Abstract
The Lower Silurian Longmaxi Formation in the southern Sichuan Basin is composed of a series of dark carbonaceous shales deposited in a hydrostatic shelf reduction environment. In this study, the ratio of uranium to thorium (U/Th), the total organic carbon (TOC), and the biological silicon content (SiBio) were selected as the characteristic parameters to precisely analyze the sedimentary environment and its impact on reservoir quality. The results show that the Weiyuan area in the Early Silurian Longmaxi period experienced two transgression-regression cycles, forming two third-class sequences, SSQ1 and SSQ2, which can be divided into six sedimentary microfacies: organic-rich siliceous argillaceous shelf, organic-rich silicon-containing argillaceous shelf, organic-rich silty argillaceous shelf, deep-water silty argillaceous shelf, shallow-water silty argillaceous shelf, and shallow-water argillaceous silty shelf microfacies. The organic-rich siliceous argillaceous shelf and organic-rich silicon-containing argillaceous shelf microfacies developed in the deepest transgressive system tract (TST1), with high U/Th, high TOC, and high SiBio, which are identified as the main control facies for reservoir development. These two microfacies are located in the middle of the study area, while a transition occurs in the east affected by the Neijiang Uplift. According to the classification criteria proposed in this article, the favourable shale gas reservoirs in Weiyuan area are characterized with high U/Th (>1.25), high TOC (>3%), and high SiBio (>15%). This paper proposed an evaluation method for shale sedimentary facies based on elemental and electrical logging characteristics, avoiding the limitations of core samples, which makes the quantitative division of shale sediments and the efficient recognition of high-quality reservoirs available. It is of great significance for delineating the potential production areas in the study area and beneficial for the scaled development of shale gas reservoirs.
Highlights
Shale gas refers to the accumulation of free or absorbed natural gas in reservoir rocks dominated by organic-rich mudstone and shale [1,2,3]
Shallow-water argillaceous silty shelf and shallow-water silty argillaceous shelf microfacies are developed in the inner shelf with a uranium to thorium (U/Th) ratio less than 0.5, total organic carbon (TOC) content less than 2.0%, and biogenic silicon content index less than 2.0%, representing an oxidizing to weakly oxidizing sedimentary environment corresponding to a lowstand-transgressive systems tract
The current study is of great significance for the readers to delineate the potential production areas and amplify the scaled development of shale gas reservoirs, by providing a more efficient and quantitative categorization method for sedimentary microfacies in shale
Summary
Shale gas refers to the accumulation of free or absorbed natural gas in reservoir rocks dominated by organic-rich mudstone and shale [1,2,3]. Chen et al [23] compared the upper Permian Longtan Formation in the Qinglong area of western Guizhou based on rock type, mudstone colour, biological distribution, etc., based on the analysis of sedimentary facies from typical wells and typical sections They analyzed the sedimentary environment of the organic-rich shale. Mohammed et al [25] discussed the geochemical and petrological characteristics of shale samples from two wells in the Late Jurassic Arwa member in the Al-Jawf Basin, Yemen; analyzed the abundance of organic matter, kerogen type, thermal maturity, and biogenicity; and evaluated the organic matter source and environmental conditions during shale deposition using biomarkers and stable isotope analysis results. A comprehensive division of sedimentary microfacies is carried out based on elemental characteristics, electrical properties, and core analysis and laboratory data from 12 shale evaluation wells in the Lower Silurian Longmaxi Formation in the Weiyuan area of the southern Sichuan Basin, China. In-depth analysis of the relationship between microfacies and locations of high-yield wells is directed, the classification standard of the sedimentary microfacies in the Weiyuan area is recognised, the main control facies and belts for high shale gas production are elucidated, and the empathetic of the distribution rules of highquality reservoirs is excavated, providing a theoretical basis for the selection of favourable shale gas zones and welldrilling locations in this area
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