Types and distribution patterns of complex turbidite sandstone reservoirs in the upper reaches of deep-water canyons—A case study of the Lingshui gas field in the Central Canyon of Qiongdongnan Basin

The Lingshui gas field,located in the upper reach of the Central Canyon in the Qiongdongnan Basin,shows the occurrence of turbidite sandstones.However,the turbidite sandstone reservoirs of diverse types,jointly affected by both sedimentation and diagenesis,present challenges in characterization.This study aims to forecast the distribution of favorable reservoirs in the deep-water areas of the Lingshui gas field.Based on the morphologies and burial depths of the Central Canyon,we categorize the upper reach of the canyon head into the adjustment section,the low-tortuosity section,and the curved section.This division allows for a detailed examination of the reservoirs’ fracture and filling types in the reservoirs.Furthermore,we analyze the deposition and origin of the reservoir rocks based on the quantitative relationships between rock fabric and physical properties.The results indicate that the adjustment section is dominated by coarse-grained gravity flow deposits,with reservoir physical properties gradually improving downstream as the intergranular matrix content declines.The low-tortuosity section is primarily composed of fine-grained to silty turbidite sand bodies,with grain sizes marginally varying along the canyon and cementation emerging as a factor affecting the quality of reservoirs consisting of alternating thin-and thick-bedded sandstone.The curved section principally comprises silty turbidite sand bodies,with reservoir physical properties gradually deteriorating downstream with an increase in the burial depth.Therefore,it can be inferred that the reservoir quality is governed by the sedimentary process in the adjustment section and the diagenesis in the low-tortuosity and curved sections.