The Barra Velha Formation is a prolific Aptian (Lower Cretaceous) oil and gas producer located in the Santos Basin, southeastern Brazilian coast. We used structural core description, petrography, geochemistry, and geochronology to show evidence from drill cores that the Barra Velha Formation was affected by several events of brittle deformation and interacting hydrothermal alteration. The U-Pb geochronology provides an age of ca. 116 Ma for depositional host rock calcites, although recrystallization is possible. An early brecciation event occurred at ca. 108 Ma. Recracked fractures (>10 cm wide) are interpreted as spring mound vents. A dolomite fault mineralization is dated at ca. 95 Ma. Events affecting fractures include widespread dissolution, with secondary porosity filled by barite, quartz, and solid bitumen. Fluid inclusion geochemistry suggests that diagenesis occurred with the participation of seawater. We show a timeline of diagenetic events and infer that fracture porosity is more likely to be preserved where deep faults enabled fluids from greater depths to ascend into the reservoir, where they created large vuggy cavities by dissolution of host rock.

Some of the most spectacular outcrops of Cretaceous rocks in Texas, USA, are exposed in the Trans-Pecos region. This trip focuses on the organic-rich marlstones and limestones of the Cenomanian–Turonian Boquillas Formation, which is equivalent to the Eagle Ford in central and east Texas and the producing region of south Texas. The Boquillas was deposited during the sea-level transgression that led up to Ocean Anoxic Event 2 (OAE2) and the subsequent regression. Unlike much of the global oceans, bottom-water redox conditions on the Texas shelf prior to the Middle Cenomanian Event up to OAE2 were anoxic to euxinic but became oxygenated during OAE2. This area also provides the opportunity to examine the effects of post-depositional tectonism on brittle mudrocks that were buried down to the early oil window, including faults, folds, and fractures. The trip also examines karsting in the rudist-rich upper Albian Georgetown Limestone (Devils River and Salmon Peak facies), the effects of topography on thickness and stacking patterns of the lower Cenomanian Del Rio Clay and Buda Limestone, fractures and scours in the Coniacian Austin Chalk, and the link between deep-seated faults related to Ouachita (Paleozoic) compression and subsequent Laramide (Cretaceous) tectonism.

The Upper Cretaceous Austin Chalk (AC) Group is an unconven-tional reservoir that extends across Texas and Louisiana. It is composed of interbedded layers of marly chalks to calcareous-siliciclastic mudrocks that vary in the degree of lamination, bioturbation, mineral abundance, and organic matter richness. Inte-grating lithologic observations with geochemistry is critical for interpreting depositional environments and modeling reservoir properties. Central to this integration is the ability to characterize the geochemistry of core samples at a resolution that captures thin-layered heterogeneity common to mudrock systems. Here, we developed a training data set using a semisupervised chemofa-cies clustering approach that is explored with a deep neural net-work model to predict chemofacies across multiple cores of the AC Group. Eight chemofacies are identified that capture differences in inorganic geochemistry, mineral abundance, rock fabric, and or-ganic matter richness; three classify differences in the marly chalks, four classify differences in the calcareous-siliciclastic mudrocks, and one is transitional between marly chalk and calcareous-siliciclastic mudrocks. Two distinct siliciclastic-carbonate mixing trends are identified that differ in modal abundances of tectosilicates and total clay. Two chemofacies are distinguished based on differences in Mo and V trace element enrichment, suggesting differences in bottom-water redox chemistry. Collectively, this approach pro-vides a means to integrate geochemical measurements and litholog-ical observations to interpret the depositional environments of mudrock systems and is an important step toward upscaling core data to characterize reservoir quality.

The Devonian-Carboniferous transition marks a fundamental shift in the surface environment primarily related to changes in ocean-atmosphere oxidation states(1,2), resulting from the continued proliferation of vascular land plants that stimulated the hydrological cycle and continental weathering(3,4), glacioeustasy(5,6), eutrophication and anoxic expansion in epicontinental seas(3,4), and mass extinction events(2,7,8). Here we present a comprehensive spatial and temporal compilation of geochemical data from 90 cores across the entire Bakken Shale (Williston Basin, North America). Our dataset allows for the detailed documentation of stepwise transgressions of toxic euxinic waters into the shallow oceans that drove a series of Late Devonian extinction events. Other Phanerozoic extinctions have also been related to the expansion of shallow-water euxinia, indicating that hydrogen sulfide toxicity was a key driver of Phanerozoic biodiversity.

Global warming during the Paleocene-Eocene Thermal Maximum (PETM) is hypothesized to have had a profound effect on the paleohydrologic cycle, including enhanced seasonality and increased water and sediment discharge. Although the PETM may represent the closest geologic analog for future global climate changes, the effects of this event on ancient coastal systems are poorly understood. We examined drill core from two locations in eastern Texas that preserve a record of tidally influenced deltaic sedimentation associated with the paleo-Colorado River that drained up to 2 x 10(6) km(2) of central North America, approximately two-thirds of the area of the modern Mississippi River catchment. In these cores, the development of a regionally extensive sand-rich unit (the Carrizo Formation) at the onset of the PETM is identified from a negative carbon isotope excursion and supported by detrital zircon U-Pb geochronology and pollen occurrence data. The basal Carrizo Formation indicates that the onset of the PETM was characterized by an increase in the delivery of coarser-grained sediments and progradation of the coastline, which occurred despite rising sea level. Using a mass-balance framework for equilibrium deltaic systems, we estimate that sediment delivery to the coastline increased by ca. 46% (2-sigma: 13% to 167%). Our findings of enhanced sediment delivery to the coast are consistent with proxy climatic and sedimentologic data indicating heightened precipitation seasonality in the interior of North America during the PETM. Thus, the effects of a regional change in climate forced by greenhouse events were transmitted downstream by large river systems to produce shifts in coastal sediment supply, progradation, and coastline evolution.

The characterization of subsurface reservoirs that are dominated by mudrock facies is hindered by the inherent heterogeneity and high degree of spatial variability typical of mudrock depositional systems. Subsurface reservoir properties that include porosity and permeability, fluid saturations, stratigraphic thicknesses of reservoir units, and source rock potential are ultimately controlled by the spatial distribution of sedimentary rock facies, which supports efforts to improve subsurface characterization workflows. Although core-based data provide direct measurements of rock attributes that are used to inform static reservoir models, capturing high-resolution core-based rock facies and downscaling these observations to tie to lower-resolution wireline logs remains a challenge. The effort to integrate core-based facies to reservoir-scale models is especially difficult when trying to capture thin-bedded heterogeneity that is common to mudrock systems. Herein, a workflow is developed and applied to visualize and integrate multivariate and spatially complex core-based data sets with wireline logs. Formation-specific core-based chemofacies training data sets are developed by integrating core descriptions with chemofacies clusters developed from high-resolution X-ray fluorescence core scanning. Core-based rock attribute data (e.g., XRD mineralogy, total porosity, and total organic matter content) are used to describe the chemofacies, providing a means to upscale low-resolution rock attribute measurements to high-resolution core-based chemofacies. Supervised core-based chemofacies training data sets are then used with neural network multiclass classification machine-learning tools to train triple combo wireline logs (gamma ray, deep resistivity, bulk density, and neutron porosity) to predict rock facies from wireline logs, providing a new approach to apply core-based facies classifications to wireline log studies. A basin-scale case study that applies this workflow is described for the Third Bone Spring Sand and units of the Wolfcamp Formation in the Delaware Basin of West Texas, United States.

The Early-Middle Jurassic (similar to 201-161.5 Ma) was characterized by major fluctuations in Earth's climate, tectonic activity, and large-scale magmatic events. There has been considerable focus on understanding the mechanisms that triggered the Early Jurassic Toarcian Oceanic Anoxic Event (TOAE; similar to 183 Ma)-a warming episode associated with a perturbation of the carbon cycle. This was marked by a negative delta C-13 isotope excursion (CIE) and CO2 release from the Karoo-Ferrar Large Igneous Province (KF-LIP)-resulting in widespread organic carbon burial under strong euxinic marine conditions. Despite extensive study of the TOAE from the NW European epicontinental shelf, less focus has been placed on the influence of North Sea Dome (NSD) uplift (late Toarcian-early Aalenian) and eruption (Bajocian-Bathonian) on Jurassic paleoceanography. The NSD is a local feature commonly invoked as a driver of regional paleoclimatic conditions-a bathymetric barrier between warm Tethyan Ocean and cooler Boreal Sea waters. This study contributes new, high-resolution geochemical data derived from Viking Corridor (VC) cores and cuttings that fills a critical spatial and temporal gap in records spanning the Early-Middle Jurassic. Redox-sensitive trace elements (RSTEs; Mo/Al, U/Al, V/Al) as well as Cd*Mn and Cd/Mo ratios were used to reconstruct paleoceanographic and redox conditions. We track magmatic events and their implications using the relative enrichment of Hg, and attempt to decouple global versus regional/local volcanic systems with the enrichment factor values of mafic trace metals (MTM-EFs; Co, Cu, Ni, and Cr). Varying concentrations of RSTEs and a particulate shuttle mechanism during the Early-Middle Jurassic suggest that the VC had fluctuating paleoredox conditions, and that the NSD did not fully restrict the VC. We document a negative CIE as well as high Hg influxes during the TOAE-likely associated with KF-LIP eruptions (Early Jurassic). In addition, pulses of elevated Hg and MTM-EF values were observed during the Aalenian and Bajocian (Middle Jurassic) that were likely a result of NSD uplift and eruptions. The subaerial exposure of the NSD created progressively restricted conditions during the Aalenian-Bajocian, but the seaway between the VC and European epicontinental shelf remained open. Together, this work provides evidence that the NSD had a critical role in driving paleobathymetric and paleoceanographic conditions during the Early-Middle Jurassic. Furthermore, this study provides a geochemical insight into the interplay between delta C-13 isotope excursions, the KF-LIP, the NSD, and Hg-RSTE-MTM systems during an important transition in the Earth's biogeochemical history.

X-ray fluorescence analysis and chemostratigraphy are fundamental tools used to characterize core, core plug, and drill cutting materials that are collected for oil and gas exploration. This chapter provides an overview of the fundamental methods required to conduct analyses and interpret X-ray fluorescence data and will familiarize the user with the development, application, and interpretation of pXRF methods, workflows, and research projects. The basic requirements necessary to clean and prepare core materials, the calibration techniques required to standardize datasets, and the multivariate statistical approaches that can be used to interpret collected datasets are discussed. These tools can be integrated into a robust workflow that will result in consistent elemental concentration datasets that can be interpreted by experienced geochemists looking to better characterize a rock unit of interest.

A novel integrated approach for chemofacies characterization of organic-rich mudrocks was developed using principal components analysis of 25 elements from core-based energy-dispersive x-ray fluorescence (ED-XRF) measurements and k-means clustering. Using this approach, three chemofacies were identified that capture the spread of the geochemical data sets (= 5000 ED-XRF measurements from three cores) of the organic-rich Cline Shale in the Midland Basin: (1) oxic-suboxic detrital-enriched argillaceous mudrocks, (2) anoxic siliceous mudrocks, and (3) oxicsuboxic intrabasinal carbonates. Chemofacies defined by the covariances of elemental concentrations can be correlated to depositional environments and primary grain assemblages in a predictable way, which has significant implications for the evolution of bulk rock properties, such as total organic carbon (TOC), rock mechanical property, porosity, and permeability. The stratigraphically thin anoxic siliceous mudrocks, which are also characterized by the highest TOC concentrations, moderate to high brittleness, and the highest porosity and permeability, are the "sweet spot" for shale oil production in the Cline Shale. Elevated TOC values of the anoxic siliceous mudrocks are most likely caused by anoxic benthic conditions. This integrated approach for chemofacies characterization of mudrocks is an important step to fully use all the geochemical data obtained through highresolution ED-XRF analyses and provide an economic, efficient, and nondestructive method to study core.

Exposed in large, continuous outcrops in Provence, southern France, the Castellas fault formed as a normal fault in the Upper Cretaceous and was reactivated as a left-lateral fault in the Eocene. Structural, petrographical, isotopic, and geochronological analyses shed light on the sequence of deformational-diagenetic stages of the fault zone and help identify properties that controlled fluid-flow behavior through time. Abrupt contrasts in fracture abundance compared to the undeformed host rock define a damage zone containing fractures arranged in regularly spaced clusters. We identified eleven episodes of calcite cementation within opening-mode fractures and host rock primary pores. Cement fabrics, sediment fills, and geochemistry show evidence of shallow burial environments. U-Pb geochronology of calcite cements indicate two main deformational phases of the fault zone during the Albo-Cenomanian Durancian uplift and Eocene Pyrenean orogeny at ca. 90 Ma and 50-40 Ma, respectively. Deformation created poro-permeability, but cementation followed shortly after, pervasively occluding most of the fault zone porosity by the end of the Eocene.

Severe global climate change led to the deterioration of environmental conditions in the oceans during the Toarcian Stage of the Jurassic. Carbonate platforms of the Western Tethys Ocean exposed in Alpine Tethyan mountain ranges today offer insight into this period of environmental upheaval. In addition to informing understanding of climate change in deep time, the effect of ancient carbon cycle perturbations on carbonate platforms has important implications for anthropogenic climate change; the patterns of early Toarcian environmental deterioration are similar to those occurring in modern oceans. This study focuses on the record of the early Toarcian Oceanic Anoxic Event ( ca 183.1 Ma) in outcrops of the north‐west Adriatic Carbonate Platform in Slovenia. Amidst environmental deterioration, the north‐west Adriatic Platform abruptly transitioned from a healthy, shallow‐water environment with diverse metazoan ecosystems to a partially drowned setting with low diversity biota and diminished sedimentation. An organic carbon‐isotope excursion of −2.2‰ reflects the massive injection of CO 2 into the ocean‐atmosphere system and marks the stratigraphic position of the Toarcian Oceanic Anoxic Event. A prominent dissolution horizon and suppressed carbonate deposition on the platform are interpreted to reflect transient shoaling of the carbonate compensation depth to unprecedentedly shallow levels as the dramatic influx of CO 2 overwhelmed the ocean’s buffering capacity, causing ocean acidification. Trace metal geochemistry and palaeoecology highlight water column deoxygenation, including the development of photic‐zone anoxia, preceding and during the Toarcian Oceanic Anoxic Event. Ocean acidification and reduced oxygen levels likely had a profoundly negative effect on carbonate‐producing biota and growth of the Adriatic Platform. These effects are consistent with the approximate doubling of the concentration of CO 2 in the ocean‐atmosphere system from pre‐event levels, which has previously been linked to a volcanic triggering mechanism. Mercury enrichments discovered in this study support a temporal and genetic link between volcanism, the Toarcian Oceanic Anoxic Event and the carbonate crisis.

The Austin Chalk Group is a very fine grained carbonate mudrock composed predominantly of microorganisms. To geologically characterize such a unit, the components and depositional and diagenetic features must be examined at the nano- to microscale, using micropetrography. Analytical methods include scanning electron microscopy, energy-dispersive X-ray spectroscopy, and thin-section analysis. Through micropetrographic analysis, we determined Austin Chalk composition to be mainly a matrix of coccoliths and coccolith elements in 20- to 30-micron peloids (probably marine snow). The most common larger components are planktic foraminifers and bottom-dwelling inoccramids. Austin Chalk mineralogy is predominantly calcite and silicidastic components that are mainly clay minerals, quartz, and albite. Grain sizes of Austin Chalk components are generally in the clay- to medium-silt range. Four basic lithofacies define the Austin Chalk: (1) lithofacies 1: highly bioturbated, organic-matter-poor marly chalk, (2) lithofacies 2: highly bioturbated, moderately organic-matter-rich, marly chalk to chalky marl, (3) lithofacies 3: sparsely burrowed, poorly to moderately laminated, organic-matter-rich marly chalk, and (4) lithofacies 4: well-laminated, organic-matter-rich manly chalk to calcareous silicidastic mudstone. These four lithofacies are distinguishable by their different fabrics, as well as by ranges in mineral composition and TOC content. Mean of organic-matter content ranges from 032 to 2.56 wt%, and the kerogen is generally Type I and Type II, except in lithofacies 1, in which Type III is more common. The abundance and type of organic matter suggest that the Austin Chalk can be self-sourcing. This investigation provides the basic nano- to micro-observations that will aid in other studies of the Austin Chalk Group and geologically similar formations relative to rock-strength/brittleness, source-rock potential, and reservoir quality. (C) 2020 Elsevier B.V. All rights reserved.

We present new carbon isotope ( δ 13 C), redox-sensitive trace metal (RSTM), and Hg records from the Viking Corridor— Norwegian Continental Shelf (NCS)—to assess the spatial and temporal evolution of redox conditions between Early-Middle Jurassic (186-169 Ma). We assess a global and a regional event —the Toarcian Oceanic Anoxic Event (T-OAE) and the localized uplift of the North Sea Dome (NSD; ~182-170 Ma), respectively. The T-OAE is characterized by a global warming event and a perturbation of the carbon cycle—expressed as a negative δ 13 C shift—linked to CO 2 release from the Karoo-Ferrar LIP (KF-LIP). The NSD acted as a large bathymetric barrier, disrupting regional paleocurrents and basin architecture. Despite extensive studies of the T-OAE, its expression across the NCS is still limited. Similarly, while we understand the role that the NSD played in the tectonostratigraphic configuration of the NCS, the impact on redox conditions during its onset and evolution remains elusive. The scarcity of continuous Early-Middle Jurassic geochemical records has contributed to this gap —the focus of this study. Results confirm the T-OAE presence with a negative δ 13 C excursion that coincides with the onset of KF-LIP along with elevated Hg and RSTM. Our data demonstrates a spatial Hg trend across the basin, where proximal settings are elevated in Hg and RSTM. Temporally, in proximal settings, post-T-OAE successions are Hg-enriched

A dataset of bulk and position-specific isotope compositions of shale gases from the Late Cretaceous Eagle Ford Shale, south Texas is reported. The chemical and bulk isotopic compositions of the seven samples in this study and those available from the literature show that across the play, produced natural gas from the Eagle Ford Shale preserves a wide range of gas compositions (wetness, similar to 5 to >90%) and delta C-13 values of C-1-C-3. The depth profiles of the isotopic compositions suggest that the western region of the Eagle Ford Shale deposit had experienced as much as 700-800 m of uplift-erosion or gas migration. The timing of and lost-gas fractions by gas expulsion events very likely affected their gas compositions and bulk isotope compositions. Some deep (>3000-3500 m), matured (%R-o > 1.5) gases with heavy delta C-13 (C-1-C-3) values indicate significant loss (>50%) of the early-stage gases. For the position-specific isotope deviations of propane, the Delta C2-1 values of the five samples show small decrease with well depths, while the Delta H2-1 values have a general increasing trend. Although some samples fall very close to the equilibrium model trajectories with reasonable calculated temperatures (138-148 degrees C), the position-specific isotope compositions of propane from the Eagle Ford Shale gases are likely results of thermal cracking of various organic molecules within the source rocks, which have different activation energy for cracking and non-statistical distributions of C/H isotope within them.

Mixtures of water and inorganic acids show differences in isotopic composition between the vapor and liquid phases, reportedly even at an azeotrope. A recent reactive azeotropy analysis rationalized this by allowing for differences in stable isotope composition between bound solvation complexes and the bulk liquid. The objective of this study is to determine whether or not significant differences in stable isotope composition exist between bound solvation complexes involving inorganic acids and the bulk solution. Rayleigh distillation of a solution of nitric acid and water was conducted. During the distillation, it was expected that bulk water would be vaporized first while (bound) water involved with hydrating the acid would be vaporized at later times along with the acid. Samples of both distillate and residual liquid were collected over time intervals and analyzed to determine oxygen and nitrogen stable isotope composition using isotope ratio mass spectrometry. As a reference, comparative samples were collected from Rayleigh distillation of water alone. The measured oxygen fractionation factors suggest that the acid is preferentially hydrated with 18O-enriched water, compared to the unbound, or bulk, water. Preferential coordination of 18O-enriched water with the dissolved acid is consistent with heavier water forming stronger bonds relative to light water. The results, although obtained under non-equilibrum conditions, suggest that isotope-dependent partitioning between bulk solvent and dissolved complexes may prove necessary in developing accurate descriptions of isotope fractionation during vapor-liquid equilibrium.

A rock-based geologic characterization was completed on a continuous core through the Austin Chalk Group section, an active exploration target in southern Texas. Because this core (located in the Pearsall field) is continuous and includes both lower and upper contacts, it can be used as the type cored section for the southern Texas Austin Chalk. Several general lithofacies are defined. Two lithofacies (lithofacies 1 and 2) are highly bioturbated and have low to moderate organic matter content and are poor to moderate source rocks. Two other lithofacies (lithofacies 3 and 4) are laminated, lithofacies 3 containing some burrows and lithofacies 4 containing no burrows. Both lithofacies are good source rocks. Organic matter in the laminated lithofacies 3 and 4 and some of the organic matter in the bioturbated lithofacies 2 are dominated by type II kerogen, suggesting the section can be self-sourcing. A general trend is observed whereby lithofacies that were deposited under more-oxygenated conditions increase in abundance up section and is related to a decrease in total organic carbon and, hence, source-rock quality. The primary oil and gas production is from fractures with possible later production from the nano- to microporous matrix after pressure drawdown. Pore types are predominantly interparticle nano- to micropores and lesser intraparticle nanopores as defined by Loucks et al. (2012). Porosity is less than 8%, and permeability is less than 0.05 md and ranges down into the nanodarcy range. The type cored section adds to the understanding of the southern Texas Austin Chalk by providing a geological description of lithofacies, stacking patterns, source-rock quality characterization, and reservoir quality. The type core can be used as a comparison section to other cored sections of the Austin Chalk.