Characteristics of Two Types of Longmaxi Gas-Shale Reservoirs in the Southern Sichuan Basin

Abstract

With the booming exploration and development of unconventional hydrocarbon resources in tight reservoirs, the comprehensive evaluation of shale gas reservoirs has become increasingly important. Fractures have a significant effect on the shale gas production because they not only provide storage space, but also are main flow paths. In this paper, a petrophysical model called dual-porosity medium system was built to calculate the fracture porosity in shales. The porosity in this model consists of matrix porosity (brittle minerals porosity, clay minerals porosity and organic matters porosity included) and fracture porosity. Based on the drilling wells data from the southern Sichuan Basin, the porosity components of the Lower Silurian Longmaxi shale (the Upper Ordovician Wufeng Formation included) have been analyzed by using the dual-porosity medium model. There are some differences in gas-shale reservoir quality between Changning gas field and Fuling gas field. Two types of gas-shale reservoirs, “matrix” pores type and “matrix + fracture” pores type, are fundamentally identified in this area. The characteristics of the first type, like Changning shale gas reservoirs, consists of medium total porosity (4%~6%), lower fracture porosity (0~0.22%) and lower permeability (0.00026mD~0.005mD). The clay minerals pores and organic pores are main storage space for this gas-shale reservoir type. However, the second type, such as Fuling shale gas reservoirs, contains medium-high porosity (5%~7.8%), high fracture porosity (0.54%~3.28%) and high permeability (0.65mD~7.21mD). These slippage effect fractures have linked with organic matter pores and clay minerals pores to form a 3D connection system. Considering two types of gas-shale reservoirs, the “matrix + fracture” pores shale gas reservoirs should be developed in the area with a special tectonic setting to from net fractures. It is difficult to explore another shale gas reservoir like Jiaoshiba gas-shale in the southern Sichuan Basin. As for the “matrix” pores shale gas reservoirs, they are widely developed in the slope zones and synclinal zones and predicted to be predominating in the marine shale gas distribution areas. To summarize, the dual-porosity medium model can help us to evaluate correctly shale gas reservoir and then gain high production in future.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017