Characterizing Charge Risk Using Oil and Gas GeochemistryKey questions in prospect evaluation include: Has the trap received economic quantities of petroleum? What types of hydrocarbons are likely to be present (oil and/or gas and in what relative proportion)? What are the oil or gas properties (e.g. viscosity !"# gravity sulfur content wa$iness)? #s reservoir compartmentali%ation an issue?For a prospect to be charged with economic quantities of petroleum, a variety of physical and timing elements must occur in thebasin. These elements collectively comprise a petroleum system. They include:Physical components "etroleum source rock (to generate the petroleum) &eservoir (to hold the petroleum) 'eal (to preserve the accumulation) (verburden (to mature the source rock)Processes/Timing components )rap formation (il generation (il e$pulsion (il migration (il accumulation &elative timing of these eventsOil geochemistry (oil ngerprinting! can be used in con"unction with basin modeling to quantify ris# associated with many of the components in the petroleum system. $lthough a comprehensive review of all applicable geochemical approaches to ris# assessment cannot be provided in this article, we outline the OilTracers approach here and discuss some of the applications in detail.%ome of the most powerful oil ngerprinting tools we use are based on biomar#ers, which are molecular fossils present in oils and roc# e&tracts. The biomar#er distribution in an oil can be used to infer characteristics of the source roc# that generated the oil without e&amining the source roc# itself. %pecically, biomar#ers can reveal ('! the relative amount of oil(prone vs. gas(proneorganic matter in the source #erogen, ()! the age of the source roc#, (*! the environment of deposition as marine, lacustrine, +uvio(deltaic or hypersaline, (,! the lithology of the source roc# (carbonate vs. shale vs. coal!, and (-! the thermal maturity of the source roc# during generation (e.g., .eters and /oldowan, '00*!. The attached Tables list e&amples of oil biomar#er parameters and the information they provide about the oil source roc#.To characteri1e charge ris#, these biomar#er parameters can be used in a variety of innovative ways. For e&ample, specic biomar#er parameters can be calibrated against specic #erogen quality parameters in a given basin. Then, the biomar#er ratiosare measured in an oil sample from the basin, and the values are pro"ected onto calibration curves to quantitatively predict characteristics of the source roc#. This approach, pioneered by the founders of OilTracers, allows e&plorationists to assess whether an oil was generated primarily from an oil(prone or gas(prone organic facies (2ahl et al., '00,3 /c4a5rey et al., '00,!. The information gained from oil biomar#ers (source type, age, maturity, #erogen quality! when integrated into a basin model. This information has substantial economic impact because it provides early estimates of oil quantity and 6O7 for e&ploration targets in the area of interest.4onsidered collectively, the geochemical and basin modeling evaluation of each element in a petroleum system results in an assessment of total charge ris# for a given prospect. 4harge ris# refers not only to petroleum quantity, but also to petroleum quality (e.g., $.8 gravity, viscosity, 9%!. .etroleum(system evaluation is intended to be an early decision(ma#ing tool, and OilTracers recommends using the charge ris# considerations described below for the most e5ective use of time and money. For e&ample, use of the approaches described here could lead you not to buy seismic data in an overcoo#ed (post(mature! basin when you are loo#ing for oil. %imilarly, oil geochemistry could lead you not to evaluate reservoirs or map traps in an area that cannot possibly be charged with hydrocarbons. :elow, our approach is described in more detail.Has the trap received economic quantities of petroleum?To evaluate the ris# associated with trapping economic amounts of oil or gas, OilTracers recommends assigning pro"ects to one of the following three categories:Category (1)8f a signicant petroleum charge e&ists in nearby accumulations, and there is a #nown hydrocarbon source roc# in the basin, then a low to moderate charge ris# usually e&ists. 8n this case, we suggest completing the following technical ob"ectives (based on data availability! to insure the prospect is not in a high(ris# sector the basin. *onstruct a diagram showing field+si%e distributions of oil condensate and gas reserves for each petroleum system in the basin. ,etermine whether the source of the petroleum in the prospect is similar to oil in nearby fields using biomarker fingerprints of petroleum seeps oil shows and/or recovered oils/condensates (e.g. "eters and -oldowan .//0). 1or gases use composition and isotope data of gas seeps and discovered gases (e.g. 'choell ./20 ./23). #dentify whether the local sector contains an effective source rock (rock known to have generated and e$pelled oil) using basic source+rock screening tools such as 4)(* analyses &ock+5val pyrolyses and vitrinite+reflectance measurements (geochemical logging). -ap regional source+rock richness and complete isopach maps of the source+rock interval which may be used to estimate lateral source variations to help calculate a regional 'ource "otential #nde$ ('"#6 ,emaison and Hui%inga .//7). *onstruct a hydrocarbon kitchen map showing prospect fetch areas and determine whether the local source rock maturity and timing are favorable (basin modeling) for the prospect. -ake a timing+risk chart for each prospect or play. !ssess whether the prospect may have been charged with a significant quantity of petroleum.Category (2)8f the presence of a signicant petroleum charge is supported by prolic, mature source roc#, but no economically signicant elds occur in the basin as analogs, then a moderate to high charge ris# usually e&ists. 8n this case, evaluating ris# involves applying all of the techniques described below based on sample availability. ,etermine the hydrocarbon source of any available nearby seeps shows or oil from non+commercial fields as described above. #dentify whether the basin contains a quantitatively significant hydrocarbon source rock using basic source+rock screening analyses such as 4)(* analysis &ock+5val pyrolysis and vitrinite+reflectance measurements. -ap regional source+rock richness and complete isopach maps of the source+rock interval which may be used to estimate lateral source variations to help calculate a regional 'ource "otential #nde$ ('"#). *onstruct hydrocarbon kitchen map showing prospect fetch areas and determine whether the local source maturity and timing are favorable (basin modeling) for the prospect. -ake a timing+risk chart for each prospect or play. !ssess whether the prospect may have been charged with a significant quantity of petroleum.Category ()8f the presence of signicant petroleum charge is hypothesi1ed only due to analogs in nearby basins, then a high to moderate charge ris# usually e&ists. ;valuating ris# in these circumstances involves demonstrating that a source(reservoir relationship e&ists which is similar to those in a nearby producing basin. Oil geochemistry (oil ngerprinting! and source roc#)?/ft0 mi$ed 9"< potential and condensate yield should also be measured. "redict oil or gas properties indirectly from knowledge of source type thermal maturity and secondary alteration (e.g. biodegradation water washing and preferential gas loss) when fluid samples are not available for direct analysis.$s reseroir compartmentalization an iss%e? #magine possible reservoir compartmentali%ation issues that might be encountered given the reservoir distribution and trap style. "oor reservoir connectivity may economically break an e$ploration play particularly in deep offshore reservoirs or in structurally or stratigraphically comple$ reservoirs. &eservoir compartmentali%ation issues should be considered in initial screening economics and again be addressed in the pre+drill e$ploration phase based on data available from nearby fields in the same type of play. 5valuate vertical reservoir continuity between reservoir %ones if the initial well is a discovery. #ntegrate gas (compositionand isotope) and oil (chromatography) data with geological and engineering information (e.g. wireline log information &1) pressure data) to corroborate vertical compartmentali%ation. 5valuate lateral reservoir continuity in successful delineation wells using gas chromatography of oils or condensates and isotope analysis of gases. #ntegrate with other geological and engineering information (e.g. 0+, seismic interpretation pressure data) to corroborate lateral compartmentali%ation. 5valuate vertical/lateral reservoir compartmentali%ation in inadequately constrained development pro@ects. 'imple models of reservoir connectivity are often not correct and lead to errors in reserve+si%e calculations and increased development and production costs. -any development pro@ects carry as much risk (albeit different kinds) as e$ploration pro@ects yet the dollar stakes for development pro@ects are much higher.For more information on the techniques described here, or to discuss a specic pro"ect, e(mail us at info=oiltracers.com, or call us at >.%. ()',! -?,(0'@0.References2ahl A. ;., /oldowan A. /., Teerman %. 4., /c4a5rey /. $., %undararaman .., .ena /. and %telting 4. ;. ('00,!. %ource roc# quality determination from oil biomar#ers 8. ( $n e&ample from the $spen %hale, %cullyBs 6ap, Cyoming. $merican $ssociation of.etroleum 6eologists :ulletin D? ('E!, '-ED('-)@.2emaison, 6., and :. A. Fui1inga ('00'! 6enetic classication of petroleum systems: $merican $ssociation of .etroleum 6eologists :ulletin, v. D-, p. '@)@('@,*./c4a5rey /. $., 2ahl A., %undararaman .., /oldowan A. /. and %choell /. ('00,!. %ource roc# quality determination from oil biomar#ers 88. ( $ case study using Tertiary(reservoired :eaufort %ea oils. $merican $ssociation of .etroleum 6eologists :ulletin D? ('E!, '-)D('-,E..eters, K. ;., and A. /. /oldowan ('00*! The :iomar#er 6uide, 8nterpreting molecular fossils in petroleum and ancient sediments, .rentice Fall, *@* p.%choell, /. ('0?*! 6enetic characteri1ation of natural gases: $merican $ssociation of .etroleum 6eologists :ulletin, v. @D, p. )))-())*?.%choell, /. ('0?,! %table isotopes in petroleum research, in A. :roo#s, and 2. F. Celte, eds., $dvances in .etroleum 6eochemistry, v. ': Gondon, $cademic .ress, p. )'-(),-.Thompson, K. F. /. ('0?D! Fractionated aromatic petroleums and the generation of gas(condensates: Organic 6eochemistry, v. '', p. -D*(-0E.Thompson, K. F. /. ('0??! 6as(condensate migration and oil fractionation in deltaic systems: /arine and .etroleum 6eology, v. -, p. )*D(),@.