Fine characterization and target window optimization of high-quality shale gas reservoirs in the Weiyuan area, Sichuan Basin

Abstract

Sinopec's Weiyuan Shale Gas Block is structurally located at the Baimazhen syncline held by the Weiyuan paleo-uplift and Ziliujing anticline in the Sichuan Basin. In this block, the Wufeng Fm of Upper Ordovician–Longmaxi Fm of Lower Silurian is an organic-rich dark shale deposit of deepwater shelf facies, whose litho-electric characteristics of geophysical logging are obviously different vertically and reservoir heterogeneity is strong. For providing a guidance for target window optimization and drilling trajectory tracking and adjustment of horizontal wells in the Weiyuan Shale Gas Block, parameter indexes were evaluated by refining the reservoir classification based on well logging subdivision, fine characterization of core laminae, high-precision geophysical prediction and genetic analysis of sedimentary microfacies. Furthermore, the “sweet spots” of shale gas reservoirs were predicted. Then, the target window was optimized and the trajectory of a horizontal well was designed. Finally, the effects of the target window of a horizontal well on shale gas productivity were evaluated. And the following research results were obtained. First, three types of laminae are developed in the high-quality shale reservoir at the bottom of Wufeng Fm–Long 1 Member, and they are vertically staggered and overlapped, which reflects the microscopic difference of sedimentary environment and reservoir quality. Second, shale gas reservoirs in this block can be divided into high-quality reservoirs, better reservoirs, general reservoirs and poor reservoirs. Third, the sublayer 2–31 at the bottom of Longmaxi Fm is biogenic sedimentary microfacies and it has the characteristics of “geological + engineering” sweet spot, e.g. high TOC contents, high porosity, high brittleness, high gas content and low in-situ stress difference, so it is classified as a high-quality reservoir. Fourth, actual drilling results show that the location selection of the target window of a horizontal well has a significant impact on single-well shale gas productivity, and the penetration rate of a high-quality reservoir is the key geological factor to achieve high-yield shale gas. The research results provide support for the evaluation of shale gas productivity and lay a foundation for the commercial development of shale gas in the Weiyuan Shale Gas Block.