Quantitative characterization on shale-hosted oil reservoir: A case study of argillaceous dolomite reservoir in the Jianghan Basin

Abstract

This article quantitatively investigated the microscopic pore structure of the Eocene argillaceous dolomite reservoir from the Jianghan Basin in China to further the understanding of shale oil and gas storage mechanism in these rocks, considering that extensive studies have been conducted on the shale reservoir but seldom focused on argillaceous dolomite reservoir. Image analysis technology (containing Areal porosity method, CT reconstruction calibration method and focused ion beam-scanning electron microscope) and intrusive method including mercury intrusion porosimetry were used to discuss the size, spatial distribution and connectivity of the pores, as well as to reveal the contribution rate of the pores to porosity. Dolomite and clay minerals are the main minerals in the samples (including mudstones, argillaceous dolomite and dolomite mudstones), with the average values are 27.8% and 28.5% respectively. Inorganic pores (mineral-associated pores) rather than organic pores (organic matter-hosted pores) are dominant in the argillaceous dolomite reservoir, which contain intercrystalline pores, intergranular pores and dissolved pores. The inorganic porosities calibrated by SEM range from 7.82% to 18.58%, close to the measured porosities. The argillaceous dolomites mainly develop intercrystalline pores in the dolomite mineral with good connectivity, while mudstones or dolomite mudstones mainly contain intergranular pores in the clay minerals with poor connectivity. Large throats always distribute in the parts where pores are well developed and connected. The connectivity of the pores in the argillaceous dolomite improves with the increase of the dolomite mineral content. The argillaceous dolomites have relatively high porosity containing more macropores while mudstones have lower porosity occupied by much more micropores. Pores with relatively larger throat radii contribute a larger fraction of the porosity of the argillaceous dolomite, while pores with smaller throat radii mainly contribute to porosity in the mudstones. These characteristics may be beneficial for understanding reservoir mechanism of argillaceous dolomite reservoir.