青西油田裂缝性油藏研究

青西油田位于酒西盆地西部的青南凹陷,油田主体构造格局为窟窿山背斜和柳沟庄断鼻,储集体主要是下白垩统下沟组半深湖-深湖亚相泥云岩和扇三角洲前缘亚相砾岩.下沟组地层现今埋深大于4000m,处于晚成岩阶段,岩石胶结致密,属于特低渗储层.由于泥云岩具有纹层状结构和富含长石、石英等特殊矿物成分,以及铁白云石胶结的砾岩具有较强的刚弹性,在多期构造运动和成岩作用下,易形成顺层或切层裂缝网络、粘土矿物转化收缩缝网络、成烃演化微裂缝网络、溶蚀缝洞网络和构造裂缝网络等,构成油气运聚的网络,在流体封存箱和流体异常高压成藏机制作用下形成裂缝性油藏,油气富集在构造转折带、弧形断裂外侧、断裂交互部位、背斜构造等应力集中和释放区.油藏含油气层段长,下沟组整体含油,油气富集程度差别大,裂缝是油气主要的渗流通道和重要的储集空间.由于岩石矿物成分复杂、裂缝类型及组合形式多样,故有效裂缝发育段的识别难度较大.该油藏具有井间产能相差悬殊、产量递减速度快、弹性产率差异大、酸化改造效果不一等特点.

RESEARCH OF THE FRACTURE RESERVOIRS IN QINGXI OILFIELD

Qingxi oilfield locates at Qingnan sag that lies in the west of Jiuxi basin.Liugouzhuang fault nose and Kulongshan anticline are the main structures in Qingxi oilfield.and whose reservoir is Xiagou group.Reservoirs of Qingxi oilfield mainly comprise of the argilliferous dolostone in the moderate deep lacustrine facies to deep lacustrine facies and the conglomerate in the fan-delta-front subfacies in lower Cretaceous.The present depth of Xiagou formation is more than 4000m,and it is at the later stage of diagenesis and is tightly compacted to reservoirs with very low permeability.As the argilliferous dolostone with laminated structures and large amount of feldspars and quartz can easily form fracture networks along and cutting through layers,contracting fracture networks caused by the changing between clay minerals,fracture networks caused during the generation of oil and gas,dissolved fracture and cave networks,and structural fracture networks,which act as the network for oil and gas migration and accumulation.According to the liquid-save-compartment theory and the oil and gas accumulation mechanism under abnormally high pressure,the oil and gas accumulation should be densely distributed at the structural transition belts,the outer flanks of arc faults,the intersection of faults and anticline structures where the structural stresses are focused and released.Xiagou formation in Qingxi oilfield wholly contains oil and gas,but the enrichment levels differ greatly within it.Fractures contribute a lot to the flow and accumulation of oil and gas.There are some difficulties in the recognition of sections with effective fractures,which are caused by the complex mineral constituents of the strata and the variety of fractures and their assemblages.Qingxi oilfield is chracterized by great differences in production and elastic recovery ratio between wells,high rate of production decreases,and different result of acid treating.