碳酸盐岩储集层隔夹层地质特征及成因——以伊拉克西古尔纳油田白垩系Mishrif组为例

Geologic features and genesis of the barriers and intercalations in carbonates: A case study of the Cretaceous Mishrif Formation, West Qurna oil field, Iraq

通过岩心、薄片、测井等资料综合分析,研究伊拉克西古尔纳油田白垩系Mishrif组碳酸盐岩内隔夹层类型、孔渗关系、测井响应特征及识别标准,并从层序地层、沉积相及成岩作用的角度分析隔夹层的成因和分布特点。研究区发育颗粒灰岩、泥粒灰岩及粒泥灰岩3种隔夹层。隔夹层一般发育在局限台地相和蒸发台地相,部分发育在开阔台地相;在海侵体系域及早期高位体系域,形成广泛发育的隔夹层,且在层序边界处形成大规模、连续分布、物性较差的隔挡层。准同生期胶结作用、埋藏压实作用、埋藏期胶结作用等成岩作用造成孔隙度不断减小,从而破坏了储集空间,导致层内夹层的形成。表生期古潜水面以下的潜流环境由于CO2的脱气作用致使Ca CO3大量沉淀出来并形成方解石胶结物,形成了区域内广泛分布的泥粒、颗粒灰岩隔夹层。通过测井综合分析确定了隔夹层测井识别标准并预测了隔夹层的展布特征,隔层主要发育在CRI段、CRII段内,夹层主要集中在m B1段内。就不同类型隔夹层而言,泥粒灰岩隔夹层数量比例最大,其次为颗粒灰岩隔夹层,最后为粒泥灰岩隔夹层。

The type, poroperm relationship, log response characteristics and identification criteria of the barriers and intercalations in the carbonates of the Cretaceous Mishrif Formation in the West Quma oil field of Iraq are studied through comprehensive analysis of cores, thin sections and well logs. The genesis and distribution of the barriers and intercalations are analyzed from the perspectives of sequence stratigraphy, depositional facies and diagenesis. The barriers and intercalations can be classified into three types： grainstone, packstone and wackestone. The barriers and intercalations generally exist in restricted platform facies and evaporative platform facies, some in open platform. They are common in transgressive cycles and early regressive cycles, forming continuously, extensive barriers and intercalations near the sequence boundaries. Penecontemporaneous cementation, burial compaction, and burial cementation led to the decrease of the porosity, damaged the space of the reservoir pores and became the important factors for the genesis of the barriers and intercalations. In the epidiagenetic phase, a large number of CaCO3 precipitated in the phreatic water zone below free-water table, leading to the formation of packstone and grainstone barriers and intercalations in a large scale. Through comprehensive log analysis, log identification criteria and the distribution of the barriers and intercalations are determined. The barriers are mainly distributed in the sections of CRI and CRII, and the intercalations are concentrated in the section of nab 1. For different types of barriers and intercalations, packstones are the most in quantity, followed by grainstones and wackestones.