Characteristics of the deep and ultra-deep shale reservoirs of the Wufeng-Longmaxi formations in the southeastern Sichuan Basin and the significance of shale gas exploration

Abstract

Fruitful achievements have been made in shale gas exploration in the middle and deep areas from the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation in the southeastern Sichuan Basin, and in the research on shale reservoir characteristics and main controlling factors. As the key direction of current shale exploration, the deep and ultra-deep areas are limited by drilling and other factors, and the research on reservoir development characteristics and differences with the middle and deep shale reservoirs is insufficient. In order to clarify the characteristics of the deep and ultra-deep shale reservoirs of the Wufeng-Longmaxi formatiosns, four typical shale gas wells at the depth of 2 000-6 000 m in the southeastern Sichuan exploration area were selected to systematically carry out a comparative study of deep and ultra-deep shale reservoir development characteristics and differences, and the causes of pore development were discussed. The results show that: (1) With a burial depth of less than 6 000 m, the shale reservoirs of the Wufeng-Longmaxi formations are effective reservoirs with high porosity, and the porosity has no obvious change with the change of burial depth. However, there are certain differences in the morphology, structure and connectivity of organic matter pores, that is, with the increase of burial depth, the size of organic matter pores relatively decreases, and the pore connectivity deteriorates; (2) It has been clarified that biogenic silica is the foundation of pore development, and fluid overpressure is the key to maintaining reservoir pores. Under the combined action of biogenic silica and biogenic silica, the deep and ultra-deep shale reservoirs with high porosity and high quality can be developed and maintained; (3) Based on the research of reservoir development, shale gas exploration will be extended to 6 000 m, which provides a clear direction for shale gas exploration in the next step. It is preliminarily assessed that the deep and ultra-deep shale gas (buried depth of 4 000-5 000 m) resources in the Wufeng-Longmaxi formations in the Sichuan Basin and its surrounding areas exceed 2×1012 m3.

Keywords

• shale reservoir
• deep and ultra-deep shale gas
• wufeng-longmaxi formations
• upper ordovician
• lower silurian
• southeastern sichuan basin