Fort Worth Oil Basin

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Introduction

The Fort Worth Oil Basin, located from northern Austin to Wichita Falls, north to south, and from sweet-water to eastern Dallas, east to west, located East of the permian basin has been under production since 1917. The Mississippian Shale serves as source, seal, and reservoir to a world-class unconventional natural-gas accumulation in the Fort Worth Basin of north-central Texas. The formation is a lithologically complex interval of low permeability that requires artificial stimulation to produce. At present, production is mainly confined to a limited portion of the northern basin where the Barnett Shale is relatively thick, organic rich, thermally mature, and enclosed by dense limestone units able to contain induced fractures. Recent assessment by the U.S. Geological Survey suggests a mean volume of 26.2 tcf of undiscovered, technically recoverable gas in the central Fort Worth Basin. Recovery of a significant portion of this undiscovered resource will require continued improvements in geo-scientific characterization and approaches to stimulation of the Barnett reservoirs. Also currently referred to as the "Bend-Arch" basin, its important to remember that because it is partially related to the burial history of the hydrocarbon[[1]] formations and deposition. Most formations that oil and gas is produced from were formed in the Mississippian Era over 300 million years ago.

Map-of-the-Permian-and-Palo-Duro-Basins-and-Bend-arch-Fort-Worth-Basin-in-Texas-New.png

History of the Basin

The North central location of the Fort Worth Basin has been a sort of mystery in terms of formation and recognition. The Fort Worth Basin is a major shale-gas producer formed by thrust fold belts from separate tectonic formations. The depositional patterns of the basin during the late Paleozoic were studied by correlating well logs and constructing structure and isopach maps. Then, using one-dimensional and two-dimensional subsidence history of the basin and constrained its relationship to the Ouachita orogen. Because the super-Middle Pennsylvanian strata were largely eroded in the region, adding uncertainty to the subsidence reconstruction, using 1-D to conduct thermal-maturation modeling to constrain the post-Middle Pennsylvanian burial and exhumation history by matching the modeled vitrinite reflectance with measured vitrinite reflectance along five depth profiles[1].

                                                                                                        Fort worth hydrocarbon layers.jpeg

The depositional patterns show that the tectonic uplift of the Muenster uplift to the northeast of the basin influenced subsidence as early as the Middle Mississippian, and the Ouachita orogen became the primary tectonic load by the late Middle Pennsylvanian when the depocenter shifted to the east[1]. The results show that the basin experienced (12,100–17,100 ft) of burial during the Pennsylvanian, and the burial depth deepens toward the east. The causes of deep Pennsylvanian burial and its spatial variation to flexural subsidence that continued into the Late Pennsylvanian in response to the growth of the Ouachita orogen and southeastward suturing of Laurentia and Gondwana[1]. The modeling results also suggest that the Mississippian Barnett Shale reached the gas maturation window during the Middle–Late Pennsylvanian[1]. Which explains why the hydrocarbons produced are over hundreds of millions years old.

Geological Risks

A problem in the past and what could be present in the future is the increased seismic activity within faults that could possibly lead to a small earthquake. Recent research results from the "TexNet Seismic Monitoring Network" and "Center for Integrated Seismic Research(CISR)" (located at the University of Texas at Austin and SMU) provide new insights into the potential for earthquakes in the Fort Worth Basin in north-central Texas. While the new map provides a better understanding of the seismic hazards beneath one of the nation’s most heavily populated areas, according to Hennings there is still more resolution to be gained in their study of seismic hazards[2]. With this being said, it's not much of a worry as earthquakes are a rare sighting in Texas however with some of this new information, small seismic fault slips could lead to small tectonic activity. The below image gives a contour layout of the Basin itself. With the technology of modern day fracking, it has been proven to serve incredible returns of oil we use everyday. However the down side to this is the off chance of horizontal fracking into a fault causing seismic activity possibly leading to a small earthquake. This being an issue as the DFW metroplex is one of the largest and still continuing to grow in population and in return brings in people from out of state, (California Mostly) that bring in ideas about fracking that are false and want all horizontal programs expelled.

Countour of ftw basin.png

Petroleum System

SEAL- Seal rocks in the basin are mostly shale units and dense, low permeability carbonate rock that are distributed on both regional and local scales. Although these formations are not considered seal rocks in areas where they are tight and not water wet, they serve as barriers confining hydraulic-induced fracturing (frac barriers) and help retain formation pressures during good stimulation.

SOURCE ROCK- Range from clastic to carbonate rocks, oil sourced straight from the Barnett shale, Caddo formation, canyon group, and more, different formations within have given off more hydrocarbons than others. Mississippian Barnett shale is the most producing shale play in the Fort Worth Basin and releases the most amount of oil and gas to use.

TRAPS- Traps for conventional hydrocarbon accumulations are mostly stratigraphic for carbonate rock reservoirs and both structural and stratigraphic in clastic-rock reservoirs. Stratigraphic traps in carbonate rocks result from a combination of facies and depositional topography and erosion of facies, and enhanced permeability and porosity zones. A good example of a carbonate stratigraphic trap is the pinnacle reef traps of the Chappel Limestone, where local porous grain-stone and packstone are restricted to isolated buildups or reef clusters on the eroded Ellenburger Group. Chappel pinnacle reefs are draped and sealed by the overlying Barnett Shale[3].

Source rock fort worth basin.jpeg

Geologic Uncertainties

With the presence of the Fort Worth Basin being since early 1900s its been a proven oil and gas basin and has produced a healthy amount for American consumption. however it is still estimated there be 26 TCF of natural gas remaining and is only available with the advancements in geological technology. The U.S. Geological Survey estimated a mean volume of about 26 tcf of undiscovered, technically recoverable hydrocarbon gas in the Barnett Shale. The Mississippian Barnett Shale is the primary source rock for oil and gas produced from Paleozoic reservoir rocks in the Bend arch Fort Worth Basin area and is also one of the most significant gas-producing formations in Texas. It seems that going forward in every day extraction is going to start increasing in difficulty as the days pass by. New technology is needed to truly be able to fully extract every last possible resource from this basin which makes it a slightly risky play. As politics start to take a swing in the direction of anti-fracking, this makes it tough for large corporations to continue what they have started under new legislation.

Future State of the Basin

In a recent interview with Ed Ireland, executive director of the Barnett Shale Energy Education Council and economics professor at Texas Christian University, he speaks on the current situation in the Barnett shale play in the Fort Worth Basin. In this interview ED conveys that the drilling has been on a slow decline since 2008, he states that no one is coming for their natrual gas and that if wells are being produced lately it is for liquid natural crude oil. When asked he states, "The problem is with a large portion of the Barnett Shale is dry natural gas – there aren’t any liquids. And in the major part of the Barnett Shale, in Montague County, there is some crude oil production and in some other parts of the Barnett Shale there is some natural gas liquids production but a lot of the Barnett Shale is dry natural gas and that’s just not economical at this stage"[4]. Stated by Ed he begins to share the ideas or refracking wells to see if they can bring some more results during their dry times of not finding as much liquid crude oil.

This may come at quite a cost though financially and image wise to big oil companies considering fracking is trending in the negative direction. People form their own opinions which are mostly not true and are persistent on trying to de rail companies from further horizontal fracking.

Petroleum/Facility Engineering Aspects

With the main target being the Barnett Shale of the Fort Worth Basin, it has made fracking to drill for the large areas of gas slightly difficult. Since the Barnett shale is low in porosity, it has taken excessive amounts of enhanced production techniques like fracking to be able to remove this substance from the ground. Fluid injection is extremely common, proppant such as sand is used most to take over hydraulic frack jobs to be able to reach all the resources available underground. These induced fractures are on the order of 0.1 to 0.25 in. thick and filled with sand or other high permeability materials (called proppant) so that they remain open and can conduct the gas to the well .

With the novel idea of gel fracking being discovered in the 1980's, this brought profitable results back to the basin after a short dry period. The idea of re-fracking wells has been in order and may be the next big thing to keep the basin competing with the rest. Since COVID-19 most rigs have been inoperable however with the few rigs still running the Barnett shale of the Fort Worth Basin have been able to still compete as a top 5 natural gas well. As the prices of oil come back after a devastating drop, rigs should be up and running again as well as profits.

The starting costs of the well are much cheaper than the continuing process of hydraulic fracking, due to the amount of water and proppant needed to retrieve these carbon rich resources. The pay off never seems to be worth it which is why most wells are Natrual gas wells, which seem to produce more although being at a lower cost.

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Further Reading

https://rc.library.uta.edu/uta-ir/bitstream/handle/10106/24705/AlSalem_uta_2502M_12753.pdf?sequence=1

https://energi.media/markham-on-energy/state-oil-patch-aint-pretty-barnett-shale-future-looks-better/

https://www.sciencedirect.com/topics/engineering/barnett-shale

References

1. https://archives.datapages.com/data/bulletns/2005/02feb/0155/0155.HTM

2. https://pubs.geoscienceworld.org/aapgbull/article-abstract/101/11/1813/520592/Late-Paleozoic-subsidence-and-burial-history-of?redirectedFrom=PDF

3. https://en.wikipedia.org/wiki/Bend_Arch%E2%80%93Fort_Worth_Basin#Fort_Worth_Basin

4. https://www.beg.utexas.edu/node/4443

5. https://www.researchgate.net/publication/249897890_Geologic_framework_of_the_Mississippian_Barnett_Shale_Barnett-Paleozoic_total_petroleum_system_Bend_arch-Fort_Worth_Basin_Texas

6. https://www.nrc.gov/docs/ML1131/ML11314A197.pdf

7. https://pubs.er.usgs.gov/publication/70029077

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