Pore space characterization of organic-rich shales using BIB-SEM

Shales are the most abundant sedimentary rocks and as such they are important constituents of sedimentary basins. The characterization of the pore space and mi-crostructure of shales is crucial for many applications in geosciences. For example in the reservoir characterization of gas shales, in basin modeling studies, in understanding of sealing behavior and in hydrocarbon generation in organic-rich shales. Because these fine-grained rocks have relatively low porosity and permeability, under-standing of their petrophysical properties is difficult and requires new methods and techniques. One such newly developed method is the combination of Broad-Ion-Beam (BIB) milling and Scanning Electron Microscopy (SEM). This method allows imaging pores with resolution down to 5 nm in equivalent diameter, and quantitative measurement of porosity in representative elementary area on the scale of a few mm2.The aim of the dissertation (Chapter 1) is to investigate the porosity and microstruc-ture in organic-rich shales of the Posidonia Shale, Haynesville Shale and Bossier Shale formations with different maturities, using BIB-SEM. In more detail, the effect of grain size, mineralogy and maturity on pore space is investigated and the connectivity of the pore space is assessed. Chapter 2 describes complementary techniques like Focused Ion Beam (FIB)-SEM, Micro Computed Tomography (MicroCT), Mercury Intrusion Porosimetry (MIP) and Wood’s Metal Injection (WMI). The BIB-SEM method and workflow is explained in detail in Chapter 3.The first BIB-SEM study (Chapter 4) reports on two early mature (VRr = 0.59 and 0.61) samples of Posidonia Shale from the Hils Syncline in Germany. Pore morphologies and pore sizes are clearly related to the mineral phases. The porosity resolvable by BIB-SEM is 2.75 and 2.74 %. Pore size distribution can be described by a power law function. The pores in the carbonate fossils show a dual-power law distribution. By extrap-olating the power-law distribution for a sample, total porosity is estimated and can be compared with values gained by MIP. Comparison between the imaged porosity and the MIP porosity suggest a very high pore body to pore throat ratio. This results in a pore model where large pores, represented mainly by pores in fossils and calcite grains, are connected via a low-porous clay-rich matrix with pore throats below 10 nm.In the following chapter (Chapter 5) four Haynesville Shale and four Bossier Shale samples of different maturity, fabric and mineralogy are described using BIB-SEM. This variability within the samples enabled to study the controls on the porosity distribution in these shales. Pores exist as intraparticle pores mainly in carbonate grains and py-rite framboids and as interparticle pores, mainly in the clay-rich matrix. Pore sizes are power law distributed and a positive trend of organic-matter porosity with maturity was found. Porosities measured by BIB-SEM are significantly lower than porosities obtained by MIP. In Chapter 6 an alternative method of MIP is presented. WMI in combination with BIB-SEM allows imaging of the solid Wood’s Metal (WM)-filled pores. WMI was conducted at 316 MPa. The samples were BIB polished followed by high resolution SEM imaging. SEM investigations showed WM-filled pores down to at least 10 nm and almost full impregnation of the WM in the Boom Clay sample and heterogeneous impregnation in the Bossier Shale sample. The Opalinus Clay and Haynesville Shale samples showed WM mainly in cracks and in larger pores directly adjacent to those cracks. These first results suggest that many published MIP data on mudstones could contain serious methodological artifacts and reliable metal intrusion porosimetry requires a demon-stration that the metal has entered the pores. Moreover, results show that, in particu-lar for the Opalinus Clay, the pore throats are actually smaller than suggested by Mercury data, resulting in higher capillary resistance and thus a higher sealing capacity. Chapter 7 reports on the microstructural characterization of porosity, mineralogy and organic matter in 2 and 3 dimensions of an organic-rich Haynesville Shale sample. Sample size varies from several centimeters down to 10 micrometers in size using op-tical microscopy, BIB/FIB-SEM, MicroCT and WMI. The sample consists of a heterogeneous mineralogy and fabric, uniformly distributed on the centimeter scale except for one millimeter-size carbonate-rich layer. Organic matter shows the largest pore networks, up to 8 μm in length. The organic matter is connected throughout the sample and is therefore the most probable controlling factor for gas transport.In Chapter 8 presents the microstructural characterization of porosity of two organic-rich Posidonia Shale samples with different maturates (VRr = 0.91 and 1.52), using BIB-SEM. Results show that both samples have a similar pore size distribution, but the ma-ture sample contains a lower visible porosity (0.82 % vs. 2.47 %). The postmature sample contains intra-organic-matter pores, which are notably different from the mature sample that contains crack-type porosity at the organic-matter – mineral interface. The latter are interpreted either to be due to shrinkage because of devolatization or hydraulic fracturing because of hydrocarbon generation. The transition from these crack-type porosity into intra-organic-matter pores is interpreted because of thermal maturation. Final concluding remarks are given in the last chapter (Chapter 9).