Characteristics of buried structures in northern Longmenshan mountains and its significance to oil and gas exploration in the Sichuan Basin

Abstract

Since 2014, great gas discoveries have been achieved in the Upper Paleozoic Shuangyushi zone in the piedmont zone of the Longmenshan mountains, northwestern Sichuan Basin, where multiple gas wells with industrial flows have been drilled and completed successively in the Middle Permian Qixia Fm and Middle Devonian Guanwushan Fm, etc. Along with the steady progress of exploration there, to make a further in-depth study on the characteristics of the frontal–piedmont zones in the Longmenshan thrust–nappe structures will be of great significance to both a better understanding of Upper Paleozoic gas reservoirs in this study area and the expansion of oil and gas exploration field. In view of this, based on the newly deployed 3D seismic surveys, gravity–magnetic–electronic data, and practical drilling information from exploration wells like Well ST 9, the characteristics of the frontal–piedmont zones in the Longmenshan thrust–nappe structures are analyzed as well as the features of the buried structures, gas reservoir forming conditions and exploration prospect there. The following findings were obtained: (1) A huge buried structure belt is developed underlying the Longmenshan front fault belt No. 1 (LSFFB 1) and marine strata are thus formed below the Lower Triassic Jialingjiang Fm due to the roles of compressive folding, extrusion and deformation; (2) The detachment horizons from LSFFB 1 go from shallower in the south to deeper in the north, turning from Cambrian slipping in the south to basement-involved in the north. The Paleozoic and its overlying strata were overall uplifted with a higher degree northward and so did the strata between the up-thrown and down-thrown sides of the faults with a greater displacement distance and higher folding degree, resulting in that the Paleozoic strata in the north are 1500 higher uplifted than those in the south; (3) In the western Jiange and Guangyuan areas, the eastern LSFFB 1 presents a tectonic pattern of “three steps” as a whole. In conclusion, there are good gas reservoir forming conditions in LSFFB 1, the down-thrown sides of which cover an area of 1800 km2, where the Guanwushan Fm and Qixia Fm provide good accumulation and preservation conditions for gas reservoirs, which will become favorable gas exploration targets in the deep marine strata in NW Sichuan Basin. Keywords: Sichuan Basin, Northwest, Longmenshan nappe structure belt, Down-thrown side of a hidden fault, Buried structure, Gas reservoir forming condition, Exploration zone