Search

There are various sedimentary structures developed in the black shales of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in Luzhou area, southern Sichuan Basin, southwestern China. In order to evaluate the controlling effects of sedimentary structures on the key parameters of shale reservoir properties, collected classification of sedimentary structures, scanning electron microscope observation, analysis of shale fabric and porosity characteristics, low‐pressure nitrogen adsorption, methane isothermal adsorption, and conventional triaxial compression test were carried out based on shale samples from Wells A‐H1, B‐H1, C‐H1 and D‐H1 in the study area. The sedimentary structures recorded in these shales are categorized into five types: homogeneous bedding, weak horizontal bedding, strong horizontal bedding, discontinuous ripple bedding, and gravel‐bearing massive bedding. In general, the palaeo‐hydrodynamics and oxygen content of water columns were weak when homogeneous bedding shale was deposited. The organic matter is the most abundant and the total organic carbon content is up to 4.01% on average. The average quartz content is 43.5%, and the brittleness index is the highest. The porosity and adsorption capacity were the highest, with an average of 4.57% and 79.5 scf/ton, respectively. Combined with its mechanical properties of low Poisson's ratio and high Young's modulus, homogeneous bedding sedimentary structure is associated with the shale units possessing the highest organic matter abundance, the strongest reservoir property, and the best fracability, which is the priority target for exploration and development of the Wufeng to lower Longmaxi Formation in the Sichuan Basin, southwestern China. [ABSTRACT FROM AUTHOR]

The lower Silurian Longmaxi Formation hosts a highly productive shale gas play in the Zhaotong region of southern China. According to core observation, X-ray diffraction analyses, and scanning electron microscopy (SEM) observations, the shale comprises primarily quartz, carbonate minerals, and clay minerals, with minor amounts of plagioclase, K-feldspar, and pyrite. The clay mineral content ranges from 15.0% to 46.1%, with an average of 29.3% in the Zhaotong region. Organic geochemical analyses show that the Longmaxi Formation has good potential for shale gas resources by calculating total organic carbon, vitrinite reflectance, and gas content. Scanning electron microscope images demonstrate that reservoir pore types in the Longmaxi shale include organic pores, interparticle pores, intercrystalline pores, intraparticle pores, and fractures. Reservoir distribution is controlled by lithofacies, mineral composition, and geochemical factors. In addition, we investigated the relationships between reservoir parameters and production from 15 individual wells in the Zhaotong region by correlation coefficients. As a result, the brittleness index, total organic carbon (TOC), porosity, and gas content were used to define high-quality reservoirs in the Longmaxi shale. Based on these criteria, we mapped the thickness and distribution of high-quality reservoirs in the Longmaxi Formation and selected highlighted several key sites for future exploration and development. [ABSTRACT FROM AUTHOR]

The characterization of organic nanopores across the entire range is essential for gas storage and productivity evaluation of marine shale. To achieve a precise and comprehensive analysis of organic nanopore properties, an integration method of helium ion microscope (HIM) and scanning electron microscope (SEM), as well as focused ion beam-HIM (FIB-HIM) and FIB-SEM, was proposed. This integration method extends the organic pore imaging and quantitative analysis to below 10 nm. Results demonstrate that organic pores below 10 nm significantly impact the evaluation of shale gas storage and productivity by contributing to the porosity and specific surface area and altering pore size distribution and connectivity characteristics. The surface porosity increases by 10.73%, while the specific surface area increases by 76% according to the integration method. The integrated pore size distribution converts the unimodal PSD from the stitched SEM mosaic to a multimodal PSD, supporting the multi-stage formation mode of organic nanopores: larger bubble pores appear with oil generation at the early stage, while smaller spongy pores appear with gas generation at the later stage. The organic pores revealed by FIB-HIM have higher coordination numbers and a higher proportion of inner-connected volume, showing the more complex connectivity characteristics. Despite limitations due to heterogeneity and inadequate resolution, these novel insights by the integration method highlight the importance of exploring organic pores at the nanoscale and have the potential to inform further research in this field. • Organic nanopore size distribution in marine shale is corrected to a multimodal curve by the integration of HIM and SEM. • Organic nanopores beyond the SEM resolution significantly impact the evaluation of shale gas storage and productivity. • The integration of HIM and SEM provides more accurate and representative characteristics of organic pores in marine shale. [ABSTRACT FROM AUTHOR]