CLAYTON, J. L., U.S. Geological Survey, Denver, CO, and CHEN JIANYU, China University of Geosciences, Wuhan, China

ABSTRACT: Organic Geochemistry of Black Shale and Associated Oils of the Pennsylvanian Hermosa Group, Paradox Basin, Utah and Colorado

According to recent estimates, the Paradox basin may contain more than 700 million bbl and 1.3 tcf of producible oil and gas, respectively. Newly developed horizontal drilling methods may increase the potential for oil produced from fractured, organic-rich shale in the Paradox Formation of the Hermosa Group. The purpose of this study was to characterize the organic matter contained in the black shale, interbedded lithologies, and oils produced from associated reservoir rocks. Total organic carbon (TOC) content of the rocks is from less than 0.2 wt.% in evaporites to as much as about 25 wt.% in some of the black shales. TOC content generally increases in the order of evaporites < siltstones < carbonates < black shale. Only the carbonate rock and black shale contain sufficient OC to have generated and expelled petroleum. Varying proportions of organic matter types I, II, and III are present in the organic-rich black shales according to Rock-Eval pyrolysis (hydrogen indexes from 700 to less than 100 mg/g), visual kerogen analyses (amorphous to structured kerogen), and carbon isotope ratios (-23 to -27 o/oo PDB). Analyses of 23 oils from the major producing intervals in the basin indicate variable maturity according to molecular distributions of aromatic hydrocarbons and organosulfur compounds. Source-related differences also are evident among the oils. For example, Cane Creek oils have relatively lower pristane/phytane and tricyclic/pentacyclic terpane ratios than Desert Creek and Barker Creek oils. Understanding the distribution and maturity of the organic sou ce facies, coupled with identification of oil-source rock correlations, should help define new areas for exploration in the basin.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.