Organic geochemical characteristics of hydrocarbon source rocks in upper Ordovician Wufeng Formation of northern Guizhou and their paleo-environmental significance

Based on a study of such organic geochemical characteristics of hydrocarbon source rocks in northern Guizhou as organic carbon, isoprenoid alkenes and terpanes, the authors have found that they have the following characteristics: ①TOC>3%, and its longitudinal distribution is stable; ②the value of S/TOC is very high, with an average of 0.1; ③the Pr/Ph ratio is lower than 1.0, which shows an obvious dominance of phytane in isoprenoids; ④the gammacerane index is higher than 0.15; ⑤the δ13Corg is lower than -30‰; ⑥the Ts/(Tm+Ts) ratio ranges from 0.361 to 0.518, with an average of 0.455. These characteristics indicate abundant sources of organic matter and a reduction environment. Furthermore, a comparison between lithological and organic geochemical profiles in different areas shows that the preservation condition in the late stage of Wufeng Period was better than that of the early stage, and that the upwelling was active in the middle stage of Wufeng Period.