibasins form in a different manner entirely.

On the lower Slope, gravitationally inducedcompression wrinkles the sedimentary veneerand initiates minibasins. On the middle Slope,several other mechanisms have been proposed.26

The pattern of sedimentation controls wheresubsidence is greatest and thus molds the top ofthe salt canopies. This structural relief, in turn,creates the local bathymetry, which is the maininfluence on where sediment moves and where itamasses. Here, cause and effect blur because salttectonics and sedimentation affect each other.

Sediment works its way down the ContinentalSlope, following a sinuous path created by partlymerged minibasins, while avoiding bulges on topof salt structures. Some pathways end intemporary or permanent cul-de-sacs, wheresediment is trapped in minibasins. The mini -basins continue to subside until all underlyingsalt is expelled laterally. At this point, a salt weldis formed as sediments that were formerly aboveand below the salt are brought together duringits expulsion. Like a boat grounding at low tide, aminibasin comes to rest on unyielding sedimentinstead of on displaced mobile salt.

21. Diegel FA, Karlo JF, Schuster DC, Shoup RC andTauvers PR: “Cenozoic Structural Evolution and Tectono-Stratigraphic Framework of the Northern Gulf Coast Continental Margin,” in Jackson MPA, Roberts DG andSnelson S (eds): Salt Tectonics: A Global Perspective:AAPG Memoir 65. Tulsa: AAPG (1995): 109–151.Peel FJ, Travis CJ and Hossack JR: “Genetic StructuralProvinces and Salt Tectonics of the Cenozoic OffshoreU.S. Gulf of Mexico: A Preliminary Analysis,” in JacksonMPA, Roberts DG and Snelson S (eds): Salt Tectonics: A Global Perspective: AAPG Memoir 65. Tulsa: AAPG(1995): 153–175.

22. Hall SH: “The Role of Autochthonous Salt Inflation andDeflation in the Northern Gulf of Mexico,” Marine andPetroleum Geology 19, no. 6 (June 2002): 649–682.

23. Peel F: “Emplacement, Inflation, and Folding of an Extensive Allochthonous Salt Sheet in the Late Mesozoic(Ultra-Deepwater Gulf of Mexico),” presented at theAAPG Annual Convention, Denver, June 3–6, 2001.

24. Peel FJ: “The Geometry and Emplacement History of theMajor Allochthonous Salt Sheets in the Central US Gulfof Mexico Slope: A Regional Review,” presented at theGulf Coast Association of Geological Societies Meeting,Austin, October 30–November 1, 2002.

25. Galloway WE, Ganey-Curry PE, Li X and Buffler RT:“Cenozoic Depositional History of the Gulf of Mexico Basin,” AAPG Bulletin 84, no. 11 (November 2000): 1743–1774.

26. Hudec et al, reference 18.

Shallow

Deep

Bath

ymet

ry

> Sigsbee salt. This bathymetric image of theSigsbee salt canopy, offshore Brownsville,Texas, is projected looking east toward NewOrleans. The downslope limit of this salt canopyis known as the Sigsbee Escarpment. Thisallochthonous salt and its thin sedimentary roofare advancing southward by means of thrustfaults at the base of the escarpment. The uppersurface of the canopy is pock-marked byminibasins sinking into salt. [Courtesy of Dr.Lincoln Pratson, Duke University, from Pratson LF and Haxby WF: “What Is the Slope ofthe U.S. Continental Slope?” Geology 24, no.1(January 1996): 3–6.]


14 Oilfield Review

Source rock

Reservoir rock

Shale seal

Migration pathw

ay

Petroleum System Elements

Source rock

Seal

Reservoir

Burial

Timing

Preservation

Trap formation

Generation, migrationand accumulation

Incr

easi

ng te

mpe

ratu

re a

nd p

ress

ure

Basement

The patterns of minibasin subsidence, saltthinning and welding are complex but aregenerally viewed as combinations of three endmembers: salt-stock canopy systems, rohosystems and stepped counter-regional systems. • Salt-stock canopy systems are characterized

by evacuated clusters of funnel-shaped saltdiapirs that have coalesced.

• Roho systems are characterized by stretchedsediments that are spread on long smears ofwelded allochthonous salt. A roho system con-sists of a group of listric basinward-dippinggrowth faults that sole, or bottom out, onto anallochthonous salt sheet or weld. (Listricfaults are curved, normal faults that exhibitdecreasing dip with depth.) Sediment wedgesin the fault blocks dip and thicken landward,but become younger seaward.

• Stepped counter-regional systems are distin-guished by sagging sediments on short weldedallochthonous salt sheets. A stepped counter-regional system consists of a major listriclandward-dipping growth “fault” or leaningsalt diapir. This fault is actually a landward-dipping salt weld that passes downward into aflat salt weld and, even more deeply, intoanother landward-dipping salt weld that rootsinto the flat source layer. Sediment wedges dipand thicken seaward.

In these varied ways, thick salt sheets can betransformed into a three-dimensional network ofirregular salt bodies partly connected by thinsmears of salt and arrays of faults.

The Petroleum SystemThe earlier discussions on tectonics anddeposition touched upon key elements requiredfor creation and accumulation of hydrocarbons.These elements, long recognized by the oil and gasindustry, have been codified into a single conceptknown as the petroleum system (above). Aneffective petroleum system comprises thefollowing elements: • source rock containing organic material of suf-

ficient quality and quantity for the generationof hydrocarbons

• temperature and pressure envelope (achievedthrough burial) suitable for converting organicmaterial into hydrocarbons

• hydrocarbon migration process and pathway• reservoir rock with enough porosity to

accumulate and store hydrocarbons and suf-ficient permeability to eventually producethe hydrocarbons

• trap and seal to stop the migration processand provide containment within the reservoir

• timing that ensures hydrocarbon creation andmigration occurred while the trap and sealwere present to retain the hydrocarbons asthey migrated through the system

• preservation to preclude destruction througherosion, tectonics or temperature.

The absence of any one of these elements willcondemn the viability of a prospect. Until the mid1980s, the search for reservoir, trap and seal inthe Gulf of Mexico basin was focused on stratalying above the salt canopy. Although an effective

petroleum system had been confirmed with eachdiscovery made above the salt, there was noevidence that requisite conditions existedbeneath it.

This mindset was challenged in 1983, whenPlacid Oil Company drilled a well through twothin salt sheets before being forced to plug andabandon it in the third salt body it encountered.27

Although the borehole penetrated only 295 ft [90 m] of subsalt sediment, with no indication ofpay, this well drilled completely through twosheets—rather than diapirs. This spurredinterest from other operators that helped set thestage for further subsalt drilling. Then, in 1986,Diamond Shamrock penetrated 990 ft [302 m] ofsalt before drilling out into a 1,000-ft [305-m]reservoir-quality sand section. No hydrocarbonswere encountered in this South Marsh IslandBlock 200 well, but drilling results confirmedthat sandstone of sufficient porosity andpermeability could be found beneath salt. Fouryears later, Exxon drilled a commercial discoveryin 4,350 ft [1,326 m] of water at its Micaprospect, in Mississippi Canyon Block 211. Exxondrilled this deepwater well through 3,300 ft[1,021 m] of salt before discovering a reservoirestimated to contain 100 to 200 million bbl [15.9to 31.8 million m3] of oil equivalent—provingthat an effective petroleum system could indeedexist beneath the salt.28

Today, E&P companies have a much betterunderstanding of the subsalt region, thankslargely to data and experience gained throughdeepwater and subsalt drilling, along withimproved seismic acquisition, processing andimaging techniques.29 There is no longer anydoubt that all elements of the petroleum systemcan be found above and below the salt. Byexploring the subsalt region, E&P companies arelearning how salt affects structure and deposi -tion and how interactions between salt andoverburden influenced the development ofpetroleum systems in the Gulf of Mexico basin.

Geoscientists have come to understand that,as the basin evolved and continued to subside,burial and subsequent heating of organic-richsource rocks of Jurassic—and perhapsCretaceous—age created an excellent system forgenerating hydrocarbons. The deformation of theautochthonous Louann Salt on the floor of thebasin below the source rocks, and subsequentdeformation of the salt into pillows, diapirs andallochthonous salt bodies, resulted in numerousstructural traps. Faults created during extensionof the salt and overlying sediments would, inmany cases, provide conduits for hydrocarbonmigration to potential reservoir rocks above. Insome cases, faulting juxtaposed permeable sands

> Petroleum system. To evaluate the viability of a petroleum system, geoscientists must determinewhether all critical elements exist, such as source rock, migration route, reservoir rock, trap and seal.These elements must be weighed against the timing of key processes including generation ofhydrocarbons and their expulsion, migration, accumulation and preservation.


Autumn 2008 15

against impermeable shales to create traps andseals. Thick deltaic and turbidite clasticsediments created potential reservoir rocks ofvarying quality throughout the Tertiary, fromhigh-permeability and high-porosity quartzosesandstones in younger rocks to less-permeablearkosic sandstones in older rocks. Studies ofbasin history have also indicated favorabletiming of fluid expulsion and hydrocarbonmigration. Thus the northern Gulf’s subsalt playappears to have all the elements necessary for aneffective petroleum system.

Revolution in the Gulf For the first 40 years of drilling in the Gulf ofMexico, it was not unusual for drillers to stopshort of targeted depth once they encounteredsalt. These encounters were not predictable—seismic coverage was often sparse, and it was notuncommon for wildcats to be drilled solely on thebasis of a few lines of 2D seismic data. Moreover,sparse coverage and early processing techniques

sometimes led operators to target poorly resolvedseismic structures or seismic bright-spot anom -alies known as direct hydrocarbon indicators(DHIs). However, these seismic targets some -times turned out to be the top of salt. Havingdrilled to targeted depth without striking pay, butrather finding their bit in salt, most operatorswere reluctant to drill ahead.

One supposed DHI target helped to launchthe subsalt trend in the northern Gulf of Mexico.The Placid Oil Company No. 2 well at Ship ShoalBlock 366, mentioned previously, was drilled to aDHI target that instead encountered three saltbodies before being plugged and abandoned.30

Several other subsalt wells drilled in the 1980sand 1990s also found salt while aiming for DHIs.

In the 20 years following the dry hole at Ship Shoal, more than 140 subsalt wells have beendrilled in the Gulf of Mexico.31 Although somewere not commercial, several of these wells werenotable for extending the trend or setting recordsin their day (above).

Of these wells, only 50 were drilled inrelatively shallow waters of the Outer ContinentalShelf; their water depths ranged from 93 to 560 ft[28 to 171 m]. The rest were drilled on theContinental Slope, in water depths ranging from630 to 7,416 ft [192 to 2,260 m]. There was nogradual and deliberate move from shallow todeeper waters; in the year following its Ship ShoalBlock 366 dry hole, Placid drilled another subsaltwell in 2,004 ft [610 m] of water.32

27. Moore DC and Brooks RO: “The Evolving Exploration of the Subsalt Play in the Offshore Gulf of Mexico,”Transactions, Gulf Coast Association of Geological Societies 45 (1995): 7–12.

28. DeLuca M: “Forty-Six Wells Designated Subsalt in the Gulf of Mexico,” Offshore Magazine 59, no. 1 (January 1999): 50, 52, 145.

29. Camara Alfaro J, Corcoran C, Davies K, Gonzalez Pineda F,Hampson G, Hill D, Howard M, Kapoor J, Moldoveanu Nand Kragh E: “Reducing Exploration Risk,” OilfieldReview 19, no. 1 (Spring 2007): 26–43.

30. Moore and Brooks, reference 27.31. US Department of Interior Minerals Management Service

(MMS), Table of Subsalt Wells, http://www.gomr.mms.gov/homepg/offshore/gulfocs/subsalt/data/Subsalt_Wells.xls(accessed August 11, 2008).

32. MMS, reference 31.

211

349

128

127

292

699

116

346

778

783

12,763

16,563

18,454

14,730

17,976

19,525

21,600

11,833

23,531

16,867

1,328

113

215

192

1,038

1,869

98

96

1,844

1,423

1990

1993

1994

1995

1995

1998

1998

1998

1999

1999

Mica

Mahogany

Enchilada

Chimichanga

Gemini

Atlantis

Hickory

Tanzanite

Thunder Horse

Magnolia

—

—

Dry hole. Targeted a direct hydrocarbon indicator (DHI); drilled through salt.

Dry hole. DHI target proved to be salt; however, 1,000 ft [305 m] of wet,

reservoir-quality sand lay beneath it.

Ship Shoal

South Marsh Island

366

200

ProtractionArea Remarks

BlockNumber feet

YearDrilled

FieldName

Notable Wells of the Subsalt Play

8,203

13,500

138

145

4,356

372

705

630

3,405

6,133

323

314

6,050

4,668

Water Depth

453

475

3,890

5,048

5,625

4,490

5,479

5,951

6,584

3,607

7,172

5,141

meters feet meters

2,500

4,115

1983

1986

Total Depth

562

782

877

640

678

759

292

726

25,624

22,826

23,500

26,804

29,066

28,175

32,500

25,680

1,213

1,347

1,615

1,224

2,145

2,088

1,786

1,433

1999

2001

2001

2002

2003

2004

2006

2007

K2

Mad Dog

Redhawk

Tahiti

St. Malo

Jack

Kaskida

West Tonga

3,979

4,420

5,300

4,017

7,036

6,850

5,860

4,700

7,810

6,957

7,163

8,170

8,859

8,588

9,906

7,827

First significant occurrence of hydrocarbons in five thin pay zones beneath salt.

Put on line before Mica. Became the first commercial development in

the Gulf of Mexico subsalt play.

Salt weld discovery.

Second commercial subsalt discovery; drilled 1,300 ft [396 m] of salt, tested at

2,100 bbl/d [334 m3/d] of oil and 20 MMcf/d [566,337 m3/d] of gas.

Third commercial discovery of the play, in Pliocene-Miocene sands.

Deepest moored floating oil and gas production facility in the world

and also one of the largest.

Penetrated 8,000 ft [ 2,438 m] of salt.

First well tested 1,917 bbl/d [305 m3/d] of oil and 29.7 MMcf/d [841,100 m3/d]

of gas, one of the highest rates in the shallow-water Gulf of Mexico.

Largest field in the Gulf of Mexico.

Tension leg platform (TLP) installed in record water depth for TLPs.

Drilled through 10,000-ft [3,048-m] salt canopy of the Miocene fold trend.

300 ft [91 m] of net pay discovered in Mississippi Fan fold belt.

Produced from the world's first truss spar facility.

Middle Miocene trap beneath 11,000-ft [3,353-m] salt canopy.

First subsalt well of the Wilcox trend, drilled through 10,000 ft [3,048 m] of

Sigsbee salt canopy, with 450 ft [137 m] of net pay.

Wilcox test: deepest extended drillstem test in deepwater

Gulf of Mexico history.

First Wilcox subsalt wildcat located significantly inboard of the Sigsbee

Escarpment, northern frontier of Wilcox prospects. Found 800 ft [244 m] of net pay.

Discovered 350 ft [107 m] of net oil in three Miocene sands.

Mississippi Canyon

Ship Shoal

Garden Banks

Garden Banks

Mississippi Canyon

Green Canyon

Grand Isle

Eugene Island

Mississippi Canyon

Garden Banks

Green Canyon

Green Canyon

Garden Banks

Green Canyon

Walker Ridge

Walker Ridge

Keathley Canyon

Green Canyon

> Subsalt wells. Noteworthy wells that were drilled beneath the allochthonous salt show a range of water depths and targets. (Compiled from pressreleases and US Department of Interior Minerals Management Service, reference 31.)


The subsalt drilling campaigns of the 1980sand 1990s primarily targeted turbidite depositsof Pliocene age, with some Pleistocene andMiocene sands.33 Today, however, in a basinwhere 99% of proven oil reserves are producedfrom formations of Miocene age or younger, theGulf of Mexico subsalt trend is being rejuvenatedfrom deepwater and ultradeepwater discoveriesin much older Eocene and Paleocene sands(above). Discoveries in turbidite channel and fansystems, the deep-basin equivalent of onshoredeposits of the Wilcox formation in Texas andLouisiana, are helping to extend the subsalt play.These turbidite reservoirs have been discoveredmore than 250 mi [400 km] downdip from Wilcoxdelta systems.34

This deepwater Wilcox trend lies in 4,000 to10,000 ft [1,200 to 3,050 m] of water and isthought to cover some 30,000 mi2 [77,670 km2].Some prospects lie beneath salt canopies thatare 7,000 to 20,000 ft [2,130 to 6,100 m] thick.Interestingly, however, the early discoveries inthis trend never penetrated salt. At first,operators tried to avoid the salt by moving todeeper waters to drill outboard of the salt. There,seismic imaging was not distorted by salt, anddrillers had to contend only with familiarchallenges associated with drilling in deepwaters. However, once they established theviability of this deepwater Wilcox trend,exploration teams began chasing their prospectsupdip, beneath the salt, resulting in discoveriessuch as St. Malo and Jack.

The Lower Tertiary Wilcox play was initiatedby the second Baha well drilled at AlaminosCanyon Block 557, a prospect that originallytargeted fractured carbonates of Mesozoic age.In 1996, drilling problems forced the No. 1 well tobe abandoned before reaching total depth, but itdid encounter 15 ft [4.6 m] of pay in an UpperEocene sand, thereby suggesting that a viablepetro leum system and perhaps a commercialdiscovery might exist deeper within the structure.In 2001, the Baha No. 2 well was successfullydrilled to 19,164 ft [5,841 m] and showed theoriginal Mesozoic carbonates to be nonporous.Though this dry hole found only 12 ft [3.7 m] of oilin an Eocene sand, it also identified possiblereservoir-quality sands contained in more than4,000 ft [1,219 m] of a Wilcox turbidite section.35

Shortly thereafter, wells located south ofBaha, such as the Trident prospect (AlaminosCanyon Block 903), and Great White prospect(Alaminos Canyon Block 857), discovered Wilcoxpay sands in the Perdido fold belt of the westernGulf of Mexico. Subsequent drilling ventures tothe east resulted in Wilcox discoveries at theCascade, Chinook, St. Malo and Jack prospectslocated in the Walker Ridge area. The Kaskidadiscovery and the noncommercial Sardinia andHadrian wells in Keathley Canyon helped bridgethe gap between west and east. Data from thesewells allowed geologists to infer that Wilcoxsands extend more than 300 mi [480 km] across

the Gulf of Mexico basin. By 2007, at least 20wildcat wells had been drilled in this trend,resulting in 12 discoveries. Estimated recover -able reserves for each discovery ranged from 40to 500 million bbl [6.4 to 79.5 million m3] of oil.36

The St. Malo discovery at Walker RidgeBlock 678 is distinguished as the first well toreach Wilcox sands beneath allochthonous salt.Another subsalt Wilcox well was drilled at WalkerRidge Block 759—the much-heralded Jackdiscovery.37 A second well was drilled to appraisethe structure. This well—the only well in thesubsalt Wilcox to be tested—produced encour -aging results: it was announced that the Jackwell flowed for 23 days, sustaining a rate of6,000 bbl/d [953 m3/d] of oil, while testing 40% ofthe total net pay interval (next page). To thenorthwest, the Kaskida well marks thenorthernmost frontier for Wilcox subsaltdiscoveries to date and lies farther from the edge of the Sigsbee Escarpment than others ofthis trend.

How many more trends and fields willeventually be discovered beneath the expanse ofthe Sigsbee salt canopy? Once hampered by poorimaging, subsalt exploration has benefitted fromrevolutionary approaches to measuring proper -ties of deep formations. New seismic acquisitiontechniques, together with more accurate velocitymodels and migration algori thms, are helping tomeet the challenge of imaging beneath salt. Inparticular, wide-azimuth and rich-azimuthseismic survey techniques obtain improvedsignal-to-noise ratios in complex subsalt geologyand provide natural attenuation of certain typesof signals that cause multiple reflections.38 TheQ-Marine system capitalizes on less-attenuatedlow frequencies to better image the subsurface.It also provides large, steerable, calibratedsource arrays for better subsalt energy, single-sensor recording to improve noise sampling andattenuation, and the capability to record whilethe vessel turns to a different heading.

With the aid of complementary nonseismictechnologies that measure different properties ofthe subsurface terrain, geoscientists andengineers are building more comprehensivemodels to help E&P companies better determinethe viability of a prospect and identify drillingrisks before moving a rig onto location. Inaddition to seismic data, geoscientists areturning to marine magnetotelluric, gravimetricand electromagnetic surveys. These surveys haveadvanced well beyond first or second generation,and all are acquired by marine survey vessels.

Complementary technologies have been usedto improve delineation of prospects located above

16 Oilfield Review

Keathley Canyon

Green CanyonGarden BanksEast Breaks

Texas Louisiana

Lund

Atwater Valley

Mississippi Canyon

Keathley Canyon

Kaskida

JackChinook

Lower Tertiary TrendLower Tertiary Trend

CascadeSt. Malo

Trident

BahaGreat White

Walker RidgeAlaminos Canyon

> The Lower Tertiary trend. In pursuit of Lower Tertiary targets, operators have moved farther off theShelf into substantially deeper waters. (Modified from Richardson GE, Nixon LD, Bohannon CM,Kazanis EG, Montgomery TM and Gravois MP: “Deepwater Gulf of Mexico 2008: America’s OffshoreEnergy Future, OCS Report MMS 2008-013.” New Orleans: US Department of Interior MineralsManagement Service Gulf of Mexico OCS Region, 2008.)


Autumn 2008 17

salt, where data from marine magneto telluricsurveys help to fine-tune seismic processing byidentifying and measuring the depth andthickness of resistive strata, and by predictingreservoir fluid properties. Subsurface structurescan also be identified by combining gravitysurveys, ocean-bottom profiling surveys and 3Dseismic data to highlight salt domes anddepositional features such as buried sand-richchannels. Controlled-source electromagnetic(CSEM) surveys identify subsurface resistors andcan be integrated with seismic surveys for a morecomplete picture of hydrocarbon traps. Afterdrilling, time-lapse seismic surveys can be run totrack production and evaluate reservoir sweepefficiency. Recognizing the value of suchcomplementary technologies in suprasalt applica -tions, geoscientists are now trying to adapt themto the subsalt regime.

These advances will help geoscientists imagenew prospects and, in turn, spur newdevelopments in drilling, logging and productiontechnologies. The Lower Tertiary trend, a newfacet of the subsalt play, is just the latest wave ofrejuvenation to sweep the Gulf. Thus, explorationcontinues to evolve as advances in technologyreveal new targets and open more frontiers in asupposedly mature drilling province.

Driven by SaltTechnologies developed for exploring the subsaltprovince of the deepwater Gulf, along with theexperience gained in the process, will provehelpful in developing future subsalt plays. TheGulf of Mexico is, after all, just one of 35 basinsaround the world that contain allochthonous saltbodies.39 These salt bodies can be found offshoreAngola, Brazil, Canada, Madagascar, Mexico,Morocco and Yemen. Some basins share remark -able similarities with others around the world.

For instance, a close analog of the SigsbeeEscarpment is seen in the ultradeepwaterKwanza basin off the coast of Angola. The AngolaEscarpment is roughly 685 mi [1,100 km] longand marks the seaward limit of an allochthonoussalt fringe, although the fringe is typically less

than 12 mi [20 km] wide.40 The Scotian basin off the coast of Nova Scotia, Canada, also hassimilarities such as Cretaceous and Tertiaryturbidite deposition, along with traps controlledby lateral and vertical motion of Triassic-Jurassic salt.41

Not all salt bodies are found in offshorewaters; some are in orogenic belts that markepisodes of mountain-building, or are otherwiselandlocked. These salt bodies can be found inAlgeria, Canada, Colombia, Germany, Iran,Kazakhstan, Peru, Spain and Ukraine. Soalthough salt in the Gulf of Mexico basin hasbeen explored by drill bit and seismic boat, saltin mountain ranges has been studied extensivelyin outcrop and explored hands-on. Geoscientistsare recognizing similarities in each, and studiesof salt structures in the field yield insights intothose found beneath the sea.

Comparisons have been made between theGulf of Mexico basin and the Precaspian basin ofKazakhstan and Russia.42 The Precaspian basinwas formed in Permian times as a result of theUralian orogeny. Beyond age differences, both

basins experienced rapid salt deposition andwere dominated by a major source of sedimen -tation (the Mississippi and Volga rivers). Bothalso display salt overhangs that spread intoallochthonous salt sheets. Recognizing differ -ences between basins is also helpful, as in thepresalt play of Brazil. In the Santos basin,different processes are at work. There, largediscoveries within carbonate reservoirs arelocated beneath autochthonous salt and arestructurally and stratigraphically unaffected bysalt tectonics.

Identifying such similarities or differenceshelps E&P companies recognize the presence ofcharacteristics in one basin that may point tocorresponding, yet previously undiscoveredfeatures in analogous basins. Thus, intenselyexplored outcrops—such as the salt glaciers ofIran—or extensively explored basins—such asthose of the Gulf of Mexico, the Precaspian orWest Africa—complement each other in helpingto unlock hydrocarbon potential in other basinsaround the world. —MV

33. Moore and Brooks, reference 27.34. Lewis J, Clinch S, Meyer D, Richards M, Skirius C,

Stokes R and Zarra L: “Exploration and Appraisal Chal-lenges in the Gulf of Mexico Deep-Water Wilcox: Part1—Exploration Overview, Reservoir Quality, and SeismicImaging,” in Kennan L, Pindell J and Rosen NC (eds):Proceedings, 27th Annual Gulf Coast Section SEPMFoundation Bob F. Perkins Research Conference (2007):398–414.

35. Lewis et al, reference 34. 36. Lewis et al, reference 34.37. For more on the testing program for the Jack discovery:

Aghar H, Carie M, Elshahawi H, Gomez JR, Saeedi J,Young C, Pinguet B, Swainson K, Takla E and Theuveny B:

“The Expanding Scope of Well Testing,” Oilfield Review 19, no. 1 (Spring 2007): 44–59.

38. Camara Alfaro et al, reference 29.Multiple reflections, commonly known as multiples, are caused by seismic energy that repeats itself as it isreflected more than once from a boundary. Multiplesoften complicate attempts to discern the image of thesubsurface, and a great deal of effort in seismic dataprocessing is spent attempting to distinguish betweenprimary and multiple energy and then to remove the multiple reflections.

39. Hudec MR and Jackson MPA: “Advance of AllochthonousSalt Sheets in Passive Margins and Orogens,” AAPG Bulletin 90, no. 10 (October 2006): 1535–1564.

40. Hudec and Jackson, reference 39.41. Mukhopadhyay PK, Harvey PJ and Kendell K: “Genetic

Relationship Between Salt Mobilization and PetroleumSystem Parameters: Possible Solution of Finding Commercial Oil and Gas Within Offshore Nova Scotia,Canada During the Next Phase of Deep-Water Exploration,” Transactions, Gulf Coast Association of Geological Societies 56 (2006): 627–638.

42. Lowrie A and Kozlov E: “Similarities and Differences in Salt Tectonics Between the Precaspian Basin, Russia, and the Northern Gulf of Mexico, USA,” Transactions, Gulf Coast Association of Geological Societies 54 (2004): 393–407.

Texas Louisiana

Jack

Mississippi

> Preparing to test. The Jack 2 well, drilled by the Discoverer Deep Seas drillship, was cased andsuspended before the Cajun Express semisubmersible rig moved in for an extended well test. Bargeswere also brought in to collect fluids produced by the test.


the prize beneath the salt

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