中东伊拉克A油田Khasib组海相花斑状非均质灰岩储层特征及成因机制

Characteristics and genetic mechanism of marine nodular limestone reservoirs of Khasib Formation in Iraq A Oilfield,Middle East

中东海相花斑状非均质灰岩储层普遍发育,但其形成机制仍不清楚。以伊拉克A油田上白垩统Khasib组为例,基于岩心、薄片等基础资料,开展花斑状灰岩储层特征、沉积成岩作用及成因机制研究。结果表明:①Khasib组发育绿藻生物碎屑灰岩、泥晶生物碎屑灰岩、生物碎屑砂屑灰岩等多种岩性。Khasib组发育多段花斑状非均质灰岩储层,花斑状灰岩发育灰白色、深灰色两类斑块:灰白色斑块多呈结核状团块,致密不含油;结核状团块间为深灰色斑块,储层物性好、含油性好。②花斑状灰岩灰白色斑块中生物碎屑颗粒间孔隙被亮晶方解石几乎完全胶结,深灰色斑块中亮晶方解石胶结作用弱、颗粒间孔隙发育。③沉积作用为花斑状灰岩的形成提供物质基础,沉积间断时期早成岩阶段“硬底化”作用是花斑状灰岩储层形成的主控因素;晚成岩作用是对早成岩作用的继承和深化,进一步强化了储层非均质性。④“硬底化”程度越高,储层物性改善越明显。Khasib组高渗层属于成熟硬底,大气淡水的溶蚀作用导致孔隙结构进一步改善,晚成岩阶段硬底的抗压实作用使成熟硬底中的高渗通道得以保存。这项研究对提高储层表征精度、制定合理的开发(调整)技术对策具有重大意义。

Marine nodular limestone reservoirs are generally developed in the Middle East,some oilfields such as Halfaya,Ahdeb,Azadegan have reserves of more than 4 billion tons and the capacity to produce more than 400 million tons.Strengthening the study on the genesis of nodular limestone reservoirs is of great significance t7o improve the accuracy of characterization.In this paper,the research object is the reservoir of the Lower Cretaceous Khasib Formation in Iraq A Oilfield.Based on core,thin section,logging and other basic data,this paper studies on reservoir characteristics and sedimentary diagenesis,and clarifies the genetic mechanism of nodular limestone and its control on reservoir.The results show that:(1)Nodular limestone reservoirs are developed in various carbonate rocks such as green algal bioclastic limestone,micritic bioclastic limestone and bioclastic sandy limestone.Most cores of Khasib limestone reservoirs are characterized by nodule,which can be divided into white and dark patches according to the color.The white patch reservoir is dense and oil-free,the dark patch reservoir has good physical properties and is oil-bearing.(2)The intergranular pores of bioclastic in white patches are almost completely cemented by bright calcite,while the cementation of bright calcite in dark patches is weak and intergranular pores are developed.(3)In the sedimentary discontinuous period,the form of hardground of early diagenesis is mainly divided into four stages:biological disturbance,nodule growth,nodule fusion,and exposure dissolution.It is the main controlling factor for the formation of nodular limestone reservoirs.The late diagenesis inherits and further strengthens the reservoir heterogeneity formed by early diagenesis.(4)The more mature the hardground is,the more obvious the improvement of reservoir physical properties is.Among them,the high permeability layer developed in Khasib Formation belongs to the mature hardground.After the dissolution by atmospheric water,the pore structure is further improved.The anti-compaction effect of the hardground in the late diagenetic stage preserves the high permeability channel in the mature hardground.This study clarifies the reservoir characteristics and genetic mechanism of nodular limestone reservoirs,which is of great significance to improve the characterization accuracy of nodular limestone reservoirs and formulate reasonable development(adjustment)technology policy.