Characteristics and controlling factors of nano pores in shale reservoirs of Wufeng-Longmaxi formations in southern Sichuan Basin: insights from Shuanghe outcrop in Changning area

Abstract

Shale reservoirs within the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation(Wufeng-Longmaxi shales) have been one of the main targets for shale gas exploration and exploitation in the south of Sichuan Basin. This study focuses on the Wufeng-Longmaxi shales of the Shuanghe outcrop in Changning area, southern Sichuan Basin. Measurements including TOC, mineral composition, carbon dioxide and nitrogen adsorptions and FE-SEM were conducted on 41 samples to depict the nano porosity within the Ordovician Baota limestones and Wufeng-Longmaxi shale reservoirs and hence to clarify the regulating factors of the occurrence of nano pores. The Wufeng-Longmaxi shales are rich in organic matter. The mineral composition is dominated by biogenic quartz and carbonate minerals, followed by clay minerals and a small amount of feldspar. The nano pores in Wufeng-Longmaxi shales are dominated by silts, with pore diameters mainly distributed between 0.3-0.9, 40-50 and 100-200 nm, mainly organic pores, followed by mineral matrix pores. The occurrence of nano pores is controlled by the contents of organic matter, quartz and carbonates. Total pore volumes of selected samples are strongly correlated with TOC and quartz contents positively and carbonate contents negatively, and have no correlation with clays or feldspar. This indicates that nano pores within these samples are dominated by organic matter pores. The Ordovician Baota limestones are depleted in various nano pores, hence resulting in a good underlying sealing layer of Wufeng-Longmaxi shale reservoirs.

Keywords

• nano pore
• shale gas
• reservoir
• wufeng-longmaxi formations
• shuanghe outcrop
• sichuan basin