Generation and migration of hydrocarbons in offshore South Texas Gulf Coast sediments

The hydrocarbon content of two thick Tertiary sequences from the offshore Gulf Coast (South Padre Island and Mustang Island) was studied using a headspace technique, thermal distillation, pyrolysis and solvent extraction. The threshold of oil generation was determined to occur in the range of 3050 m (10,000 ft; 120°C) in Miocene sediments.In the South Padre Island well, the distribution of the different classes of hydrocarbons along the sedimentary column suggests some updip migration processes are occurring.     

Arsenic enrichment in unconfined sections of the southern Gulf Coast aquifer system, Texas

Groundwater arsenic concentrations exceeding the federal drinking water standard are common in the southern Gulf Coast aquifer system in Texas, including in aerobic, unconfined groundwater which provides much of the municipal and domestic water supplies for the region. The objective of this study was to determine geochemical factors affecting the occurrence and distribution of groundwater As in unconfined portions of the southern Gulf Coast aquifer system through a comparative transect study of groundwater across three major hydrostratigraphic units (the Catahoula Formation, Jasper aquifer and Evangeline aquifer) and analysis of regional water quality data. Results show that As concentrations decrease with increasing distance from the Catahoula Formation, which is consistent with Miocene volcanic ash as the main source of As to groundwater in the region. Arsenic concentrations correlate with V, SiO₂ and K, all of which were released during weathering of volcanic sediments and their degradation products. In all three units, carbonate weathering and active recharge in the unconfined zones result in circum-neutral pH and oxidizing groundwater, which are typically amenable to As immobilization by adsorption of arsenate onto mineral oxides and clays. However, As concentrations exceed 10 μg/L in approximately 30% of wells. Silica that was co-released with As may compete for sorption sites and reduce the capacity for arsenate adsorption.     

The origin and isotopic composition of dissolved sulfide in groundwater from carbonate aquifers in Florida and Texas

The δ³⁴S values of dissolved sulfide and the sulfur isotope fractionations between dissolved sulfide and sulfate species in Floridan ground water generally correlate with dissolved sulfate concentrations which are related to flow patterns and residence time within the aquifer. The dissolved sulfide derives from the slow in situ biogenic reduction of sulfate dissolved from sedimentary gypsum in the aquifer. In areas where the water is oldest, the dissolved sulfide has apparently attained isotopic equilibrium with the dissolved sulfate (Δ³⁴S = 65 per mil) at the temperature (28°C) of the system. This approach to equilibrium reflects an extremely slow reduction rate of the dissolved sulfate by bacteria; this slow rate probably results from very low concentrations of organic matter in the aquifer.In the reducing part of the Edwards aquifer, Texas, there is a general down-gradient increase in both dissolved sulfide and sulfate concentrations, but neither the δ³⁴S values of sulfide nor the sulfide-sulfate isotope fractionation correlates with the ground-water flow pattern. The dissolved sulfide species appear to be derived primarily from biogenic reduction of sulfate ions whose source is gypsum dissolution although upgradient diffusion of H₂S gas from deeper oil field brines may be important in places. The sulfur isotope fractionation for sulfide-sulfate (about 38 per mil) is similar to that observed for modern oceanic sediments and probably reflects moderate sulfate reduction in the reducing part of the aquifer owing to the higher temperature and significant amount of organic matter present; contributions of isotopically heavy H₂S from oil field brines are also possible.     

A survey of oil and gas wells in the Texas Gulf Coast,USA, and implications for geological sequestration of CO<Subscript>2</Subscript>

Subsurface sequestration of CO₂ in oil and gas provinces where permanence of hydrocarbon accumulations has proven the reliability of potential traps is rightly seen as a solid option for containment of CO₂ atmospheric concentrations. However, one of the most promising provinces for carbon storage in North America, the Texas Gulf Coast, has also been heavily drilled for more than a century, puncturing many otherwise perfectly sound seals (>125,000 wells over ~50,000 km²). As a result, boreholes and, in particular, older abandoned wells could be major leakage pathways for sequestered CO₂. This article presents statistics on well spatial and depth distribution that have been drawn from public domain sources and relates these data to historical plugging and abandonment regulations in the Texas Gulf Coast. Surface-well density averages of 2.4 wells/km² can be locally much higher—but also much lower in larger areas. Average well penetration density drops to 0.27 and 0.05 well/km² below a depth of 2,440 and 3,660 m, respectively. Natural mitigating factors such as thief zones and heaving “shales” could also play a role in limiting the impact of these direct conduits to the shallow subsurface and surface.     

CO<Subscript>2</Subscript> source-sink matching in the lower 48 United States,with examples from the Texas Gulf Coast and Permian Basin

Documenting geographic distribution and spatial linkages between CO₂ sources and potential sinks in areas with significant levels of CO₂ emissions is important when considering carbon-management strategies such as geologic sequestration or enhanced oil recovery (EOR). For example, the US Gulf Coast overlies a thick succession (>6,000 m [>20,000 ft]) of highly porous and permeable sandstone formations separated by thick, regionally extensive shale aquitards. The Gulf Coast and Permian Basin also have a large potential for EOR, in which CO₂ injected into suitable oil reservoirs could be followed by long-term storage of CO₂ in nonproductive formations below reservoir intervals. For example, >6 billion barrels (Bbbl) of oil from 182 large reservoirs is technically recoverable in the Permian Basin as a result of miscible-CO₂ flooding_. The Gulf Coast also contains an additional 4.5 Bbbl of oil that could be produced by using miscible CO₂. Although the CO₂ pipeline infrastructure is well-developed in the Permian Basin, east Texas and the Texas Gulf Coast may have a greater long-term potential for deep, permanent storage of CO₂ because of thick brine-bearing formations near both major subsurface and point sources of CO₂.     

The origin of silt particles in the loess question

Many researchers consider loess as solely the product of glacial activity. Investigations throughout South America, however, have demonstrated that loess and loess-like sediments can be formed during geological periods when glaciation was absent. Loess deposits may be derived from volcanic action, from weathering processes, and as a result of pedogenic activity. Production of silt, regardless of the geologic process, can result in loess formation under suitable climatic conditions.     

The origin of carbonate mud in isolated carbonate platforms of Belize,Central America

Carbonate mud from three small isolated carbonate platforms of Belize (Central America) is largely a product of the breakdown of skeletal grains. The composition of the 63-20- and 20-4-µm fractions of 37 samples was determined by point counting under SEM; composition of the <4-µm fraction was assessed qualitatively under SEM. The 63-4-µm fractions are dominated by fragments of mollusks, the codiacean alga Halimeda, and other skeletal types. About one-third of the particles in the 63-4-µm fractions remained unidentified, probably due to obliteration of diagnostic features by early recrystallization processes such as micritization. Nanograins (<1 µm) and short (3-5 µm) aragonite needles are most common in the <4-µm fraction. These grains are interpreted to be largely fragments of codiacean algae (Halimeda, Penicillus). The 20-4- and <4-µm sediment fractions are composed of 80 and 75% aragonite on average, respectively. Trace element composition of strontium averages 7,900 ppm in the 20-4-µm fraction and in codiacean algae samples, and 5,600 ppm in the <4-µm fraction. Geochemical (trace element) data also argue against inorganic aragonite precipitation within the water column, and favor a skeletal origin. The lower strontium contents in the <4-µm fraction as compared with the 20-4-µm fraction may also be a consequence of early recrystallization processes.     

Age and origin of clayey silt ‚brickearth’︁ in west London,England

The loamy ?brickearth’? deposits overlying coarse cold climate Kempton Park, Lynch Hill and Taplow Gravels of the R. Thames in west London are dated by thermoluminescence and mineralogical comparisons with loesses of three known ages. Although much of the ?brickearth’? resembles loess, it often contains more sand or clay than typical loess, and the mineralogical studies suggest that even the coarse silt (16–63 μm) fraction is rarely similar in composition to any known English loess. The thermoluminescence measurements suggest that most of the ?brickearth’? on the Taplow and Lynch Hill Gravels was deposited in the late Devensian, whereas that on the Park Gravel is mainly Flandrian. It is suggested that the latter is mainly floodloam, whereas the earlier ?brickearth’? is a mixed aeolian sediment derived from various local sources in and close to the Thames Valley.     

The fate of foramol (“temperate-type”) carbonate platforms

On rimmed shelves of Bahamian-type, characterized by chlorozoan associations and typical of tropical seas, carbonate production keeps pace with normal sea-level rise except when rapid rise or drastic environmental changes occurs. On the other hand, open temperate carbonate shelves are characterized by low carbonate production of the foramol association (molluscs, benthic foraminifera, bryozoans, coralline algae, etc.) and generally show seaward relict sediments, because carbonate production cannot keep pace with normal rate of sea-level change.

Several examples of recent drowning foramol carbonate platforms (e.g., large areas of the Mediterranean Sea, eastern-northeastern Yucatan Shelf) as well as analogous ancient drowned foramol-type carbonate platforms (e.g., early to middle Miocene of the Southern Apennines; Miami Terrace) may support the idea that the drowning of many ancient carbonate platforms has been favoured by their biogenic (foramol sensu lato) constitution. Because of their typically low rate of growth, foramol carbonate platforms are fated to be drowned even if the sea-level rise is one with which the normal growth of chlorozoan platforms can keep pace. Similar conditions may also occur in tropical areas where variations in environmental conditions, such as the presence of cold waters, changes in salinity and increased nutrients, preclude the development of chlorozoan associations.     

A modeling study of seawater intrusion in Alabama Gulf Coast,USA

A numerical model of variable-density groundwater flow and miscible salt transport is developed to investigate the extent of seawater intrusion in the Gulf coast aquifers of Alabama, USA. The SEAWAT code is used to solve the density-dependent groundwater flow and solute transport governing equations. The numerical model is calibrated against the observed hydraulic heads measured in 1996 by adjusting the zonation and values of hydraulic conductivity and recharge rate. Using the calibrated model and assuming all the hydrogeologic conditions remain the same as those in 1996, a predictive 40-year simulation run indicates that further seawater intrusion into the coastal aquifers can occur in the study area. Moreover, the predicted intrusion may be more significant in the deeper aquifer than the shallower ones. As the population continues to grow and the demand for groundwater pumping intensifies beyond the 1996 level, it can be expected that the actual extent of seawater intrusion in the future would be more severe than the model prediction. Better strategies for groundwater development and management will be necessary to protect the freshwater aquifers from contamination by seawater intrusion.

Organic contaminants and characteristics of sediments from Oso Bay,south Texas,USA

The Oso Bay, Texas, sediments from nine sites were analyzed by GC-MS for organics to measure contamination in the bay. In most of the sites sediments contained tetrachloroethene (87–1433 g/kg), bis (2-ethylhexyl)phthalate (40–193 g/kg), and aliphatic hydrocarbons, C₈-C₁₃ (720–2491 g/kg). Sources of these contaminants include a landfill, military facilities, and municipal and industrial discharges. Size analysis of the sediments indicates they contain a high percentage of muddy sand (50–75 percent), which suggests that Oso Bay consists of common bay margin sediments.     

Variations in shoreface progradation and ravinement along the Texas coast, Gulf of Mexico

Shoreface architecture, evolution (mid-Holocene to present) and depths of transgressive ravinement were examined from Sabine Pass, at the Texas–Louisiana border, to South Padre Island, near the Texas–Mexico border, using 30 shoreface transects. Shoreface transects extend out to 16-m water depth, each created from an echo-sounding profile and, on average, seven sediment cores. The shoreface is composed of three broad sedimentological facies: the upper shoreface, composed almost entirely of sand; the proximal lower shoreface, composed of sand with thickly to medium-bedded (50–10 cm) mud; and the distal lower shoreface, composed dominantly of mud with medium- to thinly bedded (20–3 cm) sand. Shoreface architecture and evolution is extremely variable along the Texas coast. Shoreface gradients increase from 2·25 m km^–1 in east Texas to 3·50 m km^–1 in south Texas. Shoreface sands coarsen towards south Texas. East and south Texas shoreface deposits are thin and retrograding whereas central Texas shoreface deposits are thicker and prograding. Central Texas is characterized by stacked shoreface successions, whereas in east Texas, lower shoreface sands are preserved only in offshore banks. Preservation of shoreface deposits is low in south Texas. Although eustatic fluctuations and accommodation space have a strong impact on overall mid-Holocene to present shoreface evolution and preservation potential, along-strike variations in sediment supply and wave energy are the main factors controlling shoreface architecture. The transgressive ravinement surface varies from –6 to –15 m along the Texas coast.     

A multidimensional fuzzy least-squares regression approach for estimating hydraulic gradients in unconfined aquifer formations and its application to the Gulf Coast aquifer in Goliad County,Texas

Epistemic uncertainties arise during the estimation of hydraulic gradients in unconfined aquifers due to planar approximation of the water table as well as data gaps arising from factors such as instrument failures and site inaccessibility. A multidimensional fuzzy least-squares regression approach is proposed here to estimate hydraulic gradients in situations where epistemic uncertainty is present in the observed water table measurements. The hydraulic head at a well is treated as a normal (Gaussian) fuzzy variable characterized by a most likely value and a spread. This treatment results in hydraulic gradients being characterized as normal fuzzy numbers as well. The multidimensional fuzzy least-squares regression has an exact analytical form and as such can be implemented easily using matrix algebra methods. However, the method was noted to be sensitive to round-off and truncation errors when the epistemic uncertainties are small. A closeness index based on the cardinality of a fuzzy number is used to evaluate how well the regression model fits the fuzzy hydraulic head observations. A fuzzy Euclidian distance measure is used to compare two fuzzy numbers and to evaluate how fuzziness in the observed hydraulic heads affects the fuzziness in the estimated hydraulic gradients. The Euclidian distance measure is also used to ascertain the influence of each well on the fuzzy hydraulic gradient estimation. The fuzzy regression framework is illustrated by applying it to evaluate hydraulic gradients in the unconfined portion of the Gulf Coast aquifer in Goliad County, TX. The results from the case-study indicate that there is greater uncertainty associated with the estimation of the hydraulic gradients in the vertical (Z-axis) direction. The epistemic uncertainties in the hydraulic head data at the wells have a significant impact on the gradient estimates when they are of the same order of magnitude as the most likely values of the observed heads. The influence analysis indicated that 5 of the 13 wells in the network had a critical influence on at least one of the hydraulic gradients. Three wells along the northeastern section of the study area and bordering the Victoria County were noted to have the least influence on the regression estimates. The fuzzy regression framework along with the associated goodness-of-fit and influence measures provides a useful set of tools to characterize the uncertainties in the hydraulic heads and gradients arising from data gaps and planar water table approximation.     

Deep‐burial alteration of early‐diagenetic carbonate concretions formed in Palaeozoic deep‐marine greywackes and mudstones (Bardo Unit,Sudetes Mountains,Poland)

Carbonate concretions formed in bathyal and deeper settings have been studied less frequently than those formed in shallow‐marine deposits. Similarly, concretions affected by catagenetic conditions have rarely been reported. Calcite concretions in deep‐marine mudstones and greywackes of the Bardo Unit (Sudetes Mountains, Poland) formed during early diagenesis and were buried to significant depths. Petrographic and geochemical (elemental and stable C and O isotopic) analyses document their formation close to the sediment–water interface, prior to mechanical compaction within the sulphate reduction zone and their later burial below the oil window. Although the concretions were fully formed during early diagenesis, the effects of increased temperature and interaction with late‐diagenetic interstitial fluids can be discerned. During maximum burial, the concretions underwent thorough recrystallization that caused alteration of fabric and elemental and O isotope composition. The initial finely crystalline cement was replaced by more coarsely crystalline, sheaf‐like, poikilotopic calcite in the concretions. These large calcite crystals engulf and partially replace unstable detrital constituents. The extremely low δ¹⁸O values (down to ?21·2‰ Vienna Pee Dee Belemnite) in the concretions are the result of the increased temperature in combination with alteration of volcanic glass, both causing a significant ¹⁸O‐depletion of bicarbonate dissolved in the interstitial fluids. Recrystallization led to uniform O isotope ratios in the concretions, but did not affect the C isotope signature. The δ¹³C values of the late‐diagenetic cements precipitated in the greywacke and in cracks cutting through concretions imply crystallization in the catagenetic zone and decarboxylation as a source of the bicarbonate. These late‐diagenetic processes took place in a supposedly overpressured setting, as suggested by clastic dykes and hydrofractures that cut through both concretions and host rock. All of these features show how the effects of early and late diagenesis can be distinguished in such rocks.     

The origin of fluids in the salt beds of the Delaware Basin,New Mexico and Texas

Oxygen and hydrogen isotope analyses have been made of (1) brines from several wells in the salt deposits of the Delaware Basin, (2) inclusion fluids in halite crystals from the ERDA No. 9 site, and (3) local ground waters of meteoric origin. The isotopic compositions indicate that the brines are genetically related and that they probably originated from the evaporation of paleo-ocean waters. Although highly variable in solute contents, the brines have rather uniform isotopic compositions. The stable isotope compositions of brine from the ERDA No. 6 site (826.3 m depth) and fluid inclusions from the ERDA No. 9 site are variable but remarkably regular and show that (1) mixing with old or modern meteoric waters has occurred, the extent of mixing apparently decreasing with depth, and (2) water in the ERDA No. 6 brine may have originated from the dehydration of gypsum. Alternatively, the data may reflect simple evaporation of meteoric water on a previously dry marine flat. Stable isotope compositions of all the waters analyzed indicate that there has been fairly extensive mixing with ground water throughout the area, but that no significant circulation has occurred. The conclusions bear importantly on the suitability of these salt beds and others as repositories for nuclear waste.     

Hydrodynamic convergence of hydropressured and geopressured zones, Central Texas, Gulf of Mexico Basin, USA

Freshwater moving downdip in the Carrizo–Wilcox aquifer, Central Texas, USA, and saltwater and hydrocarbons moving updip from a geopressured zone come together in a groundwater convergence zone, marked by (1) a hydraulic-gradient reversal, (2) “updip” oil fields, and (3) the downdip limit of potable water beyond which there is a marked increase in salinity. Data combined from groundwater-supply and petroleum-extraction industries document the interface between the hydropressured and geopressured zones. The hydraulic-head gradient updip of the convergence zone is 0.001 to 0.002, directed toward the coast; farther downdip it is ?0.02 to ?0.04, directed inland. Salinity increases from <400 mg/L near the outcrop, to ～3,000 mg/L at the downdip limit of potable water, to >100,000 mg/L in the geopressured zone. Upward-directed flow paths probably predominate in the convergence zone. The convergence zone in the study area lies within only 30–50 km of the outcrop because updip extensional faulting offsets permeable aquifer units against low-permeability strata and restricts the downdip flux of recharged water. The major elements of the convergence zone may have been in place since the Miocene development of circulation in the updip coastal aquifer following incision of river valleys and lowering of base level.     

Petrography and geochemistry of septarian carbonate concretions from the Boom Clay Formation (Oligocene,Belgium)

The septarian carbonate concretions from the Boom Clay (Belgium) consist mainly of authigenic minerals such as micrite ( 70% bulk volume) and pyrite framboids ( 3%). These mineral phases occur between detrital grains and fossils. The septarian cracks are lined with calcite, which is sometimes covered with pyrite. The preservation of delicate sedimentological features in the concretion matrix (hardly compacted faecal pellets, burrows and uncrushed shells) points to an early origin of the concretions. Systematic geochemical variations from concretion centre to edge suggest that growth continued during shallow burial. The13C values (–17.5 to –20.5) of the concretionary carbonate show that bacterial sulphate-reduction processes were dominant. Sulphate-reduction-derived HCO3- was diluted by marine-related HCO3-, derived from dissolved bioclasts. A slight enrichment in 13C during growth is caused by the decreasing influence of sulphate reduction because of the progressive closure of the diagenetic system due to shallow-burial compaction. The 18O values (–0.5 to +1.0) of the concretionary carbonate point to a marine origin. The slightly 18O-depleted signature with respect to time-equivalent marine-derived carbonate relates to the incorporation of an 18O-depleted component, originating from sulphate and organic matter. The slight decrease in 18O during growth relates to an increasing influence of this component and to a decreasing influence of seawater-derived oxygen during early diagenesis.     

Geochemistry and origin of Dehoo manganese deposit,south Zahedan,southeastern Iran

Dehoo manganese deposit is located 52 km to the south of Zahedan in Sistan and Baluchestan Province, southeastern Iran. This deposit that lies in the central part of the Iranian Flysch Zone is lenticular in shape and lies above the micritic limestone-radiolarite cherts of the upper Cretaceous ophiolite unit. It is hosted within the reddish to brown radiolarite cherts and in places interlinks with them, so that the radiolarite chert packages play a key role for Mn mineralization in the region. Investigated ore-paragenetic successions and the geochemical characteristics of the Dehoo deposit were studied by means of major oxide, trace, and rare earth element (REE) contents that provide information as to the mineral origin. Strong positive correlations were found between major oxides and trace elements (Al₂O₃-TiO₂, r = 0.95; TiO₂-MgO, r = 0.94; Fe₂O₃-Al₂O₃, r = 0.90; MgO-Al₂O₃, r = 0.84; MgO-Fe₂O₃, r = 0.88; Fe₂O₃-TiO₂, r = 0.91; Fe₂O₃-K₂O, r = 0.74; Al₂O₃-K₂O, r = 0.69; Al₂O₃-V, r = 0.72; TiO₂-V, r = 0.73, and MgO-V, r = 0.69) that testify to the contribution of mafic terrigenous detrital material to the deposit. Chondrite-normalized REE patterns of all ore samples are characterized by negative Ce (0.06–0.15, average 0.10) and slightly positive Eu (0.29–0.45, average 0.36) anomalies. Based on ratios of Mn/Fe (average 56.23), Co/Ni (average 0.33), Co/Zn (average 0.38), U/Th (average 3.40), La/Ce (average 1.45), La_n/Nd_n (average 2.16), Dy_n/Yb_n (average 0.33), and light REE/heavy REE (average 8.40; LREE > HREE), as well as Ba (average 920 ppm) and total REE contents (average 6.96 ppm) negative Ce and positive Eu anomalies, Dehoo could be considered a predominantly submarine hydrothermal Mn deposit complemented by terrigenous detrital mafic material.     

Geoelectric stratigraphy and subsurface evaluation of quaternary stream sediments at the Cooper Basin,NE Texas

An electrical resistivity survey of a portion of the Cooper Reservoir Basin in northeast Texas has been used to delineate the major lithostratigraphic units of Quaternary flood plain deposits and subsurface paleogeomorphic features of interest to geoarchaeologists. This investigation was conducted to evaluate the fluviatile deposits along the South Sulphur River which flows through Cooper Basin, to identify the basin morphology, and to detect buried geomorphic features. Buried stream channels were considered to be important targets since buried prehistoric site locations are thought to be related to patterns of fluvial facies. Two general techniques were used in the survey. First, soundings were conducted over borehole locations to correlate lithologies with resistivity variations. Second, multiple resistivity profiles were established along traverses across the flood plain. By repeating each traverse but with increased probe separations, a composite of profiles was developed yielding electrical cross-sections of the subsurface. These cross-sections were then interpreted in terms of the lithologic stratigraphy in the basin. Sounding data were evaluated using empirical methods, and profile data were evaluated qualitatively. the sounding data defined the major resistivity horizons which correlate to the primary sedimentary units in the Cooper Basin. Profile data, used to construct resistivity cross-sections, revealed a series of buried erosional features interpreted as channel segments and allowed for the estimation of relative time boundaries within sediments buried under the flood plain as the basin evolved.