莺歌海盆地乐东气田天然气地化特征和成藏史

莺歌海盆地乐东气田储层流体存在明显的非均质性,天然气主要成分为甲烷、二氧化碳和氮气。热成因烃气来源于中新统梅山组-三亚组泥岩,氮气是烃源岩中的有机质在成熟-高成熟阶段的生成物,无机二氧化碳气可能来自深埋沉积地层中碳酸盐矿物热分解及其与粘土矿物反应生成的二氧化碳,或许有部分前第三系碳酸盐岩热解成因二氧化碳和少量幔源二氧化碳加入。依据化学组成和碳同位素资料初步确定乐东气田至少存在3期天然气的注入,依次是生物气、热成因富烃气和无机二氧化碳,天然气的成藏时间大约距今1.2～0.1 Ma。沟通烃源岩的底辟断裂为天然气向上运移提供了良好的通道,由于高压流体的幕式排放诱发底辟断裂周期性开启和封闭,从而造就了莺歌海盆地底辟带气田的多源混合-幕式成藏特点。

Geochemical behaviors and reservoiring history of natural gas in Ledong gas field in Yinggehai basin

Reservoir fluids in the Ledong gas field in Yinggehai basin have obvious heterogeneity. The natural gas is mainly composed of methane, CO2 and nitrogen. Thermogenic hydrocarbon gases were sourced from the mudstones in Miocene Meishan-Sanya Fms. Most of nitrogen were generated from the organic materials of the source rocks during mature and post-mature stages. Inorganic CO2 might be the results of thermal decomposition of carbonate minerals and reactions between carbonate and clay minerals in deeply buried sedimentary strata. In addition, some of the CO2 might be generated by thermal decomposition of the pre-Tertiary carbonate rocks and a little amount of the CO2 might be added from the mantle. According to the chemical composition and carbon isotope data, it is preliminarily determined that there are at least three stages of gas charging, i. e. successively biogas, thermogenic hydrocarbon-rich gas and inorganic CO2. The time of gas reservoiring is about 1.2 to 0.1Ma. The diapiric faults that link up source rocks would have acted as good pathways for upward migration of natural gas. Episodic discharging of high-pressure fluids have induced the periodic opening and closing of diapiric faults, resulting in the mixing of gases from multiple sources and episodic accumulation of gases in the diapiric zone of Yinggehai basin.