Hydrocarbon Micro See Page Article

Surface geochemical exploration forpetroleum is the search for chemicallyidentifiable surface or near-surfaceoccurrences of hydrocarbons, orhydrocarbon-induced changes, asclues to the location of oil and gasaccumulations. It extends through arange of observations from clearly vis-ible oil and gas seepage (mac-roseepage) at one extreme toidentification of minute traces ofhydrocarbons (microseepage) orhydrocarbon-induced changes at theother.

Surface geochemical methodshave been used since the 1930s, but thepast decade has seen a renewed inter-est in geochemical exploration. This,together with developments in ana-lytical and interpretive methods, hasproduced a new body of data andinsights about geochemical explo-ration. Many of these developmentsare summarized in “HydrocarbonMigration and Its Near-SurfaceExpression” (AAPG Memoir 66).Geochemical surveys and researchstudies document that hydrocarbonmicroseepage from oil and gas accu-mulations (1) is common and wide-spread, (2) is predominantly vertical(with obvious exceptions in some geo-logic settings), and (3) is dynamic(responds quickly to changes in reser-voir conditions).

The principal objective of a geo-chemical exploration survey is toestablish the presence and distribu-tion of hydrocarbons in the area and,more importantly, to determine theprobable hydrocarbon charge to spe-cific exploration leads and prospects.For reconnaissance surveys, seeps andmicroseeps provide direct evidencethat thermogenic hydrocarbons havebeen generated; that is, they docu-ment the presence of a working petro-leum system and identify the portionsof the basin that are most prospective.Additionally, the composition of theseseeps can indicate whether a basin or

play is oil-prone or gas-prone. If theobjective is to evaluate individualexploration leads and prospects, theresults of geochemical surveys canlead to better risk assessment by iden-tifying those associated with stronghydrocarbon anomalies, thereby high-grading prospects on the basis of theirprobable hydrocarbon charge. Fordevelopment projects, detailedmicroseepage surveys can (1) helpevaluate infill or stepout drilling loca-tions, (2) delineate productive limitsof undeveloped fields, (3) identifybypassed pay or undrained reservoircompartments, and (4) monitorhydrocarbon drainage through use ofrepeat geochemical surveys. Hydro-carbon microseepage surveys havepotential to add value to 2-D and 3-Dseismic data by identifying those fea-tures or reservoir compartments thatare hydrocarbon-charged.

The underlying assumption of allnear-surface geochemical explorationtechniques is that hydrocarbons aregenerated and/or trapped at depthand leak in varying but detectablequantities to the surface. This has longbeen an established fact, and the close

association of surface geochemicalanomalies with faults, productive fair-ways, and specific leads and prospectsis well known. It is further assumed,or at least implied, that the anomalyat the surface can be reliably relatedto a petroleum accumulation at depth.The success with which this can bedone is greatest in areas of relativelysimple geology but becomes increas-ingly difficult as the geology becomesmore complex. The geochemicalanomaly at the surface represents theend of a petroleum migration path-way, a pathway that can range fromshort-distance vertical migration tolong-distance lateral migration. Anexample of these contrasting seepagestyles and migration pathways is illus-trated in Figure 1.

Seepage activity. Seepage activityrefers to the relative rate of hydrocar-bon seepage. Active seepage refers toareas where subsurface hydrocarbonsseep in large concentrations into shal-low sediments and the overlyingwater column. Active seeps often dis-play acoustic anomalies on conven-tional and high-resolution seismic

258 THE LEADING EDGE MARCH 2000 MARCH 2000 THE LEADING EDGE 00

Surface geochemical exploration for oiland gas: New life for an old technology

DIETMAR “DEET” SCHUMACHER, Geo-Microbial Technologies, Ochelata, Oklahoma, U.S.

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Editor’s Note: The Geologic Column, whichappears monthly in TLE, is (1) produced coop-eratively by the SEG Interpretation Committeeand the AAPG Geophysical IntegrationCommittee and (2) coordinated by M. RayThomasson and Lee Lawyer.

Figure 1. Spectrum of contrasting seepage styles and migration pathwaysfrom the Gulf of Mexico and the North Sea (modified from“Understanding geology as the key to using seepage in exploration: thespectrum of seepage style” by Thrasher et al., AAPG Memoir 66, 1996).

profiles. Such seepage occurs in basinsnow actively generating hydrocar-bons and/or that contain excellentmigration pathways. Active seeps areeasily detected by most geochemicalsampling methods. Examples of activeseeps are found in the Gulf of Mexico,offshore California, parts of the NorthSea, the southern Caspian Sea, off-shore West Africa, and offshoreIndonesia.

Areas where subsurface hydro-carbons are not actively seeping aresaid to be characterized by passiveseepage. Such seeps usually containlow molecular-weight light hydro-carbons and volatile higher molecularweight hydrocarbons above back-ground concentrations. Acousticanomalies may be present, but watercolumn anomalies are rare. Anoma-lous levels of hydrocarbon seepagemay be detectable only near majorleak points or at greater than normalsampling depths. Passive seepageoccurs in basins where hydrocarbongeneration is relict or migration is spo-radic or inhibited by a major migra-tion barrier. Areas with passiveseepage include many intracratonicbasins, offshore Alaska, the northwestshelf of Australia, central Sumatra,and parts of the North Sea.

As indicated above, there is a seep-age continuum from the lowestdetectable levels at one extreme to vis-ible oil and gas seeps at the other.Macroseepage refers to the visible oiland gas seeps. Microseepage is de-fined as elevated concentrations ofanalytically detectable volatile orsemivolatile hydrocarbons, or hydro-carbon-induced changes, in soils andsediments. The existence of micro-seepage is supported by a large bodyof empirical evidence including (1)increased concentration of lighthydrocarbons and hydrocarbon-oxi-dizing microbes in soils and sedimentsabove petroleum reservoirs, (2) anincrease, in key light hydrocarbonratios in soil gas over oil and gas reser-voirs, (3) sharp lateral changes in theseconcentrations and ratios at the edgesof the surface projections of thesereservoirs, (4) similarity with stablecarbon isotropic ratios for methaneand other light hydrocarbons in soilgases to those found in underlyingreservoirs, and (5) the disappearanceand reappearance of soil gas andmicrobial anomalies in response toreservoir depletion and repressuring.

Microseepage rates and surfacehydrocarbon concentrations can varysignificantly with time. Surface hydro-carbon seeps and soil geochemical

anomalies appear and disappear inrelatively short times—weeks tomonths to years. Results from studiesof natural seeps and undergroundstorage reservoirs, as well as repeatsurveys of fields, indicate that the rateof hydrocarbon migration andmicroseepage varies from less than 1m per day to tens of meters per day.Empirical observations and computersimulations suggest that the mecha-nism for microseepage is a buoyancy-driven, continuous-phase gas flowthrough water-wet pores and frac-tures.

Nearly all surface exploration

methods rely on the assumption thathydrocarbons migrate in a predomi-nantly vertical direction from sourcerocks and reservoirs to the surface.Evidence for vertical leakage of hydro-carbons is commonly seen on con-ventional seismic and high-resolutionseismic sections. Figure 2 illustratesan example of such a gas-leakagechimney over Ekofisk Field in theNorth Sea. There are numerous pub-lished articles showing apical (ordirect) geochemical anomalies overoil and gas fields, as well as over petro-leum storage reservoirs. A recentreview of more than 850 wildcat


Figure 2. Seismic cross-section of Ekofisk Field, North Sea, illustrating awell-developed gas chimney caused by low-velocity conditions due to gas-charged sediments (from “Ekofisk: First of the giant oil fields in westernEurope” by Van den Bark and Thomas, AAPG Memoir 30, 1990). Hovlandand Sommerville (1985) estimated gas-seepage at 1000 liters per hour.Extrapolating this estimate to the total area of gas seepage, approximately100 000 m2 containing 140 seeps, gives a net flux of 890 liters per squaremeter per year.

Figure 3. Generalized model of hydrocarbon microseepage and hydrocar-bon-induced effects on soils and sediments (from “Hydrocarbon-inducedalteration of soils and sediments” by Schumacher, AAPG Memoir 66).

wells—all drilled after completion ofsurface geochemical surveys—findsthat 79% of wells drilled in positivegeochemical anomalies resulted incommercial oil or gas discoveries; incontrast, 87% of wells drilled in theabsence of an associated geochemicalanomaly resulted in dry holes. Datasuch as these represent powerful, ifempirical, evidence for vertical migra-tion and microseepage of hydrocar-bons.

The surface geochemical expres-sion of petroleum seepage can takemany forms, including (1) anomaloushydrocarbon concentrations in sedi-ment, soil, water, and even atmos-phere; (2) microbiological anomaliesand the formation of “paraffin dirt”;(3) anomalous non-hydrocarbon gasessuch as helium and radon; (4) miner-alogical changes such as the formationof calcite, pyrite, uranium, elementalsulfur, and certain magnetic ironoxides and sulfides; (5) clay mineralalterations; (6) radiation anomalies;(7) geothermal and hydrologic anom-alies; (8) bleaching of redbeds; (9)geobotanical anomalies; and (10)altered acoustical, electrical, and mag-netic properties of soils and sediments.Figure 3 represents a generalizedmodel of hydrocarbon microseepageand their varied geochemical and geo-physical effects on soils and sedi-ments.

Survey design and interpretation.The importance of proper surveydesign and sampling density for tar-get recognition cannot be overstated.Hydrocarbon microseepage data areinherently noisy and require adequatesample density to distinguish betweenanomalous and backgroundresponses. The major causes of ambi-guity and interpretation failuresinvolving surface geochemical studiesare probably undersampling and/orselection of an improper surveymethod. To optimize the recognitionof an anomaly, the sampling patternand sample number must take intoconsideration the objectives of the sur-vey, the expected size and shape of theanomaly (or geologic target), theexpected natural variation in surfacemeasurements, and the probable sig-nal-to-noise ratio. Defining back-ground values adequately is anessential part of anomaly recognitionand delineation. For prospect evalua-tion, as many as 70% of the samplescollected should be obtained outsidethe area of immediate interest. Forproperly designed surveys, and underideal geologic conditions, the arealextent of surface geochemical anom-alies can closely approximate the pro-ductive limits of the reservoir at depth.

How does one select a method (ormethods) for a surface geochemicalexploration program? The choice ofmethod(s) depends on the kinds ofquestions you hope the survey resultswill answer. In other words, what arethe objectives of the survey? Is it todemonstrate the presence of an activepetroleum system in a frontier area, orto high-grade previously definedexploration leads and prospects, or todetermine the type of petroleum (i.e.,oil versus gas) likely to be encoun-tered? What other data are presentlyavailable in the area of interest (satel-lite imagery, aeromagnetics, gravity,seismic, etc.)? What geochemicalmethods have previously been usedsuccessfully in the area of interest, orin a geologic analog? What limitationsare imposed by the survey area(onshore or offshore, deep water orshallow, jungle or desert, mature basinor remote area, budget and personnelconstraints, etc.)? It is beyond thescope of this article to discuss theadvantages and limitations of specificmethods or sampling procedures, butsuch information is readily availablein published literature.

As a generalization, direct hydro-carbon methods are preferred overindirect methods because they canprovide evidence of the very hydro-


Figure 4. Geochemical expression of a stratigraphic trap at about 5600 ft(1.5 s)in the Cretaceous Escondido Sandstone, La Salle County, Texas (from“Exploration enhancement by integrating near-surface geochemical andseismic methods” by Rice, Oil and Gas Journal, 1989). A soil gas hydrocar-bon survey was conducted to look for evidence of hydrocarbon microseep-age from a seismically defined trap at CDP 1070 (left). Propane soil gasanomalies were detected at CDPs 1070 and 1096. A wildcat drilled at CDP1070 resulted in a new field discovery. The geochemical lead at CDP 1096was reevaluated seismically and, after additional processing, a revisedinterpretation (right) predicted porosity development coincident with thesurface geochemical anomaly. A productive well was subsequently drilled.

carbons we hope to find in our trapsand reservoirs. Additionally, chemi-cal and isotopic analysis of thesehydrocarbons, especially the highmolecular weight hydrocarbons, canprovide insight into the nature andmaturity of the source rocks that gen-erated these hydrocarbons. If surfaceconditions or budgetary constraintspreclude the use of direct hydrocarbondetection methods, the next best choice is one of those indirectmethods most closely linked to hydro-carbons and hydrocarbon accumu-lations. Whenever possible, it isrecommended to use more than onegeochemical survey method, forexample, combining a direct methodwith an indirect method. The use ofmultiple methods can reduce inter-pretation uncertainty because seep-age-related anomalies will tend to bereinforced while random highs andlows tend to cancel each other out.

The presence of hydrocarbonmacroseeps or microseeps in the areaof a geochemical survey is direct evi-dence that petroleum has been gen-erated. Hydrocarbon seepage at thesurface represents the end of a petro-leum migration pathway. Thesehydrocarbons may represent hydro-carbon leakage from an accumulation,or merely leakage along a carrier bed

or other migration pathway.Anomalies defined by multiple sam-ples from one or more survey linesmay indicate the location of discretestructural or stratigraphic targetswithin the survey area. If the basin orplay is characterized by predomi-nantly vertical migration, then the cor-relation of a strong geochemicalanomaly at the surface with a possi-ble trap at depth suggests that the trapis charged with hydrocarbons; con-versely, if the trap is not associatedwith a positive geochemical anomaly,the assumption is that the trap is notcharged with hydrocarbons. Becauserelationships between surface geo-chemical anomalies and subsurfaceaccumulations can be complex, properinterpretation requires integration ofsurface geochemical data with geo-logic, geophysical, and hydrologicdata (Figure 4).

Summary. The past decade has seena renewed interest in surface geo-chemical exploration which, togetherwith developments in analytical andinterpretive methods, have produceda new body of data and insights thatestablish the validity of many of theseexploration methods. Surface explo-ration methods cannot replace con-ventional exploration methods, but

they can be a powerful complementto them. Geochemical and other sur-face methods have found their great-est utility when used in conjunctionwith available geologic and geophys-ical information. The need for such anintegrated approach cannot beoveremphasized. Seismic data, espe-cially 3-D data, are unsurpassed formapping trap and reservoir geometry;however, only surface geochemicalmethods can consistently and reliablymap hydrocarbon leakage associatedwith those traps. Properly acquiredand interpreted, the combination ofsurface geochemical data and sub-surface exploration data has thepotential to reduce exploration anddevelopment risks and costs byimproving success rates and shorten-ing development time.

Suggestions for further reading.Hydrocarbon Migration and Its Near-Surface Expression by Schumacher andAbrams (AAPG, 1996). Soil Gas andRelated Methods for Natural ResourceExploration by Klusman (Wiley, 1993).Surface Exploration Case Histories bySchumacher and LeSchack (AAPG-SEGSpecial Publication, in preparation). LE

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