Fracture characteristics and genetic mechanism of overpressure carbonate reservoirs: Taking the Kenkiyak Oilfield in Kazakhstan as an example

Taking the Kenjiyak Oilfield in Kazakhstan as an example, the fracture characteristics and genetic mechanism of carbonate reservoirs with overpressure were discussed. The study showed that carbonate reservoirs are formed under overpressure developed hydraulic fractures besides tectonic fractures and karstic fractures. The Carboniferous sub-salt hydraulic fractures in Kenkiyak are mainly formed in the Mesozoic and Cenozoic, on a macroscopic level they cut rocks into brecciated texture or are packed by calcite veins, on a microscopic level they are in a radiant distribution around pores. The formation and evolution of the fracture system in the region experienced three main stages, including structure transforming and uplifting, structure stretching and subsiding, and hydraulic cracking. In the stage of hydraulic cracking, salt movement and tectonic movement lead to the formation of overpressure zone, with the pressure coefficient of 1.84. Overpressure leads to the formation of hydraulic fractures in the Carboniferous low permeable carbonate reservoirs under the Permian non-permeable mudstones, and also leads to reopening, widening and prolonging of former tectonic fractures. Hydraulic fractures are mainly the extensional fracture, which has late opening time and low filling degree, thus is the main effective fractures in the study area.

摘要：以哈萨克斯坦肯基亚克油田为例讨论了异常高压碳酸盐岩油藏储集层裂缝特征及形成机制。异常高压条件下形成的碳酸盐岩储集层除发育构造裂缝和岩溶裂缝外，还发育水力破裂缝。盐下石炭系水力破裂缝形成于中、新生代，宏观上表现为将岩体切割成角砾状及充填方解石脉，微观上表现为沿孔隙周围呈放射状分布。该区裂缝系统的形成与演化经历了3个主要阶段，即构造变形与抬升阶段、构造拉张与沉降阶段、水力破裂阶段。在水力破裂阶段，在盐丘活动、构造运动等多种因素作用下，盐下地层中形成异常高压带，压力系数高达1.84，在异常高压条件下，二叠系泥岩不渗透层下的石炭系碳酸盐岩低渗透层发生破裂，形成水力破裂缝，同时使早期闭合的裂缝开启、扩张和延伸。水力破裂缝以张性裂缝为主，其张开时间晚，充填程度低，是研究区主要的有效裂缝。图8参13