Abstract:

The Permian Dalong Formation in the northeastern Sichuan Basin contains a suite of high-quality marine shales， with shale reservoirs governing shale gas enrichment. In this study， we investigate the deep-shelf shales in this formation using data from scanning electron microscopy （SEM） of argon-ion milled shale samples， low-temperature nitrogen adsorption and desorption method， and mercury injection capillary pressure （MICP） technique. The geochemistry and mineralogy of shales in the Dalong Formation reveal the dominant factors controlling the occurrence of shale reservoirs in the Formation while comparing with those of other marine shale reservoirs in the Sichuan Basin， including the Silurian Longmaxi Formation， the 3^rd member of the Permian Maokou Formation （also referred to as the Mao 3 Member）， and the 2^nd member of the Permian Wujiaping Formation （also referred to as the Wu 2 Member）. The results indicate that high-quality shales occur in deep-shelf and basin facies in the northeastern Sichuan Basin. These shales show a ring-shaped distribution along the Kaijiang-Liangping shelf， with a thickness ranging from 20 to 35 m. The deep-shelf shales in the Dalong Formation comprise mixed siliceous and calcareous siliceous shales. In the Nanjiang area， the shales measure 20 to 30 m in thickness and are dominated by mixed siliceous shales. In the Puguang area， they exhibit greater thicknesses ranging from 30 to 35 m and consist primarily of calcareous siliceous shales. In contrast， near the central part of the Kaijiang-Liangping shelf， the shales are slightly thinner， around 20 m， and predominately comprise siliceous shales. The shale reservoirs in the Dalong Formation feature high total organic carbon （TOC） content， brittleness index， porosity， and gas content. As porous reservoirs dominated by organic pores， these reservoirs typically exhibit high porosity combined with ultra-low permeability， a large proportion of micropores， and excellent pore connectivity. The developmental degree and types of pores in the Dalong Formation shale reservoirs are determined by the sedimentary setting， as the pores therein preserved in the late stage under fluid overpressure. Compared to marine shales in other strata in the Sichuan Basin， the shales in the Dalong Formation exhibit a lithologic assemblage dominated by grayish-black siliceous shales intercalated with thinly laminated argillaceous limestones. The presence of numerous limestone intercalations vertically leads to high lithologic heterogeneity. While shales in different strata share similarities in their main storage space types， that is， organic pores dominate in all， the shale reservoirs in the Dalong and Longmaxi formations generally exhibit relatively lower porosity.

Key words: organic pore, reservoir characteristics, shale, Dalong Formation, Permian, northeastern Sichuan Basin, Sichuan Basin