Thermal Effect on the Distribution of Regular Sterane and Geological Significance

Abstract

In order to study thermal effect on distribution characteristics of regular sterane in humic coals, a suit of 25 coals derived from Kuqa Depression in Tarim Basin and Ordos Basin are studied to investigate the distribution of regular sterane in different maturity, which mainly discusses the thermal effect. The results show that the C 27 ααα/C 29 ααα regular sterane ratio has a linear relationship with R o (t max ) and aromatic hydrocarbon maturity parameters in the coals from mature to high-mature stage. It suggests that the distribution of the C 27 , C 29 regular steranes will fail to reflect original source input in high evolutionary stage. On the basis of thermal simulation experiment results, it further proves that the distribution patterns result from the demethylation effect of C 29 sterane in the mature to high mature evolution stage and the C 27 ααα/C 29 ααα regular sterane increases with increasing maturity. The breakage of C-C key in branched chain from C 27 , C 29 regular steranes to make C 27 ααα/C 29 ααα regular sterane ratio invariance at high and over mature stage. Therefore, the ratio can be used to distinguish the maturity in high and over mature stage.

Research Status and Progress of Genetic Mechanism of Abnormal Regular Sterane Distribution in Geological Bodies

“…C 27 , C 28 and C 29 steranes were first identified in the American green river and Pre-Cambrian shale by Burlingame (1965) [1]. Then, Eglinton and Calvin (1966) [2] and Eglinton and Murphy (1969) [3] further confirmed the existence of cholestane, ergostane and sitostane.…”

“…Kimble et al (1974) [4] carried out a systematic research on the stereoconfiguration of steranes, and then gave their schematic diagrams and typical mass spectra, laying the foundation for the steroid geochemical attribute research. Based on the study of relative abundance and composition of C 27 , C 28 and C 29 sterols in various organisms and modern sediments in Mexico Bay, Meinshein (1979, 1981) [5] [6] suggested that algae was the predominant primary producer of C 27 -sterols, while C 29 -sterols were more typically associated with terrestrial land plants. On this basis, they proposed the distribution of C 27 , C 28 and C 29 sterols could be used to differentiate the organic matter source.…”