Stuart City Research Articles

Deeper-water deposition in intrashelf basins: Example from the Lower Cretaceous (Albian) upper Glen Rose Formation in the Houston trough, eastern Texas

In easternTexas, a thick (150-200 ft) interval of Lower Cretaceous upper Glen Rose D strata was deposited in the Houston trough intrashelf basin landward of the Stuart City reef margin and seaward of the main eastern Texas intrashelf basin. The stratigraphic section in this intrashelf basin forms a primarily oil-bearing hydrocarbon system where porous grainstones in the Alabama Ferry and Fort Trinidad fields were deposited coevally with organic-rich calcareous argillaceous mudstones. Here, we present evidence to suggest that the depositional setting within the Houston trough and the contained ooid grainstone beds were of deeper-water origin (400-500 ft), below storm wave base. Further, the water column was stratified, allowing dysaerobic to anaerobic conditions to develop, fostering accumulation and preservation of organic matter. Grain-rich gravity flows consisting of ooids, intraclasts, and thin-walled mollusks, derived from the shallower shelf to the northwest, were deposited in the basin, forming the Alabama Ferry and Fort Trinidad fields. The upper Glen Rose interval is composed of lithofacies that vary laterally within the Houston trough. Five lithofacies are recognized on the basis of sedimentary features, texture, fabric, biota, and organic-matter content: (1) laminated to nonlaminated argillaceous lime mudstone to lime wackestone; (2) laminated to nonlaminated calcareous siliciclastic mudstone to siltstone; (3) intraclast–ooid skeletal lime packstone; (4) intraclast–bivalve–ooid lime grainstone; and (5) oyster lime floatstone and rudstone. Grain-rich lithofacies laterally adjacent to and interbedded with organic-rich argillaceous lime mudstones and calcareous siliciclastic mudstones were previously interpreted as being deposited in a tidally influenced, high-energy, shallow-water shoal environment; however, reconstruction of the depositional setting and interpretation of the vertical and lateral distribution of lithofacies suggest that these lithofacies were deposited as grain-rich gravity flows into the deeper-water Houston trough. This interpretation suggests a new conceptual framework integrating facies models from submarine channel and fan complexes to explore for grainstone reservoirs similar to Alabama Ferry and Fort Trinidad fields.

Dip-related changes in stratigraphic architecture and associated sedimentological and geochemical variability in the Upper Cretaceous Eagle Ford Group in south Texas

A detailed, rock-based investigation of three Upper Cretaceous Eagle Ford Group cores situated behind, at, and downdip of the Lower Cretaceous Stuart City paleoreef-shelf margin in south Texas was conducted to understand stratigraphic, sedimentological, and geochemical relationships across this buried shelf margin. An understanding of how the Eagle Ford Group lithofacies vary across the paleoreef-shelf margin is currently lacking. We therefore examined a dip section of three cores across the antecedent shelf margin and delineated seven Eagle Ford lithofacies: (1) massive argillaceous mudstone, (2) massive to laminated foraminiferal lime wackestone, (3) radiolarian and foraminiferal dolomitic to lime packstone, (4) massive to bioturbated skeletal lime wackestone, (5) laminated foraminiferal lime packstone, (6) laminated inoceramid and foraminiferal lime grainstone, and (7) massive to bioturbated claystone. A basinward decrease in calcite from 60% to 48% is accompanied by an increase in clay minerals from 12% to 20%. The low-relief raised rim of the older, buried Stuart City paleoshelf margin may have acted as a barrier, dividing the Eagle Ford Group into two sedimentological systems: (1) a restricted drowned shelf to the north, and (2) an open-marine basinal setting to the south. The lower to upper Cenomanian Eagle Ford strata on the drowned shelf are cyclic and enriched in molybdenum, suggesting anoxic to euxinic water masses. The anoxic, open-marine, basinward strata are less cyclical and have a lower molybdenum (compared with the drowned shelf) content. Ash beds and gravity-flow deposits are rare south of the margin. A depositional model was constructed of the lower and upper Eagle Ford formations.

Domal, thrombolitic, microbialite biostromes and associated lithofacies in the Upper Albian Devils River Trend along the northern, high-energy margin of the Maverick Basin

The Lower Cretaceous Devils River Trend on the northeast margin of the Maverick Basin in southwest Texas contains unique deposits of large domal, thrombolitic, microbial biostromes that have not been identified elsewhere in the Albian of the onshore Gulf of Mexico. These microbial biostromes are overlain by microbial-dominated, moderate-energy grainstones to mud-free rudstones. Above the grainstone to rudstone section is a hiatus (the Albian-19, high-frequency-sequence maximum flooding surface), and above this hiatus are large caprinid rudist bioherms. The overall setting for these carbonate deposits is the moderate- to high-energy north rim of the intraplatform Maverick Basin that is approximately 150 mi (240 km) behind the Stuart City Reef Trend. Four levels of description are necessary to describe the biostromes: (1) megalevel (overall external form)—domal biostromes, (2) macrolevel (larger internal form or layering)—sinuous convex-up layers within domes, (3) mesolevel (basic growth morphology—clotted thrombolites [subvertical to vertical]), and (4) microlevel (microbialite basic texture)—microbial peloids and mesoclots (clots of microbial-mediated material—mainly packed clusters of dark, micritic peloids). The thrombolitic peloids and mesoclots are arranged in digitate to randomly branching growth forms. The thrombolites are very microporous. Growth-framework cavities with sediment fill are common and are filled with mesoclots, peloids, clasts, miliolids, ostracods, echinoid fragments, bivalve fragments, and gastropods. The microbial biostrome is not continuous but has gaps (channels) filled with microbial-dominated grainstone and mud-free rudstones. The microbial-dominated grainstone and mud-free rudstone strata above the microbial biostrome are continuous, although irregularly bedded, with some beds being cross-stratified, and they are composed of fossiliferous, peloidal, mesoclot grainstone with abundant biota encrusted in a microbial coating. The domal, thrombolitic, microbialite biostrome is interpreted to have been deposited on a moderate-energy, semirestricted intraplatform rim at the northeast edge of the Maverick Basin during a higher order transgression. The microbial-dominated strata may have been associated with high nutrient concentrations and initial low sedimentation rates, followed by low to moderate sedimentation rates. The microbial-dominated grainstones and mud-free rudstones above were laid down in a moderate-energy environment during continued transgression. A sharp break exists with the shelf-interior, high-frequency highstand cycle above, which contains abundant rudist bioherms but no microbialites. Most of the primary megapores (growth-framework cavities and interparticle pores) are cemented with equant calcite. Nano- and micropores are extremely abundant within the microbial-related strata. These deposits would make good microporous reservoirs in the subsurface.

Documentation and characterization of the Lower Cretaceous (Valanginian) Calvin and Winn carbonate shelves and shelf margins, onshore northcentral Gulf of Mexico

The Lower Cretaceous stratigraphic section in the northern Gulf of Mexico (GOM) comprises several well-documented carbonate systems that have prominent shelf-margin buildups, including the Berriasian Knowles Limestone ramp and shelf system, Barremian Sligo Formation shelf system, Aptian Pearsall Formation ramp system, and Albian Glen Rose, Edwards, and Stuart City Formations shelf systems. Two lesser known but large-scale Lower Cretaceous Valanginian carbonate shelf to shelf-margin systems are documented in this investigation using core, wireline-log, and regional 2D seismic data. The platform-margin succession of the older Calvin section comprises a shelf-margin buildup (back reef, reef, and fore reef facies) that may be as thick as 2000 ft (610 m). The seaward margin of the shelf, as displayed by seismic data, suggests shelf-to-basin relief of as much as 1000 ft (305 m) and margin-to-lagoon relief of 200 to 500 ft (60 to 140 m) (relief estimates are not decompacted). Major reef-building organisms are Lithocodium, stromatoporoids, and corals. The Winn limestone is younger than the Calvin limestone, and at the shelf margin, it may have been 600 to 800 ft (180 to 245 m) thick. It is composed of organisms similar to those of the Calvin reef complex. Considering the vertical position of the Calvin platform, succeeding the major bypass of Calvin lowstand siliciclastic sands, the platform probably records a transgression, which is consistent with its highly aggradational depositional style. The Winn shelf margin was initiated landward of the Calvin shelf margin and did not prograde as far seaward as the Calvin shelf margin. The recognition of these two Lower Cretaceous carbonate systems is important because they further detail the stratigraphic history and architecture of this deeply buried, lowermost Lower Cretaceous section in the GOM. These formations are also potential deep-gas, tight-carbonate (low-porosity) plays.

Regional assessment of the Eagle Ford Group of South Texas, USA: Insights from lithology, pore volume, water saturation, organic richness, and productivity correlations

A comprehensive regional investigation of the Eagle Ford Shale linking productivity to porosity-thickness (PHIH), lithology ([Formula: see text]), pore volume (PHIT), organic matter (TOC), and water-saturation ([Formula: see text]) variations has not been presented to date. Therefore, isopach maps across the Eagle Ford Shale play west of the San Marcos Arch were constructed using thickness and log-calculated attributes such as TOC, [Formula: see text], [Formula: see text], and porosity to identify sweet spots and spatial distribution of these geologic characteristics that influence productivity in shale plays. The Upper Cretaceous Eagle Ford Shale in South Texas is an organic-rich, calcareous mudrock deposited during a second-order transgression of global sea level on a carbonate-dominated shelf updip from the older Sligo and Edwards (Stuart City) reef margins. Lithology and organic-matter deposition were controlled by fluvial input from the Woodbine delta in the northeast, upwelling along the Cretaceous shelf edge, and volcanic and clastic input from distant Laramide events to the north and west. Local oxygen minimum events along the South Texas margin contributed to the preservation of this organic-rich source rock related to the Cenomanian/Turonian global organic anoxic event (OAE2). Paleogeographic and deep-seated tectonic elements controlled the variations of lithology, amount and distribution of organic matter, and facies that have a profound impact on production quality. Petrophysical modeling was conducted to calculate total organic carbon, water saturation, lithology, and porosity of the Eagle Ford Group. Thickness maps, as well as PHIH maps, show multiple sweet spots across the study area. Components of the database were used as variables in kriging, and multivariate statistical analyses evaluated the impact of these variables on productivity. For example, TOC and clay volume ([Formula: see text]) show an inverse relationship that is related to production. Mapping petrophysical parameters across a play serves as a tool to predict geologic drivers of productivity across the Eagle Ford taking the geologic heterogeneity into account.

Back-Reef Depositional Environments in a Lower Cretaceous (Sligo) Shelf-Margin Complex: Insights into Ultradeep Reservoir Preservation and Controls on Stacking Patterns in an Outer Platform Setting

The Early Cretaceous Sligo and Stuart City shelf-margin trends in Texas have been a subject of interest for over 60 years, attracting attention both because of their striking complex reef lithologies and their continued potential for oil and gas exploration. Numerous gas reservoirs have been discovered since the late 1950s, with a major push for revitalization and expansion occurring in the 2000s. The shelf-margin complexes originated as steep-walled reefs rimming the Comanche Platform. They evolved into a narrow (<6 mi) highly heterogeneous belt of banks, islands, shoals, and tidal flats with an associated highly productive shallow-water carbonate factory. Most characterization efforts for these systems have focused on known reservoirs in South Texas; very little work has investigated the margins outside of this region, particularly with respect to the Sligo shelf margin. This investigation presents the first well-documented long stratigraphic section for the Sligo margin in East Texas, which serves as an initial step in understanding this major shelf-margin complex. Facies are mainly associated with back-reef to back-reef lagoon environments and exhibit significant heterogeneity on the basis of variations in water depth and wave energy, with facies including argillaceous carbonate mudstones, oncolitic packstones, rudist biostromes, reef-derived grainstones, ooid grainstones, and algal laminate bindstones. Four depositional sequences are delineated based on broad changes in facies and stacking patterns related to water depth and energy regime, yielding new insight into controls on facies partitioning in a stable long-term shelf-margin complex. The pore network, which is limited to micropores, is largely facies-controlled and is related to the presence of originally Mg–calcite allochems such as miliolids and Lithocodium/Bacinella. As East Texas has significant structural complexity, the ultradeep buried Sligo Formation is an underexplored area with the potential for undiscovered reservoirs related to structural features. Geologic characterization of this core is an important step in understanding some of the porous lithofacies present within the Sligo shelf margin in East Texas.

Commonwealth and the English Reformation: Protestantism and the Politics of Religious Change in the Gloucester Vale, 1483-1560 (review)

Reviewed by: Commonwealth and the English Reformation: Protestantism and the Politics of Religious Change in the Gloucester Vale, 1483-1560 Peter Clark Commonwealth and the English Reformation: Protestantism and the Politics of Religious Change in the Gloucester Vale, 1483-1560. By Ben Lowe. [St. Andrews Studies in Reformation History.] (Burlington, VT: Ashgate Publishing. 2010. Pp. xvi, 308. $124.95. ISBN 978-1-409-40045-5.) From this case study of the middle-rank English city of Gloucester and its surroundings, Ben Lowe argues that it was a combination of prophetic message and social and economic conditions that caused leading men in both town and county to join and support the early Tudor Reformation. Rather than seeing the disruptive nature of religious change, the analysis suggests there was a growing momentum of reform from the 1530s through to King Edward VI's reign, only to be reversed under Queen Mary. Lowe takes issues with other recent interpretations of the Reformation that have emphasized extensive Catholic resistance to change, religious indifference among the laity, or the economic interest of leading Protestants. His account returns to an older multidimensional approach in the tradition of A.G. Dickens. In considerable measure it is persuasive. Early chapters examine the economic, political, and ecclesiastical state of Gloucester before the Reformation with serious municipal [End Page 788] conflict with the nine or so religious houses in and around the city, many in decay; by comparison, parish religious life was more vibrant but (an important point) selectively so. A further chapter looks at the rise of new gentry families from the late Middle Ages, some trained in the law and officiating for religious houses, others linked to Crown administration. Intermarrying and working closely together, they took their functions seriously as local agents of the Crown and as guardians of religious life. From the 1530s they consolidated their own position by becoming chief enforcers of royal religious policies. Both in city and shire the local elite tended to prop up the existing religious order until the Reformation gave them the opportunity to seize church property for themselves and to endow a range of schools and charities—a concrete local version of a Commonwealth program much talked about at the Henrician Court. Lowe emphasizes the broad coalition of city leaders, county gentry, and Court politicians and leading clergy that promoted these initiatives in Gloucestershire. All this was reversed under Mary but at the cost of alienating a large part of the ruling class in both city and county. In the last part of the book Lowe argues that there were important parallels between the early Tudor reform movement and the early Stuart Puritan regime of the city, when acute economic and social problems and civic patronage of Puritan preachers and godly discipline turned Gloucester into a famous bastion of radical Protestantism. But this may be misleading. In the early-sixteenth century economic conditions in Gloucester (and the county) were relatively good (western cities did better than those in eastern England), and the acute pressures of large-scale poverty and immigration had yet to make much of an impact. Even more important, although Lowe shows in the early Tudor period city and landowners in close cooperation, by 1600 they were at daggers drawn over jurisdictional disputes. The early Stuart city on the hill was in part founded as a bastion against the shire. Overall, however, this is an interesting, well-written book that provides useful detailed material on both town and countryside. Hopefully, Lowe can be encouraged to write a comparative study of the English urban Reformation in the future. Peter Clark University of Helsinki Copyright © 2011 The Catholic University of America Press

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.

Deep Structure of the Texas Gulf Passive Margin and its Ouachita-Precambrian Basement: Results of the COCORP San Marcos Arch Survey (1)

This COCORP deep seismic survey provides a comprehensive image of the southeast-Texas part of the Gulf passive margin and its accreted Ouachita arc foundation. Beneath the updip limit of the Cenozoic sediment wedge, a prominent antiformal structure is imaged within the interior zone of the buried late Paleozoic Ouachita orogen. The structure appears to involve Precambrian Grenville basement. The crest of the antiform is coincident with the Cretaceous-Tertiary Luling-Mexia-Talco fault zone. Some of these faults dip to the northwest, counter to the general regional pattern of down-to-the-basin faulting, and appear to sole into the top of the antiform, suggesting that the Ouachita structure has been reactivated as a hingeline to the subsiding passive margin. The antiform may be tied via this fault system and the Ouachita gravity gradient to the similar Devils River, Waco, and Benton uplifts, interpreted as Precambrian basement-cored massifs. Above the Paleozoic sequence, a possible rift-related graben is imaged near the updip limit of Jurassic salt. Paleoshelf edges of the major Tertiary depositional sequences are marked by expanded sections disrupted by growth faults and shale diapirs. Within the Wilcox Formation, the transect crosses the mouth of the 900-m-deep Yoakum Canyon, a principal pathway of sediment delivery from the Laramide belt to the Gulf. Beneath the Wilcox, the Comanchean (Lower Cretaceous) shelf edge, capped by the Stuart City reef, is imaged as a pronounced topographic break onlapped by several moundy sediment packages. Because this segment of the line parallels strike, the topographic break may be interpreted as a 2000-m-deep embayment in the Cretaceous shelf-edge, and possibly a major submarine canyon older and deeper than the Yoakum Canyon.

The Stratigraphy of the Trinity Group, East Texas Basin

The Lower Cretaceous Trinity Group of the East Texas basin contains formations that have produced oil for a number of years, and while they have been the subject of considerable discussion, little has been said about their character and regional distribution over the entire basin. This regional treatment is critical to an understanding of the tectonic history of the East Texas basin. The Trinity Group consists of facies varying from fluvial sands to marginal marine limestones and shales. In the East Texas basin, Trinity rocks were initially deposited during basinal subsidence and form a wedge of sediments that thicken rapidly eastward. Early Trinity deposition began as clastics were shed from highlands to the north and west and then were deposited within the basin by prograding deltaic systems. Sediment was first deposited as channel-fill and point-bar deposits. Toward the basin, fluvial sands were deposited as destructive deltaic facies. True marine sedimentation was confined to the extreme southeastern part of the basin. Trinity deposition reflects a series of minor transgressive and regressive pulses, with environments ranging from deltaic and shallow open shelf to restricted lagoon. With the buildup of the Stuart City reef system along the southern margin of the basin isolationmore » occurred. During late Trinity, seas covered the entire basin and a large area of the stable Texas platform resulting in deposition of a thick, shallow, open-marine limestone and shale sequence, which marks the end of Trinity deposition.« less

Origin of micro-rhombic calcite matrix within Cretaceous reservoir rock, West Stuart City Trend, Texas

Cores from four wells from the West Stuart City Trend in La Salle and Webb Counties, Texas, were analyzed to define depositional and diagenetic facies and to determine factors controlling porosity distribution. In all, 1187 ft (362 m) of core and 220 thin sections were examined in detail and supplemented by SEM analyses of fractured surfaces and plastic casts of pore systems. A comparison of lithofacies and ecologic facies to permeability and porosity values compiled from core data revealed that zones with permeabilities greater than 0.1 md and porosities of at least 6% were associated with rudistid grainstones cemented by isopachous, submarine cement and packstones with a finely crystalline rhombic calcite matrix. Isotopic and trace element analyses of both isopachous submarine cements and micro-rhombic matrix suggest a common origin. The precursor to the micro-rhombic calcite is believed to have been peloidal, high-magnesian calcite. This internal marine sediment may be analogous to the peloidal fabrics that have been reported from Holocene and Pleistocene carbonates. Diagenetic equilibration of both submarine cements and peloidal infill is believed to have occurred during burial either in marine pore waters at elevated temperatures or in restricted flow, phreatic freshwaters.

Depositional and Postdepositional History of Stuart City Member, Edwards Limestone (Lower Cretaceous), Washburn Ranch Field, LaSalle County, Texas.: ABSTRACT

The Stuart City member of the Edwards Limestone (Lower Cretaceous) was cored in 2 wells in Washburn Ranch field, LaSalle County, Texas. Depositional sequences encountered in the cored intervals represent an alternating sequence of reef-derived grainstone, pellet-rich grainstone, near-reef wackestone, and lagoonal mud-rich facies, with a general trend toward more shallow marine conditions. Deposition of the Stuart City member ended abruptly with deepening water conditions and deposition of a pelagic foraminiferal facies. Porosity in the cored intervals can be related to incomplete cementation of the reef-derived grainstone facies. Four separate and unevenly developed generations of cementation have occurred as determined by standard petrography and cathodoluminescence. Thin, isopachous, syntaxial marine cement is present at some grain contacts. An influx of meteoric waters precipitated a nonluminescent, blocky calcite spar. Following a period of partial dissolution, a brightly luminescent cement lined cavities and indicates deeper burial. The bright cement is followed in optical continuity by an unzoned, faintly luminescent cement forming blocky spar that terminates in rhombic euhedra extending into the remaining pore spaces. It appears that invasion by hydrocarbons expelled meteoric waters and abrupt y ended cementation. End_of_Article - Last_Page 283------------

Seismic Stratigraphy of Shelf and Slope, Northeastern Gulf of Mexico

A seismic stratigraphic framework of the shelf in the northeastern Gulf of Mexico near Destin dome is established by correlating multifold seismic lines with deep wells that penetrate to the Middle Jurassic Louann Salt and with a previously published seismic stratigraphic study based on shallow core holes. Nine depositional sequences or seismic units are recognized and are designated A through I in the order of increasing age. During the Middle Jurassic to middle Cretaceous, shallow-water deposition prevailed in the area. This deposition was followed by a period of general transgression until middle Oligocene, during which deeper water deposition took place. After the middle Oligocene, a shallow-water regime returned to the area. Thinning of seismic units and onlap of ref ectors of post-middle Cretaceous age on the Destin dome suggest that the dome was uplifted in Late Cretaceous and into the early Tertiary. Six previously established seismic stratigraphic units from the deep Gulf of Mexico are traced into the lower slope near De Soto Canyon. Although several units thin and pinch out, two key boundaries can be traced onto the shelf. The important Challenger-Campeche boundary, which is recognized as a marker horizon and unconformity throughout the abyssal Gulf, is correlated to the F-E boundary, the middle Cretaceous (97 Ma) unconformity on the shelf. The base of the Sigsbee-Cinco de Mayo unit is correlated to an 8-Ma reflector on the shelf. These correlations confirm previous age estimates for the deep Gulf units. Absence of coherent reflections in the Lower Cretaceous carbonate margin indicates possible reef growth. This margin is the southeastward subsurface extension of the Stuart City reef trend in Texas and Louisiana. This trend extends farther to the southeast where the carbonate margin crops out along the Florida Escarpment.

Physical Stratigraphy and Facies Analysis, Lower Cretaceous, Maverick Basin and Devils River Trend, Uvalde and Real Counties, Texas: ABSTRACT

The formations of the Fredericksburg and lower Washita Groups in Uvalde and Real Counties, Texas, were deposited on a broad platform behind the Stuart City reef trend. Nine distinct lithofacies and corresponding depositional environments occur within this lower Cretaceous depositional regime: (1) nodular and burrowed mollusk lime wackestone lithofacies representing an open-marine platform, low wave-energy environment; (2) rudist, shell fragment, miliolid wackestone to packstone lithofacies representing a partially restricted to open-marine platform, low to moderate wave- and current-energy environment; (3) rudist, shell fragment, miliolid, grainstone to wackestone lithofacies representing an open-marine platform and bank, moderate to high wave- and current-energy environm nt; (4) evaporite collapse breccia lithofacies representing a hypersaline platform interior environment; (5) miliolid, shell fragment wackestone and Texigryphaea bed lithofacies representing a platform interior, shallow subtidal to intertidal transitional environment; (6) alternating thin evaporite collapse breccia with laminated mudstone and pellet packstone lithofacies representing an evaporite-dominated tidal mud flat environment; (7) carbonaceous lime mudstone lithofacies representing a platform interior euxenic lagoonal environment; (8) pellet intraclast lime wackestone to packstone lithofacies representing a restricted platform interior environment; and (9) pelagic lime mudstone to wackestone lithofacies representing a deep, open to slightly restricted platform interior environment The Fredericksburg-early Washita depositional history is characterized by 2 distinct cycles of deposition. The first cycle is interpreted as an initial marine transgression over the partially eroded Glen Rose tidal flat deposits, followed by a period of regional shoaling, culminating with the deposition of the Kirschberg Evaporite within the Devils River formation and northward, and deposition of the lower McKnight evaporite-dominated tidal mud flat sediments to the south. The second cycle of deposition is interpreted as a gradual marine transgression culminating in the deposition of open-marine platform rudist bank facies (upper Devils River) and partially restricted to open deep platform interior deposits (Salmon Peak) prior to middle Washita emergence and erosion forming the pre-De Rio unconformity. End_of_Article - Last_Page 508------------

Evolution and Seismic Expression of Mesozoic and Cenozoic Shelf Margins, Gulf of Mexico and Vicinity: ABSTRACT

Mapping and classification of modern and ancient shelf margins provide a basis for a concise post-rifting history of the Gulf of Mexico basin. Many hydrocarbon occurrences in the basin can be related to styles of shelf margins and their associated slope with implications for frontier exploration in deep-water facies. Seven basic types of shelf margins have been recognized. 1. Reef-dominated carbonate margins surrounded the deep basin during the Early and middle Cretaceous. This category can be subdivided into: (a) low-relief, short-lived (ca. 10 m.y.) margins in Louisiana and Texas (Stuart City and Sligo), and (b) high-relief (1 to 3 km; 3,280 to 9,843 ft), long-lived (ca. 40 m.y.) margins elsewhere (Florida, Campeche, and Blake escarpment, Golden Lane, El Abra), which are typically associated with fore-reef talus. 2. Sigmoidal progradational carbonate margins developed landward of drowned mid-Cretaceous margins of the Florida and Yucatan platforms during the Cenozoic. Large-scale gravity slides with rollover structures have occurred along the Yucatan slope contemporaneously with deposition of sigmoidal carbonate margins. 3. Carbonate ramps with little or no seismic expression characterized the Late Jurassic, when a deep marine basin was first established, an the Late Cretaceous, following drowning of mid-Cretaceous carbonate platforms. 4. Stable progradational clastic margins with well-developed, undeformed, large-scale clinoform stratification are relatively rare in the Gulf basin. Modern examples are limited to offshore Alabama and Veracruz. 5. Unstable progradational clastic margins result from gravity sliding of the continental slope, commonly associated with salt and shale diapirism, which obscures large-scale clinoform stratification. Growth faults with major expansion and rollover characterize the shelf margin; folds and/or thrust faults characterize the lower slope. This type of shelf margin has dominated the northwestern Gulf during the Cenozoic, and now extends from offshore Mississippi to offshore Veracruz. Numerous episodes of deltaic progradation to the shelf margin have been identified in Texas and Louisiana; these clastic influxes have prograded the shelf edge as much as 350 km (215 mi). 6. Tectonically active progradational clastic margins are present in Tabasco and eastern Veracruz. There the shelf margin has prograded up to 200 km (125 mi) since the early Miocene in an area of active compression and sinistral wrench faulting. Interaction of basement tectonics with diapirs and probably gravity sliding has created great structural complexity in thick Neogene deltaic sequences. 7. Tectonically active sediment-bypassing margins characterize the early Tertiary. The continental margins of Veracruz and Cuba underwent major compressional deformation and did not develop broad constructional shelves; instead, most sediment was probably bypassed to the deep basin. At the same time, a deep-water foreland basin, probably with a similar type of margin, extended into Chiapas and central Guatemala. End_of_Article - Last_Page 570------------

Outcrop-to-Basin Stratigraphy and Structure of Glen Rose Limestone of Central Texas: ABSTRACT

The Glen Rose Limestone is a proven oil and gas producer in south Texas along the Stuart City reef trend. However, it has not been productive, except in minor occurrences, in the East Texas basin, even though it is present in an area of approximately 75,000 mi2 (195,00 km2) and attains a maximum thickness of 3,500 ft (1,067 m). This absence of production is believed to result from a lack of exploration rather than a lack of potential. Therefore, a regional study of the outcrop-to-basin stratigraphy and structure of the Glen Rose in central Texas was undertaken to correlate existing stratigraphic nomenclature and to determine the depositional and diagenetic history. Be relating these to known production, it should be possible to identify potential exp oration fairways. Cross sections were used in establishing stratigraphic correlations, resulting in a division of the Trinity Group different from that commonly seen in literature. The group is divided into the Glen Rose and Travis Peak subgroups, with the division at the outcrop placed at the Hensel-lower Glen Rose contact. However, in the basin, the Pearsall-Rodessa contact is used as it is time-equivalent to the division adopted at the outcrop. Examination of cores, cuttings, and sample logs defined the depositional and diagenetic environments of Glen Rose strata. Isopach maps and seismic sections were used to map distribution of facies tracts related to production. Facies and structure of known production areas were compared to those of untested areas to define possible exploration fairways and to map recommended exploration trends. Principal of these is the Stuart City reef trend. End_of_Article - Last_Page 548------------

Burial Cements in Lower Cretaceous Pearsall Formation and Lower Glen Rose Formation, South Texas: ABSTRACT

Lower Cretaceous platform carbonates and shales were buried to depths in excess of 2,000 ft (610 m) by the end of Eocene time, and were locally affected by late-stage cementation. Burial diagenetic cements include ferroan baroque dolomite, ferroan and nonferroan calcite, anhydrite, kaolinite, barytocelestite, galena, and sphalerite. The lack of these minerals in outcrop and their occurrence in fractures are evidence for a subsurface origin. Carbonate cements are chemically and isotopically zoned; the FeCO3 content in baroque dolomite cement varies by as much as 10 wt. % across a single crystal. Stratigraphic and regional distribution of iron in baroque dolomite indicates that the iron is derived from local sources. Good negative correlation between ^dgr13C values and iron contents of baroque dolomite suggests the End_Page 572------------------------------ simultaneous reduction of iron oxides and oxidation of organic material. Some of the late subsurface carbonate cements with extremely depleted ^dgr18O values precipitated either from hot brine or from isotopically light water; both possibilities require the vertical movement of fluid along faults. Galena and sphalerite occur in small amounts in some cores; a single fluid inclusion homogenization temperature from sphalerite was 20°C (68°F) higher than the present formation temperature at that depth. Brines moving up faults after albitizing feldspars in more deeply buried formations could be the source of lead and zinc for these minerals. Strontium isotopic ratios for calcite cement in these rocks are similar to ratios for brines from the Stuart City reef trend that are believed to originate deep in the Gulf of Mexico basin. End_of_Article - Last_Page 573------------

Exploration and Development--Gulf Coast Mesozoic, 1982 and 1983: ABSTRACT

Looking at exploration and development within the Gulf Coast Mesozoic region as a whole, the trend has been toward shallow oil and away from deep gas. The capability of each area to successfully accommodate this shift is dramatically reflected in the comparison of total wells drilled in 1982 as compared to 1981: Table The southeastern states were hard hit because this area was so strongly oriented toward deep Hosston and Cotton Valley gas plays, and there were no viable shallow oil plays available as quick alternatives. South Arkansas and north Louisiana suffered some from the downturn in Smackover and Cotton Valley exploration (note total wells much greater than in the southeastern states), but there were numerous plays to be made in the Upper Cretaceous Sands, Wash.-Fred., Paluxy, Rodessa, Sligo, and Hosston. East Texas actually faired better in 1982 than 1981, in part because of the many shallow oil plays to be made, but also because, according to G.C.S. statistics, the shift to shallow oil had actually begun between 1980 and 1981, so momentum in this direction was already present. South Texas s w the demise of the Austin Chalk play within the Giddings trend through Lee and Fayette Counties. Nevertheless, the Chalk play remains active in Atascosa and Wilson Counties. The most significant trends in the southeastern states during 1982 were the Rodessa-Cotton Valley play centered in Warren and Hinds Counties, Mississippi, and the Miocene play of Baldwin and Mobile Counties, Alabama. The most significant discovery was the Movico Smackover field on the west flank of the Mobile graben in Mobile County, Alabama. In 1983, an apparent major Smackover discovery has been made by Beau Coupe Oil & Gas in extreme southeastern Escambia County, Alabama. Also, there is an important play developing in the lower Tuscaloosa on the shelf slope between the low relief anticlines of northwestern Mississippi and the Tuscaloosa growth faults of southern Louisiana. Although exploration in north Louisiana and south Arkansas was relatively strong, most activity was concentrated in or around existing fields. The west and north flanks of the Pine Island dome located in Caddo Parish, Louisiana, were the sites of very intense activity. Exploration and development are directed toward the Sligo with secondary pays in the Rodessa and Hosston. In all, 44 wells have been drilled to these pays in and around the shallow Pine Island Pettit oil field. East Texas activity was dominated by discoveries and development of Rodessa and Pettit fields in the basin and on the west and south flanks of the Sabine uplift. However, the most significant discovery last year was the East Ginger Smackover field in Rains County. In early 1983, Cities Service made headlines with what appeared to be a major discovery in eastern Cass County. A 40-ft (12-m) pre-Smackover sand made a strong flow of oil and gas before going to water. As 1982 came to a close, it appeared that south Texas would have an important play in the Sligo, seaward of the Stuart City Reef trend. However, at this time, commercial production from the Sligo has not been established definitely. End_of_Article - Last_Page 1474------------

Burial Cementation--Is It Important? A Case Study--Stuart City Trend, South-Central Texas: ABSTRACT

The Stuart City trend is a shelf-edge buildup of Lower Cretaceous bioclastic and reefal carbonates that is currently buried to depths of between 3,300 and 5,000 m (11,000 and 16,000 ft). Compaction and cementation have generally reduced rock porosities to less than 9%. Sediments were cemented in the marine environment by finely crystalline bladed, isopachous cement and volumetrically important (14 volume %) coarse to very coarsely crystalline, fibrous to bladed, isopachous, Mg-calcite cement. These cements have been neomorphically altered End_Page 536------------------------------ to low Mg-calcites forming an unusual radiaxial texture observed in the coarse to very coarsely crystalline, bladed, calcite cement. Evidence of their marine origin consists of a relative 1 mole % Mg++ memory, a marine-like isotopic character (^dgr18O ^approx -2.5 and ^dgr13C ^approx +2.0), and early relative timing of precipitation. Diagenetic alteration of these carbonate sediments by the interaction with meteoric water in lenses that formed within topographic highs along the shelf margin changed the initial marine chemical, isotopic, and textural character of the sediments. Secondary porosity formation, mineral stabilization, aggrading neomorphism, and equant spar calcite cementation are the important products of meteoric diagenesis. The equant spar calcite cements make up approximately 16% by volume of the limestones studied. They are iron and manganese poor. The majority have a ^dgr13C composition which falls in the range of modern marine carbonates, i.e., 0.9 to +3.5^pmil. The ^dgr18O compositions range from -1.3 to -6.6^pmil relative to the PDB standard. Oxygen stable isotopic and petrographic data suggest that over 50% of the equant spar calcite cements were formed in a near-surface meteoric environment. A large percentage of the remaining equant spar calcite cements formed at shallow burial depths in a water-limited system where mass transfer was dominated by diffusional processes. Thermally induced ^dgr18O depletion of the equant spar calcites, indicating significant fluid flow, was of minimal importance. Pyrobitumen pore fillings and inclusions in the outer 1.0 mm-thick rims of the very coarsely crystalline, equant spar calcite cements indicate that only minor amounts of cementation have occurred since the introduction of hydrocarbons. Deep burial diagenesis (i.e., post-hydrocarbon migration) consisted of the precipitation of minor amounts of galena, fluorite, and Sr++-rich equant spar calcites. These diagenetic products can be directly related to the present-day formation water. The chemical, isotopic, and textural characteristics of the Stuart City trend limestones contain the imprints of their initial marine composition and shallow diagenetic alterations in a hydrodynamic system. Burial diagenesis has not significantly altered these limestones. Fault and fracture control on the movement of formation waters in this system determine the location and intensity of late stage diagenetic events. End_of_Article - Last_Page 537------------

Stratigraphic and Diagenetic Controls on Porosity Distribution Within a Cretaceous Carbonate Bank: West Stuart City Trend, Texas: ABSTRACT

Cores from four wells from the West Stuart City trend in LaSalle and Webb Counties, Texas, were analyzed to define depositional and diagenetic facies and to determine factors controlling porosity distribution. In all, 1,187 ft (362 m) of core and 220 thin sections were examined in detail, supplemented by SEM analyses of fractured surfaces and plastic casts of microporosity. Attempts to correlate cores purely on the basis of lithology were unsuccessful owing to the high degree of vertical lithologic variability and the rapidity of lithofacies changes over a relatively short distance (approximately 8,000 ft, 2,400 m). However, when lithofacies were integrated with biofacies (based on gross faunal aspect) and related to submarine hardground surfaces, meaningful correlations ere possible. The depositional model envisioned for these deposits is that of a broad, discontinuous, shallow bank constructed of coarse skeletal debris (largely caprinid grainstones) with more restricted lagoonal sediments in its lee (miliolid, requienid wackestones, and mudstones). The buttress zone of this bank was periodically subjected to extensive wave action, evidenced by sheets of skeletal grainstones intertonguing with muddy lagoonal facies. Important diagenetic effects noted include extensive submarine cementation, neomorphism of aragonitic components with virtually no development of moldic porosity, and extensive pressure solution. Cumulative measurements of stylolites with amplitudes greater than 2 cm (0.8 in.), combined with microscopic observations of small-scale stylolites indicate a loss of stratigraphic section of as much as 20%. A comparison of lithofacies and biofacies logs with permeability and porosity logs compiled from perm-plug data revealed that zones with permeabilities greater than 0.1 md and porosities of at least 6% were associated with rudistid grainstones cemented by End_Page 531------------------------------ isopachous submarine cement and packstones with a finely crystalline rhombic calcite matrix. This general porosity distribution is modified by basinward tilting so that the most favorable traps lie in updip positions where grainstones intertongue with and are sealed by muddy lagoonal facies, (i.e., in a more landward position behind the bank proper). End_of_Article - Last_Page 532------------