Pore system classification of Jurassic Da’anzhai Member Lacustrine Shale: Insight from pore fluid distribution

Abstract

Lacustrine shale oil has the potential to lead the development of China's oil and gas industry. By integrating scanning electron microscopy, low-temperature CO2 and N2 adsorption, high-pressure mercury intrusion, and nuclear magnetic resonance with centrifugation at different speeds, the pore system and pore fluid distribution of Da’anzhai Member lacustrine shale in the Sichuan Basin are studied. The results show that: (1) The reservoir space is mainly inorganic pores and micro-fractures. Nano-micron scale pores are commonly found and widely distributed in the Da’anzhai shale with multiple peaks of 28 nm, 200 nm, 900 nm, and 3.5 µm. The total pore volume ranges from 0.00849 to 0.02808 cm³/g, and the pores ranging from 100 nm to 1000 nm are the main contributors to total pore volume. (2) Pore fluid can be divided into movable oil, bound oil, and adsorption oil. The proportion of movable oil, bound oil, and adsorbed oil is 21.4%, 12.4%, and 66.2% in Da’anzhai shale, respectively. Movable oil mainly occurs in pores larger than 350 nm, bound oil is 30–350 nm, while adsorbed oil mainly exists in pores below 30 nm. (3) The higher the total organic carbon content and clay minerals content, the smaller the pore size, resulting in the low content of movable oil. The higher the content of brittle minerals such as quartz, the better the development of intergranular pores and microfractures, and the higher the content of movable oil. Through the grading evaluation of shale pore structure and pore fluid, it is conducive to guide the exploration and development of Da’anzhai shale oil, which has important theoretical and practical significance.

Hydrocarbon Geological Characteristics and Factors Controlling Hydrocarbon Accumulation of Jurassic Da’anzhai Continental Shale

“…During J 1 dn deposition, the orogenic activity decreased, reducing the input of terrigenous debris and resulting in a subsiding basin [19]. The subsidence rate exceeded the sedimentation rate, forming the largest Jurassic lacustrine basin [16]. On this plane, semi-deep lacustrine facies dominated the central to eastern parts of the basin.…”

“…The single-well testing of the J 1 d shale led to significant hydrocarbon production [15]. Many studies have reported that J 1 dn has favorable conditions for shale oil accumulation, and some researchers have proposed that the lacustrine shale within the second sub-member of J 1 dn has high porosity, high brittle mineral content, and a high degree of pore fracture development, which benefit shale oil accumulation [16,17]. However, the actual exploration results indicate that the predicted potential has not been realized, and therefore further study is required to improve the poor-quality testing of the J 1 dn shale and determine the main controlling factors for shale oil enrichment.…”

Controlling factors of organic matter accumulation and lacustrine shale distribution in Lianggaoshan Formation, Sichuan Basin, SW China

“…In recent years, the practical experience of marine shale oil has been continuously applied to the geological studies of lacustrine shale hydrocarbon resources (Lu et al, 2012). Major breakthroughs in shale oil have been achieved in several sedimentary basins in western and northern China (Hu D. et al, 2021;Hu Z. et al, 2021;He et al, 2022;Cai et al, 2023;Lai et al, 2023). Breakthroughs have been obtained in the exploration of lacustrine shale hydrocarbons of the Middle Jurassic Lianggaoshan Fm.…”

Reservoir characterization of the shallow to deep Longmaxi formation in the Weiyuan Block, southwestern Sichuan Basin

“…(Zhu et al, 2021; 2022). Shale gas, which has a CH 4 content of more than 90%, remains in place as free gas in interparticle pores and microfractures as well as adsorbed gas attached to organic pores and clay nanopores (Cai et al, 2023; Chalmers and Bustin, 2015; Loucks and Dutton, 2019; Mastalerz et al, 2013). As such, gas-bearing shales belong to both self-sourced and self-stored reservoirs (Loucks and Dutton, 2019).…”