Mineralogy and Pore Structure of Marine–Continental Transitional Shale: A Case Study of the Upper Carboniferous Keluke Formation in the Eastern Qaidam Basin, China

Yingchun Guo,Na Wang,Xinxin Fang,Haifeng Wang

doi:10.3389/feart.2021.825173

Abstract

Organic-rich shale and associated fine-grained sedimentary rocks of marine-continental transitional facies were well developed in the Upper Carboniferous Keluke Formation in the Eastern Qaidam Basin, which is expected to be a set of potential shale gas exploration and development target. Mineralogy and pore structure of marine-continental transitional shale were investigated systematically based on thin-section identification, X-ray diffraction (XRD), helium porosity test and pressure-pulse permeability measurement, scanning electron microscopy (QEMSCAN), field emission scanning electron microscopy (FESEM), and high-pressure mercury injection (MICP) and nitrogen adsorption. Thin section, XRD, and QEMSCAN data suggest that marine–continental transitional shale has complex mineral compositions, resulting in mixed rocks and mixed sequences. FE-SEM images show that interparticle and intercrystalline pores are popular in the Keluke Shales, with minor dissolution pores and microfractures. No secondary organic matter pores occur in the Keluke Shales because organic macerals are dominated by vitrinite and inertinite, where only primary pores can be found among organic matter frameworks. MICP and nitrogen adsorption indicate that pore size distributions follow a bimodal pattern and proportions of micro-scale pores and macro-scale pores increase in an order: bioclastic limestone, argillaceous bioclastic limestone, silty mudstone, argillaceous siltstone. The differences in pore structure are caused by sedimentary facies and associated mineralogy and diagenesis. This study can provide a crucial theoretical guidance for sweet spots determination and deep understanding of transitional shale gas potential.

Highlights

Organic-rich shales are primary targets for unconventional oil and gas exploration and development around the world (Jarvie et al, 2007; EIA, 2013; Zou et al, 2019)
Two shale samples were mineralogically imaged by Quantitative Mineral Evaluation via Scanning Electron Microscopy (QEMSCAN) at standard (5 μm) and high (1 μm) resolution
Core observation and thin section identification show that the Upper Carboniferous Keluke Formation in the Eastern Qaidam Basin varies greatly in lithology, forming typical mixed rock and mixed sequences (Figure 3), e.g., siltstone, fine sandstone, calcareous siltstone, argillaceous siltstone, silty mudstone, mudstone, carbonaceous mudstone, coal, bioclastic limestone, marlstone, shell-bearing silty mudstone, lime-bearing argillaceous siltstone, dolomite-bearing silty mudstone, siltybearing dolomite, and dolomite-bearing limestone

Summary

Introduction

Organic-rich shales are primary targets for unconventional oil and gas exploration and development around the world (Jarvie et al, 2007; EIA, 2013; Zou et al, 2019). These shales are classified into three types based on origins, including marine shales, marine–continental transitional shales, and continental shales (Zou et al, 2019; Dong et al, 2021). Transitional shales are characterized by a frequent lithology alternation in vertical because of rapid variation in water depth during deposition

Results

Discussion

Conclusion