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

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 44 μm to 5 nm). 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 25 masses (length: 1-100 μm, thickness: 0.5-2.0 μm) and discrete spheroidal particles (0.5-20.0 μm). Organic matter forms an inter-connected network where it exceeds a concentration between 6 and 18 wt%. Different pore types have different diameters and total pore volumes: inter-mineral pores (0.2 μm, 10-94 %), organic interface pores (0.2 μm, 2-77 %), intra-organic pores (0.05 μm, 1-40 %) and intra30 mineral pores (0.05 μm 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.