四川盆地中侏罗统下沙溪庙组致密气勘探潜力及有利方向

Exploration potential and favorable direction of tight gas in Middle Jurassic Xiashaximiao Formation in Sichuan Basin

基于对四川盆地中侏罗统下沙溪庙组多年的勘探实践,研究了致密气成藏富集的关键条件,认为下沙溪庙组致密气藏为多层叠置、整体含气的次生构造—岩性气藏,具有明显的源控、相控、位控的"三元"控藏特点:(1)下侏罗统自流井组、上三叠统须家河组暗色泥岩、碳质泥岩与煤层等多套源岩提供了天然气来源;(2)冲积扇、三角洲相等沉积体系中的各类河道砂体和泥页岩组合形成的泥包砂结构是有利的富集组合体;(3)广泛分布的"叶肢介页岩"或紫红色泥页岩层可以作为区域盖层;(4)喜马拉雅运动之后形成的3大古隆起和3大坳陷,隆坳相间,发育了一系列下沙溪庙组顶部构造,具有全盆地分布形成有利含气圈闭的条件;(5)从须家河组断至下沙溪庙组的断层可以形成远源气藏的运移通道,近源的自流井组烃源可以通过小断层或微裂缝就近输导。据此,建立了下沙溪庙组致密气成藏模式,认为下沙溪庙组具备形成大中型气田的基础,川西地区勘探潜力最大,川北及川南地区次之,川中地区成藏条件配置较差,勘探潜力较差。

Tight gas is an important gas-pool type for natural gas resource and output increment in China.The Middle Jurassic Xiashaximiao Formation(J 2 x)is an important target for the exploration of tight oil and gas in the Sichuan Basin.Based on decades of exploration practice and studies on accumulation conditions of tight gas,it was indicated that the J 2 x formation gas reservoir is a secondary structural lithologic gas reservoir with multiple layers,and was constrained by source,position and sedimentary facies.The major characteristics of these reservoirs are(1)the dark mudstones,carbonaceous mudstones and coal seams in the Lower Jurassic Ziliujing and Upper Triassic Xujiahe formations provided abundant natural gas sources;(2)the mud-packed sand structure formed by the combination of various channel sand bodies and shale in sedimentary systems such as alluvial fans and deltas was favorable for the enrichment of natural gas;(3)the widely distributed“estheria shale”or purple-red mud shale layer acted as regional cap rock;(4)three major paleo-uplifts and three major depressions formed after the Himalayan Movement,developed a series of structures on the top of the Xiashaximiao Formation,which was favorable for the formation of gas-bearing traps distributed throughout the basin and(5)the faults from the Upper Triassic Xujiahe Formation to the Xiashaximiao Formation provided migration channels for far-source gas reservoirs,while gas in the Ziliujing Formation could migrate to near-source reservoirs through small faults or micro-fractures.Thus a tight gas accumulationmodel of the Xiashaximiao Formation was established.The Xiashaximiao Formation possesses the foundation for forming large and medium-sized gas fields.The western part of Sichuan Basin has the largest potential for gas exploration and followed by the northern and southern parts.In the central Sichuan Basin,the reservoir-forming conditions are poorly configured and made its exploration potential poor.