DIAGENETIC FACIES AND THEIR MICROSCOPIC CHARACTERISTICS IN UPPER TRIASSIC YANCHANG FORMATION CHANG 6 MEMBER, ORDOS BASIN, CHINA

Diagenetic activities are an important and complicated geological process for oil preservation after deposition in a fluvial–lacustrine siliciclastic sedimentary system. The studied Ordos Basin is located in the western part of the North China craton. And the Yanchang Fm. is an important oil-bearing layer and is composed of interbedded sandstones and mudstones, which are divided into 10 members. And Chang 6 member is sandstone dominated reservoir as the focus of our study. In this work, the basic geological characteristics, diagenesis and pore evolution are studied by using basic physical data, polarizing microscope, SEM and XRD. The characteristics of micro-pore structure and seepage of each diagenetic facies are studied by high pressure and constant velocity mercury intrusion, nuclear magnetic resonance and oil-water phase infiltration experiments.

The sedimentary sub-facies are delta front with microfacies of subaqueous distributary channels, estuary dams, and diversion bays. The lithology is feldspathic sandstone with main cement of carbonate minerals and matrix of clay minerals. The reservoir is a typical low to extra low porosity (8.58% in average) and extra to super low permeability (0.33×10^-3μm² in average) reservoir. It contains a series of complex diagenetic processes. ① The compaction destroys the physical properties, resulting in a decrease in porosity of 20.14%; ② The cementation includes chlorite, illite, carbonate, and siliceous cementation, resulting in a decrease in porosity of 9.88%; ③The dissolution improved the reservoir properties, resulting in an increase in porosity of 2.84%. The main pore types are intergranular pores and dissolved pores; whereas the main throat types are flaky, curved and bundled. Five types of diagenetic facies are classified: ① chlorite cemented intergranular pore, ② feldspar dissolution, ③ illite cemented pores-intergranular pore, ④ siliceous cemented pores-intergranular pore, and ⑤ carbonate cements the dense. The chlorite cemented intergranular pore facies have the best physical properties with large pore throat radius, good pore throat sorting and connectivity, high movable fluid saturation.