西湖凹陷中央洼陷带中部花港组岩性油气藏主控因素及形成模式

Main controlling factors and formation model of lithologic reservoirs of Huagang Formation in the middle of central depression belt in Xihu sag

西湖凹陷中央洼陷带发育一系列挤压大背斜,是寻找大中型油气田的勘探区域之一,一直以来以构造油气藏评价和研究为主。应用钻井、岩心、实验分析及地震资料,系统开展了中央洼陷带中部花港组岩性油气藏主控因素分析,建立了该地区岩性油气藏形成的地质模式,并预测了勘探有利区分布。研究结果表明,广覆式厚层平湖组煤系烃源岩发育为岩性油气藏的形成提供了雄厚的物质基础,辫状河三角洲—湖泊沉积背景下花港组多期水道与分流间湾交互发育有利于岩性尖灭型圈闭的形成,H1—H2层区域性盖层遮挡下可形成多套良好的储盖组合,早期NNE向油源断裂高效输导是岩性油气藏源外成藏的关键因素。大背斜北翼受北部物源控制,大背斜西翼受西部物源控制,分别发育多期水道尖灭;后期的龙井运动导致地层遭受强烈挤压,水道砂反转抬升形成上倾尖灭型岩性圈闭;同时,先存油源断层活化,沟通平湖组煤系烃源岩和上部水下分流河道砂体,最终形成多个岩性油气藏。综合分析认为,中央洼陷带中部A1、N1、Y1、N2等4个井区是H1—H2、H3—H5两套气层有利区叠合分布范围,面积达700 km 2,是最有利的勘探指向区。

A series of compressional anticlines are developed in the central depression belt of Xihu sag,which is one of the exploration areas for large-medium-sized oil and gas fields,focusing on structural reservoir evaluation and research all along.Based on drilling,core,experimental analysis and seismic data,the main controlling factors of Huagang Formation lithologic reservoirs in the middle of central depression belt are analyzed systematically,the geological model of lithologic reservoir formation in this area is established,and the distribution of favorable exploration areas is predicted.The results show that widely-covered thick coal-measure source rocks in Pinghu Formation provide an abundant material basis for the formation of lithologic reservoirs;under the braided river delta-lacustrine sedimentary background,the interactive development of multi-stage channel and interdistributary bay deposit in Huagang Formation is favorable for the formation of lithologic pinchout traps;under the shelter of regional cap rocks in H1—H2 layers,many sets of good reservoir-cap rock assemblages can be formed;the efficient transportation of early NNE trending oil source faults is the key factor for the formation of lithologic reservoirs.The northern and western wings of the geoanticline controlled by the northern and western provenances respectively developed multi-stage channel pinchouts;the late Longjing movement resulted in strong compression of the strata,and the channel sand reversed and uplifted to form the updip pinchout lithologic trap;at the same time,the activation of pre-existing oil source faults connected the coal measure source rocks of Pinghu Formation with the upper underwater distributary channel sandbodies,forming multiple lithologic reservoirs at last.Comprehensive analysis shows that the four well areas of A1,N1,Y1 and N2 in the middle of the central depression belt are the superimposed distribution range of the two sets of favorable gas zones of H1—H2 and H3—H5,with an area of 700 km 2,which are the most favorable exploration areas.