川东红星地区二叠系吴家坪组页岩纹层特征及其对储集性能的影响

Laminae characteristics of shale and their influence on reservoir performance in Permian Wujiaping Formation of Hongxing area, eastern Sichuan Basin

  • 摘要: 川东红星地区二叠系吴家坪组页岩在纵向上存在多种纹层类型,目前针对吴家坪组页岩纹层特征及其对储集性能的影响,相关研究结论尚不明确。基于岩心观察描述、薄片鉴定、微区扫描电镜矿物定量评价(QEMSCAN)、氩离子抛光扫描电镜、常规物性、氮气吸附等分析测试手段,对吴二段不同类型纹层、纹层组合的特征及其对储层品质的影响展开研究。结果表明,吴二段页岩发育凝灰质纹层、硅质纹层、放射虫纹层、混合生屑纹层、泥晶方解石纹层5种纹层类型,以及“放射虫+硅质”、“泥晶方解石+凝灰质”2种纹层组合类型。其中,“放射虫+硅质”纹层组合总有机碳(TOC)含量均值10.87%,脆性矿物含量均值86.85%,储集物性及含气性均值分别为4.80%和6.67 m3/t,主要分布于③小层上部以及④小层下、中部;混合生屑纹层TOC含量均值7.37%,脆性矿物含量均值86.15%,储集物性及含气性均值分别为3.91%和4.35 m3/t,主要分布于③小层的中部;“泥晶方解石+凝灰质”纹层组合TOC含量均值4.21%,脆性矿物含量均值60.69%,储集物性及含气性均值分别为2.19%和2.57 m3/t,主要分布于③小层中、下部。此外,吴二段整体上还发育不具有纹层构造的块状硅质页岩、灰质页岩以及混合质页岩,其分别主要分布于④、⑤、③小层。不同类型页岩在脆性矿物含量、TOC含量、储集物性及含气性方面的对比结果显示,以“放射虫+硅质”纹层组合页岩最优,其次为硅质页岩、混合质页岩和灰质页岩,混合生屑纹层页岩、“泥晶方解石+凝灰质”纹层组合页岩最差。综上可知,吴二段③小层上部以及④小层下、中部为页岩气勘探开发的工程地质综合甜点层段。

     

    Abstract: Multiple shale laminae types exist vertically in the Permian Wujiaping Formation of the Hongxing area, eastern Sichuan Basin. Currently, research on the characteristics of shale laminae in the Wujiaping Formation and their influence on reservoir performance remains unclear. Based on core observation and description, thin-section identification, micro-area quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), argon ion-polished scanning electron microscopy, conventional physical property measurements, and nitrogen adsorption, the characteristics of different laminae types and laminaecombinations in the second member of the Wujiaping Formation and their influence on reservoir quality were studied. The results showed that the shale in the second member of the Wujiaping Formation developed five laminae types: tuffaceous laminae, siliceous laminae, radiolarian laminae, mixed bioclastic laminae, and mud crystal calcite laminae, as well as two laminae combination types: radiolarian + siliceous and mud crystal calcite + tuffaceous. Among them, the average total organic carbon (TOC) content of the radiolarian + siliceous combination was 10.87%, the average brittle mineral content was 86.85%, and the average reservoir properties and gas content were 4.80% and 6.67 m3/t, respectively, mainly distributed in the upper part of sub-layer ③ and the lower and middle parts of sub-layer ④. The average TOC content of the mixed bioclastic laminae was 7.37%, the average brittle mineral content was 86.15%, the average reservoir properties and gas content were 3.91% and 4.35 m3/t, respectively, mainly distributed in the middle part of sub-layer ③. The average TOC content of the mud crystal calcite + tuffaceous laminae combination was 4.21%, the average brittle mineral content was 60.69%, and the average reservoir properties and gas content were 2.19% and 2.57 m3/t, respectively, mainly distributed in the middle and lower parts of sub-layer ③. In addition, the second member of the Wujiaping Formation as a whole also developed blocky siliceous shale, calcareous shale, and mixed shale without laminae structures, mainly distributed in sub-layers ④, ⑤, and ③, respectively. Comparisons of different shale types in brittle mineral content, TOC content, reservoir properties, and gas content showed that the radiolarian + siliceous laminae combination shale has the best reservoir quality, followed by siliceous shale, mixed shale, and calcareous shale, while the mixed bioclastic laminae shale and the mud crystal calcite + tuffaceous laminae combination shale were the worst. In summary, the upper part of sub-layer ③ and the lower and middle parts of sub-layer ④ in the second member of the Wujiaping Formation are comprehensive engineering-geological sweet spot intervals for shale gas exploration and development.

     

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