Rock features and sedimentary environment of the fourth member of Dengying Formation in Hujiaba section of Ningqiang, northern Sichuan Basin

In the central Sichuan Basin, significant progress has been made in the exploration of natural gas around the Dengying Formation. However, the exploration in the northern Sichuan Basin is relatively low and the drilling data is scarce. The petrological characteristics of the field geological section and the analysis of the sedimentary environment are of great significance for oil and gas exploration. Based on the survey and systematic sampling analysis of the Dengying Formation, Hujiaba section, sedimentary stratigraphy, rock classification and sedimentary environment analyses were carried out using sedimentology and petrology. Our research showed that:(1) According to the rock types, algae content and shale content, the 4th member of Dengying Formation, Hujiaba section can be divided into upper and lower sub-sections, of which the upper sub-section is the main part of the bioherm-beach and reservoir development; (2) The carbonate rock types of the 4th member of Dengying Formation can be divided into three sub-classes of crystalline dolomite, algae dolomite and granular dolomite, and eight micro-classes; (3) The 4th member of Dengying Formation, Hujiaba section is generally deposited on the edge of the platform, which can be divided into four subfacies of table, interbank sea, algal mound and grain beach. (4) During the deposition period of the fourth member of Dengying Formation, the sea level is characterized by two cycles of transgression and regression. The types of sedimentary rocks are controlled by the sedimentary cycle and sea level which shows a sedimentary evolution process of table facies→interbank sea facies→bioherm-beach facies→interbank sea facies→bioherm-beach facies.