Sedimentary facies and reservoir characteristics of fourth Member of Dengying Formation in Well DB1 area， north-central Sichuan Basin

Abstract

The reservoir lithology of the fourth Member of Dengying Formation in the north-central Sichuan Basin is complex with strong heterogeneity， which limits the gas reservoir exploration in the fourth Member of Dengying Formation. To further deepen the understanding of sedimentary facies and reservoir characteristics in Well DB1 area in north-central Sichuan， the sedimentary facies and reservoir characteristics of the fourth Member of Dengying Formation in Well DB1l area in the north-central Sichuan were studied based on the data of drilling core， thin section observation， well logging， log， physical property test， and drilling results. The results show that two sedimentary subfacies of platform margin and restricted platform are mainly developed in the fourth Member of Dengying Formation in the study area， and sedimentary microfacies such as algal mound， grain beach， and inter-beach sea were further identified. Among them， algal mound microfacies mainly develop algal clot dolomite and algal stromatolite dolomite， grain beach microfacies mainly develop calcarenite dolomite， and inter-beach sea microfacies mainly develop micritic dolomite and powder crystal dolomite. The algal mounds and grain beaches in the fourth Member of Dengying Formation in the study area are mainly developed on the side of the platform margin， and the deposition of algal mounds and grain beaches in the platform gradually decreases mainly inter-beach sea deposition. The development of algal mounds in the fourth Member of the Dengying Formation in the study area is characterized by longitudinal superposition， and the horizontal extension is relatively close. Meanwhile， the algal mounds are concentrated in the Upper Submember of the fourth Member of Dengying Formation with large monomer thickness， while the Lower Submember of the fourth Member of Dengying Formation is less developed and thinner. The reservoir rock types of the fourth Member of Dengying Formation in the study area are mainly algal clot dolomite， algal stromatolite dolomite， and calcarenite dolomite. The reservoir space is mainly algal binding framework pore， intergranular dissolved pore， and intergranular dissolved pore. The core porosity of the fourth Member of Dengying Formation in the study area is distributed between 2.02% and 6.03% with an overall average porosity of 3.11%， the core permeability is distributed between 0.007 04 and 9.78 mD， and the overall average permeability is 0.692 64 mD. The overall physical properties are characterized by low porosity and ultra-low permeability， and some samples show low porosity and high permeability due to the influence of fractures.

Keywords

• sedimentary microfacies
• distribution characteristic
• reservoir characteristic
• fourth member of dengying formation
• north-central sichuan basin