南鄱阳坳陷海相烃源岩晚期生烃论证

南鄱阳坳陷为一“上张下压”双层结构的叠合盆地,存在海相—海陆过渡相的上二叠统茅口组和龙潭组以及上三叠统安源组三套烃源岩。烃源岩的热演化程度随上覆白垩纪—古近纪区域盖层厚度成正比增加,中燕山期不整合面附近不存在Ro值“上低下高”的跳跃式变化,白垩系沉积期末,二叠系、上三叠统烃源岩达到了地质历史上的最大埋藏深度,表明晚期叠置的白垩纪—古近纪伸展沉积盖层是导致这些烃源岩达到现今热演化程度并出现晚期生烃的根本原因。中燕山期不整合面附近存在活跃的油气显示,这些原油从地化特征上可分为两大类型。产自LT1井周家店组底部的原油是由龙潭组树皮煤(Ⅱ型干酪根)形成的典型煤成油,且以轻质油为主,未遭受过地表水氧化与生物降解改造,从而证明中燕山期以后确实存在海相烃源岩的晚期生烃。盆地模拟分析表明,坳陷内海相烃源岩晚期生烃现象普遍,生烃强度可达油(150～280)×104t/km2,气(35.5～52.4)×108m3/km2,预示着该叠合盆地晚期成藏具备烃源基础。

Demonstration of Late Hydrocarbon Generation from Marine Source Rocks in Nanpoyang Depression, Poyang Basin

Nanpoyang Depression of Poyang Basin, located in Jiangxi province, is a superimposed basin, where source rock mainlyare the marine-neritic Upper Permian Maokou and Longtan Formations and the Upper Triassic Anyuan Formation.Level of ther-mal evolution of the source rocks rises with increasing thickness of overlying Cretaceous-Paleogene regional cap.During the end ofCretaceous sedimentation, the Permian and Upper Triassic source rocks were buried to largest depth, which demonstrates that latesuperposition of Cretaceous-Paleogene regional expensive cap results in the reaching to nowadays thermal evolution and late hy-drocarbon generation of source rock. A number of hydrocarbon indications exist near the Mid-Yanshanian unconformity.Oil fromthe source rock can be divided two types in geochemistry. The oil (commonly light oil) yielded from the bottom of CretaceousZhoujiadian Formation of Well LT- 1 is a typical coal-originating oil from the Longtan "bark coal" (Type Ⅱ kerogen) and did notundergo with oxidation of surface water and biodegradation,which indicates that late hydrocarbon generation of marine sourcerocks really occurred after mid-Yanshanian stage.Analysis of basin modeling also supports this conclusion, The source rock indexshows(150～280) ×104t/km2 (oil) and (35.5～52.4) ×108m3/km2(gas), which is predicted that this basin possesses basic source forlate hydrocarbon accumulation