Formation mechanism and distribution pattern of high-quality reservoirs in the X area of Wenchang B Sag

The shallow layer of the Wenchang B Sag in the western part of the Pearl River Mouth Basin has a high exploration degree. At present, the middle and deep layers of the Enping (Lower Oligocene) and Wenchang (Eocene) formations are key exploration layers. In recent years, multiple exploration wells in the X Zone of the sag have achieved good oil and gas discoveries in the middle and deep layers, but the formation mechanism and distribution pattern of high-quality reservoirs have not been clearly studied, which restricts the achievement of a breakthrough in oil and gas exploration in the middle and deep layers of the X Zone. To address these issues, comprehensive use of logging, core, cuttings, petrographic thin sections, and fluid inclusions analysis data from the study area, through the “three-line combination” method (the three lines refer to the maximum normal porosity evolution curve, the average normal porosity evolution curve, and the actual porosity envelope curve), determined the middle and deep layer high-quality reservoirs, and explored their evolution and formation mechanism. Results show that the pore development of the middle and deep layer high-quality reservoirs in the X Zone is mainly controlled by organic acid dissolution for pore enlargement and oil and gas injection for pore preservation. The subaqueous distributary channel deposits at front edge of fan delta provided rich material sources for the development of good reservoirs in the X Zone. Inhibition due to compaction and cementation by hydrocarbon injection was the key factor for the formation of high-quality reservoirs, and the early carbonate cementation prevented large-scale carbonate cementation in the middle and deep layers. Meanwhile, affected by organic acid dissolution and closed environment of strata water, the middle and deep layers developed mainly secondary solonoclastic pores of mixed origin. The "deposition - diagenesis - oil injection" multi-factor comprehensive analysis was used to predict the horizontal distribution range of the high-quality reservoirs in this area. It is believed that thick sand bodies in thick fan delta front adjacent to source rocks or near-oil-source faults and far from volcanic rock basement are the key development areas of high-quality reservoirs.