摘要
为了深入研究四川盆地南部下寒武统麦地坪组-筇竹寺组烃源岩发育环境及源岩沉积地球化学特征，通过分析川南井研-犍为地区金页1井麦地坪组与筇竹寺组烃源岩岩矿组成、有机碳含量、成烃生物组合、氧化还原性指标及各项指标之间的内在联系，发现麦地坪组与筇竹寺组烃源岩岩矿组成与成烃环境存在很大差别：麦地坪组烃源岩为含硅质云岩，有机碳丰度中等，δU大于1.7，指示超强还原环境，其成烃生物主要为具有浅水生活习性的小壳生物形成的微生物席，推测其强还原性可能与大量繁盛并死亡的小壳生物迅速消耗海底水中氧气有关，并非由深水相环境造成，虽还原性很强，但烃源岩有机碳丰度并不高，各种沉积地球化学参数关联性分析认为麦地坪组应形成于潮坪沉积环境。筇竹寺组烃源岩主要由硅质与粘土质页岩组成，下、上两段有机碳丰度高低不一，成烃生物以无定形絮状有机聚集体为主，δU指数0.8~1.7，有机碳含量与氧化还原性之间对应关系明显，沉积环境应为氧化至较强还原环境的浅水-深水陆棚相。川南地区早寒武世时期的构造-沉积-岩相古地理环境、原始生物类型、岩石矿物质组成等多种因素共同决定了烃源岩品质的优劣及其沉积地球化学特征。
Abstract
An in-depth study was carried out on the settings and sedimentary geochemical characteristics of source rocks in the lower Cambrian Maidiping-Qiongzhusi Formations of southern Sichuan Basin.The analysis of the rock compositions, organic carbon content, hydrocarbon generation biological combinations, and redox indexes as well as their correlations, on source rock samples from the Maidiping and Qiongzhusi Formations in Well Jinye 1 of Jingyan-Qianwei area, shows that the two formations are quite different in terms of mineral composition and hydrocarbon generation environment.The Maidiping Formation is dominated by siliceous dolomite and has a moderate organic carbon content.Hydrocarbon was mostly generated by microbial mats composed of small shallow-water shell creatures.A strong reducing environment can be inferred from its high Uranium index (greater than 1.7).It is suggested that, instead of being caused by deep water, the strong reducing environment is due to the thriving and rapid consumption of oxygen of the shell creatures in the sea bed.A combination of these understandings with a correlation analysis of sedimentary geochemical parameters indicates a tidal flat sedimentary setting for the deposition of the formation.The Qiongzhusi Formation is dominated by siliceous and clayey shale and varies in organic carbon abundance from the lower to upper members.Different from the Maidiping Formation, its hydrocarbon-generation organisms are mainly amorphous flocculent aggregates of organic matter.The Uranium index varying between 0.8 and 1.7 and a positive correlation between the organic carbon content and the redox properties of the samples all indicate a shallow-to-deep shelf setting of oxidation to strong reduction for the formation.In summary, various factors, including the tectonic-sedimentary-lithofacies paleogeographical environment, primitive biological type, and rock mineral composition during the Early Cambrian in southern Sichuan Basin, serve to jointly determine the quality of the source rocks and their sedimentary and geochemical characteristics.
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