DIAGENESIS AND POROSITY DEVELOPMENT IN THE CHANG 8 OIL GROUP SANDSTONE OF UPPER TRIASSIC YANCHANG FORMATION, SOUTHWESTERN ORDOS BASIN, CHINA

The Chang 8 oil group sandstone of upper Triassic Yanchang Formation is the most important oil exploration and development target layer in the southwestern Ordos basin, China. There are rapidly lithology changes and intense diagenesis heterogeneity in the reservoir. This study aims to recognize the diagenetic types, diagenetic process and influence degree between diagenesis and porosity development of the reservoir based on 315 thin sections, porosity and permeability determination, scanning electron microscope, microprobe analysis, cathode luminescence and X-ray diffraction. The diagenesis types of the reservoir mainly include compaction, cementation, dissolution and replacement. Diagenetic stage is mainly in the stage of middle diagenetic phase B. The early diagenetic stage A was mainly dominated by mechanical compaction, developing leaf shaped chlorite coat on particle surface in form of thin film. The original porosity decreased sharply under intense mechanical compaction. In early diagenetic stage B, illite-smectite, secondary enlargement of quartz, and the appearance of carbonate cementation were largely developing, resulting in poor reservoir properties. During the middle diagenetic phase A, organic matters entered into the mature stage. Hydrocarbon filling brought a large amount of organic acids. The feldspar, others aluminium silicate and early formating carbonate cements were dissolved in great quantities. A large number of secondary intragranular dissolution pores, intergranular dissolved pore, mold hole, native hole and a small amount of microcracks were forming. With the increasing of buried depth, organic matters were highly matured in middle diagenetic phase B. Hydrocarbon filling had been diminished significantly. Diagenetic environment transformed into the weak alkaline, while the quartz secondary enlargement continued. Moreover, there was locally appearing the filling of later carbonate cements. Finally, cementations together with compaction eventually resulted in the present property features. The pore loss controlled by compression is average for 58% and by cementation is average for 42%. Compaction takes the main responsibility for decreasing of the reservoir porosity and permeability, while the cementation takes as the second important role.