Identifying and Characterizing Mass-Transport Complexes in the Longmaxi Shale in Sichuan Basin, China

Abstract

Abstract The Lower Silurian Longmaxi Shale Formation of Sichuan Basin is the largest shale gas reservoir in China. It was formed mainly in a deepwater shelf depositional environment with a relatively stable tectonic background. More and more cores and borehole images data from the Longmaxi Shale revealed that the sedimentary features show considerable changes in mineralogy or texture in different regions. Mass-transport complexes (MTC) were identified for the first time in the Longmaxi Shale based on the integration of logs, borehole images, and seismic data. The MTC have led to a clearly abnormal formation thickening in both well and seismic data. Identifying and characterizing the MTC in the Longmaxi Shale is the objective of this case study. The integration of the logs, borehole images, and seismic data brings more insights into different aspects of MTC. The repeat section that results from the MTC can be identified based on the well logs directly. Detailed analysis of the borehole images yields additional evidence such as lithofacies and textures in the MTC. The faults and fractures in the MTC can also reveal the controlling factors on the shale gas enrichment. The analogs between the concept model and seismic reflections have improved our understanding of when and how the MTC are likely to form in the Longmaxi Shale. The MTC in the Longmaxi Shale are developed above the detachment surface between the calcite-cemented sandstone body and the high-clay-volume shale. It brings more than 20 m of repeat section inside the Longmaxi Formation. The boundary between the original and repeat sections is a low-angle thrust fault. The repeat sections can be divided into four zones based on their log responses. The gamma ray (GR) response and the borehole image features have validated the presence of the repeat section. The minerology differences between the original and repeat sections have highlighted the facies variations in the Longmaxi Shale. This is also the reason why the MTC could occur in this area. Based on the calibrations of well data, the geometry and architecture of the MTC are readily observed in 3D seismic data. The MTC in the Longmaxi Shale are formed in a slope background. It helps to get more understanding of the sedimentary variations in the Longmaxi Formation. The process of MTC forming may have a significant impact the reservoir quality of the Longmaxi Shale.