Reservoir rock characteristics and main controlling factors of the Feixianguan Formation in the Maoba-Dawan-Tieshanpo area, Sichuan basin

Abstract：The 21st century is a great development period of China's oil and gas exploration and development of marine carbonate rocks. For the moment, the majority of China's marine carbonate oil and gas fields are buried below 4000 m depth in the Paleozoic sequence. How to look for favorable reservoirs in deep carbonate rocks has become the focus of attention of petroleum geologists. The Puguang gas field discovered recently is a gas accumulation with the best developed secondary pores among carbonate reservoirs discovered in China at present and is strictly controlled by the distribution of sedimentary facies, dolomitization and buried dissolution. In this paper, a detailed study and anatomy of the “reservoir facies”-favorable sedimentary facies and diagenetic facies-is conducted under the guidance of the“facies control theory”, which controls the formation and evolution of the reservoir of the Lower Triassic Feixianguan Formation in the Dawan - Maoba - Tieshanpo area in the surroundings of Puguang. According to the outcrop section, drilling, logging and seismic data, combined with the “single-factor analysis and multifactor comprehensive mapping method”, four sedimentary units are proved to be developed in the Feixianguan Formation in the Dawan - Maoba - Tieshanpo area; they are platform slope, platform shoal, open platform and restricted platform. Based on the detailed description of cores and indoor observations and studies of large numbers of core and cutting thin sections and casting slices, the authors think that the key factors which control the reservoir formation are dolomitization, buried dissolution, and so on. According to casting slice identification, scanning electron microscopy, analysis of mercury and other means of analysis, it is found that the favorable reservoir is characterized by large sets of porous finely and very finely crystalline residual oolitic dolostone. Solution pores in rocks are very well developed, consisting dominantly of the large pore and coarse throat type, medium pores and medium throat type. The physical properties of the gas reservoir are better, marked mainly by high porosity and permeability and medium porosity and permeability. The correlation of porosity and permeability is obvious. The role of “facies control” is pronounced in this reservoir. Vertically, the reservoir is mainly distributed from the Second Member to the top of the First Member of the Early Triassic Feixianguanan Formation.. Laterally, the favorable area for reservoir distribution is the platform-margin shoal. The platform-margin shoal facies zone controls the lithology and distribution of the favorable reservoir, while dolomitization and buried dissolution control the final distribution limits and performance of the favorable reservoir. Through a study of the “facies control theory” in the reservoir of the platform-margin shoal in the Dawan - Maoba - Tieshanpo area.