渝东南地区志留系黑色页岩层序地层研究

Sequence Stratigraphy of Silurian Black Shale and Its Distribution in the Southeast Area of Chongqing

中国南方地区主要发育下寒武统、上奥陶统—下志留统、下二叠统、上二叠统4套区域性黑色页岩,分布广泛,其中下志留统龙马溪组在渝东南地区最为发育。由于渝东南地区是我国页岩气勘探的先导试验区,因此分析渝东南地区龙马溪组黑色页岩的分布规律对研究该区页岩气分布特征尤为重要。通过对渝东南地区志留系进行岩石地层对比,对野外露头进行精细分析,建立研究区志留系层序地层格架,将志留系划分为8个三级层序,即ssq1、ssq2、ssq3、ssq4、ssq5、ssq6、ssq7、ssq8,再通过分析其横向展布特征,最终确立研究区北部和西部2个黑色页岩沉积中心。笔者主要从2个方面分析黑色页岩沉积富集的控制因素:①通过分析古地貌特征,分析有利于黑色页岩沉积的沉积环境;②通过分析龙马溪组黑色页岩短期相对海平面变化,得出短期相对海平面变化对有机碳含量和有机质类型的控制规律。该控制因素对于寻找页岩气赋存的有利层段具有重要的指示意义。

Four sets of black shale in lower Cambrian,upper Ordovician-lower Silurian,lower Permian and upper Permian are distributed in south China,especially the lower Silurian shale widely distributed in the southeast area of Chongqing.Since the southeast area of Chongqing is the pilot test district of the shale gas exploration in China,the distribution of lower Silurian Longmaxi Formation black shale is considerably important to find the industrial shale gas.By means of correlation of litho-stratigraphy and fine description of outcrop sequence stratigraphy in the southeast area of Chongqing,we setup the sequence stratigraphic framework of Silurian and divide into 8 third-order sequences,i.e.ssq1,ssq2,ssq3,ssq4,ssq5,ssq6,ssq7,and ssq8.Based on the horizontal distribution of black shale,we determine northern and western depocenters of lower Silurian shale.There are two controlling factors for the sedimentary distribution of black shale as follows：（1） favorable sedimentary environment and palaeogeomorphology,（2） short-term relative sea-level change which constrained the total organic carbon（TOC） and kerogen type.These controlling factors are significant to confirm the favorable section of shale gas accumulation.