--> --> Controlling Factors for Shale Gas Geological Sweet Spots in Wufeng-Longmaxi Fm, Southern Sichuan Basin, China

2018 AAPG International Conference and Exhibition

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Controlling Factors for Shale Gas Geological Sweet Spots in Wufeng-Longmaxi Fm, Southern Sichuan Basin, China

Abstract

At the end of 2017, shale gas production from Wufeng-Longmaxi formation in China has reached 90×108m3. Great heterogeneity still exists in Wufeng-Longmaxi formation as most shale gas were produced from the lower part of formation. Therefore, it’s very necessary to understand the characteristics and controlling factors of the sweet spots to make sure the following drilling wells are targeted at the right intervals. Little change of sedimentary environment makes it difficult to discriminate the fine-grained shale lithology and also brings comparison difficulty in both horizontal and vertical direction. However, graptolites are well developed and preserved in the whole Wufeng-Longmaxi formation, which are useful for reservoir parameters comparison under the isochronous stratigraphic framework. In this paper, thin section examination, XRD, SEM determination with cathodoluminescence and geochemical analyses were combined to investigate Wufeng-Longmaxi shale. We follow the graptolite biozonation reported by Chen et al (2000). The TOC measurement shows that organic-rich shale are deposited in Katian and Rhuddanian stages with TOC ranging from 2.2% to 5.6%. Siliceous shale is the major component for Rhuddanian and Katian stage, while argillaceous and silty shales are more dominant for other stages. Both Katian and Rhuddanian stages are featured by high Babio ranging from 1800 to 10000μg/g. But there is no correlation between TOC and Babio. Redox-sensitive element and trace elemental redox indices suggest that the water reducibility in Katian and Rhuddanian stages is much stronger than other stages. And element ratios generally display a positive correlation with TOC, which means preservation rather than paleoproductivity controls the TOC enrichment. The cathodoluminescence analysis reveals that up to 60% of quartz in siliceous shale has an authigenic origin. Positive correlation between authigenic quartz and TOC suggests the migrated organic matter (OM) fill in quartz aggregates is the principal matrix for OM pore development. These authigenic quartz can also resist to compaction and preserve the internal pore structure, resulting in larger pore size and more shale gas storage in siliceous shale. In summary, the high paleoproductivity, strong reducing environment, and high content of authigenic quartz aggregates control the sweet spots development which makes shales of Katian and Rhuddanian stages are favorable targets for horizontal well drillings.