Lower Cretaceous Shelf Edge Research Articles

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.

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.

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.

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.

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------------

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------------

Deep Stratigraphy and Evolution of Baltimore Canyon Trough Based on Multifold Seismic Reflection, Refraction, Gravity, and Magnetic Data: ABSTRACT

A recent 48-channel seismic reflection profile (U.S. Geological Survey line 25) extends 330 km southeast off southern New Jersey and crosses the widest and deepest part of the Baltimore Canyon Trough (10 km southwest of the COST B-3 well). The profile has been migrated and converted to depth to reveal the deep sedimentary and basement structures across the ocean-continent transition zone. The sedimentary wedge thickens from 5 km nearshore to 17 km just landward of the East Coast Magnetic Anomaly (ECMA; 20 km landward of the shelf edge in this area). A strong, flat reflector about 10 km wide exists at a depth of 14 km, directly beneath ECMA. Acoustic basement becomes obscure and appears to rise to a depth of 5 km over the next 40 km to the southeast, beneath a Jurassic and lower Cretaceous carbonate shelf-edge complex which extends 20 km seaward of the present shelf edge. Landward-dipping continental rise sediments exist to a depth of at least 13 km on the seaward side of the Jurassic shelf edge. The top of oceanic basement is first seen as a set of prominent hyperbolic reflectors about 50 km seaward of the Jurassic shelf edge, where it occurs at 11 km depth and dips gently landward. It is obscured landward of this point by the prominent middle Jurassic (J3) horizon. A Jurassic and lower Cretaceous shelf-edge carbonate platform or reef complex prograded 40 km out over oceanic crust in this area. Greater differential subsidence and compaction of the basin west of the ECMA have produced back-tilted and arched horizons in the Jurassic and lower Cretaceous shelf edge units, creating a 20-km-wide with 500-m closure beneath the upper continental slope. Other lines to the southwest indicate the anticlinal arch extends at least 40 km to the southwest. Similar slope anticline structures have been reported off northwest Africa. End_of_Article - Last_Page 715------------

Louisiana Tuscaloosa Versus Southeast Texas Woodbine: ABSTRACT

The deep Tuscaloosa play extending across south-central Louisiana has resulted in the recent discovery of large gas reserves. This same downdip lower Upper Cretaceous interval, known as the Woodbine in southeast Texas, extends into Texas across Newton, Jasper, Polk, and Tyler Counties. Although well control is sparse through this interval in southeast Texas, available data suggest a different depositional and structural setting for the lower Upper Cretaceous interval. No thick units of sands are within the interval, and most of the production has been found in stratigraphic traps exhibiting thin sands, as in the Seven Oaks field. Subsurface studies of the lower Upper Cretaceous interval across south-central Louisiana and into southeast Texas indicate it is unlikely the Louisiana Tuscaloosa play will extend into the southeast Texas area. However, there is sparse downdip control in Texas south of the Lower Cretaceous shelf edge and it is, therefore, possible that some sands have been deposited subparallel to the shelf. The best area to explore for this possibility would be south of the 1,000-ft Tuscaloosa or Woodbine isopachous contour which extends across southeast Texas. End_of_Article - Last_Page 1558------------

Description and Sedimentology of Submarine-Fan Gas Reservoir in Woodbine-Eagle Ford Interval (Upper Cretaceous), Sugar Creek Field, Tyler County, Texas: ABSTRACT

Abundant gas and some condensate are being produced from fractured sandstones of the Upper Cretaceous Woodbine-Eagle Ford interval at depths of 10,800 to 11,350 ft (3,240 to 3,405 m) in the Sugar Creek field area of Tyler County, Texas. The reservoir sandstone units are complex, single to multiple bodies 15 to 40 ft (4.5 to 12 m) thick and less than a few thousand feet wide within a mud-dominated clastic wedge. The wedge thickens from about 50 ft (15 m) near the Lower Cretaceous shelf edge to more than 1,500 ft (450 m) within 15 mi (24 km) downdip to the south. Subsurface correlation and mapping of the discontinuous, lenticular sandstone bodies indicate that they are best delineated as a series of coalescing, dip-oriented lobes. Deposition appears most likely to have been as prograding submarine-fan lobes, with sediment being channeled from updip delta and nearshore deposits across a narrow shelf and through shelf-edge breaks and then dumped downslope. Within the major sandstone units, individual beds commonly are 1 to 3 ft (0.3 to 1 m) thick and display sharp contacts with interbedded, thin (1 to 2 in.; 2.5 to 5 cm) shale layers. As viewed in polished core slabs, the sandstones are mostly massive; however, radiography reveals abundant lamination and cross-stratification and some ripple-bedding and soft-sediment-deformation structures. A detailed analysis of sedimentary features and sandstone fabric suggests periodic rapid End_Page 528------------------------------ deposition of sand by low-concentration to high-concentration, submarine density currents and associated tractive currents. The fine to very fine-grained sandstones are well-cemented quartz arenites with porosities commonly 10 to 15% and permeabilities commonly around 0.1 to 1.0 md. Abundant fractures in the brittle sandstone provide the necessary reservoir permeability to allow commercial gas production. End_of_Article - Last_Page 529------------