Organic facies

Source and Migration Processes and Evaluation Techniques
Series	Treatise Handbook
Part	Petroleum Generation and Migration
Chapter	Petroleum Source Rocks and Organic Facies
Author	S. R. Jacobson
Link	Web page
PDF	PDF file (requires access)
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Organic facies have been described from rocks of diverse ages and from various areas. The primarily chemical criteria used to characterize organic facies reflect numerous complex biological, physical, and chemical processes. The term organic facies has had an evolving etymology (see Jones). Organic facies are not palynomorph facies, palynofacies, pollen facies, kerogen facies, organic matter facies, or maceral facies. They cannot be determined by microscopy alone. The definition of the term as used here is from Jones and Demaison: "An organic facies is a mappable subdivision of a designated stratigraphic unit, distinguished from adjacent subdivisions on the basis of its organic constituents, without regard to the inor anic aspects of the sediments." Organic facies are determined by their elemental (C, H, and O) composition and, in ambiguous circumstances, are further differentiated by their microscopically determined maceral (organic particle) abundances. The concept of mappability presupposes that organic facies represent sufficiently large stratigraphic thicknesses and areal extent and have similar organic geochemical properties. This homogeneous character is rarely found on a fine scale, and the larger "mappable" scale requires a geological interpretation. To apply any facies concept, a geologist must (1) determine the scale at which variations are important and (2) carefully examine and interpret perturbations in data trends.

Method of organic facies determination

In thermally immature rocks, organic facies can be determined by kerogen typing. According to Jones, this technique mainly relies on (1) kerogen identification adequately reflecting petroleum generative potential, (2) kerogen distribution being "neither random nor capricious," and (3) the distribution of similar kerogen extending over tens to hundreds of meters vertically and hundreds to thousands of square kilometers areally. The latter criterion is sometimes recognizable in massive dark shale units, but must be evaluated with care. There has to be enough stratigraphic sampling to recognize volumetrically significant variations in organic facies within lithostratigraphic units. Petroleum source rocks have organic facies variations both vertically and horizontally.

Kerogen typing is based on elemental analyses for hydrogen, carbon, and oxygen. By plotting atomic H/C versus O/C of coal constituents (macerals), Van Krevelen recognized that each kind of coal maceral had consistent ratios. These ratios reflect the hydrocarbon generative potential of the maceral type. Van Krevelen's work was extended to potential petroleum source rocks by Tissot and his co-workers. The somewhat time-consuming elemental analyses of kerogen were replaced by Rock-Eval data for whole rock samples. The Rock-Eval derived hydrogen index (HI) and oxygen index (OI) were substituted for H/C and O/C.

From data published by many workers, main pathways emerged in Van Krevelen-type plots that reflected the dominance of four main organic matter components. These four types have evolved to kerogen types I, II, III, and IV. These narrowly defined kerogen types are found with surprising frequency, although intermediate values are common. Jones presents a scheme (Table 1) for separating this range of data into seven organic facies, which he designates with letters to distinguish them from kerogen types (Figures 2 and 3). He suggests that organic facies reflect both the original biological input and the preservational processes. Jones supports his criteria with numerous published examples.

Alteration of petroleum source rocks and organic facies

Alteration affects organic material before it is buried. This can include biodegradation, physical abrasion, and chemical changes related to Eh and pH in the water column and in sediments. Individual organic matter constituents react differently to each of these factors. Also, selective alteration of particular constituents can occur after burial and kerogen formation. The measurable properties of petroleum source rocks and their organic facies change after weathering and thermal maturation. In both cases, organic matter quantity and quality are diminished.

Table 1. Generalized geochemical characteristics of organic facies A through D.^a

Organic Facies	H/C (at % Ro = 0.5)	H (mg HC/g TOC)	Ol (mg CO2/g TOC)
A	>1.45	>850	<30
AB	1.35-1.45	650-850	20-50
B	1.15-1.35	400-650	30-80
BC	0.95-1.15	250-400	40-80
C	0.75-0.95	125-250	50-150
CD	0.60-0.75	50-125	40-150+
D	>0.60	>50	20-200+

^a_{The H/C values define the organic facies. Hydrogen index (HI) and oxygen index (Ol) values from Rock-Eval may vary slightly from indicated values for specific organic facies. (After Jones.)}

An example of extreme weathering occurs in numerous thin (millimeters to a few centimeters thick), thermally immature to marginally mature chocolate brown oil shales of the Galena Group (Middle Ordovician) of the North American Mid-Continent. In shallow cores (<1000 ft, or ~300 m, deep) from east-central Iowa, these thin brown shales have TOC values occasionally exceeding 40 wt. % and Rock-Eval hydrogen indices (HI) near 1000. These thin layers contain more than 85% organic matter (mainly carbon, hydrogen, oxygen, sulfur and nitrogen) by volume. Along the Mississippi River at Guttenberg and Specht's Ferry, Iowa, these chocolate brown shales have selectively weathered away completely, leaving recesses between the more resistant carbonate layers. Preferential weathering of most fine-grained sedimentary rocks occurs whether or not rocks are organic rich and whether or not the fine-grained sediments are well cemented. Weathering often penetrates beyond the observable changes in color and induration and reduces TOC and HI values. It appears that the quantity and quality of all are diminished when exposed.

Generating and expelling petroleum reduces organic matter quantity and quality of source rocks (e.g., Daly and Edman). 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. 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 (already given), the organic "constituents" that distinguish organic facies might be macerals (organic particles) or molecular components. Jones, 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. 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 <0.60 and HI <50. This facies contains highly oxidized organic matter, which may represent burnt wood (charcoal), recycled terrestrial material, and thermally postmature constituents. It may include fragments of larger woody components recycled from porous sandy units where oxidation prevailed. This facies is usually encountered in small concentrations and has no hydrocarbon generative capacity. It occurs in prograding sediments associated with sea level high stands and redeposited sediments of lowstands. Organic facies D is the regionally distributed type IV kerogen. It is frequently found in fluvial paleoenvironments, on offshore toes of deltas, and where organic carbon has been recycled. Sediments containing this kerogen can be found in all systems tracts, including the fine-grained parts of turbidites in lowstand systems tracts and the silts and muds of highstand prograding sediments.

Summary

Organic facies are mappable units. Hydrogen-rich organic facies reflect hydrocarbon generative potential that helps exploration geologists map the distributions of possible petroleum source rock candidates. The criteria for assigning rocks to organic facies are hydrogen richness combined with maceral analysis of sedimentary organic matter. The method is based on elemental analyses (C, H, and O) of the organic matter, but may be effectively applied using hydrogen and oxygen indices from Rock-Eval pyrolysis.

The distribution of organic facies reflects varying influences of biological productivity, preservation, and geological processes. These processes are complexly woven into a fabric of water (oceanic and lacustrine) and atmospheric paleocirculation, nutrient and mineral source and supply, water depth, water temperature and chemistry, wave and storm activity, and bioturbation related to oxygen availability in bottom sediments.

Petroleum source rocks form where appropriate conditions for organic productivity and preservation of organic matter occur. Condensed sections often record these conditions because they reflect conditions of rising sea level and reduced detrital input. Thus, oil-prone source rock candidates often occur where condensed sections formed in poorly oxygenated sedimentary environments. These condensed sections are often associated with transgressive system tracts. Gas-generative organic facies such as coals may occur in the prograding deltaic deposits of some highstand system tracts.

Organic facies can be mapped with relatively small amounts of data in sequence stratigraphic frameworks. Furthermore, by increasing their familiarity with organic facies and source rock potential in stratigraphic sequences, geologists can more accurately predict petroleum source rock distributions in geological time and space.

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