Characteristics Of Karst Dolomite Reservoirs In The Middle Permian Maokou Formation In Central Sichuan

In the central region of Sichuan basin,the middle Permian Maokou Formation bears a set of high quality dolomite reservoirs with unstable distribution,which seems to be related to karstification.It is not clear that the main controlling factor of the formation of dolomite reservoir is dolomitization or karstification,and their relationship.So this study is aimed at the characteristics and genesis of dolomite in the Maokou Formation,based on more than 20 cored wells,combining with the predecessors' achievements,outcrop,drilling/well logging,seismic and wafer materials and permeability of the late,rare earth elements(REE)analysis,analysis of carbon and oxygen isotopes,strontium isotope analysis.This study suggests that the dolostone in the Maokou Formation in central Sichuan is mainly composed of fine crystalline dolomite and leopard limy dolomite fine-medium crystalline dolomite,which is the main reservoir.The main storage space types of Maokou Formation include:l)Pores(intergranular pores,intercrystalline pores and kernels);2)Karst cave;3)Fractures(structural joints and corrosion joints).According to the study of reservoir space types,the reservoir type of the Maokou Formation in the study area is dominated by fracture-vug reservoir,which is mainly controlled by karstification,it falls into a type of karst dolomite reservoir.Effects of karstification and dolomitization on reservoirs have been carried out in further investigation in this study.Analysis of abundant cores and sections indicates that not all the dolomites could be reservoir,and only dolomite underwent early alteration by karstification could be good reservoir,because after early karstification the reservoir model has taken shape and later dolomitization just carries conservation effects on the reservoir.At the same time by means of two kinds of typical dolomite reservoir rocks in the study area of rare earth elements,carbon and oxygen isotopes,strontium isotope are studied and compared with original rock of the limestone for dolomite diagenesis and diagenetic fluid are studied:l)it shows the loss of the sigma ?LREE of the tessellated dolomite,and the negative anomaly of Ce/Ce*in the relative enrichment of ?HREE and the distribution of rare earth elements;The semi-self-shape silicon fine crystal dolomite is relatively high,and the ?LREE is relatively enrich.The?HREE shows a loss,and the rare earth is divided into the negative anomaly of Eu/Eu*.2)it is inlaid with fine crystal dolomite and hypidiomorphic the self-shaped silicon fine crystalline dolomite have higher 613C values than the seawater and limestone in the same period.3)the Mosaic fine grain dolostone 87Sr/86Sr values are significantly higher than original rocks(i.e.,limestones)and half hypidiomorphic siliceous fine grain dolostone,and limestone protolith,half the shape-the shape siliceous fine grain dolostone similar 87Sr/86Sr.According to the these geochemical characteristics above,this study suggests that the Maokou Formation limestones have been underwent multiphase dolomitization by mixing hydrothermal solution,including fresh water,sea-sourced and mantle-sourced fluid.Finally,taking the dolomitization and karstification influence on reservoir into consideration,the study has established a genetic model of karstic dolomite reservoir,and pointed to that the early karst systems could be the pathways of later dolomitization which could improve the ability of reservoir.So the type of karst reservoir should be given priority when carrying out prediction of the Maokou Formation in future.