ABSTRACT: Lacustrine Source Rock Systems--Is the Green River Formation an Appropriate Analog?

KATZ, B. J., Texaco Inc., Houston, TX

The Eocene Green River Shale is a commonly invoked analog for lacustrine source rock systems. The hydrocarbon source potential of the Green River Shale is well known. Organic carbon content within the unit is variable but often exceeds 30 wt.%. Pyrolytic yields are commonly greater than 20 mg HC/g rock and often exceed 50 mg HC/g rock. Atomic H/C ratios obtained on isolated kerogens are greater than 1.4. The oils generated from these rocks are usually paraffinic. The use of the Green River Shale as an analog may have occurred largely as a consequence of its excellent source rock attributes rather than as a result of direct or indirect evidence of either geologic or geochemical similarities. A comparison, however, of geochemical and sedimentologic data for several source rock systems a d modern lacustrine settings reveals that the Green River Formation is an inappropriate analog.

Although the generated products from most lacustrine source rock systems appear similar, their ability to generate hydrocarbons varies substantially. For example, the generation potential of the Green River Formation is substantially greater than that observed for modern muds of Lakes Edward, Kivu, and Tanganyika, or in the source rocks of the Songliao (China), Wind River (Wyoming), and Central Sumatra (Indonesia) basins. These differences in hydrocarbon yield appear to be, in part, a result of the amount of both the allochthonous organic material and the effects of clastic dilution. There are, however, indications that controls on organic productivity and preservation for these systems were also different.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)