四川盆地南部向斜区二叠系岩溶孔隙型石灰岩储层成因与天然气勘探新领域

Permian karst porous limestone reservoir in the syncline area of southern Sichuan Basin:Origin and new field of natural gas exploration

随着油气勘探程度不断提高,勘探方向逐渐从构造高部位往向斜区拓展。但是,传统认识认为向斜区岩性一般较致密,寻找向斜区优质储层成为其油气获得勘探突破的关键因素之一。为此,以四川盆地南部向斜区中二叠统茅口组石灰岩为例,基于钻井岩心、薄片、测井以及地球化学等资料综合分析,开展了向斜区孔隙型石灰岩储层特征及成因研究,并建立了储层发育模式,指出了天然气勘探新方向。研究结果表明:(1)四川盆地南部向斜区茅口组发育岩溶孔隙型碳酸盐岩储层,储层岩性主要为泥晶—亮晶生屑石灰岩和亮晶生屑石灰岩,储集空间主要为生物体腔孔、生物铸模孔及粒内溶孔等组构选择性溶蚀形成的孔隙;(2)研究区茅口组储层平均孔隙度为4.07%,平均渗透率为0.273 mD,孔隙度与渗透率有明显的正相关关系,为典型的孔隙型储层;(3)茅口组储层段的铝、钪、锆及总稀土元素含量较低,锰和铁含量也较低,碳同位素值位于中二叠世海水沉淀方解石碳同位素值分布范围内,氧同位素呈现出一定的负偏;(4)茅口组岩溶孔隙型储层的形成与沉积期微地貌及高频层序控制的准同生期岩溶有关,溶蚀流体主要为大气淡水。结论认为,岩溶孔隙型储层在盆地内普遍发育,且该类储层远离不整合面,受沉积古地貌、沉积相带控制的准同生期岩溶改造,因此,天然气勘探思路需要向“有利沉积相带规模控储”方向转变,向斜区将是盆地内未来天然气勘探的重要新领域。

With the continuous advancement of oil and gas exploration,the exploration direction gradually expands from structural highs to syncline areas.According to the traditional understanding,the reservoirs in syncline areas are generally tight,so how to find high-quality reservoirs is one of the key factors to achieve breakthroughs in oil and gas exploration in syncline areas.Taking the limestone of Middle Permian Maokou Formation in the syncline area of southern Sichuan Basin as an example,this paper comprehensively analyzes the drilling cores,thin sections,well logging and geochemical data.Based on this,the characteristics and genesis of porous limestone reservoirs in syncline areas are researched.In addition,a reservoir development model is established,and the new direction of natural gas exploration is pointed out.And the following research results are obtained.First,the karst porous carbonate reservoir developed in the Maokou Formation in the syncline area of southern Sichuan Basin is lithologically dominated by micritic-sparry bioclastic limestone and sparry bioclastic limestone,and its reservoir space is mainly acted by the pores formed through fabric selective dissolution,such as biological coelomopores,biological moldic pores and intragranular dissolved pores.Second,the average porosity of the Maokou Formation reservoir in the study area is 4.07%,and the average permeability is 0.273 mD.There is a significant positive correlation between porosity and permeability,so it is a typical porous reservoir.Third,the Maokou Formation reservoir is in a low content of Al,Sc,Zr and total rare earth(ΣREE) as well as the content of Mn and Fe.Its carbon isotope values are within the distribution range of those of calcite precipitated in the Middle Permian seawater,and the oxygen isotope shows a certain negative bias.Fourth,the formation of the karst porous reservoir in the Maokou Formation is related to the penecontemporaneous karst,which is controlled by the sedimentary microgeomorphology and the highfrequency sequence,and the dissolution fluid is mainly meteoric water.In conclusion,karst porous reservoirs are generally developed in the Sichuan Basin,and this kind of reservoir is far away from unconformity and modified by penecontemporaneous karstification controlled by sedimentary palaeogeomorphology and facies belt.Therefore,the exploration target should be changed to "favorable sedimentary facies belts controlling reservoirs in a large scale",and syncline area will be an important new field for future natural gas exploration in the Sichuan Basin.