SURFACE GEOCHEMISTRY APPLICATIONS IN OIL AND GAS EXPLORATION06/06/19940000Kenneth R. Sundberg Phillips Petroleum Co. Bartlesville, Okla.Geochemical exploration presumes that oil or gas reservoirs leak petroleum to the surface, and thatthese seeping hydrocarbons can be related to possible reservoirs in the subsurface !s an exploration techni"ue, surface geochemistry assumes neither that every reservoir actively leaks and #ill be expressed geochemically nor that every geochemical anomaly is associated #ith a commercial reservoir $t does assume that seepage is common enough to be useful%ver the last several years, &hillips &etroleum 'o has executed geochemical exploration pro(ects inits #orld#ide exploration program )*ig 1+ ,ome uses, in -orth !merica, are prospect evaluation %thers, in .gypt for instance, #ere part of #ork commitments associated #ith opening and operating ne# exploration concessions ,ome are part of technical agreements #ith international partners and research organi/ations$n fact, the use of seeps in hydrocarbon exploration is #idely accepted and practiced throughout the industry $ndependents and ma(ors all use a variety of techni"ues aimed at seep detection and characteri/ation &articular methods vary, but the general ob(ectives of the various surveyors are about the same0 locate hydrocarbon seeps, map the seeps to relate them to subsurface prospects, characteri/e the petroleum type seen in a play1s seeps, re2ne economic evaluations before entering ne# plays, and aid explorationists in making lease relin"uishments$n the follo#ing, #e #ill illustrate geochemical applications to these problemsSCIENTIFIC OBJECTIVESMICROSEEPAGE-SURVEY METHODS,cienti2cally, our survey ob(ectives are to detect and map seeps and to relate them to prospects!ssuming hydrocarbons seeping from a reservoir, through a caprock, 2nd their #ay to the surface, some analytical method is used to detect them, and they are plotted as linear pro2les or mapped int#o dimensions !t &hillips #e have conducted research in and actively used four methods01 3ight hydrocarbon survey 4 detecting hydrocarbons themselves 5ethane to pentane are analytical ob(ectives, and methods include headspace analyses and 2xed phase measurements like the 6orvit/ )1978+ acid digestion9 5easurements of helium and oil associated gases, #hich can be present in elevated concentrations over petroleum reservoirs that are charged #ith helium or other associated gas :his charge is common enough to make the method useful; 5easurements of microbial blooms of light gas consuming microorganisms 4 an indirect euvial in the southeast trending to marine to#ard the north and north#est .xploration in the east, central, and far #estern areas of the block con2rmed this general trendRELINQUISHMENTS$n concessions like ,outh Imbarka, operators typically must relin"uish large portions of the tract after a speci2ed timeGuides to these relin"uishments are typically pro(ections made from the geological, geophysical, and normally modest production data Geochemical maps are a simple ad(unct to these considerations*ig 7a sho#s large areas that are relatively barren of hydrocarbon seepage :he general southeast of the ,outh Imbarka is geochemically "uite barren :hough a concession manager might not base relin"uishment decisions on geochemical data, some comfort could be taken from data like those in*ig 7a &hillips 2rst relin"uishment in this concession #as consistent #ith these dataPETROLEUM TYPE IN CAMEROON?hen macroseeps are located, by accident or by reconnaissance survey, a much richer suit of geochemical analyses can be brought to bear on them&hillips explorationists located several such seeps in 'ameroon, and these seeps #ere sub(ected to a full organic geochemical characteri/ation )!ckerman and Boat#right, 199;+ *ig 9a locates these seeps:his area normally has been classi2ed as a gas prone area, and it has been largely neglected by explorationists 6o#ever, the seep geochemistry has suggested there may indeed be oil prone source rocks in this area, and this observation #as part of a &hillips decision to ac"uire acreage in 'ameroon and to begin exploration there*ig 9b is a ternary diagram of the gas prone light alkanes )11 to 14 carbons+, the #axy alkanes )98 to 97 carbons+, and the aromatic indicators )ben/ene, toluene, and xylene+ :he 'ameroon seeps plot to#ard the center of the light oil side of the diagram and trend to#ard the #axy side of the diagram%nly t#o samples sho# de2nite gas tendencies :hese are the 3ogbaba seep, from a long established gas province, and the -1:ota seep, found near a ne#ly opened gas province:ypically, seeps are badly biodegraded and #ater #ashed :he 'ameroon seeps #ere no exception -evertheless, analytical data can be obtained from these materials &yrolysis/G' overcomes many of the sample damage problems $t thermally isolates the heaviest, most polar, most insoluble, chemically undisturbed portion of the petroleum and breaks it do#n into light fractions that can be analy/ed via conventional G' methods *ig 9c compares a conventional G' and a pyrolysis/G' trace of one of these oilsINTERNATIONAL WORK COMMITMENTSGeochemical exploration methods are not ne#, and they have al#ays en(oyed a varied degree of support #ithin the exploration community&hillips typically runs some kind of geochemical survey as part of its international exploration #ork ?e have also #ritten these geochemical programs into a number of our #ork options and international teaming agreements:he acceptance is broad enough that some of our lessors and partners expect such #ork to be doneACKNOWLEDGMENTS:he author is grateful to his colleagues $n particular he thanks =' Boat#right for his diligence in studying the organic geochemistry of surface samples, and he notes a debt to the earlier #ork of his colleagues and former co#orkers =r !G 6olba and =r ?B 6ughes :he author is also deeply grateful to &hillips &etroleum 'o for permission to submit the paperBIBLIOGRAPHYBeghtel, *?, 6it/man, =%, and ,undberg, DC, 5icrobial %il ,urvey :echni"ue )5%,:+ evaluation of ne# 2eld #ildcat #ells in DansasA !&G. Bull, Jol ;,197@, pp 1414Garcia, C, =eibis, ,, and ,undberg, DC, 3ight hydrocarbon survey data and accumulation in the Imbarka4,outh Imbarka area, ?estern =esert, .gyptA &roceedings 9th &etroleum .xploration 'onference, 'airo, .gypt, 19776orvit/, 3, Geochemical exploration for petroleum, ,cience, Jol 999, 1978, pp 791479@6ughes, ?B, and 6olba, !G, Celationship bet#een crude oil "uality and biomarker &atterns, !dvan %rg Geochem, Jol 1;, -os 14;, 197@a, pp 184;06ughes, ?B, and 6olba, !G, Celationship bet#een crude oil "uality and biomarker &atterns in samples from the &oint !rguello area, 'aliforniaA corporate report, 197@b5atthe#s, 5=, Kones, J:, and Cichers, =5, Cemote sensing and hydrocarbon leakageA $nt ,ympCemote ,ens of .nvironment, 'olorado ,prings, 'olo, 1994-ikanov, J*, =istribution of methane homologs in gas and oil 2eldsA !kad -auk ,,C, =oklady, Jol 906, 19@1, pp 9;449;6Cichers, =5, Ceed, CK, 6orstman, D', 5ichels, G=, Baker, C-, 3undell, 3, and 5ars, C?, 3andsat and soil4gas geochemical study of &atrick =ra# oil 2eld, ,#eet#ater 'ounty, ?yoA !!&G Bull, Jol 66, -o @, 1979, pp 90;4990,undberg, DC, 5ultispectral imagery )3andsat+ hydrocarbon alteration signature0 =e2nition of the signature based on studies of probable hydrocarbon microseepage in the I, 5id'ontinentA !&G. Bull, Jol 6, 1990, pp 1999,undberg, DC, and :iedemann, 6!, 5ultispectral imagery )3andsat+ hydrocarbon alteration signature0 &rospect leads in the 6as#ell4Dit 'arson area of .astern 'oloradoA !&G. Bull, Jol 6, 1990, pp ;0447?alters, K&, and ,undberg, DC, ,oil4gas helium surveys for petroleum exploration in Dansas, !&G. Bull, Jol 7, -o 1, 1999, pp 8846;Copyright 1994 Oil & Gas Journal. All ights eserve!.FacebookTwitter LinkedIn ShareMore Oil & Gas Journal Current Issue ArticlesMore Oil & Gas Journal Archives Issue ArticlesRLAT! 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