濮城油田南区沙二上4-7砂层组低渗储层特征及成因分析

Characteristics and genesis of low-permeability reservoir,4-7 sand sets of the upper 2nd member of Shahejie group in south of Pucheng Oilfield

利用岩心观察描述、薄片、铸体薄片、扫描电镜、岩心分析化验等资料,对濮城油田南区沙二上4-7砂层组沉积特征及储层低渗透成因进行了研究。结果表明,该区沙二上4-7砂层组为小型闭塞陆相断陷湖盆三角洲前缘沉积,其岩石成分和结构成熟度低,塑性岩屑和杂基含量高,属于低渗透储层,目前处于晚成岩A1亚期。沉积因素对低渗透储层的形成起决定性作用,它不仅直接导致储层低孔、低渗,极易被压实,而且储层单层厚度小,粒度细,胶结物含量高,原生孔隙不发育,使得地下酸性流体难以进入,影响了溶蚀作用的进程。成岩作用是储层低渗透成因的最主要的影响因素,中-强的压实和胶结作用使原生孔隙空间不断缩小,孔渗性降低,岩石致密;而溶蚀作用不充分,对储层没能发挥明显的改造作用,也是低渗储层形成的一个重要原因。

The strata of 4-7 sand sets of upper 2nd member of delta frontal facies of limited terrigenous faulted depression lake Shahejie group in south of Pucheng Oilfield are deposited in Reservoir characteristics and its genesis of low-permeability were studied by means of core observation and description, sections, cast sections, scanning electron microscope（SEM） and core lab analysis data. The results indicate the reservoir is characterized by low-permeability, low compositional and structural maturity, abundant plastic lithic fragment and matrix. The diagenesis may be ascribed to the A1 sub-stage of late diagenetic stage. Sedimentation has deciding effect on the forming of the low-permeability reservoir. It results in forming low- porosity and low-permeability reservoir directly, and makes the reservoir compacted easily. Thin sand bed, small grain size, abundant cements and under-development primary pore are factors for making acid fluid penetrate into the reservoir difficultly. So the dissolution in the reservoir is slow and weak. Diagenesis is the key factor for genesis of the low-permeability reservoir. Meaning intensive compaction and cementation make the primary pore and pore-throat narrower, the porosity and permeability lower, and the sandstone tighter. The dissolution is not so thorough as not to improve the porosity and permeability of the reservoir obviously. So it is also an important factor for forming low permeability reservoir.