Wilcox Sandstones Research Articles

Heavy Mineral and Zircon Age Constraints on Provenance of Cenozoic Sandstones in the Gulf of Mexico Subsurface

Combined heavy mineral analysis and detrital zircon geochronology have enabled us to track detritus supplied by the ancestral river systems draining the North American continent into the deep subsurface of the Gulf of Mexico, in both the coastal plain and the offshore deep water areas. During deposition of the Paleocene–Eocene Wilcox Group, sandstones in the western part of the area are interpreted as the products of the Rosita system derived via paleo-Rio Grande material, with a large component of sediment shed from the Western Cordillera. By contrast, samples from wells further east have high proportions of zircons derived from the Yavapai-Mazatzal Province and are attributed to the Rockdale system with sediment fed predominantly by the paleo-Colorado or paleo-Colorado-Brazos. There is evidence that sediment from the Rosita system occasionally extended into the central Gulf of Mexico, and, likewise, data indicate that the Rockdale system sporadically supplied sediment to the western part of the basin. During the Late Eocene of the central Gulf of Mexico (Yegua Formation) there was a distinct shift in provenance. The earlier Yegua sandstones have a large Grenville zircon component and are most likely to have had a paleo-Mississippi origin, whereas the later Yegua sandstones are dominated by zircons of Western Cordilleran origin, similar to Wilcox sandstones fed by the Rosita system via the paleo-Rio Grande. The switch from paleo-Mississippi to paleo-Rio Grande sourcing implies there was a major reorganisation of drainage patterns during the Late Eocene. Miocene sandstones in the deepwater Gulf of Mexico were principally sourced from the paleo-Mississippi, although the paleo-Red River is inferred to have contributed to the more westerly-located wells.

Origin, migration, and characterization of petroleum in the Perdido Fold Belt, Gulf of Mexico basin

Herein, the characteristics and origin of petroleum in the Perdido Fold Belt (PFB) were studied and the petroleum migration processes and system elements are discussed. The molecular compositions and stable carbon isotopes indicate that natural gases in the PFB are mainly organic and have similar origins. Further, they are mainly sapropel type gases and originated from kerogen cracking. The oil in this region has a high API gravity and contains a relatively high sulfur content. The light hydrocarbon values indicate that most of the oil samples are sapropel type oil. Based on the geochemical characteristics of petroleum, Tithonian is considered to be the dominant source rock in the PFB. Various parameters regarding petroleum maturity suggest that the natural gases and oil are mature. The calculated temperature and vitrinite reflectance of the oil samples indicate that petroleum charging occurred from the Middle to Late Eocene. API gravity and sulfur content are related to maturity and herein, indicate that the Tithonian source rocks are mainly type II-S kerogen and the associated high API gravity of petroleum can be primarily attributed to this kerogen. Petroleum migration in the PFB was jointly controlled by salt activity and petroleum generation, wherein. The active faults and the Wilcox sandstones acted as petroleum migration pathways, allowing large petroleum reservoirs to form from the Middle to Late Eocene.

Vertical effective stress and temperature as controls of quartz cementation in sandstones: Evidence from North Sea Fulmar and Gulf of Mexico Wilcox sandstones

Abstract We present quantitative petrographic data, high spatial resolution oxygen isotope analyses of quartz cement, basin modelling and a kinetic model for quartz precipitation for two Paleocene-Eocene Wilcox Group sandstones from Texas and two Jurassic Fulmar Formation sandstones from the Central North Sea. At each location, one sandstone has been buried to ca. 145 °C and one to ca. 185 °C. A key difference between the Wilcox and Fulmar burial histories is that the Wilcox sandstones are currently at higher vertical effective stresses and, from basin modelling studies, have been subjected to generally higher vertical effective stresses through their burial history. The amounts of quartz cement in the Wilcox sandstones are between 12 and 18%, and between 2 and 6% in the Fulmar sandstones. High-spatial-resolution oxygen isotope data obtained from the quartz cements suggest temperature ranges for quartz precipitation from 60 to 80 °C to values approaching maximum burial temperature. Factors such as grain coatings or the timing of petroleum emplacement cannot explain the differences in the amounts of quartz cement. Petrographic data show that most of the silica for quartz cement can be derived from intergranular pressure dissolution. Although the sample set is small, we interpret the results to suggest that the differences in quartz cementation in Fulmar and Wilcox sandstones can be explained better by differences in their vertical effective stress history than their temperature history; in this case, the supply of silica rather than the precipitation of quartz becomes an important control on the rate and extent of cementation.

Insights into deep, onshore Gulf of Mexico Wilcox sandstone pore networks and reservoir quality through the integration of petrographic, porosity and permeability, and mercury injection capillary pressure analyses

In the Paleocene to Eocene Wilcox Group in the northern Gulf of Mexico, exploration targets are reaching into deep to ultradeep burial depths. At these great depths, reservoir quality (porosity and permeability) becomes an important risk factor in determining the chance of encountering an economic reservoir. Major controls on reservoir quality are pore types and abundances, pore-throat sizes, and pore network composition. These factors can be analyzed by integrating petrographic, core plug porosity and permeability, and mercury injection capillary pressure (MICP) analyses. The Wilcox sandstones are mostly lithic arkoses and feldspathic litharenites that contain primary interparticle pores, secondary dissolution pores, and micropores. However, these pore types evolve with depth and temperature. As temperature increases, the relative abundance of primary interparticle pores decreases, whereas the relative abundance of secondary dissolution pores and nano- to micropores increases. Associated with this evolution of pore networks with increasing temperature, there is a decrease in reservoir quality. This decrease in reservoir quality is caused by a transition to finer pore-throat sizes that correspond to changes in pore types. Petrographic analysis provides information on pore types, core plug porosity and permeability analysis provides information on volume of pores and effectiveness of flow, and MICP analysis provides information on pore-throat radius distribution. Through forecasting the pore network in the target temperature zone, a realistic porosity versus permeability transform can be selected to estimate permeability from wire-line log porosity.

Predicting flow properties in diagenetically-altered media with multi-scale process-based modeling: A Wilcox Formation case study

Two approaches to capture the complex texture of Wilcox sandstone samples in a pore-scale model are employed: a process-based method and an image-based method. In the process-based method, petrographic analyses from transmitted light microscopy are used to infer the diagenetic history of the rock. Key steps in the diagenesis are quantitatively followed using a multi-scale pore-network modeling method to reconstruct the porous medium at different times in its history, including the final state. Further information about pore sizes is gathered from scanning electron microscopy and nitrogen adsorption hysteresis measurements. The ability to incorporate texture information of each diagenetic stage of a tight gas sandstone into a predictive multi-scale pore-network model is a novel aspect of this work. In the image-based method, a micro-CT image of the sample is acquired and a pore-network model is extracted based on skeletonizing the resolvable pore space and mapping clusters of micropores to the unresolved porous areas. The paragenesis of the formation was not captured using the image-based method since sufficient details of its history could not be extracted. The process-based and image-based methods produce comparable drainage capillary pressure curves. While the drainage relative permeability curves in both methods display permeability jail effect, they considerably differ from each other quantitatively. The process-based method produced a higher porosity and permeability compared to the image-based method which is potentially due to the lower degree of its disorder. High uncertainties in the petrophysical properties of tight gas sandstones is a costly challenge in their reservoir development. Our work provides insights into the effect of pore texture from different scales on rock fluid properties. Furthermore, the methods demonstrated in this work can be pursued to extrapolate tight gas sandstone petrophysical properties between wells and throughout paragenetic sequences. • An algorithm for the inclusion of oversize pores in process-based modeling is introduced and implemented. • A comparison of process-based and image-based multi-scale pore-network modeling methods in predicting several petrophysical properties is conducted. • Sedimentary rock fragments and dissolving feldspars can be crucial for fluid flow for the Wilcox Formation. • Permeability jail effect is observed during primary drainage using both methods.

Reprint of: Diagenetic controls on evolution of porosity and permeability in lower Tertiary Wilcox sandstones from shallow to ultradeep (200–6700 m) burial, Gulf of Mexico Basin, U.S.A.

Reservoir quality is a critical risk factor in deep to ultradeep reservoirs at depths >4.5 km. Analysis of Paleogene Wilcox sandstones on the upper Texas Gulf Coast provides insight into the evolution of reservoir quality during shallow to ultradeep burial diagenesis. Reduction of porosity and permeability with burial in Wilcox sandstones was evaluated using subsurface samples from 200 to 6700 m, at temperatures of 25–230 °C. Diagenesis and petrophysical properties were interpreted from petrographic data and core analyses. Wilcox sandstones are mostly lithic arkoses and feldspathic litharenites having an average composition of Q 59F 22R 19. Provenance did not change significantly during Wilcox deposition in this area, nor does average sandstone composition vary among lower, middle, and upper Wilcox sandstones. However, composition does vary with sequence-stratigraphic position; lowstand slope-fan deposits contain more rock fragments than do deposits from highstand or transgressive systems tracts. Given observations from this onshore dataset, Wilcox sandstones deposited in deepwater environments in the Gulf of Mexico are likely to contain more rock fragments than their linked highstand equivalents. Pore types in Wilcox sandstones change from a mix of primary and secondary pores and micropores at shallower depths to predominantly secondary pores and micropores in deeper sandstones. Wilcox sandstones show steady porosity reduction from an average of 33% at 38 °C to 12% at 132 °C, with little additional loss at higher temperatures. By 132 °C, most primary pores have been lost by mechanical compaction or occluded by quartz cementation. Volume of secondary pores remains nearly constant despite dissolution of 3.5% average potassium feldspar during deep burial. Volume of this late dissolution of potassium feldspar is offset by precipitation of ankerite, albite, illite, and minor quartz in secondary pores. The proportion of microporosity, which occurs in authigenic clays, altered grains, and matrix, increases in the deepest sandstones. Change in proportion of pore types and reduction in total porosity with increasing temperature changes porosity–permeability transforms. Because most deep to ultradeep Wilcox sandstones are dominated by secondary pores and micropores, porosity–permeability transforms have lower slopes than those for shallower sandstones.

Diagenetic controls on evolution of porosity and permeability in lower Tertiary Wilcox sandstones from shallow to ultradeep (200–6700 m) burial, Gulf of Mexico Basin, U.S.A.

Reservoir quality is a critical risk factor in deep to ultradeep reservoirs at depths >4.5 km. Analysis of Paleogene Wilcox sandstones on the upper Texas Gulf Coast provides insight into the evolution of reservoir quality during shallow to ultradeep burial diagenesis. Reduction of porosity and permeability with burial in Wilcox sandstones was evaluated using subsurface samples from 200 to 6700 m, at temperatures of 25–230 °C. Diagenesis and petrophysical properties were interpreted from petrographic data and core analyses. Wilcox sandstones are mostly lithic arkoses and feldspathic litharenites having an average composition of Q 59F 22R 19. Provenance did not change significantly during Wilcox deposition in this area, nor does average sandstone composition vary among lower, middle, and upper Wilcox sandstones. However, composition does vary with sequence-stratigraphic position; lowstand slope-fan deposits contain more rock fragments than do deposits from highstand or transgressive systems tracts. Given observations from this onshore dataset, Wilcox sandstones deposited in deepwater environments in the Gulf of Mexico are likely to contain more rock fragments than their linked highstand equivalents. Pore types in Wilcox sandstones change from a mix of primary and secondary pores and micropores at shallower depths to predominantly secondary pores and micropores in deeper sandstones. Wilcox sandstones show steady porosity reduction from an average of 33% at 38 °C to 12% at 132 °C, with little additional loss at higher temperatures. By 132 °C, most primary pores have been lost by mechanical compaction or occluded by quartz cementation. Volume of secondary pores remains nearly constant despite dissolution of 3.5% average potassium feldspar during deep burial. Volume of this late dissolution of potassium feldspar is offset by precipitation of ankerite, albite, illite, and minor quartz in secondary pores. The proportion of microporosity, which occurs in authigenic clays, altered grains, and matrix, increases in the deepest sandstones. Change in proportion of pore types and reduction in total porosity with increasing temperature changes porosity–permeability transforms. Because most deep to ultradeep Wilcox sandstones are dominated by secondary pores and micropores, porosity–permeability transforms have lower slopes than those for shallower sandstones.

Simulation of burial diagenesis in the Eocene Wilcox Group of the Gulf of Mexico basin

Diagenesis may be evaluated quantitatively by using petrographic observations and results of paleohydrologic reconstructions in combination with geochemical reaction path model calculations. The authors have applied a reaction path method by simulating diagenesis in the Eocene Wilcox sandstones in the Gulf of Mexico basin to evaluate the effects of variable Pco 2, fluid composition, amount of rock reaction and burial history. The results show that increases in Pco 2 cause the amount of carbonate phases to increase, instead of creating secondary porosity, and closed system reactions with a chemically evolved pore fluid cause a reduction in the amount of carbonate phases, thereby preserving primary porosity. Diagenesis resulting from increased rock reaction per pore volume is characterized by a dominance of Fe-free mineral phases, and albite forms in simulations at temperatures above 100°C with neutral pH evolved fluids. The results approximate petrographic observations of previous workers on the Wilcox with only a few exceptions. Continued simulations using different fluid compositions and organic acid anions may increase the capability to reproduce observed paragenetic sequences.

Radiogenic Heat Production in Sedimentary Rocks of the Gulf of Mexico Basin, South Texas

Radiogenic heat production within the sedimentary section of the Gulf of Mexico basin is a significant source of heat. Radiogenic heat should be included in thermal models of this basin (and perhaps other sedimentary basins). We calculate that radiogenic heat may contribute up to 26% of the overall surface heat-flow density for an area in south Texas. Based on measurements of the radioactive decay rate of a-particles, potassium concentration, and bulk density, we calculate radiogenic heat production for Stuart City (Lower Cretaceous) limestones, Wilcox (Eocene) sandstones and mudrocks, and Frio (Oligocene) sandstones and mudrocks from south Texas. Heat production rates range from a low of 0.07 ±0.01 µW/m3 in clean Stuart City limestones to 2.21 ±0.24 µW/m3 in Frio mudrocks. Mean heat production rates for Wilcox sandstones, Frio sandstones, Wilcox mudrocks, and Frio mudrocks are 0.88, 1.19, 1.50, and 1.72 µW/m3, respectively. In general, the mudrocks produce about 30-40% more heat than stratigraphically equivalent sandstones. Frio rocks produce about 15% more heat than Wilcox rocks per unit volume of clastic rock (sandstone/mudrock). A one-dimensional heat- conduction model indicates that this radiogenic heat source has a significant effect on subsurface temperatures. If a thermal model were calibrated to observed temperatures by optimizing basal heat-flow density and ignoring sediment heat production, the extrapolated present-day temperature of a deeply buried source rock would be overestimated.

Thermal Conductivity of Wilcox and Frio Sandstones in South Texas (Gulf of Mexico Basin)

Thermal conductivity and petrographic data are presented for verifying mechanistic models of sandstone thermal conductivity. We measured the thermal conductivity of 83 Wilcox and Frio sandstones from south Texas in the Gulf of Mexico sedimentary basin, and correlated conductivity to petrographic variables. Thermal conductivities of water-saturated sandstones at 20°C (68°F) and 3 MPa (435 psi) were measured on core plugs using a divided-bar apparatus. Thermal conductivity ranges from 2.06 to 5.73 W/m/K over a porosity range of 2.4 to 29.6%. Because of a higher quartz content, Wilcox sandstones at a given porosity are more conductive than Frio sandstones. A grain-matrix conductivity of 5.9 W/m/K is estimated for Wilcox sandstones; matrix conductivity is adequately described with an arithmetic mixing model. Thermal conductivities of clean ( 35% of the solids) sandstones, the dependence on quartz content can be dropped and thermal conductivities can be predicted with a linear decrease in conductivity with increasing porosity. These sandstones appear isotropic with respect to thermal conductivity.

Recent Horizontal Drilling in Lockhart Crossing Field, Livingston Parish, Louisiana: Its impact on Geological Interpretation

ABSTRACT Recent horizontal completions drilled by Callon Petroleum Company in the Lockhart Crossing Field have further optimized production in the Lower Eocene First Wilcox Sandstone, a marine bar composed of 40-45 feet of very fine to fine-grained glauconitic sandstone. This reservoir has produced a total of 16 MMBO and 17 BCFG from 40 wells since 1982. Well site geology was a key factor in these successful horizontal completions because these wellbores were not mudlogged; were not electrically logged; and cores were not taken. Over 2,800 feet of drill cuttings, taken at 10 foot intervals, were examined to maintain the drilling wellbore near the top of the sandstone where the highest values of permeability and porosity are present. By drilling horizontally, First Wilcox Sandstone structure was encountered that was not previously mapped using existing subsurface well control. Callon's International Paper Company (IPCO) #6 well, the first of the three horizontal wells drilled in the Lockhart Crossing Field, flowed at a rate of 527 BOPD and 400 MCFGPD on a 15.5/64 choke with a flowing tubing pressure of 650 psi from the First Wilcox Sandstone in March of 1992. Current flowrates continue to average 600+ BOPD on a 32/64 choke. This well, located upthrown to a down-to-the-south fault, has produced over 227 MBO and should adequately drain this area of the reservoir. A second horizontal well, the IPCO #15, was sidetracked as a horizontal injector and is located near the geographical center of the field. The third and northernmost horizontal well in the field, the IPCO #7, has produced in excess of 47 MBO and continues to produce at rates in excess of 600 BOPD on choke.

Lower Wilcox Submarine Canyon Channel Sandstones, Sheridan Field, Colorado County, Texas

ABSTRACT Sheridan Field was discovered in 1940 by Shell Oil Company. The field is located along the Wilcox fault zone (Figure 1) and produces gas from Wilcox sandstones in an ovate, anticlinal structure on the downthrown side of a normal, down-to-the-southeast fault. Five wells in the field have been drilled below the main Lower Wilcox sandstone sequence and reveal some enigmatic sandstones in an otherwise very thick shale sequence (Figure 2). This sequence is at least 2500 feet thick at Sheridan Field and has been interpreted by regional correlation as part of the Lavaca submarine canyon fill (Galloway et al., 1989) (Figure 3). Sedimentological analysis of several Sheridan Field conventional cores, coupled with determination of vertical and lateral facies relationships, indicates that the canyon sandstones were deposited by mass-gravity flow mechanisms in channel and associated interchannel levee environments. Individual channel sandstones range from 3 to 30 feet thick but may locally form thicker sequences where they are stacked. These sandstones are very fine- to fine-grained, have sharp lower contacts, and are either 1) massive-appearing with floating mudstone clasts or 2) exhibit a combination of cross-stratification and severely contorted bedding (Figures 4 and 5). Levee sandstones are thin-bedded, horizontal planar-stratified to ripple-stratified, and form units similar to Bouma bcde and cde sequences (Figure 6). Associated levee mudstones are delicately-graded laminites. All lithologies commonly exhibit soft-sediment deformation resulting from gravity-induced, mass movement such as slumping, flowing, and sliding. The above sedimentary features and inferred depositional processes support a continental slope setting. The channel sandstones are potential exploration targets in the field and surrounding areas because of their enormous stratigraphic trap potential. They form the reservoirs in the Lavaca Shale interval at several fields in the Hallettsville area (Chuber and Howell, 1986). The sandstones have geometries that are elongate down depositional slope and truncate updip against adjacent canyon shale fill. In cross section, the sandstones are lenticular and terminate abruptly into laterally-equivalent levee deposits (Figure 7). The recognition of these submarine canyon channel sandstones also implies that potential submarine fan reservoir sandstones may be present downdip at the mouth of the canyon. Figure 1. Regional map showing location of Sheridan Field in the Wilcox fault zone. End_Page 488------------------------ Figure 2. Sheridan Field type log through the Lower Wilcox and Lavaca Canyon fill. Figure 3. Isopach map of the decompacted fill of the Lavaca Canyon with respect to Sheridan Field (Modified from Galloway et al. 1989). End_Page 489------------------------ Figure 4. Core description of Core #1 for the SGU No. 75 Well. End_Page 490------------------------ Figure 5. Core description of Core #2 for the SGU No. 75 Well. End_Page 491------------------------ Figure 6. Core description for the SGU No. 54 Well. End_Page 492------------------------ Figure 7. Stratigraphic cross section showing lenticular channel sandstones. End_Page 493------------------------

Diagenetic modification of Sm-Nd model ages in Tertiary sandstones and shales, Texas Gulf Coast

Sm-Nd model ages of sandstones and shales from the Oligocene Frio Formation of south Texas and the Paleocene-Eocene Wilcox Group of south-central Texas increase with increasing depth of burial. Depleted-mantle model ages (Tdm) of Frio shales increase from ca. 800 Ma at 1 km of burial to ca. 1000 Ma at ∼4km; in Frio sandstones, Tdm increases from ca. 750 Ma at 1 km to over 1200 Ma at 4 km of burial. In Wilcox shales, Tdm increases from ca. 1300 Ma to ca.1500 Ma over a burial depth of 1.5 to 4.5 km; Tdm of Wilcox sandstones increases from ca. 1450 Ma at 2.5 km of burial to as much as ca. 2000 Ma at 4.0 km. The sampling scheme employed minimizes, but does not eliminate, the possibility that these trends in Tdm are due to detrital variability. Close correlation of these trends with diagenetic changes in the rock and evidence of rare earth element mobility preserved in the diagenetic products suggests that diagenesis has modified Tdm. Diagenetic modification of Tdm in Frio sandstone and Wilcox shale is due primarily to loss of radiogenic Nd, whereas increased 147Sm/144Nd produces elevated Tdm in deeper Wilcox sandstones. The Tdm of Frio shales does not change appreciably with depth, owing to the opposite effects on Tdm of decreased 147Sm/144Nd and decreased ϵNd(0). Loss of radiogenic Nd is due primarily to reactions involving unstable volcanic detritus, which is more abundant in the Frio than in the Wilcox. Decreased 147Sm/144Nd in authigenic illite, and consequent increased 147SM/144Nd in pore fluids, results in elevated Tdm in Wilcox and Frio sandstones containing late-diagenetic cement, and in decreased 147Sm/144Nd in deeper Frio shale. The similar Tdm trends observed in both units, which have different detrital characteristics, suggest that modification of Sm-Nd model ages in siliciclastic rocks may be commonplace during burial diagenesis.

Fe Substitution for Al in Glauconite with Increasing Diagenesis in the First Wilcox Sandstone (Lower Eocene), Livingston Parish, Louisiana

AbstractGlauconites in early ankerite concretions, ferroan calcite-cemented sandstones, and uncemented sandstones in the first Wilcox sandstone of the Lockhart Crossing field, Livingston Parish, Louisiana, show a progressive substitution of Fe for octahedral Al with increasing diagenesis. An octahedral Fe content of 0.50 atoms was calculated from glauconite located in early ankeritic concretions. Octahedral Fe averaged 0.60 and 0.90 atoms in later ferroan calcite-cemented sandstone and uncemented sandstone, respectively. Corresponding octahedral Al averages were 1.16, 1.03, and 0.67, respectively. A systematic increase in average interlayer K from 0.49 to 0.54 to 0.61 was also observed, with apparent increases in diagenesis. All element determinations were made with an electron microprobe and recast on an anion equivalent basis to structural formulae based on the O10(OH)2 unit. The clay preserved in the early ankerite concretions was found to be an illite/smectite containing about 20% expandable layers, and the mineral in the glauconite pellets from uncemented areas of the sandstone, an ordered glauconite. “Minus cement” porosities of the sandstone indicate that glauconitization may have taken place at burial depths greater than 0.6 to 1.8 km, but the mechanism for the incorporation of Fe3+ in the glauconite at that depth is not apparent.

Environment of Deposition of Downdip Lower Wilcox Sandstones, Provident City Field, Lavaca County, Texas: ABSTRACT

ABSTRACT The Lower Wilcox section at Provident City Field produces dry gas from thin-bedded, silty sandstones, at depths of 12,500 to 14,100 ft (3810 to 4298 m). The sandstones lie downdip of the Colorado and Guadalupe deltas of the Rockdale Delta System, within the Wilcox fault zone. Regional stratigraphy and structural trends indicate that the sandstones were deposited in a deep marine environment. Cores show that sandstone cosets range 0.1 to 2.7 ft (0.03 to 0.82 m) and average 0.58 ft (0.18 m) in thickness. Sedimentary structures within the cosets range upward from a massive unit (A) to a planar-laminated unit (B) to a ripple-laminated unit (C). The cosets have an average composition of lithic arkose and show textural grading indicative of deposition from turbidity flows. A growth fault, having approximately 1000 ft (3048 m) of throw at a depth of 12,300 ft (3750 m), bounds the field to the northwest and largely controls the distribution of lithofacies. Stacked, AB-type, turbidite cosets indicate channel facies. The M Sandstone was deposited as a constructional channel, with abrupt lateral grading to overbank facies, where turbidites of the BC- and C-type are dominant. The S Sandstone was deposited as a series of thin, constructional channels, mostly with turbidites of the AB- and ABC-type, that are generally stacked, causing superimposed, dip-trending lobes on an otherwise strike-trending sandstone.

Log Response as Model for Production: Deep Wilcox Sandstones, Provident City Field, Lavaca County, Texas: ABSTRACT

ABSTRACT Producibility of overpressured middle and lower Wilcox sandstone reservoirs along the central Texas Gulf Coast is notoriously difficult to determine from standard logging suites. Typically however, this is the only means available. The middle Wilcox S sandstone of Provident City field, examined in terms of porosity, resistivity and shaliness explained highly variable rates of gas, water and formation fines produced in the thirteen wells penetrating this section. Structure and stratigraphic maps further delineated the reservoir and helped reconstruct the original trapping mechanism. Log data, plotted on resistivity-porosity crossplots (Pickett, 1966) as a means of comparative analysis, defined a water saturation equation. Cutoffs of 65% Sw for wet wells were applied using Rw=0.1, a=0.7, m=1.90 and n=2 in the standard Archie equation. Whole core taken from the 50-65% Sw interval in one producing well shows most of the reservoir porosity to be secondary, with a wide variety of precipitated materials suspended in the pore throats. A palinspastic structure map, constructed by omitting late faulting, revealed a simple downthrown rollover anticline. Net feet of section with Sw < 50%, contoured for each of three S sandstone lobes, defined commercially productive limits of the reservoir and confirmed the timing of hydrocarbon migration into this structure. This implies gas migrated into secondary pore spaces, which were created just prior to hydrocarbon formation, and inhibited late precipitation in the main reservoir; while off-structure pore throats contain water, gas and mobile fines. Using the derived Sw equation, reconstructed structure maps and production figures, a recovery factor of 1720 mcf/acre-foot was calculated for reservoirs with Sw < 50%.

Compaction of Wilcox Sandstones to 14,500 Feet: ABSTRACT

Compaction of Wilcox Sandstones to 14,500 Feet: ABSTRACT

Oil and Gas Developments in South Texas in 1988

The downward slide in drilling activity in south Texas began to level off in 1988 as overall drilling activity declined at lower rates than in 1987. Exploratory drilling in south Texas (Texas Railroad Commission--RRC--Districts 1, 2, and 4) in 1988 declined 14% from 1987; development drilling declined 6%. New-field discoveries totaled 56, an increase of 3 from 1987, with gas discoveries accounting for 87.5% of the new fields. Frio and Wilcox sandstones continue to be the most sought after wildcat targets, accounting for 37% of all exploratory drilling. RRC District 4 had a 61% exploratory success rate, the highest in south Texas.

Diagenetic history of Eocene Wilcox sandstones, South-Central Texas

Diagenetic history of Eocene Wilcox sandstones, South-Central Texas

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