川中八角场构造大安寨段凝析气藏形成机制再认识

八角场构造下侏罗统大安寨段凝析气藏是四川盆地中部地区目前发现的唯一规模较大的凝析气藏，研究其成藏机制对认识整个川中该段的成藏特征具有一定的指导意义。为此，根据天然气及原油的碳同位素特征，分析了大安寨段的油气成因；结合地震资料解释成果上断裂的展布以及大安寨段油气的分布特征，指出了八角场构造大安寨段凝析气藏的形成机制，进而探讨了天然气对大安寨段油气富集的影响。结果表明：①该构造靠近断裂的井，其天然气、原油的碳同位素值以及凝析油的轻烃特征异常，明显存在来自下部上三叠统须家河组天然气的侵入；②断裂的分布及规模控制着下部须家河组天然气混入的规模，大安寨段之所以能形成凝析气藏，在于存在断穿须家河组烃源岩与大安寨段储层的断裂，使须家河组天然气得以大量侵入，从而改变了大安寨段油藏的原始气油比，形成气侵型凝析气藏；③邻近的金华构造断裂规模较小，天然气侵入较少，仅在局部形成高气油比的油气井，而未形成凝析气藏；④天然气是川中大安寨段致密储层中石油运聚的主要动力，影响着低幅度无水致密储层中石油的富集程度及油井的产能。

Restudy of the formation mechanism of the Da’anzhai condensate gas reservoir in the Bajiaochang structure,Middle Sichuan Basin

The Da’anzhai condensate gas reservoir in the Bajiaochang structure is the only one large scale condensate gas reservoir in Middle Sichuan Basin, so the study of its formation mechanism is of great significance to a better understanding of the hydrocarbon accumulation there. In view of this, according to carbon isotopic compositions of crude oil and natural gas, this paper studied the origins of hydrocarbons, analyzed the formation mechanism based on the distribution of faults and hydrocarbons, and finally discussed the influences of natural gas on the enrichment of hydrocarbons in this study area. The following results were obtained. (1) In those wells close to the faults, abnormal characteristics were presented from carbon isotopes of natural gas and crude oil as well as C₄~C₇ light hydrocarbons in condensates, which discloses that the gas obviously came from the underlying Upper Triassic Xujiahe Fm. (2) The intensity of such gas invasion is controlled by the distribution and size of faults. The formation of the condensate gas reservoir in the Bajiaochang structure benefited from the faults which connect the source rock of the Xujiahe Fm and the Da’anzhai reservoir. The resulted mass invasion of gas from the Xujiahe Fm into the Da’anzhai oil reservoir changed the original gas oil ratio, so that a gas invasion condensate gas reservoir was formed. (3) Comparatively, the gas invasion appears weak in the adjacent Jinhua structure with small scale faults, where hydrocarbons were found with high gas oil ratios instead of condensate gas in some wells. (4) Natural gas is the primary drive for the migration and accumulation of oil in the Da’anzhai tight reservoir, influencing oil enrichment degree and oil well productivity of such a low amplitude water free tight reservoir in this study area.