Geochemical Processes Controlling Sulfur, Carbon, and Metals in Pennsylvanian Source Rocks of the Paradox Basin, Utah and Colorado

Abstract

During the Pennsylvanian Period, marine evaporate-carbonate-shale units were cyclically deposited in the Paradox basin, Utah and Colorado. To understand the formation and preservation of petroleum source rocks in these cycles, the authors chemically, mineralogy, and isotopically analyzed 100 shale samples from cores penetrating cycles 3 and 5 of the Paradox Formation. The geochemistry of sulfur, carbon, and metals in these carbonaceous shales records the heterogeneity of depositional and diagenetic conditions during formation of these rocks. Variations in sulfur, carbon, and metal geochemistry between and within shale units reflect differences in organic matter source, source of clastic material and its potential for iron sulfidization, salinity, sedimentation rates, and redox conditions, during deposition. Tentative conclusions are that: (1) organic matter production and preservation was enhanced in areas with low clastic input and a stable water column (hydrogen index values increase away from clastic source); (2) large amounts of hydrocarbon-rich organic matter (hydrogen index value of up to 440) and sulfate-enriched brines maximized sulfate reduction by bacteria in the anoxic sediment in the northern part of the basin; (3) sulfide-mineral formation was limited by the abundance of iron-oxyhydroxide coating on fine-grained particles; (4) during long-term stratification of the water column, anoxic bottom watermore » promoted isotopic evolution of sulfur reservoirs ([delta][sup 34]S of sulfide increased from [minus]36% to [minus]17%) and increased accumulation of Cr and Ni (up to 450 ppm and 150 ppm, respectively); and (5) euxinic conditions within the water column were unlikely during stratification.« less