Evidence for Biogenic Silica Occurrence in the Lower Silurian Longmaxi Shale in Southeastern Chongqing, China

Yutao Liang,Zhen Li,Yang Liu,Xuan Tang,Sen Yang,Jianghui Ding,Yang Wang,Jinchuan Zhang

doi:10.3390/min10110945

Abstract

The gas shale in the Lower Silurian Longmaxi Formation contains a considerable amount of biogenic silica. Various originated silicas in shale, derived from different depositional environment, are commonly associated with different degrees of organic matter enrichment, resulting in different mechanical and physical properties of shale reservoirs. Thin section identification, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), total organic carbon (TOC) analysis, X-ray diffraction (XRD) analysis, and X-ray fluorescence (XRF) spectroscopy were used to investigate the Lower Silurian Longmaxi shale from Well Yuye 1 in southeastern Chongqing, China to obtain a better understanding of the origin of silica in the Longmaxi Shale. The results show ubiquitous cryptocrystalline silicas with poorly crystalline morphology, which differs from that of the detrital silica, authigenic silica, and hydrothermal silica, proving that the cryptocrystalline silicas may have a biogenic origin. Major element and mineral composition analysis indicate no correlations between K2O/Al2O3 and SiO2/Al2O3 and between illite and SiO2, and negative correlations between TiO2 and SiO2/Al2O3, between illite and quartz and excess Si, and between Al2O3 and excess Si, and all samples being located in the area of non-hydrothermal origin in the Al-Fe-Mn diagram, excluding silicas of terrigenous detrital origin, clay mineral transformed origin, and hydrothermal origin. Moreover, the fact that almost all samples plot above the illite Si/Al line in the cross-plot of Si versus Al and the mean values of Al/(Al + Fe + Mn) and Si/(Si + Al + Fe + Ca) are close to the values of biogenic silica prove that the silicas are primarily of biogenic origin. Positive correlations between TOC and quartz and excess Si and numerous siliceous organisms are observed, indicating that the silicas are associated with siliceous organisms. The postmortem siliceous organisms underwent silica diagenesis via a dissolution-precipitation mechanism following the sequence of opal-A → opal-CT → cryptocrystalline biogenic silica as the burial depth and temperature increased.

Highlights

After the Hirnantian glaciation at the end of Ordovician, a global eustatic transgression led to deposit a group of black shales in many regions of the world [1]
The vitrinite reflectance (Ro) value is in the range of 1.62%−2.26%, with an average of 2.04% [35], indicating that the shales of Longmaxi Formation belong to high-over mature shales
It is obviously different in the morphology from terrigenous detrital silica, the authigenic silica formed during smectite-illite transformation, and the hydrothermal silica [11,12,47]

Summary

Introduction

After the Hirnantian glaciation at the end of Ordovician, a global eustatic transgression led to deposit a group of black shales in many regions of the world [1]. Lower Silurian organic-rich shales were deposited during Rhuddanian times In China, the black shales are widely distributed in the Upper Yangtze region and usually exist in the Longmaxi Formation of the lower part of the Silurian Llandovery strata, which were mainly the products of large-scale anoxic events [1,3]. The Lower Silurian Longmaxi shale in southern China is characterized by widespread gas-bearing, large-area enrichment, and high yield [4]. Dong et al (2014) suggested that the controlling factors of shale gas enrichment and high yield in Longmaxi Formation included: (1) stable distribution of deep-water shelf facies shale; (2) high organic matter abundance and moderate thermal evolution;

Methods

Discussion

Conclusion