Source Rocks. I Source rocks. II Discussion of geochemical parameters. III Global distribution of source rocks

Transcription

1 Source Rocks I Source rocks II Discussion of geochemical parameters III Global distribution of source rocks

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5 Source rock A petroleum source rock is generally recognized as a fine grained sedimentary rock that has naturally generated and released enough hydrocarbons to form a commercial accumulation of oil and/or gas (Tissot and Welte, 1984). Implicit in this definition is that a source rock meets the following geochemical requirements (Peters and Cassa, 1994): the source rock contains sufficient quantity of organic matter the organic matter is of sufficient quality to generate oil and/or gas, and the source rock attained a level of thermal maturity capable of generating and expelling hydrocarbons

6 Organic Geochemistry and Source Rocks 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 determine 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).

7 Terms Quantity, or amount of organic matter Quality, or type of organic matter Thermal maturity, or extent of burial heating TOC & abundance Kerogen: it is the particulate fraction of organic matter after extraction of pulverized rock with organic solvents Vitrinite: a material derived from land plant.

8 The Origin of Petrole eum Organic rich rich Source Rock The ermally Matured Organic Matter Oil

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10 Thermal Maturation Hist tory Less Hydrogen More Hydrogen Diagenesis R o = 0.5% K K Kerogen Onset of Oil Generation Burial to Greater and Hotter Depths Catagenesis K 2 K 1 Oil Oil Oil Gas Gas Gas Oil window R o = 2.0% K 4 K 3 Oil Phase Out Cond Gas Gas window Metagenesis Gas Horsfield and Rullkotter, 1994

11 Types of Petroleum Oil and gas are form med by the thermal cracking of organic compounds buried in fine grained rocks. Algae = Hydrogen rich = Oil prone Wood = Hydrogen poor = Gas prone

12 Kerogen Types sapropelic vs. humic

13 The Transfo ormation of Organic Matter

14 What Is Kerogen? Precursor to oil and gas Insoluble in organic solvents Complex mixture of high molecular weight organic materials Bulk composition det termined by source and environment General composition may be described as: (C 12 H 12 2ON 0.16 )x

15 Keroge en Type It is important to identify the type of kerogen in a sourc ce rock Determines the typ pe of hydrocarbon produced, if at all

16 Kerogen Types Type I : algal kero ogen best oil source Type II: herbaceou us kerogen Good oil source Type III: woody ke erogen (coaly) Good gas source Type IV: amorpho ous kerogen

17 Type I Kerogen Rare High grade grade algal sediment Generally lacustrin ne Contains sapropeli ic OM Oil shales, coorong gite & tasmanite (Oz), Boghead coa als, torbanites H:C =

18 Type II Kerogen Intermediate deriv vation Commonly margin nal marine Mixture of continen ental and aquatic (planktonic) OM Algal tissue, pollen n, spores Principal source fo or oil H:C = about 1.4

19 Type III Kerogen Sediment containin ng primarily humic OM Terrestrial (woody y) origin Equivalent of coal vitrinite Deposited at the oxic water/ sediment interfacee Gas prone H:C < 1.0 (more C than H)

20 Type IV Kerogen From any source Oxidized, recycled or altered during an earlier thermal event Inert carbonaceou us material H:C < 0.4 No evolutionary pa ath left: no hydrocarbons gene erated.

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22 S1 measures hydrocarbon shows as the amount of free hydrocarbons that can be volatilized out of the rock without cracking the kerogen (mg HC/g rock). S1 increases at the expense of S2 with maturity. S2 measures the hydrocarbon yield from cracking of kerogen (mg HC/g rock) and heavy hydrocarbons and represents the existing potential of a rock to generate petroleum. S2 is a more realistic measure of source rock potential than TOC because TOC includes "dead carbon" incapable of generating petroleum. S1 + S2 is a measure of genetic potential (Tissot and Welte, 1984) or the total amount of petroleum that might be generated from a rock.

23 Hydrogen index [HI = (S2/TOC) x 100, mg HC/g TOC] Oxygen index [OI = (S3/TOC) x 100, mg CO2 /g TOC] is related to the amount of oxygen in the kerogen. Tmax measures thermal maturity and corresponds to the Rock Eval pyrolysis oven temperature (C) at maximum S2 generation.

24 Kerogen is generally defined as sedimentary organic matter that is insoluble in common organic solvents and aqueous alkaline solvents (Tissot and Welt, 1984).

25 (After Peters and Cassa, 1994)

26 (After Peters and Cassa, 1994)

27 Source Rock for Petrole eum Organic Rich Thin Laminae 1 Inch Measured Values Total Organic Carbon In Place Petroleum S LOMPOC Quarry Sample Monterey Formation, CA Hydrogen Index Pyrolytically Generated Petroleum S 2

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29 immature and mature source rocks in the Upper and Lower Cretaceous

30 Source Rocks I Source rocks II Discussion of geochemical parameters III Global distribution of source rocks

31 TOC Myth IF I HAVE HIGH TOC, I HAVE A GOOD SOURCE ROCK Although a good source rock should have a high TOC, not all organic matter is created equal. Some organic matter will generate oil, some will generate gas, and some will generate nothing (Tissot et al., 1974). Both TOC and Rock Eval S2 need to be used to determine a source rock s richness and maturity needs to be factored in. (Dembicki, 2009)

32 TOC Myth A crossplot showing two populations of source rocks with similar total organic carbon (TOC) values but different The population with the higher S2 values shows greater hydrocarbon generating potential. (Dembicki, 2009)

33 TOC Myth As a source rock generates and hydrocarbon migrates off, the amount of organic matter in the source rock will decrease with a corresponding decrease in total organic carbon (TOC), and the amount of reactive kerogen will decrease (the amount of hydrogen will decrease) resulting in a decrease in Rock Eval S2 (from data described by Jarvie and Lundell, 1991). (Dembicki, 2009)

34 Rock Eval fallacy The Rock Eval data tell me what kind of kerogen is in my source rock. ) Rock Eval data need to be supplemented to accurately identify the kerogen types present. This can be done using PGC to evaluate the composition of hydrocarbons that can be generated.

35 Typical source rock hydrogen and oxygen index data set plotted on a pseudo Van Krevelen diagram.

36 In addition to the three main kerogen types, several other kerogen types have been defined. The two most common are Types IIS and IV. Type IIS kerogen has high initial H/C and low initial O/C atomic ratios and is derived from autochthonous organic matter deposited under highly reducing conditions in marine environments (Orr, 1986). Sulfur (8 14 wt.%) has substituted for oxygen in the kerogen structure resulting in the early generation of high sulfur naphthenic ( 环烷 )oil (Orr, 1986). (allochthonous) Type IV kerogen has a very low initial H/C ratio and a variable initial O/C atomic ratio. Type IV kerogen is a product of severe alteration and/or oxidation of organic matter in the depositional environment and is essentially inert with no hydrocarbon generating potential (Tissot and Welte, 1984).

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38 Although Type IIS kerogen plots along the Type I trend on Rock Eval pseudo Van Krevelen diagrams, it is found in marine source rocks, whereas Type I kerogen is associated mainly with lacustrine sediments. So if a marine source rock is plotting along the Type I kerogen trend on pseudo Van Krevelen diagrams, it likely contains Type IIS kerogen. However, this needs to be confirmed with supplemental geochemical data for certainty.

39 Results of mixing Types I and II kerogens with Type III kerogen plotted on pseudo Van Krevelen diagrams. Kerogen mixtures shown by the trends are in the following proportions: 100/0, 75/25, 50/50, 25/75, and 0/100.

40 Results of diluting Types I, II, and III kerogens with Type IV kerogen plotted on pseudo Van Krevelen diagrams. Kerogen mixtures shown by the trends are in the following proportions: 100/0, 75/25, 50/50, 25/75, 0/100.

41 One source of supplemental information has been the microscopic examination of kerogen, commonly referred to as visual kerogen analysis or organic petrography. Although optical kerogen descriptions can provide some useful information, their function in determining the type(s) of hydrocarbon that may be generated by a source rock is limited. A better solution to interpreting kerogen mixtures is to use supplemental geochemical data, such as pyrolysis gas chromatography (PGC).

42 Pyrolysis gas chromatographic analysis of representative samples of the Types I, II, and III kerogen. Type IV kerogen is essentially inert and provides little or no signal during pyrolysis gas chromatographic analysis.

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44 Pyrolysis gas chromatographic results showing the results of mixing Types I and II kerogen with Type III kerogen. Progressive changes in the pyrolysis gas chromatograms provide a means of estimating the contribution of each of the reactive kerogen components to the hydrocarbon generating potential of the sediment.

45 By combining the two sets of data, the source rock would be interpreted as containing a mixture of 25% Type II and 75% Type IV kerogen.

46 VITRINITE REFLECTANCE DEFICIENCY: VITRINITE REFLECTANCE WILL TELL ME IF MY SOURCE ROCK IS GENERATING Vitrinite reflectance is commonly measured on populations of randomly oriented particles in a kerogen concentrate. Mean values are calculated for the populations of vitrinite particles from each sample and reported as percentage reflectance in oil immersion (%Ro). The mean values are plotted on a log scale versus linear depth, commonly resulting in a linear trend. Vitrinite reflectance data need to be put into a geologic context for proper interpretation of maturity.

47 A typical vitrinite reflectance data set with interpretation.

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49 A B Two possible burial histories for the vitrinite trend in Figure 9. (A) The burial is fairly continuous to present day. The vitrinite reflectance indicates current maturity level. (B) An alternative burial history with a surface unconformity. The vitrinite reflectance indicates the maximum maturity level reached before the uplift and erosion.

50 Vitrinite reflectance is an indicator of the cumulative time and temperature history of the sediments. Vitrinite reflectance can indicate if generation could have occurred and suggest the types of hydrocarbons that may have been formed. Maturity inferences from vitrinite reflectance can help identify which source rocks are potential contributors to a petroleum system. Also, vitrinite reflectance can be used in basin modeling to tell if the modeled thermal history is reasonable. To address the issues of migration and expulsion, direct geochemical indicators and basin modeling are needed.

51 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 = 435C) and gas prone (HI = 150 mg HC/g TOC). Organic petrography shows a TAI of 2.5, an Ro of 0.5%, 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 gasprone character.

52 Source Rocks I Source rocks II Discussion of geochemical parameters III Global distribution of source rocks

53 Stratigraphic Effective Source Rocks given as a percentage of world s original petroleum reserves generated by these rocks

54 mid K Oli Mio Jr3 P C S

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56 (Klemme and Ulimsheck, 1991)

57 Low latitude

58 C3 P1 Source rock

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60 middle K

61 Oligocene Miocene Source Rocks

62 Main factors controlling source rock deposition and effectiveness Geological age Paleolatitude Structural form Biological evolution Eustatic transgression, global climate and ocean hydrodynamics Maturation of source rocks

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66 Reasons for high abundance of reserves in J and K

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70 Vertical migration of petroleum given as percentage of the world s original petroleum reserves

71 graptolite

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75 Summary

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79 Origin of Oil: non marine origin Oil generation by non marine and marginal mature source rocks Oil generation by coals