Relationship between structure and evolution of the Early Carboniferous rifted basin and shale gas preservation: A case study of the Rongshui area in the Guizhong Depression

Abstract

Objective Many important discoveries have been made in shale gas exploration in the Lower Carboniferous Luzhai Formation in the central and northern Guizhong Depression, southern margin of the Xuefengshan Uplift, and commercial test production has achieved successful gas ignition. However, the structure and evolution of the depression in this area have not been clearly defined, which restricts an accurate understanding of the formation conditions for organic-rich mud shale, the distribution of favourable sedimentary facies zones, and shale gas enrichment law. Methods Taking the Rongshui area as an example, this paper studies the main structures and their relationships with shale gas preservation through geological investigation and analysis, 2D-3D seismic section structural interpretation, evolutionary equilibrium section interpretation and other technologies. Results It is believed that there are three main faults and four main NE-NNE synclines in the study area. The preexisting NNE-trending Sanjiang-Rong'an fault is the main factor involved in extensional fault depression and compression-strike-slip transformation. In the study area, the extensional fault depression developed in the Late Devonian, strengthened in the Early Carboniferous and stopped in the late Early Carboniferous. The compression-strike-slip effect of the depression reconstruction developed in the Indochinese and Yanshanian stages, and the local tensioning effect developed in the Alpine stage. The main part of the present fold system was formed in the Indosinian period. Influenced by the increasing thrust of the Sanjiang-Rong'an fault from west to east, the structure pattern of broad and slow synclinal-compound synclinal-faulted folds is distributed. Complex synclines are favourable structural units for shale gas preservation, and the favourable structural styles for shale gas preservation are synclines and anticlines. Conclusion This understanding clearly defines the direction of shale gas exploration and deployment in the next step, and provides a beneficial idea for the structure and evolution of the Liucheng slope in the north-central part of the Guizhong Depression.

Keywords

• early carboniferous
• fault basin
• tectonic evolution
• guizhong depression
• shale gas preservation
• shaping complex syncline