含火山灰的致密砂岩储层成岩相及其分布——以鄂尔多斯盆地定北地区上古生界为例

Diagenetic facies and distribution of tight sandstone reservoirs containing volcanic ash:Case study of the Upper Paleozoic in Dingbei area,Ordos Basin

鄂尔多斯盆地定北地区上古生界致密砂岩具备一定的油气勘探潜力,但纵横向上成岩差异较大,且富含火山灰,成岩相及其展布较常规碎屑岩更为复杂。综合运用普通薄片、铸体薄片、扫描电镜、X射线衍射及测井响应等资料,对定北地区上古生界太2段、盒1段含火山灰的致密砂岩储层的成岩作用、成岩演化过程进行研究,划分成岩相类型并厘清成岩相平面展布规律。研究结果表明:①研究区储层经历了火山灰蚀变、压实—压溶、胶结、交代、自生矿物充填等对储层不利的成岩作用及溶蚀、破裂等对储层有利的成岩作用。不同火山灰经过成岩作用演化形成不同成岩相类型,进一步影响储层储集性能,其中研究区中—基性火山灰易形成溶蚀相,储集性能较好。②综合成岩环境、矿物成分、成岩作用等多因素,依据“成岩环境—岩性—成岩作用”复合因素的成岩相划分标准,划分出5种成岩相:大气淡水塑性颗粒砂岩致密压实相、酸性环境可溶颗粒砂岩溶蚀压实相、酸性环境可溶颗粒砂岩溶蚀蚀变相、酸性环境可溶颗粒砂岩溶蚀相及碱性环境碳酸盐矿物胶结交代相,其中酸性环境可溶颗粒砂岩溶蚀相为有利成岩相。③依据各成岩相测井判别特征确定了研究区成岩相分布特征,太2段和盒1段各小层优质成岩相的分布纵向上受河道砂体厚度影响,平面上受沉积微相展布的控制。

The Upper Paleozoic tight sandstone in the Dingbei area has certain oil and gas exploration potential,but its diagenetic facies and distribution are more complex than conventional clastic rocks.The diagenesis and diagenetic evolution of the tight sandstone reservoirs containing volcanic ash in Tai 2 Member and He 1 Member in the north of Dingbei area are studied by using the data of ordinary thin slice,cast thin slice,scanning electron microscope,X-ray diffraction and logging response.The results show that:(1)The tight sandstone reservoir in the study area has undergone diagenesis such as volcanic ash alteration,compaction-pressure dissolution,cementation,metasomatism and autogenetic mineral filling which are unfavorable to the reservoir and diagenesis such as dissolution and fracture which are favorable to the reservoir.Different volcanic ash forms different diagenetic facies through diagenetic evolution,which further affects the reservoir performance.In the study area,the medium-basic volcanic ash can easily lead to the formation of dissolution phase,and the reservoir performance is good.(2)Based on the comprehensive factors such as diagenetic environment,mineral composition and diagenesis,five types of diagenetic facies are divided according to the diagenetic facies division standard of“diagenetic environment-lithology-diagenesis”:The compact diagenetic facies of atmospheric freshwater plastic granular sandstone,the dissolution compaction diagenetic facies of soluble granular sandstone in acidic environment,the dissolution diagenetic facies of soluble granular sandstone in acidic environment into rock facies,the dissolution diagenetic facies of soluble granular sandstone in acidic environment and the cemented metasomatic diagenetic facies of carbonate minerals in alkaline environment,among which the dissolution diagenetic facies of soluble granular sandstone in acidic environment is the most favorable.(3)The distribution characteristics of diagenetic facies in the study area are determined according to the logging discrimination characteristics of each diagenetic facies.The distribution of high-quality diagenetic facies in the small layers of Tai 2 Member and He 1 Member is influenced by the thickness of channel sand body in the longitudinal direction and the distribution of sedimentary microfacies in the plane.