Black shale formation environment and its control on shale oil enrichment in Triassic Chang 7 Member, Ordos Basin, NW China

Abstract

Based on geochemical analysis results of core samples from the Triassic Chang 7 Member of Well Feng 75 drilled in the northwest margin of Ordos Basin, combined with geological characteristics of this region, the formation environment of the black shale and its control on shale oil enrichment are comprehensively studied. From the Chang 73 to Chang 71 Sub-members, the black shale have organic carbon contents decreasing from 5.70% to 3.55%, hydrogen indexes decreasing from 345 mg/g to 269 mg/g, while the oxygen indexes increasing gradually from 6 mg/g to 29 mg/g, indicating that the sedimentary environment during the depositional period of Chang 72 and Chang 73 Sub-members was anoxic. Biomarkers in the black shale change regularly, and have an obvious “inflection point” at the depth of 2753–2777 m in the Chang 73 Sub-member, indicating that the input of terrigenous organic matter increased. However, there is a negative drift about 2% of organic carbon isotopic composition near the “inflection point”, which is in conflict with the results of biomarker compounds. This is because the extreme thermal and anoxic events caused by continental volcanic activity in the ancient Qinling region caused negative drift of carbon isotopic composition of the black shale in the Ordos Basin. The volcanic activity caused rise of temperature, polluted air, extreme weathers, more floods and thus more input of terrigenous organic matter, and gave rise to extremely anoxic environment conducive to the preservation of organic matter. Terrigenous organic matter is more conducive to the formation of light oil than aquatic organic matter, so these sections in Yanchang Formation are major “sweet spots” for shale oil enrichment.

Keywords

• black shale
• Triassic
• sedimentary environment
• volcanic ash
• anoxic event
• shale oil