Multi-scale 3D characterisation of porosity and organic matter in shales with variable TOC content and thermal maturity: Examples from the Lublin and Baltic Basins, Poland and Lithuania

Lin Ma,Kevin G Taylor,Patrick J Dowey,Loic Courtois,Ali Gholinia,Peter D Lee

doi:10.1016/j.coal.2017.08.002

Lin Ma, Kevin G Taylor + Show 4 more

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https://doi.org/10.1016/j.coal.2017.08.002

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Abstract

Understanding the distribution of pores and organic matter with varying organic matter concentrations and maturity is essential to understanding fluid flow in shale systems. Analysis of samples with low, medium, and high total organic carbon (TOC) and varying maturities (gas-mature and oil-mature) enables the impact of both organic matter concentrations and thermal maturation on organic matter porosity to be examined. Three gas-mature samples of varying TOC (Lublin Basin) and one oil-mature sample (Baltic Basin), both with similar mineral compositions, were selected from the same formation. Samples were imaged in 3D over four orders of magnitudes (pixel sizes from 44μm to 5nm). A combination of X-ray computed tomography (XCT) and Focus Ion Beam Scanning Electron Microscopy (FIB-SEM) enabled the morphologic and topological characteristics of minerals, organic matter and pores to be imaged and quantified.In the studied samples, organic matter primarily has two geometries: lamellar masses (length: 1–100μm, thickness: 0.5–2.0μm) and discrete spheroidal particles (0.5–20.0μm). Organic matter forms an inter-connected network where it exceeds a concentration between 6 and 18wt%.Different pore types have different diameters and total pore volumes: inter-mineral pores (0.2μm, 10–94%), organic interface pores (0.2μm, 2–77%), intra-organic pores (0.05μm, 1–40%) and intra-mineral pores (0.05μm diameter, 1–2% of total porosity). The major pore system in the studied shales is composed of inter-mineral pores which occur between clay mineral grains. TOC concentration influences the total volume of organic matter-related pores while maturity controls the presence of intra-organic pores. The study improves the understanding of the relationship of organic matter concentrations, maturity and pore systems in shales. This study characterises porosity and organic matter distributions in 3D; it also improves the understanding of the relationship of organic matter concentrations, maturity and pore systems in shales.

Highlights

Technological advances in horizontal drilling and hydraulic fracturing have paved the way for the exploration and production of shale gas and shale oil
Between the two high total organic carbon (TOC) samples, L1 has less granular minerals, higher clay minerals and similar organic matter content compared with B1
The high TOC L1 and B1 samples possess thick laminations (~ 5.0 mm), and partially-aligned clay minerals, with elongated organic matter particles present in the matrix (Fig. 2A and D)

Summary

Introduction

Technological advances in horizontal drilling and hydraulic fracturing have paved the way for the exploration and production of shale gas and shale oil. Organic matter morphology and structure can vary on a micron to sub-micron scale (Curtis et al, 2014) and the extent and connectivity of organic matter has been reported to affect both storage capacity and permeability in shales (Curtis et al, 2014). This is due to the perceived importance of organic matter-related pores and the gas sorption and diffusion properties of organic matter (Curtis et al, 2014; Sondergeld et al, 2013). The small sizes of these components coupled with the highly heterogeneous nature of shales make imaging and accurate quantification of pore and organic structures challenging

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