Magoon, L. B, and W. G. Dow, eds,, 1994, The petroleum system-from source to trap: AAPG Memoir( Chapter 5 Applied Source rock Geochemistry Kenneth E. Peters Mary Rose Cassa hezron Ove rseas petroleum It California Environnental Protection Agency San Ramon, California, U.s.A Department of To.vic Substances Control U.S.A Abstract Applied organic geochemistry provides the information needed to make maps of the richness, the stratigraphic and geographic extent of a pod of active source rock in a petroleum system, and ey are based on geochemical analyses of rock samples from outcrops and wells that are displayed on logs. These geochemical well logs are based on Rock-Eval pyrolysis, total organic carbon, vitrinite reflectance, and other id, inexpensive"screening"methods. The logs define(1) otential, effective, and spent petroleum source rock; (2)the thermal maturation grad A/ tatietul including immature, mature, and postmature zones, and (3)in situ and migrated petroleum shor geochemical logs require proper sample selection, preparation, analysis, and interpretation Detailed studies, including oil-source rock correlations by biomarker and supporting techniques, are undertaken on selected samples only after the screening methods are completed san he goal of this chapter is to show how geochemical dures and methods of analysis and interpretation Pipe 11 INTRODUCTION ll site sampling, type of samples(core, sidewall, cuttings), sample spacing, sample preparation reening"technology is applied to petroleum explo- ration. This chapter provides a conceptual framework for later discussions in this book by defining key terms used SOURCE ROCK PROPERTIES AND to describe source rock characteristics and reviewin TERMS principles and recent developments in source rock geochemistry. Major emphasis is placed on()criteria for organic matter with the pore space occupied by water, sampling, preparation, and analysis of rocks and oils; (2) tumen oil and c rogen is the particulate eochemical logs; and(3)geochemical maps The main contribution of organic geochemistry to fraction of organic matter remaining after extraction of verized rock with organic solvents. Kerogen can be sedimentary basin analysis is to provide analytical data isolated from carbonate- and silicate-bearing rocks by to identify and map source rocks. These maps include treatment with inorganic acids, such as HCl and HF(e. g he richness, type, and thermal maturity of a source rock Durand, 1980). This is only an operational definition and are a necessary step toward determining the strati- because the amount and composition of insoluble graphic and geographic extent of a pod of active source organic matter or kerogen remaining after extraction rock in a petroleum system. The volume, richness, and depends on the types and polarities of the organic thermal maturity of this pod of active source rock deter solvents. Kerogen is a mixture of macerals and reconsti- mines the amount of oil and gas available for traps. tuted degradation products of organic matter. Macerals Because of this, maps that show the pod of active source are the remains of various types of plant and animal rock reduce exploration risk( e. g, Demaison, 1984) matter that can be distinguished by their chemistry and eochemical well logs are essential for mapping active by their morphology and reflectance using a petro- source rocks. These logs plot various geochemical para graphic microscope(Stach et al., 1982). This term was meters versus depth and can be made from surface originally applied to components in coal but has been sections and during or after drilling. Certain criteria that extended to sedimentary rocks. Palynomorphs are are largely omitted from the literature must be met to resistant, organic-walled microfossils such as spores, ensure useful geochemical logs. These criteria include pollen, dinoflagellate cysts, and chitinozoa "Present address: Mobil Exploration and Producing Technical Center, Dallas, Texas, U.S.A
Magoon, L. B, and W. G. Dow, eds., 1994, The petroleum system—from source to trap: AAPG Memoir 60. Chapter 5 I""* Applied Source Rock Geochemistry Kenneth E. Peters* Mary Rose Cassa Chevron Overseas Petroleum Inc. California Environmental Protection Agency San Ramon, California, U.S.A. Department of Toxic Substances Control Berkelet/, California, U.S.A. Abstract Applied organic geochemistry provides the information needed to make maps of the richness, type, and thermal maturity of a source rock. These maps are a necessary step toward determining the stratigraphic and geographic extent of a pod of active source rock in a petroleum system, and they are based on geochemical analyses of rock samples from outcrops and wells that are displayed on logs. These geochemical well logs are based on Rock-Eval pyrolysis, total organic carbon, vitrinite reflectance, and other rapid, inexpensive "screening" methods. The logs define (1) potential, effective, and spent petroleum source rock; (2) the thermal maturation gradient, including immature, mature, and postmature zones, and (3) in situ and migrated petroleum shows. Useful geochemical logs require proper sample selection, preparation, analysis, and interpretation. Detailed studies, including oil-source rock correlations by biomarker and supporting techniques, are undertaken on selected samples only after the screening methods are completed. INTRODUCTION The goal of this chapter is to show how geochemical "screening" technology is applied to petroleum exploration. This chapter provides a conceptual framework for later discussions in this book by defining key terms used to describe source rock characteristics and reviewing principles and recent developments in source rock geochemistry. Major emphasis is placed on (1) criteria for sampling, preparation, and analysis of rocks and oils; (2) geochemical logs; and (3) geochemical maps. The main contribution of organic geochemistry to sedimentary basin analysis is to provide analytical data to identify and map source rocks. These maps include the richness, type, and thermal maturity of a source rock and are a necessary step toward determining the stratigraphic and geographic extent of a pod of active source rock in a petroleum system. The volume, richness, and thermal maturity of this pod of active source rock determines the amount of oil and gas available for traps. Because of this, maps that show the pod of active source rock reduce exploration risk (e.g., Demaison, 1984). Geochemical ivell logs are essential for mapping active source rocks. These logs plot various geochemical parameters versus depth and can be made from surface sections and during or after drilling. Certain criteria that are largely omitted from the literature must be met to ensure useful geochemical logs. These criteria include well site sampling, type of samples (core, sidewall, cuttings), sample spacing, sample preparation procedures, and methods of analysis and interpretation. SOURCE ROCK PROPERTIES AND TERMS Sedimentary rocks commonly contain minerals and organic matter with the pore space occupied by water, bitumen, oil, and/or gas. Kerogen is the particulate fraction of organic matter remaining after extraction of pulverized rock with organic solvents. Kerogen can be isolated from carbonate- and silicate-bearing rocks by treatment with inorganic acids, such as HC1 and HF (e.g., Durand, 1980). This is only an operational definition because the amount and composition of insoluble organic matter or kerogen remaining after extraction depends on the types and polarities of the organic solvents. Kerogen is a mixture of macerals and reconstituted degradation products of organic matter. Macerals are the remains of various types of plant and animal matter that can be distinguished by their chemistry and by their morphology and reflectance using a petrographic microscope (Stach et al., 1982). This term was originally applied to components in coal but has been extended to sedimentary rocks. Palynomorphs are resistant, organic-walled microfossils such as spores, pollen, dinoflagellate cysts, and chitinozoa. 'Present address: Mobil Exploration and Producing Technical Center, Dallas, Texas, U.S.A. 93
94 Peters and Cassa Bitumen in rocks is that fraction of the organic matter pressure, and salinity), Oxic water(saturated with hat is soluble in organic solvents. Small amounts of oxygen) contains 8.0-2.0 mL O2/L H2O (Tyson and bitumen originate from lipid components in once-living Pearson, 1991). Dysoxic water contains 2.0-0.2 mL O2/L organisms, but most is generated by cracking(thermal H2O, suboxic, 0.2-0.0 mL O2/L H2O, and anoxic water dissociation) of the kerogen. Lipids are oil-soluble, water lacks oxygen. When referring to biofacies, the corre- insoluble organic compounds, including fats, waxes ponding terms are aerobic, dysaerobic, quasi-anaerobic, and pigments, steroids, and terpenoids, that are m anaerobic precursors for petroleum(Peters and Moldowan, 1993) Below the 0.5 mL O2/L H2O threshold, the activity of Petroleum is a complex mixture of gas, liquid, and multicellular organisms as agents in the oxidative naturally in the earth (Magoon and Dow, Chapter 1, fs destruction of organic matter is severely limited solid hydrocarbons and nonhydrocarbons occurrin volume). The term hydrocarbon is commonly used in the typically thinly laminated(distinct alternating layers <2 petroleum industry to indicate crude oil or natural ga mm thick)because of the lack of bioturbation by In the chemical sense, hydrocarbons are compounds burrowing, deposit-feeding organisms. Pederson and containing only carbon and hydrogen. Nonhydrocarbon Calvert(1990) contend that anoxia is less important than contain elements in addition to hydrogen and carbon. primary productivity in determining quantities of For example, NSO compounds contain nitrogen, sulfur, organic matter preserved. However, Peters and or oxygen, and porphyrins contain metals such as Moldowan(1993)stress the effect of on the vanadium or nickel. For this volume, petroleum, oil and quality rather than quantity of organic matter preserved gas, and hydrocarbons, used without modifiers, have that is, anoxia favors preservation of all matter similar meanings including hydrogen-rich, oil-prone organi may explain the positive relationship between petroleum Depositional Environment source rocks and the faunal, sedimentologic, and geochemical parameters indicating anoxia Descriptions of oils or source rocks using the terms marine or terrigenous are unclear without specifying Alteration of Organic Matter whether these terms refer to provenance (origin)or tional environment. G Diagenesis refers to all chemical, biological, and these terms to refer to organic matter derived from physical changes to organic matter during and after and land plants, respectively, whereas geologists deposition of sediments but prior to reaching burial usually refer to marine or terrigenous depositional envi temperatures greater than about 600-80oC. The quantity ronments For exam ple, when geologists refer to a and quality of organic matter preserved and modified marine" sedimentary rock, they are discussing deposi- during diagenesis of a sediment ultimately determine the tional environment, not provenance of the mineral petroleum potential of the rock (Horsfield and grains. Likewise, a geologist might equate a marine Rullkotter, Chapter 10, this volume source rock with marine depositional conditions, Catagenesis can be divided into the oil zone, which although the included organic matter or kerogen might corresponds to the oil window, where liquid oil genera be of marine, terrigenous, or mixed origin. For similar tion is accompanied by gas formation, a e more reasons, the meaning of the terms marine oil, lacustrine oil, mature wet gas zone, where light hydrocarbons are terrigenous oil is unclear without further explanation. generated through cracking and their proportion Misunderstandings can occur because a marine oil might increases rapidly (Tissot and Welte, 1984). Wet gas(< e(1)generated from land plant organic matter methane)contains methane and significant amounts of deposited in a marine environmen nt, (2) generated from ethane, propane, and he easier hydrocarbons. The gas marine organic matter, or (3)produced from a reservoir window corresponds to the interval from the top of the rock deposited in a marine environment. Rather than just wet gas zone to the base of the dry gas zone "marine"oil, it must be specified whether the oil is Metagenesis corresponds to the dry derived from a source rock deposited under marine gas is generated(2.0-4.0%Ro). Dry gas consists of 98% or conditions or from marine organic matter nore of methane(Tissot and Welte, 1984), Dry gas is also Various factors play a role in the preservation of found as deposits of bacteriogenic (microbial) gas organic matter, notably the oxygen content of the water cenerated during diagenesis of organic matter by lumn and sediment (oxic ver oxic), primary methanogenic bacteria under anoxic conditions (Rice and productivity of new organic matter by plants, water ClaypooL, 1981) circulation, and sedimentation rate(Demaison and Thermal maturity refers to the extent of temperature- ent sediments, the oxygen content of the matter(source rock) to oil, wet gas, and finally to dry nic Moore, 1980: Emerson 1985 overlying water column is unknown, but it can be inter- and pyrobitumen. Thermally immature source rocks preted from the presence or absence of laminated or have been affected by diagenesis without a pronounced bioturbated sediments and organic matter content in the effect of temperature(<0.6%Ro)and are where microbial pediment(Demaison and Moore, 1980). The oxygen gas is produced. Thermally mature organic matter is(or content of water is determined by availability and solu- was)in the oil window and has been affected by thermal lity of oxygen(which depends upon the temperature, processes covering the temperature range that generates
94 Peters and Cassa Bitumen in rocks is that fraction of the organic matter that is soluble in organic solvents. Small amounts of bitumen originate from lipid components in once-living organisms, but most is generated by cracking (thermal dissociation) of the kerogen. Lipids are oil-soluble, waterinsoluble organic compounds, including fats, waxes, pigments, steroids, and terpenoids, that are major precursors for petroleum (Peters and Moldowan, 1993). Petroleum is a complex mixture of gas, liquid, and solid hydrocarbons and nonhydrocarbons occurring naturally in the earth (Magoon and Dow, Chapter 1, this volume). The term hydrocarbon is commonly used in the petroleum industry to indicate crude oil or natural gas. In the chemical sense, hydrocarbons are compounds containing only carbon and hydrogen. Nonhydrocarbons contain elements in addition to hydrogen and carbon. For example, NSO compounds contain nitrogen, sulfur, or oxygen, and porphyrins contain metals such as vanadium or nickel. For this volume, petroleum, oil and gas, and hydrocarbons, used without modifiers, have similar meanings. Depositional Environment Descriptions of oils or source rocks using the terms marine or terrigenous are unclear without specifying whether these terms refer to provenance (origin) or depositional environment. Geochemists commonly use these terms to refer to organic matter derived from marine and land plants, respectively, whereas geologists usually refer to marine or terrigenous depositional environments. For example, when geologists refer to a "marine" sedimentary rock, they are discussing depositional environment, not provenance of the mineral grains. Likewise, a geologist might equate a marine source rock with marine depositional conditions, although the included organic matter or kerogen might be of marine, terrigenous, or mixed origin. For similar reasons, the meaning of the terms marine oil, lacustrine oil, or terrigenous oil is unclear without further explanation. Misunderstandings can occur because a marine oil might be (1) generated from land plant organic matter deposited in a marine environment, (2) generated from marine organic matter, or (3) produced from a reservoir rock deposited in a marine environment. Rather than just "marine" oil, it must be specified whether the oil is derived from a source rock deposited under marine conditions or from marine organic matter. Various factors play a role in the preservation of organic matter, notably the oxygen content of the water column and sediment (oxic versus anoxic), primary productivity of new organic matter by plants, water circulation, and sedimentation rate (Demaison and Moore, 1980; Emerson, 1985). For ancient sediments, the oxygen content of the overlying water column is unknown, but it can be interpreted from the presence or absence of laminated or bioturbated sediments and organic matter content in the sediment (Demaison and Moore, 1980). The oxygen content of water is determined by availability and solubility of oxygen (which depends upon the temperature, pressure, and salinity). Oxic water (saturated with oxygen) contains 8.0-2.0 mL O2/L H2O (Tyson and Pearson, 1991). Dysoxic water contains 2.0-0.2 mL O2/L H2O, suboxic, 0.2-0.0 mL O2/L H2O, and anoxic water lacks oxygen. When referring to biofacies, the corresponding terms are aerobic, dysaerobic, quasi-anaerobic, and anaerobic. Below the 0.5 mL 02 /L H2 0 threshold, the activity of multicellular organisms as agents in the oxidative destruction of organic matter is severely limited (Demaison and Moore, 1980). Anoxic sediments are typically thinly laminated (distinct alternating layers <2 mm thick) because of the lack of bioturbation by burrowing, deposit-feeding organisms. Pederson and Calvert (1990) contend that anoxia is less important than primary productivity in determining quantities of organic matter preserved. However, Peters and Moldowan (1993) stress the effect of anoxia on the quality rather than quantity of organic matter preserved, that is, anoxia favors preservation of all organic matter, including hydrogen-rich, oil-prone organic matter. This may explain the positive relationship between petroleum source rocks and the faunal, sedimentologic, and geochemical parameters indicating anoxia. Alteration of Organic Matter Diagenesis refers to all chemical, biological, and physical changes to organic matter during and after deposition of sediments but prior to reaching burial temperatures greater than about 60C -80°C. The quantity and quality of organic matter preserved and modified during diagenesis of a sediment ultimately determine the petroleum potential of the rock (Horsfield and Rullkotter, Chapter 10, this volume). Catagenesis can be divided into the oil zone, which corresponds to the oil window, where liquid oil generation is accompanied by gas formation, and the more mature wet gas zone, where light hydrocarbons are generated through cracking and their proportion increases rapidly (Tissot and Welte, 1984). Wet gas (<98% methane) contains methane and significant amounts of ethane, propane, and heavier hydrocarbons. The gas window corresponds to the interval from the top of the wet gas zone to the base of the dry gas zone. Metagenesis corresponds to the dry gas zone where dry gas is generated (2.0-4.0% RQ). Dry gas consists of 98% or more of methane (Tissot and Welte, 1984). Dry gas is also found as deposits of bacteriogenic (microbial) gas generated during diagenesis of organic matter by methanogenic bacteria under anoxic conditions (Rice and Claypool, 1981). Thermal maturity refers to the extent of temperaturetime driven reactions that convert sedimentary organic matter (source rock) to oil, wet gas, and finally to dry gas and pyrobitumen. Thermally immature source rocks have been affected by diagenesis without a pronounced effect of temperature (<0.6% RJ and are where microbial gas is produced. Thermally mature organic matter is (or was) in the oil window and has been affected by thermal processes covering the temperature range that generates
5. Applied Source Rock Geochemistry 95 Table 5.1. Geochemical Parameters Describing the Petroleum Potential(Quantity)of an Immature Source Rock Petroleum TOc Rock-Eval Pyrolysis Bitumen Hydrocarbons Potential Poor 005 00.5 2.5 0-500 0.5-1 2.55 0.050. 500-1000 0.10-020 10002000 600-120 Very Good 200.40 2000-4000 2002400 >4000 2400 amg HCig dry rock distilled by pyrolysis. Dmg HCig dry rock cracked rom kerogen by pyrolysis are descrbed as c1s. hydrocarbons. "Lighter hydrocarbons can be at least partially retained by avoiding complete evaporation of the soent (..on-c1s.Thus,most extracts Table 5. 2. Geochemical Parameters Describing Kerogen Type(Quality) and the Character of Expelled Productsa Main Expelled Product Kerogen Type S2/S3 Atomic H/C at Peak Maturity >600 300600 10-1 12-1.5 /|b 200300 5-10 10-1.2 Mixed oil and gas 50-200 1-5 0.7-1.0 Ga <0.7 Based on a thermaly immature soute rock Fanges are approximate bType Iwill designates kerogens with composion between type ll and il pathways (e.g Figure 5. 1)that show intermediate HI(see Figures 5. 4-5. 11). oil (-0.6-1.35%Ro)or about 600-150oC. Thermally post- relationships are never proven because some level of mature organic matter is in the wet and dry gas zones uncertainty always exists depending on the available window)and has been heated to such high temper data. Nonetheless, effective source rocks satisfy three tures(about 150%-2000 C, prior to greenschist metamor- geochemical requirements that are more easily defined phism) that it has been reduced to a hydrogen-poor (Tables 5.1-5.3 residue capable of generating only small amounts of hydrocarbon gases Quantity, or amount of organic matter(Table 5.1) It is generally accepted that oil is unstable at higher Quality, or type of organic matter(Table 5.2) temperatures and progressively decomposes to ga ases Thermal maturity, or extent of burial heating and pyrobitumen a thermally-altered solidified bitumen (Table 5.3) that is insoluble in organic solvents(e.g, Hunt, 1979; Tissot and Welte, 1984). Mango (1991)shows evidence A potential source rock contains adequate quantities of that hydrocarbons in oil are more thermally stable than organic matter to generate petroleum, but only becomes their kerogenous precursors. He believes that oil and gas an effective source rock when it generates bacterial gas at are generated by direct thermal decomposition of low temperatures or it reaches the proper level of kerogen, but that hydrocarbons in oils show no evidence thermal maturity to generate petroleum. An actiue source of decomposing to gas in the earth. This scenario does rock is generating and expelling petroleum at the critical not exclude some oxidative decomposition of hydrocar- moment, most commonly because it is within the oil bons during thermochemical sulfate reduction(.g, window(Dow, 1977a). An inactive ock has Krouse et al. 1988) stopped generating petroleum, although it still shows petroleum potential (Barker, 1979). For example, an Source Rock Terms nactive source rock might be uplifted to a position where temperatures are insufficient to allow further be and Welte, 1984). An effective source rock is generating or further oil on, but may still be capable of gener has generated and expelled petroleum. This definition ating wet and dry gas excludes the requirement that the accumulations be Active source rocks include rocks or sediments that "commercially significant, "because(1)the terms signifi- are generating petroleum without thermal maturation cant and commercial are difficult to quantify and change For example, a peat bog might produce microbially depending on economic factors, and (2)oil-source rock generated gas (marsh gas consisting mostly of bacterio-
5. Applied Source Rock Geochemistry 95 Table 5.1. Geochemical Parameters Describing the Petroleum Potential (Quantity) of an Immature Source Rock Petroleum Potential TOC (wt. %) Organic Matter Rock-Eval Pyrolysis S^ S2 b (wt. %) Bitumen^ (ppm) Hydrocarbons (ppm) Poor Fair Good Very Good Excellent 0-0.5 0.5-1 1-2 2-4 >4 0-0.5 0.5-1 1-2 2-4 >4 0-2.5 2.5-5 5-10 10-20 >20 0-0.05 0.05-0.10 0.10-0.20 0.20-0.40 >0.40 0-500 500-1000 1000-2000 2000-4000 >4000 0-300 300-600 600-1200 1200-2400 >2400 amg HC/g dry rock distilled by pyrolysis. hmg HC/g dry rock cracked from kerogen by pyrolysis. cEvaporation of the solvent used to extract bitumen from a source rock or oil from a reservoir rock causes toss of the volatile hydrocarbons below about n-Cis. Thus, most extracts are described as "C^^. hydrocarbons."Lighter hydrocart)ons can beat least partially retained by avoiding complete evaporation of the solvent (e.g., Cio+). Table 5.2. Geochemical Parameters Describing Kerogen Type (Quality) and the Character of Expelled Products3 Kerogen Type I II ll/lll° III IV HI (mg HC/g TOC) >600 300-600 200-300 50-200 <50 S2/S3 >15 10-15 5-10 1-5 <1 Atomic H/C >1.5 1.2-1.5 1.0-1.2 0.7-1.0 <0.7 Main Expelled Product at Peak Maturity Oil Oil Mixed oil and gas Gas None aBased on a thermally immature source rock. Ranges are approximate. Type ll/lll designates kerogens with compositions between type II and III pathways (e.g., Figure 5.1) that show intermediate HI (see Figures 5.4-5.11). oil (-0.6-1.35% RQ) or about 60°-150°C. Thermally postmature organic matter is in the wet and dry gas zones (gas window) and has been heated to such high temperatures (about 150°-200° C, prior to greenschist metamorphism) that it has been reduced to a hydrogen-poor residue capable of generating only small amounts of hydrocarbon gases. It is generally accepted that oil is unstable at higher temperatures and progressively decomposes to gases and pyrobitumen, a thermally-altered, solidified bitumen that is insoluble in organic solvents (e.g., Hunt, 1979; Tissot and Welte, 1984). Mango (1991) shows evidence that hydrocarbons in oil are more thermally stable than their kerogenous precursors. He believes that oil and gas are generated by direct thermal decomposition of kerogen, but that hydrocarbons in oils show no evidence of decomposing to gas in the earth. This scenario does not exclude some oxidative decomposition of hydrocarbons during thermochemical sulfate reduction (e.g., Krouseetal.,1988). Source Rock Terms Sedimentary rocks that are, or may become, or have been able to generate petroleum are source rocks (Tissot and Welte, 1984). An effective source rock is generating or has generated and expelled petroleum. This definition excludes the requirement that the accumulations be "commercially significant," because (1) the terms significant and commercial are difficult to quantify and change depending on economic factors, and (2) oil-source rock relationships are never proven because some level of uncertainty always exists depending on the available data. Nonetheless, effective source rocks satisfy three geochemical requirements that are more easily defined (Tables 5.1-5.3): • Quantity, or amount of organic matter (Table 5.1) • Quality, or type of organic matter (Table 5.2) • Thermal maturity, or extent of burial heating (Table 5.3). A potential source rock contains adequate quantities of organic matter to generate petroleum, but only becomes an effective source rock when it generates bacterial gas at low temperatures or it reaches the proper level of thermal maturity to generate petroleum. An active source rock is generating and expelling petroleum at the critical moment, most commonly because it is within the oil window (Dow, 1977a). An inactive source rock has stopped generating petroleum, although it still shows petroleum potential (Barker, 1979), For example, an inactive source rock might be uplifted to a position where temperatures are insufficient to allow further petroleum generation. A spent oil source rock has reached the postmature stage of maturity and is incapable of further oil generation, but may still be capable of generating wet and dry gas. Active source rocks include rocks or sediments that are generating petroleum without thermal maturation. For example, a peat bog might produce microbially generated gas (marsh gas consisting mostly of bacterio-
Peters and Cassa Table 5.3. Geochemical Parameters Describing Level of Thermal Maturation Maturation Generatio Stage of Themal Maturity for Oil TAla ToCb (mgg rock) [S1/ (S1+ S2) Immature 02-06 152.6 Mature 0.60.65 435445 2.62 0.05-0.10 50-100 0.100.15 Peak 0.65-09 445450 0.150.25 50250 0.25040 09-1.3 450470 2.9-3.3 1.35 >470 >33 mAture oih-prone source rocks with type i or ll kerogen commonty show bitumen foc ratios in the range 0.05-0.25. Caution should be applied when interpreting extract yields naized to TOC is low(<0 mg HC/g TOC) Bitumen TOC ratos over 0.25 can ndcate contamination or migrated oil or can be artifacts caused by ratios of small, inaccurate numbers genic methane)without significant heating due to shallow burial. By this definition, trapped methane and I Oil-Prone nearby unconsolidated swamp muds from which it was derived represent a petroleum system Criteria for describing the quantities of extractable 兑 l Oil-Pror organic matter in source rocks Tables 5. 1 and 5.3)can be used to map the pod of active source rock where data are available from several wells For examp sIe, source rock bitumen yields normalized by weight of rock or by tota organic carbon toC) generally increase from immature eak thermal maturity(Table 5.3). The principal ons of oil accumulation in many petroleum provinces are confined to areas showing the greatest 工U=o) .d Gas-Prone 05 、3。25▲ Jurassic, Saudi Arabia L Eocene, Green R, U.S.A normalized bitumen yields (e. g, figure 21 in ● Toarcian, France Kontorovich, 1984) ● Tertiary Organic Matter Classifications ATOMIC O/ en t The amount and maceral composition of kerogen 900 I Oil-Prone or laterally within a source rock. No universally accepted 750 classification for kerogen types exists in the literature. In this chapter, we use types I, IL, Ill (Tissot et al., 1974), and IV(Demaison et al., 1983)to describe kerogens (see o Chapter Appendix A) A Jurassic, Saudi Arabia H/C versus O/C or oan Krevelen diagram (Figure 5. 1A), g ■E。cene, Green R.,USA originally developed to characterize coals(van Krevelen, 1961: Stach et al. 1982). Tissot et al. (1974)extended the use of the van Krevelen diagram from coals to include kerogen dispersed in sedimentary rocks. Modified van Gas-Prone Krevelen diagrams(Figure 5.1B)consist of hydrogen index(HD) versus oxygen index (on plots generated from Rock-Eval pyrolysis and ToC analysis of whole OXYGEN INDEX (mg CO, g TOC) rock. HI versus OI data can be generated more rapidly and at less expense than atomic H/C versus O/C data for van Krevelen diagrams. Figure 5.1. (A) Atomic HC versus O/C or van Krevelen H/C, HD generally corresponds to higher oil-generative versus Ol diagram based on Rock-Eval pyrolysis of whole potential Gas(methane, or CH and oil are enriched in rock can be used to describe the type of organic matter in hydrogen compared to kerogen During thermal matura source rocks. tal, thermal alteration index Jones and tion, generation of these products causes the kerogen to Edison, 1978). The type IV(inertinite)pathway is not become depleted in hydrogen and relatively enriched in shown.( From Peters, 1986)
96 Peters and Cassa Table 5.3. Geochemical Parameters Describing Level of Thermal Maturation Stage of Thermal Maturity for Oil Immature Mature Early Peak Late Postmature Ro (%) 0.2-0.6 0.6-0.65 0.65-0.9 0.9-1.35 >1.35 Maturation 'max (°C) <435 435-^45 445-^50 450-470 >470 TAIa 1.5-2.6 2.6-2.7 2.7-2.9 2.9-3.3 >3.3 Bitumen/ TOO <0.05 0.05-0.10 0.15-0.25 — — Generation Bitumen (mg/g rock) <50 50-100 150-250 — — p|c [S,/(S1 + S2)] <0.10 0.10-0.15 0.25-0.40 >0.40 — aTAI, themnal alteration index. "Mature oil-prone source rocks with type I or II kerogen commonly snow oftumer/TOC ratios in the range 0.05-0.25. Caution should be applied when interpreting extract yields from coals. For example, many gas-prone coals show high extract yields suggesting oil-prone character, but extract yield normalized to TOC is low (<30 mg HC/g TOC). Bitumen/TOC ratios over 0.25 can indicate contamination or migrated oil or can be artifacts caused by ratios of small, inaccurate numbers. cpi, production index. genie methane) without significant heating due to shallow buriaL By this definition, trapped methane and nearby unconsolidated swamp muds from which it was derived represent a petroleum system. Criteria for describing the quantities of extractable organic matter in source rocks (Tables 5.1 and 5.3) can be used to map the pod of active source rock where data are available from several wells. For example, source rock bitumen yields normalized by weight of rock or by total organic carbon (TOC) generally increase from immature to peak thermal maturity (Table 5.3). The principal regions of oil accumulation in many petroleum provinces are confined to areas showing the greatest normalized bitumen yields (e.g., figure 21 in Kontorovich, 1984). Organic Matter Classifications Kerogen Type The amount and maceral composition of kerogen determine petroleum potential and can differ vertically or laterally within a source rock. No universally accepted classification for kerogen types exists in the literature. In this chapter, we use types I, II, HI (Tissot et al., 1974), and IV (Demaison et al., 1983) to describe kerogens (see Chapter Appendix A). Kerogen types are distinguished using the atomic H/C versus O/C or van Krevelen diagram (Figure 5.1A), originally developed to characterize coals (van Krevelen, 1961; Stach et al„ 1982). Tissot et al. (1974) extended the use of the van Krevelen diagram from coals to include kerogen dispersed in sedimentary rocks. Modified van Krevelen diagrams (Figure 5.1B) consist of hydrogen index (HI) versus oxygen index (OI) plots generated from Rock-Eval pyrolysis and TOC analysis of whole rock. HI versus OI data can be generated more rapidly and at less expense than atomic H/C versus O/C data for van Krevelen diagrams. Higher relative hydrogen content in kerogen (atomic H/C, HI) generally corresponds to higher oil-generative potential. Gas (methane, or CH4) and oil are enriched in hydrogen compared to kerogen. During thermal maturation, generation of these products causes the kerogen to become depleted in hydrogen and relatively enriched in 1.5 (J I y 1.0 s o I - < 0.5 (A) I Oil-Prone 5 - II Oil-Prone Thermal Maturation Pathways 2.0 4.0 '3.7 • 4.0 III Gas-Prone 3 V 0 2.5 A Jurassic, Saudi Arabia • Eocene, Green R., U.S.A. ft Toarcian, France • Tertiary, Greenland 0.1 0.2 ATOMIC O/ C 900 X UJ aCT750 ?o a o) >- E 300 I — 150-1 (B) f. I Oil-Prone II Oil-Prone A Jurassic, Saudi Arabia • Eocene, Green R., U.S.A. • Toarcian, France • Tertiary, Greenland T V Gas-Prone "•"Till - 5 0 100 150 200 250 OXYGEN INDEX (mg COj/g TOC) Figure 5.1. (A) Atomic H/C versus O/C or van Krevelen diagram based on elemental analysis of kerogen and (B) HI versus OI diagram based on Rock-Eval pyrolysis of whole rock can be used to describe the type of organic matter in source rocks. TAI, thermal alteration index (Jones and Edison, 1978). The type IV (inertinite) pathway is not shown. (From Peters, 1986.)
5. Applied Source Rock Geochemistry 97 Groups ll % TYPE X H/c=△H/C 0.20 1.3=0.26 Telocollinite 1.5 S--- Telinite 060x0.85=0.51 020x0.50=0.l0 ESTIMATED H/C =0.90 TAI Zone of Oil Generation 0.1 0.15 Atomic o/c Figure 5. 2. Combined use of organic petrography, elemental analysis, and Rock-Eval pyrolysis and TOC improves conf dence in assessment of the quality and maturity of kerogen in rock A sample a by rock-Eval pyrolysis was characterized as being marginally mature(max=435oC)and gas prone(HI= 150 mg HClg TOC). Organic petrography shows a TAl of 2.5, an Ro of 0. 5%(supporting the maturity assessment from pyrolysis), and the following maceral composition type ll 20%, type l 60%, and type IV 20%. The calculated atomic HC(0.90)corresponds with that determined by elemental analysis, supporting a dominantly gas-prone character. (Concept for figure courtesy of T. A. Edison carbon. During catagenesis and metagenesis, all is the structureless constituent of vitrinite, whereas kerogens approach graphite in composition(nearly pure telinite is the remains of cell walls of land plants. Figure carbon)near the lower left portion of both diagrams 5.2 shows two types of collinite: telocollinite contains (Figure 5.1) inclusions and is the maceral recommended for vitrinite reflectance measurements. whereas desmocollinite shows Maceral Groups submicroscopic inclusions of liptinite and other mentary rocks are liptinite (exinite), vitrinite, and inertinite (Figure 5.3), and commonly fluoresces under ultraviolet (Stach et al., 1982). Liptinite macerals, such as alginite, light, unlike telocollinite. Inertinitic macerals, such porinite, cutinite, and resinite, generally mature along semi-fusinite and fusinite mature along the type IV the type I or I kerogen pathways on the van Krevelen kerogen pathway. Because of the combined effects of diagram(Figure 5. 2). Preserved remains of the algae diagenesis, thermal maturity, and differing organic Botryococcus and Tasmanites are examples of structure matter input, a kerogen c can plot anywhere alginite. Vitrinite macerals originate from land plants Krevelen diagram and need not fall on any of the and mature along the type Ill kerogen pathway. Collin indicated maturation curves
5. Applied Source Rock Geochemistry 97 o-f %R 0 ^ Botryococcus Resinite Tasmaniles Maceral Groups %TYPE x H/ C =AH/ C I 0.20 x 1.3 = 0.26 i 0.60 x 0.85 = 0.51 - 0.20 x 0.50 = 0.10 ESTIMATED H/ C = 0.90 0.1 0.15 Atomic O/ C 0.20 Figure 5.2. Combined use of organic petrography, elemental analysis, and Rock-Eval pyrolysis and TOC improves confidence in assessment of the quality and maturity of kerogen in rock samples. A sample analyzed by Rock-Eval pyrolysis was characterized as being marginally mature (Tmax = 435°C) and gas prone (HI = 150 mg HC/g TOC). Organic petrography shows a TAI of 2.5, an R0 of 0.5% (supporting the maturity assessment from pyrolysis), and the following maceral composition: type II20%, type III 60%, and type IV 20%. The calculated atomic H/C (0.90) corresponds with that determined by elemental analysis, supporting a dominantJy gas-prone character. (Concept for figure courtesy of T. A. Edison.) carbon. During catagenesis and metagenesis, all kerogens approach graphite in composition (nearly pure carbon) near the lower left portion of both diagrams (Figure 5.1). Maceral Groups The three principal maceral groups in coal and sedimentary rocks are liptinite (exinite), vitrinite, and inertinite (Stach et al., 1982). Liptinite macerals, such as alginite, sporinite, cutinite, and resinite, generally mature along the type I or II kerogen pathways on the van Krevelen diagram (Figure 5.2). Preserved remains of the algae Botryococcus and Tasmanites are examples of structured alginite. Vitrinite macerals originate from land plants and mature along the type HI kerogen pathway. Collinite is the structureless constituent of vitrinite, whereas telinite is the remains of cell walls of land plants. Figure 5.2 shows two types of collinite: telocollinite contains no inclusions and is the maceral recommended for vitrinite reflectance measurements, whereas desmocollinite shows submicroscopic inclusions of liptinite and other materials. Because of the inclusions, desmocollinite shows a higher atomic H/C, has a lower reflectance (Figure 5.3), and commonly fluoresces under ultraviolet light, unlike telocollinite. Inertinitic macerals, such as semi-fusinite and fusinite, mature along the type IV kerogen pathway. Because of the combined effects of diagenesis, thermal maturity, and differing organic matter input, a kerogen can plot anywhere on the van Krevelen diagram and need not fall on any of the indicated maturation curves