barnett shale

Petroleum Frontiers Excerpt

PI/Dwights PLUS Drilling Wire™

The following is excerpted and adapted from Barnett Shale: A New Gas Play in the Fort Worth Basin, authored byScott Montgomery, the latest in IHS Energy’s Petroleum Frontiers series, a quarterly investigation into the mostpromising hydrocarbon horizons and provinces. The 78-page publication includes more than 75 figures and anextensive bibliography. The material presented here is from Chapter 4.

For more information on IHS Energy’s Petroleum Frontiers series, call (888) 645-3282.

Barnett Shale: Detailed Discussion

Figure 4.1. Regional isopach data, Barnett Shale. Crosssection lines refer to Figure 4.2. Source: Pollastro

Barnett StratigraphyRegional stratigraphic relationships for the Barnett

Shale are fairly well established. As noted in the previouschapter, the formation is overlain by interbedded shaleand limestone of the lower Marble Falls/Comyn interval.It overlies the regional unconformity capping the LowerPaleozoic (Ellenburger, Viola/Simpson), except in thenorthwestern part of the basin. Here, extending eastwardfrom the Bend Arch, the basal portion of the Mississip-pian is formed by a sequence of carbonate bank depositsknown as the Chappel Shelf (Figure 4.1). The Barnettthins rapidly over these deposits, but remains conform-able above them. These relationships are shown by thecross sections in Figure 4.2.

Stratigraphic divisions within the Barnett are less wellestablished. Several schemes have been proposed, withvarying degrees of complexity and basin-wide applicabil-ity (see, for example, Henry, 1982, for the northern basinarea). Current usage by operators involved in the Barnettplay is shown on Figure 4.3. Divisions include an upperBarnett interval (50-150 ft thick), the Forestburg Lime-stone (where present, 10-200 ft thick), and a lowerBarnett interval (50-400 ft). Definition of upper andlower Barnett units has been made on the basis of twoseparate intervals of high radioactivity and resistivity,which occur regionally throughout the basin where theBarnett is more than about 100 ft thick in total. TheForestburg Limestone, which divides the upper and lowerBarnett, is confined to the northeastern part of the basin.The Forestburg pinches out near the southern limit ofNewark East Field (southern Wise County). South of this



NE

A Denton Wise Denton Tarrant Parker Hood Somervell Erath Hamilton Lampasas

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A'FORT WORTH BASIN

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BThrockmorton Stephens Palo Pinto Hood Somervell Johnson Hill

CHAPPEL LIMESTONE SHELF BEND ARCH FORT WORTH BASIN

SE

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THIC

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Chappel LsLower Barnett

PinnacleReef

BarnettShelf

(Bank Deposits)Forestburg Ls

Marble FallsW E

Viola-Simpson

NewarkEast Field

EllenburgerLs

Figure 4.2. Regional cross sections, illustrating thicknesstrends and stratigraphic relationships for the BarnettShale, Fort Worth Basin. Modified from Pollastro, 2004.

Figure 4.3. Stratigraphic diagram showing commondivisions in Newark East Field and areas to the west, FortWorth Basin-Bend Arch.

field, division between the upper and lower Barnett is notstandardized and is defined somewhat arbitrarily, on thebasis of one or another log marker.

Where it outcrops along the northern flanks of theLlano Uplift, the Barnett is relatively thin (<50 ft) andundivided. It is also undifferentiated over the Bend Archand Chappel Shelf (see Figure 4.1). In the northeastern-most part of the basin, where the Barnett reaches thick-nesses of over 900 ft, the formation contains a number oflimestone intervals that complicate the simple upper/lower divisional scheme.

Stratigraphic relationships and general lithology

suggest that the Barnett was deposited on a gentle south-southwest facing slope, which developed a somewhatrapidly subsiding deep adjacent to the incipiently risingMuenster Arch. As such, the Barnett is interpreted torepresent the last stage of quiet-water, shelf-type deposi-tion prior to initiation of the Fort Worth Foreland Basin.

LithologyOver most of the Fort Worth Basin and eastern Bend

Arch, the Barnett Shale is an organic-rich, siliceous shalewith variable amounts of limestone, minor dolomite, anda scattering of exotic minerals. Though often described asa “black shale,” this term can be misleading when applied

(Petroleum Frontiers Excerpt, cont’d)



to the Barnett. In fact, the formation is rich in silica(35%-50%, by volume) and relatively poor in clayminerals (less than 35%, usually). It is, therefore, litho-logically distinct from various well-known black shalessuch as the Antrim, Bakken, Chattanooga or Woodford.Silica in the Barnett is predominantly derived fromradiolarian tests.

Important lithologic changes occur in the Barnettfrom south to north. Where it outcrops along the LlanoUplift, the Barnett is 35-50 ft thick and contains highlypetroliferous intervals (Turner, 1957), with up to 13%total organic carbon by weight (Jarvie, 2001). In thesubsurface, the formation thickens northeastward from50 ft, close to the Llano Uplift and along the Bend Arch,to 400 ft in Newark East Field, to over 1,000 ft adjacentto the Muenster Arch. South and west of a line throughnorthern Palo Pinto, Parker and Tarrant counties, theBarnett is generally poor in carbonate material, contain-ing only a few thin limestones. Northeast of this line,however, in Wise, Denton and Montague counties, theamount of carbonate increases rapidly. The ForestburgLimestone changes from a feather edge in southern WiseCounty to being over 200 ft thick about 15 miles north.Even further northeast, especially in Montague County,the Barnett contains multiple debris-flow limestoneintervals, along with calcareous shale, testifying to erosionof exposed Paleozoic carbonates from the Muenster Archin the late Mississippian.

The subsurface Barnett is generally dominated bysiliceous shale. The basal portion of the Barnett in thecentral and eastern part of the basin frequently contains athin zone (<10 ft) of highly phosphatic material (Bowker,2002), making for easy distinction on logs and in mudlog samples from underlying Viola or Ellenburgerdeposits. Organic content is highest (3%-10%, byweight) in the most silica-rich intervals. These intervalsare also the primary producing facies of the Barnett. Theyare generally more abundant in the lower part of theinterval, but are significant in the upper Barnett as well.According to Bowker (2002), the average composition ofthis facies (by volume) is as follows: 45% quartz; 27%

illite, with minor smectite; 8% calcite + dolomite; 7%feldspar; 5% organic matter; 5% pyrite; and 3% siderite,with trace amounts of native copper and phosphaticminerals. In the northern basin, where the Barnettthickens rapidly, the formation contains an increasingnumber of carbonate-rich zones, including what appearto be debris flow-type deposits (Bowker, 2002). In thewestern part of the basin, meanwhile, along the flanks ofthe Bend Arch, fine-grained calcareous material is fairlyabundant in the lower Barnett, due to wave and currentdistribution of debris from Chappel reefs (Henry, 1982).

In addition, the Barnett appears to change characterlocally over erosional and karst-related highs. Differentialcompaction apparently has caused thinning and en-hanced alignment of clay particles in the formation acrosssuch features (Zhao, 2004). This seems to have reducedporosity/permeability in the Barnett still further, and tohave increased rock brittleness. Whether other changes—such as relate directly to composition—occur in suchsettings is not known.

Log CharacterIn general, the Barnett is distinguished by very high

radioactivity and high resistivity log signatures, whichdifferentiate it easily from overlying Marble Falls/Comynand underlying Viola/Ellenburger carbonate-bearingintervals. Gamma ray values of up to 300 API units andresistivities of 100-1,000 ohm-meters are common.Gamma ray values, in particular, tend to be highest whereorganic material is especially abundant.

As shown on Figure 4.4, the lowermost portion ofthe Marble Falls, which consists of interbedded carbonateand shale, contains a rather prominent limestone markerknown informally as the “Barnett Limestone.” Thismarker lies about 25-40 ft above the first high-radioactiv-ity shale bed used to designate the top of the Barnett. Theupper Barnett interval commonly includes several zoneswith gamma ray values above 100 API units and most ofthe entire interval above 75 API units (Figure 4.4). Whenpresent at significant thickness (>20-25 ft), the Forest-burg appears on logs as a distinct decrease in gamma ray




Barnett LsMarker

TopBarnett

UpperBarnett

ForestburgLs

Figure 4.4. Type log showing detailed character of upper Barnett. Courtesy: Star of Texas Energy.




ForestburgLs

LowerBarnett

Viola Figure 4.5. Type log, lower Barnett. Courtesy:Star of Texas Energy.

values to less than 40 API units. Below the Forest-burg, the lower Barnett appears as a secondinterval of high gamma ray/high resistivity signa-tures (Figure 4.5). Also notable on Figures 4.4 and4.5 is the decrease in bulk density through thenon-Forestburg Barnett (RHOB values as low as2.4-2.5 g/cc).

Formation evaluation using log data in theBarnett has proved challenging, due to the uncon-ventional nature of the reservoir. One series ofapproaches has been published by Johnston (2004)and is shown in Table 4.1. This author notes that,in addition to high gamma ray and resistivitymeasurements, Barnett reservoirs typically showdensity porosity of 0-16 units and neutron poros-ity slightly to much higher than this (see Figure4.4 and 4.5). Johnston (2004) emphasizes the useof resistivity and FMI data for identifying fractureswith sufficient aperture to contribute to productiv-ity, and he also outlines the use of calibrated logdata for identifying clay-rich and silica-richintervals. An example of such calibrated data, usedto help locate potentially productive intervals, isshown in Figure 4.6. It should be mentioned,however, that the importance of natural fracturesto well performance is strongly contested by otherworkers familiar with the play, who stress that suchfractures are normally mineralized and thus act asflow barriers. Thus, the discussion offered byJohnston (2004) may not be representative.

Basic Reservoir CharacteristicsThe Barnett Shale is an unconventional reservoir

with low porosity and very low permeability.Porosities in productive siliceous shale intervalsaverage 6%, with permeabilities varying from 0.01md down to the nano-darcy range. The reservoirhas no free water; measured water saturations of




Figure 4.6. Calibrated and interpreted log response,Barnett Shale, correlated against perforated zones(shown on left). Source: Johnston, 2004

Log Type

Resistivity

Gamma ray

Density

Neutron

Sonic

Micro-resistivity/Electric images

Spectroscopy

Properties Measured

Bound water volume, both clay and pores

Clay and organic material volume

Minerals and fluids content

Clay and gas content

Clay and gas content

Identify natural and drilling-induced fractures,pyrite, calcite nodules, other geologic features

Organic carbon content, clay and carbonaceous minerals

Table 4.1. Log type and reservoir characteristics. Source:Johnson, 2004

35% correspond to water bound up in clay minerals. Gasis held within the Barnett reservoir in two basic ways. Itoccurs as free gas, whether in matrix porosity, anyremnant fracture porosity and—possibly—in micro-porosity. Gas also exists in sorbed form on organicmatter; such gas is therefore particularly abundant in highTOC intervals.

It is assumed that production from the Barnettinvolves two main stages. Initial flow into inducedfractures consists of free gas from matrix porosity, anyopen natural fracture apertures and—possibly—frommicroporosity. The formation is known to be somewhatoverpressured (0.49-0.54 psi/ft pressure gradient), andsince water saturations are generally rather low (e.g.35%), this overpressuring is interpreted to be a result offree gas. A second stage of production begins once asignificant portion of the free gas has been withdrawnand reservoir pressure decreases, stimulating increaseddesorption of gas from organic carbon in the Barnett.The latter portion of this second stage involves long-livedproduction (flattened rate of decline) of desorbed gas.

Hydrocarbons within the Barnett reservoir changeeast to west and also north to south across the basin(Figure 4.7). In the far eastern portion, dry gas predomi-nates. This changes westward to wet gas, gas with oil




BAYLOR

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chita

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ldBe

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Oil

Gas w/Oil

Dry Gas

Figure 4.7. Patterns of hydrocarbon occurrence in theBarnett Shale, Fort Worth Basin-Bend Arch area. Modi-fied from Jarvie, 2001.

and—finally, along the flanks of the Bend Arch—tomainly oil. Figure 4.7 also shows that oil is present withinthe Barnett in the northern part of the basin, changingsouthward to wet gas/gas w/oil, and finally to dry gas.These patterns of occurrence do not correspond well topresent-day depth/burial patterns for the Barnett, sug-gesting that thermal history in the basin has been com-plex and perhaps multi-phased (Jarvie, 2001; Pollastro,2004).

For most gas wells, the upper Barnett yields 20-25%of the total production, with the lower Barnett delivering75-80%. Perforated intervals commonly range fromabout 35-75 ft in the upper Barnett and 100-200 ft inthe lower Barnett, depending on thickness of eachinterval. Data from stress tests indicate that siliceousintervals respond better to hydraulic fracturing than domore clay-rich intervals (Johnston, 2004).

It is known that the upper Barnett has a higher fracgradient than the lower Barnett, particularly where theForestburg Limestone is present. This gradient is 0.70psi/ft or above for the upper Barnett and 0.50-0.60 (orslightly higher) for the lower Barnett (Martineau, 2003).In addition, where the Barnett is especially thick (>450ft) and productive, in northern Newark East Field,different productive zones in the lower Barnett haveshown different frac gradients. The specific reasons forsuch variation are not entirely understood. However, it isassumed by many workers that such differences arerelated to changes in source potential and gas generationwithin each zone, and to sealing potential of surroundingshales. Some geologists have also proposed that naturalfracturing may be involved.

Considerable debate exists over the role of naturalfractures with regard to Barnett productivity. Some recentpublications (see, for example, Johnston, 2004) haveemphasized the crucial role of such fracturing. Earlyreports from “data wells” in the Barnett also focused onthe assumed probability that natural fractures wererequired for economical production (CER Corp., 1992).These reports, however, indicated that fractures werelargely or entirely mineralized with calcite and that




stimulation did not open these structures but insteadgenerated new fracture sets with orientations that differedfrom natural sets by 90-120 degrees. Workers withconsiderable experience in the play have stated that coresreveal only sealed fractures, which would effectivelyreduce deliverability (by acting as local flow barriers)(Bowker, pers. comm., 2004).

Bowker, K.A., 2002, Recent developments of the Barnett Shaleplay, Fort Worth Basin; in Law, B.E. and Wilson, M., eds.,Innovative Gas Exploration Concepts Symposium: RockyMountain Association of Geologists and Petroleum TechnologyTransfer Council, October, 2002, Denver, CO, 16 p.

CER Corporation, 1992, Geological, petrophysical andengineering analysis of the Barnett Shale in the MitchellEnergy Corporation T.P. Sims No. 2, Wise County, Texas: GasResearch Institute Contract Report No. 5091-212-2242, 83 p.

Henry, J.D., 1982, Stratigraphy of the Barnett Shale (Missis-sippian) and associated reefs in the northern Fort Worth Basin;in C.A. Martin, ed., Petroleum Geology of the Fort WorthBasin and Bend Arch Area: Dallas Geological Society, p. 157-178.

Jarvie, D.M. and L.L. Lundell, 1991, Hydrocarbon generationmodeling of naturally and artificially matured Barnett shale,Ft. Worth Basin, Texas, Southwest Regional GeochemistryMeeting, Sept. 8-9, 1991, The Woodlands, Texas, 1991, oralpresentation.

Johnston, D., 2004, Barnett Shale-1: Technological advancesexpand potential play; Oil and Gas Journal, Jan. 19, 2004, vol.102, no. 3, p. 51-59.

——— , 2004, Barnett Shale-Conclusion: Reservoir charac-terization improves stimulation, completion practices; Oil andGas Journal, Jan. 26, 2004, vol. 102, no. 4, p. 35-39.

Martineau, D., 2003, Newark East, Barnett Shale Field, Wiseand Denton Counties, Barnett Shale frac gradient variances;(abs.); 2003 AAPG Southwest Section Meeting, Fort Worth,TX; available online at: http://www.fwgs.org/swsec/techsessions.htm. Accessed: February 15, 2004

Pollastro, R.M., 2004, Geologic and production characteristicsutilized in assessing the Barnett Shale continuous (unconven-tional) gas accumulation, Barnett-Paleozoic Total PetroleumSystem, Fort worth Basin, Texas.

Turner, G.I., 1957, Paleozoic stratigraphy of the Fort Worthbasin; in Bell, W.C., ed., Abilene and Fort Worth GeologicalSocieties Joint Field Trip Guidebook, p. 57-77.

Zhao, H., 2004, Thermal maturation and physical propertiesof Barnett Shale in Fort Worth Basin, North Texas (abs);American Association of Petroleum Geologists AnnualMeeting, 2004; Session on Unconventional Gas; availableonline at: http://aapg.confex.com/aapg/da2004/techprogram/A87090.htm.

Select Bibliography

At present, this important issue remains to beresolved. Most core data from the Barnett are still propri-etary, due to the ongoing nature of the play. A futurestudy of fracturing in the Barnett, appropriately sup-ported by both analytical data and relevant images, wouldcontribute significantly to an overall understanding ofthis complex reservoir.


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