- Green River (Mahogany Zone) oil shale is visualized and quantified in 2-D and 3-D.
- The impact of temperature on oil shale pore growth is studied from 300 to 500 °C.
- 3-D visualization of the pore space in organic-rich and organic-lean regions.
- Quantified the representative sample size at which porosity remains constant.
Tarik Saif, Qingyang Lin, Branko Bijeljic, Martin J. Blunt
Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK
Received 13 August 2016 – Accepted 13 February 2017
The microstructural evaluation of oil shale is challenging which demands the use of several complementary methods. In particular, an improved insight into the pore network structure and connectivity before, during, and after oil shale pyrolysis is critical to understanding hydrocarbon flow behavior and enhancing recovery. In this experimental study, bulk analyses are combined with traditional and advanced imaging methods to comprehensively characterize the internal microstructure and chemical composition of the world’s richest oil shale deposit, the Green River Formation (Mahogany Zone).
Image analysis in two dimensions (2-D) using optical and scanning electron microscopy (SEM), and in three dimensions (3-D) using X-ray microtomography (µCT) reveals a complex and variable fine-grained microstructure dominated by organic-rich parallel laminations of the order of 10 µm thick which are tightly bound in a highly calcareous and heterogeneous mineral matrix. We also report the results of a detailed µCT study of the Mahogany oil shale with increasing pyrolysis temperature (300–500 °C) at 12 µm and 2 µm voxel sizes. The physical transformation of the internal microstructure and evolution of pore space during the thermal conversion of kerogen in oil shale to produce hydrocarbon products was characterized.
The 3-D volumes of pyrolyzed oil shale were reconstructed and image processed to visualize and quantify the volume and connectivity of the pore space. The results show a significant increase in anisotropic porosity associated with pyrolysis between 400 and 500 °C with the formation of micro-scale connected pore channels developing principally along the kerogen-rich lamellar structures. Given the complexity and heterogeneity of oil shale, we also characterize the representative size at which porosity remains constant. Our results provide a direct observation of pore and microfracture development during oil shale pyrolysis and the petrophysical measurements from this study serve as valuable input parameters to modeling oil shale pyrolysis processes.
The increasing global demand for liquid fuels has driven unconventional petroleum resources to play an important role in the world’s energy portfolio . Oil shale, an organic-rich fine grained impermeable sedimentary rock represents a large and mostly untapped unconventional hydrocarbon resource with many known deposits across the world , , . The lacustrine Eocene Green River Formation, located in Colorado, Utah, and Wyoming, is the largest known oil shale deposit in the world with an estimated 4.3 trillion barrels of oil originally in place . In the context of petroleum systems, organic-rich rocks have traditionally been considered as non-reservoirs, often fulfilling the critical role as source rocks for hydrocarbon accumulations.
However, the potential for oil shale as a direct hydrocarbon resource has been realized, creating an emphasis on effectively releasing this fuel , , , . Oil shale consists of highly cross-linked macromolecular organic material in the form of kerogen, a complex structure of carbon, hydrogen, and oxygen with lesser amounts of sulfur and nitrogen that is essentially insoluble in organic solvents or aqueous solutions , . To convert kerogen that has not thermally matured beyond the diagenesis stage into shale oil and combustible gas, it must be heated, in the absence of oxygen (pyrolysis), to high temperatures 300–500 °C where chemical bonds are broken yielding lighter hydrocarbon molecules , .
Oil shale can either be mined and retorted under anaerobic conditions in large processing facilities at temperatures near 500 °C for times on the order of an hour, or processed in place using downhole heaters at 300−350 °C from days to months , , , , .
Oil shale pyrolysis process involves complex physical and chemical reactions , , , . Many experimental studies have been conducted on oil shale pyrolysis to better understand the effect of fundamental parameters on shale oil yield and quality to enhance the reaction conditions. This includes composition of source material , , pyrolysis temperature , , , heating rate , , , , residence time for pyrolysis reaction , , particle size , , , mineral matrix ,  and composition of pyrolysis atmosphere , . Pyrolysis under vacuum conditions has been shown to improve both oil yield and quality compared to pyrolysis under atmospheric pressure , , , . Vacuum pressures accelerate the transport of pyrolysis products by providing faster escape of primary oil from the reaction zone, therefore reducing the occurrence of secondary cracking reactions , .
As our understanding of the physical and microstructural properties of oil shale rocks continues to be enriched, it has become clear that these rocks comprise compositional and structural heterogeneity; especially at small length scales. These heterogeneities include variations in mineralogy, organic matter distribution, and pore properties, which makes the characterization of physical parameters of oil shale challenging. In particular, there are key questions related to the structure and connectivity of the pore space during oil shale pyrolysis which directly impact hydrocarbon flow behavior through the pore channels and ultimate recovery.
Therefore, it is important to comprehensively explore and quantify the evolution of the pore space with increasing pyrolysis temperature. Previous experimental studies have used nitrogen adsorption-desorption isotherms to estimate the pore properties of Green River (Western USA) , New Albany (Eastern US)  and Huadian (China) oil shales , , , .
A broad range of imaging methods can also be used to quantify pore structure. Traditional optical and scanning electron microscopy (SEM) can only reveal the surface or two dimensional (2-D) morphology, rather than internal microstructures , , , , , . The development of pores and microfractures during the process of oil shale pyrolysis needs to be assessed in three dimensions (3-D) due to the heterogeneous internal architecture. There are several techniques available for quantitatively characterizing the 3-D microstructure of oil shales including serial sectioning which involves obtaining a series of scanning electron or optical micrographs and then computationally assembling these digital images , .
Although such a method enables the measurement of connectivity and other structural properties such as pore volume and surface area, once sectioned, this destructive process means that the sample is no longer available for other static or dynamic analyses. In addition to being destructive, this technique is time-consuming and may damage the sample during preparation.
X-ray micro-computed tomography (µCT) has emerged as a powerful tool for the visualization and quantification of the internal structure of geological materials, in particular being effective at studying complex pore-scale processes , , , , , , , , , , , , . µCT offers several advantages: it is non-destructive, provides 3-D imaging, achieves high spatial resolutions at scales down to the micron level, gives good contrast between phases, and is adaptable to many types of experimental procedures. In the case of oil shales, a 3-D approach allows us to obtain important information on the spatial distribution of organic matter and inorganic minerals to monitor the evolution of the pore space during pyrolysis.
Quantitative analysis can provide useful insights into the mechanical and transport behaviors during the oil shale pyrolysis process by characterizing component volume fractions and fundamental pore space geometric attributes, including pore size, shape, tortuosity and connectivity. μCT has been applied to characterize oil shale samples from the United States (Green River) , China (Fushun)  and Australia (Queensland) . Tiwari et al.  characterized pore structures before and after pyrolysis based on 42 µm voxel size scans reporting pores as large as 500 µm after pyrolysis. More recently, dynamic imaging of oil shale pyrolysis using a synchrotron X-ray tomography was conducted on a Green River (Mahogany Zone) oil shale sample presenting a direct visualization of the temporal evolution of the pore space during pyrolysis .
This paper reported microscale disconnected pores at 390 °C; with porosity increasing dramatically between 390 °C and 400 °C where the vast majority of the pore space became connected.
In this study, we complement our previous work  by initially characterizing the bulk properties of the Mahogany oil shale followed by an evaluation of the microstructure in 2-D using optical and SEM methods and in 3-D using static μCT to reveal the compositional and structural heterogeneity. We then apply vacuum pyrolysis conditions to visualize and quantify the pore structure development using μCT at 12 µm and 2 µm voxel sizes at temperatures representative of surface retorting technologies (300-500 °C). The representative sample size was also characterized using porosity as the target parameter.
Oil shale samples
Oil shale samples were obtained from an outcrop of the organic-rich Mahogany zone of the Green River Formation (Uinta Basin, Utah). The stratigraphy, geochemistry, mineralogy and hydrocarbon potential of the Green River Formation has been extensively studied , , , , , , , . The richest oil shale horizon is the Mahogany zone (R-7) which is a primary target for shale oil production due to its high oil yield which can exceed 250 L per tonne of rock recovered as liquid fuel by pyrolysis . To initially characterize the Mahogany oil shale, samples were prepared and analyzed to characterize bulk properties including mineralogy (XRD), total organic carbon (TOC), thermal maturity (Rock-Eval and Source Rock Analyzer (SRA)), elemental analysis (CHNOS) and oil yield (Modified Fischer Assay (MFA)).
However, given the complexity and heterogeneity of oil shale, samples were also cut and prepared for petrographic analysis, SEM-EDX and µCT image characterization. These imaging techniques offer local information on the spatial distribution of organic material, mineral phases and pore structures. With these bulk analytical techniques and imaging methods, a full characterization of oil shales can be achieved to better understand the internal microstructure and chemical composition which allows for improved modeling and prediction of fluid flow during oil shale pyrolysis.
Oil shale composition analysis
X-ray diffraction (XRD)
X-ray diffraction (XRD) is a laboratory-based analytical technique for identifying minerals and other crystalline phases in a wide range of materials, including oil shale samples , , . In this work, the oil shale rock sample was first disaggregated gently using a pestle and mortar. A 2 g split of this material was then micronized using a McCrone Micronizing Mill to obtain a powder with a mean particle diameter of between 5 and 10 µm. The fine powder was then backpacked into an aluminum cavity mount producing a randomly orientated sample suitable for XRD. Samples were analyzed using a Bruker D8 Advance X-ray diffractometer with scattering angles, 2θ, between 5° and 75° with a step size of 0.05°/s using CuKα radiation (λ = 1.54 Å) at a tube voltage of 35 kV and a current of 30 mA. Quantitative identification of the minerals in the oil shale sample was achieved using the full pattern analysis technique known as the Rietveld method  using the Inorganic Crystal Structure Database (ICSD) .
Fig. 1. XRD pattern (diffractogram) for the Green River oil shale sample (Mahogany Zone) with characteristic mineral peaks identified.
XRD analysis of oil shale from the Green River Formation (Mahogany Zone) reveals a complex mineral signature. The dominant mineral phases were dolomite, calcite, quartz, and feldspars with small amounts of illite, analcime and pyrite (Fig. 1 and Table 1). Although kerogen is the most important component of oil shale, it cannot be detected by direct XRD analysis. However, XRD results provide valuable information in characterizing bulk oil shale mineralogy with previous studies establishing that minerals in oil shale can have both catalytic and inhibitory effects on pyrolysis reactions , .
Table 1. Composition analysis for the Green River oil shale sample (Mahogany Zone) including mineralogy, TOC and elemental analysis.
Total organic carbon (TOC)
The oil shale sample was ground, homogenized and sieved to 60 mesh (<250 µm). 100 mg of the crushed sample was then treated for two hours with concentrated hydrochloric acid to remove carbonate minerals. The acid was removed from the sample with a filtration apparatus fitted with a glass microfiber filter paper. The filter was placed in a LECO crucible and dried at 110 °C for one hour. After drying, the sample was analyzed with a LECO 744 Carbon Analyzer with detection limits to 0.01 wt%. The result reveals a TOC of 30.15% for the Mahogany zone oil shale indicating an organic-rich sedimentary rock.
Programmed pyrolysis (Rock-Eval, Source Rock Analyzer)
Programmed pyrolysis (Rock-Eval II and SRA) was performed to assess source rock quality and thermal maturity. A 100 mg crushed rock sample was heated in an inert environment to obtain the parameters S1, S2, S3, Tmax, hydrogen index (HI), oxygen index (OI) and production index (PI). Sample heating at 300 °C for 3 min produced the S1 peak by vaporizing the free (unbound) hydrocarbons. The temperature was then increased by 25 °C/minute to 600 °C and the S2 and S3 peaks were measured from the pyrolytic degradation of the kerogen in the sample.
The S2 peak is proportional to the amount of hydrogen-rich kerogen in the oil shale rock, and the S3 peak measures the amount of carbon dioxide released (to 390 °C) providing an assessment of the oxygen content of the rock. The temperature at which the S2 peak reaches a maximum, Tmax, is a measure of the source rock maturity. The pyrolysis data indicates a type I kerogen and the results obtained from the Rock-Eval II instrument showed good agreement with the results obtained from the SRA method.
Kerogen was isolated from the raw oil shale using a standard hydrochloric and hydrofluoric acid treatment which is effective in removing carbonate and silicate minerals. The remaining material contained primarily kerogen as well as pyrite (FeS2) which is particularly resistant to these methods. Carbon, hydrogen and nitrogen were determined using a Flash EA 1112 Series Analyzer according to ASTM D5291.
Oxygen was determined using a Leco RO-478 Oxygen Analyzer with the method described in ASTM D5622. Sulfur and iron contents were determined by analysis on a Perkin Elmer DV 5300 ICP-AES following a modified EPA 200.7 method for ICP-AES analysis. Iron in isolated kerogen is considered to exist entirely as pyrite and the pyrite sulfur content can be determined given the stoichiometric relationship between the sulfur and iron contents of pyrite (Spyrite/Fepyrite = 1.148). The organic sulfur content is then calculated by subtracting the pyrite sulfur from the total sulfur (Table 1).
Modified Fisher Assay (MFA)
A Modified Fischer Assay (MFA) method was used to measure the potential oil yield from the Mahogany oil shale sample outlined in ASTMD3904. A 100 g of 8 mesh (<2.38 mm) oil shale was heated in an aluminum retort to 500 °C at 12 °C/min and held at this temperature for 40 min. The evolved hydrocarbons were passed through a condenser system and analyzed. The quantity of oil was measured directly and the gas fraction was analyzed by gas chromatography. The MFA results are summarized in Fig. 2.
Fig. 2. Modified Fisher Assay (MFA) results for the Green River oil shale sample (Mahogany Zone) including mineral decomposition and organic products, oil density, and oil yield. The clean oil density represents the density of the oil recovered as liquid in the receiver following the test. The oil yield represents the amount of potentially extractable shale oil present in the sample as determined the MFA method.
Visualization using optical microscopy
Petrographic analyses provide a starting point to digitally characterize the mineral composition, texture and the fine-grained structure of shale rocks. In this study, geological thin sections from Green River Formation were prepared perpendicular to the laminations. The oil shale samples were air dried followed by epoxy vacuum impregnation. The samples were then hand lapped and polished using progressively finer alumina suspension (1 μm, 0.3 μm and 0.05 μm) alongside a high-purity hydrocarbon lubricant. Thin sections of 20 µm thickness were optically examined using a Zeiss SteREO Discovery.V12 microscope with accompanying AxioVision LE64 software (Carl Zeiss, Germany) using plane-polarized light (PPL). A mosaic photomicrograph was created by the automated stitching of 1152 individual images at 100× magnification (Fig. 3).
Fig. 3. Thin section mosaic images of the Green River oil shale sample (Mahogany Zone) generated by digitally stitching 1152 individual images into one all-encompassing view.