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Organic Geochemistry and Petroleum Potential of Pennsylvanian Black Shales, Powder River and Denver Basins

J. L. Clayton, C. M. Lubeck, J. D. King, T. A. Daws

Thin, Middle Pennsylvanian (Desmoinesian) organic-rich black shales (cumulative thickness generally less than 7 m) underlie much of the northern Denver basin and the southeastern Powder River basin. In the Powder River basin, these shales are part of the middle member of the Minnelusa Formation. During Desmoinesian time, the present-day area of the southeastern Powder River basin and the Nebraska panhandle was a shallow, at times highly saline restricted sea. In contrast, the black shales were deposited in a marine environment with normal salinity in the present-day area of northeastern Colorado, which was probably continuous with the Mid-Continent Pennsylvanian sea. Total organic carbon content of all the black shales is high (up to 20 wt. %) except along the margins of he ancient seaway and along the boundary between the open sea and the restricted basin (TOC < 2 wt. %). Extractable organic matter in the black shales is predominantly of marine origin. Evidence for marine organic material includes a predominance of low-molecular-weight alkanes (<C20) and absence of long-chain alkanes (>C25), very low saturated/aromatic hydrocarbon ratios (avg. 0.5), and relatively low pristane/phytane ratios (0.7-1.3). Although the black shales were apparently deposited in uniformly low-oxygen marine conditions, the restricted basin and open marine environments can be differentiated by organic geochemical analysis. Chloroform extracts from black shales from the restricted basin environment contain abundant red pigments, most likely derived from bacteria or algae requiring high salinity for growth. In contrast, extracts from the open marine shales, although equally organic rich, are devoid of pigments, suggesting deposition under conditions of normal seawater salinity. In bulk, organic matter in black shales from both environments is type II and has good potential for generation of liquid hydrocarbons during catagenesis. Pyrolysis yields of 50 kg/ton (50,000 ppm) are common, and some shales yield as much as 100 kg/ton (100,000 ppm).

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.