Sedimentary And Reservoir Characteristics Of The Feixianguan Formation In The Yilong-Pingchang Area,Sichuan Basin

Recent exploration and development have shown that the Sichuan Basin in China is rich in oil and gas resources.The Yilong-Pingchang area in the study area is located in the northeast portion of the Sichuan Basin,where a large number of marine carbonate rocks have developed and natural gas resources are abundant.With further increase in oil and gas exploration,a series of large-scale marine carbonate oil and gas fields have been discovered in the Lower Triassic Feixianguan Formation in the northeastern Sichuan Basin.The sedimentary facies and reservoir characteristics of the Feixianguan Formation in the study area restricts further exploration and development.Therefore,it is necessary to strengthen the study of sedimentary facies and reservoir characteristics in this area.This paper is based on various geological data such as field outcrop observation,logging,thin section,and physical property analysis,with focuses on the description of field outcrops and cores,identification of castings and thin sections,and comprehensive analysis of various physical property data to determine and analyze the sedimentary and reservoir characteristics of the Feixianguan Formation in the study area.The main finding are as follows:(1)The Feixianguan Formation in the study area shows highest thickness in the middle trough and thins toward both sides of the trough margin.The top of the Feixianguan Formation is dominated by argillaceous gypsum and dolomitic gypsum,distinguishable from the bottom of the Jialingjiang Formation.The lowest Feixianguan Formation largely consists of marls which are different from the bioclastic limestone and bioclastic dolomite in the Changxing Formation.Grainstone and packstone are common in the second(Fei-2)and third(Fei-3)member of the Feixianguan Formation.(2)The Feixianguan Formation in the study area was deposited in an open to restricted platform environment.The deposition of the Lower Triassic Feixianguan Formation shows a unique distribution pattern of presence of two platforms and two troughs.The platform marginal belt during the Fei-2 is characterized by the development of oolitic beaches,while the Fei-3 sedimentary is characterized by the deposition of inner-platform high land beaches.The platform gradually closed and increased in salinity during the Fei-4 period,which formed an excellent regional caprock for subsequent reservoirs.(3)The favorable lithofacies types in the Feixianguan Formation in studied area are oolitic dolomite,grain dolomite,and oolitic calcitic dolomite.The space in the reservoirs of the Feixianguan Formation are largely dominated by porosity and includes intercrystalline dissolution pore and intergranular dissolution pore.Reservoirs are mainly distributed in platform margin beaches and inner-platform beaches.Among them,the “platform oolitic beach” has the best performance because of high porosity generated by significant weathering and leaching.(4)The development of the platform marginal beach is strongly controlled by factors such as dolomitization,dissolution,atmospheric freshwater leaching related to sea level decline,constituting the most favorable reservoir in the study area.The distribution of the platform marginal belt controls the distribution of favorable reservoirs.