Research on Diageneses of Cambrian Shoal Facies Carbonate Rocks in the Xiadong Area, Hubei Province

With continuous outcrops, developed shoal facies rocks, complete types of diagenesis and changeable diagenetic environments, Cambrian strata are well developed in the Xiadong area, Yichang, Hubei Province. Under the combined influence of numerous diageneses, secondary pores can be formed, which result in better reservoir properties of the rock strata.The Cambrian rocks in this area consist of mainly carbonate rocks and secondarily detrital rocks. The carbonate rocks are dominated by grainstones including wormkalk, calcirudite-calcarenite, oolitic limestone and oncolitic limestone. Graded bedding and cross bedding are well developed in the strata, which indicates that the formation environment is of a high-energy shoal facies.In this area, there has developed a sequence of stable Cambrian platform carbonate deposition. The evolution trend is as follows: open sea shelf facies? intertidal low-energy restricted sea facies? intertidal high-energy shoal facies? coastal shoal facies? evaporite tidal-flat facies.The diageneses that the strata have experienced mainly include dolomitization, dedolomitization and dissolution, which are constructive diageneses for the formation of secondary pores, such as intercrystal pores, intercrystal solution pores, gypsum mold pores and caverns of dolomite.The diagenetic facies intervals can be divided into the unitary and the compound ones, totalling 22 in the area.In the early atmospheric fresh-water diagenetic environment and the late epidiagenetic environment, Cambrian rocks, especially dolomite of the Middle and Upper Cambrian, experienced extensive and profound fresh water corrosion, forming pore intervals with a porosity ranging from 5% to 15%.

Quality characterization of tight sandstone reservoirs in the Yanchang Formation of the Honghe oilfield, Ordos Basin, central China

Reservoir quality is one of the important geological factors controlling the development of tight oil in the Honghe oilfield,Ordos Basin,northwestern China.Analyses of core,well logging,mud logging and geophysical data as well as thin sections(casting and fluorescence)were combined with testing methods(such as grain size analysis,constant-rate mercury injection,and scanning electron microscopy)to characterize the microporeethroat development in reservoirs of the eighth member of Yanchang For-mation in the oilfield.From the perspective of sedimentation and diagenesis,the mechanisms causing reservoir quality difference were explored and a method for characterizing reservoir quality difference and distribution was proposed.The results show that complex and diverse poreethroat configurations and multi-scale throat development are the microscale manifestations of reservoir quality differences in the member.Three types of poreethroat combinations are recognized,including intergranular pore-wide lamellar throat,intergranular and intragranular pore-wide lamellar throat,and intergranular/clustered micropore-wide lamellar throat.Different diagenesis processes and intensities under the control of sedimentary conditions determine the differential development of the reservoirs.Diagenetic facies are the indicators of reservoir quality.Diagenetic facies with chlorite cementation-moderate dissolution indicates reservoirs with the most ideal physical properties for hydrocarbon accumulation,while that with moderate calcite and kaolinite cementations are usually observed in reservoirs with less ideal physical properties.Reservoirs with the worst physical properties often correlate with diagenetic facies with strong calcite cementation and compaction facies.A multi-level constrained method under the control of sedimentation and diagenetic facies is proposed for characterizing tight reservoir quality difference in the member.The spatial distribution of sedimentary elements is analyzed through sedi-mentary configurations,the diagenetic facies distribution is constrained by sedimentary elements and the reservoir quality distribution is predicted with constraint of diagenetic facies.It suggests that the high-quality reservoirs in the member occur vertically in the middle of thick channel sandstone,while poor reservoirs occur in the upper or lower parts of the channel sandstone due to intensive compaction and cementation.Laterally,the quality of reservoirs enhances along the channels with a change in shape to lens or strips along the middle and downstream sections.