Structural division of granite weathering crusts and effective reservoir evaluation in the western segment of the northern belt of Dongying Sag, Bohai Bay Basin, NE China

Abstract

With improvements in exploration, increasing numbers of large and medium-sized granite reservoirs have been discovered in many parts of the world. However, there is no unified agreement about the structural division of granite weathering crusts and the characteristics of favorable oil and gas reservoirs have not been clarified. Based on cores and thin section observations, outcrop surveys, conventional logging data, image logging analysis and seismic interpretations, our study analyzes the vertical structural characteristics of granite weathering crusts and the characteristics of reservoirs in the western segment of the northern belt of Dongying Sag, Bohai Bay Basin, NE China. The results indicate that the vertical structure of granite weathering crusts in the study area is a double layer composed of a top eluvium or dissolution layer (completely weathered layer) and a bottom disintegration layer (semiweathered layer) and the dissolution and eluvium layers are the products of different evolution stages on top of the disintegration layer. The types of effective reservoirs can be divided into three categories: type I (dissolution layer), with a porosity of 2–8% (averaging 4%) and permeability of 0.1–25 md (averaging 0.5 md), which are mainly distributed in the middle and bottom parts of the structure and yield high oil and gas productivity; type II (eluvium), with a porosity of 3–5% (averaging 4%) and permeability of 0.5–5 md (averaging 1 md), which are mainly distributed in the gentle slope area of the uplift and have poor oil- and gas-bearing properties; and type III (disintegration layer), with a porosity of 1–3% (averaging 2%) and permeability of 0.1–0.5 md (averaging 0.11 md), which are widely distributed and found in the high and middle to low parts of the structure and present oil spots in the cores. The lateral distribution of effective reservoirs in the study area can be divided into the top section of the uplift, the edge section of the uplift and the bottom section of the slope. The top mainly develops a disintegration layer, the edge mainly develops eluvium, and the bottom of the slope develops a dissolution layer. The dissolution layer in the middle and bottom sections of the structure is the most favorable reservoir development zone. The dynamic genetic pattern of the double layer structure of granite weathering crust and the distribution characteristics of reservoir differences proposed in this paper have important theoretical and application value for determining the geological genesis and performing oil and gas exploration of granite weathering crust reservoirs in other rift basins.