Maverick basin

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Fig.1 Location of Maverick basin


History of the basin

Located in south central Texas, the Maverick basin is a small carbonate sedimentary basin which is located at the southern edge of the Eagle Ford Basin and is a product of the jurassic to the late cretaceous geological time period. As seen in Fig.1, The Maverick encompasses the eastern part of Coahuila, a state in Mexico and ranges across the Rio Grande River covering areas in Val Verde, Uvalde, Kinney, Zavala, Dimmit, and Maverick counties in south central Texas. Shown in Fig.2, the Maverick basin is a neighboring formation to the more established and active Gulf Coast and Eagle Ford basins of south Texas and has long been overlooked by oil companies. Due to it's extremely small range compared to these more readily available plays located in the nearby Gulf Coast basin, Permian basin and Eagle Ford basin, the Maverick basin has not received much interest in recent years. [1] Similar to these adjacent basins, the Maverick can display various sandstone, carbonate, shale, and coal sedimentary formations which create many areas for potential reserves. [2] The Maverick possesses a unique set of formations which display a broad range of various depositional environments and potential reservoirs. Though multiple of the Maverick formations present potential reserves, the highests prospects of this basin are the Eagle Ford formation and the Pearsall Shale Formation. [3] Due to the introduction of newly developed drilling technologies such as 3-D seismic and directional drilling, since 2006, the Maverick basin has become increasingly active in recent years.

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Fig.2 Maverick Basin neighboring Gulf Coast Basin

Geological Risks

The two highly targeted formations of the Maverick basin are the Eagle ford shale and the Pearsall shale formations. The Eagle Ford formation contains source rock with generally higher percentage carbonate concentration than clay. This characteristic of the formation results in a very brittle formation and therefore makes it easy for companies to access the formation hydrocarbons through hydraulic fracturing. The ease of access and high availability makes the Eagle ford formation a top target for companies drilling in the Maverick basin. Subsequently to the high production rates that are generated from the Eagle Ford formation, steep declines rates pose a serious threat to operators. Shale plays such as the Eagle Ford are well known for extremely steep decline rates. The steep decline rates of wells results in continued production replacement by companies and can eventually result in nearby wells affecting one another (parent-child well interference). When one well is drilled (parent) and being produced, drilling a nearby well (child) can create different underground disturbances in pressure and other characteristics which can in turn affect the parent well. Though the Pearsall shale formation has not been as highly developed as the Eagle Ford, the two formations share the same characteristics and pose the same threats to operators. In r

Petroleum Geology

Seal

As seen in Fig.3, the Eagle Ford formation is rested below the Austin Chalk and above the Buda formations. Both of these neighboring formations possess low permeability and porosity creating an excellent seal for the Eagle Ford. This carbonate seal created by the Austin Chalk is easily fractured to allow the flow of hydrocarbons from the below, Eagle Ford formation. The other common target in the Maverick, the Pearsall shale formation is below the Glen Rose and above the Sligo formations found throughout the Maverick basin. Similarly to the Austin Chalk, the Glen Rose formation contains low permeability and porosity and acts as a carbonate seal to the Pearsall formation.

Fig.3 Eagle Ford source rock stages

Source Rock & Migration

[5] Though both the Eagle Ford and the Pearsall are named "shale" formations, they primarily consist of carbonate materials (limestone). This makes these formations have low permeability and porosity. The organic rich limestone source rock of the Eagle Ford formation provides exploration companies with an easy and sure target in the area of the Maverick Basin. Hydrocarbons within the Eagle Ford formation are sealed off by the surrounding formations, Austin Chalk and Buda. These hydrocarbons are forced to migrate from deep in the ground, high pressure areas and are pushed upwards towards the surface through the formation. The hydrocarbons are trapped until they reach a fracture in the formation where they escape and are extracted and produced.

Reservoir

The low permeability and low porosity characteristics present in the reservoirs throughout the Maverick basin make them great areas for the storage of hydrocarbons. The Austin Chalk formation lies above the hydrocarbon rich Eagle Ford formation and provides a great reservoir seal, keeping hydrocarbons contained in the Eagle Ford. The high carbonate content within the formations and low presence of clays makes for very brittle formations, making these locations ideal for hydraulic fracturing and easily accessible hydrocarbons. The source rocks present in the target formations of the Maverick contain high quality contents and provide quailty oil/gas contents.

Trap

Both structural and stratigraphic traps can be found throughout areas of the Maverick basin. The more common method of traps that is found is stratigraphic trapping which requires companies to implement hydraulic fracturing and the low permeability/porosity of most of these formations requires large amounts of fresh water in fracking. [6] Shown in Fig.3, is the trapping of Eagle Ford hydrocarbons during updip migration. The low permeability and porosity of the Austin Chalk formation and the Buda formation keep the hydrocarbons of the Eagle Ford trapped for storage. [5.] Aside from this natural trap, hydrocarbons within the formation naturally migrate upwards until they encounter a fracture in the formation.

Future Petroleum Potential of the Maverick basin

[1] Being that the Maverick basin has been so underdeveloped compared to surrounding basins, there is still much to be determined from the underlying formations. Studies from the U.S. geological survey indicate that of the 5.5 billion cubic ft. of natural gas already produced from the Maverick Basin, there still remains an estimated 8.8 trillion cubic ft. to be discovered. Along with the high volumes of gas left to be discovered in this play, there is just as much room for oil exploration in the Maverick. Recent advancements of 3-D seismic and directional drilling has made it possible for companies to explore more undeveloped areas such as the Maverick. These technologies reduce the exploration risk and uncertainties for companies, allowing them to target these smaller areas more accurately and cost effectively. [7] The production mix of the Maverick basin tends to be about 50% crude oil, 20% natural gas liquids, and 30% natural gas production.

Petroleum/Facility Engineering Aspects of the Maverick basin

With the main target formation being the Eagle Ford formation and it being such a shallow, easily accessible formation, this makes the Maverick basin one for easy drilling and easily accessible hydrocarbons. Recent implementation of drilling technologies like hydraulic fracturing and horizontal drilling has made it possible for companies to easily target formations such as the Eagle Ford. The Pearsall formation however lies substantially lower the the Eagle Ford and poses more threats to drilling operations. Both of these formations contain low permeability and porosity which makes them easily fracturable and producible, the only downside to the Pearsall formation is it being a much deeper operation and more complex drilling operation. Companies who drill in the area tend to turn their attention more towards the Eagle Ford being that it is much shallower and a very hydrocarbon rich zone. With the low permeability/porosity Austin Chalk formation lying above the target formation, this presents engineers with an easy frac job to reach the target hydrocarbons. The hydrocarbons within the Eagle Ford are forced to rise to the surface by a pressure differential created by the fracturing of the formation, these hydrocarbons escape the formation through the fractures created where they are extracted by the production company.

Further Reading

References

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  1. 1.0 1.1 Clarke, R. (2009, January 1). Maverick Basin Feature. Hart Energy. https://www.hartenergy.com/news/maverick-basin-feature-77845
  2. Cook, T. D. (1979, January 1). Exploration History of South Texas Lower Cretaceous Carbonate Platform. AAPG Bulletin. https://pubs.geoscienceworld.org/aapgbull/article-abstract/63/1/32/36029/Exploration-History-of-South-Texas-Lower?redirectedFrom=PDF.
  3. Scott, R. J. (1970, January 1). EXTENDED ABSTRACT: The Maverick Basin: New Technology - New Success. AAPG Datapages/Archives. http://archives.datapages.com/data/gcags/data/054/054001/603.htm.
  4. More operators eye Maverick shale gas, tar sand potential. StackPath. (2007, August 13). https://www.ogj.com/general-interest/companies/article/17228708/more-operators-eye-maverick-shale-gas-tar-sand-potential.
  5. Eagle Ford Shale Geology. Eagle Ford Shale Play. (n.d.). https://eaglefordshale.com/geology/.
  6. PennPetro Energy. Go to PennPetro Energy. (n.d.). https://www.pennpetroenergy.co.uk/operations-assets/geology-reserves/.
  7. Dukes, R. T., & DuBose, K. (2012, February 22). Maverick Basin - Eagle Ford Shale News. Eagle Ford Shale Play. https://eaglefordshale.com/efs-news/tag/Maverick+Basin.
  8. Whaley, J., 2017, Oil in the Heart of South America, https://www.geoexpro.com/articles/2017/10/oil-in-the-heart-of-south-america], accessed November 15, 2021.
  9. Wiens, F., 1995, Phanerozoic Tectonics and Sedimentation of The Chaco Basin, Paraguay. Its Hydrocarbon Potential: Geoconsultores, 2-27, accessed November 15, 2021; https://www.researchgate.net/publication/281348744_Phanerozoic_tectonics_and_sedimentation_in_the_Chaco_Basin_of_Paraguay_with_comments_on_hydrocarbon_potential
  10. Alfredo, Carlos, and Clebsch Kuhn. “The Geological Evolution of the Paraguayan Chaco.” TTU DSpace Home. Texas Tech University, August 1, 1991. https://ttu-ir.tdl.org/handle/2346/9214?show=full.