Depositional and Diagenetic Controls on Sandstone Reservoirs with Low Porosity and Low Permeability in the Eastern Sulige Gas Field, China

In order to determine the genesis and the factors that control the low-porosity and low- permeability sandstone reservoirs in the eastern Sulige Gas Field in the Ordos Basin, systematic studies on the sedimentary facies and diagenesis were conducted by means of analysis of cores, thin sections, fluid inclusions, X-ray diffraction, cathode luminescence and scanning electron microscope. It was found that the sand bodies of the major gas reservoirs in the Shan1 section （P1S1） and the He8 section （P2H8） were formed during the Permian as sedimentary facies such as braided-channel bars, braided-river channels and point bars of a meandering river. Four types of diagenetic facies developed subsequently： in order from the best to the poorest properties these are type A （weak compaction, early calcite cement-chlorite film facies）, type B （moderate compaction, quartz overgrowth-feldspar corrosion-kaolinite filling facies）, type C （strong compaction, late calcite cement-quartz corrosion facies） and type D （matrix filling and strong compaction facies）. This diagenesis is undoubtedly the main reason for the poor reservoir properties of sandstone reservoirs, but the sedimentary facies are the underlying factors that greatly affect the diagenesis and thus the reservoir performance. Favorable diagenetic facies developed mainly in relatively small lithofacies such as braided-river channels, channel bars and point bars. The vertical distribution of the physical properties and the diagenetic facies of the reservoirs are related to the stratigraphic succession. Most of the sandstones between mudstones and thin beds of sandstone are unfavorable diagenetic facies. Analyses indicate that siliceous cementation can hardly be stopped by hydrocarbon filling. Authigenic chlorite could hardly protect the primary porosity. It not only occupies pore space, but also blocks pathways through sandstone reservoirs, so that it has significant influence on the permeability. Authigenic chlorite cannot be used as a marker for a specific sedimentary facies because it can be formed in different sedimentary facies, but it indicates high hydrodynamic conditions and presence of favorable reservoirs.

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.

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.

Genesis of the low-permeability reservoir bed of upper Triassic Xujiahe Formation in Xinchang gas field,western Sichuan Depression

The genesis of a reservoir is a result of the combined action of deposition, diagenesis, tectonic reworking, and interaction of rock and fluid and the evolutionary environment. We discuss the genetic and evolution mechanism of a low-permeability reservoir bed of the Xujiahe Formation in the western Sichuan Depression on the basis of the study of diagenesis, diagenetic reservoir facies and the diagenetic evolution sequence. The research indicated that this reservoir bed can be divided into five types of diagenetic reservoir facies, namely strong dissolution, chlorite-lined intergranular pores, compaction and pressure solution, carbonate cementation and secondary quartz increase. There are, however, just two diagenetic reservoir facies which provide low-permeability reservoir beds, namely strong dissolution and chlorite-lined intergranular pores. We also analyzed their diagenetic evolution sequences and the origin of the low-permeability reservoir bed. Besides, it was also indicated that the composition and structure of sandstones, types of sedimentary microfacies, diagenesis history as well as the tectonic reworking in later periods are the main factors controlling the formation of the low-permeability reservoir bed. The above- mentioned factors establish the foundation for the forecasting the distribution of high quality reservoir beds.