Characterization and evaluation of fractal dimension of intersalt shale oil reservoirs in Qianjiang Depression

Abstract

At present, studies of the shales in Qianjiang Formation from the Qianjiang Depression has been widely documented and significant progress has been made in the aspects of geochemical characteristics and reservoir physical properties.However, the complexity of pores, the main minerals forming pore throats and their influencing factors is still poorly understood. In this work, based on fractal theory and combined with nitrogen adsorption and high-pressure mercury injection experiments, Wells BX7 and BYY2 at different deposition locations were selected to evaluate the specific surface fractal characteristics, pore structure fractal characteristics and pore throat fractal characteristics of the Eq34-10 rhythmic shale of the Qianjiang Formation and analyses the influencing factors.The results show that the pore surface of shale in study are a is relatively flat, with the fractal dimension D1 being close to 2, and the roughness of the pore surface is mainly affected by the characteristics of the clay minerals. Compared with Well BX7 at the sedimentary edge, Well BYY2 at the sedimentary center has simpler fractal characteristics of reservoir pore structure. The larger the proportion of pore volume with a small pore size is, the more complex the fractal characteristics of the pore structure are. The fractal dimension D2 of the reservoir pore structure in Well BX7 is mainly affected by clay minerals and quartz, while dolomiteis the dominant controlling factor for the fractal dimension D2 of the reservoir pore structure in Well BYY2. Dolomite is the main mineral that constitutes the pore throat of the Well BX7 reservoir. With the increase in dolomite, the pore throat features are complex, and the pore connectivity becomes poor. The pore throat diameter formed by quartz in the Well BYY2 reservoir is small.The increase in quartz will worsen the pore connectivity, and the pore throat diameter formed by calcite is large. The occurrence of shale oil will reduce the fractal dimension of pores and have little influence on the fractal characteristics of pore throats. The presence of salt minerals can block the pores and make the pore connectivity poor