Cretaceous Shelf Edge Research Articles

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.

Geological and geochemical characterization of the Lower Cretaceous Pearsall Formation, Maverick Basin, south Texas: A future shale gas resource?

As part of an assessment of undiscovered hydrocarbon resources in the northern Gulf of Mexico onshore Mesozoic section, the U.S. Geological Survey (USGS) evaluated the Lower Cretaceous Pearsall Formation of the Maverick Basin, south Texas, as a potential shale gas resource. Wireline logs were used to determine the stratigraphic distribution of the Pearsall Formation and to select available core and cuttings samples for analytical investigation. Samples used for this study spanned updip to downdip environments in the Maverick Basin, including several from the current shale gas-producing area of the Pearsall Formation. The term shale does not adequately describe any of the Pearsall samples evaluated for this study, which included argillaceous lime wackestones from more proximal marine depositional environments in Maverick County and argillaceous lime mudstones from the distal Lower Cretaceous shelf edge in western Bee County. Most facies in the Pearsall Formation were deposited in oxygenated environments as evidenced by the presence of biota preserved as shell fragments and the near absence of sediment laminae, which is probably caused by bioturbation. Organic material is poorly preserved and primarily consists of type III kerogen (terrestrial) and type IV kerogen (inert solid bitumen), with a minor contribution from type II kerogen (marine) based on petrographic analysis and pyrolysis. Carbonate dominates the mineralogy followed by clays and quartz. The low abundance and broad size distribution of pyrite are consistent with the presence of oxic conditions during sediment deposition. The Pearsall Formation is in the dry gas window of hydrocarbon generation (mean random vitrinite reflectance values, Ro = 1.2–2.2%) and contains moderate levels of total organic carbon (average 0.86 wt. %), which primarily resides in the inert solid bitumen. Solid bitumen is interpreted to result from in-situ thermal cracking of liquid hydrocarbon generated from original type II kerogen that was prevented from expulsion and migration by low permeability. The temperature of maximum pyrolysis output (Tmax) is a poor predictor of thermal maturity because the pyrolysis (S2) peaks from Rock-Eval analysis are ill defined. Vitrinite reflectance values are consistent with the dry gas window and are the preferred thermal maturity parameter. A Maverick Basin Pearsall shale gas assessment unit was defined using political and geologic boundaries to denote its spatial extent and was evaluated following established USGS hydrocarbon assessment methodology. The assessment estimated a mean undiscovered technically recoverable natural gas resource of 8.8 tcf of gas and 3.4 and 17.8 tcf of gas at the F95 and F5 fractile confidence levels, respectively. Significant engineering challenges will likely need to be met in determining the correct stimulation and completion combination for the successful future development of undiscovered natural gas resources in the Pearsall Formation.

Cretaceous to Recent Rifting in the Gulf of Mexico Basin

ABSTRACT There is evidence in, and, around the periphery of the northern Gulf of Mexico Basin that indicate the basin was formed by tectonic plate readjustment. This rifting started in the late Early Cretaceous and is continuing today. Over 40 articles of evidence support Gulf rifting. A geologic model for this type basin formation is also presented. Evidence of rifting includes: Arcuate linear magnetic signatures, recorded across the northern Gulf of Mexico, outline possible plate movement. Reflection and refraction data along with deep well control revealed a deep basement ridge and valley system in the northern Gulf that matches closely the above linear magnetic trend. Radiating from the DeSoto Canyon high is a triple juncture system formed by the Cretaceous Shelf Edge, West Florida Escarpment and Suwannee Strait. The Trans-Mexico neovolcanic belt is offset by a right-lateral transform fault resulting from plate readjustment. The possible connection of the New Madrid and Middleton Place-Summerville earthquake zones to this rift system. Plate readjustment created compressional forces that caused a series of east-west trending mountain ranges in Guatemala, Honduras and Nicaragua along with Caribbean seamounts. Sea floor spreading in the Cayman Trench that could be results of, and/or part of, initiation of plate movement. The northern Gulf's Exterior Basin salt mass, along with its confining basin, are possibly the results of plate movement. This tectonic model has implications for future petroleum exploration that will be discussed.

ABSTRACT: Geologic Overview of the NE Mississippi Fan and Shelf to West Florida Terrace Region, Offshore Gulf of Mexico

Abstract This study is based on the joint venture (Schlumberger and TGS NOPEC) Phase 46, 16,000-line-mile 2D seismic and gravity/magnetic program that was acquired, processed, and interpreted from 1997-1999. It consists of a 2 2 mile grid (16,100 mi2), covering parts of Main Pass, Destin Dome, Viosca Knoll, Mississippi Canyon, Desoto Canyon, Atwater Valley, and Lloyd Ridge. Water depths range from 75 feet to over 10000 feet. Geological control from 44 biostratigraphy wells helped calibrate seven mapped horizons. In addition, a velocity cube was generated from this dataset. A Cretaceous carbonate shelf, a prominent escarpment to the southeast, dominates the east and northeast portions of the survey. It is overlain by primarily regressive Cenozoic clastics. West of the carbonate shelf is a Miocene to Plio-Pleistocene shelf /slope/fan complex. The shelf facies is developed in the northwest and progresses through slope to deepwater fan deposits toward the southeast. This complex contains numerous allochthonous salt bodies. In general, autochthonous salt structures are present east of the detached salt, underlying the slope fan complex. These continue eastward to the Cretaceous shelf edge. This region is becoming an increasingly attractive area to explore. For example, Crazy Horse in Mississippi Canyon (MC) Block 778 is a recent subsalt discovery within the study area.

Ferry Lake, Rodessa, and Punta Gorda Anhydrite Bed Correlation, Lower Cretaceous, Offshore Eastern Gulf of Mexico: ABSTRACT

ABSTRACT The Lower Cretaceous Ferry Lake and Punta Gorda anhydrite has been used as marker beds throughout the Gulf of Mexico, and divided into 9 individual anhydrite beds traceable from southern Mississippi to southern Florida. The underlying Rodessa Formation has an equally well developed anhydrite section in the offshore Eastern Gulf of Mexico and is divided into 12 basin-wide anhydrite bed groupings. The anhydrites of Ferry Lake and Rodessa Formations are traceable to the anhydrites of the Punta Gorda and Lehigh Acres Formations of South Florida. Anhydrite beds appear to thicken in the central part of the trend interfingering with carbonates of the Lower Cretaceous shelf edge reef. Carbonates and anhydrites may be deposited simultaneously with carbonate patchreefs developing on crests of paleo highs with evaporites precipitating out of a hyper-saline solution on the flanks. Areas where poor anhydrite bed development occurs may indicate areas of patch reefs and, therefore, the best potential for hydrocarbon reservoirs. Individual anhydrite beds have been correlated and color-coded on photocopies of compensated neutron density logs to determine their geographic distribution. Prepared cross sections show some beds to be area wide while others are more restricted. Isopach maps show .the configuration of the basin in which these beds were deposited with the basin's long isopach axis parallel to the reef trend. Anhydrite deposition occurred with evaporation of restricted highstand waters behind reefs that rimmed the shelf edge.

PROBABLE CRETACEOUS-TO-RECENT RIFTING IN THE GULF OF MEXICO BASIN An answer to Callovian salt deformation and distribution problems? Part 1

Abundant evidence from the Northern Gulf of Mexico Basin suggests that late Early Cretaceous to Recent sea-floor spreading is the principal factor in the formation of the structural components of this basin. This would classify the Gulf of Mexico Basin as an active tectonic basin as opposed to the more accepted passive basin status. The following two-part paper presents a model for this lithospheric plate adjustment together with the geological evidence to support this movement. Part I discusses the rift model and supporting evidence. Some of this evidence is: a proposed triple junction rift zone radiating from a large dome centered in the DeSoto Canyon area; the division of the Gulf of Mexico Basin Jurassic salt province into three salt basins (the Interior Exterior and Challenger Basins); the linearity and structurally-positive attitude of the Cretaceous Shelf Edge; an excessive Exterior Basin salt mass that thickens towards the Sigsbee escarpment; large areas of non-diapiric salt found within the Exterior Basin; an extensional fault zone found underlying the Abyssal Plain; and the absence of a Late Mesosoic section from the Exterior Basin a section that would be needed to cover the salt until it was overlain by younger Tertiary sediments.

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.

Controls on Facies Distribution of the Downdip Jurassic in Eastern Texas

As the East Texas region becomes increasingly mature, deep wells are testing the downdip Jurassic. Well log and seismic data indicate the section was deposited in a basin, hereafter referred to as the Brazos basin, located between the Lower Cretaceous shelf edge and the East Texas basin. The Brazos basin is separated from the East Texas basin by a major northeast-southwest trending feature called the Houston arch. The stratigraphy and facies distribution of these two basins are distinctly different.

Evidence for a Deep Mesozoic Basin in Central East Texas: Implications for Hydrocarbon Exploration

ABSTRACT A region in central east Texas outlined by a thickened Mesozoic section is described as a new feature, called the Brazos basin. The basin is approximately 120 mi (190 km) long and 50 mi (80 km) wide, extending from Houston and Trinity counties in the northeast to Austin and Fayette counties in the southwest. It is separated from the East Texas basin by the Houston arch, a northeast-southwest trending positive feature located just southeast of the Elkhart-Mt. Enterprise fault zone. The southeast margin of the basin is formed by the Angelina-Grimes terrace, located just north of the Cretaceous shelf edge. Compared to the East Texas basin and Houston embayment, the Brazos basin represents a relatively under-explored region, completely lacking Jurassic production. However, thermal and subsidence histories indicate the potential for significant hydrocarbon generation and entrapments. The Brazos basin originated as a faulted half-graben in association with the rifting that opened the Gulf of Mexico. Maximum subsidence within the Brazos basin occurred during the Jurassic, and gradually diminished through the Cretaceous. Time-temperature indices indicate that potential source rocks reached maturity during the Early Cretaceous. During this time, hydrocarbon migration over much of the basin was to the south. Starting in the Paleocene, the southern limb of the Brazos Basin subsided, causing a reversal of the northward dip of strata along the Angelina-Grimes terrace. Diagenetic seals could have prevented further migration when the region tilted south during the Tertiary, thus forming several types of traps that have no relationship to the basin's present structure.

Megaregional Project Progress Report

When completed, the first data segment will extend from the Arkoma basin of Arkansas to the East Cameron area, offshore Louisiana and will be approximately 500 miles long. The line will cross the Arkoma basin, the Ouachita thrust belt, the State Line graben trend, the North Louisiana salt basin, the Lower Cretaceous shelf edge and complex faults of southwest Louisiana. Additional Megaregional dip and strike lines are planned for the Gulf of Mexico Basin as the project advances.

Organic Geochemistry of Lower Tertiary Shales of Southern Louisiana: Regional Distribution of Source Potential: ABSTRACT

Outer continental shelf shales of the Paleogene/Eocene Wilcox Group and middle Eocene Sparta Formation are potential source rocks for hydrocarbons in southern Louisiana. Eight hundred and thirty-seven shale samples (Sparta 219) Wilcox, 618 from 24 wells across south Louisiana were analyzed using Rock-Eval pyrolysis; 24 organic-rich samples were analyzed using pyrolysis-gas chromatography and gas chromatography-mass spectrometry (GC-MS). Regional trends in organic geochemistry are related to two main factors: sedimentary facies transition from proximal deltaic to shelf/slope environments, and thermal maturity. According to geothermal gradient maps, thermal effects increase from east to west across southern Louisiana and from north to south as the lower Tertiary section is buried to greater than 30,000 ft. The Wilcox, and to a lesser extent the Sparta, are major regressive deltaic sequences whose primary deltaic depocenters are located in subsurface Texas. Volumetrically, smaller deltas prograded beyond the ancient Cretaceous shelf edge by as much as 50 mi across southern Louisiana. Source potential in both rock units becomes more oil prone from west to east across south Louisiana, with an apparent maximum in the shelf/slope flexure zone in south central Louisiana. In this area, Sparta Formation shales show good to excellent potential in shales overlain by shelf-edgemore » barrier sands. Wilcox source potential is fair to good in shales deposited in submarine fan environments. Source units are interpreted to be a product of transgressive episodes occurring within primarily regressive sequences. The close proximity of source units to the shelf-edge suggest that they may represent conditions deeper in the Eocene Gulf of Mexico basin.« less

Structure of the Hawasina Window culmination, central Oman Mountains

ABSTRACTDetailed mapping, stratigraphic logging and structural analysis of the Hawasina Window culmination in the central Oman Mountains of Arabia reveals an extremely complex thrust geometry and structural history. The initial thrust sequence involved a southwestward propagating stacking during telescoping of the Arabian continental margin slope (Sumeini complex), and time-equivalent, more distal Tethyan basin (Hawasina and Haybi complexes) facies rocks. The Semail thrust, carrying the 12 km-thick ophiolite sequence, progressively overlaps Haybi and Hawasina duplexes towards the SW. Late stage “leap-frog” thrusts have punched Sumeini duplexes higher up into the earlier thrust stack locally reversing the normal stacking order. SW-directed thrusts and SW-facing folds in the SW and NE-directed backthrusts and NE-facing backfolds in the NE have created a fan structure cored by the Jebel Rais “pop-up” composed of Sumeini slope facies rocks.The palaeogeographic presence of a large NE-facing promontory in the Cretaceous shelf edge is inferred beneath the Window. The frontal ramp of this promontory was sufficiently large to inhibit the overthrusting of large volumes of Hawasina and Haybi complex rocks. The whole central part of the Window shows NE-facing and verging backfolds and backthrusts affecting all duplexes from the lowest Sumeini up to the Semail ophiolite. The promontory is bounded by major lateral ramps to the NW (Wadi Shafan area) and SE (Jebel Milh area). A map and four balanced cross-sections are presented here to promote a complex model for the structural evolution of the Hawasina Window.

Exploring Lower Tuscaloosa in Southwest Mississippi: ABSTRACT

The updip Tuscaloosa trend of southwest Mississippi is north of both the Lower Cretaceous shelf edge and the currently developing intermediate lower Tuscaloosa play in Florida, Louisiana, and Wilkinson County, Mississippi. The updip trend first became active in the 1940s with the discovery of major reserves at Brookhaven and Mallalieu fields in Lincoln County and Cranfield field in Adams County. Today, the trend is considered mature; however, it has become more active with the discovery of several new fields since 1981, including Olive, Liberty, and Friendship Church fields. A primary reason for several of these discoveries is the development of new seismic-stratigraphic methods integrated with detailed subsurface geologic interpretations. The fact that oil can be found at relatively shallow depths with reasonable drilling costs and minimal engineering problems has provided incentive to pursue this play. End_of_Article - Last_Page 1422------------

Fluid-Flow Patterns in Central Tuscaloosa Trend, Louisiana: ABSTRACT

Contoured hydraulic-head cross sections constructed from well logs in the central Tuscaloosa gas trend of Louisiana provide information about fluid flow near the hydropressure-geopressure interface. Extensive head inversions correspond to long, dip-oriented convolutions of the interface. The inversions occur where permeable, hydropressured, massive sandstone facies in the lower Tuscaloosa underlie geopressured Eagle Ford Shale updip of fault zones that preserve geopressures downdip. Thus, regional fluid pressure regimes in the Tuscaloosa and Eagle Ford are predominantly structurally controlled, with some lithofacies control updip. Hydraulic-head trends indicate an overall pattern of regional upward flow from depth, with highest hydraulic gradients corresponding to the top of the Austin Chalk in most places. This pattern is complicated by inversions--which are nearly horizontal and tend to show high hydraulic gradients--and by a sharp, upward-protruding head peak that becomes nearly vertical along a trend above the Lower Cretaceous limestone shelf edge. This peak represents the escape of highly pressurized fluids from depth along a preferred path, which may be fault controlled. Highest hydraulic gradients occur locally and regionally where fluids flow from geopressured shale toward permeable, hydropressured sandstone. Salinities are also reported to show a regional increase toward End_Page 1427------------------------------ hydropressured, sandy, updip facies of the Tuscaloosa. Thus, reverse osmotic effects are probably unimportant. The head peak above the Lower Cretaceous shelf edge, although characterized by relatively low hydraulic gradients, could be a locus of membrane filtration at the top of Tuscaloosa sandstone if bypassing of reverse osmosis by fluid escape along faults has been minimal. The role of permeable Tuscaloosa sandstone as a sink for fluids near the top of geopressure may have localized gas along the producing trend. End_of_Article - Last_Page 1428------------

Regional Distribution of Wave- and Fluvial-Dominated Deltaic Deposits of Olmos Formation (Upper Cretaceous) in Maverick Basin, Southwest Texas: ABSTRACT

Regional subsurface analysis in southwest Texas indicates that the Olmos Formation (Gulfian) was deposited by a complex of wave- and fluvial-dominated delta systems in two depocenters. Sediment influx was from the north and northwest. Five deltaic subunits, A through E, were deposited in the western depocenter. Three other deltaic wedges (F, G, H) formed the second depocenter farther east in present-day Frio and LaSalle Counties. Subsidence was greater in the western half of the Maverick basin where thickest (1,300 ft; 395 m) deltaic sediments were deposited. Lower Olmos strata represent a succession from wave-reworked, strike-elongate deltas of subunit A, similar to those of the underlying San Miguel Formation, to fluvial-dominated, dip-elongate deltas of subunits B and . Extensive (1,200 mi2 or 3,100 km2 in Texas) aggradational floodplain deposits of B and C are characterized by diverse electric-log patterns; variation in log character is a response to complex depositional facies on the delta platform. Downdip, toward the Cretaceous shelf edge, delta-plain facies merge with upward-coarsening delta-front sandstones. Uppermost subunits D and E were deposited by a prograding barrier-island system in an interdeltaic embayment marginal to high constructive deltas of the eastern depocenter. Lagoonal and fluvial-channel deposits are recognized from cores. Eastward migration of deposition was accompanied by an abrupt change of depositional style in the western depocenter from deltaic to coastal-interdeltaic. End_of_Article - Last_Page 448------------

Geothermal Gradients of Texas--A New Look: ABSTRACT

The heat flow equation shows that geothermal gradient varies directly with conductive heat flow and inversely with thermal conductivity. Because of the second of these functions, geothermal gradient values have limited validity wherever bottom-hole temperature (BHT) readings are reduced without regard for ambient rock type. Thus, I present a geothermal gradient map of Texas using new data from selected geologic formations of generally uniform rock type. Four carbonate rock units provide the data base for most of the state inland of the Lower Cretaceous shelf edge. These include, for the inner Gulf Coast Basin region, the Jurassic Smackover Formation and the Cretaceous Sligo and Edwards Formations. For most of the remainder of the state west of the Balcones/Ouachita trend, data were obtained for the Ordovician Ellenburger Group. Coastward of the Cretaceous shelf edge, data from the 1976 AAPG and U. S. Geol. Survey map were employed owing to the paucity End_Page 540------------------------------ of carbonate-rock horizons at reasonable depths there. Despite this selective use of BHT data, the geothermal gradient contours do not necessarily represent areal variations in heat flow. Since all the horizons charted are petroleum reservoirs (and in their updip reaches, some are also aquifers), hydrodynamics also must be involved. High gradient anomalies may thus be due to upwelling of basinal waters; low anomalies may be a result of recharge. Hydrodynamic influences also provide a local means for delimiting structures, with uplifts commonly being the loci of upwelling and thus registering relatively high geothermal gradients. High anomalies are even more consistently correlated with fault zones, as seen along the Luling-Mexia trend in south-central Texas where there is an overlap of gradient highs among the three horizons contoured.End_of_Article - Last_Page 541------------

Geology of De Queen Formation of Arkansas: ABSTRACT

Beds of the De Queen Formation are exposed in gypsum quarries within Pike and Howard Counties, Arkansas. The formation lies within the Lower Cretaceous Trinity Group that crops out in an east-west band across southwestern Arkansas. The De Queen Formation consists of a lower sulfate facies and an upper predominantly siliciclastic facies. The lower facies is approximately 40 ft (12 m) thick and is composed of interbedded gypsum, claystone, and limestone. This lower facies is equivalent to the subsurface Ferry Lake Anhydrite of the Gulf coastal plain. The upper facies is also approximately 40 ft (12 m) thick and contains interbedded clastics, limestones, and minor evaporites. Upper beds of the De Queen are equivalent to the lowermost beds of the subsurface Mooringsport Forma ion. During Ferry Lake-De Queen deposition, a wide lagoon was located behind an extensive reef stretching around the Early Cretaceous shelf edge. This reef formed a barrier that restricted circulation and led to the deposition of the gypsum beds of the De Queen Formation and the Ferry Lake (later recrystallized to anhydrite during burial). Individual evaporite beds may be traced downdip from the outcrop across southern Arkansas into Louisiana and Texas. The regional extent of these evaporite beds reflects the variable geographic breadth of the lagoon during deposition of the Ferry Lake. Gypsum beds of the outcrop are the equivalent of the more widespread anhydrite beds of the Ferry Lake Anhydrite. Faunal assemblages, sedimentary structures, and trace fossils (which include numerous dinosaur tracks) are important to the interpretation of depositional environments of rocks of the De Queen Formation. Much of the lower half of the formation was deposited in a shallow subaqueous setting, whereas depositional environments of beds within the upper half of the De Queen varied between subaqueous and exposed conditions. End_of_Article - Last_Page 1218------------

Deposition of Prograding Carbonate Sand Shoals and Their Subsequent Diagenesis--Lower Glen Rose (Cretaceous), South Texas: ABSTRACT

End_Page 546------------------------------The lower Glen Rose in southwest Texas is a widely explored, but oil- and gas-barren, carbonate sequence (200 to 300 m thick) that was deposited on a broad, shallow-marine shelf. Three cyclic, shoal-water complexes, consisting of high-energy grainstone and coral-stromatoporoid-caprinid boundstone, developed over the Pearsall arch of south Texas. Facies distributions, determined from core and electrical logs, show that these linear complexes trend east-west for at least 125 km, and are located about 80 km inland of the Cretaceous shelf edge and 70 km seaward of the Cretaceous shoreline. The shoal-water, cyclic sequences change upward from: (1) sandy, fossiliferous mudstone/wackestone deposited in an open-shelf environment; (2) echinoid-mollusk and oncolite-caprinid packstone deposited in intertidal shoals and subtidal grain flats; (3) coated-grain and echinoid-mollusk grainstone deposited in sand flats, tidal channels, spits, and bars; and (4) coral-stromatoporoid-caprinid boundstone and packstone deposited as patch reefs and flanking deposits. Lagoonal deposits, consisting of toucasiid-oyster-miliolid wackestone, boundstone, and mudstone enclose each of the shoal-water sequences, and indicate successive seaward progradations, interrupted by transgressions of open-shelf facies. The patch reefs may have prograded out across the shelf and formed the initial buildup of he Stuart City shelf margin. Four gradational phases of grainstone diagenesis have caused almost continuous loss of porosity during burial. Micritic envelopes and isopachous crusts are early submarine cements. Next, development of an extensive meteoric-water system during burial to shallow depths led to dissolution or neomorphism of aragonite and precipitation of equant, isopachous cement, syntaxial cement, and nonferroan, equant calcite. As burial increased, subsurface brine displaced the original connate fluids and caused complex cementation and replacement by zoned ferroan and nonferroan calcite, lutecite and megaquartz, anhydrite, and saddle dolomite. The highest porosity is found in mappable facies of shoal-water grainstone. End_of_Article - Last_Page 547------------

Development and Hydrocarbon Potential, Carbonate Platforms Offshore Northeastern America: ABSTRACT

The Jurassic-Lower Cretaceous carbonate platforms and banks form a discontinuous belt extending from the Grand Banks to the Bahamas (over 6,000 km). The thickness of the carbonate buildups progressively increases southward along the margin, attaining a thickness of more than 5 km on the Bahamas. The platforms also become younger southward, which has been interpreted as an indication of a northward motion of the North American plate. Six types of carbonate buildups recognized document variability of depositional, paleo-oceanographic, and tectonic processes along the margin. The composition of the buildups closely resembles the recent deposits of the western Great Bahama Bank, since oolitic shoals were present near the shelf edge and skeletal, peloid wackestones and biomicr tes were deposited in the inner part of the platform. Coral-hydrozoan, sponge and algal stromatolite bioherms and reefs are important constituents of the Late Jurassic-Early Cretaceous shelf edge. The hydrocarbon prospectivity of the carbonate front differs for the individual carbonate platform types which prevents construction of a single model for evaluation of their hydrocarbon potential. Porosity has locally developed as a result of secondary dolomitization in a mixing zone. Hydrocarbons were at least locally generated and migrated through the porous carbonate rocks. The major critical factor is presence of a rich source rock. The latest deep sea drilling on the northwestern African and eastern North American margins, together with the interpretation of the Mesozoic paleogeography of the shelf, allows the elucidation of this important factor in a time and space framework of margin development. End_of_Article - Last_Page 585------------

Delta-Front Shelf Storm Deposits of Subsurface Woodbine-Eagle Ford Interval (Upper Cretaceous), Damascus Field, Northern Polk County, Texas: Success from Combined Development Geology and Sedimentologic Core Analysis: ABSTRACT

Gas production from several, 6 to 23 ft (2 to 7 m), single to multistory sandstone bodies of the Woodbine-Eagle Ford interval, 160 to 200 ft (49 to 61 m) thick at 9,000 to 9,600 ft (2,743 to 2,926 m) in the Damascas field has been developed since discovery in 1976. Subsequent offset drilling resulted in a few gas wells and several dry holes. In February 1979 the entire Woodbine-Eagle Ford interval was cored in the No. 7A Dorrance well. Sedimentologic core study generated a predictive depositional model which has guided field development of the subtle stratigraphic traps at a 5 to 1 well success ratio. Present gas reserves are 40 Bcf with 440,000 bbl of condensate. The productive area is located slightly southwest of the Sabine uplift and just updip from the Lower Cretaceous continental shelf edge. Seismic sections and foraminiferal paleoecology establish a middle-shelf depositional setting. Bioturbated, silty, shelf shales comprise the upper and lower Woodbine-Eagle Ford interval. The middle is a complex of (1) graded, medium to very fine-grained, massive to laminated sandstone beds; (2) contorted, soft-sediment-deformed intervals; (3) swirled and sheared siltstone beds; and (4) thin diamict conglomerate beds. Genetic units indicate periodic rapid deposition by debris flows and low to high-concentration density currents. The several distinct productive sandstone bodies (porosities 9 to 14%; permeabilities 2 to 10 md) are northward-thickening, d p-oriented lobes. The localized deposition in the shelf setting was controlled by delta development slightly to the north. Periodic major storms generated delta flooding which contributed high-energy reservoir-quality deposits to the shelf. Similar shelf sand buildups should occur throughout the area; however, recognition must rely on detailed sedimentologic study of core sequences. End_of_Article - Last_Page 992------------