A study of tight reservoir diagenesis in the Chang 7 Member of Triassic Yanchang Formation, Jiyuan-Longdong area, Ordos Basin

Chang 7 Member is a tight reservoir in Triassic Yanchang Formation, Jiyuan-Longdong area, Ordos Basin. The tight reservoir is formed mainly in the deep lake sub-facies, and partly in the delta front sub-facies. The study of tight reservoir diagenesis has been very insufficient, and it is important to find out the effect of diagenesis on the development of reservoir pore. The samples collected from the core were observed and analyzed by casting lamella and scan electron microscope to reveal the influence of the main diagenetic activities such as compaction, cementation and dissolution on the pore type and pore size of the tight reservoir in the study area. Compaction is the main factor responsible for the formation of tight reservoir. The sand body in the bay of delta front distributary and the edge of turbidite fan is especially easier to form calcite cementation when the pore water is excluded from the mudstone under pressure. Illite cementation and calcite cementation makes the reservoir more dense. A small amount of chlorite films cementations form the residual intergranular pores and promote the mineral dissolution. Dissolution is the main function causing the reservoir pore formation in the study area. The diagenesis study constitutes the basis for the research on reservoir physical properties in the study area.