Sedimentary characteristics of the Lower Cambrian Shuijingtuo Formation in the E’Xi trough and its petroleum geological significance
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摘要:
中国南方下寒武统富有机质页岩具有整体厚度大、热演化程度高的特点,近期的页岩气勘探已经在中扬子地区获得下寒武统页岩气工业气流。为了进一步调查中国南方下寒武统页岩气资源潜力,以中扬子地区下寒武统水井沱组为重点,结合地表露头观察和钻井资料,研究分析了其沉积特征和沉积过程,划分了下寒武统页岩三级地层层序格架,构建了中扬子地区鄂西海槽早寒武世的沉积模式,并指明了中扬子地区下寒武统页岩气勘探方向。研究表明:中扬子地区下寒武统水井沱组主要发育浅水陆棚相和深水陆棚相沉积,其底部和顶部发育浅水碳酸盐岩沉积;下寒武统富有机质页岩可划分为5个三级层序,并可与上扬子地区进行对比;水井沱组发育于震旦纪—早寒武世鄂西海槽的充填阶段,主要为泥质碎屑岩沉积。富有机质页岩分布于深水相区,在鄂西海槽识别出恩施、宜昌长阳2个次级凹陷,是页岩气勘探的重点地区;下寒武统水井沱组与下伏震旦系灯影组和陡山沱组形成“三明治”式常规油气组合,是下一步鄂西常规油气勘查的重点领域。
Abstract:Objective The organic-rich shales of the Lower Cambrian in South China are characterized by significant thickness and high thermal maturity. Recent explorations have yielded industrial gas flows from the Lower Cambrian shales in the Middle Yangtze region. To further investigate the shale gas resource potential of the Lower Cambrian in South China, this study focuses on the Lower Cambrian Shuijingtuo Formation in the Middle Yangtze region.
Methods Through an integrated analysis of outcrop observations and drilling data, we analyzed its sedimentary characteristics and processes. A third-order stratigraphic sequence framework for the Lower Cambrian shale was established, and a sedimentary model of the Early Cambrian E’Xi Trough in the Middle Yangtze region was constructed, clarifying exploration targets.
Results The Lower Cambrian Shuijingtuo Formation in the Middle Yangtze region primarily consists of shallow-water and deep-water shelf facies, with shallow carbonate deposits at both the bottom and top. The Lower Cambrian organic-rich shales can be divided into five third-order sequences, which are correlatable with those in the Upper Yangtze region. The Shuijingtuo Formation formed during the filling stage of the E’xi Trough, primarily consisting of argillaceous clastic sedimentary rocks.
Conclusion Organic-rich shales are distributed in deep-water facies, with two identified subsags within the E’xi Trough (the Enshi and Yichang–Changyang depressions) serving as key exploration targets. The Lower Cambrian Shuijingtuo Formation, together with the underlying Sinian Dengying and Doushantuo Formations, forms a "sandwich-like" conventional hydrocarbon assemblage, which is a priority area for future conventional oil and gas exploration in Western Hubei.
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图 1 扬子地区早寒武世古地理图(据Wang et al.,2012;刘忠宝等,2018;赵建华等,2019修改)
Figure 1.
图 4 中扬子地区下寒武统层序地层划分柱状图(剖面位置见图1;据王传尚等,2012修改)
Figure 4.
图 9 中上扬子地区下寒武统富有机质页岩沉积模式图(上扬子地区数据据刘忠宝等,2018修改)
Figure 9.
表 1 中上扬子地区下寒武统地层对比表
Table 1. Stratigraphic correlation of the Lower Cambrian in the Middle–Upper Yangtze region
系 四川盆地 黔北 中扬子 寒武系 沧浪铺组 沧浪铺组 石牌组 筇竹寺组 上段 牛蹄塘组 三段 水井沱组 上段 下段 二段 下段 麦地坪组 一段 岩家河组/天柱山段 震旦系 灯影组 灯影组 灯影组 -
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