Wilcox Sandstones Research Articles

Depositional and Diagenetic Controls on Reservoir Quality in First Wilcox Sandstone, Livingston Field, Louisiana

Livingston field is a large oil field (approximately 25 million bbl) in southeastern Louisiana. The field produces from the lower Eocene First Wilcox sandstone, a 40 to 50-ft (12 to 16-m) thick barrier island deposit, from subsea depths of 9,917-10,146 ft (3,023-3,092 m). The presence and quality of the reservoir are the direct result of diagenetic events that were strongly influenced by depositional facies. Early primary porosity was significantly reduced by clay and quartz overgrowths and by carbonate cements. Dissolution of the carbonate cement and leaching of feldspars and other unstable grains restored porosity to 65-75% of original values. The highest degree of secondary porosity was created in the facies of highest primary porosity--the eolian, beach, and upper sho eface--while the initially less porous middle and lower shoreface developed little or no secondary porosity. The formation of late-stage diagenetic clays (kaolinite, illite, chlorite, and smectite) have decreased permeability by reducing or blocking pore-throat openings.

Wilcox sandstone diagenesis, Texas Gulf Coast: a regional isotopic comparison with the Frio Formation

Summary The burial diagenesis of Eocene Wilcox sandstones differs significantly from the burial diagenesis of other Gulf Coast terrigenous clastic formations. Differences between the onshore Wilcox and Oligocene-Miocene Frio Formations are larger than regional variations within either formation. Younger, offshore units have undergone less diagenetic alteration than either of the older, onshore units, whereas Mesozoic sandstones are generally more extensively altered. Cementation by quartz and calcite was the first diagenetic modification of volumetric significance to affect either the Wilcox or Frio Formations. The average δ 18 O of Wilcox quartz cement is approximately +25‰ SMOW, in contrast to + 31‰ for the Frio. In both formations, quartz cement is more abundant in the overpressured zone. Calcite has an average δ 18 O of −10.8‰ PDB in the Wilcox Formation (compared to −7.2‰ for the Frio). 87 Sr/ 86 Sr values suggest that calcite was derived from coeval nannofossils, carbonate rock fragments of Mesozoic age, or by local input of fluids from underlying Mesozoic carbonates. Most calcite cement (and therefore quartz which pre-dates calcite cement) was apparently emplaced prior to extensive alteration of detrital silicates which would have released strontium with a high 87 Sr/ 86 Sr ratio. The most common carbonate cement in the Wilcox Formation is ankerite which replaces calcite with increasing depth. The δ 13 C of ankerite is essentially identical to that of calcite at the same depth, and is also similar to Frio and younger carbonate cements. In contrast, δ 18 O values of Wilcox ankerite (average −9.8‰) indicate emplacement at higher temperature or from more depleted water than was true of Wilcox calcite. 87 Sr/ 86 Sr values for Wilcox ankerite are very radiogenic (>0.7100), indicating that ankerite emplacement occurred during or after active silicate diagenesis. Both ankerite and dolomite are very uncommon phases in Frio and younger sandstones despite massive smectite stabilization, suggesting that the conversion of smectite to illite was not the source of iron and magnesium for the late cements. The volume of secondary porosity is similar in both Wilcox and Frio sandstones. Albitization and K-feldspar removal from both formations are essentially complete below 3000 m and essentially no unaltered detrital feldspar occurs below that depth. Differences between the Gulf Coast formations are attributed to different geothermal gradients, differences in the basinal sediments over which the units prograded, and to the changing nature of connate fluids in the units themselves. Diagenesis of the Wilcox Formation, like other Gulf Coast terrigenous wedges, is understandable only in terms of interaction with underlying units during the large-scale evolution, in both time and in space, of the Gulf Coast diagenetic system.

Occurrence of Pseudophragmina sp. as a Possible Indication of Carbonate Bank Deposition in Upper Wilcox (Paleocene-Eocene), Pointe Coupee Parish, Louisiana: ABSTRACT

Paleontologic and sedimentologic analysis of whole-diameter cores from two upper Wilcox sandstones in south-central Louisiana suggests the presence of carbonate bank deposition during the late Wilcox. The larger foraminifera Pseudophragmina sp. occurs in two stratigraphically unrelated sandstones from the upper Wilcox (Paleocene-Eocene) in Pointe Coupee Parish, Louisiana. In Bayou Fordoche field, Pseudophragmina sp. is dispersed throughout an uppermost Wilcox sandstone, which is a 6-m thick unit found 82-m below the top of the Wilcox. In Fordoche field, which is south and down dip of Bayou Fordoche field, numerous individuals of Pseudophragmina sp. are concentrated in two distinct zones within the W-12 sandstone, a relatively thicker (33 m) sandstone that occurs 320 m below the top of the Wilcox. The down dip W-12 sandstone is stratigraphically unrelated to the updip sandstone; however, the two sandstones are lithologically and faunally similar. The updip sandstone and the Pseudophragmina sp. bearing intervals in the down dip W-12 sandstone are quartzose and glauconitic. In addition to numerous Pseudophragmina sp., they contain globorotalid and globigerinid planktonic foraminifera and smaller benthic foraminifera from such groups as miliolids, lagenids, and nodosarids. Echinoids and mollusks are also present. These fossils are dispersed throughout the updip sandstone and are concentratedmore » in two separate zones within the down dip W-12 sandstone. This Pseudophragmina facies in the upper Wilcox is faunally and lithologically similar to facies that are related to modern submarine carbonate banks in the Gulf of Mexico and to reef facies reported for subsurface Tertiary units in the Gulf Coast.« less

Computerized Quantitative Petrographic Image Analysis of Wilcox Sandstone, Louisiana Gulf Coast: ABSTRACT

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Diagenetic history of Eocene Wilcox sandstones, South-Central Texas

Sandstones and shales of the lower Eocene Wilcox Group extend from outcrop to depths greater than 5 km where temperatures exceed 200°C. Eight authigenic minerals have been identified. Quartz, kaolinite, calcite, ankerite and albitized plagioclase are volumetrically significant, whereas chlorite, illite, albite overgrowths and dolomite are present only in trace amounts. Quartz, kaolinite, and ankerite cements are present over the entire depth range sampled (1528 to 4446 m). Calcite cement occurs only in sandstones shallower than 3300 m. Albitized plagioclase is the predominant feldspar in samples deeper than 2600 m whereas detrital potassium feldspar has been completely dissolved out of sandstones deeper than about 3200 m. No systematic variations in the chemical composition of calcite with depth, no zoning within calcite or ankerite, and no compositional differences between pore-filling and grain-replacing calcite or ankerite were found. Calcite cements have 87Sr 86Sr ratios generally less than 0.708, whereas ankerite cements are significantly enriched in 87Sr. The oxygen isotopic composition of carbonate cement ranges between −9 and −14%. (PDB) for calcite and between −8 and −12%. for ankerite with no systematic variation with depth. Calculated δ 18O values for authigenic quartz and kaolinite range from +22 to +31%. (SMOW) and from +14 to +17%., respectively. Volumetrically important amounts of authigenic quartz and kaolinite precipitated during early burial before calcite cementation, secondary porosity generation, or smectite to illite transformation in encasing shales. The isotopic composition of strontium in calcite cement shows most of it to be of Eocene or older marine carbonate origin. Ankerite cementation followed the generation of secondary porosity and occurred after the products of organic maturation and shale diagenesis, plagioclase albitization or potassium feldspar dissolution significantly affected the sandstone-pore water system.

Oil and Gas Developments in Louisiana Gulf Coast Onshore in 1984

A decrease in operating and exploration costs prompted a rise in drilling, leasing, and geophysical activity in the south Louisiana onshore region in 1984. However, because of a persistent gas surplus and uncertain oil prices, operators adopted a conservative strategy for reserve replacement that included an increase in development drilling, an acceleration of enhanced oil recovery projects, and a concentration of extension and exploration drilling in the established Miocene and Oligocene trends. Operators drilled 1,447 wells in 1984, an increase of 15% over the 1,230 wells drilled in 1983. Total footage also rose from 13,453,752 to 15,324,733 ft. Oil companies drilled 245 wildcats, 302 other exploratory tests, and 900 development wells (development drilling increasing 18%). Seven newfield discovery wells were completed in 1984, of which 4 produced from Wilcox (Eocene) sandstones, 2 were perforated in the Oligocene, and 1 well was completed in the Austin Chalk (Cretaceous). There were 36 significant extensions, of which 33 found production in rocks of Miocene and Oligocene age. Leasing activity increased 10% to 885,903 ac. This level is far below that of 1980, 1981, and 1982 when more than 1 million ac were leased each year. Leasing activity was heaviest in Lafourche, St. Tammany, Vermilion, Calcasieu, and Beauregard Parishes. Geophysical activity also had a slight increase of 4% in 1984. A total of 1,259 crew-weeks of seismic shooting was recorded as compared to 1,212 crew-weeks in 1983. Cameron, St. Martin, Jefferson, Lafourche, Plaquemines, and Vermilion Parishes recorded the highest levels of geophysical activity.

Reservoir Characteristics of Lower Wilcox Sandstones, Lobo Trend, Webb and Zapata Counties, Texas: ABSTRACT

To date, over 340 bcf of gas have been produced from the Lobo sandstones in the Laredo field area at depths of less than 10,000 ft (3,050 m). Gas accumulation is controlled by faulting and erosional truncation. The resulting structural complexity has made accurate prediction of reservoir sandstones difficult. Cored sections display repetitive ordered sequences of sedimentary structures and textural and compositional gradations indicative of turbidity-current deposits. The reservoir sandstones were deposited as constructional channels having vertical and lateral variation from channel-fill to channel-margin to overbank deposits. Channel-fill units are 2-10 ft (0.61-3.05 m) thick and composed of AB, AE, and ABE bedsets. Channel-margin units are 1-3 ft (0.31-0.92 m) thick an contain thinner, more complete ABC, ABE and ABCE sequences. Overbank deposits consist of highly bioturbated, thinly interbedded sandstones and shales. Sandstones are feldspathic litharenites that have 15% matrix and 15% calcite cement. Porosities average 16% and permeabilities range from 0.54 to 12 md, decreasing with increased matrix, cement, and bioturbation. The channel-fill sandstones are linear, dip-trending bodies less than 3,000 ft (915 m) wide, which bifurcate downdip into distributary channels. High-intensity, small-scale, soft-sediment deformation indicates the sandstones were deposited in an unstable outer-shelf to upper-slope environment. A slumped, dip-trending channel-fill interpretation for the Lobo sandstones provides a mechanism for sediment transport beyond the present downdip limits of the trend. End_of_Article - Last_Page 265------------

Maturation of Organic Matter in Paleocene-Eocene Wilcox Group, South Texas: Relationship to Clay Diagenesis and Sandstone Cementation: ABSTRACT

Thirty-three mudstone core and cuttings samples ranging in depth of 1.6-4.7 km (5,200-15,400 ft) and in temperature from 80° to 210°C (175-410°F) were obtained from wells in south Texas. The results of closed-system pyrolyses and bitumen and kerogen analyses are related to available data on clay mineralogy and sandstone cement. This study examines the diagenesis of Wilcox organic matter, the migration of hydrocarbons, and the importance of organically derived CO2 in sandstone cementation. The samples average 1% TOC, contain type III kerogen, and generally show bitumen contents less than or equal to about 150 mg/gC. Modeling indicates that primary migration of methane and light hydrocarbons in aqueous solution is capable of producing giant (>= 1 tcf) gas and condensate fields. Because hydrocarbon generation does not occur until after the main stage of illitization, smectite accounts for less than 25% of the total water involved. Pyrolysis experiments indicate that as much as 150 mg/gC CO2 may be liberated by Wilcox organic matter during diagenesis to present-day temperatures of 100°C (212°F). The main zone of oil generation occurs at subsurface temperatures of 95°-125°C (203°-257°F). The timing of these processes suggests that CO2 could play an important role in creating secondary sandstone porosity for hydrocarbon migration. The ^dgr13C values indicate that 25% of the carbonate cement present in Wilcox sandstones may originate from decomposition and diagenesis of organic matter. End_of_Article - Last_Page 492------------

Stratigraphy and Structure of Rosita Gas Fields, Duval County, Texas: ABSTRACT

Deep Wilcox sandstones in the Rosita field produce large quantities of gas from highly faulted reservoirs at depths of 9,500 to 15,300 ft (2,895 to 4,660 m). The producing sandstones were deposited downdip from a large, active growth-fault system. Six full-diameter cores and 25 electric logs were used to interpret structure and environment of deposition. Structural movement along listric normal faults resulted in structural closures and stratigraphic thickening. These faults and rollover structures form hydrocarbon traps. Thick sequences of sandstones and shales show a basal shear zone, overlain by a folded and contorted zone, and finally, a normal bioturbated section. The sequence ranges from 10 to 50 ft (3 to 15 m) in thickness and may indicate soft-sediment deformation by slip along nearly horizontal glide planes. The extensive deformation suggests that the entire Wilcox section moved intermittently shortly after deposition under the force of gravity. Numerous deformed slides are present within major fault blocks. High pore pressure probably facilitated movement of these slide blocks. The depositional environment of Wilcox sandstones in the Rosita field was probably outer neritic to upper bathyal. This interpretation is supported by regional stratigraphic location and trace fossil morphology. The style of deformation is also compatible with an outer-shelf or upper-slope morphology. The producing sandstones are classified as lithic and feldspathic graywackes. Variation of texture and composition is small. Low permeabilities of 0.001 to 2 md can be attributed to high authigenic-clay content. End_of_Article - Last_Page 1680------------

Subunconformity Seismic Stratigraphic Exploration: ABSTRACT

Seismic stratigraphic exploration is particularly applicable to exploration for unconformity traps. Typical upstructure truncation and angularity can often be detected with conventional seismic displays. More subtle traps created by lateral truncation of secondary porosity in either limestone or chert are more difficult to explore for using the seismic tool. Often sophisticated data processing techniques like SeislogR aid in identification of the reservoir-seal relation. Comparison of the Seislog expression of a structural unconformity with a low relief erosion surface provides insight into seismic stratigraphic exploration for subunconformity fields. South Laredo field, Webb County, Texas, serves as a classic upstructure unconformity. Eocene Wilcox sandstones are either progressively truncated at the flanks of the Salado uplift or preserved in rotated fault blocks. On Seislog, unique high velocity sandstone beds progressively lose thickness updip beneath a low-velocity shale. This truncation provides clear evidence for the trap. In contrast, erosional plays that search for shale overlying porous carbonate rocks or chert often need to recognize units with similar velocity. At Star Lacey field in Kingfisher County, Oklahoma, porous Hunton is not a distinct unit but, on Seislog, exhibits velocity comparable with the overlying Woodford Shale. The seismic trap expression is created by anomalous Woodford thicks which represent the superposition of shale over Hunton porosity. The chert trap at Nicols field, Kiowa County, Kansas, similarly is based on a subtle slope change on the Seislog at the shale reservoir interface. Seislog colors separate the gentle character formed when shale overlies porous chert to the steep slope generated by shale over nonporous carbonate rock. The expression of the fields on Seislog provides encouragement that seismic data can successfully locate unconformity-type reservoirs. The subtle nature of the features, however, suggests that careful integration with subsurface data is required to maximize exploration efforts. End_of_Article - Last_Page 1501------------

Depositional Systems and Lignite Prospecting Models--Wilcox Group and Meridian Sandstone (Eocene), Northern Mississippi: ABSTRACT

ABSTRACT The first year of a three year Mississippi Mineral Resources Institute project to map the surface and subsurface terrigenous clastic depositional systems and lignite seams of the Eocene Wilcox-Meridian stratigraphic interval in Mississippi has been completed. Data from 620 oil and water well electric logs, 65 sample logs, and 35 surface exposures have been utilized to determine the distribution of the principal sandstone bodies in the northern third of the state. Evidence from eight regional cross sections indicates that the combined Wilcox and Meridian study interval can be subdivided into a minimum of four distinct units: (1) a basal Wilcox progradational interval, (2) a lower Wilcox fluvial-deltaic unit, (3) an upper Wilcox fine-grained fluvial unit, and (4) a Meridian coarse-grained fluvial unit. In extreme northwestern Mississippi the highest 200 feet of the Wilcox is made up of massive sandstone and is genetically related to the overlying Meridian Sandstone (Claiborne Group). This is the Meridian-Upper Wilcox Aquifer System of hydrologists. Hence, in the northern-most counties of the study area, the upper Wilcox and Meridian become a single, very massive, coarse-grained fluvial unit. Sandstone-body geometry and lignite distribution within the Wilcox-Meridian interval have been evaluated using net-sandstone isolith, net sandstone percent, thickness of most massive sandstone unit, and lignite isopleth maps. Vertical curve shapes for individual wells furnish additional information for interpreting specific environments of deposition. Dendritic, contributory sandstone belts of low to moderate sinuosity comprise the dominant geometry displayed by these rock bodies, even for the basal Wilcox progradational interval. Nine distinct sandstone belts 6 to 10 miles wide are noted in the lower Wilcox; seven of these same belts are also present in the upper Wilcox. The Meridian has a much higher proportion of sandstone, averaging approximately 70 percent, but the 90 percent sandstone contour line highlights 8 sandstone belts occupying roughly the same trends as the sandstone maxima for the lower Wilcox. A down-dip bifurcating (distributary) net-sandstone pattern characteristic of the upper delta-plain facies has been mapped in the basal Wilcox, lower Wilcox, and upper Wilcox units in southern Bolivar, Yazoo, Sharkey, Issaquena, and Washington Counties. Lignite isopleth maxima in central Washington County of 17 and 18 for the lower and upper Wilcox lend further support for identifying the proximal delta plain. The principal distal deltaic sandstone bodies of the Wilcox high-constructional Holly Springs Delta System, however, have been mapped by other researchers as being 75 to 125 miles downdip from this area. Lignite prospecting in the Wilcox-Meridian section of North Mississippi can be greatly aided through an understanding of the geographic and stratigraphic distribution of the major depositional systems. Regarding fluvial systems, organic debris preferentially accumulates with fine-grained sediment. Significant lignite deposits most likely formed in overbank deposits away from active stream channels. As a result, commercially exploitable seams associated with ancient fluvial systems should be sought in interstream basins of fine-grained meanderbelt systems where values of lower than 50% sandstone occur. Such areas are present in the lower Wilcox of Calhoun, Webster, and Choctaw Counties and in the upper Wilcox as far north as Marshall County. Coarse-grained meanderbelt and incised valley-fill fluvial systems represent much less satisfactory settings for lignite formation, inasmuch as most of the fine-grained material is transported downdip without being deposited. Consequently, the Meridian coarse-grained meanderbelt system throughout North Mississippi is barren of lignite. High-constructive deltas furnish optimum conditions for the accumulation of organic material. In North Mississippi, the thickest individual seams and the greatest number of seams occur at the junction of the alluvial plain and proximal delta plain. This junction is the position where channels first bifurcate to exhibit a distributary pattern and is well demonstrated by lower and upper Wilcox sandstone bodies in Washington, Sharkey, and Yazoo Counties. End_Page 283------------------------

Environment of Deposition of Upper Wilcox Sandstones, Katy Gas Field, Waller County, Texas: ABSTRACT

ABSTRACT At Katy gas field, sandstones of the upper Group produce gas at depths of 10021 to 11000 ft (3054 to 3353m) in reservoirs controlled by stratigraphic and structural characteristics. Producing zones are from 6 to 42 ft (1.8 to 12.8m) in the upper first lower massive sandstones, and D, C, B, A and second Wilcox interbedded sandstones and shales. The reservoir sandstones are dip-trending with production being localized on the top of the anticline. The upper sequence has been interpreted as delta-front grading upward to bay-marsh transitional deposits (Williams, et al., 1974) and, alternatively as deep-water turbidite deposits (Berg and Findley, 1973). The field is located downdip from the fault zone, downdip from known delta-destructional deposits in the upper Wilcox, and is as much as 45 miles (75 km) downdip from the postulated late Sabinian shoreline. Full-diameter cores from the upper sequence show the sandstones are submarine, constructional-channel turbidites, giving way vertically to thinner turbidite sandstones in a predominantly shale section. The sandstones are representative of submarine fan deposits, having bedset associations characteristic of channel deposits (A, AB and ABD) becoming middle fan associations (AE, BE, ABCE, and BCE) and then outer fan associations (ABE, BDE, CDE and DE) upward in the section. The thicker channel sandstones show limited lateral extent along strike, grading to thin, overbank sandstones. Sandstones are sparsely bioturbated, and shales are bioturbated only when they immediately overlie sandstones. The burrows are characteristic of a wide range of water depths from middle neritic to bathyal. Benthonic forams found in the cores are abraded by transport and represent a range of water depths from middle to outer neritic. Therefore, water depths during deposition were probably bathyal, as indicated by deeper-water trace fossils. The deposition of the upper Group is associated with transgression during late Sabinian and incipient uplift of a deep-seated, diapiric mass under the field. Electric log correlations and sandstone isopach maps suggest that the sands were deposited as parts of a submarine fan that shifted northwestward through time.

Clay Minerals as Indicators for Depositional Environment in South Halletsville Field, Lavaca County, Texas: ABSTRACT

The South Halletsville field, Lavaca County, Texas, has gas and condensate production from lower Wilcox sandstones and shales which have been interpreted as either channel turbidite deposits in outer-shelf to slope locations or deltaic and strand-plain sands and muds. Twenty-four core samples from the General Crude Oil Co. 1 A. G. Henkes Gas Unit were analyzed by X-ray diffraction methods to determine whether a semiquantitative estimate of clay mineral content would aid in determining the depositional environment. Discrete illite and chlorite are of particular interest because the presence of these minerals is interpreted as being due to original deposition. Three shale samples, from 10,194 to 10,206 ft (3,107 to 3,111 m) and 11 sandstone samples, 10,180 to 10,194 ft (3,103 to 3,107 m) were selected from one core section. This sequence of samples is particularly important because it contains a shale and the overlying sandstone. In addition, a deeper sandstone was sampled in the interval of 11,032 to 11,072 ft (3,363 to 3,375 m). If a turbidity-type event had occurred, the weight percent of non-diagenetic clays should (1) decrease significantly as the boundary is crossed between the shale and the overlying sandstone, and (2) gradually increase in progressively shallow samples within the sandstones. However, the weight percent for chlorite does not vary significantly regardless of a change in lithology, shale to sandstone, or a change in depth. The illite content gradually decreases with shallower depths in both core intervals. This sequence is more compatible with a transgressive deltaic environment. End_of_Article - Last_Page 1557------------

Reservoir Morphologies for Gulf Coast Turbidite Sandstones, Texas: ABSTRACT

Turbidite sandstones form gas reservoirs in the deeper Tertiary section of the Gulf Coast province of Texas. Turbidite reservoirs are of three types: (1) thin-bedded sandstones of middle-shelf origin, (2) constructional-channel-fill sandstones, and (3) stacked-channel turbidites. Production in all these reservoirs is associated with structures, but it appears that stratigraphic control exerts a fundamental influence on the accumulation of gas. Oligocene Frio sandstones at Nine Mile Point field are turbidites which formed in water depths of about 300 ft (100 m). The thin-bedded sandstones are interbedded with bioturbated shales. Sandstones are largely channel turbidites of the AE and ABE types, but more complete sequences are also present. Bed associations resemble those of submarine fans, but Frio turbidites are concentrated on the downthrown sides of major normal faults, and adjacent anticlinal crests are nearly barren of porous sand. The sandstones are clayey, and maximum permeabilities are in the range of 10 to 50 md. Eocene lower Wilcox sandstones at Northeast Thompsonville field are massive turbidites of channel End_Page 418------------------------------ origin. Bed sets are of the AE and ABE types. The channel-fill sandstones appear to have been deposited in constructional channels, narrowly bordered by more thinly bedded levee deposits. Multiple-channel sandstones cross anticlinal structure at Northeast Thompsonville field, but in others areas the occurrence of oil and gas is not related to structural closure. The sandstones are highly quartzose, and maximum permeabilities are in the range of 100 to 200 md. Oligocene Vicksburg sandstones at McAllen Ranch field form a thick series of channel turbidites of the AE and ABE types with interbedded, more complete, turbidite sequences. These stacked-channel sandstones probably represent inner-fan deposits. Associated anticlinal structures are probably the result of differential compaction over thick concentrations of channel turbidites. The sandstones are clayey, and maximum permeabilities are in the range of 10 to 50 md. The relatively abundant turbidites in these deep sections indicate that sand transport in a dip direction was a common mechanism that operated through much of the Tertiary. Although many of these sandstones have low permeabilities, they have the potential to form important, stratigraphic accumulations of natural gas through most of the deep, basinal section. End_of_Article - Last_Page 419------------

Clay Diagenesis in Wilcox Sandstones of Southwest Texas: Implications of Smectite Diagenesis on Sandstone Cementation

ABSTRACT Sandstones and shales of the Wilcox Group (lower Eocene) in southwest Texas were examined by X-ray powder diffraction, electron microprobe, and petrographically to interpret their diagenetic history. Samples analyzed are from depths of 975 to 4650 m, representing a temperature range of 55°C to 210°C. No consistent trend of depositional environments is recognized with increasing depth, and mineralogic changes observed are interpreted as diagenetic. Major mineral distribution patterns are (1) disappearance of discrete smectite at temperatures >70°C, (2) gradation of mixed-layer illite/smectite to less expandable (more illitic) illite/smectite over the entire temperature range, (3) disappearance of kaolinite from 150-200°C accompanied by an increase in chlorite, and (4) replacement of calcite cement at about 117 120°C by ankerite. Calculations based on data of Hower and others (1976) indicate that the stability of smectite layers may be a function of composition. Smectites with high ratios of octahedral (Fe + Mg)/Al appear to resist conversion to illite until temperatures high enough to produce ordering are attained. A diagenetic model is proposed which involves the breakdown of detrital K-feldspar and of some smectite layers in illite/smectite to convert other smectite layers to illite. Silica and calcium released by the illitization of smectite is transferred from shales to sandstones to produce quartz overgrowths and calcite cements at temperatures as low as 60°C. Iron and magnesium released by the illitization reaction are transferred from shales to sandstones at temperatures >100°C and react with kaolinite to produce high-alumina chlorite and/or with calcite to produce ankerite.

Active ankerite cementation in the subsurface Eocene of southwest Texas

In subsurface samples of Wilcox (Eocene) sandstones, calcite cements occur above 2315 m depths, whereas ankerites occur at depths from 2560 m (temperatures 125 ° C) to at least 4650 m (temperatures 210 ° C). Microprobe analyses indicate that some shallow ankerites have appreciable excess calcium, analogous to protodomites. Ankerites at depths greater than 3200 m have compositions of about CaMg0.5Fe0.5(CO3)2.

Developments in South Texas in 1977

The South Texas report includes 58 counties in Texas Railroad Commission Districts 1, 2, and 4, and the offshore area. District 1 exploration activity increased 32% from 1976, with significant discoveries from the Cretaceous in Dimmit, Maverick, and Milam Counties. Although the total exploration effort was significantly greater, the success rate decreased 21%, mainly because of a drop in successful oil completions. Development activity in District 1 increased slightly with a continued 87% success rate. District 2 exploration activity remained consistent with 1976 figures. Oil completions increased by 60% with a total exploration effort of 338 wells. The overall exploratory success rate decreased from 42% in 1976 to 39% in 1977. The heaviest sustained activity continues to be in the Frio trend of Jackson, Victoria, Refugio, and Calhoun Counties. Development activity remained constant in 1977 with 488 total wells and a 70% success rate. District 4 exploration activity was up 33.5% from 1976, with significant discoveries in the Frio trend in Aransas and San Patricio Counties, and in upper and lower Wilcox sandstones in Webb and Zapata Counties. Oil discoveries were up significantly in 1977 with 33 successful exploration-class oil completions, including 6 new-field wells. District 4 development activity was up 29.3% in 1977 with 903 wells drilled and a total footage of 5,349,831. This increase is due mainly to activity in southern Webb County in the lower Wilcox south Laredo gas trend and in the Navarro Escondido and Olmos sandstone trend of northwestern Webb County. Exxon led in development activity, primarily with Frio-Vicksburg plays on King Ranch acreage. The South Texas offshore area had a 22.4% increase in activity, with a total of 71 wells drilled in 1977. Significant discoveries were drilled in the Matagorda and Mustang Island areas by Corpus Christi Oil & Gas and McMoRan. Development activity increased from 3 wells in 1976 to 12 wells in 1977 with 4 successful completions.

Clay Diagenesis in Wilcox Sandstones of Southwest Texas--Implications of Smectite-Illite Reaction for Sandstone Cementation: ABSTRACT

Clay Diagenesis in Wilcox Sandstones of Southwest Texas--Implications of Smectite-Illite Reaction for Sandstone Cementation: ABSTRACT

Characteristics of Wilcox Gas Reservoirs, Northeast Thompsonville Field, Jim Hogg and Webb Counties, Texas: ABSTRACT

ABSTRACT Gas was discovered in upper Wilcox sandstones in 1959 at Northeast Thompsonville field. Reservoirs are found on a faulted anticline at depths ranging from 9,400 to 13,700 ft. The Wilcox is dominantly shale, and stratigraphic units thicken off structure. At the crest of the structure is a growth fault that has about 1100 ft of throw, downthrown to the basin, and has a low dip of about 30 degrees, decreasing in the deep section near 12,000 ft. Sandstones are thin-bedded, 1 to 2 ft thick, with interlaminated black shale. Bed sets display a sequence of graded, structureless sandstone overlain by laminated sandstone. This sequence appears to represent an ABE turbidite, probably of channel origin. Thinner sandstones are laminated and ripple-bedded, representing more complete turbidite sequences of the ABCDE type, but some thin sandstones are less complete BCE and CDE turbidites. The thinner sandstones are probably of overbank origin. Average grain size is 0. 13 mm (fine-grained), and bed sets are commonly graded from 0. 15 mm at the base to 0.06 mm at the top. Average detrital composition is 64 percent monocrystalline quartz, 6 percent feldspar, 2 percent rock fragments including polycrystalline quartz, 27 percent matrix, and 1 percent other grains. Common cement is silica as grain overgrowths and, more rarely, siderite. Total cement averages 17 percent of bulk composition. The best average porosities and permeabilities are on the order of 23 percent and 200 md and are found in thicker, channel turbidites. The morphology of reservoirs is illustrated by the uppermost Wilcox, Hinnant 1 sandstone that has a gross thickness of about 90 ft. Within the Hinnant are massive sandstone sections composed largely of AE and ABE turbidite sets. The massive units range from 30 to 75 ft in thickness and form narrow, dip-trending concentrations of channel turbidites that are the principal reservoir sandstones.

Factors Influencing Porosity and Permeability of Lower Wilcox (Eocene) Sandstone, Karnes County, Texas: ABSTRACT

Factors Influencing Porosity and Permeability of Lower Wilcox (Eocene) Sandstone, Karnes County, Texas: ABSTRACT