四川盆地致密砂岩天然气成藏特征及规模富集机制——以川中地区上三叠统须家河组气藏为例

Accumulation characteristics and large-medium gas reservoir-forming mechanism of tight sandstone gas reservoir in Sichuan Basin:Case study on the Upper Triassic Xujiahe Formation gas reservoir in central Sichuan Basin

四川盆地川中地区须家河组致密砂岩气藏含水饱和度普遍较高。基于天然气地球化学分析测试数据和核磁共振高压充注模拟实验,研究了天然气运聚特征及气藏高含水机制。结果表明:(1)须家河组烃源岩—储层大面积交替叠置发育奠定了油气近距离聚集的基础,随烃源岩成熟度增高,天然气湿度系数变小、δ^(13)C_(1)值增高,为近源成藏;(2)须家河组气藏含气饱和度主体分布在50%~65%之间,储集空间的主要孔隙半径为0.1~10μm,相对大孔径占比越大,含气饱和度越高;(3)随充注压力增大,天然气在致密砂岩中由大孔径向小孔径空间"渐进式"聚集,并呈现出快速增加、缓慢增加和基本稳定的"三段式"充注特点,小压力驱动与相对大孔隙储集的耦合是川中地区须家河组天然气可以形成大中型气藏但含水饱和度高的主要原因。研究成果将为致密砂岩领域的拓展勘探提供理论技术支撑。

Tight sandstone gas reservoirs of Xujiahe Formation in central Sichuan Basin has generally high water saturation.Based on the test data of natural gas geochemistry analysis and nuclear magnetic resonance(NMR)high pressure filling simulation experiment,the characteristics of natural gas migration and accumulation and the gas reservoirs with high water saturation formation mechanism were studied.The results show that:(1)The Xujiahe Formation has laid the foundation for oil and gas near-source accumulation by interactiving superimposed of large area of source rock and reservoir.As the maturity of source rocks increases,the wetness becomes smaller and theδ^(13)C_(1) value of natural gas becomes heavier,indicating a near-source accumulation.(2)The gas saturation of Xujiahe Formation gas reservoirs is mainly distributed between 50%and 65%,and the main pore radius of the reservoir space is 0.1μm to 10μm.The larger the proportion of relatively large pore size,the higher the gas saturation will be.(3)With the increase of charging pressure,natural gas accumulation in tight sandstone by progressive aggregated from large pore diameter to small pore diameter space and presents the"three-stage"charging characteristics of rapid increase,slow increase and basically stable.The coupling between low pressure drive and relatively large pore reservoir is the main reason why the Xujiahe Formation of natural gas can form large-medium gas reservoir but high water saturation in central Sichua Basin.The research results will provide theoretical and technical support for the exploration of tight sandstone.