Research Of Sedimentation And Reservoir Characteristics On The Cambrian Loushanguan Group In South Sichuan-North Guizhou

Preliminary studies indicate that there are multiple sets of source rocks in and under the Cambrian in south Sichuan.The study area is close to the southern section of Mianyang-Changning ancient rift trough developed from Sinian to Early Cambrian,and the deep and large faults can provide effective transportation channels for oil and gas migrations.The Loushanguan Group is at the top of the Cambrian,deposited huge thick dolostone,developed multiple sections of reservoirs,and has superior petroleum accumulations conditions.Therefore,the Cambrian Loushanguan Group in South Sichuan-North Guizhou is expected to become the focus of exploration in the future and has certain research value.In this paper,based on the basic data of outcrops,thin sections of rocks,analysis of samples,well logging data,under the guidance of carbonate sedimentology,with the research methods of combining field and indoor,macro and micro,underground and outcrop,the formation distribution,sedimentary facies,reservoirs characteristics,diagenesis and reservoirs control of Cambrian Loushanguan group in South SichuanNorth Guizhou were discussed.The stratum thickness of the Loushanguan Group in the study area shows a general trend of increase from northwest to southeast,with an average stratum thickness of about 600 meters.Affected by sedimentary paleostructures and the uplifting and erosion,the formation thickness in the area of Weixin-Jinsha-Zunyi is 800-1000 meters,and in other areas the formation thickness mainly varies between 300-600 meters.A brief and rapid transgression occurred in the early deposition of the Loushanguan Group,resulting in a significant positive drift of the 13 C,and then began to slowly recede with secondary sea level high-frequency oscillations.The Loushanguan period was mainly a warm or hot subtropical climate,and the temperature variation was not large.The diagenetic fluid was high salinity seawater and partly salted seawater.The hydrodynamic conditions varied frequently,and the environment was low-energy and reductive overall.The longitudinal lithology,layer thickness,and color of the Loushanguan Group are variable.According to the field sections observation and thin sections identification,the Loushanguan Group in the study area can be divided into four types of lithofacies based the comprehensive study,and can be divided into two sections from bottom to top.It shows that the Loushanguan Group were mainly deposited in shallow-sea limited platform,which can be further subdivided into 3 subfacies and 13 microfacies.The reservoirs of Loushanguan Group are mainly developed in granular dolostone and crystalline dolostone.The secondary pores such as intergranular dissolution pores,intragranular dissolution pores and intercrystalline dissolution pores are the main reservoirs space types,and the matrix porosity distributes between 1%-4.74%,with an average porosity of 3.75%,and the matrix permeability mainly varies between 0.0046 m D-15.3m D,with an average permeability of 1.381 m D,which show a characteristics of low porosity-low permeability.The pore structure of the reservoirs is dominated by mesopores and fine throats.The Loushanguan Group mainly developed type III reservoirs and relatively few type I and type II reservoirs.Most of the reservoirs are pore type,with a small number of fracture-pore type.The dolostones of Loushanguan Group mainly had 8 types of diagenesis,such as dolomization,mud crystallization,cementation,dissolution,filling,compaction and dissolution,siliceous metasomatism and tectonic rupture.Among them,dolomitization,dissolution and structural rupture were the main constructive diagenesis for reservoirs developments.The control factors of reservoirs developments in Loushanguan Group are relatively complex.Studies show that reservoirs developments were mainly controlled by sedimentary facies,diagenesis and tectonic movements,and sedimentary facies was the critical factor.Sedimentary facies controlled the lithology,pore types and physical properties of reservoirs,and laid the foundation for the constructive diagenesis.Therefore,the sedimentary facies was the key to the reservoirs developments of the Loushanguan Group.