| With the deepening of shale gas exploration and development, shale gas fields in the Weiyuan area of the
southwestern Sichuan Basin have shown great resource potential. However, gas wells with high test production reveal
rapid decline rate and low estimated ultimate recovery (EUR) of single well. To clarify the geological reasons, the main
factors affecting the enrichment and high production of deep shale gas are discussed from the perspective of source-cap
controlling hydrocarbon based on the deep shale gas accumulation conditions as well as the exploration and development
practices in the Weiyuan area. The results show that: (1) The sedimentary environment of Wufeng Formation-Longmaxi
Formation in Weiyuan area is controlled by the central Sichuan paleo-uplift and the Ziliujing underwater highland, and
multiple graptolite shale intervals is thinned or missing in varying degrees. The LM1-LM4 graptolite shale interval
developed in anoxic environment with relatively large paleowater depth is generally thin (1.8-4.5 m). The lithology is
black gray carbonaceous shale and gray siliceous shale with high carbonate minerals content and low siliceous minerals
content. The anti-compaction ability of reservoir is weak, and the shale quality is slightly worse than that in Jiaoshiba and
Changning areas. (2) The dissolution pores of carbonate minerals in shale of Wufeng Formation-Longmaxi Formation in
Weiyuan area are developed and formed after the maximum oil generation period of kerogen, which is not conducive to
shale gas enrichment. (3) In the later stage, the strata were uplifted and denuded strongly, part of the Triassic gypsum salt
rock acting as regional cap was denuded, so the regional sealing was damaged to a certain extent, but the preservation
conditions were better in the area far from the paleo-uplift. (4) The deep shale gas reservoir in Weiyuan area has large
buried depth, high temperature and pressure, and the proportion of micropores, mesopores and other small pores with large
specific surface area is reduced due to the influence of higher overlying formation pressure, resulting in the reduction of
methane adsorption capacity of shale, and the gas occurs mainly as free gas. It is considered that the source-cap condition
is an important geological reason for the high trial yield but poor stable production capacity of deep shale gas of Wufeng
Formation-Longmaxi Formation in Weiyuan area. It is pointed out that the area with large thickness of high-quality shale
of LM1-LM4 graptolite intervals, relatively stable local structure and complete preservation of gypsum salt caprock of
Middle and Lower Triassic is a favorable area for shale gas enrichment. |
