甘泉油田长8油藏水下分流河道砂体钙质夹层特征及成因

Characteristics and Genesis of Calcareous Interlayers in Underwater Distributary Channel Sandbodies in Chang 8 Reservoir,Ganquan Oilfield

钙质夹层研究对油田开发初期井网布置、后期增储上产等具有重要意义。通过岩心观察、铸体薄片观察、扫描电镜观察、测井资料分析等手段,对甘泉油田延长组长8段钙质夹层岩性、电性、分布成因及其对储集层非均质性的影响开展了系统研究。结果表明,钙质夹层发育受水下分流河道微相控制,主要发育在砂体顶部和底部,具有稳定性较高、厚度较大等特征,此外,具有低自然伽马、低声波时差、高电阻率的测井响应特征。钙质夹层发育主要受控于2期方解石胶结,第一期与同生成岩期方解石的过饱和沉淀有关,此时期,壳类生物残体的溶解、河流携带的Ca^(2+)及CO_(2)^(3-)、大气中CO_(2)的溶解等是方解石胶结的物质来源;第二期方解石胶结为中成岩阶段B期的孔隙式胶结,地层中残留流体的Ca^(2+)及CO_(2)^(3-)、不稳定组分溶蚀、黏土矿物转化等为钙质胶结提供了Ca^(2+)及CO_(2)^(3-)。钙质夹层的发育增强了储集层砂体的层内非均质性,导致砂体垂向连通性变差和油水关系复杂。在油藏注水开发过程中,钙质夹层与相邻非渗透性隔夹层斜交形成有效隔挡,形成局部剩余油富集区。

The study on calcareous interlayers is of great significance to well pattern deployment in the early stage of oilfield development and to the increase of reserves and production in the late stage.By means of core observation,well logging data analysis,casting thin sec⁃tion and SEM observation,the lithology,electrical properties,distribution,and genesis of calcareous interlayers in the eighth member of Yanchang formation(Chang 8 member)in the Ganquan oilfield and the influence of such calcareous interlayers on reservoir heterogeneity were systematically studied.The results show that the calcareous interlayers were developed under the control of underwater distributary channel microfacies,mainly at the top and bottom of sand bodies.The calcareous interlayers exhibit the geological characteristics of high stability and large thickness,and the logging response characteristics of low gamma,low acoustic slowness and high resistivity.The devel⁃opment of the calcareous interlayers was mainly controlled by calcite cementation of two periods.The calcite cementation of the first period was related to the supersaturated precipitation of calcite in the syndiagenetic stage,during which the dissolution of crustal biological resi⁃dues,Ca^(2+)and CO_(2)^(3-)carried by rivers,and the dissolution of CO_(2)from the atmosphere provided materials for calcite cementation.The cal⁃cite cementation of the second period was the porous cementation in the Phase B of the middle diagenetic stage,and the residual Ca^(2+)and CO23-in the original formation fluids,the dissolution of unstable components,and Ca^(2+)and CO_(2)^(3-)precipitated from mineral transformation provided materials for calcareous cementation.The development of calcareous interlayers enhanced the intralayer heterogeneity of sandbod⁃ies,leading to poor vertical sandbody connectivity and complex oil⁃water relationships.During the water flooding of the reservoir,calcare⁃ous interlayers obliquely intersect with the adjacent non⁃permeable interlayers,forming effective barriers and remaining⁃oil enriching areas locally.