Sedimentology and geochemistry of Cretaceous source rocks from the Tiancao Sag, Yin'e Basin, North China: Implications for the enrichment mechanism of organic matters in small lacustrine rift basins

摘要

Petrography and compositions of organic matters of Cretaceous source rocks from the Tiancao Sag, the Yin'e Basin, Northern China, have been presented in order to determine the Cretaceous sedimentary environment and processes that enriched organic matters. The mudstones of the Bayangobi Formation have medium-high total organic carbon (TOC) (0.27-2.59%) and S-1 + S-2 (0.30-12.23 mg HC/g rock) values. Vertically, three sedi-mentary units (2460-2600 m; 2640-2860 m; and 3020-3050 m) present good hydrocarbon potentials. The macerals, the C-27-C-29 regular sterane distribution, and the isotopic and element geochemistry of the kerogen prove that the organic matter has a mixed origin of plankton/algae and terrestrial plants. The vitrinite reflectance, Tmax, and biomarker parameters collectively suggest that the source rocks have reached the mature stage. The mudstones were deposited in a reducing brackish-saline environment, and the salinity-sensitive proxies indicate a gradual evolution of the sedimentary conditions from low to high salinity. The positive correlation between the hydrogen index value and TOC content and the negative correlation between Pr/Ph ratio and TOC content suggest that the primary productivity and preservation in reducing water collectively contributed to organic matter enrichment of the Bayangobi Formation. The relationship between sedimentation rate and TOC content suggests that a sedimentation rate of 60-70 m/Myr is conducive to the formation of effective source rocks. Therefore, the organic matter enrichment of Bayangobi Formation was controlled by moderate-high paleoproductivity, reducing conditions, and an appropriate sedimentation rate. This paper proposes a "supply preservation-dilution" model for the source rocks in the Tiaocao Sag, Yin'e Basin.