Abstract: Qixia Formation gas reservoir in Shuangyushi structure is the first large-scale marine carbonate gas reservoir discovered in Longmenshan piedmont belt, with faults extremely well developed and complex structural patterns, and structural features are a key factor influencing gas reservoir development. The structures in this area were jointly controlled by multi-stage tectonic movements and multiple detachment layers, while there was insufficient discussion on the controlling effects of multi detachment system in previous studies. A progressive technical route of “physical simulation–numerical simulation–geological verification” has been adopted, and various models with different properties and thicknesses of detachment layers have been designed to reveal the controlling mechanism of multi detachment layers on structural deformation, then the accuracy of the simulation results was verified by actual drilling data. By analyzing the geological characteristics of different structural styles, awe can further optimize the deployment of gas reservoir development. The results show that: (1) Three sets of detachment systems were developed in Shuangyushi area, namely the Cambrian (deep main detachment layer), Triassic (shallow main detachment layer), and Silurian (secondary detachment layer). The “upper–lower double detachment” combination was the key to forming the thrust imbricate–steep fold structure in this area. (2) The properties of detachment layers significantly affected structural styles. For example, the weak detachment layers tended to form continuous imbricate thrust structures, while strong detachment layers enhanced the deformation complexity of overlying strata and weakened the continuity of imbricate structures. (3) The thickness of detachment layers controlled structural evolution. Thin detachment layers are characterized by “fold dominant, and secondary faults”. The medium-thick detachment layers formed ejective box folds–imbricate fan structures, and the thick detachment layers developed “detachment layer dominated” giant fold–thrust systems, with better sealing performance of thick plastic detachment layers. (4) Based on the differences in structural styles, differentiated gas reservoir development models have been proposed, that is, “sparse well pattern + high-angle deviated well” model for the pop-up structural zone, “long horizontal well (at the high point)” model for the monoclinal anticline zone, and “high-angle deviated well + horizontal well combination” model for the faulted anticline zone. The research results have clarified the structural formation mechanism under the control of multiple detachment layers, which provides theoretical support and technical reference for the exploration and development of similar gas reservoirs in Longmenshan piedmont belt.

Key words: Shuangyushi structure, Qixia Formation gas reservoir, detachment layer, physical simulation, numerical simulation, structural style