Variations in hydrocarbon generating potential of the Chang 7 shale: Evidence from pyrite morphology and sulfur isotope

Abstract

Hydrocarbon generating potential in source rocks is influenced by various controlling factors. Bottom water redox condition during source rock deposition is an important one among these factors and can directly control the organic matter enrichments in source rocks. The study of pyrite morphology and sulfur isotopic compositions in pyrite provides useful information on depositional redox conditions in ancient marine and lacustrine systems. The results of our study in the Ordos Basin, North China show that pyrite morphology varies from framboid to euhedral crystals throughout the Chang 7 Shale. Further, the sulfur isotopic compositions of pyrite (δ34Spy) display a close linkage with the framboid size distributions of throughout the Chang 7 Shale deposition. The presence of relatively small pyrite size (average = 8.7 μm) and lighter sulfur isotopic compositions of pyrite (average = 4.9‰) in the Chang 73 suggests the bottom water redox conditions were anoxic to dysoxic. In contrast, pyrites in the Chang 72 and Chang 71 are relatively larger (average = 12.7 μm), and have heavier sulfur isotopic compositions (average = 7.1‰) as compared of those in the Chang 73 shales, suggesting a long-term oxic-dysoxic bottom water conditions. Previous paleontological and geochemical data suggest that marine incursion may have supplied ample sulfate into the basin water promoting bacterial sulfate reduction and led to the forming of anoxic-dysoxic lake bottom water redox conditions during Chang 73 shale deposition. In addition, the coupled organic matter enrichment in the Chang 73 and oxygen-depletion conditions is also reflected by the coincidence of higher total organic carbon content in the Chang 73 deposits. A decreasing marine influence with subsequent restriction of the lake basin led to distillation of sulfate, followed by a gradual return to oxic-dysoxic bottom waters during the deposition of the Chang 72 and Chang 71.