川中地区下二叠统栖霞组白云岩储层特征和成因

Characteristics and genesis of dolomite reservoir of the Lower Permian Qixia Formation in central Sichuan Basin

栖霞组白云岩储层是四川盆地中部地区重要的勘探领域之一。基于岩石学、成像测井、地球化学及U-Pb同位素年龄分析,明确了川中地区栖霞组白云岩储层特征及成因。研究认为:①白云岩储层主要发育于栖霞组高位体系域,单层厚度薄、层数多,并与石灰岩交替叠置发育,整体构成多个旋回;横向上,白云岩分布在一定区域内具有较好的连续性。②白云岩储层的岩石类型主要包括粉细晶白云岩和中-粗晶白云岩;储集空间以晶间(溶)孔、溶蚀孔洞为主;储层孔隙度平均值为4.07%,渗透率平均值为3.12×10^(-3)μm^(2),呈现中等孔隙度、中高渗透率、孔渗弱相关的特点。③白云岩储层的成因以准同生白云石化作用为主,茅口末期受热液轻微改造;微古地貌高部位颗粒滩是白云岩储层发育的物质基础,高频层序界面控制了早期孔隙的形成,早期白云石化作用有利于早期孔隙的保存。基于上述认识预测了川中合川三维地震工区的储层分布,预测结果与已钻井吻合。

The dolomite reservoir of Qixia Formation is one of the important exploration fields in central Sichuan Basin.Based on the analysis of petrology,imaging logging,geochemistry and U-Pb isotopic age,the characteristics and genesis of dolomite reservoir of Qixia Formation in central Sichuan Basin have been clarified.It is believed that:(1)Dolomite reservoir is mainly developed in the highstand system tract of Qixia Formation,with thin single layer thickness and many layers,and alternately superimposed with limestone,forming multiple cycles as a whole.Horizontally,dolomite is distributed in a certain area with good continuity.(2)The rock types of dolomite reservoir mainly include very fine-fine crystalline dolomite and medium-coarse crystalline dolomite;the reservoir space is dominated by intercrystalline(dissolved)pores and dissolved vugs;the average porosity is 4.07%,and the average permeability is 3.12×10^(-3)μm^(2),showing the characteristics of medium porosity,medium-high permeability,and weak correlation between porosity and permeability.(3)The formation of dolomite reservoirs is dominated by quasi-contemporaneous dolomitization,and slightly reformed by hydrothermal fluid at the end of Maokou.The granular shoal in the high part of the micro paleogeomorphology is the material basis for the development of dolomite reservoirs.The high-frequency sequence interface controls the formation of early pores,and early dolomitization is conducive to the preservation of these pores.Based on the above knowledge,the reservoir distribution in Hechuan 3D seismic survey in central Sichuan Basin was predicted,and the prediction results were consistent with the drilled wells.