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Summary The focus of this work was to obtain reliable kerogen and solid bitumen density data and to establish robust descriptions of the organic matter responsible for liquid hydrocarbon generation in the Devonian organic-rich Duvernay shale. Five wells were selected from Alberta, Canada, for investigation of the organic matter (OM) properties, particularly its density. A comprehensive workflow was designed and executed that includes the selection of representative samples for detailed evaluation of possible vertical heterogeneity of organic facies and on which to perform separation of organic matter and density measurements. The major goals were to understand the various controls on organic matter types (macerals), kerogen density, and to develop a predictive tool for calibrating density to maturity and organic matter distribution. We showed that the sole utilization of Rock-Eval pyrolysis data as the maturity proxy, following published correlations, may lead to substantial overestimation of the actual maturity, as compared to other maturity proxies. We also demonstrated that the organic petrology and the vitrinite reflectance equivalent (VRE) values derived from solid bitumen reflectance and published correlations (Jacob, 1985; Schoenherr et al., 2007) much more closely reflect the maturity of actual fluids being produced in the subject area. The rocks studied vary from 2 to10% in total organic carbon (TOC) content, with VRE values of 1 to 1.2%, and are producing light fluids that correlate with the middle upper maturity range. Geochemical markers corroborate the petrographic maturity estimations observed. The organic matter is dominated by two main constituents—amorphous organic matter (AOM) and solid bitumen—with traces of other liptinites. Kerogen density varies from 1.25 to 1.35 g/cm3, depending on the influence of solid bitumen on overall composition; however, the average density of the kerogen in this area, and taking into consideration the maturity range, was established at 1.28 +/- 0.3 g/cm3. We have also captured kerogen density variation driven by maturity, with one well showing consistently lower-density kerogen than the other four wells. Measured kerogen densities were subsequently used in advanced petrophysical analysis, with an effort to distinguish between bitumen and kerogen volumes and to determine the porosity associated with each OM constituent. While this paper uses the Duvernay shale as an example, the conclusions have universal application to all unconventional resource plays. In addition, our work can be used to better understand OM variability and its control on the type of fluids generated and produced, and it provides measured rather than assumed kerogen properties as direct input to formation evaluation and modeling software.
Ko, Lucy Tingwei (University of Texas at Austin) | Zhang, Tongwei (University of Texas at Austin) | Loucks, Robert G. (University of Texas at Austin) | Ruppel, Stephen C. (University of Texas at Austin) | Shao, Deyong (Lanzhou University)
Abstract Low-maturity Boquillas (Eagle Ford equivalent) outcrop samples (carbonate-rich facies) were heated anhydrously under confining pressure to study the evolution of organic-matter (OM)-hosted pores and mineral pores. Oil and gas yields, Rock-Eval and Leco TOC analyses were used to characterize kerogen type, organic matter conversion, and thermal maturation. Calculated vitrinite reflectance (from Tmax) indicated that the outcrop Boquillas samples have reached the thermal maturity level of about 0.65 to 0.7%Ro, implying that they have reached the stage of early-hydrocarbon generation. Samples were also prepared using Ar-ion polishing to look at pore evolution under field-emission scanning electron microscopy (FE-SEM). Based on SEM petrography, we observed several diagenetic features that formed prior to hydrocarbon migration. These features include carbonate cement, quartz cement, kaolinite cement, chlorite cement and dolomite replacement. Two major pore types were observed in outcrop samples: OM-hosted pores and mineral pores. We interpreted the occurrence of OM-hosted pores to be the result of hydrocarbon phase transformation. With increasing laboratory heating temperature, the character of OM changed through different phase transformation stages. OM connectivity also changed and new types of OM-pores were formed. Several different OM-hosted pore types coexisted at liquid generation stage, and pores were in association with solid kerogen, liquid oil, solid bitumen, solid pyrobitumen, and products from secondary cracking. The abundance and morphology of OM-hosted pores changed from no pores to large micrometer-sized OM-hosted pores, and then finally to small equant nm-sized OM-hosted pores. The small equant pores are similar to those observed in naturally matured samples and are most probably associated with methane generation (dry gas stage).
Cudjoe, Sherifa (University of Kansas) | Barati, Reza (University of Kansas) | Marshall, Craig (University of Kansas) | Goldstein, Robert (University of Kansas) | Tsau, Jyun-Syung (University of Kansas) | Nicoud, Brian (Chesapeake Energy) | Bradford, Kyle (Chesapeake Energy) | Baldwin, Amanda (Chesapeake Energy) | Mohrbacher, David (Chesapeake Energy)
Abstract Microscopic analysis including transmitted light, UV epifluorescence, BSE, and FIB-SEM carried out on Lower Eagle Ford (LEF) shale samples, selected from similar depths, show complex depositional fabrics, kerogen, migrated organic matter, and diagenetic history. It is well known that LEF samples contain depositional kerogen and migrated organic matter. Much of the migrated organic matter occupies diagenetically reduced primary porosity. Some of this organic matter is not porous, while some contains large pores and other contains a fine network of nanopores. Where thermal maturity is one control on porosity in organic matter, there is also a control of composition and origin. This paper investigates the chemistry of organic matter in-situ using Raman spectroscopy, to begin to understand what, other than thermal maturation, leads to porosity in both depositional kerogen and migrated organic matter. This is used to evaluate the nature of the pores in LEF, and to assess the impact of hydrocarbon gas injection on organic porosity. Thin sections of the lower Eagle Ford shale samples are examined with transmitted light microscopy to select samples for Raman spectroscopy, after studying with FIB-SEM to analyze distribution of porosity in organic matter. In the Raman spectra, the separation between the D and G bands, the width of the G-band, and the intensity ratio of the D-to-G-bands are typically ascribed to maturity-related changes. However, composition and origin of the organic matter may also have an effect. The Raman spectra are analyzed to characterize the different types of porous and non-porous organic matter at the same depth. Then, samples are subjected to gas injection in the laboratory in preparation for a gas huff-n-puff operation, and changes in Raman spectra are analyzed once again. BSE images show depositional kerogen is found as isolated bodies, lamellar forms, and fine material disseminated in the matrix. Transmitted light and UV microscopy reveal that some of this is non-fluorescent and some is fluorescent. Cement-reduced intraparticle pores, other primary pores, intercrystalline pores, and micro-fracture and micro-breccia pores contain migrated organic matter (OM), none of which fluorescences in UV. FIB-SEM images show the migrated OM has either spongy nanopores, larger bubble/meniscate pores, or no pores, all in the same sample. Raman spectroscopy analysis on the different types of organic matter show examples where both G- and D- bands are visible with distinctive separation, intensity ratio, or width, or where the D-band is absent. Moreover, the effect of gas injection on the different types of organic matter is inferred from the G- and D- bands. This work improves our understanding of organic pore generation and modification, which influences pore size distribution and pore tortuosity, the underlying factors in gas huff-n-puff recovery in shales. It expands the utility of Raman micro-spectroscopy as a tool in understanding the evolution of pore systems and organic constituents in shale. It also presents an in-situ molecular structural study of the effect of hydrocarbon gas huff-n-puff on the different types of organic matter.
Knapp, Levi J. (Japan Oil, Gas and Metals National Corporation (JOGMEC)) | Nanjo, Takashi (Japan Oil, Gas and Metals National Corporation (JOGMEC)) | Uchida, Shinnosuke (Japan Oil, Gas and Metals National Corporation (JOGMEC)) | Haeri-Ardakani, Omid (Geological Survey of Canada) | Sanei, Hamed (Aarhus University)
Organic matter exerts a fundamental control on porosity and permeability in organic-rich tight hydrocarbon reservoirs. The complexities of these relationships are not well defined however and may be influenced by a variety of factors including organic matter richness, thermal maturity, kerogen type, original kerogen structure and primary organic-hosted porosity, compositional fractionation, interaction with mineral catalysts, compaction, and occlusion by generated products such as solid bitumen.
This abstract presents preliminary results from the first of three wells analyzed in the play fairway of the Duvernay Formation of western Canada –a prolific source rock and rising star as an unconventional reservoir. Distinct porosity morphology groups have been observed in SEM and ongoing work has shown that organic matter porosity morphology may be influenced by organic matter composition and degree of isolation from nearby macerals. Integration of porosity calculated from image analysis of focused ion beam scanning electron microscopy (FIB-SEM) images with results from nuclear magnetic resonance (NMR), mercury injection capillary pressure (MICP), and helium porosimetry has demonstrated that a significant portion of the porosity is below typical SEM imaging resolution and that even methods such as He-porosimetry are challenged to access nanometer-scale pores. Variations in sample preparation and analysis procedures can significantly alter porosity results.
Cudjoe, Sherifa (University of Kansas) | Barati, Reza (University of Kansas) | Goldstein, Robert (University of Kansas) | Tsau, Jyun-Syung (University of Kansas) | Nicoud, Brian (Chesapeake Energy) | Bradford, Kyle (Chesapeake Energy) | Baldwin, Amanda (Chesapeake Energy) | Mohrbacher, David (Chesapeake Energy)
Abstract Huff-n-puff gas injection has proven to be effective for recovering more liquid hydrocarbons from hydraulically fractured and horizontally drilled wells in ultra-tight unconventional shales. The complexity of shales, however, inherent in the variation of mineral microstructure and heterogeneous pore space, makes accurate simulation of the huff-n-puff process for optimum recovery challenging. Therefore, this study deals with the visualization and quantification of the microstructure of Lower Eagle Ford (LEF) shale samples before and after hydrocarbon gas huff-n-puff recovery. This is used to produce reliable estimations of petrophysical (porosity, permeability) and intrinsic rock properties (tortuosity). The petrophysical and intrinsic property estimations measured provide accurate inputs for reservoir simulation for the huff-n-puff process. The integrated workflow for pore-scale characterization includes scanning electron microscopy/backscattered electron microscopy (SEM/BSE), energy-dispersive X-ray spectroscopy (EDS), and focused ion beam-scanning electron microscopy (FIB-SEM). It includes mineral and maceral identification through elemental analysis, pore size and pore throat distribution, in addition to pore network development. A high pressure, high temperature (HPHT) system is used to expose the samples to hydrocarbon gas for 3 days before repeating the SEM measurements. The 2-D SEM/BSE images are particularly useful at the micrometer scale, and show sediment particles, wispy seams of kerogen, discrete particles of depositional kerogen, and migrated organic matter embedded in a fine-grained matrix of clays, quartz, coccoliths, foraminifera, organic matter, and unidentifiable particles. Together, the SEM/BSE and FIB-SEM images show nano-scale pores in organic matter, as well as micro-scale intraparticle, interparticle, intercrystalline, breccia and fracture pores of varying sizes and geometries. Much of the pore space is impregnated with variously arranged porous organic matter which comes in later in the paragenesis. FIB-SEM images were utilized in the generation of pore network models. The EDS combined with SEM/BSE reveals spatially distributed diagenetic textures indicating calcite precipitation before pyrite, kaolinite precipitation, compactional fracturing and late migration of organic matter into open pore space. Significant findings include the differentiation of depositional kerogen from migrated organic matter (bitumen or solid bitumen/pyrobitumen). Most porosity is in the migrated organic matter, which has spongy or bubble pores, rather than in depositional kerogen. The pore-scale tortuosity in organic pores averages 1.56, 1.94, and 1.73 for the Lower Eagle Ford (LEF) samples A, B, and C, respectively. The tortuosity of the inorganic pore network is estimated at 1.60. Furthermore, the equivalent pore diameters from pore network models of both the organic and inorganic pores range from 13 nm – 580 nm and 20 nm – 4 μm, respectively. This is important because organic pores developed in both migrated solid bitumen (most common) and depositional kerogen (less common). These organic pore networks create permeability and provide diffusion pathways for gas molecules during the huff-n-puff process. After the hydrocarbon gas injection experiments, the gas exposure was observed to have displaced some of the migrated organic matter. In-situ interaction of injected hydrocarbon gas with bitumen/solid bitumen enhances our understanding of the recovery process.