Pore structure difference of shale in different structural units and its petroleum geological implications: A case study on deep shale in the Luzhou area, southern Sichuan Basin

Different structural units present obvious differences in shale reservoir quality and gas content. Tectonic reworking action has a controlling effect on shale exploration and development, but the structure controlling mechanism of deep shale gas is less researched and not understood clearly, which restricts the exploration and development of deep shale gas. In this paper, the differences in pore structures and reservoir characteristics of deep shale from Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in different structural units in the Luzhou area of southern Sichuan Basin are comparatively analyzed by means of core-thin section-SEM multi-scale observation, whole-rock mineral X-ray diffraction analysis and NMR analysis. In addition, the coupling relationships between shale pore structural difference and reservoir quality in different structural units are discussed, the internal geological reasons for the difference in shale gas production are identified, and the favorable areas for shale gas exploration and development under tectonic reworking action are defined. And the following research results are obtained. First, the shale in the syncline area is dominated by organic pores and abnormal-pressure fractures with larger pore sizes, while the shale pore structure in the anticlinal area suffers strong reworking and is dominated by mineral intergranular pores and tectonic fractures. The mineral intergranular pores are arranged directionally in a narrow and long pattern, and small in size. Second, in the syncline area, the structures are relatively stable, organic pores, mineral intergranular pores and diagenetic fractures are well preserved, and the proportion of macropores is higher, so the reservoir performance is good, and the gas content and production rate are high. In the anticline area, a large number of tectonic fractures and faults are developed in shale reservoirs, pores and diagenetic fractures are compacted, and pore size decreases, so the reservoir performance deteriorates, and the gas content and production rate are low. Third, the farther away from the basin marginal denudation area, the better the preservation conditions of the unit with stabler structure;the more developed the shale reservoir in this type of structure unit, the higher the gas content and production rate, and the greater the potential of shale gas exploration and development. In conclusion, the influence of tectonic movement on the pressure system is an important factor for different pore structure characteristics of deep shale in different structural units, and controls the gas content and production of shale reservoirs. The deep reservoir in the syncline area has a good preservation condition and is far away from the denudation area, so it is the most favorable area of exploration and development. The research results can provide a technical support for deep shale gas exploration and development. © 2024 Natural Gas Industry Journal Agency. All rights reserved.