Influence of the sedimentary environment of the Wufeng-Longmaxi shale on organic matter accumulation in the Dingshan area, Sichuan Basin

IF 2 3区地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY

Frontiers in Earth Science Pub Date : 2024-08-06 DOI:10.3389/feart.2024.1457377

Qiang Wang, Yue Feng, Ping Gao, Guangming Meng, Chengang Lu, Qizhang Fan, Gang Li, Yineng Tan, Xianming Xiao

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Abstract

The sedimentary environment and organic matter (OM) accumulation are vital indicators for shale gas exploration. However, research on deep shale gas systems is relatively limited; moreover, the exploration of deep shale gas in the southeastern Sichuan Basin has entered a period of stagnation. In this study, systematic geochemical analysis of Wufeng (WF) and the first member of the Longmaxi (Long-1) deep shale samples from the recently drilled DY7 well in the Dingshan area of the Sichuan Basin is carried out, and the longitudinal variations in major and trace elements are revealed. The differences in the WF, lower section of the Long-1 (Long-11) and upper section of the Long-1 (Long-12) shales are studied in terms of redox conditions, paleoproductivity, terrigenous detrital input, sedimentation rate and paleoclimate, and the different main controlling factors of OM accumulation for these three layers are discussed. The WF shale has a higher TOC content (mean: 5.73%), the Long-11 shale has a high TOC content (mean: 2.89%), while the Long-12 shale has a low TOC content (mean: 1.44%). For the WF shale, due to complex geological events and large fluctuations in element contents, its TOC content is poorly correlated with these indices, redox and paleoproductivity proxies have a positive association with the Long-11 shale’s TOC content, but negatively correlated with terrigenous input and sedimentation rate indices. The formation of these two sets of organic-rich shales (TOC > 2%) is jointly controlled by good preservation conditions. In contrast, the TOC content of the WF shale is higher than that of the Long-11 shale as the result that terrigenous input and sedimentation rate of the Long-11 shale represent the dilution and destruction of OM, which is different from the former. During the Long-12 depositional period, the water column experienced weak reducing conditions and low productivity, and its high terrigenous debris input further diluted the OM, leading to a low TOC content.

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四川盆地顶山地区五峰-龙马溪页岩沉积环境对有机质积累的影响

沉积环境和有机质（OM）积累是页岩气勘探的重要指标。然而，对深层页岩气系统的研究相对有限，而且四川盆地东南部深层页岩气勘探已进入停滞期。本研究对四川盆地鼎山地区近期钻探的 DY7 井的五峰（WF）和龙马溪（龙-1）深层页岩第一组样本进行了系统的地球化学分析，揭示了主要元素和微量元素的纵向变化。研究了WF页岩、龙-1（龙-11）下段页岩和龙-1（龙-12）上段页岩在氧化还原条件、古生产率、土著碎屑岩输入、沉积速率和古气候等方面的差异，讨论了三层页岩OM积累的不同主要控制因素。WF 页岩的 TOC 含量较高（平均值：5.73%），Long-11 页岩的 TOC 含量较高（平均值：2.89%），而 Long-12 页岩的 TOC 含量较低（平均值：1.44%）。对于 WF 页岩，由于地质事件复杂，元素含量波动较大，其 TOC 含量与这些指数的相关性较差，氧化还原和古生产率代用指标与 Long-11 页岩的 TOC 含量呈正相关，但与土著输入和沉积速率指数呈负相关。这两组富含有机质的页岩（TOC > 2%）的形成是由良好的保存条件共同控制的。相比之下，WF页岩的TOC含量高于Long-11页岩，这是因为Long-11页岩的土著输入和沉积速率代表了OM的稀释和破坏，与前者不同。在长-12沉积期，水体的还原条件较弱，生产力较低，其大量的土著碎屑输入进一步稀释了 OM，导致 TOC 含量较低。

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来源期刊

Frontiers in Earth Science Earth and Planetary Sciences-General Earth and Planetary Sciences

CiteScore

3.50

自引率

10.30%

发文量

2076

审稿时长

12 weeks

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