川西坳陷上三叠统油气运移特征

OIL AND GAS MIGRATION IN UPPER TRIASSIC IN WEST SICHUAN DEPRESSION

川西坳陷是四川盆地上三叠统的生气中心 ,由于其超致密、超埋深、超压及圈闭形成历史复杂 ,勘探开发难度极高。文章分析了川西上三叠统油气的运移相态及油气疏导条件 ,据天然气样品碳同位素和轻烃指纹分析 ,天然气运移相态有油溶相、水溶相和游离相 3种 ;油气运移疏导条件也较有利 ,在致密化之前还存在厚大的孔隙性砂岩 ,与烃源岩沟通的断裂系统及流体增压形成的微缝是良好的运移通道 ,流体低势区是油气运移的有利指向带。应用BASINS 4 .5盆地模拟软件对川西坳陷上三叠统进行了埋藏史、热史、生烃史和运移史模拟 ,进行了流体势计算 ,盆地模拟结果表明中坝印支期古隆起 ,绵竹—丰谷、平落坝—新津燕山期古隆起 ,在上三叠统烃源岩生排烃主要时期 ,始终处于流体低势区 ,是有利的天然气运移指向带 ,与现今勘探实际吻合。

West Sichuan Depression was the gas generation center of Upper Triassic in Sichuan Basin.It is very difficult to explore and develop because of its super-compactness,ultra burial depth,hyperpressure and complicated trap formation history.In the paper,in the light of analyzing the phase states and transport conditions of oil and gas migration in Upper Triassic in West Sichuan Depression,It is pointed out that,according to the carbon isotope and light-hydrocarbon fingerprint analyses of gas samples,the phase states of natural gas migration include oil-soluble state,water-soluble state and free state;and the transport conditions were beneficial,i.e.there still existed massive porous sandstones befor compactification;the fault systems communicating with hydrocarbon source rocks and the microfissures formed by fluid boosting were the good migration channels;and the fluid low-potential areas were the favourable directing zones of oil and gas migration.On the basis of simulating the burial history,thermal history,hydrocarbon generation history and migration history of Upper Triassic in West Sichuan by applying BASINS 4 and 5 simulation softwares,and of calculating the fluid potentials,it was indicated that the Zhongba Indosinian epoch paleo-uplift,Mianzhu—Fenggu and Pingluoba—Xinjin Yanshan epoch paleo-uplifts were the favourable directing zones of oil and gas migration,being in the fluid low-potential areas from beginning to end during the main hydrocarbon-generating and-expelling periods of Upper Triassic source rocks.Such a conclusion is coincident with the present exploration results.Therefore,it is very necessary for determing the directing zone of oil and gas migration,surveying gas enrichment zone and guiding oil and gas exploration to study the oil and gas migration in Upper Triassic.