Geochemical characteristics and implications of hydrocarbon in source rocks of Chagan Sag, Yin'e basin of Inner Mongolia

Abstract

Chagan Sag is one of the most significant tectonic unit in the Yin'e Basin. The special component of the organic macerals and biomarkers in the Chagan sag suggests great difference of its hydrocarbon generation process. In this paper, forty samples of source rocks are subjected to carry out on the geochemical characteristics by methods of rock-eval analysis, organic petrology and gas chromatography mass spectrometry (GC-MS) to reveal the origin, depositional environment and maturity of organic matter in Chagan Sag, Yin'e Basin of Inner Mongolia. The total organic matter of the analyzed samples ranges from 0.4 wt%∼3.89 wt% with an average of 1.12 wt%, indicating fair to excellent hydrocarbon generation potential. The rock-eval results show that the S1+S2 and hydrocarbon index range from 0.03 mg/g∼16.34 mg/g (avg.3.6 mg/g) and 6.24–521.32 mg/g (avg. 199.63 mg/g), suggesting most of the kerogen types are type II and III, with small amount of type I. The Tmax ranges from 428 to 496 °C, suggesting low mature to mature stage. The macerals component of morphological macerals presents with certain amount of vitrinite, liptinite and some inertinite. However, the amorphous component takes the dominant part of the macerals with account of 50%–80%. The amorphous components are dominated by sapropelite in the source rock, indicating bacteriolytic amorphous promote the organic generation process. Hopanes and sterane are widely distributed in the source rocks. The biomarker results suggest a mix of planktonic-bacterial and higher plant origins, with wide range of thermal maturity and relatively reducing depositional environment. Abnormal high content of hopanes were observed in the biomarkers, and some special biomarkers such as monomethylalkanes, long-chain-alkyl naphthalenes, aromatized de A-triterpenes, 8,14-seco-triterpenes, and A, B-cyclostane are detected in Chagan Sag. The presence of these compounds suggests that bacterial and microorganisms are of great significance to the generation of hydrocarbon in the source rock in Chagan Sag.