成都凹陷上侏罗统蓬莱镇组天然气运移特征与富集主控因素

成都凹陷上侏罗统蓬莱镇组气藏为次生气藏,其中的天然气来自纵向上距其3 000多米的下伏上三叠统,储层和烃源岩层的远距离纵向分隔使得运移条件在成藏诸要素中尤为关键。蓬莱镇组中含气砂体较为发育且分布非均质性较强,认识其非均质性特征需要深入分析天然气富集的主控地质因素。通过气源分析,纵向上各气藏产出天然气的同位素组成、原始组分、干燥系数、iC₄/nC₄等地球化学特征分析表明,蓬莱镇组气藏天然气运移通道主要为气源断层和储集砂体,运移方式为借助断裂系统和纵向上较为发育的多套渗透性砂体进行立体式高效运移。勘探开发成果分析表明:发育气源断层是成都凹陷蓬莱镇组成藏的必要条件;气源断层与储层的合理配置、砂岩的储集性好坏进一步影响天然气富集程度,储层下倾方向与气源断层相接或与之邻近、砂岩物性好的地区天然气富集程度高。天然气在气源断层与储层合理配置带内的有利储层中最为富集的部署思想,大大提高了钻获高产井和工业气井的成功率。

Characteristics and controlling factors of natural gas migration and accumulation in the Upper Jurassic Penglaizhen Formation of Chengdu Sag

The Upper Jurassic Penglaizhen Formation gas pool in Chengdu sag is a secondary gas pool with its gas being sourced from the Upper Triassic source rocks over 3 km beneath the reservoirs.Due to the long vertical distance between the reservoir and source rocks,the gas migration path and mechanism are assumed to be of particular importance.The gas-bearing sandstones are widely distributed in the Penglaizhen Formation and have great heterogeneity,thus study of the principal controlling factors on gas accumulation is critical to understanding its heterogeneity.Analyses of gas sources,original gas compositions,isotopic compositions,dry coefficient,and iC₄/nC₄ demonstrate that the major hydrocarbon migration pathways are source-rock-connected faults and permeable carrier beds through which gas is capable of migrating with high efficiency.Hydrocarbon exploration and exploitation indicate that source-rock-rooted faults and good-quality reservoirs.The existence of source rock-rooted faults is the indispensable condition for gas accumulation in the Penglaizhen Fm in Chengdu sag.The rational combination between the source rock-rooted faults and reservoirs and the poroperm charateristics of reservoirs further influence the enrichment of gas.Gas is highly enriched in the downdip areas where the sandstones have good poroperm characteristics and are connected with or near to the source rock-rooted faults.The study results are valuable for gas exploration and development and can increase success rate in drilling prolific and commercial wells.