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International audience; 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.

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.

Enrichment of redox-sensitive trace elements in ancient marine shales is conventionally believed to be controlled by marine benthic redox conditions, whereas the influence of hydrographic conditions on trace element enrichment pattern has been rarely considered. Here, we present newly obtained data sets from the Upper Pennsylvanian organic-rich Cline Shale in the Midland Basin, Texas, to illustrate the influence of hydrographic circulation on the trace-element enrichment pattern and the stratigraphic record of mudrocks. Various lithofacies, including siliceous mudrocks, argillaceous mudrocks, skeletal-bearing argillaceous mudrocks, calcareous mudrocks, and wackestone, are identified in the Cline Shale. Significant changes in the trace-element enrichment pattern, mineral composition, texture of framboidal pyrite, and other bulk geochemical parameters in different lithofacies are interpreted to have been caused by high-amplitude and high-frequency glacio-eustatic sea-level oscillations in the silled Midland Basin during the late Paleozoic ice age. Specifically, glacio-eustatic sea-level falls generally resulted in the severe isolation of the Midland Basin from the Panthalassic Ocean, highly restricted hydrographic circulation, long deep-water renewal time, euxinic bottom-water conditions, depleted seawater Mo (molybdenum) in the silled basin, and low sediment Mo/TOC (total organic carbon), coupled with significant extrabasinal detrital quartz input, forming siliceous mudrocks. Enhanced phosphorus cycling and excellent preservation conditions are considered to be responsible for the high TOC observed in siliceous mudrocks. In contrast, glacio-eustatic sea-level rises substantially enhanced water exchange between the basin and the Panthalassic Ocean, created overall suboxic to anoxic bottom-water conditions, resupplied seawater Mo, elevated sediment Mo/TOC, and increased platform carbonate production in the basin, forming calcareous mudrocks and wackestone.

The spatiotemporal variability of soil water content (SWC) at the remote sensing scale requires dense monitoring for calibration and validation. Here, we present an overview of the Texas Soil Observation Network (TxSON), an intensively monitored area in the semiarid rangelands of the central Texas Hill Country. TxSON is a dense network consisting of 40 in situ locations nested at 36, 9, and 3 km within the Equal-Area Scalable Earth Grid and serves as a Core Calibration and Validation Site for NASA’s Soil Moisture Active Passive mission. The 4-yr dataset consists of hourly SWC measured at 5, 10, 20, and 50 cm. The SWC data are upscaled using arithmetic, Voronoi, and inverse distance weighting for 36-, (2) 9-, and (5) 3-km grid cells. We present the site selection, environmental characteristics, network design, quality assurance (QA), upscaling algorithms, and data structure. Ancillary data include bulk density, particle size, and carbon content for each site and depth. We also provide automated QA for each location and scripts to use our binary flagging in upscaling methods. To summarize, the 36-km grid cell has a mean bulk density of 1.34 ± 0.18 g cm and a loam textural class. The in situ SWC has a root mean square error of 0.029 m m against gravimetric data from 14 field campaigns. TxSON continues to add new locations with additional dense networks planned in other ecotones of the Edwards Plateau. The time series data along with scripts to import, plot, and upscaled SWC are available at .

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, ~5 to >90%) and δ13C values of C1–C3. 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 (%Ro > 1.5) gases with heavy δ13C(C1–C3) values indicate significant loss (>50%) of the early-stage gases. For the position-specific isotope deviations of propane, the ΔC2-1 values of the five samples show small decrease with well depths, while the ΔH2-1 values have a general increasing trend. Although some samples fall very close to the equilibrium model trajectories with reasonable calculated temperatures (138–148 °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.

Canonical models for speleothem formation and the subsurface carbon cycle invoke soil respiration as the dominant carbon source. However, evidence from some karst regions suggests that belowground CO 2 originates from a deeper, older source. We therefore investigated the carbon sources to central Texas caves. Drip-water chemistry of two caves in central Texas implies equilibration with calcite at CO 2 concentrations (P CO2_sat ) higher than the maximum CO 2 concentrations observed in overlying soils. This observation suggests that CO 2 is added to waters after they percolate through the soils, which requires a subsoil carbon source. We directly evaluate the carbon isotope composition of the subsoil carbon source using δ 13 C measurements on cave-air CO 2 , which we independently demonstrate has little to no contribution from host rock carbon. We do so using the oxidative ratio, OR, defined as the number of moles of O 2 consumed per mole of CO 2 produced during respiration. However, additional belowground processes that affect O 2 and CO 2 concentrations, such as gas-water exchange and/or diffusion, may also influence the measured oxidative ratio, yielding an apparent OR (OR apparent ). Cave air in Natural Bridge South Cavern has OR apparent values (1.09 ± 0.06) indistinguishable from those expected for respiration alone (1.08 ± 0.06). Pore space gases from soils above the cave have lower values (OR apparent = 0.67 ± 0.05) consistent with respiration and gas transport by diffusion. The simplest explanation for these observations is that cave air in NB South is influenced by respiration in open-system bedrock fractures such that neither diffusion nor exchange with water influence the composition of the cave air. The radiocarbon activities of NB South cave-air CO 2 suggest the subsoil carbon source is hundreds of years old. The calculated δ 13 C values of the subsoil carbon source are consistent with tree-sourced carbon (perhaps decomposing root matter), the δ 13 C values of which have shifted during industrialization due to changes in the δ 13 C values and concentrations of atmospheric CO 2 . Seasonal variations in P CO2_sat in most of the drip waters suggest that these waters exchange with ventilated bedrock fractures in the epikarst, implying that the subsoil CO 2 source contributes carbon to speleothems.

Six limestone assemblages along the North Anatolian Fault (NAF) Niksar pull-apart basin in northern Turkey were analyzed for δ18OPDB and δ13CPDB using bulk isotope ratio mass spectrometry (IRMS). Matrix-vein differences in δ18OPDB (−2.1 to 6.3‰) and δ13CPDB (−0.9 to 4.6‰) suggest a closed fluid system and rock buffering. Veins in one travertine and two limestone assemblages were further subjected to cathodoluminescence, trace element (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) and δ18OPDB (Secondary Ion Mass Spectrometry, SIMS) analyses. Fluid inclusions in one limestone sample yield Th of 83.8 ± 7.3 °C (±1σ, mean average). SIMS δ18OPDB values across veins show fine-scale variations interpreted as evolving thermal conditions during growth and limited rock buffering seen at a higher-resolution than IRMS. Rare earth element data suggest calcite veins precipitated from seawater, whereas the travertine has a hydrothermal source. The δ18OSMOW-fluid for the mineralizing fluid that reproduces Th is +2‰, in range of Cretaceous brines, as opposed to negative δ18OSMOW-fluid from meteoric, groundwater, and geothermal sites in the region and highly positive δ18OSMOW-fluid expected for mantle-derived fluids. Calcite veins at this location do not record evidence for deeply-sourced metamorphic and magmatic fluids, an observation that differs from what is reported for the NAF elsewhere along strike.

Assessing natural vs. anthropogenic sources of methane in drinking water aquifers is a critical issue in areas of shale oil and gas production. The objective of this study was to determine controls on methane occurrences in aquifers in the Eagle Ford Shale play footprint. A total of 110 water wells were tested for dissolved light alkanes, isotopes of methane, and major ions, mostly in the eastern section of the play. Multiple aquifers were sampled with approximately 47 samples from the Carrizo-Wilcox Aquifer (250-1200 m depth range) and Queen City-Sparta Aquifer (150-900 m depth range) and 63 samples from other shallow aquifers but mostly from the Catahoula Formation (depth 1 mg/L, 10 samples >10 mg/L). No dissolved methane samples exhibit thermogenic characteristics that would link them unequivocally to oil and gas sourced from the Eagle Ford Shale. In particular, the well water samples contain very little or no ethane and propane (C1/C2+C3 molar ratio >453), unlike what would be expected in an oil province, but they also display relatively heavier δ13 Cmethane (>-55‰) and δDmethane (>-180‰). Samples from the deeper Carrizo and Queen City aquifers are consistent with microbial methane sourced from syndepositional organic matter mixed with thermogenic methane input, most likely originating from deeper oil reservoirs and migrating through fault zones. Active oxidation of methane pushes δ13 Cmethane and δDmethane toward heavier values, whereas the thermogenic gas component is enriched with methane owing to a long migration path resulting in a higher C1/C2+C3 ratio than in the local reservoirs.

Clusters of elevated methane concentrations in aquifers overlying the Barnett Shale play have been the focus of recent national attention as they relate to impacts of hydraulic fracturing. The objective of this study was to assess the spatial extent of high dissolved methane previously observed on the western edge of the play (Parker County) and to evaluate its most likely source. A total of 509 well water samples from 12 counties (14,500 km2 ) were analyzed for methane, major ions, and carbon isotopes. Most samples were collected from the regional Trinity Aquifer and show only low levels of dissolved methane (85% of 457 unique locations 20 mg/L) are limited to a few spatial clusters. The Parker County cluster area includes historical vertical oil and gas wells producing from relatively shallow formations and recent horizontal wells producing from the Barnett Shale (depth of ∼1500 m). Lack of correlation with distance to Barnett Shale horizontal wells, with distance to conventional wells, and with well density suggests a natural origin of the dissolved methane. Known commercial very shallow gas accumulations (

Understanding the source of dissolved methane in drinking-water aquifers is critical for assessing potential contributions from hydraulic fracturing in shale plays. Shallow groundwater in the Texas portion of the Haynesville Shale area (13,000 km2 ) was sampled (70 samples) for methane and other dissolved light alkanes. Most samples were derived from the fresh water bearing Wilcox formations and show little methane except in a localized cluster of 12 water wells (17% of total) in a approximately 30 × 30 km2 area in Southern Panola County with dissolved methane concentrations less than 10 mg/L. This zone of elevated methane is spatially associated with the termination of an active fault system affecting the entire sedimentary section, including the Haynesville Shale at a depth more than 3.5 km, and with shallow lignite seams of Lower Wilcox age at a depth of 100 to 230 m. The lignite spatial extension overlaps with the cluster. Gas wetness and methane isotope compositions suggest a mixed microbial and thermogenic origin with contribution from lignite beds and from deep thermogenic reservoirs that produce condensate in most of the cluster area. The pathway for methane from the lignite and deeper reservoirs is then provided by the fault system.

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 inoceramids. Austin Chalk mineralogy is predominantly calcite and siliciclastic 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 marly chalk to calcareous siliciclastic 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 0.32 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.