Shale reservoir characteristics of Ordovician Wufeng-Silurian Longmaxi formations in Dangyang synclinorium, Middle Yangtze region

The petrological and organic geochemical characteristics, reservoir porosity types and features, and gas-bearing characteristics of organic-rich shale were systematically studied using FIB-SEM, mineral contentanalysis, organic geochemical analysis and low-temperature and low-pressure nitrogen adsorption porosity determination, and drilling data of typical wells in the Dangyang synclinorium. (1) Vertically, shale reservoirs from the Wufeng Formation(O₃w) are quite different from the first section of the first member of Longmaxi Formation (S₁l₁¹). High-quality shale reservoirs are developed from S₁l₁¹⁽¹⁾ to S₁l₁¹⁽²⁾ sublayers, with a high abundance of organic matter (2.45%-6.98%), type Ⅱ₁-Ⅱ₂ organic matter, rather high thermal evolution degree, high brittle mineral content (34.0%-85.5%), low clay mineral content (9.0%-40.3%), and high total gas content (1.76-4.30 m³/t). Organic pores are well developed. In the S₁l₁¹⁽³⁾ sublayer, affected by the intensified Caledonian tectonic activity, terrigenous material input increased, and reservoir quality began to deteriorate. (2) Horizontally, organic-rich shale of the Wufeng-Longmaxi formations in the Dangyang synclinorium become thicker from southeast to northwest, and the reservoir quality becomes better. (3) Shale reservoirs in the S₁l₁¹⁽¹⁾-S₁l₁¹⁽²⁾ sublayers in the study area are the "sweetest" and the "most brittle".However, considering the convenience of real-time monitoring and adjustment of well trajectory during horizontal drilling, the upper and lower half of GR peak in the S₁l₁¹⁽¹⁾ sublayer should be selected as the best horizontal target of horizontal wells.