Organic geochemistry and petroleum potential of Pennsylvanian black shales, Powder River and Denver basins

Conference · Fri May 01 04:00:00 EDT 1987 · AAPG Bull.; (United States)

OSTI ID:6184385

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

Thin, Middle Pennsylvanian 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 the 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 depositional 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.

• Research Organization: Geological Survey, Denver, CO
• OSTI ID: 6184385
• Report Number(s): CONF-870606-
• Conference Information: Journal Name: AAPG Bull.; (United States) Journal Volume: 71:5
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
020200* -- Petroleum-- Reserves
Geology
&amp; Exploration
BITUMINOUS MATERIALS
BLACK SHALES
CARBONACEOUS MATERIALS
CHATTANOOGA FORMATION
CHEMISTRY
COLORADO
DEPOSITION
ENERGY SOURCES
FEDERAL REGION VIII
FOSSIL FUELS
FUELS
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC FORMATIONS
MATERIALS
MATURATION
MINERAL RESOURCES
MONTANA
NORTH AMERICA
OIL SHALES
PALEOZOIC ERA
PENNSYLVANIAN PERIOD
PETROLEUM DEPOSITS
POWDER RIVER BASIN
RESOURCES
SOURCE ROCKS
USA
WYOMING