山前冲断带砂砾岩储层发育机制及控制因素:以柴达木盆地昆北油田切12区为例

Mechanism and Controlling Factors of Sandy Conglomerate Reservoir in Piedmont Thrust Belt: A Case of Kunbei Oilfield, Qaidam Basin, NW China

柴达木盆地昆北断阶带为大型山前压扭冲断构造带,该构造带上的昆北油田切12区下干柴沟组下段发育厚层冲积扇沉积的砂砾岩储层.明确储层微观特征及发育控制因素,对冲断带砂砾岩储层评价与油藏治理具有重要意义.运用铸体薄片、X-衍射、扫描电镜、常规压汞与恒速压汞等测试分析资料,系统研究冲积扇砂砾岩储层微观特征,明确储层发育的主控因素.结果表明,切12区砂砾岩储层具有成分成熟度低、结构成熟度低的岩石学特征,储集空间类型具双重孔隙介质特征,发育2类储层结构模态、4类储集空间类型、4类储层孔隙结构,储层物性为低孔、特低渗型储层,成岩作用阶段为早成岩阶段B期.在砂岩动力成岩作用相同的地质背景下,成岩作用对储层改造的影响是相对均衡的,储层发育程度的差异主要受控于沉积作用差异而导致的碎屑组成与结构差异,泥质含量是储层发育程度差异的主控因素,明确泥质含量上限为8%,颗粒分选对储层性质起重要控制作用.建立了基于泥质含量、物性、孔隙结构和产能指标的储层评价分类标准,将砂砾岩储层划分为4类;运用该认识指导油藏综合治理,并取得应用实效.

The Kunbei thrust belt of Qaidam Basin is a large piedmont compression-torsion thrust belt.On the belt,the thick sandy conglomerate reservoir formed in alluvial fan is developed in the lower member of the lower Ganchaigou Formation in Qie12 block of Kunbei Oilfield.Understanding of the microcosmic characteristics and the main development controlling factors of this sandy conglomerate reservoir is significant to reservoir evaluation and the comprehensive reservoir management.In this study,the microcosmic characteristics of alluvial fan sandy conglomerate reservoir were systematically studied by using casting thin sections,XRD,SEM,conventional mercury injection and constant velocity mercury injection,and the main controlling factors of reservoir development were identified.The results show that the sandy conglomerate reservoirs in the area have the petrological characteristics of low compositional maturity and low structural maturity.The reservoir space type is characterized by dual porosity medium.There are 2 types of reservoir structure modes,4 types of reservoir space types and 4 types of reservoir pore structures.The reservoir is of low porosity,ultra-low permeability and early diagenesis stage B.Under the same geological background of reservoir dynamic diagenesis,the influence of diagenesis on reservoir reconstruction is relatively balanced.The difference of reservoir development degree is mainly controlled by the difference of clastic composition and structure caused by the difference of sedimentation.The shale content is the main controlling factor of the reservoir development degree,and the upper limit of the shale content for reservoir is 8%,while the particle sorting plays an important role in controlling the reservoir properties.The reservoir classification standard based on shale content,physical property,pore structure and productivity index was established,and the sandy conglomerate reservoir was divided into four types.The research findings have guided the comprehensive reservoir management and achieved practical application results.