Ouachita Foldbelt Research Articles

Reservoir systems of the Pennsylvanian lower Atoka Group (Bend Conglomerate), northern Fort Worth Basin, Texas: High-resolution facies distribution, structural controls on sedimentation, and production trends

This study defines the depositional systems of mature lower Atoka Group reservoirs, structural influence on their sedimentation, and sand-transport patterns at a higher degree of resolution and over a significantly larger part of the play area than previously conducted. The reservoir systems are characterized by pronounced variations in depositional style, even between stratigraphically adjacent systems. They represent a variety of on-shelf siliciclastic depositional facies, including gravelly braided river, fluvial-dominated delta, and low-sinuosity incised river deposits. Penecontemporaneous, high-angle, basement-rooted reverse faults and genetically associated folds of the Mineral Wells–Newark East fault system exerted direct control on the orientation of complex fluvial-channel and delta-distributary sand-transport pathways and the geometry of deltaic depocenters. Multiple contemporaneous source areas, including the Ouachita fold belt to the southeast, the Muenster arch to the northeast, and the south flank of the Red River arch, also contributed to the complexity of sandstone trends in the lower Atoka play area. Bubble maps of normalized per-well first-year production and total cumulative production allow qualitative conclusions regarding geologic controls on production distribution. Most wells with optimal gas production occur within two northwest-trending production fairways that coincide with primary sandstone trends of one or more reservoir systems. Highest per-well oil production exists where lower Atoka reservoir facies occur above oil-prone Barnett Shale source rocks (vitrinite reflectance 1.1% Ro) in the western and northwestern parts of the study area. Widespread fault-bounded, karst-produced sag structures that extend vertically from the source rocks through the lower Atoka Group most likely served as hydrocarbon-migration conduits and formed traps for both oil and gas.

Sea level changes and rapid crustal movements in cratonic areas in the Late Paleozoic

In the Phanerozoic, the sea depth in epeiric sedimentary basins showed considerable variations, often accompanied by regression. In periods of regression and erosion, the subaerially exposed shelf and the adjacent parts of the marine basins gave rise to numerous nonstructural (stratigraphic) hydrocarbon traps. Sea depth variations with a magnitude of up to 100–200 m and 1–3 myr long (third-order cycles) are usually attributed to the eustatic fluctuations of the sea level. To estimate their possible range, a model is proposed which describes the water depth variations as a function of eustatic fluctuations in tectonically subsiding carbonate platforms. We take into account the crustal isostatic response to the changing water load and the finite time necessary for soil and karst formation in the exposed shelf or its upper part. This model allowed analyzing data on the reference sections of the North Timan shallow-water sediments. According to the analysis, the third-order sea level changes in the Middle Carboniferous, Late Carboniferous, and Early Permian did not exceed several tens of meters. During the same period, shorter fluctuations (∼100 kyr) occurred owing to the waxing and waning of large Gondwanan ice sheets. In the first half of the Bashkirian Age (Early Pennsylvanian), regression took place in the East European and North American cratons and then shallow-water sedimentation resumed. This regression is usually attributed to a considerable sea level fall. In some other areas, slow shallow-water sedimentation continued throughout the Bashkirian. This suggests that the Bashkirian regression was due to the crustal uplifts. Short-term uplifts can be explained by ascending convective currents beneath the asthenosphere. In southern North America, they brought an active fluid into the lithosphere. This caused rapid eclogitization-related crustal subsidence in the Arkoma and Anadarko basins as well as intense lithospheric weakening and shortening in the Ouachita Fold Belt.

Burial, Maturation, and Petroleum Generation History of the Arkoma Basin and Ouachita Foldbelt, Oklahoma and Arkansas

Removed overburden, burial, maturation, and petroleum generation analysis indicates that maturity in the Arkoma Basin and the Ouachita Foldbelt is explained effectively using simple burial models that account for the significant surface erosion that has occurred and assuming geothermal gradients similar to present-day gradients have been approximately constant through geologic time. Regional models, based on analysis at 115 well locations, indicate that from 5,000 to 15,000 ft (1.5–4.5 km) of section, differing with location from north to south and west to east, has been removed from the Arkoma Basin region, and as much as 25,000–40,000 ft (7.5–12 km) have been removed from areas of the Ouachita Foldbelt. Based on burial and thermal history reconstruction, increasing maturation from west to east across the basin is primarily the result of increasing overburden and subsequent surface erosion from west to east. The models predict most publicly available vitrinite reflectance data within a factor of 1.5 at two standard deviations. Comparison of model and measured reflectance-depth trends in six wells indicates that hydrothermal fluid movement should not have modified reflectance by more than approximately 20% in the center of the basin. Analysis indicates that most of the basin is overmature for oil production from intervals below the Spiro Sandstone, except to the north and northwest. Although thermal maturities are high, methane is stable throughout the basin. Except for the basal Arbuckle Group, all formations were thermally immature for oil generation prior to burial by the Mississippian and Morrowan in the Ouachita Foldbelt of Oklahoma and by the Atokan and Desmoinesian over most of the basin and study area. In the deeper part of the present basin, all strata entered and passed through the oil window during or within 10 My after Atokan time. Because no additional major quantities of hydrocarbons were generated after Atokan time, the hydrocarbons must have been emplaced and trapped during this brief time interval.

Abstract: Removed Overburden and Maturation Analysis of the Arkoma Basin and Ouachita Foldbelt, Oklahoma and Arkansas 

Abstract: Removed Overburden and Maturation Analysis of the Arkoma Basin and Ouachita Foldbelt, Oklahoma and Arkansas 

Gravity constraints on lithosphere flexure and the structure of the late Paleozoic Ouachita orogen in Arkansas and Oklahoma, south central North America

Spectral analysis of Bouguer gravity anomalies in western central Arkansas and eastern Oklahoma indicates that the thickness of the crust in the Ouachita fold and thrust belt increases from 38 km in the western Ouachitas to 44 km in the eastern Ouachitas. The change in crustal thickness occurs near the western end of the Broken Bow uplift and coincides with an abrupt decrease in the flexural rigidity of the lithosphere from 1.8×1024 N m in the western Ouachitas to 5.0×1023 N m in the eastern Ouachitas. The flexural rigidity in the western Ouachitas is similar to values determined in the Appalachian fold and thrust belt and coincides with the depth of the 450°C isotherm predicted by conductive cooling models for the thermal evolution of the early Paleozoic southern Laurentian rifted continental margin. The thick crust in the eastern Ouachitas results in lithosphere that is anomalously weak for rifted continental crust of this age. The thicker crust is attributed to an eastward transition from a rift segment to a transform segment of the Paleozoic continental margin. A layered density model derived from the gravity data shows that strata interpreted to be deformed Ouachita facies rocks are thickest in the eastern Ouachitas and are consistent with a greater amount of shortening in the central thrust belt in Arkansas as compared to Oklahoma. The opposite relationship is observed in the frontal Ouachita province, where shortening appears greater in Oklahoma. The cross‐strike changes in the locus of shortening, crustal thickness, flexural rigidity, and the inferred transition from rift to transform segments of the early Paleozoic continental margin all coincide with the location of a previously hypothesized zone of diffuse rightlateral shear located at the western end of the Benton uplift. Flexural modeling indicates that the load required to produce the observed Bouguer gravity low in the Arkoma foreland basin trends parallel to the Benton and Broken Bow uplifts but is located 114 to 276 km farther south. In the western Ouachitas, the position of the load coincides with the northern edge of the Sabine uplift and is interpreted to mark the southern extent of Ouachita facies rocks that were emplaced on the Laurentian continental margin and/or attached remnant oceanic crust.

Geometry of Thrusting in the Wilburton Gas Field and Surrounding Areas, Arkoma Basin, Oklahoma: Implications for Gas Exploration in the Spiro Sandstone Reservoirs: ABSTRACT

Abstract The Arkoma basin is an arcuate structural feature located in southern Oklahoma and western Arkansas. It is recognized as a foreland basin of the Ouachita fold and thrust belt and is one of the most prolific gas producing basins in North America. The Choctaw fault is the leading-edge thrust of the Ouachita frontal belt. Several south-dipping leading imbricate fan thrust faults are present in the hanging wall block of the Choctaw. The Wilburton gas field is located in the central part of the basin and produces mostly from the Pennsylvanian (Atokan) Spiro Sandstone. We have constructed many balanced structural cross sections in the Wilburton gas field and adjacent areas to determine the detailed geometry and structural history of the thrusting. The cross sections are perpendicular to the tectonic transport direction and are based on updated surface geology by the Oklahoma Geological Survey, wire-line well log data, and our interpretation of several seismic profiles donated by the EXXON oil company. The restored cross sections suggest about 40% shortening in the Wilburton gas field area. There is a good correlation between the presence of Chamosite clay and the porosity and permeability in the Spiro Sandstone. The original distribution of the Chamosite bearing facies in the Spiro sandstone was plotted on the cross sections to determine the extent of potential reservoir rock. In the Wilburton area, the cross sections suggest a shallow triangle zone floored by the Lower Atokan detachment and flanked by the south-dipping Choctaw thrust to the south and north-dipping Carbon fault to the north. The foot wall of the Choctaw contains duplexes that are located between Springer detachment (the lower detachment) and the Lower Atokan Detachment (the upper detachment). These duplexes contain overpressured Spiro sandstone gas reservoirs. There is a rough correlation between the pressure data in the Spiro reservoirs and the duplex structures. The highest pressure-depth gradients are found in the Spiro reservoirs that were brought to structurally higher positions by the thrust faults in the duplex structures. Structurally lower reservoirs exhibit lower pressure-depth gradients. Therefore, we suggest that the thrusting in the duplex structures may have formed seals that isolated the Spiro Sandstone gas reservoirs.

Three-Dimensional Geometry and Kinematics of the Gale-Buckeye Thrust System, Ouachita Fold and Thrust Belt, Latimer and Pittsburg Counties, Oklahoma

The Gale-Buckeye thrust system consists of a series of blind thrusts and associated folds near the boundary between the Ouachita fold and thrust belt and the Arkoma basin in Oklahoma. A dense grid of high-resolution seismic reflection data, augmented by data from exploration and production wells, provides a unique opportunity to analyze the three-dimensional geometry and kinematics of these structures in detail. Results from our interpretation provide new insight into the extent, style, and sequence of thrusts within the Gale-Buckeye thrust system, and contrast with previous models for the region. Specifically, we recognize two previously unmapped transverse structures in the Gale-Buckeye thrust system: a sharp, left-stepping zone of oblique ramps, and a tear fault within the Gale thrust sheet. These transverse structures are spatially coincident with similar structural trends in lower Paleozoic strata beneath the thrust system as well as outcropping structural trends on the surface. This interpretation extends thrusts in the Gale-Buckeye system to minimum strike lengths of 7-9 mi (11-15 km) and fault length-to-displacement ratios of about 10:1. Furthermore, interpreted structural relationships suggest that the Gale and Buckeye thrusts formed in a backward-stepping manner, a new interpretation for faults in this system. Measurement of differential transport angles from our interpretations of the Gale-Buckeye thrust system indicates that along-strike displacement variations of the Gale and Buckeye thrusts are consistent with thrusts from other nonmetamorphic fold and thrust belts. Using the differential transport model, we infer that the tear fault in the Gale thrust sheet indicates a region that became segmented to accommodate excessive along-strike differences in displacement. In addition, we use the model to explain errors in previous interpretations of the Gale-Buckeye thrust system that resulted in the drilling of dry holes. Integrating realistic differential transport angles with our interpretations for extent, style, and sequence of thrusting provides new strategies for hydrocarbon exploration hroughout the region.

Ouachita-Appalachian Juncture: a Paleozoic Transpressional Zone in the Southeastern U.S.A.

The northern margin of Gondwana collided with the southern margin of the North American craton in the late Paleozoic (330-265 Ma). Near-normal compressional stresses, generated where the trend of the North American margin was nearly perpendicular to Gondwana's plate-motion vector (closure direction), caused extensive crustal shortening and decollement thrusting within the sedimentary section. The Appalachian and Ouachita fold belts are the erosional remnants of this collision on the North American margin. Transecting the Ouachita fold belt are several zones of late Paleozoic transcurrent faulting: the Val Verde, Ardmore-Anadarko, and Reelfoot zones. They occur where the precollision margin of the North American craton was oriented oblique or parallel to the closure direct on with Gondwana. Stresses generated within these zones during the collision were transpressional rather than simply compressional, and gave rise to high-angle faulting, high-amplitude vertical displacements, and the emplacement of positive flower structures. In the Val Verde and Ardmore-Anadarko basins, these features have proven to be major structural traps for oil and gas. On the basis of seismic and well data, this study identifies a zone of complex structures, including positive flower structures, in the subsurface Paleozoic section of Kemper County in east-central Mississippi. The region is considered to be another transpressional zone analogous to the Val Verde, Ardmore-Anadarko, and Reelfoot zones. The Kemper County zone separates and truncates the Appalachian Valley and Ridge fo d belt in the subsurface from the Mississippi Ouachita deformed belt to the west and is, therefore, also identified as the juncture between the Ouachita and Appalachian fold-belt systems in the southeastern U.S.A.

Structural Styles and Exploration Strategles along the Leading Edge of Fold and Thrust Belts: Examples from the Ouachita Fold and Thrust Belt, Southeast Oklahoma

Structural Styles and Exploration Strategles along the Leading Edge of Fold and Thrust Belts: Examples from the Ouachita Fold and Thrust Belt, Southeast Oklahoma

A Glimpse of Provenance Through the Diagenetic Haze in Smackover Sandstones of the Gulf Coast: ABSTRACT

Late Jurassic Smackover sandstones deposited in the four interior salt basins of the Gulf Coast have been extensively altered, yet their provenance signature has not been grossly modified on QFL and QmFLt plots. Recalculating framework compositions to account for diagenesis suggests that Smackover sandstones were derived from the Ouachita fold and thrust belt and the interior portion of the North American craton. Differences in framework composition between the four basins result from both diagenesis and provenance. Detrital quartz and rock fragments have been lime modified by diagenesis. The south Texas and Mississippi basin sandstones contain more polycrystalline quartz than the other two basins. Rock fragments in the three eastern basins consist primarily of SRFs and low-grade MRFs; south Texas sandstones contain mostly gneiss and schist. Feldspar composition is controlled by the pore fluid chemistry. The South Texas and Mississippi basins contain potassium-rich K-spar whereas the east Texas and Louisiana basins contain mostly albite. A uniform 6% loss (whole rock) of feldspar can be recognized petrographically in all four basins but this loss does not significantly shift the provenance fields on QFL and QmFLt plots. Mississippi basin sandstones plot primarily in the craton interior provenance field supporting derivation of these sandsmore » from the ancestral Mississippi river, East Texas sandstones are also dominated by a cratonic provenance area. Louisiana sandstones plot in the recycled orogen field and were primarily derived from a local source in the Ouachita fold belt. The provenance of south Texas sands was the interior zone of the Ouachitas, as indicated by the presence of high-grade metamorphic rock fragments and Rb-Sr dates of detrital muscovite.« less

Middle Pennsylvanian recurrent uplift of the, Ouachita fold belt and basin subsidence in, the Arkoma basin, Oklahoma

Recurrent uplift of the Ouachita fold belt in Oklahoma coincided with the disruption of the Arkoma basin following the deposition of the Boggy Formation (early Desmoinesian time). The Boggy, composed of sandstone-shale sequences that record southerly progradation of coal-bearing, fluvially dominated deltaic complexes into the Arkoma basin, was folded at the time of uplift of the Ouachita fold belt. The uplift ended the progressive subsidence of the Arkoma basin and shifted the depocenter to the northwest. Subsequently, the Thurman Formation (middle Desmoinesian), which had a source in the southeast, was deposited in the smaller resurgent foreland basin over the folded and eroded surface of the Boggy. Chert-pebble conglomerates in the Thurman were derived from the erosion of newly elevated Ordovician and Devonian cherts in the core of the Ouachita foldbelt. Sandstone-shale packages are found in both formations. The origin of the coal-bearing cycles in the Boggy are enigmatic, but they probably were controlled by a combination of factors such as glacio-eustatic changes in sea level and delta-lobe abandonment. In contrast, cycles in the Thurman probably were strongly influenced by episodic thrust faulting and uplift in the Ouachitas.

Late Mississippian and Pennsylvanian depositional history in the Arkoma basin area, Oklahoma and Arkansas

The Arkoma basin was depositionally part of a broad, stable shelf along a passive continental margin during much of its history. During the Chesterian, Morrowan, and early Atokan, the depositional patterns on the shelf varied greatly, depending on the inconsistent development of carbonate environments and the intermittent introduction of terrigenous elastics (quartz arenites) from the north. Beginning with the middle Atokan, flexural downwarping of the south margin of the shelf was accompanied by down-to-the-south syndepositional normal faults developed sequentially to the north, as a result of continued collapse of the Ouachita trough. Lithic arenites were introduced into this developing trough from the east, apparently derived from a tectonic provenance southeast of the Ouachita trough, on the southwest margin of the Black Warrior basin. Further closure plus rapid deposition resulted in the closing and filling of this incipient foreland basin by the end of deposition of the middle Atoka. With further compressional deformation, the axis of deposition shifted farther northward with the development of a fully formed and continually subsiding foreland basin (beginning in late Atokan). Lithic arenites were transported westward along the axis of the basin (documented in earliest Desmoinesian). In Arkansas some of the early Desmoinesian sediments apparently came from the uplifted Ouachita thrust belt immediately to the south. During the rest of the early Desmoinesian (most of Krebs Group), with the continued subsidence of the foreland basin, extensive deltaic deposits (sublitharenites) were introduced from the north and provided the primary source of sediments to the foreland basin in Oklahoma. They apparently came from the continental interior to the west and north of the Ozark dome. Although there is evidence of a limited source of sediments from the Ouachita fold belt in Arkansas during the deposition of the Hartshorne Sandstone (earliest Desmoinesian), the fold belt to the west in Oklahoma was apparently quiescent and presumably standing at or near sea level throughout the time of deposition of the upper Atoka and the whole of the early Desmoinesian Krebs Group (Hartshorne, McAlester, Savanna, and Boggy Formations). Although added field confirmations are needed, it is concluded that renewed folding and uplift of the Ouachita fold belt following the deposition of the early Desmoinesian Krebs Group involved also the compression and folding of the Arkoma basin. This ended the progressive downwarping of this basin and shifted the depocenter still farther to the northwest. The core area of the Ouachita fold belt was extensively elevated for the first time resulting in the erosion and transportation of chert pebbles and other sediments to the northwest (Thurman Sandstone). Beginning with the middle Desmoinesian, Cabaniss Group deposition was in a narrow successor foreland basin located to the north-west of the Arkoma basin and to the northeast of the Hunton arch. The Ouachita fold belt was the primary source for terrigenous sediments, periodically including chert-pebble conglomerates, to this area throughout the remainder of the Pennsylvanian.

Potential for Subthrust Gas Fields: Results of Recent Deep Drilling in the Ouachitas: ABSTRACT

The Broken Bow anticline, with dip closure of over 170 mi/sup 2/, is the Oklahoma culmination of a long regional trend extending from Arkansas to central Texas. The Standard Oil 1-22 Weyerhaeuser well, drilled to 18,980 ft, is the first crestal well on this feature to rest the subthrust section beneath the Ouachita fold and thrust belt. The wellspudded in and drilled a substantial section of sloates, phyllites, and calcareous quartzites of presumed Ordovician and Cambrian(.) age. The basal Ouachita thrust was penetrated at 11,714 ft, where an interpreted Ordovician Simpson-Arbuckle section was encountered. Reflectance methods using pyrobitumen and visual determination of metamorphic grade indicated the presence of a significant maturity reversal at the thrust boundary. Although economic quantities of hydrocarbons were not found in this well, the following observations may influence future work. (1) A para-autochthonous section of shelf carbonates and sands exists in the subthrust, at least 60 mi from the leading edge of the Ouachita thrust belt. (2) The abundance of pyrobitumen observed in the Simpson section suggests the former presence of significant hydrocarbons. (3) The well encountered good dolomite reservoir at high maturities (R/sub 0/ = 7-8%) that flowed formation water and unexpectedly contained dissolved C/sub more » 1/ to C/sub 3/. These observations suggest that high maturities alone may not be a condemning factor and should be considered in future exploration plays. « less

Genetic link between Ouachita foldbelt tectonism and the Mississippi Valley–type lead-zinc deposits of the Ozarks

Fluids expelled from the Arkoma basin in response to Late Pennsylvanian–Early Permian orogenesis in the Ouachita foldbelt may have been ultimately responsible for formation of the Mississippi Valley–type lead-zinc deposits in the Ozark region of Missouri, Arkansas, Kansas, and Oklahoma. Fluid inclusions in both mineralized and barren rock record the passage of hot, saline fluids regionally throughout most of the Paleozoic section in the Ozarks. Dating studies in addition to structural and geologic observations provide further evidence for fluid migration and mineralization approximately coincident with the Ouachita orogeny. Formation of foreland basins, such as the Arkoma, during convergent tectonism creates conditions exceptionally favorable to the migration of fluids from deep sedimentary basins. Proximity to a basin whose margin has undergone some form of tectonic deformation or uplift may be the unifying factor in the genesis of Mississippi Valley–type deposits in geologically diverse settings.

Geologic Description of East Velma West Block, Sims Sand Unit, for an Enhanced Oil Recovery Project

Summary East Velma West Block, Sims Sand Unit (EVWBSSU), located in Stephens County, OK, is currently beingflooded with CO2. A detailed, quantitative, reservoirdescription was constructed for a simulation model thatwill aid in the evaluation and operation of the CO2 flood. The Sims sandstone member of the Upper-Mississippian/Lower Pennsylvanian Springer formation was deposited in a deltaic environment. Four deltaic facies are present: the delta plain, proximal and distal delta-front, present: the delta plain, proximal and distal delta-front, and abandonment phase. The delta-plain facies forms ashale barrier to vertical fluid flow between the abandonment and proximal delta-front sandstones. The proximaldelta-front facies forms the bulk of the reservoir. Porosity (16%) and permeability (70 md) in this facies are high Porosity (16%) and permeability (70 md) in this facies are high because of minor clay and cement content. The proximaldelta-front facies grades laterally downstructure into the distal delta-front facies. Authigenic clays and carbonateand silica cement have reduce porosity (14%) and permeability (30 md) in the distal delta-front sandstones. permeability (30 md) in the distal delta-front sandstones. A reservoir description was built from the geologic analysis for a four-component, three-dimensional (3D)full-field model. Reservoir zonation was based on the arrangement of deltaic facies. Porosity from gamma-ray/densitylogs was calibrated to core porosity to determine porosity in uncored wells. A linear correlation between core porosity in uncored wells. A linear correlation between core porosity and permeability was established for each facies. porosity and permeability was established for each facies. Permeability and gamma ray cutoffs were statistically Permeability and gamma ray cutoffs were statistically determined to define net pay. Vertical-to-horizontal permeability ratios were determined from measurements on permeability ratios were determined from measurements on oriented core plugs. Computer isopach maps of gross pay, net pay, porosity, and horizontal and verticalpermeability were generated for individual reservoir zones and permeability were generated for individual reservoir zones and then entered into the reservoir model. Introduction ARCO Oil and Gas Co. is conducting an enhanced-oil-recoveryprogram on EVWBSSU in Stephens County, program on EVWBSSU in Stephens County, OK. Continuous injection Of CO2 into the updip portion of the reservoir began in Sept. 1983. EVWBSSU wasdiscovered in 1949 with an estimated 220 × 10 STB[34 958 × 10 m3] of original oil in place (OOIP). Primary depletion was by gravity drainage. Crestal gas Primary depletion was by gravity drainage. Crestal gas injection for pressure maintenance began in 1962 andwater injection began in 1972. Approximately 32% ofOOIP was recovered by these methods of production. Success Of CO2 flood depends on distribution ofreservoir lithology, porosity, and permeability. Distributionand continuity Of sandstone reservoirs are controlled bydepositional environments. Clay and cement contentdetermine the subsurface distribution of porosity andpermeability. Thus the purpose of this study was to analyze permeability. Thus the purpose of this study was to analyze mineral composition and to determine the depositionalsetting of Sims sandstone. On the basis of this geologicanalysis, a reservoir description was constructed for asimulation model. Stratigraphy The Upper-Mississippian/Lower-Pennsylvanian Springerformation consists of approximately 2,500 ft [7670 m] ofalternating sandstone and shale. The Goddard formationconformably underlies the Springer formation, which isunconformably overlain by Morrowan-Age rocks. Four subsurface Springer sandstone members-indescending order, the Markham, Aldridge, Humphreys, andSims-have been defined by drilling in the westernArdmore basin. The Sims sandstone member, ranging inapparent gross thickness from 300 to 600 ft [91.4 to 182.9m], is the major oil-producing horizon within the Velmareservoir. It consists of three sandstone units: indescending order, C1, C2, and C3. Sandstones are white to buff, fine-grained, and subrounded to subangular. They gradebasinward into clay-rich sandstones and finally into shales. Approximately 250 ft [77.1 m] of shale overlies the Simssandstone member. Geologic Setting The Sims member of the Springer formation was depositedon moderately subsiding shelves in theAnadarko-Ardmore basin, which is bounded by the Arbuckle upliftto the north, by the Wichita-Criner uplift to the south, and by the Ouachita foldbelt to the east (Fig. 1). The EVWBSSU is part of the Velma reservoir, locatedin the northwestern part of the Ardmore basin. TheEVWBSSU is approximately 3.5 miles [5.6 km] long and0.75 miles [1.2 kin] wide (Fig. 2). This reservoir strikesin a southeast-northwest direction. It dips steeply (30 to60* [0.52 to 1.04 rad]) to the northeast. To the south, the EVWBSSU is bounded by an erosional unconformityand, to the north, by poor reservoir quality and a water/oilcontact (WOC). Folding and faulting of the Velma reservoir, whichoccurred in Early Pennsylvanian times during collision ofNorth American, South American, and Africancontinental masses, is postdepositional. JPT P. 1420

Depositional Systems and Stratigraphic Relationships of Strawn Group (Pennsylvanian), Colorado River Valley, Central Texas: ABSTRACT

The Strawn Group (late Atokan-early Missourian) comprises a thick sequence of terrigenous clastic and carbonate facies that crop out in central Texas in two roughly triangular areas separated by Cretaceous overlap. The exposures in the Brazos River valley have been studied in detail and interpreted as fluvial, deltaic, and shallow marine deposits. The Strawn Group of the Colorado River valley has been cited in the literature, but little detailed work regarding subdivision or interpretation has been done. The Strawn Group exposed in the Colorado River valley can be informally subdivided into two general stratigraphic units. The lower Strawn represents submarine fan and basin depositional systems that prograded southwestward. At least two but possibly several cycles of fan progradation are recognizable based on vertical succession of distal to more proximal middle-fan facies associations. In marked contrast, the upper Strawn represents three regressive and transgressive cycles of typical Pennsylvanian deltaic, perideltaic, and shallow submarine depositional systems. Stratigraphic relationships of the Strawn Group are complex and reflect changes in the evolving Ouachita foldbelt and its associated tectonic features. The lower Strawn occurs only in the Fort Worth basin and apparently was deposited with a clinoformal relationship to the underlying basinal Smithwick Shale. It records foreland basin subsidence and regional block faulting synchronous with orogenesis. As active tectonic subsidence shifted westward, upper Strawn deltaic systems prograded to the west, onlapped the Concho platform, and truncated older units particularly on horst blocks. In the southeastern portion of the Concho platform, the overlying Canyon Group completely onlaps the Strawn Group and rests unconformably on the Marble Falls Limestone. End_of_Article - Last_Page 147------------

Early Opening of Initially Closed Gulf of Mexico and Central North Atlantic Ocean: ABSTRACT

ABSTRACT Regional structures beneath the northern Gulf of Mexico coastal plain clearly reveal the fit of the American continents following Late Paleozoic Appalachian-Ouachita orogeny. Most diagnostic is a wrench fault zone within the former Gondwana megacontinent, recognizable from the Florida panhandle to western Mississippi, along which future South America moved northwest against the southern edge of eastern North America to form the Ouachita foldbelt, while future Africa was already subducting that continent along the Appalachian belt. Extending westward from this in southwestern Alabama is the Wiggins arch, underlain by granite and phyllite of Late Pennsylvanian into Early Permian age, apparently part of a volcanic arc. Along its north side and that of its counterparts farther west are shallow-marine strata of similar age. These appear to occupy a remnant ocean on North American oceanic crust, which was uncoupled by the close approach of future South America and subducted very briefly beneath North America, while the sediments on its leading edge were peeled off and thrust onto the continent. The inferred volcanic arc and remnant ocean, and the Late Triassic Interior rift system that separates them from the Ouachita foldbelt, terminate abruptly in East Texas against a wrench fault that transferred this rifting south-southwest to the Rio Grande embayment area. Taking the Pickens-Quitman-Gilbertown-Pollard and the Mexia-Talco fault zones, which mark the approximate updip limit of appreciable Louann Salt, to represent these northward-converging wrench faults, northern South America's Guyana shield fits snugly against them, with slight overlap onto the intervening Wiggins and Sabine uplifts. With Africa conventionally joined to South America, this continental fit also places West Africa's two prominent capes opposite pronounced salients of the Appalachian foldbelt, while capes Cod and Hatteras face the principal West African basins, which occupy salients in the equivalent African foldbelt. Also, pre-Mesozoic Florida lies in the reentrant between the southern continents, seemingly separated from each by an arm of a triple rift system whose third arm passes between them. From this tightly-closed condition the Gulf of Mexico opened by relative right-lateral translation of eastern North America (Florida to Nova Scotia) against the bulge of northwestern Africa, actually a rotation about a pole near the central southern edge of the Sahara. The Interior rift system formed while the sialic block that includes the Wiggins and Sabine uplifts remained briefly with South America as North America moved north-northeast along the East Texas transform and a parallel one at the east end of the Wiggins arch. The rifting jumped south of these uplifts after extension of perhaps 50 mi (80 km) to open the present Gulf basin. Almost 200 mi (320 km) of such translation brought Cape Hatteras solidly against Africa's Cap Blanc, ending this movement in earliest Jurassic time. Magnetic anomaly lineaments off the southeastern states, used as crude isochrons, show that the central North Atlantic Ocean (CNAO) began to open by southern North America swinging westward to disengage the obstructing capes. The Blake Plateau basin and Carolina trough opened first, then the spreading jumped east of the latter and propagated northward along the entire CNAO, followed by a second jump marked by development of the Blake Spur magnetic anomaly lineament. Changes in North America's movement direction throughout this period were recorded by the Great Abaco fracture zone, as reinterpreted, along which the early CNAO terminated southward against the Florida-Bahama platform. Right-lateral offsets of this fracture zone totalling about 65 mi (100 km) are associated with the two spreading-center jumps. The Late Triassic intercontinental transform continued across northwestern Morocco into the Tethyan sea, which then extended west into what's now the northern CNAO. As the CNAO opened to its full length it appropriated this tip of Tethys and left the intervening Moroccan Meseta with Africa. Tethys' venture into the future CNAO followed extensive earlier Triassic rifting in eastern North America, which formed an inferred graben system responsible for the Brunswick magnetic anomaly lineament and prepared the way for subsequent intercontinental translation.

Textures of Chert and Novaculite: An Exploration Guide: ABSTRACT

Textures of chert and novaculite observable in scanning electron micrographs (SEMs) are useful as a practical, geologic thermometer for estimating the maximum temperature of those rocks since deposition. Such information may be further applied during exploration for hydrocarbons (maturation or degradation), and for metallic and nonmetallic minerals, in those and associated rocks. Scanning electron micrographs taken of cherts and novaculites that crop out in the Ouachita Mountain foldbelt of Arkansas and Oklahoma, and in areas adjacent to exposed and buried intrusives, show a sequential range in textures from cryptocrystalline, anhedral quartz in the nonmetmorphosed chert and novaculite to coarse euhedral, polygonal, triple-point quartz 60 µm in diameter. A similar range of textures occurs in chert of a contact metamorphic aureole on the Isle of Skye, Scotland, where classic metamorphic mineral suites from talc through tremolite, diopside, and forsterite grades are represented. Hence, some chert and novaculite of the Ouachita Mountain foldbelt shows textures morphologically correlative with classic representatives of varied metamorphic conditions. Measurements were made of grain sizes of the quartz along transects across the SEMs of chert and novaculite from the Ouachita foldbelt. From them an isopleth map was made showing mean grain sizes of the polygonal triple-point texture developed. The map defines a linear 25 to 65 km (15 to 40 mi) wide belt that extends from Little Rock, Arkansas, about 250 km (155 mi) west to Broken Bow, Oklahoma. The texture increases from the margins to the core of the Ouachita Mountain foldbelt and contains two coarse-grain anomalies, one near Little Rock (35 µm diameter) and another near Broken Bow (15 µm diameter). This textural belt, with anomalies, conforms to the most intense, predominant late Paleozoic, structural deformation in the Ouachita Mountains. Previous interpretations have co sidered the rocks in the core of the foldbelt to have attained a maximum metamorphic grade in the zeolite to lower greenschist facies. Cherts and novaculites adjacent to Magnet Cove, a Cretaceous age pluton in the eastern Ouachita Mountains of Arkansas, illustrate a superposed overprinting of polygonal triple-point texture. It ranges from a background of about 2 µm (talc grade) in chert 1,370 m (4,500 ft) from the pluton to about 45 µm (forsterite grade) from near the contact. Private drilling operations indicate that the pluton contact dips about 45° beneath much of the sedimentary rock that exhibits locally anomalous crystallinity. Homogenization temperatures of vein quartz, determined by H. Jackson in 1973, show a gradient along this profile of slightly more than 200°C (390°F) in quartz 1,370 m (4,500 ft) from the pluton, to about 440°C (825°F) near the contact. Novaculite xenoli hs in the adjoining Potash Sulphur Springs intrusive are coarser in texture, 60µm or larger, and represent the higher temperature periclase metamorphic grade, approximately 760°C, 1,400°F, at Crestmore, California, according to Carpenter. The triple-point texture and coarseness of chert and novaculite are related to the degree of thermal maturation resulting from various heating events. The crystal morphology is equivalent in the two processes described (regional and contact metamorphism), but the changes due to individual agents, temperature, physical deformation, time, depth of burial, and mineralizers have not yet been resolved separately. Very small quantities of chert and novaculite, by using SEM techniques, can serve as a guide to areas that have undergone elevated rock temperatures resulting from deep burial, mechanical stresses, intrusions, exhalations, and other thermal events. These investigations are relevant in determining temperature levels that may mature or degrade hydrocarbons, and offer clues in exploration for thermally related metallic and nonmetallic minerals. SEM studies of cherts and novaculite now provide another method of ascertaining the thermal maturation of rocks. End_of_Article - Last_Page 1466------------

Depositional History of Strawn Group (Pennsylvanian), Fort Worth Foreland Basin, Colorado River Valley, Central Texas: ABSTRACT

In the Colorado River Valley of central Texas about 1,200 ft (366 m) of Pennsylvanian (Desmoinesian) terrigenous clastic and carbonate facies were deposited in the Fort Worth basin. Paleocurrent and petrographic data indicate that the clastic provenance, to the west and northwest, was the rising Ouachita orogenic foldbelt. Surface study suggests that the Strawn Group includes two main stratigraphic divisions: (1) a lower sequence representing basin and submarine fan sediments deposited as active tectonic subsidence substantially increased water depths in the Fort Worth basin; and, (2) an upper progradational sequence of fluvial-deltaic and carbonate platform sediments deposited as passive isostatic subsidence or regional uplift to the east accompanied progressive shoaling of the sediment surface. The lower Strawn is characterized by sediment gravity flows suggestive of submarine fan deposition. It is subdivided into four facies (from proximal to distal): (1) massive channeled sandstone; (2) massive amalgamated sandstone; (3) turbidite; and (4) shale facies. The upper Strawn constitutes a delta-platform ssemblage that includes: (1) delta facies including channel-mouth-bar sands, delta-front sands, delta-slope shales, and interdistributary fine clastics; and (2) carbonate platform facies that includes phylloid algal mounds and perideltaic bioclastic limestones. The lower Strawn basinal shale and submarine fan sequence is gradational with the underlying basinal Smithwick Shale (Atokan) and records foreland basin subsidence synchronous with orogenesis in the Ouachita foldbelt. In contrast, the basal contact of the upper Strawn is variable. Generally, the upper Strawn is gradational with the lower Strawn but locally it rests disconformably on the Marble Falls Formation (Morrowan-Atokan). This stratigraphic relationship suggests that regional compressive forces from the Ouachitas were depositionally more important than previously supposed. End_of_Article - Last_Page 637------------

Middle Strawn (Desmoinesian) Cratonic Delta Systems, Concho Platform of North-Central Texas: ABSTRACT

Terrigenous clastic- and carbonate-rock units forming the middle third of the Strawn Group were deposited in the Fort Worth basin and on the adjacent Concho platform of north-central Texas. Four significant transgressive-regressive cycles comprising the interval between the top of the Brannon Bridge Limestone and the top of the Brazos River Formation have been evaluated employing data gathered from 4,000 well logs and 35 measured sections. Subsurface maps indicate that four discrete, vertically persistent, deltaic depocenters, two carbonate banks, and an embayment strand-plain complex are present within the area. The average thickness for the total vertical stratigraphic interval is 1,000 ft (300 m), or approximately 250 ft (75 m) per cycle of deltaic progradation and aba donment. When active tectonic downwarping diminished in the central part of the Fort Worth basin, middle Strawn cratonic deltas prograded across the filled foreland basin and out onto the stable, gradually subsiding Concho platform. Deltaic facies present on the platform for all four cycles involve thin, usually less than 140 ft (42 m) thick, multilateral, high-constructive elongate and lobate systems. For the lower two cycles, the Buck Creek and Dobbs Valley sandstones of outcrop, deltaic progradation extended to the western margin of the platform more than 200 mi (320 km) downdip from the source area. Carbonate-bank deposition subsequently was established on the distal ends of these oldest delta sands and westward deltaic progradation was less extensive with the upper two cycles. A strand-pl in--embayment system composed of mudflats, chenier sandstone bodies, and thin bay-head deltas developed between the two principal deltaic depocenters on the platform. The Midland basin on the west was a poorly defined, gradually deepening, depression; no true Desmoinesian shelf-edge or slope systems have been discovered. High-constructive delta systems attributed to the Dobbs Valley (cycle II) and Brazos River (cycle IV) units have sandstone accumulations in excess of 200 ft (60 m) at one depocenter along the northwestern margin of the platform and at a second one on the northwestern rim of the Fort Worth basin. These thicker deltaic complexes contain linear, multistoried sandstone bodies whose geometries resemble bar-finger sands of the modern Mississippi delta. Valley-fill fluvial deposits incise the high-conservative deltaic facies. The Arbuckle and Wichita Mountains were the major sources for the more arkosic, northern delta systems. The Ouachita foldbelt furnished the chert-rich detritus for the fluvial-deltaic facies on the Concho platform. End_of_Article - Last_Page 1601------------