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Generating and expelling petroleum reduces organic matter quantity and quality of source rocks (e.g., Daly and Edman, 1987). Therefore, organic facies maps must reflect the original organic constituents and changes caused by weathering and thermal maturation. Experienced organic geochemists can often reconstruct organic facies for thermally mature samples by tracking the kerogen types back to thermally immature values along pathways of Van Krevelen diagrams.
Generating and expelling petroleum reduces organic matter quantity and quality of source rocks (e.g., Daly and Edman, 1987). Therefore, organic facies maps must reflect the original organic constituents and changes caused by weathering and thermal maturation. Experienced organic geochemists can often reconstruct organic facies for thermally mature samples by tracking the kerogen types back to thermally immature values along pathways of Van Krevelen diagrams.
==Types of organic facies==
In the following brief review of organic facies, their geochemical and microscopic characteristics are taken from the conclusions of Jones (1987, p. 78-80). Some new examples are also presented. Organic facies A through D represent the range of sedimentary histories from no oxidation to severe oxidation.
In the definition of organic facies by Jones and Demaison (1982) (already given), the organic "constituents" that distinguish organic facies might be macerals (organic particles) or molecular components. Jones (1987), however, only employs the elemental atomic ratios of C, H, and O and to some extent equivalent Rock-Eval parameters (HI and OI). Macerals are used to identify the components of mixtures and this has a bearing, for example, on whether relatively low-quality organic facies have oil- or gas-generative potential. Thermally immature mixed maceral assemblages with H/C values of 0.8 might contain some oil-generative type I or II kerogen with predominantly inert type IV kerogen or might contain all gas-generative type III kerogen.
===Organic Facies A===
Criteria for this relatively rare facies are H/C > 1.45 and HI > 850. The rocks are usually laminated and organic rich and are found in alkaline lakes and marine paleoenvironments. The organic matter, often brightly fluorescent, is derived primarily from a single type of algae or bacteria. Organic facies A commonly occurs in carbonate settings. It is found in condensed sections of lakes and marine margins that are protected from oxygenated waters. Examples include the lacustrine Green River oil shale of Eocene age in Wyoming, Colorado, and Utah; and the marine Galena oil rock of Ordovician age in Illinois, Iowa, Minnesota, and Wisconsin.
===Organic Facies AB===
Criteria for this facies are H/C = 1.45-1.35 and HI = 850-650. Rocks forming this facies are often laminated and organic rich. The organic matter is similar to that in organic facies A, except that it is diluted either with organic input of lesser quality or by partial degradation. Organic facies AB is found in both carbonates and shales. One example is the Upper Jurassic marine source rocks of Saudi Arabia.
===Organic Facies B===
Criteria for this facies are H/C = 1.35-1.15 and HI = 650-400. This facies is the source of petroleum for the majority of the world's oil fields, although organic facies AB may have contributed more of the world's oil (Jones, 1987). It is often laminated and may contain some terrestrial organic matter. It can be interbedded with less oil-prone facies, reflecting either fluctuations in bottom water anoxia or introduction of sediments with associated oxygen or poorer quality organic matter. Organic facies B and its systematic neighboring facies AB and BC can be mixtures representing biological source variation, some transported organic matter, or variations in preservation. Organic facies B encompasses most of the earth's best petroleum source rocks and is predominantly found in marine ocks, especially in deep water paleoenvironments associated with upwelling. Examples include the marine Kimmeridgian Clay source rocks of Jurassic age of the North Sea; the marine Monterey Formation of Miocene age in California; and the marine Phosphoria Formation of Permian age in Wyoming and Montana.
===Organic Facies BC===
Criteria for this facies are H/C = 1.15-0.95 and HI = 400-250. This facies is found in both marine and lacustrine paleoenvironments. It is often deposited in fine-grained siliciclastics where rapid deposition captures small oxygen volumes in the sediments. This "sedimentary oxygen" encourages biological activity in the sediments. Terrestrial organic matter can be a significant contributor, but bioturbation of bottom sediments may be sufficient to degrade marine organic matter to this quality. Examples include the marine Mowry Formation of Cretaceous age in Wyoming and prodelta muds from many deltaic and lacustrine deposits.
===Organic Facies C===
Criteria for this facies are H/C = 0.95-0.75 and HI = 250-125. This facies is predominantly gas prone. The organic matter is primarily woody and terrestrial and makes up most coals. Organic facies C is found in marine environments on Tertiary and Mesozoic shelf margins where it includes mixtures of hydrogen-rich and hydrogen-poor macerals or degraded hydrogen-rich macerals. Organic facies C and neighboring facies BC and CD are found in coal-forming swamp deposits, deltaic deposits, and bioturbated marine mudstones. The types of environments in which this facies occurs often correspond to the transgressive and early highstand systems tracts where some oxidation occurs and where different kerogen components can be deposited together (Vail et al., in press). One example is the lower Tert ary of Labrador.
===Organic Facies CD===
For this facies, the criteria are H/C = 0.75-0.60 and HI = 125-50. It is heavily oxidized and frequently represents terrestrial organic matter that has been transported through oxidizing environments. This facies may represent recycled organic matter that has been eroded one or more times from sediments. An example is in Cretaceous sediments lying on the Atlantic shelf of North America.
===Organic Facies D===
Criteria for this final facies are H/C