Pore Structure and Gas Content Characteristics of Lower Jurassic Continental Shale Reservoirs in Northeast Sichuan, China

Abstract

The Jurassic shale in the northeastern Sichuan Basin is one of the main target intervals for continental shale gas exploitation. Research on the pore structure and gas-bearing properties of shales is the key issue in target interval optimization. Through core observation, geochemistry, bulk minerals, scanning electron microscopy, nitrogen adsorption, and isothermal adsorption experiments, various lithofacies with different pore structure characteristics were clarified. In addition, the factors that control gas-bearing properties were discussed, and a continental shale gas enrichment model was finally established. The results show that the Jurassic continental shale in the northeastern Sichuan Basin can be classified into six lithofacies. Organic pores, intergranular pores, interlayer pores in clay minerals, intercrystalline pores in pyrite framboids, and dissolution pores can be observed in shale samples. Pore structures varied in different shale lithofacies. The contact angle of shales is commonly less than 45°, leading to complex wettability of pores in the shales. Free gas content is mainly controlled by the organic matter (OM) content and the brittleness in the Jurassic shale. The adsorbed gas content is mainly controlled by the OM content, clay mineral type, and water saturation of the shales. The enrichment mode of the Lower Jurassic continental shale gas in the northeastern Sichuan Basin is established. Paleoenvironments control the formation of organic-rich shales in the center part of lakes. The “baffle” layer helps the confinement and high pressure, and the complex syncline controls the preservation, forming the enrichment pattern of the complex syncline-central baffle layer.

Control of Physical Properties by the Matching between Rock Components and Pore Structure in Shale Reservoirs of the Permian Lucaogou Formation, Jimusar Sag

“…In lacustrine shale oil reservoirs, the thermal maturity of OM tends to be relatively low, , resulting in poorly developed OM pores (Figure a). Hydrocarbon residual pores in solid bitumen are the main OM pores in the continental shale oil reservoirs. , Pores remaining in solid bitumen after hydrocarbon generation are a significant type of OM pore within these reservoirs. , They generally form at the contacts between OM and inorganic minerals or within solid bitumen in a distributed bubble-like manner, with small pore sizes, mainly as P mic (Figure b,e). The evolution of such hydrocarbon residual pores is significantly influenced by the level of maturity, with increased maturity correlating with enhanced pore development. ,, The resilience of brittle minerals to compression plays a crucial role in the preservation of these residual pores adjacent to solid bitumen. , Alongside the formation of hydrocarbon residual pores, organic acids are expelled, a process that contributes to the dissolution of certain less stable minerals, ,, further influencing the PS (Figure e).…”

“…25,59,60 The resilience of brittle minerals to compression plays a crucial role in the preservation of these residual pores adjacent to solid bitumen. 60,61 Alongside the formation of hydrocarbon residual pores, organic acids are expelled, a process that contributes to the dissolution of certain less stable minerals, 27,55,58 further influencing the PS (Figure 4e).…”

Study on the Differential Distribution Patterns of Fine-Grained Sedimentary Rocks in the Lower Third Member of the Shahejie Formation in Zhanhua Sag, Bohai Bay Basin

“…Classification schemes for fine-grained sedimentary rocks often use clay, felsic, and carbonate minerals as the three end members. These schemes divide fine-grained sedimentary rocks into four categories, namely claystone, siltstone, carbonate rock, and mixed fine-grained sedimentary rock, with the boundary set at a content of 50% for each end member [2,12,22,23]. However, the above scheme makes it difficult to accurately reflect the rocks' sources.…”

Clay mineral transformation mechanism modelling of shale reservoir in Da’anzhai Member, Sichuan Basin, Southern China

“…The mineral composition of the Da'anzhai Member in northeastern Sichuan is mainly quartz, clay minerals and calcite, among which clay minerals and quartz are generally high, and the relative content is greater than 35%, the content of calcite minerals varies widely, with high content in limestone and local enrichment (Xu et al, 2019;Liu et al, 2020;Jiang et al, 2023b).…”