百色盆地浅层气成藏机制分析

GENETIC TYPES AND RESERVOIRING MECHA-NISM ANALYSIS OF SHALLOW GAS IN BAISE BASIN,GUANGXI

百色盆地是一个典型的陆相第三系残留型盆地。浅层气的分布,在盆地东部坳陷围绕生烃中心呈环带状分布,区带上沿主干断裂呈珠状出现;在盆地西部主要分布在南斜坡的江泽地区并沿断层呈带状分布。含气层位主要是始新统百岗组和渐新统伏平组,次为始新统那读组,埋深一般小于1500 m,大部分气层埋深在300-700 m之间。浅层气在地球化学特征上大多数呈现为甲烷含量高(超过95％,干气)、甲烷碳同位素组成轻(-55‰^-70‰)典型的生物气特征,其成因类型可划分为生物气、生物气-热解气及油层菌解气三类。其成藏机制与烃源岩的早、晚两期生物气的形成作用有关(成岩作用早期和构造运动抬升后的晚期),并以晚期复合成藏为主。

Baise Basin in Guangxi is a typical continental Tertiary remnant basin. Shallow gas is mainly distributed as a circular belt around hydrocarbon generating center and occurs as pearls along the main fault regionally in the east depression of the basin, and it is also distributed as a belt along the faults in Jiangzhe region of south slope in the west part of the basin. The gas-bearing strata are mainly the Baigang Formastion of Eocene and Fuping Formation of Oligocene, then the Nadu Formation of Eocene, and their buried depths are less than 1500 m in general, most of them being 300-700 m. The geochemical characteristics of the shallow gas are indicated as typical biogas with high methane content (>95% , dry gas) and light carbon isotopic composition (55%0- 70%0), and its genetic types can be divided into three types, i. e. biogas, biogas-pyrolysis gas and reservoir bacterial degradation gas. The gas reservoir formation was related to the two-stage biogas generation (the early diagenetic stage and the late stage after source rocks' being uplifted because of tectonic movement) , the late complex reservoiring being in the majority.