伊拉克中部上白垩统Khasib组岩溶储层演化及异常高渗层成因

The karst reservoir evolution and genesis of abnormal high permeability zone of the Upper Cretaceous Khasib Formation in central Iraq

针对伊拉克艾哈代布（Ahdeb）油田Khasib组早期的注水突破难题,本文以地球化学资料及取心薄片分析为基础,结合区域沉积—构造演化背景,对储层演化及异常高渗层的成因进行了研究,并将储层演化划分为3个阶段：沉积同生期、低沉降速率浅-中埋藏期和快速沉降深埋藏期.沉积同生期具两层高孔段：即受同生岩溶改造形成的以砂屑粒间孔为主的Kh2-2-1砂屑颗粒灰岩和以藻屑铸模孔为主的Kh2-3-2藻屑泥粒灰岩.在此基础上,浅-中埋藏期产生以走滑断裂为流体的运移通道,以深部源岩降解形成的有机酸为溶蚀介质,以同生期后形成的高孔层段为溶蚀的载体,以非组构选择溶蚀作用为特征的埋藏有机酸岩溶,它与同生期岩溶作用在时间上具有连续性,形成的孔隙分布在空间上具有继承性.晚期地层快速沉降深埋藏期,发生大规模烃类充注成藏,阻止孔隙内水岩反应,减弱储层内压实胶结破坏性成岩作用,最终使得这两段具异常高渗透率特征.Kh2-2-1砂屑颗粒灰岩段以砂屑粒间孔、粒间溶孔、针状溶洞为主要孔隙类型,以孔隙缩小型为主要喉道类型,孔喉连通性好,岩相区域分布稳定,是造成早期注水突破的层段.总体而言,本区异常高渗层的成因主要是：优质储层的沉积基础及同生岩溶改造,长期浅-中埋藏期有机酸溶蚀对储层的叠加和改造优化,以及快速深埋藏期与烃类充注极好的耦合关系使其得以保存。

Aiming at the early waterflood breakthrough which caused by the ＂ abnormal high permeability zone＂ in the development of the Kh2 reservoir of Khasib Formation of AHDEB oilfield in Iraq, we carried out the research on the Khasib reservoir evolution and genesis of the abnormal high permeability zone by using drill core, imaging logging and geochemistry data. The results indicate that the karst reservoir of Kh2 forming is controlled by the depositional karst and multiple phase deep buried karst. The reservoir evolution can be divided into three stages： Syndiagenetic stage, shallow-medium buried of organic acid dissolution and alteration stage, and rapid settlement of deep burial period of hydrocarbon filling stage. The depositional karst featured as the fabric selective dissolution of vertical infiltration, which forming the two high porosity zones which are the Kh2-1-2 of the intraclast intergranular pore bearing zone by the primary high connectivity pore of the intraclast grainstone, and the Kh2-3-2 of the algal moldic pore bearing zone by well solubility of algal debris. During the diagenesis period, the reservoir is under through the buried karst that is inherited karstification in non-fabric selective dissolution way based on the post-depositional pore of the reservoir, and featured as the strike- slip fault（connected the source rock and reservoir）as the fluid migration path, the acidizing fluid by from organic substance degradation as the dissolution media, the post-depositional high porosity zone as the dissolution carrier（the Kh2-1-2 and Kh2-3-2 zone）. Owe to the different pore character of vertical zones in reservoir have the different karstification characteristic, and the openness of strike-slip faults controlled by the tectonic activity, after the depositional karst and at least two stages deep buried karst, finally formed the two ＂abnormal high permeability zones＂ that are intraclast grainstone zone of Kh2-1-2 show as the weak compaction and cementation, main pore type is intergranular and intergranular dissolved pore with excellent connectivity, pinhole vug, and moderate-poro high-perm property, and the algal debris packstone zone of Kh2-3-2 show as the weak cementation, high plane porosity, coexistence of different pore types including the moderate-micro vug, dissolution fracture, and micro matrix micro-pore with high heterogeneity and high-poro moderate-perm property. The intraclast grainstone zone is fluid flow passage to cause the early water breakthough in water-flooding. In summary, high connectivity origin intraclast intergranular pore, the pore integrity was conserved during the depositional karst period, long-term shallow buried diagenetic environment（90-20 Ma）, favorable potential area for fluid migration （the top end of the fault） , good coupling relationship of the hydrocarbon fluid filling and fast formation burying period, all of these, which make the Kh2-1-2 zone to forming the abnormal permeability zone.