I Feature Article

Surface Exploration for Oil and Gas: Applications for Field Development and Production

by Dietmar "L)eetJ' Schwnache~ Geo-Microbial Technologies, Inc., Ochelata. Oklahoma

G eochemical exploration for petroleum is the search for chemically identifi-

able surface or near-surface occurrences of hydrocarbons as clues to the location of oil and gas accumulations. Observations range from clearly visible oil and gas seepage (macroseepage) at one extreme to the iden- tification of minute traces of hydrocarbons (microseepage) or hydrocarbon-induced changes at the other.

The past decade has seen a renewed interest in geochemical exploration which, when coupled with developments in analytical and interpretive methods, has produced a new body of data and insights in this area. Many of these developments and new insights are published AAPG Memoir 66. "Hydrocarbon Migration and its Near-Surface Expression" (Schumacher and Abrams, 1996). Recent geochemical and geomicrobiological surveys and research stu&es document that hydrocar- bon microseepage from oil and gas accu- mulations is common and widespread, is predominantly vertical (with obvious exceptions in some geologic settings) and dynamic (responds quickly to changes in reservoir conditions). These characteristics

expand the suite of applications for surface geochemical surveys to include field development, reservoir characterization, and monitoring patterns of hydrocarbon drainage. Combined with more established uses of surface geochemistry like high- grading concessions, exploration leads, and prospects, the new applications show great promise for the wider use of surface exploration methods in exploration and field development.

Assumptions The underlying assumption of all near-sur- face geochemical exploration techniques is that hydrocarbons are generated, trapped at depth, and leak in varying but detectable quantities to the surface. This is an estab- lished fact and the close association of surface geochenlical anomalies with faults, productive fairways, and specific leads and prospects is well known. It is further assumed that the anomaly at the surface can be reliably related to a petroleum accu- mulation at depth. This is successful in areas of relatively simple geology and becomes increasingly difficult as the geology becomes more complex. A geochemical or microbial anomaly at the surface represents the end of a petroleum migration pathway, a pathway that can range from short distance vertical migra- tion at one end of the spectrum to long

distance lateral migra- tion at the other extreme. Relationships between surface geo- chemical anomalies and subsurface accumula- tions can be complex; proper interpretation requires integration of seepage data with geo- logical, geophysical, and hydrological data. Understanding geology, and hence petroleum dynamics, is the key to using seepage data in exploration. Figure 1 illus- trates some common seepage styles and migration pathways.

Microseepage Microseepage is defined as high concentra- tions of detectable light hydrocart>ons, or hydrocarbon-induced changes, in soils and sediments. The existence of microseepage is supported by a large M y of empirical evidence. This includes (1) increased concentration of light hydrocarbons and hydrocarbon-oxidizing microbes in soils and sediments above hydrocarbon reser- voirs; (2) an increase in key light hydrocar- bon ratios in soil gas over oil and gas reservoirs; (3) sharp lateral changes in the concentrations and ratios at the edges of the surface projections of IAe reservoirs, (4) similarity of stable carbon isotopic ratios

Figure 1: Euunple of common seepage styles crrd migration pathwuys from rlze Gulf of Mexico and North Sea (Thmsher el al.. 1996 AAPG Memoir 66).

June 1998 Houston Geological Society Bulletin

for methane and other light hydrocarbons in soil gases to those found in underlying memoirs; and (5) the disappearance and reappearance of soil gas and microbial anomalies in response to reservoir deple- tion and repressuring.

Microseepage rates and surface hydro- carbon concentrations can vary signif~cantly with time. Surface hydrocarbon seeps and soil geochemical anomalies have been shown to appear and disappear in relative ly short times, weeks to months to years. Results from studies of natural seeps, as well as seeps from underground storage reservoirs indicate that the rate of h y b c a n migration and microseepage varies from less than one meter per day to tens of meters per day. Microseepage occurs by a vettical migration mechanism of displacing water by ascending gas bubbles, that is, the "buoyancy of microbubbles." Computer modeling of this mechanism explains the surface observations. Furthermore, the close correspondence of surface anomalies to surface projections of a reservoir and the rapidly disappearing surface anomalies after the start of production are predicted by this model.

Anomaly Recognition Hydrocarbon microseepage data, whether soil gas or microbial or other i n k t mea- surements, is inherently noisy and requires adequate sample density to distinguish between anomalous and background areas. Undersampling is probably the major cause of ambiguity and interpretation fail- ures involving surface geochemical studies. To optimize the recognition of an anomaly, the sampling pattern and sample number must take into consideration the objectives of the survey. the expected size and shape of the anomaly (or geologic target). the expected natural variation in surface measurements, and the probable signal-to-noise ratio. Defining background values adequately is an essential part of anomaly recognition and delineation. For prospect evaluation, we suggest that as many as 70% of the samples collected be obtained outside the area of immediate interest. However, for very small targets such as pinnacle reefs or channel sandstones, optimum results are obtained when numerous samples are collected in a closely spaced grid pattern over the feature.

Figure 2 Sacramento basin gas @ld. Low microbial values aajacew to wells W e c t probable pressure depletion due to production or d r a i n e d reservoir compamnents are suggested by isolated anomalies between wells and by the large anomaly in nottheasr corner of the area

New Applications for Field Development and Production Detailed geochemical and geomicrobiolog- ical surveys and research studies have doc- umented that petroleum microseepage is a dynamic and predominantly vertical process which responds quickly to changes in reservoir conditions. These important characteristics make possible a new suite of applications for soil gas and microbial surveys, including: (1) early delineation of field limits; (2) finding by-passed oil and gas; (3) contributions to reservoir charac- terization; (4) evaluation of infiIl and step- out well locations; (5) documentation of hydrocarbon drainage over time; and (6) monitoring of waterflood operations.

Because hydrocarbon microseepage is nearly vertical, the extent of a soil gas or microbial anomaly at the surface can approximate the productive limits of the reservoir at depth. Furthermore, the detailed pattern of microseepage over a field can reflect reservoir heterogeneity

Houston Geological Society Bulletin

and distinguish hydrocarbon-charged com- partments from drained or uncharged com- partments. Additionally, since hydrocarbon microseepage is dynamic, seepage patterns can change rapidly in response to produc- tion-induced changes. These applications require close sample spacing and are most effective when results are integrated with subsurface data, especially 3-D seismic data. The need for such integration cannot be overemphasized. Seismic data will remain unsurpassed for imaging trap and reservoir geometry, but, especially on- shore, only detailed microbial or soil gas surveys can reliably image hydrocarbon microseepage from those same reservoirs. The following example illustrates the

hydrocarbon microseepage pattern over a large gas field in the Sacramento basin, California (Figure 2). In this survey, the seepage anomalies are recognized by high concentration of hydrocarbon-oxidizing microbes in the soil using a method Ionown au Microbial Reservoir Characterization.

June 1998

T ime Lapse Mon i to r ing of Rmcrvo i r Dra inaer Thew k \vidccprc;~d interest in the industry in thz possibility of repeating 3-D seismic surveys ;I! intcnxls during hydrocarbon prwluctinn imnt a field. in order In iblluw I l u ~ d rnnwnwnl through the reservoir and prcscure cli;tn:es withtn it. This is called 4-D sriwiic. and i s most effective with marinc seismic d:~t;~. Result\ of detailed rnicrohi;tl IMRCI surveys suggest gre;d polt'ntial ior wch time l;tpse rnrmiloring of reservoir dm~ni~pe usmg repeilt high-reso- lut~crn n~icm\rrpape surveys. l lnl ike 4-D seismic w w e y . nl!crnreep;lge surveys are n iwt effective on hnd ;tnd may represent a vinhlc ;~ltern;~livc to J-D seismic in some situ;~lion\. Inlormation p i ned from such surveys w ~ l l en:~hle rescrvoir geologists and cnginrers to cemstruct mnrc accurate licld nwdrlr. leuding to improvcd de~clop- nlcnt p l .~nn i~~g and LIII~IIIIIIL.~~ to increase ICLIIVC~~. U I c ;II~ vrry excited :thwt the potenrial n i wch lime hpse studies and h;~v+ se\.cr.~l dcmonstr;~tion projects underway. 11 wil l he some mnnths before rewlts wi l l he ;tvailahle for puhlic scrutiny and ~ I ~ I I S S ~ I ~ .

Conclu4ons Recent sntdics dcc~~n~ent that h y d r w u h ~ n n~icn)serp;tge fmm oil 21nd gas nccumula- l ions i\ common and widesprei~d. predominantly vertical (with ohvious exceptions in some geolrrgic settings), and Jynarnic (responds quickly to chlnges in reservoir conditions). Thrse chmcnsr ics create :I n c w su i t e o f application.; Iix soil g:w imd microhi;d suweys: field devel- opment. reservoir chxrxterizalions, and mcmitoring patterns o f hydrwuhon <Ininage. Seirmic dim wil l remain unsur- p i ~ . w I for detailed imaging of tr.lp and reservoir geomctry. however. only high- resolution microbial and soil gas st~rveys can image hydrocarbon microsccpi~gc f ~ o m many of thuse traps i ~nd reservoirs.

Acknowledgements T h i s : ~ r t i c l e i s an abbreviated version o f one puhlishcd in "Applications of Emerg-

ing Technol~~pirs~'", SMll 1'n.s~. Octokr IW7. Cqiie\ d ~ h c :!nick or of the list of references ;are ;~vail;~hle lrom the author.

BI~~\PIIICAI. Sfit~~.(.tt l h v t Schumnch~r i c Dimctcw v i Gewhmm- istv inr Ge~~>l icmhwl 'li*chnologics. Inc.. in 0chel:ltn. Oklnh~~m;~. He h:~r Inore than 25 years experience with Phillips Petrnlcum. Pennnril. :md in ;~c;lrlemia. Deet i s a pxst-president o f HGS ;md editor. with Michael Ahmns. of A,\/'(; Mwmir C,I,.

i l r ~ r t ~ r t : r t nrtd I . Nmr- .Swfi~rr E v p r ~ ~ s i o ~ ~ .

June 1998 Houston Geological Society Bulletin