川中地区中上寒武统洗象池组颗粒滩储集层特征及主控因素

Characteristics and main controlling factors of shoal reservoirs in the Middle-Upper Cambrian Xixiangchi Formation,central Sichuan Basin

川中地区中上寒武统洗象池组油气勘探一直未取得大规模突破,仅在威远—资阳地区获得了少量产能,研究其储集层形成过程和主控因素可以为勘探提供更可靠的地质依据。本研究基于川中地区的野外露头、岩心、薄片和钻测井等地质资料,对川中地区洗象池组储集层特征进行了详细的研究,在此基础上分析了不同类型颗粒滩沉积组合、准同生溶蚀作用和表生岩溶作用对储集层的影响。结果表明:(1)研究区洗象池组储集岩主要为颗粒白云岩,根据原始结构保存程度,可将颗粒白云岩划分为残余颗粒白云岩和粉晶—细晶白云岩,储集空间以粒间溶孔、晶间溶孔及小型溶洞为主,整体表现为低孔低渗的特征;(2)研究区洗象池组发育潟湖—颗粒滩(—台坪)、颗粒滩—台坪(混积潮坪)、多期颗粒滩垂向叠置3种颗粒滩沉积组合类型,其中多期颗粒滩垂向叠置可形成厚度较大的颗粒岩段,其储集层物性最好;(3)准同生溶蚀作用是川中洗象池组储集层形成的关键,加里东期表生溶蚀作用改善了储集层质量。

Within the Middle and Upper Cambrian Xixiangchi Formation,large-scale breakthroughs of the oil and gas exploration never came true and only a small amount of productivity were obtained in the Weiyuan-Ziyang area.Hence,investigating the formation and associated controlling factors of those reser-voirs can provide geological basis for the future hydrocargon exploration.Based on outcrops,cores,thin sections,as well as drilling and logging data of the central Sichuan Basin,this study made a detailed de-scription for the characteristics of the Xixiangchi reservoir and then analyzed the influences of the shoal depositional combinations,eogenetic karstification and the telogenetic karstification on the reservoir.The results show that:(1)The Xixiangchi reservoir in the study area is mainly granular dolostone.Based on whether the inner texture of grains can be preserved,the granular dolomite is divided into residual clastic texture dolomite and powder-fine crystal dolomite,whose reservoir spaces come from intergranular dis-solved pores,intercrystalline dissolved pores and vugs,and thus,those reservoirs are characterized by low porosity and low permeability as a whole;(2)There are three types of shoal depositional combina-tions in the Xixiangchi Formation,which are lagoon-shoal(-tidal flat),shoal-tidal flat or mixed tidal flat,and multi-stage shoal vertical overlapping,respectively.The multi-stage shoal vertical overlapping can form a stratal succession with the most grainstone,whose reservoir properties are the best;(3)The eogenetic karstification is crucial for the formation of the reservoir and the telogenetic karstification could also improve the reservoir quality.