The Bend Arch–Fort Worth Basin Province is a major petroleum producing geological system which is primarily located in North Central Texas and southwestern Oklahoma. It is officially designated by the United States Geological Survey (USGS) as Province 045 and classified as the Barnett-Paleozoic Total Petroleum System (TPS).
Introduction Oil and gas in Province 045 are produced from carbonate and clastic rock reservoirs ranging in age from the Ordovician to the Permian. The 1995 USGS Assessment of undiscovered, technically recoverable oil and gas identified six conventional plays in Province 045, which are listed below in Table 1:Ball, 1996 One continuous unconventional play, the "Mississippian Barnett Shale" (4503), was also considered. The cumulative mean of undiscovered resource for conventional plays was: of oil, of natural gas liquids, associated gas, and non-associated gas.
Table 1 1995 Play No. 1995 Play Designation 2003 AU 2003 Proposed AU Designation 4501 Pre-Mississippian 1 Ordovician Carbonate 4502 Mississippian Carbonate 2 Mississippian Pinnacle Reef 4504 Low-Pennsylvanian Sandstone & Conglomerate 3 Pennsylvanian Fluvial-Deltaic Sandstone & Conglomerate 4505 Strawn (Desmoinesian) 4 Pennsylvanian Fluvial-Deltaic Sandstone & Conglomerate 4505 Post Desmoinesian 5 Upper Pennsylvanian/Permian Clastic 4503 Mississippian Barnett Shale (Hyp) 6 Greater Newark East Fractured Siliceous Shale 4503 Mississippian Barnett Shale (Hyp) 7 Ellenburger Subcrop Fractured Barnett Shale 4503 Mississippian Barnett Shale (Hyp) 8 North Basin and Arch Fractured Shale Notes: 1. Assessment unit number also indicates time span of stratigraphic units.
The United States Geological Survey's assessment of undiscovered conventional oil and gas and undiscovered continuous (unconventional) gas within Province 045 resulted in estimated means of (Tcf) of undiscovered natural gas, of undiscovered oil, and a mean of of undiscovered natural gas liquids. Nearly all of the undiscovered gas resource (98%, or is considered to be in continuous accumulations of nonassociated gas trapped in strata of two of the three Mississippian-age Barnett Shale Assessment Units (AUs) - the Greater Newark East Frac-Barrier Continuous Barnett Shale Gas AU and the Extended Continuous Barnett Shale Gas AU combined). The remaining of undiscovered gas resource in the province is in conventional nonassociated gas accumulations or and associated/dissolved gas in conventional oil accumulations ( or . The Barnett-Paleozoic TPS is estimated to contain a mean of of conventional gas, or about 88% of all undiscovered conventional gas, and about of conventional oil, or about 65% of all undiscovered oil in Province 045.
Continuous-type accumulations include fractured shale and fractured limestone oil and gas, basin-centered gas, coal bed gas, and tight reservoir gas. They typically cover large areas, have source rocks in close association with these unconventional reservoir rocks, and are mostly gas (and in some cases oil) charged throughout their extent.Schmoker, 1996 Continuous accumulations commonly have transition zones that grade into more conventional accumulations.Pollastro, 2001
Boundary
The Fort Worth Basin and Bend Arch lie entirely within North Central Texas covering an area of . The southern and eastern boundaries are defined by county lines that generally follow the Ouachita structural front, although a substantial portion of this structural feature is included near Dallas. The north boundary follows the Texas-Oklahoma State line in the east, where the province includes parts of the Sherman Basin and Muenster Arch. In the west, the north boundary follows the north-east county lines of Oklahoma's three southwestern counties (Harmon, Jackson and Tillman Counties), which include the south flank of the Wichita Mountains and the Hollis Basin. The western boundary trends north-south along county lines defining the junction with the Permian Basin where part of the eastern shelf of the Permian Basin lies in Province 045.
Structural elements Major structural features include the Muenster and Red River Arches to the north, and the Bend and Lampasas Arches along the central part of Province 045. Along the east portion is an area that includes the Eastern Shelf and Concho Arch, collectively known as the Concho Platform. The Mineral Wells fault runs northeast-southwest through Palo Pinto, Parker, Wise, and Denton Counties and joins with the Newark East fault system. The fault system bisects the Newark East Field (NE-F) creating a zone of poor production in Barnett Shale gas reservoirs. Several faults that cut basement and lower Paleozoic rocks in the southern part of the province are identified at the Ordovician Ellenburger Group stratigraphic level. These faults and associated structures formed during development of the Llano Uplift and Fort Worth Basin with faulting ending by the early Missourian.Flippen, 1982
Tectonic history
Fort Worth Basin Evolution of the Fort Worth Basin and Bend Arch structures are critical to understanding burial histories and hydrocarbon generation. The asymmetrical, wedge-shaped Fort Worth Basin is a peripheral Paleozoic foreland basin with about of strata preserved in its deepest northeast portion adjacent to the Muenster Arch and Ouachita structural belt. The basin resembles other basins of the Ouachita structural belt, such as the Black Warrior, Arkoma, Val Verde, and Marfa Basins that formed in front of the advancing Ouachita structural belt as it was thrust onto the margin of North America. Thrusting occurred during a late Paleozoic episode of plate convergence.
Bend Arch The Bend Arch extends north from the Llano Uplift. It is a broad subsurface, north-plunging, positive structure. The arch formed as a hingeline by down-warping of its eastern flank due to subsidence of the Fort Worth Basin during early stages of development of the Ouachita structural belt in the Late Mississippian and west tilting in the late Paleozoic which formed the Midland Basin. There is disagreement on the structural history of the Bend Arch. Flippen (1982) suggested it acted as a fulcrum and is a flexure and structural high and that only minor uplift occurred in the area to form an erosional surface on the Chester-age limestones that were deposited directly on top of the Barnett. In contrast, Cloud and Barnes (1942) suggested periodic upwarp of the Bend flexure from mid-Ordovician through Early Pennsylvanian time resulted in several unconformities. The Red River Arch and the Muenster Arch also became dominant structural features during the Late Mississippian and Early Pennsylvanian.
General stratigraphy Hydrocarbon production from Ordovician and Mississippian formations is mostly from carbonate reservoirs, whereas production in the Pennsylvanian through Lower-Permian Wolfcamp) is mostly from clastic reservoirs. The sedimentary section in the Fort Worth Basin is underlain by Precambrian granite and diorite. Cambrian rocks include granite conglomerate, sandstones, and shale that are overlain by marine carbonate rocks and shale. No production has been reported from Cambrian rocks. The Silurian, Devonian, Jurassic, and Triassic are absent in the Fort Worth Basin.
From Cambrian through Mississippian time, the Fort Worth Basin area was part of a stable cratonic shelf with deposition dominated by carbonates. Ellenburger Group carbonate rocks represent a broad epeiric carbonate platform covering most of Texas and central North America during the Early Ordovician. A pronounced drop in sea level sometime between Late Ordovician and earliest Pennsylvanian time, perhaps related to the broad, mid-North American, mid-Carboniferous unconformity, resulted in prolonged platform exposure. This erosional event removed any Silurian and Devonian rocks that may have been present.Henry, 1982 The Barnett Shale was deposited over the resulting unconformity. Provenance of the terrigenous material that constitutes the Barnett Shale was from Ouachita thrust sheets and the reactivation of older structures such as the Muenster Arch. Post-Barnett deposition continued without interruption as a sequence of extremely hard and dense limestones were laid down. These limestones have often been confused with the lower part of the overlying Marble Falls Formation (Early Pennsylsvnian), and they have never been formally named, although they are widely referenced in the literature as the "Forestburg Formation."Henry, 1982 Since the underlying Barnett is generally assumed to be Late Mississippian Chester in age, the superposed Forestburg is occasionally referred to informally as "the Chester Limestones."
As the shallow Late Mississippian seas spread southward and westward from the subsiding Southern Oklahoma Aulacogen, they inundated an uneven Lower Paleozoic surface and almost immediately initiated the growth of reef-forming organic communities. All of the Mississippian-age reef complexes whose bases have been penetrated by boreholes have been found, without exception, to be resting directly upon the underlying Ordovician rocks. But although reef growth began at the same time as Barnett Shale deposition, the reefs did not survive to the end of Barnett time; all known Chappel reefs are immediately overlain by the typical Barnett Shale facies except for a very few in central Clay County that have been very deeply breached by pre-Atokan erosion. The reef complexes are subdivisible into three constituent facies: the reef core, the reef flanks, and the inter-reef area. The reef cores are porous enough to serve as stratigraphic traps for oil and gas, and they have yielded excellent production in the northern part of the Fort Worth Basin for three-quarters of a century. The Chappel buildups are often referred to as "pinnacle reefs," but that is a misnomer. They may appear as pinnacles on a cross section with an exaggerated vertical scale (see cross sections A-A′ and B-B′ above), but in reality they have almost exactly the same height/width aspect ratio as a fried chicken egg sunny side up. The reef core, of course, is represented by the egg yolk, and the reef flank debris are represented by the egg white. The inter-reef facies is represented by a black, calcareous, bituminous shale. Where it occurs in Jack County it is typically 30 to 40 feet (9 to 12 meters) thick, and it is synonymous with the calcareous basal shale member of the Barnett. Consequently, the proximity of a given borehole to a nearby reef complex can be qualitatively estimated by the degree to which this lower member of the Barnett has been impregnated with calcite. Henry, 1982
Clastic rocks of provenance similar to the Barnett dominate the Pennsylvanian part of the stratigraphic section in the Bend Arch–Fort Worth Basin. With progressive subsidence of the basin during the Pennsylvanian, the western basin hinge line and carbonate shelf, continued migrating west. Deposition of thick basinal clastic rocks of the Atoka, Strawn, and Canyon Formations occurred at this time.Walper, 1982 These Mid- and Late Pennsylvanian rocks consist mostly of sandstones and conglomerates with fewer and thinner limestone beds.
Petroleum production history