The organic petrology of graptolites in sections parallel and perpendicular to the bedding: Implication for thermal maturity evaluation

IF 5.7 2区 工程技术 Q2 ENERGY & FUELS
Jin Wu , Qingyong Luo , Ningning Zhong , Fariborz Goodarzi , Yang Bai , Václav Suchý , Meijun Li , Lipeng Yao , Imran Khan , Ye Zhang , Haoran Wang , Haixu Liu , Yinghang Tang
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引用次数: 0

Abstract

Graptolites are widely presented in the Ordovician–Silurian Wufeng–Longmaxi Formations and other Lower Paleozoic marine shales throughout the world, and their reflectance is regarded as an effective index for the thermal maturity assessment of the graptolite-bearing shales devoid of vitrinite. Graptolites display distinct optically biaxial features, and the optical characteristics and reflectance of graptolites reveal significant differences depending on orientation of respective graptolite fragments relative to the bedding planes. However, specific studies on these optical discrepancies remains limited, resulting in confusion and/or substantial errors in thermal maturity assessment of graptolite-bearing shales. In this study, the organic petrological method was employed to compare the optical characteristics of graptolites in sections parallel and perpendicular to the bedding. Graptolites and in-source solid bitumen (SB) are the dominant organic components in the Alum and Wufeng–Longmaxi graptolite-bearing shales, and the liptinites are only detected in low maturity samples. In the studied sediments, both non-granular and granular graptolites were identified in abundance, but non-granular graptolites, which exhibit higher reflectance and stronger anisotropy than granular graptolites, were the predominant species. The intense positive correlations between the mean random reflectance (GRo) and true maximum reflectance (Rmax) and bireflectance of non-granular graptolites indicate their increasing anisotropy with maturation. Rmax values in sections parallel to the bedding are slightly lower than or equal to those in sections perpendicular to the bedding, and the mean maximum reflectance (R¯max), the intermediate reflectance (Rint), and bireflectance are lower than those in sections perpendicular to the bedding. GRo values in sections parallel to the bedding display a strong positive correlation with GRo in sections perpendicular to the bedding, and the former are higher than the latter. GRo exhibits stronger positive correlations with Rmax, R¯max, and bireflectance in sections perpendicular to the bedding than those in sections parallel to the bedding, revealing the stronger anisotropy of non-granular graptolites in sections perpendicular to the bedding. According to the reflectance discrepancies of non-granular graptolites in sections parallel and perpendicular to the bedding, several equivalent equations are proposed to evaluate the thermal maturity of Ordovician–Silurian graptolites-bearing sediments.
平行和垂直于层理剖面笔石的有机岩石学:热成熟度评价的意义
笔石广泛存在于奥陶系—志留系五峰组—龙马溪组及其他下古生界海相页岩中,其反射率被认为是评价无镜质组含笔石页岩热成熟度的有效指标。笔石具有明显的光学双轴特征,其光学特性和反射率随笔石碎片相对于层理面取向的不同而有显著差异。然而,对这些光学差异的具体研究仍然有限,导致含笔石页岩热成熟度评估的混乱和/或严重错误。本研究采用有机岩石学方法,比较了平行于层理和垂直于层理剖面上笔石的光学特征。明矾页岩和五峰—龙马溪页岩的主要有机成分为笔石和源内固体沥青(SB),只有在低成熟度样品中才检测到脂质岩。在沉积物中,非粒状笔石和粒状笔石均有丰富的分布,但非粒状笔石以反射率较高、各向异性较强的非粒状笔石为主。非粒状笔石的平均随机反射率(GRo)与真实最大反射率(Rmax)和双反射率之间呈显著正相关,表明其各向异性随成熟度的增加而增加。平行层理剖面的Rmax值略低于或等于垂直层理剖面的Rmax值,平均最大反射率(R¯max)、中间反射率(Rint)和双反射率均低于垂直层理剖面。平行层理剖面的GRo值与垂直层理剖面的GRo值呈正相关,且垂直层理剖面的GRo值大于垂直层理剖面的GRo值。垂直于层理剖面的GRo与Rmax、R¯max、双反射率的正相关较平行于层理剖面的强,说明垂直于层理剖面的非粒状笔石各向异性较强。根据平行层理剖面和垂直层理剖面上非颗粒笔石的反射率差异,提出了若干评价奥陶系—志留系笔石沉积层热成熟度的等效方程。
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来源期刊
International Journal of Coal Geology
International Journal of Coal Geology 工程技术-地球科学综合
CiteScore
11.00
自引率
14.30%
发文量
145
审稿时长
38 days
期刊介绍: The International Journal of Coal Geology deals with fundamental and applied aspects of the geology and petrology of coal, oil/gas source rocks and shale gas resources. The journal aims to advance the exploration, exploitation and utilization of these resources, and to stimulate environmental awareness as well as advancement of engineering for effective resource management.
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