Thomas E. Ewing

Mississippian Barnett Shale, Fort Worth basin, north-central Texas: Gas-shale play with multi–trillion cubic foot potential: Discussion

Montgomery et al. (2005) have written a very useful, information-filled review article on the state of knowledge of the Barnett Shale play in north Texas, a topic of great current interest and importance. One error exists, however; the burial history that they present shows no uplift during the early and middle Mesozoic and strong uplift after the Cretaceous, whereas the geologic record indicates major pre-Cretaceous uplift. This error substantially affects the discussion of the maturation history of the Barnett and should be corrected in the literature. I will also briefly discuss the implications of pre-Cretaceous erosion and Ouachita thrusting to Barnett maturity in the deep Fort Worth basin. In Montgomery et al.s (2005) figure 7, they show a time-depth burial history diagram for Eastland County that is contrary to what is known about the area. In that figure and in the text, they indicate that the Barnett was rapidly buried during the Pennsylvanian and Early Permian, remained at depth with no uplift or subsidence except for minor subsidence in the Early Cretaceous, then was uplifted some 1.8 km (6000 ft) between the middle Cretaceous and the Eocene (with very slight subsidence to the present). The surface geology of Eastland County and surrounding areas (Barnes, 1972) shows that flat-lying Lower Cretaceous strata (Antlers Sand and overlying Edwards Group marine carbonates) lie unconformably above units, ranging from the Strawn Group (Mingus Formation, Desmoinesian regional age) in the southeast to the lower Cisco Group (Graham Formation, Virgilian regional age) in the northwest. The west-northwest–dipping Pennsylvanian strata form part …

The Frio River Line in South Texas--Transition from Cordilleran to Northern Gulf Tectonic Regimes

ABSTRACT A northwest-southeast linear zone, here named the Frio River line, about 60 miles (100 km) southwest of San Antonio separates two areas of contrasting structural and stratigraphic history. To the northeast, structures include the uplifted Llano area, the Balcones and Luling zones of normal faulting, and a narrow graben which closely outlines the landward edge of Jurassic salt. All of these features disappear southwest of the Frio River line. To the southwest are northwest-southeast low-amplitude folds of the Rio Grande foldbelt, which are similar in orientation and age to Laramide folding in northeastern Mexico. Northeast of the line, Jurassic salt was deposited over broad areas; to the southwest, seismic data suggests an erratic distribution of salt. Lower Cretaceous subsidence was marked southwest of the line, and Gulfward sliding predominated northeast of the line. Upper Cretaceous alkalic volcanics are restricted to a belt northeast of the line, and are most abundant where that belt intersects the line. Even Tertiary growth-fault styles show distinct changes across this line, probably due to a greater thickness of Upper Cretaceous shale in the south. Fragmentary evidence indicates that the line may have a pre-Late Jurassic origin, possibly as the northeastern boundary of a Mesozoic strike-slip system, or as the southeastward continuation of the Devils River Uplift. Oil and gas plays in Cretaceous rocks are different north and south of the Frio River line; the Northern Gulf is dominated by traps related to normal fault systems, while Southwest Texas contains mostly fold-related and cross-fault traps and stratigraphic traps.

Growth Faults and Salt Tectonics in Houston Diapir Province--Relative Timing and Exploration Significance: ABSTRACT

ABSTRACT Oil and gas accumulation in Gulf Coast Tertiary strata is mainly controlled by regional growth faults and by salt-related structures. Salt forms the most prominent set of structures in the Houston diapir province of southeast Texas. Recent work in three study areas shows that the Tertiary growth-fault trends so well displayed along strike to the southwest continue through this salt basin as well, but have been deformed by later salt movement. In the Katy area, seismic data disclose early (pre-Wilcox) salt pillows downdip of the Cretaceous reef trend. Progradation of the lower Wilcox Rockdale delta system created a linear growth-fault trend above and seaward of the pillows. Salt stocks were injected upward from the pillows in Claiborne time and were flanked by deep withdrawal basins and turtle structures. Major oil accumulations occur over an inferred turtle structure and over deep-seated salt domes. The lower Wilcox growth-fault trend deformed by the later salt flowage is virtually unexplored, although geopressured gas production from these low-permeability deltaic reservoirs exists in adjacent areas. In Brazoria County, a major lower Frio growth-fault trend affecting the Houston delta system was deformed by later salt domes, by a salt-withdrawal basin, and by a possible turtle structure at Chocolate Bayou. A productive geopressured aquifer exists in the salt-withdrawal basin bounded by the previously formed growth faults. In Jefferson County, in contrast, salt tectonic activity and growth faulting appear to have been coeval. Early salt-cored ridges continued to rise throughout Frio deposition; growth faults occur both updip and downdip. Salt diapirism may have occurred throughout Frio time at Orange and Port Neches salt domes, but other domes such as Spindletop formed in post-Frio time. Hydrocarbons accumulated over the salt domes in growth-fault anticlines and in stratigraphic traps. Contemporaneous, low-intensity growth faulting and salt movement may be ascribed to the minimal loading imposed by the sand-poor lower and middle Frio section. Recognition that shelf-margin growth faulting preceded the development of the present pattern of domes and basins has important implication for hydrocarbon exploration. Growth faults may be migration paths for hydrocarbons; furthermore, early-formed traps, distorted by salt movement, may still be found to contain hydrocarbons. Figure 1. Regional shelf-margin trends of the Gulf Coast Basin and location of the Katy, Pleasant Bayou, and Port Arthur study areas. Map from Winker and Edwards (1983). End_Page 83-------------------------

Assessment of undiscovered conventional oil and gas resources, onshore Claiborne Group, United States part of the northern Gulf of Mexico Basin

The middle Eocene Claiborne Group was assessed for undiscovered conventional hydrocarbon resources using established U.S. Geological Survey assessment methodology. This work was conducted as part of a 2007 assessment of Paleogene–Neogene strata of the northern Gulf of Mexico Basin, including the United States onshore and state waters (Dubiel et al., 2007). The assessed area is within the Upper Jurassic–Cretaceous–Tertiary composite total petroleum system, which was defined for the assessment. Source rocks for Claiborne oil accumulations are interpreted to be organic-rich, downdip, shaley facies of the Wilcox Group and the Sparta Sand of the Claiborne Group; gas accumulations may have originated from multiple sources, including the Jurassic Smackover Formation and the Haynesville and Bossier shales, the Cretaceous Eagle Ford and Pearsall (?) formations, and the Paleogene Wilcox Group and Sparta Sand. Hydrocarbon generation in the basin started prior to deposition of Claiborne sediments and is currently ongoing. Primary reservoir sandstones in the Claiborne Group include, from oldest to youngest, the Queen City Sand, Cook Mountain Formation, Sparta Sand, Yegua Formation, and the laterally equivalent Cockfield Formation. A geologic model, supported by spatial analysis of petroleum geology data, including discovered reservoir depths, thicknesses, temperatures, porosities, permeabilities, and pressures, was used to divide the Claiborne Group into seven assessment units (AUs) with three distinctive structural and depositional settings. The three structural and depositional settings are (1) stable shelf, (2) expanded fault zone, and (3) slope and basin floor; the seven AUs are (1) lower Claiborne stable-shelf gas and oil, (2) lower Claiborne expanded fault-zone gas, (3) lower Claiborne slope and basin-floor gas, (4) lower Claiborne Cane River, (5) upper Claiborne stable-shelf gas and oil, (6) upper Claiborne expanded fault-zone gas, and (7) upper Claiborne slope and basin-floor gas. Based on Monte Carlo simulation of justified input parameters, the total estimated mean undiscovered conventional hydrocarbon resources in the seven AUs combined are 52 million bbl of oil, 19.145 tcf of natural gas, and 1.205 billion bbl of natural gas liquids. This article describes the conceptual geologic model used to define the seven Claiborne AUs, the characteristics of each AU, and the justification behind the input parameters used to estimate undiscovered resources for each AU. The great bulk of undiscovered hydrocarbon resources are predicted to be nonassociated gas and natural gas liquids contained in deep (mostly 12,000-ft [3658 m], present-day drilling depths), overpressured, structurally complex outer shelf or slope and basin-floor Claiborne reservoirs. The continuing development of these downdip objectives is expected to be the primary focus of exploration activity for the onshore middle Eocene Gulf Coast in the coming decades.

Abstract: Surface to Subsurface Correlation of the Claiborne and Jackson Groups, Colorado River to Trinity River

ABSTRACT Detailed correlation of dip and strike sections discloses the stratigraphic architecture of the Claiborne and Jackson Groups, long studied in surface outcrop because of well-exposed fossiliferous horizons. Carrying sections updip to within a few miles of outcrop allows the detailed surface stratigraphic work to be carried into the subsurface and compared with names and ideas used in petroleum and natural gas exploration. The stratigraphic sequence is analyzed and divided by recognition of flooding surfaces (Galloway, 1989). These surfaces can commonly be recognized on electrical logs and objectively correlated. Sequence boundaries can in some cases be recognized between flooding surfaces but are commonly cryptic and inferred. Major flooding surfaces (MFS) with underlying thick transgressive deposits (greensandstone, marl, and thin reworked sandstones) divide the succession into four major genetic stratigraphic units (see Fig. 1). The Queen City genetic unit begins at the MFS at the top of the Wilcox, marked by the top of a persistent marly zone equivalent to the Newby Member of the Reklaw Formation in outcrop. The Queen City is usually represented by two units formed by major sand deposition near the present outcrop, grading to equivalent silts and shales downdip. At its top is a thin unit of reworked and transgressive deposits, equivalent to part of the Weches Formation of outcrop. The Sparta genetic unit begins at a low-resistivity shale within the Weches. A basal progradational shale is succeeded by a thick, sharp-based sandstone. The upper part of the unit consists of a thick transgressive deposit correlative with the Stone City Beds of Stenzel (1938; Stenzel et al., 1957), which is usually placed as the basal member of the Cook Mountain (equivalent to Crockett) Formation in outcrop. Both Queen City and Sparta cycles show little evidence of internal sequence boundaries, such as incised channels, lowstand delta complexes, or slope fans. The Yegua genetic unit begins at the clearly defined top of the Stone City transgressive deposits. The unit is internally complex, with 10 regional flooding surfaces. The lowest internal cycle (equivalent to Yegua 90 in the Wharton County nomenclature [see Ewing, 1994]) includes the entire outcrop of the Cook Mountain Formation. Downdip this sequence is represented by about 200 ft of shelfal siltstone and shale; insertion of a silty or limy unit on a scoured surface yields a distinctive marker, which can be traced widely in a downdip position. Overlying this cycle are nine cycles that are either eroded toward the basin margin or pass landward into deltaic and nonmarine phases. Six of these packages (all above Yegua 70) appear to contain sequence boundaries, as evidenced by incised channels and downdip shelf-edge lowstand systems tracts. All of this complexity is represented in outcrop only by the nonmarine sandstone and lignitic shale of the Yegua Formation. The top of the Yegua genetic unit is a thin transgressive deposit, which is correlative to the Moodys Branch marl of the eastern Gulf Coast Basin. This unit, although distinctive in the subsurface, is not noted from surface exposures in Texas. The Fayette genetic unit (Fayette is used instead of Jackson to avoid homonymy of stage/group and genetic units) begins at the top of the Moodys Branch. Marine shales at the base (Caddell Formation of outcrop) grade upward into marginal marine sandstones (the Wellborn Sand of outcrop) and landward into lignite-bearing nonmarine deposits of the Manning Formation. Locally, a basinward-shifted sandstone in the Textularia hockleyensis zone overlying an unconformity suggests a minor sequence boundary, possibly correlative to the Cocoa Sand of the eastern Gulf (Baum and Vail, 1988). Marine deposits overlie the Fayette clastic wedge in the subsurface, but they are truncated by the sub-Vicksburg and sub-Frio/Catahoula unconformities before reaching outcrop. Two important shelf processes may be noted on the cross sections. First, a large amount of mud was deposited far seaward of the sand depocenters of all units. Because of this, genetic units do not thin markedly seaward (although they do contain internal clinoforms). Second, discrete packages of highly resistive materials (limy shale or siltstone) with sharp bases are inserted into the distal parts of the genetic units. These units can be tens to several hundred feet thick, and they form useful correlation markers for substantial areas. They may represent scour or slump fill deposits. The surface outcrops along the basin margin are useful, if limited, windows into basin sedimentation and have provided fertile ground for paleontological and sedimentological research. The full potential of these studies can only be realized when outcrop data can be placed in the basinwide context provided by detailed subsurface correlation. End_Page 233------------------------ Figure 1. Regional stratigraphic section from outcrop in Lee County to a point near the Yegua shelf margin in Wharton County, Texas. Vertical exaggeration is 25. MFS = maximum flooding surface; outline names = major genetic units; boxed names = transgressive units at MFS. Foraminifer tops: iTw = in Textularia warreni; Jx = First Jackson; Mc = Marginulina cocoaensis; Mp = Massilina pratti; Th = Textularia hockleyensis; Td = Textularia dibollensis; Gsi = Globigerinatheka semiinvoluta; Nc = Nonionella cockfieldensis; Dy = Discorbis yeguaensis; Ey = Eponides yeguaensis; Nm = Nodosaria mexicana; Au = Anomalina umbonatus; Gb = Globorotalia bullbrooki; Ce = Ceratobulimina eximia; Cg = Clavulinoides guayabalensis; Tt = Truncrotalinoides topilensis; Os = Operculinoides sabinensis; AB = Anomalina B; BB = Bifarina B; Gbr = Globorotalia broedermanni; Tsm = Textularia smithvillensis; GA = Gyroidina A; CE = Cyclammina E; Hys = Hystracospheridium stenzeli; CF = Cibicides F; Gar = Globorotalia aragonensis. End_Page 234------------------------

Late Paleozoic Structural Evolution of Permian Basin: ABSTRACT

The Permian basin of West Texas and New Mexico is one of the premier hydrocarbon provinces of the world; nonetheless, little regional subsurface structural information about it has been published. Mapping at 1:250,000 on the Ellenburger horizon (Lower Ordovician), compiled for the Tectonic Map of Texas, discloses the overall geometry of Paleozoic deformation in the area. The southern Permian basin is underlain by the NNW-trending Central Basin disturbed belt of Wolfcamp age (Lower Permian), the deep Delaware basin to its west, and the shallower Midland basin to its east. The disturbed belt is highly segmented with zones of left-lateral offset. Major segments from south to north are: the Puckett-Grey Ranch zone; the Fort Stockton uplift; the Monahans transverse zone; the Andector ridges and the Eunice ridge; the Hobbs transverse zone; and the Tatum ridges, which abut the broad Roosevelt uplift to the north. East-west compression is inferred, with shortening increasing from the Tatum ridges south to the Fort Stockton uplift. The segment boundaries and transverse elements are inferred zones of strike-slip faulting. These fault zones extend both southeast and west of the disturbed belt into discrete strike-slip faults with local uplifts in compressive bends (such as the Big Lake uplift). The Midland basin is much shallower than the Delaware basin, and the uplift-to-basin transition is gradual. A belt of subtle domes and anticlines, extending northeast from Andrews County, overlies a major basement discontinuity (the Grenville Front). The disturbed belt may have originated along rift zones of either Precambrian or Cambrian age. The extent of Lower and Middle Pennsylvanian deformation is unclear; much of the Val Verde basin-Ozona arch structure may have formed then. The main Wolfcamp deformation overthrust End_Page 474------------------------------ the West Texas crustal block against the Delaware block, with local denudation of the uplifted edge and eastward-directed backthrusting into the Midland basin. Later in the Permian, the area was the center of a subcontinental bowl of subsidence--the Permian basin proper. The disturbed belt formed a pedestal for the carbonate accumulations which created the Central Basin platform. The major pre-Permian reservoirs of the Permian basin lie in large structural and unconformity-bounded traps on uplift ridges and domes. Further work on the regional structural style may help to predict fracture trends, to assess the timing of oil migration, and to evaluate intrareservoir variations in the overlying Permian giant oil fields. End_of_Article - Last_Page 475------------

Hackberry Oil and Gas Fields in Southeast Texas: Channel/Fan Depositional Systems and Structural Controls: ABSTRACT

Deep-water sandstones of the Hackberry Formation (Oligocene) host significant quantities of oil and gas. They remain one of the most important deep exploration targets in southeasternmost Texas; new fields producing from the Hackberry have been discovered at a steady rate from 1946 to the present. The Hackberry contains two hydrocarbon plays. The updip play is relatively shallow, oil-rich, and lies near the updip limit of deep-water deposition. Some of the fields in this play actually produce from shallow-water Frio sandstones of Hackberry age rather than from Hackberry sandstones. The downdip play is gas rich and generally geopressured. The reservoirs lie either within or on the flanks of major channel systems and are often bounded updip by small growth faults. The discontinuous distribution and complex lithofacies of these channel and fan sands demand a careful understanding of the component depositional environments in order to discover and efficiently produce hydrocarbons. The Hackberry Formation is a wedge of sand and shale with bathyal fauna that separates upper Frio sandstone and shale from middle and lower Frio shale and sand. The main sandstone lies atop a channeled unconformity at the base of the formation; some sandstones are also found locally within the shale wedge. Sandstones in a typical sand-rich channel evolve upward from a basal channel-fill sand to more widespread valley-fill deposits of interbedded sand and shale. Topmost are proximal to medial fan deposits with slightly meandering channels and overbank turbidites. This sequence suggests that the Hackberry sands were laid down by an aggrading, onlapping submarine canyon-fan complex that eroded headward into the contemporaneous Frio barrier bar-strand plain. Regional mapping and seismic i terpretation outlines a network of partly sand-filled channels extending from the strand plain toward the southeast. The downdip limits of lower Hackberry sand are not defined by available well data. The early structural history of the area is obscure, but Vicksburg-age faulting associated with continental slope sedimentation is possible. Small growth faults displace the Hackberry section less than 500 ft (150 m) and extend upward into the Miocene strata. Isopach and isolith maps indicate that the Orange, Port Neches, and Fannett salt domes were active uplifts during Frio and Anahuac deposition. Near Spindletop dome, however, only a north-south trending salt-cored ridge was present. The Hackberry channels are somewhat influenced by salt activity, but major channel axes extend across the uplifts. The genesis of the deep-water Hackberry embayment is obscure. Middle Frio strata underlying the Hackberry are neritic shelf deposits to the west but may include deeper water shales in the central and eastern parts of the area. The embayment may have formed by subsidence of a large part of the Frio-Vicksburg continental shelf with consequent canyon erosion. Alternatively, the Hackberry canyons may be analogous to canyons currently forming on the flanks of the Niger delta in an entirely deep-water regime. End_of_Article - Last_Page 458------------