Geochemical characteristics and sedimentary environment of Chang 7 source rocks, Southern Ordos Basin

Key words:  Ordos Basin  Tongchuan area  organic geochemical characteristics  source rock evaluation  source of organic matter  sedimentary environment

Abstract:

The study focused on the typical shale profiles of the Chang 7 member extensively exposed along the southern margin of the Ordos Basin. To gain insights into the formation of source rocks and the prevailing sedimentary environment,detailed geochemical analyses were conducted on 35 outcropped source rock samples collected from Tongchuan areas.These examinations include total organic carbon (TOC) analysis, rock-eval pyrolysis, GC-MS of saturated hydrocarbons, and trace elements analysis. The findings suggest that the organic-rich black shales were deposited within a semi-deep lacustrine environment characterized by freshwater and reducing bottom water conditions. The climate was warm and humid fostering the thriving of algaet during the sedimentary period of Chang 7. The presence of highly enriched Mo elements indicated lake eutrophication and high primary productivity, with organic matter primarily contributed by algae. The source rocks are highly enriched in organic matter, with 23.51% of TOC and 112.93 mg/g of S₁₊S₂ in average. Furthermore, the organic matter within the source rocks predominantly comprised a sapropelic-mixed type, indicative of substantial hydrocarbon generation potential. The peak temperature of rock-eval and biomarker ratios suggested that the source rock has reached the threshold of hydrocarbon generation, indicating promising exploration potential.