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 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 This chapter analyzes the different lithofacies of the Vaca Muerta Formation using macroscopic observations, standard microscopy textural studies, scanning electron microscopy (SEM), and x-ray diffraction (XRD) techniques and presents them in a paleo-environmental and a sequence stratigraphic context. Microfacies-scale analyses in the Vaca Muerta Formation reveal a wide range of sedimentary and diagenetic processes that are essential for the understanding of this heterogeneous self-sourced unconventional reservoir. Seventeen of the more representative lithofacies in the Vaca Muerta Formation are described based on textures, grain type, and matrix composition. XRD data indicate that most of the mudstones microfacies show quartz–feldspar values above 40%, carbonate between 20% and 40%, clay lower than 35%, and total organic carbon (TOC) between 2% and 8%. Porosity of crushed rock samples shows common values between 10% and 16%. Porosity observed in SEM can be divided into interparticle, intraparticle, and organic-hosted type. Coccoliths are commonly found as calcareous fecal pellets that can make up to 35% of the rock volume. Pores within pellets range from hundreds of nanometers to more than 5 μm. Lateral and vertical distribution of lithofacies is controlled by the sedimentary environments and the systems tracts. Transgressive systems tracts (TST) show an enrichment in siliceous particles, the highest TOC values, and abundant organic-hosted porosity. Highstand systems tracts (HST) are carbonate rich, with lower TOC values and interparticle and intraparticle porosity. Comparison of the lithological-dependent characteristics from the Vaca Muerta Formation with other well-known unconventional plays shows that the Vaca Muerta Formation shares many characteristics with the Oxfordian–Tithonian Haynesville/Bossier Shale and the Cenomanian–Turonian Eagle Ford Shale, such as lithofacies, grains composition, mineralogical composition, diagenesis, percentage of TOC, porosity volume, and pore types.

Abstract By comparing new detrital zircon provenance analysis of Triassic synrift sediments from the Tallahassee graben (FL), the South Georgia rift basin (GA), and Deep River rift basin (NC) with our previous detrital zircon provenance data for the Jurassic Norphlet Formation erg in the Eastern Gulf of Mexico, we have developed a regional model of Triassic-Jurassic erosion and sediment transport. In the Eastern Gulf of Mexico, detrital zircon ages observed in Triassic synrift clastics from the Tallahassee graben and southern South Georgia rift system contain not only Gondwanan-aged and Grenville-aged zircon grains but also an abundance of Paleozoic detrital zircon grains, reflecting sediment influx from rocks associated with the Paleozoic orogens of eastern Laurentia. Although Paleozoic detrital zircon grains are present in the younger Norphlet deposits, they are less abundant than in Triassic rift sediments. In southwest Alabama, the most abundant detrital zircon age population in the Norphlet Formation is Grenville-aged (950-1,250 Ma). In the Conecuh embayment of southeastern AL and western FL panhandle, Norphlet samples show a marked decrease in Grenville detrital zircon and an increase in 525-680 Ma zircon ages, interpreted to represent influx from rocks associated with the Gondwanan Suwannee terrane. In the Apala-chicola Basin, the proportion of Gondwanan zircon ages increases to nearly 40% of the total population and Grenville-aged grains constitute just ~20% of the population. We suggest that the difference between Triassic and Jurassic detrital zircon signatures in the Eastern Gulf of Mexico reflects significant unroofing of Paleozoic rocks during early Mesozoic rifting of the easternmost Eastern Gulf of Mexico, possibly including rocks equivalent with those exposed in the Talladega slate belt units. Subsequent erosion of rift-flanking highlands to expose older Gondwanan and Grenville rocks and/or input from northern sediment sources supplied the older Grenville-aged detrital zircon grains present in the Norphlet erg in the area to the west and within the Conecuh embayment.

Abstract Although the Upper Jurassic Haynesville Formation is a proven hydrocarbon reservoir in the onshore eastern Gulf of Mexico, the unit remains understudied because exploration has been focused on the older and more productive Norphlet and Smackover Formations. In this study, we use cores, gamma ray logs, and spontaneous potential logs from 49 wells in southern Alabama to analyze the Haynesville Formation. We point-counted 18 sandstone samples from seven cores, and six sandstone samples from four of those cores were analyzed for detrital zircon age distributions. Core and well log analyses indicate that the Haynesville Formation can be subdivided into anhydrite, sandstone, and carbonate lithofacies. The thickness and distribution of these lithofacies suggests that relict basement topography derived from the opening of the eastern Gulf of Mexico during Late Triassic-Early Jurassic time is the primary influence on Upper Jurassic sediment distribution. Framework grain compositions indicate that the sandstone lithofacies was derived from a recycled orogenic provenance, indicating a primarily Laurentian terrane source with some mixing from the Gondwanan Suwannee terrane. Detrital zircon age distributions from Haynesville Formation sandstones contain major age populations that correspond with derivation from both the Laurentian Grenville Province and Appalachian Mountain source rocks, with some mixing from the Gondwanan Suwanee terrane. Haynesville Formation detrital zircon ages and sandstone compositions are similar to that of the underlying Norphlet Formation, indicating that the provenance and sediment transport pathways remained similar through deposition of the Upper Jurassic units.

Abstract Porosity and pore size distribution (PSD) are required to calculate reservoir quality and volume. Numerous inconsistencies have been reported in measurements of these properties in shales (mudrocks). We investigate these inconsistencies by evaluating the effects of fine grains, small pores, high clay content, swelling clay minerals and pores hosted in organic content. Using mudrocks from the Haynesville, Eastern European Silurian, Niobrara, and Monterey formations, we measured porosity and pore or throat size distribution using subcritical nitrogen (N 2 ) gas adsorption at 77.3 K, mercury intrusion, water immersion, and helium porosimetry based on Gas Research Institute standard methodology. We used scanning electron microscope (SEM) images to understand the pore structure at a microscopic scale. We separated the samples from each formation into groups based on their clay and total organic carbon (TOC) contents and further investigated the effects of geochemical and mineralogical variations on porosity and PSD. We find that differences in the porosity and PSD measurement techniques can be explained with thermal maturity, texture, and mineralogy, specifically clay content and type and TOC variations. We find that porosity and PSD measurement techniques can provide complementary information within each group provided the comparison is made between methods appropriate for that group. Our intent is to provide a better understanding of the inconsistencies in porosity measurements when different techniques are used.