Reservoir types and well logging identification of the Sinian Dengying Formation in the Central Sichuan region

The Sinian Dengying Formation, situated in the Central Sichuan region, lies at the eastern high point of the Longnvsi ancient uplift axis in Leshan. This formation possesses both oil source and preservation conditions, making it a promising site for exploration. It is an important target and successor layer for oil and gas exploration in the Sichuan Basin. In 2011, the China National Petroleum Corporation (CNPC) initiated the exploration of oil and gas resources at Well Gaoshi-1 in the Dengying Formation. This endeavor resulted in the successful discovery of oil and gas reservoirs within the Dengying Formation. Therefore, many scholars have conducted a lot of relevant research. However, they have only reached a consensus on the lithology and stratigraphic classification of the strata. There is still great controversy over the results of classifying reservoir types. In the process of oil and gas exploration, understanding the type of reservoir is an important step and a key factor that will directly affect the success of oil and gas exploration and development.

The Dengying Formation in the Sichuan Basin has undergone seven major tectonic movements over a geological history of about 600 million years. These movements have led to significant changes in burial depthes and a range of diagenetic processes, resulting in a variety of reservoir space types. As a result, the combination of reservoir spaces is also very complex, leading to significant controversy in the classification of reservoir types. Understanding the type of reservoir is an important link in oil and gas exploration and development, as it directly affects the success of these endeavors. Therefore, this article utilizes rock cores and drilling logs from more than 30 wells in Central Sichuan and its surrounding areas as first-hand data. It conducts in-depth analysis of the reservoir types and logging identification characteristics of the Dengying Formation.

In the research process, the existing drilling cores, field profiles, and thin sections of rocks in Central Sichuan and its surrounding areas were used as the main research subjects to study the rock types and storage spaces that constitute the reservoir. The results show that in the Dengying Formation, the lithology is predominantly dolomite, with a small amount of mudstone and siliceous rock. The rock types that form the reservoir are diverse, mainly including grain dolomite, sandstone dolomite, algal laminated dolomite, and karst breccia. The observation of thin sections and rock cores indicates that the current reservoir space is mainly composed of pores, holes, caves, and fractures formed by secondary processes. The pore-type space refers to that with a pore diameter less than 2 mm, mainly including intercrystalline dissolution pores, intergranular pores, and intergranular dissolution pores. The hole-type space refers to that with a pore diameter greater than 2 mm and less than 100 mm, mainly including karst holes along the direction of algal layers, porous karst holes, fractured karst holes, intergranular holes, and residual holes bordered with the shape of grape. The caves-type space refers to that with a pore diameter greater than 100 mm. The fracture-type space refers to a fracture formed by tectonic action, which significantly enhances the storage capacity and permeability of reservoirs in the Dengying Formation.

In Central Sichuan, the storage spaces of the Dengying Formation are interconnected and distributed in a combined manner. Based on the primary or dominant reservoir spaces, reservoir types are classified into four categories, pore-holes type, fracture-pore-holes type, pore type, and cave type reservoirs. Each reservoir type exhibits distinct logging response characteristics. With the use of existing conventional logging data and micro-resistivity scanning imaging logging data in the study area, logging identification standards for different reservoir types have been established. This provides new insights for the exploration and development of oil and gas in the Dengying Formation in Central Sichuan.