Frio Formation Research Articles

Facies Variability in Paleogeographic Trends from Core Data in the Lower and Middle Frio Formation in South Texas

This core-based study of the Oligoene lower and middle Frio formation in South Texas documents facies variability, depositional systems, and sandstone-body architecture in the lower and middle Frio formation in South Texas. A previous study by the Bureau of Economic Geology (BEG) in 1982 characterized the Frio Formation in terms of depositional systems, structural framework, hydrocarbon origin, migration, and exploration potential. Paleogeographic trends in the 1982 study were delineated for the lower, middle, and upper Frio Formation, based mostly on wireline-log data. Core data in this study augment and refine facies interpretations and clarify depositional styles in four paleogeographic trends from the 1982 study in the lower and middle Frio Formation. These trends include (1) lower delta plain/margin, (2) aggradational fluvial, (3) aggradational (multistoried) barrier, and (4) floodplain. With minor exceptions, this study validates and refines interpretations of the regional distribution of lower and middle Frio paleogeography in South Texas in the 1982 study. Four (4) main results of this study include (1) documenting lithology and facies types in these paleogeographic trends, (2) demonstrating that these paleogeographic trends are a composite of multiple transgressive-regressive (T–R) cycles, (3) providing evidence that these fluvial systems are mixed-load, meander belt in origin rather than coarse-grained bedload in origin, based on vertical grain-size profiles and the presence of paleosols that indicate stable, well-developed floodplains, and (4) identifying anomalous occurrences of sandstone-rich fluvial-channel systems in areas previously mapped as sandstone-poor and interpreted to be primarily floodplain in origin. Finally, this study serves as an example of the value of core data in refining interpretations of paleogeography and depositional systems in stratigraphic units that are a composite of multiple T–R episodes.

Elastic properties of unconsolidated sandstones of interest for carbon storage

AbstractUnconsolidated sandstones are attractive targets for underground storage of carbon due to their high porosity and permeability. Monitoring of injection and movement of CO2 in such formations using elastic waves requires an understanding of the acoustic properties of the sandstone. Current approaches often use the so‐called soft‐sand model in which a Hertz–Mindlin model of the acoustic properties at high porosity is mixed with the acoustic properties of the mineral phase to predict the acoustic properties over the entire porosity range. Using well‐log data from two unconsolidated sand formations of interest for CO2 storage, we discuss the limitations of this model and provide an alternative approach in which the mechanical properties of grain contacts are obtained by inversion, and the properties of infill material lying within the pore space are estimated. The formations considered are the Paluxy Formation in Kemper County, Mississippi, and the Frio Formation near Houston, Texas. The ratio of the normal to shear compliance of the grain contacts is found to be significantly less than unity for both formations. This implies that the grain contacts are more compliant in shear than in compression. However, the grain contact compliance is higher and the ratio of the normal to shear compliance is lower for the Frio example than for the Paluxy case, and this may lead to sliding at grain contacts with low shear compliance and transport of grains during fluid flow, particularly if CO2 acts to weaken any cement that may be present at the grain contacts. Such transport was suggested by Al Hosni et al. in explaining why the magnitude of the time‐lapse effect due to the injection of CO2 at the Frio CO2 injection site is greater than predicted using conventional rock physics models. A simple model of the mechanical properties of infill material lying within the pore space suggests that the bulk and shear moduli of infill material in the Paluxy case are significantly higher than the Frio case, consistent with the lower grain contact compliance in the Paluxy case.

Triple Oxygen Isotopes in Evolving Continental Crust, Granites, and Clastic Sediments

This Chapter considers triple oxygen isotope variations and their 4 Gyr temporal evolution in bulk siliciclastic sedimentary rocks and in granites. The d18O and D'17O values provide new insights into weathering in the modern and ancient hydrosphere and coeval crustal petrogenesis. We make use of the known geological events and processes that affect the rock cycle: supercontinent assembly and breakup that influence continent-scale and global climate, the fraction of the exposed crust undergoing weathering, and isotopic values of precipitation. New data from a 5000 m Texas drillhole into the Oligocene Frio Formation demonstrate minimal isotopic shifts from mudrocks to shales during diagenesis, mostly related to expulsion of water from smectite-rich loosely cemented sediment and its conversion to illite-rich shale. Inversion of triple oxygen isotope fractionations return isotopic values and temperatures along the hole depth that are more consistent with weathering conditions in the Oligocene and modern North America (d18O = -7 to -15‰, and T of +15 to +45°C) rather than d18O from 8 to 10‰ diagenetic water in the drill hole at 175-195°C. More precise T and d18Owater are obtained where the chemical index of alteration (CIA) based detrital contribution is subtracted from these sediments. Triple oxygen isotopes from suspended sediments in major world rivers record conditions (T and d18Ow) of their watersheds, and not the composition of bedrock because weathering is water-dominated. In parallel, the Chapter presents new analyses of 100 granites, orthogneisses, migmatites, tonalite-trondhjemite-granodiorite (TTG), and large-volume ignimbrites from around the world that range in age from 4 Ga to modern. Most studied granites are orogenic and anatectic in origin and represent large volume remelting/assimilation of shales and other metasediments; the most crustal and high-d18O of these are thus reflect and record the average composition of evolving continental crust. Granites also develop a significant progressive increase in d18O values from 6-7‰ (4-2.5 Ga) to 10-13‰ (~1.8-1.2 Ga) after which d18O stays constant or even decreases. More importantly, we observe a moderate -0.03‰ step-wise decrease in D'17O between 2.1 and 2.5 Ga, which is about half of the step-wise decrease observed in shales over this time interval. We suggest that granites, as well as shales, record the significant advent and greater volumetric appearance of low-D'17O, high-d18O weathering products (shales) altered by meteoric waters upon rapid emergence of large land masses at ~2.4 Ga, although consider alternative interpretations. These weathering products were incorporated into abundant 2.0-1.8 Ga orogens around the world, where upon remelting, they passed their isotopic signature to the granites. We further observe the dichotomy of high-D'17O Archean shales, and unusually low-D'17O Archean granites. We attribute this to greater contribution from shallow crustal hydrothermal contribution to shales in greenstone belts, while granites in the earliest 3.0-4.0 Ga crust and TTGs require involvement of hydrothermal products with lower-D'17O signatures at moderately high-d18O, which we attribute to secondary silicification of their protoliths before partial melting. The Chapter further discusses evolution of the shale record through geologic history and discusses the step-wise change in d18O and D'17O values at Archean/Proterozoic transition. Denser coverage for shales in the past 1 billion years permits investigation of the rocks and their weathering in the last supercontinent cycle, with observed lighter d18O values, characteristic for the mid-Phanerozoic at the initiation of Gondwana breakup. The continuing increase in d18O values of the shales since 4 Ga is interpreted to reflect accumulation of weathering products via shale accretion to continents, as low-density and buoyant shales tend to not subduct back into the mantle. The rock cycle passes triple oxygen isotopic signatures from precipitation to sedimentary, metasedimentary, and finally to anatectic igneous rocks. Continental crust became progressively heavier in d18O, lighter in D'17O due to incremental accumulation of high-d18O sediments in accretionary wedges. Second-order trends in d18O and D'17O are due to supercontinent cycles and glacial episodes.

Lithic Raw Materials in the Lower Rio Grande Valley, South Texas and Northeast Mexico

ABSTRACT Analysis of 976 lithic artifacts from twelve museum and private collections in the Lower Rio Grande Valley revealed a preference for seven rock types. Sixty nine percent of all tools were made from gravel chert, which is locally the most abundant rock type on the Frio and Goliad Formations, as well as on the gravels of the Rio Grande. Representing less than 10% each were, the local El Sauz Chert, a black banded metamorphic rock, volcanic rocks, agates, silicified wood, limestone and black chert. Variations in the relative proportion of each rock type are observed by location, suggesting a tendency to use other suitable rocks that were locally available. Contrary to what has been suggested an abundance of lithic resources were available to stone tool makers in the Lower Rio Grande Valley. This study underscores the value of working with collectors in regions where little archaeological research has been conducted.

Comparative analysis of transport and storage options from a CO2 source perspective

This study evaluated integrated CCS costs from the perspective of a CO2 source. A source will have captured CO2 requiring suitable storage and affordable transportation between source and storage. Capture, transport, and storage are the links of the CCS value chain. For this study, capture costs were modeled for six hypothetical sources (various industrial and power-generation facilities) with annual CO2 capture rates ranging from 0.65 to 3.90 million tonnes (Mt). Seven storage reservoirs located in the Appalachian, Illinois and Gulf Coast Basins—and of various quality with respect to CO2 storage—were modeled: two within the Rose Run Formation, three within the Mount (Mt.) Simon Formation, one in the Lower Tuscaloosa Formation, and one in the Frio Formation. Storage costs were calculated with the FE/NETL CO2 Saline Storage Cost Model for the seven selected storage reservoirs under dome and regional dip structural settings. For transportation, a dedicated pipeline system and a trunkline pipeline system were modeled. Transportation costs from source to storage reservoirs were evaluated using the FE/NETL CO2 Transport Cost Model. A source may not always be able to find suitable proximal storage. Low CCS costs reflect a source’s mass of captured CO2 finding suitable storage within an affordable transportation distance. Economies of scale are present in each link of the CCS value chain. These economies of scale are limited with respect to storage or the distance captured CO2 can be transported to storage.

Sedimentologic and sequence-stratigraphic characteristics of wave-dominated deltas

Wave-dominated deltaic strata form prolific hydrocarbon plays in many mature basins across the world. Examples include the Jurassic Brent Group play in the North Sea, offshore United Kingdom and Norway (e.g., Husmo et al., 2003); Eocene Jackson Group and Oligocene Frio Formation plays, Texas, onshore United States (e.g., Fisher et al., 1970; Galloway and Morton, 1989); and Tertiary plays in the Niger Delta province, offshore Nigeria (e.g., Evamy et al., 1978), the Baram Delta province, offshore Brunei (e.g., Rijks, 1981), and the Columbus Basin, offshore Trinidad and Tobago (e.g., Sydow et al., 2003). Wave-dominated deltas also form key exploration targets in frontier basins (e.g., Triassic Snadd Formation, Barents Sea, offshore Norway; Klausen et al., 2014, 2016). The overall architecture of many wave-dominated deltaic reservoirs is determined by their sequence-stratigraphic framework together with their structural configuration. Sequence-stratigraphic frameworks contain elements such as shoreface–shelf parasequences bounded by flooding surfaces and fluvio-estuarine complexes that fill incised coastal valleys developed at sequence boundaries. The internal facies distributions and stacking patterns of such elements are ordered, which aids prediction of reservoir character and behavior. Outcrop analogs of wave-dominated deltaic reservoirs display aspects of ordered stratigraphic architecture across a range of spatial scales that are equivalent to those observed in core, well-log, and seismic data and can therefore be used to support interpretation of their subsurface counterparts. The Cretaceous Blackhawk Formation contains wave-dominated deltaic strata that are superbly exposed in the Book Cliffs of eastcentral Utah (Figure 1A). The extensive, continuous exposures of the Book Cliffs outcrop belt have allowed the wave-dominated deltas and related deposits to be mapped for tens of kilometers (e.g., Young, 1955; Balsley, 1980), and, consequently, these strata were the “birthplace” of high-resolution sequence stratigraphy using outcrop data (Van Wagoner et al., 1990). The Blackhawk Formation and related …

Application of thin-bed indicator and sweetness attribute in the evaluation of sediment composition and depositional geometry in coast-perpendicular subbasins, South Texas Gulf Coast

My focus of study is the nature of sediments within late Eocene to middle Oligocene, coast-perpendicular subbasins located on the southwest side of the San Marcos Arch in Refugio County, near the South Texas Gulf Coast. Methods of investigation consist of 3D seismic interpretation, seismic attribute, and isochron mapping. Depositional geometry, inferred sediment composition, and bed-thickness data are examined by extraction of amplitude-envelope, sweetness, and thin-bed-indicator (TBI) attributes from the 3D seismic volume. Four subbasins, numbered SB1 through SB4 from northeast to southwest, are recognized. Although the coast-perpendicular subbasins formed at the same time and by the same processes, the sediment distribution, basin-fill history, and tectonics differ from subbasin to subbasin within the Jackson, Vicksburg, and Frio Formations. Amplitude envelopes extracted at base Jackson and at near-base Vicksburg Formation show that, although the distribution pattern of interpreted sandstone-prone sediments within each subbasin is mainly of sheet geometry, SB2 displays a pattern of short, narrow, linear features. Based on available well control, the sweetness and TBI attributes correlate well with [Formula: see text]. Crossplots between [Formula: see text] and sweetness exhibit a strong, linear, negative relationship — the sweetness value increases as the [Formula: see text] value decreases, suggesting an increasing sandstone content. In contrast, crossplots between [Formula: see text] and TBI exhibit a positive linear relationship — the TBI value increases as the [Formula: see text] value increases, suggesting an increasing shale content. Because the sweetness attribute may in some cases not detect some thin shale bed within a stratigraphic interval, it may overestimate the net:gross ratio. Therefore, a combined TBI and sweetness procedure is required to determine the net:gross ratio. The results show that the net:gross ratio decreases from SB1 closest to the San Marcos Arch, to SB4 farthest into the Rio Grande Embayment. The approach adopted in this study can be used to estimate net:gross ratios in frontier areas.

Mineral dissolution and precipitation during CO2 injection at the Frio-I Brine Pilot: Geochemical modeling and uncertainty analysis

During the Frio-I Brine Pilot CO2 injection experiment in 2004, distinct geochemical changes in response to the injection of 1600tons of CO2 were recorded in brine samples collected from the monitoring well. Previous geochemical modeling studies have considered dissolution of calcite and iron oxyhydroxides, or release of adsorbed iron, as the most likely sources of the increased ion concentrations. In this modeling study we explore possible alternative sources of the increasing calcium and iron, based on the data from the detailed petrographic characterization of the Upper Frio Formation “C”. Particularly, we evaluate whether dissolution of pyrite and oligoclase (anorthite component) can account for the observed geochemical changes. Due to kinetic limitations, dissolution of pyrite and anorthite cannot account for the increased iron and calcium concentrations on the time scale of the field test (10 days). However, dissolution of these minerals is contributing to carbonate and clay mineral precipitation on the longer time scales (1000 years). We estimated that during the field test dissolution of calcite and iron oxide resulted in ∼0.02wt.% loss of the reservoir rock mass. The reactive transport models were constructed for 25 and 59°C temperature and using Pitzer and B-dot activity correction methods. These models predict carbonate minerals, dolomite and ankerite, as well as clay minerals kaolinite, nontronite and montmorillonite, will precipitate in the Frio Formation “C” sandstone as the system progresses toward chemical equilibrium during a 1000-year period. Cumulative uncertainties associated with using different thermodynamic databases, activity correction models (Pitzer vs. B-dot), and extrapolating to reservoir temperature, are manifested in the difference in the predicted mineral phases. However, these models are consistent with regards to the total volume of mineral precipitation and porosity values which are predicted to within 0.002%.

Depositional architecture of growth-fault related wave-dominated shelf edge deltas of the Oligocene Frio Formation in Corpus Christi Bay, Texas

Growth faults within the Frio Formation define six subbasins on the South Texas shelf and add to the complexity of the sediment dispersal along the shelf margin. These growth faults influenced sediment pathways, controlled sediment partition and provided increased accommodation for deltaic depocenters. Rollover and thickening of sediments occur on the downthrown side of growth faults with offsets of at least 150 m and up to 750 m. Growth strata are indicated by intervals that thicken landward (by several tens of meters) from anticline crests. Individual 4th-order (100–500 ky) regressive cycles expand about five to seven times across growth faults; expansion ratios as great as 10 are observed. The shelf edge is associated with the largest displacement, expansion ratios and thickness of prograding deltaic sequences. Sedimentary structures and trace–fossil associations identified in cores from Nueces, Encinal and Red Fish Bay subbasins indicate deposition on a wave-dominated deltaic shoreline. The cores exhibit highly bioturbated muddy sandstones alternating with decimeter-thick clean sandstones with hummocky cross-stratification. Alternation of fair-weather wave deposits with sandy storm beds in two successive fault zones suggests that all subbasins developed under similar conditions during growth-fault development. High sediment supply conditions favored accumulation of thicker sediments on the downthrown sides of the faults. Wave dominance of deltas on the outer shelves in the growth compartments suggest that building of the upper slope rather lowers slope margin, and decreases the likelihood of the presence of sandy deep water fans.Correlation of about 700 well logs integrated with seismic data provided the areal distribution and relative timing of sediment filling of the growth-faulted compartments to unravel the evolution of higher-order (fourth and fifth) sequences affected by these growth faults. Core data were used to recognize depositional environments, to calibrate lithology to well logs and to characterize seismic amplitude anomalies. Within this shoreface succession the clean sandy storm beds will make attractive hydrocarbon reservoirs especially on the crest of rollover anticlines in downthrown compartments. These sandstone bodies pinch out seaward into open marine shelf mudstones and are sealed by overlying shelf mudstones deposited during transgressive episodes.

CCU&S via Stacked Storage—case Studies from CO2-EOR Basins of the United States

Most compilations of global CO2 storage capacity separate volumes of storage in oil fields from volumes of storage in nonproductive saline formations. In this study we begin to define accessible volumes by strategically combining CO2 use for enhanced oil recovery with large-volume storage in associated saline formations. This variant of carbon capture, use, and storage (CCU&S) is known as stacked storage because it uses the three-dimensional properties of the subsurface to achieve multiple complementary objectives. The motivation for combining CO2 enhanced oil recovery (EOR) and saline-storage types is that where EOR is possible, it provides an attractive market for CO2 but does not alone achieve emission- reduction targets. By spatially and temporally linking CO2 EOR with vertically and horizontally adjacent saline storage volumes, the infrastructure, experience, and community acceptance developed for EOR can provide a substantive boost for using the entire subsurface for storage of very large volumes.Representative fields in major areas of current commercial CO2 EOR served as test cases and were matched with mapped subregional saline-storage potential. Test cases included (1) selected Permian and Pennsylvanian CO2 EOR fields in the Permian Basin of west Texas and New Mexico that were matched with regional, nonproductive Permian saline formations, (2) Cretaceous-age fields (e.g., the SECARB study area at Cranfield) in the Mississippi salt basin that were matched with regional Cretaceous Tuscaloosa saline formations, and (3) salt-dome marginal and growth-fault rollover fields in the Oligocene Frio Formation of the Texas Gulf Coast that were matched with regional Tertiary-age saline formation. In each case, good matches were obtained between CO2 EOR markets and adjacent saline formations. Other saline formations underlie and are separated from the productive zone by their own confining systems. Limitations on the application are also considered.

Seismic interpretation of mass-transport deposits within the upper Oligocene Frio Formation, south Texas Gulf Coast

We integrated well logs and three-dimensional seismic data to describe a wedge of deformed shallow Frio rocks lying above a major bed-parallel decollement within the upper Oligocene Frio Formation located between the Houston and Norias deltas on the south Texas Gulf Coast. Our analyses show that the identified deformed shallow Frio rocks can be divided into proximal clay-rich and low-permeability sandstones characterized by discontinuous, mounded, and chaotic seismic events; near-proximal, clay-poor, and high-permeability sandstones characterized by parallel to subparallel bedded seismic events; and distal sand-, silt-, and mudstones composed of a mix of proximal and near-proximal rocks. All of the deformed rocks are composed of acoustic-impedance materials that are lower than those of the undeformed shallow Frio and are underlain by low-velocity, overpressured, shale-rich rocks. The mechanism that triggered the collapse of the shallow Frio and subsequent development of mass-transport deposits is attributed to an uprising, overpressured, shale-rich high and the development of a shelf-edge listric fault. Proprietary biostratigraphic data show that the collapse of the shallow Frio in areas between the Houston and Norias deltas occurred between 27.5 and 25.3 Ma—approximately the same time as the Hackberry collapse in the Mississippi delta. In the proximal area, interpreted paleowater depths from biostratigraphic data based on benthic foraminifers range from 60 to 120 ft (20–40 m) in a shallow neritic environment. In contrast, the distal area lies in paleowater depths interpreted to be between 120 and 300 ft (40–90 m) in a middle neritic environment.

Location and Felt Reports for the 25 April 2010 mbLg 3.9 Earthquake near Alice, Texas: Was it Induced by Petroleum Production?

Abstract This study examines seismograms and felt reports for the 25 April 2010 Alice, Texas, earthquake and explores its possible relationship with gas and oil production in the Stratton field. We identified P arrivals at seven broadband stations situated within ∼100 km of the epicentral region and determined a location of 27.72° N, 97.95° W, about 11 km east of the location reported by the National Earthquake Information Center but coincident with the region of highest intensity (modified Mercalli intensity V–VI) felt reports. We compare arrivals for observed secondary P and S arrivals with predictions from a published Gulf Coast velocity model. At nearby stations, the secondary arrivals are much stronger than primary arrivals; the arrival times and the presence of high‐amplitude phases traveling at the Love‐wave velocity of the uppermost model layer suggest the focal depth was shallow, 3 km or less. This places the 2010 hypocenter approximately along the mapped trace of the Vicksburg fault zone and at the depth of the Frio formation, the principal productive member in the Stratton field, which has produced at least 2.7 trillion cubic feet of gas and about 100 million barrels of oil since production commenced in 1938. We conclude it is plausible, although not proven definitively, that production in the Stratton field contributed to the occurrence of the 2010 Alice earthquake and an earlier similar earthquake that occurred on 24 March 1997.

The impact of geologic variability on capacity and cost estimates for storing CO2 in deep-saline aquifers

While numerous studies find that deep-saline sandstone aquifers in the United States could store many decades worth of the nation's current annual CO2 emissions, the likely cost of this storage (i.e. the cost of storage only and not capture and transport costs) has been harder to constrain. We use publicly available data of key reservoir properties to produce geo-referenced rasters of estimated storage capacity and cost for regions within 15 deep-saline sandstone aquifers in the United States. The rasters reveal the reservoir quality of these aquifers to be so variable that the cost estimates for storage span three orders of magnitude and average>$100/tonne CO2. However, when the cost and corresponding capacity estimates in the rasters are assembled into a marginal abatement cost curve (MACC), we find that ~75% of the estimated storage capacity could be available for<$2/tonne. Furthermore, ~80% of the total estimated storage capacity in the rasters is concentrated within just two of the aquifers—the Frio Formation along the Texas Gulf Coast, and the Mt. Simon Formation in the Michigan Basin, which together make up only ~20% of the areas analyzed. While our assessment is not comprehensive, the results suggest there should be an abundance of low-cost storage for CO2 in deep-saline aquifers, but a majority of this storage is likely to be concentrated within specific regions of a smaller number of these aquifers.

Variation in Energy Available to Populations of Subsurface Anaerobes in Response to Geological Carbon Storage

Microorganisms can strongly influence the chemical and physical properties of the subsurface. Changes in microbial activity caused by geological CO(2) storage, therefore, have the potential to influence the capacity, injectivity, and integrity of CO(2) storage reservoirs and ultimately the environmental impact of CO(2) injection. This analysis uses free energy calculations to examine variation in energy available to Fe(III) and SO(4)(2-) reducers and methanogens because of changes in the bulk composition of brine and shallow groundwater following subsurface CO(2) injection. Calculations were performed using data from two field experiments, the Frio Formation experiment and an experiment at the Zero Emission Research and Technology test site. Energy available for Fe(III) reduction increased significantly during CO(2) injection in both experiments, largely because of a decrease in pH from near-neutral levels to just below 6. Energy available to SO(4)(2-) reducers and methanogens varied little. These changes can lead to a greater rate of microbial Fe(III) reduction following subsurface CO(2) injection in reservoirs where Fe(III) oxides or oxyhydroxides are available and the rate of Fe(III) reduction is limited by energy available prior to injection.

A New Automated MPD System Used in an Onshore Depleted Gas Field

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 130319, ’Experience and Results with a New Automated MPD System while Drilling and Cementing Liner in an Onshore Depleted Gas Field,’ by Paul Fredericks, SPE, and Ossama Sehsah, SPE, At Balance, and Julio Montilva, SPE, and Phil Vogelsberg, SPE, Shell E&amp;P Company, originally prepared for the 2010 SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition, Kuala Lumpur, 24-25 February. The paper has not been peer reviewed. An automated and modular managed-pressure-drilling (MPD) system recently was used in two wells to hold constant bottomhole pressure (BHP) in a very narrow pressure window without a backpressure pump. In the first well, the system was used to drill in 740 ft of 75/8-in. liner in an 8½-in. hole with 15.0-lbm/gal mud statically underbalanced relative to wellbore stability. Introduction The McAllen and Pharr fields are in Hidalgo County in south Texas. Those fields were acquired by the opera-tor in 2006, and over the 7 decades since discovery, nearly 1.4 Tcf of gas has been produced from reservoirs in the Frio formations. Initial production was from shallow zones, many of which now are depleted by as much as 5,000 psi. The fields have an extended history of commingled production and complex patterns of difficult-to-map faults that make pore pressure and depletion difficult to predict. Isolating the severely depleted zones with liners is impractical because often they are found between overpressured virgin sands. Compared with nearby fields in the Vicksburg formations, those in the Frio have higher permeability and require heavier mud weight to avoid influx. Another limiting factor in the McAllen/Pharr fields is the ease with which losses occur in many of the reservoir sands as a result of the lower minimum horizontal stress caused by depletion. The resulting narrow pressure profile and the inaccurate pore-pressure predictions complicated early drilling efforts with excessive lost circulation and well-control events.

Potential environmental issues of CO2 storage in deep saline aquifers: Geochemical results from the Frio-I Brine Pilot test, Texas, USA

Sedimentary basins in general, and deep saline aquifers in particular, are being investigated as possible repositories for large volumes of anthropogenic CO2 that must be sequestered to mitigate global warming and related climate changes. To investigate the potential for the long-term storage of CO2 in such aquifers, 1600 t of CO2 were injected at 1500 m depth into a 24-m-thick “C” sandstone unit of the Frio Formation, a regional aquifer in the US Gulf Coast. Fluid samples obtained before CO2 injection from the injection well and an observation well 30 m updip showed a Na–Ca–Cl type brine with ∼93,000 mg/L TDS at saturation with CH4 at reservoir conditions; gas analyses showed that CH4 comprised ∼95% of dissolved gas, but CO2 was low at 0.3%. Following CO2 breakthrough, 51 h after injection, samples showed sharp drops in pH (6.5–5.7), pronounced increases in alkalinity (100–3000 mg/L as HCO3) and in Fe (30–1100 mg/L), a slug of very high DOC values, and significant shifts in the isotopic compositions of H2O, DIC, and CH4. These data, coupled with geochemical modeling, indicate corrosion of pipe and well casing as well as rapid dissolution of minerals, especially calcite and iron oxyhydroxides, both caused by lowered pH (initially ∼3.0 at subsurface conditions) of the brine in contact with supercritical CO2. These geochemical parameters, together with perfluorocarbon tracer gases (PFTs), were used to monitor migration of the injected CO2 into the overlying Frio “B”, composed of a 4-m-thick sandstone and separated from the “C” by ∼15 m of shale and siltstone beds. Results obtained from the Frio “B” 6 months after injection gave chemical and isotopic markers that show significant CO2 (2.9% compared with 0.3% CO2 in dissolved gas) migration into the “B” sandstone. Results of samples collected 15 months after injection, however, are ambiguous, and can be interpreted to show no additional injected CO2 in the “B” sandstone. The presence of injected CO2 may indicate migration from “C” to “B” through the intervening beds or, more likely, a short-term leakage through the remedial cement around the casing of a 50-year old well. Results obtained to date from four shallow monitoring groundwater wells show no brine or CO2 leakage through the Anahuac Formation, the regional cap rock.

STRUCTURAL INTERPRETATION OF THE MIDDLE FRIO FORMATION USING 3D SEISMIC AND WELL LOGS: AN EXAMPLE FROM THE TEXAS GULF COAST OF THE UNITED STATES

The Oligocene middle Frio Formation is a major hydrocarbon producer in the U.S. Gulf Coast. In the area of the Stratton and Agua Dulce fields, the middle Frio Formation consists of multiple vertically-stacked reservoir sequences (Figures 1 and 2). The target for this study was the basal part of the middle Frio Formation (BMF) interval between the G2 (top lower Frio) and the F11 horizons (Figure 3). The structural setting of the middle Frio in the study area is predominantly growth faults and associated rollover anticlines. The objective of this work is to show how 3D seismic data acquired on the Texas Gulf Coast images subsurface structure that was impossible to fully interpret on 2D seismic and well logs.

Early clay diagenesis in Gulf Coast sediments: New insights from XRD profile modeling

AbstractSamples from different depths in the Oligocene Frio formation (offshore Gulf of Mexico) were studied by X-ray diffraction (XRD), thermal analyses, and scanning electron microscopy. The experimental XRD patterns recorded from oriented and ethylene glycol (EG) solvated clay fractions of the samples were similar to those typical of random, mixed-layered illite-smectite (R0 I-S). The experimental XRD patterns recorded in air-dried (AD) and EG states were simulated using three different models. One of them corresponds to R0 I-S for which thickness and content of the interstratified layers were determined by the Środoń technique. The second model is represented by a single homogeneous I-S in which illite and smectite layers are interstratified with a tendency to segregation. The expandability of the segregated I-S model varies from 48% to 75% without any rational relationship between the smectite layer content and depth.The third model assumes that the clay fraction is a physical mixture of smectite and an R0 I-S. In this model the I-S contains 65% illite and 35% smectite layers independent of depth, whereas the smectite content varies from 28% to 63%. This model has consistently smaller profile factors, Rp, for both EG and AD XRD scans compared with the Rp values determined for the other two models.The mineralogical association, volcanic origin, narrow stratigraphic interval (427 m), and low maximum temperature (42°C) of the studied Frio Formation are considered. These features are completely consistent with the two-phase model and so the segregation model must be rejected. An authigenic origin of the pure smectite and an alternative detrital or authigenic origin of the R0 I-S are discussed.

High-Pressure/High-Temperature Consolidated Completion in the Gulf of Mexico

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18976, "High- Pressure, High-Temperature Consolidated Completion in the Continental Shelf Environment of the Gulf of Mexico: Case History," by Richard E. Patterson, SPE, and Trevor J. Willms, SPE, El Paso E&amp;P, and Keith Foley, SPE, and Jason Edwards, SPE, Halliburton, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May. The paper has not been peer reviewed. Decades after the first completion in the Gulf of Mexico (GOM) continental shelf, the logical expansion of these mature assets has extended into reservoirs that are deeper, hotter, and higher-pressured than wells completed previously. These high-pressure/high-temperature (HP/HT) wells can cause extreme completion challenges. HP/HT formations also tend to have low permeabilities, which is the opposite of most GOM reservoirs. To make these low-permeability formations economical in an offshore environment, it is imperative that stimulation treatments be completely effective. Introduction The West Cameron 62 field, on the continental shelf just south of Louisiana at a water depth of 35 ft, had its first well completed more than 20 years ago. Since that time, more than 30 different intervals have been completed at depths ranging from a few thousand feet to more than 10,000 ft. In recent years, it has become necessary to focus on reservoirs that exist in the 18,000 to 20,000 ft true vertical depth (TVD) range. At these depths, the pressures and temperatures of the formation trend toward the limit of what current technology allows when completion equipment and fluids are considered. The West Cameron 62 Well A-2 was completed at these depths in the Cris R formation. The Cris R sand is located between 17,843 and 18,021 ft TVD, which correlates to 19,789 and 19,976 ft measured depth (MD), respectively. The initial bottomhole pressure (BHP) for the Cris R formation was measured at 16,500 psi at midperforation. The bottomhole temperature (BHT) at midperforation was 356°F. Contrary to conventional GOM reservoirs, which typically have average permeabilities greater than 50 md, the permeability of the Cris R sand averaged only 0.64 md. The 18% porosity also was significantly lower than that of the well-sorted formations found at shallower depths. In fact, along with permeability and porosity, the rock mechanics falls more in line with typical south Texas "hard rock" reservoirs such as the Wilcox and Frio formations. Despite the low permeability and porosity, the production of the well before the hydraulic-fracturing treatment was 12.0 MMscf/D and 396 B/D of condensate at a 8,572-psi flowing tubing pressure (FTP). Use of fracture models and nodal analysis led to the belief that the production rates could be increased by at least two-fold through implementation of a hydraulic fracture. However, very little information on HP/HT fracturing treatments in the GOM was available.

Time-lapse crosswell seismic and VSP monitoring of injected CO2 in a brine aquifer

Seismic surveys successfully imaged a small scale CO2 injection (1,600 ton) conducted in a brine aquifer of the Frio Formation near Houston, Texas. These time-lapse borehole seismic surveys, crosswell and vertical seismic profile (VSP), were acquired to monitor the CO2 distribution using two boreholes (the new injection well and a pre-existing well used for monitoring) which are 30 m apart at a depth of 1,500 m. The crosswell survey provided a high-resolution image of the CO2 distribution between the wells via tomographic imaging of the P-wave velocity decrease (up to 500 m/s). The simultaneously acquired S-wave tomography showed little change in S-wave velocity, as expected for fluid substitution. A rock physics model was used to estimate CO2 saturations of 10–20% from the P-wave velocity change. The VSP survey resolved a large (∼70%) change in reflection amplitude for the Frio horizon. This CO2 induced reflection amplitude change allowed estimation of the CO2 extent beyond the monitor well and on three azimuths. The VSP result is compared with numerical modeling of CO2 saturations and is seismically modeled using the velocity change estimated in the crosswell survey.