Characteristics And Identification Of Diagenetic Facies Of Low Permeability And Ultra-low Permeability Dolostone Reservoirs

The natural gas resources of the Ordovician Majiagou Formation are abundant in the Lower-Paleozoic of Ordos Basin,and low permeability and ultra-low permeability dolomite reservoirs are important reservoir types of natural gas in Majiagou Formation,while diagenetic facies controls the distribution of low-permeable and ultra-low-permeable reservoirs to some extent.It is of great significance to study the diagenetic facies characteristics and its identification of low-permeable and ultra-low-permeable dolomite reservoirs.There are typical low permeability and ultra-low permeability dolomite reservoirs developed in the Ma55?Ma51 sub-members of the Majiagou Formation.This paper take the low permeability and ultra-low permeability dolomite reservoirs of the Ma5₅ to Ma5₁sub-members of Majiagou Formation in the central eastern Ordos Basin as an object,the main reservoir rock types,reservoir space types,reservoir microporous structural characteristics,diagenetic environment and diagenesis were analyzed,and the diagenetic facies types and the markers of petrographic identification in the study area were further clarified by means of observation of field outcrop and drilling core,conventional and cast thin section identification,cathodoluminescence and scanning electron microscope analysis,X diffraction experiment,mercury pressure experiment combined with fractal theory.Geochemical identifying characteristics of different types of diagenetic facies were studied by test of isotopes,major and trace elements as well as rare earth elements.Quantitatively logging identification techniques of different diagenetic facies were studied by logging data with Forward software programming method.Based on diagenetic facies analysis of single well,the spatial and temporal distribution features of each diagenetic facies were clarified by using logging technique,and the controlling function on reservoir distribution of diagenetic facies was analyzed by the way of overlap between the diagenetic facies and reservoir thickness distribution map.The result shows that the main diagenetic facies types of dolomite reservoirs in the study area include early atmospheric freshwater dissolution,epiphytic atmospheric freshwater dissolution,shallow buried active reflux seepage dolomitization and shallow buried latent reflux seepage dolomitization diagenetic subfacies.The petrographic identification markers of the four types are micrite-powder crystalline dolomite with gypsum and salt dissolving,moldic pore and geopetal pore and geopetal structure;the karst breccia micrite-powder crystalline dolomite,inter breccia pores,denudation pores and karst caves,and breccia structure;the powder-fine crystalline dolomite with residual psammitic structure,dolomite intergranular pores,residual and clear brim and cloudy centers structures,and the leopard fine limy dolomite,bioboring and intercrystal pores,leopard and bioboring textures,respectively.Test methods such as?¹⁸O and?¹³C,⁸⁷Sr/⁸⁶Sr,major and trace elements such as Fe?Mn?Al?Ti?Sc and Cs,and rare earth elements including?REE??LREE??HREE??LREE/?HREE??Ce and?Eu,and distribution patterns of REE are effective geochemical pathways to identify different diagenetic facies.Techniques of cross plot of DEN-Pe,RLLD-Pe?which can be replaced by RLLD-AC when the absence of Pe and DEN occurs?is an effective logging approach for quantitatively identifying different diagenetic facies.The Ma5₅ sub-member of the study area mainly develope shallow buried active reflux seepage dolomitization diagenetic subfacies,which is widely distributed in the north,middle and southeast of the study area,followed by shallow buried latent reflux seepage dolomitization subfacies,which mainly distributed in the southwest of the region.The Ma5₄ and Ma5₃sub-members are mainly developed epigenetic atmospheric freshwater dissolution diagenetic subfacies,but the former is only distributed in the southwest,while the latter is expanded to the north,southeast and southwest.The main type of diagenetic facies of the Ma5₂sub-member is early atmospheric freshwater dissolution diagenetic subfacies.The development scale of the two atmospheric freshwater dissolution diagenetic subfacies in the Ma5₁ sub-member is similar,and the epiphytic atmospheric freshwater dissolution diagenetic subfacies distribute isolately in the northern study area,but the southwest,south and southeast area have better connectivity of distribution.The most favorable diagenetic facies for the development of high quality reservoir is the early atmospheric freshwater dissolution diagenetic subfacies,followed by the shallow buried active reflux seepage dolomitization diagenetic subfacies.