Well log evaluation of shale gas reservoirs and preservation conditions of Lower Cambrian shale succession in Cengong Block of southeast Sichuan basin, south China

• Shale gas reservoir was identified by log evaluation in CGB.
• Tectonic characterization impacts on shale gas accumulation.
• Properties of formation water affect gas storage capacity.
• Lower Cambrian formation pressure coefficient associated with shale gas production.

The abundant lower Cambrian marine shale gas resources discovered in south China is of great potential for exploitation. In particular, Cengong Block, which is located in a complex tectonic deformation region of southeast Sichuan basin, south China, is one of the marine shale gas exploring areas currently. In this area, folds and faults are well developed, mainly strike in NE or NNE directions. Lower Cambrian marine shale is characterized by great thickness and regional extensive distribution. Lower Cambrian strata in this region is mainly consisted of deep shelf deposits. Sandstone and limestone are determined based on GR log and neutron-density cross plot. Lower Cambrian shales in Cengong Block are associated with high GR value and TOC. Relatively low content of Thorium (Th) and Potassium (K) indicates low clay content. Neutron-density crossover is usually associated with gas-bearing shale beds. Brittle shale beds are well developed and the gas storage capacity of lower Cambrian shale reservoir has been affected by faulting and folding to some extent. Small-scale water-bearing caves are developed in the limestones and dolomites of lower Cambrian strata which usually displays low-pressure anomalies. Although the lower Cambrian shale reservoir in study area is characterized with distinct gas-bearing properties and shale gas flow has been obtained after fracturing, it is essential to recognize that the preservation conditions of shale gas accumulation are affected by tectonics, formation water and pressure coefficient.