Geochemical characteristics of lower cretaceous lacustrine organic matter in the southern sag of the Wuliyasitai depression, Erlian Basin, China

Abstract

The Wuliyasitai southern sag is an oil-rich area in the Erlian Basin. The geochemical characteristics of the organic matter were comprehensively analyzed by a combined investigation of Rock-Eval pyrolysis, molecular geochemistry, vitrinite reflectance, and thin section observation. The studied samples have vitrinite reflectance (Ro) values of 0.33–1.22% and pyrolysis parameter Tmax values of 424–456 °C, suggesting that organic matter is mainly present in the low mature to mature stage. The values of the abundance indicators including total organic carbon (TOC), chloroform bitumen “A,” potential hydrocarbon generation amount (PG), hydrogen index (HI), and total hydrocarbon (HC) suggest a good to very good hydrocarbon generation potential. The pyrolysis parameter Tmax and HI obtained imply that the samples are dominated by the type Ⅱ1-Ⅱ2 organic matter. The composition characteristics of n-alkanes, steranes, and terpanes confirm that the organic matter has mixed input sources. The Aershan Formation (K1ba) in the southeastern part of the sag show higher contributions of algae and bacteria due to fertilization by volcanic ash. The hydrocarbon generation threshold was determined from the hydrocarbon index (HCI), production index (PI), chloroform bitumen “A"/TOC, and HC/TOC. By analyzing the relationships between TOC and pyrolysis parameter HCI, chloroform bitumen “A”/TOC, the lower limits of the TOC values for effective source rocks were established to be 2.5 wt % in the K1bt1 and 1.5 wt % in the K1ba. The pristane/phytane (Pr/Ph), gammacerane/C31 22R hopane, C35/C34 homohopane and trace element ratios suggest that the source rocks are primarily deposited in a freshwater sub-oxidizing environment. The TOC and HI are negatively correlated with Pr/Ph, indicating that redox conditions play an important role in source rock preservation. Most molecular maturity parameters are not recommended owing to their poor correlations with Ro.