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

Abstract

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 TOC 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 [sup [minus]]00 to less than 100 mg/g), visual kerogen analyses (amorphous to structure kerogen), and carbon isotope ratios ([minus]23 to [minus]27% PDB). Analyses of 23 oils from the major producing intervals in the basin indicate variable maturity according to molecular distributions of aromaticmore » 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 source facies, coupled with identification of oil-source rock correlations, should help define new areas for exploration in the basin.« less