An enhanced understanding of the Basinal Bowland shale in Lancashire (UK), through microtextural and mineralogical observations
An enhanced understanding of the Basinal Bowland shale in Lancashire (UK), through microtextural and mineralogical observations
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DOI：
10.1016/j.marpetgeo.2017.07.030
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发表时间：
2017-09
影响因子：
4.2
通讯作者：
A. Fauchille;Lin Ma;E. Rutter;M. Chandler;P. Lee;K. Taylor
中科院分区：
文献类型：
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作者：
A. Fauchille;Lin Ma;E. Rutter;M. Chandler;P. Lee;K. Taylor
本文首次利用光学和扫描电镜（OM， SEM）、x射线衍射（XRD）和总有机碳含量（TOC）测量，从厘米到微米尺度研究了Lower Bowland页岩微观结构的变化。在英国兰开夏郡Preese Hall-1井中，Lower Bowland页岩以及下伏的Pendleside石灰岩和Worston页岩地层的岩石中观察到大量的微观结构、有机质颗粒和裂缝类型。识别出4种微结构类型：无层状富石英泥岩；富石英、黄铁矿层状泥岩；富石英、黄铁矿层状泥岩；弱夹层富方解石泥岩。有机质颗粒根据其大小、形状和位置可分为四种类型：有或没有大孔隙的多微米颗粒；水泥中和粘土矿物之间的微米颗粒；multi-micrometre层;以及大孔隙中的有机物。裂缝分为碳酸盐岩封闭裂缝；bitumen-bearing骨折;resin-filled骨折;空洞的裂缝。研究认为，在热成熟过程中，水平沥青裂缝主要受超压、应力松弛、压实和侵蚀卸荷作用的影响，而垂向含沥青裂缝、充树脂裂缝和空裂缝可能受前期脉纹发育过程中产生的垂向弱节理的影响。对于大多数样品来说，高TOC （20 wt%）、低粘土含量（<20 wt%）、高石英比例（50 wt%）和多尺度裂缝网络的存在支持了人们对Bowland页岩作为潜在可开发油气来源的兴趣。本研究中所做的微观结构观察突出了埋藏期间鲍兰德页岩层序中流体通道或循环的初步证据。
Variability in the Lower Bowland shale microstructure is investigated here, for the first time, from the centimetre to the micrometre scale using optical and scanning electron microscopy (OM, SEM), X-Ray Diffraction (XRD) and Total Organic Carbon content (TOC) measurements. A significant range of microtextures, organic-matter particles and fracture styles was observed in rocks of the Lower Bowland shale, together with the underlying Pendleside Limestone and Worston Shale formations encountered the Preese Hall-1 Borehole, Lancashire, UK. Four micro-texture types were identified: unlaminated quartz-rich mudstone; interlaminated quartz- and pyrite-rich mudstone; laminated quartz and pyrite-rich mudstone; and weakly-interlaminated calcite-rich mudstone. Organic matter particles are classified into four types depending on their size, shape and location: multi-micrometre particles with and without macropores: micrometre-size particles in cement and between clay minerals; multi-micrometre layers; and organic matter in large pores. Fractures are categorized into carbonate-sealed fractures; bitumen-bearing fractures; resin-filled fractures; and empty fractures. We propose that during thermal maturation, horizontal bitumen-fractures were formed by overpressuring, stress relaxation, compaction and erosional offloading, whereas vertical bitumen-bearing, resin-filled and empty fractures may have been influenced by weak vertical joints generated during the previous period of veining. For the majority of samples, the high TOC (>2 wt%), low clay content (<20 wt%), high proportion of quartz (>50 wt%) and the presence of a multi-scale fracture network support the increasing interest in the Bowland Shale as a potentially exploitable oil and gas source. The microtextural observations made in this study highlight preliminary evidence of fluid passage or circulation in the Bowland Shale sequence during burial.