下扬子巢湖地区中二叠统孤峰组富有机质硅质岩有机地球化学特征

Organic Geochemistry of Organic-rich Cherts in the Middle Permian Gufeng Formation in Chaohu, Lower Yangtze Area

华南下扬子巢湖地区孤峰组黑色硅质岩中富含有机质,是中国油气和页岩气勘探的潜在源岩,研究孤峰组有机质富集规律对开展油气勘探工作具有重要意义。文章对巢湖平顶山地区中二叠统孤峰组硅质岩进行总有机碳含量(TOC)和饱和烃分子标志物特征分析,探讨其中有机质富集规律。结果表明孤峰组有机质热成熟度参数C29ααα甾烷20S/(20S+20R)比值分布在0.37至0.46之间,C31升藿烷22S/(S+R)比值分布在0.53至0.59之间,热成熟度水平处于生油窗的早期。有机质正构烷烃主峰为C18,以短链烷烃为主,藿烷含量较高,甾烷以C27ααα20R胆甾烷为主,有机质主要来自海洋浮游藻类和细菌。孤峰组有机质Pr/Ph比值小于0.6,伽玛蜡烷指数平均值为0.2,说明古沉积水体为中等分层缺氧环境。孤峰组普遍富含有机质可能与炎热气候下古沉积水体长期分层和缺氧有关。随着气候在孤峰组沉积早期和晚期突然变冷,高强度的海洋洋流上涌提高了初级生产力水平,造成有机质更大规模的富集。

The black cherts in the middle Permian Gufeng Formation in Chaohu area in Lower Yangtze,South China are rich in organic matter.This formation is the potential source rocks of petroleum and shale gas exploration in China.The study on the organic matter accumulation of the Gufeng Formation is important for the petroleum exploration.Here we analyze total organic carbon(TOC)content and biomarkers characteristics of organic matter in the Gufeng cherts of middle Permian in the Pingdingshan,Chaohu area,so as to address the organic matter accumulation mechanism.Our results show that the organic matter thermal mature proxy C29ααα20S/(20S+20R)ratios range from 0.37 to 0.46,and C3122S/(S+R)ratios range from 0.53 to 0.59.These suggest that the thermal mature level is the early stage of“oil window”.The n-alkane compounds are mainly composed of short-chain n-alkanes and peak at C18.Hopane contents are higher than the terpane.C27ααα20R sterane is higher than the C28 and C29ααα20R.These suggest that the organic matter of the Gufeng Formation originates from phytoplankton algae and bacteria with a small amount of terrestrial high plant.The Pr/Ph ratios of the organic matter in the Gufeng Formation is less than 0.6,and the average gamacerane index is 0.2.These suggest that the redox conditions of paleo-water column during the deposition of the Gufeng Formation were moderate stratification and anoxia.The organic-rich GufengFormation is probably related to this stratification and anoxia during warm climatic conditions.Associated with the suddenly cooling,enhanced upwelling increased greatly the primary productivity greatly and yielded more large-scale organic matter accumulation.