Diagenesis of Deep Low-permeability Reservoirs in Yangjiang Depression and Its Impact on Reservoirs

The oil and gas resources in the deep of Yangjiang Depression of Pearl River Mouth Basin are huge. However,the study on the diagenesis of deep reservoir is relatively week. Based on rock cores, this paper provided a comprehensive analysis of the diagenesis and its effects on the reservoir physical property of deep low-permeability sandstone reservoir of Enping Formation in the research area. The result of research shows that the composition maturity of Enping Formation reservoir in Yangjiang Depression is relatively high and the reservoir space is dominated by dissolution pores, accounting for more than 50%. The reservoir is generally porous with low permeability and porosity. The strata of Enping Formation are in the middle diagenetic stage (IIA-IIB). The diagenetic sequence is: Prodiagenetic stage A (IA) - mechanical compaction/chlorite film → Prodiagenetic stage B (IB) - mechanical compaction/tuffaceous alteration/feldspar dissolution → Mesodiagenetic stage A (IIA) - mechanical compaction/calcite cementation/feldspar dissolution/debris dissolution/kaolinite cementation → Mesodiagenetic stage B (IIB) - chemical compaction/dolomite cementation/pore chlorite/quartz enlargement. The reservoir space of deep low permeability reservoir in Enping Formation is obviously affected by diagenesis. Compaction changes the porosity of Enping Formation reservoir significantly, which reduces porosity by 11.8% to 26.3%, with an average of 19.5%, and has a pore reduction of 2.3% to 30.1%, with an average of 12.3% in the pore space by cementation. The porosity of Enping Formation reservoir is increased by 1.3% to 24.8%, with an average increase of 9.4%, due to the dissolution impact. Based on a comprehensive analysis of the matching relationship between oil and gas filling and reservoir pore evolution, it is concluded that the deep reservoirs in this area belong to "tight first and then reservoir formed".