Abstract:

Significant progress has been made in the exploration and development of shale gas within the Lower Silurian Longmaxi formation of the thrust-nappe tectonic belt in the Nanchuan block. By the end of 2022, this area saw the drilling of 170 wells, achieving a cumulative gas production of 51.6 billion cubic meters. As development has progressed, the focus has shifted from the structurally simpler main area to the more complex peripheral regions. Previously, the structural interpretation of the Nanchuan block primarily identified faulted anticlines constrained by major reverse faults, without adequately addressing the structural deformation across the entire belt. Recent evaluations have revealed significant discrepancies between well data and seismic interpretations, leading to numerous drilling failures and hindering further shale gas development. This study revisits these challenges using advanced drilling insights and forward modeling analysis. It suggests that the apparent thrust faults with large displacements and high angles in the seismic profiles of the Longmaxi formation are likely structural misinterpretations, rather than true geological features. Instead, the fault areas are characterized by high-steep knee-fold structures with smaller fault distances and steeply inclined strata. By integrating high and steep structural illusion analysis with fault-related fold theory, the well-seismic joint structural modeling has been refined to better represent the structural characteristics of the Longmaxi formation gas reservoir in the Nanchuan block. This improved understanding has identified the high-steep knee fold zone as a potentially favorable target for further exploration and development, supported by positive results from five recently deployed evaluation well groups.

Key words: well-seismic contradiction, faulted fold, knee fold belt, structural illusion, seismic forward modeling