ABSTRACT Organic-rich mudstones of the Appalachian Basin hold a sizable portion of the natural gas produced in the United States. Indeed, in 2015, Pennsylvania and West Virginia accounted for 21% of produced natural gas, driven in part by production from the Point Pleasant Limestone. The critical role that unconventional reservoirs will play in future global energy use necessitates the need for an enhanced understanding of those geological aspects that shape and influence their reservoir architecture. Foremost among these is a clearer understanding of the preservation and accumulation of organic carbon, as it is the source of hydrocarbons, and often provides the dominant host of interconnected porosity and hydrocarbon storage. To this end, pyrite morphology can offer insight into the redox conditions of the bottom and pore water environment at the time of sediment deposition and early diagenesis and can be especially useful in the analysis of deposits devoid of redox sensitive trace metals. Pyrite contained in cuttings and core chips retrieved from vertical and horizontal Point Pleasant Limestone wells were analyzed by scanning electron microscope. Results demonstrate a dearth of pyrite in the Point Pleasant (0.02–1.7% of the surface area analyzed). Pyrite morphology is dominated by euhedral grains and masses (~80% of pyrite encountered) co-occurring with infrequent framboids. Framboids are uniformly small (average = 4.7 μm) with just a few examples >10 μm. The presence of small amounts of euhedral pyrite grains and masses is consistent with accumulation under a dysoxic water column. Conversely, the size of the framboids suggests that they formed in a water column containing free hydrogen sulfide. A model invoking a lack of reactants necessary to sustain diagenetic pyrite growth in anoxic pore waters may explain this apparent paradox. In such a case, the framboid size distribution may reflect newly forming diagenetic framboids competing for a finite amount of reactants resulting in a population of small framboids and few large examples. Indeed, the low total iron/aluminum (Fe/Al) content of the Point Pleasant (average Fe/Al = 0.45) would indicate a low delivery of reactive iron to the seafloor during Point Pleasant deposition. The data suggests a model in which organic carbon preservation occurred by rapid burial and removal from oxygen-bearing water. In turn, more organic-rich and potentially higher quality reservoir facies of the Point Pleasant Limestone occur in areas of higher clastic delivery to basin.

ABSTRACT An evaluation of an integrated data set collected over the past 12 years designed to identify the parameters controlling reservoir quality and production properties in organic, siliceous mudrocks reveals the key diagenetic processes affecting the development of brittleness in siliceous mudrocks. This work was motivated by the failure of early efforts to correlate brittleness to x-ray diffraction (XRD) mineralogy. The outcome of this analysis has been the recognition of two, often overlapping, pathways to brittleness that are determined at the time of deposition by the relative proportions of clay, detrital quartz, and biogenic silica present in the original sediment and are later affected by burial history. One pathway begins with a phyllosilicate–mud-dominated sediment, and the other begins with a sediment containing common or abundant biogenic silica (opal-A). Both pathways are characterized by compactional porosity loss and both eventually include the generation of authigenic quartz cement; however, the source of that authigenic quartz is different between the two pathways. The authigenic quartz that characterizes the first pathway is developed from the illitization of smectite and is precipitated as a cement within the argillaceous matrix. This authigenic quartz is detectable along with the detrital quartz by XRD analysis. All other factors being equal, the volume of brittle, authigenic quartz cement derived from the alteration of smectite is proportional to the volume of original clay. As a result, the effectiveness of this cement to increase the brittleness of the rock may be impacted by the presence of the ductile clays. In the alternate pathway, authigenic quartz is derived from the transformation of biogenic opal-A and is independent of the amount of clay. Much of the XRD quartz volume in rocks derived from biogenic–silica-rich sediment that contained little or no detrital quartz will comprise a brittle, authigenic cement.

ABSTRACT Kinetic barriers inhibit quartz nucleation and growth at lower temperatures (<50°C [<122°F]). Thus, under ordinary geothermal gradients, the formation of authigenic quartz in fine-grained systems is preceded inevitably by the early stages of compaction. Nucleation sites for quartz precipitation and the abundance and sizes of pores into which quartz cement can be emplaced are limited by the compactional state at the time of precipitation. The two main types of grain alteration that are proposed to yield authigenic quartz, dissolution of biogenic opal and illitization of smectite, occur in different temperature ranges and contrasting compactional regimes. This chapter summarizes petrographic observations on quartz components (grains and cement) by high-resolution cathodoluminescence (CL) and X-ray elemental mapping in 11 mudrock units ranging in age from Ordovician to Oligocene. The amounts of quartz cement observed provide constraints on the sources of silica for the formation of authigenic quartz and mass and volume balances of silica generation and precipitation in mudrock diagenesis. The size (1–3 μm), spatial distribution, and abundance (typically 30–40% of rock volume) of authigenic microquartz that arise from the biogenic opal pathway are consistent with the compactional state of mud in the temperature range of the opal-A to opal-CT transition. Mudrocks that are clearly cemented by authigenic microquartz contain a volume of quartz in excess of amounts potentially generated by illitization (up to about 13% of rock volume). In the absence of abundant biogenic silica and consequent early cementation that inhibits compaction, the most common mudrock in the temperature range of illitization (>~80°C [>~176°F]) have few available nucleation surfaces for quartz precipitation and little available pore space (mostly nanometer scale) to accommodate pore-filling crystal growth. Sites for higher temperature quartz precipitation, synchronous with illitization, are mostly restricted to localized packing flaws at contacts between silt-size particles and constitute a trivial volume of the rock. Thus, tarls tend to feature diagenesis dominated by compaction that dramatically reduces pore space prior to the onset of significant reactions of the grain component such as albitization and illitization. The absence of discernible cementation in most deep basinal mudrocks raises the possibility that mechanical compaction persists as a mechanism of porosity decline to greater depths in mud than in sand.

ABSTRACT This chapter demonstrates a nondestructive, multispectral approach to evaluating chemical and spatial heterogeneities within mudstone fabrics. A combination of laser scanning confocal microscopy (LSCM) and scanning electron microscopy (SEM) was used to document nanometer- to millimeter-scale microtextures in mudstones. Additionally, micro-Fourier transform infrared (micro-FTIR) spectroscopy was used to identify both clay minerals and compositional structures, such as aromatic and aliphatic components in kerogen. A set of organic-rich mudstones with thermal maturities ranging from immature to oil prone were analyzed and used as examples to document the multispectral, multiscale approach. This work demonstrates the different spectral approaches and their applicability to the analysis of organic-rich mudstones. Single-channel fluorescence images collected with various excitation/emission wavelengths were used to access microtextural details in mudstones, whereas multichannel composite fluorescence images were used to evaluate relative thermal maturity among samples. In addition, SEM backscatter and energy dispersive X-ray microscopy were used to calibrate fluorescence signals to mineralogy and provide submicron information on grain boundaries and microfabrics. Micro-FTIR chemical maps represent the spatial distribution of chemical information related to properties of interest such as the presence and character of hydrocarbons and clay minerals. The infrared (IR) spectra associated with organic matter were also analyzed for quantitative indicators of thermal maturity. Opportunities for image processing and analysis that have the capability to integrate these multiscale, multispectral approaches are discussed for a more robust understanding of mudstone microfabrics, heterogeneity, and their impact on mudstone reservoir quality.

ABSTRACT Detailed diagenetic studies of the late Cambrian Alum Shale in southern Sweden were undertaken across an interval that includes the peak Steptoean Positive Carbon Isotope Excursion (SPICE) event to evaluate the pyrite mineralization history in the formation. Samples were collected from the Andrarum-3 core (Scania, Sweden); here the Alum was deposited in the distal, siliciclastic mudstone-rich end of a shelf system. Abundant cryptobioturbation is observed in the Alum, which points to oxic–dysoxic conditions prevailing during deposition. Petrographic examination of polished thin sections (n = 65) reveals the presence of numerous texturally distinct types of pyrite, including matrix framboids, two different types of framboid concretions (those with rims of iron-dolomite and those lacking rims), disseminated euhedral pyrite crystals, concretions of euhedral pyrite crystals, overgrowths of pyrite on these different pyrite generations, anhedral pyrite intergrown with bedding parallel mineralized fractures (i.e., “beef”), and massive vertical/subvertical accumulations of pyrite. Paragenetic relationships outline the relative timing of formation of the texturally distinct pyrite. Framboids and framboid concretions formed prior to precipitation of any euhedral pyrite crystals, and these pyrite generations precipitated prior to the pyrite overgrowths on them. As Alum Shale sediments are all distorted by these texturally different pyrite generations, they are likely to have formed early in the postdepositional history of the formation. In contrast, pyrite associated with “beef” is likely temporally related to the onset of hydrocarbon generation, which in this part of Sweden is thought to have been many tens of millions of years after deposition. Because vertical/subvertical massive pyrite features distort “beef,” they clearly postdate it. Of all these pyrite textures, only framboid concretions appear to be restricted to the SPICE interval. The texturally distinct nature of the pyrite generations, along with evidence of their formation at different times in the postdepositional history of the Alum Shale, is the key outcome of this petrographic study. Because the petrographic data presented herein point to a postdeposition origin for all generations of pyrite, diagenetic processes—not those processes associated with deposition—were responsible for the complex pyritization history observed in the Alum, in the Andrarum-3 core.

ABSTRACT The Devonian Woodford Shale and Cretaceous Mowry Shale consist of relatively deep (below storm wave base) intracratonic basin deposits commonly referred to as “shales” because of their dark gray to nearly black color, very fine-grained nature, pelagic fossils such as radiolarians, and common amorphous marine kerogen. These shales typically contain less than 30% detrital clay by weight and more than 50% quartz (locally up to 80%). The quartz is a mix of biogenic grains, mainly radiolarians, and authigenic silica along with some detrital quartz silt of extrabasinal origin. The authigenic silica is dominantly microcrystalline (< 1 micron) and forms a major component of the matrix in these formations, but the rocks also contain authigenic pyrite, commonly as framboids, minor carbonates including magnesite, and quartz overgrowths, but together these authigenic minerals form less than 10% of the rock. Authigenic quartz in the Woodford and Mowry samples commonly takes the form of silica nanospheres, a type of microquartz less than a half micron in diameter. Textures of this microquartz are best observed directly with a high-resolution electron microscope. In many Woodford and Mowry samples, the silica nanospheres, which tend to be associated with organic matter, form more than 50% of the rock. The large volume of the authigenic quartz, together with “floating” detrital components and the close association with pyrite framboids, indicates that the silica nanospheres formed very early, perhaps in association with microbial activity on or in the seafloor sediments. These early silica nanospheres, which are only weakly luminescent, helped create a lithified sediment during or soon after deposition. Where the silicification process ceased prior to complete silica cementation, the early silica nanospheres are associated with up to 15% interparticle microporosity. This gives the Woodford and Mowry good potential reservoir quality, at least locally. The authigenic silica nanospheres also enhance the mechanical properties and brittleness of these siliceous mudrocks to a degree much greater than the presence of the detrital quartz particles alone.

ABSTRACT Shales exhibit a wide range of textures, compositions, and mechanical properties, which are interlinked by their diagenetic history. During hydraulic fracturing of shales, the matrix is subjected to shear deformation, which may create microfractures and enhance hydrocarbon transport from nanoscale, organic matter (OM)-hosted pores to the larger, induced fracture network. To study the nanoscale response to shear deformation of shale pore systems with different diagenetic histories, we deformed shale samples from a formation in the Northern Rocky Mountains and the Eagle Ford Group in Texas, using confined compressive strength tests. N 2 and CO 2 adsorption were performed to quantify fracture effects on pore morphology including pore size distribution, porosity, surface area, and surface fractal dimensions. Most samples increased their gas adsorption quantity, pore volume, and surface area after failure. The surface fractal dimensions were less sensitive to shear deformation. Results show that varying nanometer-to-micron-scale fracture patterns are in part caused by contrasting rock fabrics that are preconditioned by their distinctive diagenetic histories. For example, fractures tend to propagate along the OM laminae, whereas others cut across OM grains and access OM pores. Other possible mechanisms for porosity increase include the deformation of relatively uncemented clay aggregates and contrasting amounts of intra-OM pores between samples. Thus, the mechanisms for syn-deformational porosity changes at the micro scale are highly dependent on diagenetic history, particularly the maturation of OM, and the cementation history relative to clay content.

ABSTRACT Scanning electron microscopy (SEM) has revolutionized our understanding of shale petroleum systems through microstructural characterization of dispersed organic matter (OM). However, as a result of the low atomic weight of carbon, all OM appears black in SEM (BSE [backscattered electron] image) regardless of differences in thermal maturity or OM type (kerogen types or solid bitumen). Traditional petrographic identification of OM uses optical microscopy, where reflectance (%R o), form, relief, and fluorescence can be used to discern OM types and thermal maturation stage. Unfortunately, most SEM studies of shale OM do not employ correlative optical techniques, leading to misidentifications or to the conclusion that all OM (i.e., kerogen and solid bitumen) is the same. To improve the accuracy of SEM identifications of dispersed OM in shale, correlative light and electron microscopy (CLEM) was used during this study to create optical and SEM images of OM in the same fields of view (500× magnification) under white light, blue light, secondary electron (SE), and BSE conditions. Samples (n = 8) of varying thermal maturities and typical of the North American shale petroleum systems were used, including the Green River Mahogany Zone, Bakken Formation, Ohio Shale, Eagle Ford Formation, Barnett Formation, Haynesville Formation, and Woodford Shale. The CLEM image sets demonstrate the importance of correlative microscopy by showing how easily OM can be misidentified when viewed by SEM alone. Without CLEM techniques, petrographic data from SEM such as observations of organic nanoporosity may be misinterpreted, resulting in false or ambiguous results and impairing an improved understanding of organic diagenesis and catagenesis.

ABSTRACT The lower Permian Wolfcamp Shale in the Permian Basin is a major unconventional resource play composed of organic-rich, siliceous and calcareous mudstones interbedded with carbonate turbidites and debrites. Using two cores that comprise the Wolfcamp Shale near the eastern margin of the Midland Basin, this study reconstructs the complex diagenetic history of both the mudstone and carbonate facies. These cores were analyzed using petrographic and SEM techniques to test if the Wolfcamp Shale was an open or closed system and to characterize diagenetic processes that impact reservoir characteristics, such as porosity types, porosity distribution, permeability pathways, and mechanical brittleness. Early, middle, and late phases of chemical diagenesis are defined in this study. Mineral precipitation and dissolution events occur from the passage of fluids through both interstitial and fracture pore space. Early authigenic mineral precipitation (calcareous and phosphate concretions, sphalerite, barite, framboidal pyrite, quartz, dolomite, and ferroan dolomite) resulted in destruction of primary porosity within the mudstone facies, before and during the mechanical compaction event. Destruction of porosity in the carbonate turbidites facies occurred through carbonate cementation (calcite, ferroan calcite, dolomite, and ferroan dolomite) during early to middle diagenesis. An episode of dissolution and dolomitization in the carbonate facies resulted in the creation of moldic and intercrystalline porosity respectively. Within mudstones intercrystalline porosity is observed between pyrite framboids and clay sheets of chlorite. Diverse fracture types occur in all facies within the Wolfcamp Shale and play a critical role in the migration of diagenetic fluids and hydrocarbons. Horizontal fractures are filled by “beef”-type calcite, and vertical fractures are filled with equant calcite and/or celestine-barite. Mineralized fractures contain porosity, some of which contain ferroan dolomite rhombs within pores, which supports diagenetic fluid movement through fractures after an initial stage of mineralization. Fluid inclusion data suggest that some mineralized fractures acted as fluid conduits for externally derived, warm, high-salinity brines, suggesting the Wolfcamp Shale was an open system during it burial history.

ABSTRACT Characterizing unconventional reservoirs involves the investigation of a wide range of potential source-rock targets at various stages of thermal maturity. These samples may contain a mixture of kerogen, bitumen, oil, and pyrobitumen within their fabric. Thus, it is critical that we properly identify and examine each organic phase to better understand reservoir properties. In the present study, we have selected samples of gilsonite from a naturally occurring solid hydrocarbon deposit to serve as an analog for characterizing the bitumen phase of generation. Gilsonite is an aromatic-asphaltic solid bitumen found in vertical veins along the eastern portion of the Uinta Basin, Utah. It is thought to be an early generation product from oil-prone Green River Shale source beds and is similar to low-maturity crude oil in composition. It has a high nitrogen content, low sulfur content, and high melting point (fusibility) and is soluble in organic solvents. We have used a variety of analytic methods to characterize this material, including standard optical organic petrology and scanning electron microscopic imaging to examine the occurrence of organic porosity. Optical organic petrology analyses using both air and oil immersion objectives show that the polished gilsonite surfaces are typically dark grey and featureless. Optical evidence for the presence of macerals and inorganic constituents is absent. Visual estimates suggest that fractures make up approximately 1% of the conchoidal fracture plane, whereas the pencillated variety contains approximately 2% fractures along with 5% shallow pits. Scanning electron microscopic images also show the occurrence of fractures within gilsonite, but the matrix contains no evident organic porosity. The results of our analyses suggest that, unlike pyrobitumen, pre-oil solid bitumen represented by gilsonite was found to contain no significant occurrences of organic nanoporosity within its matrix. Gilsonite does have minor pitting and fractures, but these do not represent an effective interconnected pore network and are probably artifacts of weathering/sampling. Thus, this material would not represent a potential candidate for in-situ petroleum storage capacity. Whether this is typical of all naturally occurring solid bitumen is debatable, considering that gilsonite has undergone some secondary alteration via devolatilization and limited biodegradation. Nevertheless, the pore-scale imaging of this solid bitumen provides potentially important new insights for unconventional reservoir characterization.

ABSTRACT Shales are enigmatic rock types with compositional and textural heterogeneity across a range of scales. This work addresses pore- to core-scale mechanical heterogeneity of Cretaceous Mancos Shale, a thick mudstone with widespread occurrence across the western interior of the United States. Examination of a ~100 m (~328 ft) core from the eastern San Juan Basin, New Mexico, suggests division into seven lithofacies, encompassing mudstones, sandy mudstones, and muddy sandstones, displaying different degrees of bioturbation. Ultrasonic velocity measurements show small measurable differences between the lithofacies types, and these are explained in terms of differences in allogenic (clay and sand) and authigenic (carbonate cement) mineralogy. Variations in ultrasonic velocities can be related to well log velocity profiles, which allow correlation across much of the eastern San Juan Basin. A quarry block of Mancos Shale from eastern Utah, USA, a common target for unconventional exploration and ultrasonically, compositionally, and texturally similar to the laminated muddy sandstone (LMS) lithofacies of the San Juan core, is examined to sublaminae or micro-lithofacies scales using optical petrographic and electron microscopy. This is mapped to results from axisymmetric compression (ASC) and indirect tensile strength testing of this facies at the core-plug scale and nanoindentation measurements at the micron scale. As anticipated, there is a marked difference in elastic and failure response in axisymmetric and cylinder splitting tests relating to loading orientation with respect to bedding or lamination. Shear bands and Mode-I fractures display contrasting fabric when produced at low or high angles with respect to lamination. Nanoindentation, mineralogy distribution based on MAPS (modular automated processing system) technique, and high-resolution backscattered electron images show the effect of composition, texture phases, and interfaces of phases on mechanical properties. A range of Young’s moduli from nanoindentation is generally larger by a factor of 1–4 compared with ASC results, showing the important effect of pores, microcracks, and bedding boundaries on bulk elastic response. Together these data sets show the influence of cement distribution on mechanical response. Variations in micro-lithofacies are first-order factors in determining the mechanical response of this important Mancos constituent and are likely responsible for its success in hydrofracture-based recovery operations as compared with other Mancos lithofacies types.

ABSTRACT A new model is proposed to predict porosity in organic matter for unconventional shale reservoirs. This model is based on scanning electron microscopic (SEM) observations that reveal porosity in organic matter is associated with secondary porosity developed within organic matter cement that fills void space preserved prior to oil generation. The organic matter cement is interpreted as solid bitumen resulting from the thermal alteration of residual oil retained in the source rock following oil expulsion. Pores are interpreted to develop within the solid bitumen as a result of thermal cracking and gas generation at increased levels of thermal maturity, transforming the solid bitumen to pyrobitumen. The pyrobitumen porosity model is an improvement over existing kerogen porosity models that lack petrographic validation. Organic matter porosity is predicted by first estimating the potential volume of organic matter cement by deriving the matrix porosity available at the onset of oil generation from extrapolations of lithologic specific compaction profiles. The fraction of organic matter cement converted to porosity in the gas window is then calculated by applying porosity conversion ratios derived from SEM digital image analysis of analogous shale reservoirs. Further research is required to refine and test the porosity prediction model.

ABSTRACT Organic matter (OM) in petroleum source rocks is a mixture of organic macerals that follow their own specific evolutionary pathways during thermal maturation. Understanding the transformation of each maceral into oil and gas with increasing thermal maturity is critical for both source rock evaluation and unconventional shale oil/gas reservoir characterization. In this study, organic petrology was used to document the reflectance, abundance, color, and fluorescence properties of primary organic macerals and solid bitumen (SB) in 14 Upper Devonian New Albany Shale samples (kerogen type II sequence) from early mature (vitrinite reflectance [VR o ] of 0.55%) to post-mature (VR o 1.42%). Micro-Fourier transform infrared (micro-FTIR) spectroscopy analyses were conducted on these samples to derive information on the evolution of the chemical structure of organic macerals and SB with increasing thermal maturity. Primary OM (amorphous organic matter, alginite, vitrinite, and inertinite) and secondary organic matter (SB) were identified in early mature samples. Amorphous organic matter (AOM) was the dominant organic component in early mature samples and was observed up to the maturity equivalent to VR o 0.79% but could not be identified at VR o 0.80%. An organic network composed of AOM and SB was observed from VR o 0.55 to 0.79%, which, together with the decrease in AOM content being accompanied by an increase in SB content, suggests that with the onset of petroleum generation, SB gradually replaced the original AOM. Alginite, represented by Tasmanites cysts, started to transform to pre-oil bitumen at a maturity corresponding to VR o 0.80%. It shows weak orange-yellow fluorescence at this maturity, a change from strong greenish-yellow fluorescence in early mature samples. Alginite could not be identified at VR o 0.89%, and generated bitumen remained in place or migrated over short distances. Petrographic observations and micro-FTIR study of alginite indicate that substantial hydrocarbon generation from alginite does not start until alginite is completely transformed to pre-oil bitumen. In contrast to AOM and alginite, vitrinite and inertinite derived from terrestrial woody materials occur as dispersed particles and do not change significantly during thermal maturation. A linear relationship between vitrinite and SB reflectance exists for the studied samples. The reflectance of vitrinite is higher than that of SB until VR o 0.99%, and at higher maturities, SB reflectance exceeds vitrinite reflectance. The inclusion of pre-oil SB converted from alginite in reflectance measurements could result in a lower average SB reflectance and, therefore, caution should be applied when using SB reflectance as an indicator of thermal maturity.