Lithofacies types and reservoir characteristics of Jurassic shale in the Sichuan Basin revealed by the Da’anzhai Member, Well Y2, Yuanba area

The continental shale strata are generally lithologically complicated and highly heterogeneous, making it difficult to pinpoint sweet spots and deploy exploratory wells. To evaluate the reservoir characteristics and gas-bearing capacity of different lithofacies types of Jurassic shale sequences in the Sichuan Basin, the Da’anzhai Member(Well Y2)in the Yuanba area was taken as an example. Experimental methods including TOC content measurement, whole-rock mineral composition analysis, thin section observation, FIB-SEM, mercury injection-N2 adsorption measurements as well as tests for physical properties were performed to classify the lithofacies types of shale and interlayers in the second sub-member of the Da'anzhai Formation(hereinafter referred to as the J1da2)and then to single out the lithofacies or lithofacies assemblages with the highest hydrocarbon potential based on physical properties, pore structure, gas content and occurrence as well as fracability. Results show that the shale can be divided into three categories and six sub-categories of lithofacies types, and the interlayers into two categories and six sub-categories for lithofacies types, which can be further grouped into three lithofacies assemblage types from a macro perspective. The shale contains mostly inorganic pores such as interlayer pores within clay minerals and dissolved pores of calcite, providing storage space for gas. The total shale gas content is calculated to be 2. 59−4. 38 m3/t, in which the free gas accounts for an average of 67 %, indicating a good exploration potential. However, the brittle mineral content of shale lithofacies is barely 50 %, indicating a poor fracability. It is concluded that type AB−Ⅰ lithographic assemblages in the sub-member are the most promising exploration targets as they are observed to contain well developed cleavage and lamellation fractures, favorable hydrocarbon generation conditions, higher gas content and proportion of free gas as well as brittle interlayers, all indicating the most promising exploration targets of all types. © 2022 Editorial Department of Oil and Gas Geology. All rights reserved.