Quantitative evaluation of geological conditions for shale gas preservation based on vertical and lateral constraints in the Songkan area, Northern Guizhou, southern China

Abstract

Organic rich Wufeng-Longmaxi shales (WLS) occur widely in the Upper Ordovician–Lower Silurian strata in southeastern Sichuan Basin, SW China. However, complex structural deformation and faulting activity in this area since the Mesozoic time has had significant impact on shale gas preservation. Thus, difference in the structural deformation strength is an important constraint for differential shale enrichment in a shale reservoir. Fracture analysis and bedding surface normal stress data obtained from the outcrop, core and fluid inclusion samples were used to determine the vertical and lateral constraints on shale gas preservation conditions. Using the finite element method, 2D mechanical models were established in the Songkan area, based on seismic inversion, rock mechanics and acoustic emission tests. Paleotectonic stress fields were estimated for the Middle Yanshanian (K11/K12), Late Yanshanian (K22/K23) and Himalayan （E-N）times. Fracture type distribution was determined from the rock failure criterion, and the shale bedding surface normal stress calculated by dip angle and burial depth. The results indicate that optimal conditions for shale gas preservation occur where the maximum principal stress values reach the fracture development stage (around 63.96–73.8 MPa). When the value increases to over 65 MPa, shale bedding surface closure favors the shale gas preservation. Based the vertical and lateral constraints, the best preservation area occurs where fracture development coincides with the shale bedding closure.