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.

Abstract The Middle Devonian Geneseo Formation and its lateral equivalents in the Northern Appalachian Basin are regarded as crucial secondary targets to the extensively explored Marcellus subgroup. High-resolution sedimentology, stratigraphy, and petrography have yielded differentiation of genetically related packages, comprised of distinct lithofacies with characteristic physical, biological, and chemical attributes. In addition, argon ion milling and nanoscale scanning electron microscopy of shale sections has shown that the pore structure of the Geneseo derives from pores defined by phyllosilicate frameworks, carbonate dissolution, and within organic matter. Intervals of silt-rich mudstones and muddy siltstones occur in multiple facies types and “interrupt” facies, reflecting background sedimentation. These deposits and their sedimentary features are interpreted as products of high-density fluvial discharge events. Pore morphology and distribution correlates with distinct mudstone lithofacies as a result of small-scale compositional and textural characteristics. Phyllosilicate framework pores are small triangular openings (100-1500 nm wide) and are the dominant pore type observed in hyperpycnites. Organic matter porosity is common (10-500 nm pore size) and dominates the organic-rich facies that represents “background” sedimentation with high organic content. Carbonate dissolution pores (50-500 nm wide) are observed in calcareous intervals and reflect partial dissolution of carbonate grains during catagenetic formation of carboxylic/phenolic acids.

Abstract Although typically considered with a focus on high-resolution petrography, shale porosity should not be thought of as a stand-alone petrographic feature. Shale and mudstone porosity is the outcome of a long succession of processes and events that span the continuum from deposition through burial, compaction, and late diagenesis. For the Eagle Ford Shale this journey began with accumulation in intra-shelf basins at relatively low latitudes on a southeast-facing margin during early parts of the late Cretaceous. To understand the factors that generated and preserved porosity in this economically important interval, a scanning electron microscope study on ion-milled drill-core samples from southern Texas was conducted to understand the development of petrographic features and porosity and place them in stratigraphic context. The studied samples show multiple pore types, including pores defined by mineral frameworks (clay and calcite), shelter pores in foraminifer tests and other hollow fossil debris, and pores in organic material (OM). In many instances, framework and shelter pores are filled with OM that has developed pores due to maturation. Large bubble pores in OM suggest that hydrocarbon liquids were left behind in or migrated into these rocks following petroleum generation and that the bubbles developed as these rocks experienced additional thermal stress. These larger OM pores indicate deeper seated interconnection on ion-milled surfaces and in three-dimensional image stacks. The largest pores occur in the infills of foraminifer tests. The framework of crushed carbonate debris in planktonic fecal pellets shows intermediate levels of porosity, and the silicate-rich matrix that encloses framework components has the smallest average porosity. The distribution of pore types is not uniform. Our hypothesis is that facies association is an important factor that determines bulk porosity and influences reservoir performance. The observed variability in the attributes of the described distal, medial, and proximal facies associations is thought to translate into significant variability of rock properties such as total organic carbon and porosity. In turn, this variability should control the quality and distribution of the intervals that are optimum sources and reservoirs of hydrocarbons in the Eagle Ford Shale. The medial facies association most likely has the best porosity development when a favorable combination of more commonly abundant calcareous fecal pellets and organic material versus clay content is present. The systematic arrangement of facies associations into parasequences provides the basis for testing and predicting the best development of optimal reservoir facies within a sequence-stratigraphic framework in the Eagle Ford Shale.

Abstract Quartz (SiO 2 ) is the most abundant mineral in terrigenous sedimentary rocks and is exceedingly durable (surviving multiple generations of weathering and deposition). Quartz and silica occur in many varieties—true quartz in the form of megaquartz, chert, microquartz, or chalcedony and various other forms of silica, mainly opal (opal-A and opal-CT [cristobalite]).

Abstract Feldspars (XAl (1-2) Si (2-3) O 8 ) are the most common rock- forming minerals in the Earth’s crust, and they occur in many varieties — ranging from sodium- and calcium-rich (plagioclase) to potassium-rich (K-feldspar or alkali feldspar). K-feldspars may also contain significant amounts of sodium in their crystal lattices. Feldspars are far less resistant than quartz to chemical and physical destruction and thus are altered or removed by weathering, transport and diagenesis, yielding secondary pores or alteration products (illite, white mica/sericite, albite or kaolinite). Even so, they are the second most abundant grains in sandstones, and identifying their mineralogy is crucial for accurate sandstone classification and provenance studies.

Abstract Rock fragments (also called lithic fragments or composite grains) can be derived from a wide variety of lithotypes and commonly have source-specific textures and compositions that can be recognized in thin section. Because of their multicrystalline/granular nature, rock fragments tend to be more common in the coarser grain-size modes of clastic terrigenous rocks (although, under the right circumstances, they can even be seen in mudrocks). Given the composite character of lithic fragments, many petrographers use the Gazzi-Dickinson method of point counting to record the constituent crystals within the fragments, rather than counting the fragments as such (Ingersoll et al., 1984). Rock fragments should be very common in sediments, and they are in many deposits, but because of their multi-crystalline or multi-granular nature, many succumb to the effects of weathering, abrasion or later mechanical or chemical diagenesis. But because the surviving rock fragments yield some of the most direct evidence of contributions from igneous, metamorphic or sedimentary terranes, it is especially important that such grains be accurately identified.

Abstract Accessory minerals include all the many detrital minerals that are found in clastic terrigenous rocks that do not contribute directly to rock classification (thus, primarily minerals other than quartz and feldspar). Although thousands of minerals could potentially fall under that definition; practically, a limited number are found with any great frequency. Accessory minerals as a whole typically make up less than 1% (rarely more than 2%) of most terrigenous sedimentary rocks. Quartz arenites commonly have the fewest accessory minerals (as little as 0.05% in some cases); arkoses are somewhat richer in accessories, and lithic arenites generally have the highest levels. This results from the fact that most accessory minerals, like some feldspars and lithic fragments, lack the abrasion resistance or chemical stability to survive erosion, transport and diagenesis. Accessory minerals can be examined in thin sections; alternatively, they can be concentrated by mechanical (shaker table) or flotation (heavy liquid) methods (see, for example, Munsterman and Kerstholt, 1996; Koroznikova et al., 2008) and can then be viewed with stereoscopic microscopes, SEM or other methods. Thin- section examination shows the grains in the context of rock fabric, but such minerals can be quite scarce in any single section. Disaggregation and concentration is much more effective for evaluating the full assemblage of such minerals in rock or sand samples and also allows identification by x-ray or geochemical methods. Because accessory minerals are so commonly studied as separates, they generally are divided into light and heavy minerals with a boundary drawn by various workers at specific gravities between 2.85 and 3 (they also are commonly divided into opaque and nonopaque minerals). The most commonly encountered detrital light accessory minerals are micas (mainly muscovite and also biotite). Heavy minerals are vastly more numerous, and can be grouped into ultrastable, intermediate stability, and unstable categories. The ultrastable minerals are the ultimate survivors, even more stable than quartz under most conditions — thus, they are found in most clastic terrigenous rocks. The intermediate group has varied levels of survivability, but most such minerals can be degraded or removed under specific conditions; minerals in the unstable group survive only under very favorable conditions (minimal mechanical and chemical stresses). There are so many detrital accessory minerals that occur in clastic terrigenous rocks that it is simply impossible to provide a brief yet usable summary of the mineralogical features and optical characteristics of all these minerals. We provide instead a chart of the relative stabilities of the most common accessory minerals (Table 4.1). In addition, characteristic mineral properties are described in the individual photo captions for each mineral illustrated. For additional information readers are encouraged to consult the references at the end of this chapter or the more general mineralogy texts listed in the bibliography in the introduction to this book.

Abstract Many clastic terrigenous rocks contain variable, but in some cases substantial, amounts of primarily nondetrital constituents. These include biogenic/skeletal grains such as calcareous shells, siliceous tests, phosphatic vertebrate or invertebrate material and organic matter (from plant remains down to plankton and microbial filaments). Nonskeletal, but still biogenic grains, primarily fecal pellets, also can be abundant in some deposits, especially bioturbated ones. Other materials, such as phosphate, gypsum, green marine clays (glauconite, berthierine, chamosite) and ferrous oxides and hydroxides, are found in terrigenous deposits as minerals formed by direct precipitation, through alteration of other minerals or as detrital grains. Most of these grains, in some circumstances, can be sufficiently abundant to be the major constituents of rocks. Even where such grains are not the major rock constituent, however, it is important to recognize them and, if deemed important, one can add a descriptive adjective to any rock name (e.g., glauconitic quartz arenite or radiolarian-bearing arkosic siltstone).

Abstract The term “texture” encompasses a wide range of attributes of sediments/rocks and their constituent grains, including grain size and sorting, particle morphology (form and sphericity, rounding, and surface texture), grain orientation, imbrication and packing. All of those properties have significance in interpreting transport processes and depositional settings of sedimentary rocks, but they also have economic importance in a wide variety of fields ranging from engineering of construction and road materials to understanding and predicting the porosity and permeability relationships of such materials in petroleum exploration/production or hydrologic contexts. Most textural properties mentioned above are best measured in unconsolidated or easily disaggregateable materials where grains can be size-sorted by sieving, settling, laser particle analysis or other techniques or where individual grains can be viewed in three dimensions. These properties generally are far more difficult to measure accurately in thin sections of consolidated rocks. To use just one example, the size of a grain in thin section can never exceed the longest axis of the grain but it can easily be shorter because most cuts through grains are tangential or oblique to that axis. So, in most cases, the basic size-, sorting- and shape-related properties of grains are merely estimated in thin sections through the use of visual comparators. Even there, the most useful comparators are ones that have been specifically corrected for thin-section use. Detailed direct measurements on grain size and shape can, of course, be done using thin-section microscopy, especially through point-counting large numbers of grains, but there too, correction factors must be used to overcome, to the degree possible, the two-dimensional (2-D) view of three-dimensional (3-D) grains afforded in thin sections (see Harrell and Eriksson, 1979; Johnson, 1994). Advances in computerized photomicrographic image analysis can make the process of measurement and measurement correction both more accurate and far less time consuming (see, for example, Schäfer and Teyssen, 1987; Seelos and Sirocko, 2005; Syvitski, 2007). An enormous amount of effort was made in the period from the 1930s to the 1970s to perfect textural measurements and to find reliable statistical measures that could be used to identify specific environments of deposition. That work still finds application in soft and unconsolidated sediments, but it will not be discussed in detail in this book, because it is of lesser applicability to petrographic studies of hard rocks. A number of papers in the bibliography at the close of this section can be used to follow up on unconsolidated sediment studies (especially the excellent overview of statistical measures provided in Folk, 1980) and almost all textbooks on sands and sandstones include discussions of these topics.