Western Gulf Basin
- 1 Introduction
- 2 History
- 3 Geologic History
- 4 Conventional and Unconventional Play Characteristics
- 5 Current Drilling Activity
- 6 Austin Chalk Formation
- 7 Eagle Ford Play
- 8 Petroleum and Facility Engineering
- 9 References
- 10 See Also
The Western Gulf Basin is a foreland basin spanning the area of Louisiana south of the Lower Cretaceous shelf edge as well the area south and east of the Ouachita Fold Belt in Texas. Furthermore, the Western Gulf Basin extends southwest to the Texas-Mexico border. In all, the basin is 116,599 square miles. At the moment, the Western Gulf Basin is considered one of the most heavily explored sedimentary basins in the United States. As of 2019, the Western Gulf Basin, also commonly referred to as the Gulf Coast Basin, had proved natural gas reserves of 26.6 trillion cubic feet. The primary and current drilling play in the Western Gulf Basin is the Eagle Ford Shale play, which has redefined Southern Texas as a breeding ground for hydrocarbon production. Moreover, the Eagle Ford play resource potential is estimated to be as high as 25 to 30 billion bbl of oil.
Commercial Production of the Western Gulf Basin was largely exclusive in the Austin Chalk formation. Prior to the discovery of the Eagle Ford Shale formation, the Austin Chalk was the main play within this area. However, this changed when Petrohawk Energy discovered the Eagle Ford shale play in 2008. Petrohawk Energy Corporation is considered the pioneer operator in this basin, as they were the first company to successfully drill into the Eagle Ford Shale formation in the Western Gulf Basin. Shortly after this discovery, operators flooded to the Western Gulf Basin to begin drilling. Following Petrohawk’s successful production in 2008, the Texas Railroad Commission issued 1,010 drilling permits in 2010 alone. Since initial production began, the basin has seen unprecedented reserve and production growth.
History of the Eagle Ford Formation
Believed to have been formed in the late Cretaceous period, the Eagle Ford Shale formation is estimated to be 89 to 95 million years old. Prior to 2008, the Eagle Ford Shale had yet to be commercially explored. Attention in this area was primarily directed towards the Austin Chalk formation, which was famous in the 1970’s and 1990’s for inhabiting large amounts of crude oil. However, in 2008, Petrohawk Energy diverted their attention below the Austin Chalk, to a formation now known as the Eagle Ford Shale formation. On October 21st, 2008, Petrohawk Energy announced their plan to drill into the Eagle Ford formation. The well, which was a 3,200-foot horizontal lateral, had an initial flow rate of about 7,600 MMBtu of wet natural gas. Following the completion, and more importantly, success of Petrohawk’s well, Exploration and Production companies flooded to South Texas to drill into the hydrocarbon-rich formation. Since production in the Eagle Ford began in 2008, the Eagle Ford has become the second-most productive oil basin in the United States.
The geologic history of the Western Gulf Basin can be traced back to the formation of the greater Gulf of Mexico basin, as the Western Gulf Basin resides within it. In the early stages of Continental Separation, five rift basins were formed on thinned Continental Crust and became the inland segment of the Gulf of Mexico basin. These five rift basins are known as the Rio Grande embayment, the East Texas basin, the North Louisiana basin, the Mississippi Interior basin, and the Apalachicola embayment. Put together, these basins make up the now known Western Gulf Basin. Separating these rift basins are the San Marcos arch, Sabine arch, Monroe arch, and northeast extension of the Wiggins arch. Across the Western Gulf basin, a continental shelf developed as a result of mid-Jurassic to Late Cretaceous subsidence. Additionally, a substantial influx of sediments during the Cenozoic time caused a thick accumulation of sediment in depocenters seaward of the Mesozoic shelf edge. As these depocenters shifted from South Texas to South Louisiana, this shift caused the continental shelf to be best developed off Louisiana by the Neogene geologic period.
Conventional and Unconventional Play Characteristics
The Western Gulf Basin plays can be distinguished by several varying characteristics. Within this basin, plays are classified by either Conventional or Unconventional, and have characteristics that distinguish the two.
• Well-Defined structural and stratigraphic traps
• Hydrocarbon migration into traps from distant source rock
• Initial high production rates that decline following well maturation
• Pressured Reservoirs
• Good Reservoir Porosity
• Good Reservoir Permeability
• Lack of traditional seals and/or traps
• Large in-place resources, yet low production rates
• Lack of Hydrocarbon Water contacts
• Low Reservoir Porosity and Permeability
• Low or absent water production
Current Drilling Activity
Drilling activity in the Western Gulf Basin as risen in recent years. So much so, that the Eagle Ford formation is now considered the second most productive oil basin in the United States, behind the Permian basin. Recent 2019 data shows that the Western Gulf Basin has proved reserves of 4,297 million barrels of crude oil and produced 451 million barrels in the 2019 fiscal year. Furthermore, as of November 2021, in the Eagle Ford formation alone, oil production per rig was 2,383 bbl/day. Additionally, gas production in the formation reached 8,559 cubic feet/day.
Operators currently drilling in the Eagle Ford Formation with the most drilling permits are:
• EOG Resources, Inc.
• ConocoPhillips Co.
• BPX Operating Co.
• Marathon Oil EF LLC
• Chesapeake Operating LLC
Austin Chalk Formation
Prior to the discovery of the Eagle Ford play, operators in the Western Gulf Basin focused their drilling efforts on the Austin Chalk formation. Production in the Austin Chalk formation has been active since the 1920’s and as of 2016, the Austin Chalk has cumulative production of 948 million barrels of crude oil and 5.6 trillion cubic feet of natural gas. The Austin Chalk lies directly on top of the Eagle Ford Shale which is the primary source of oil in the Austin Chalk formation. Prior to the 1973 Middle East Oil Embargo, drilling in the Chalk was primarily centered around naturally fractured areas. However, shortly after the Oil Embargo, operators ramped up the use of fracture stimulation. However, once multi-stage hydraulic fracturing in horizontal wells was introduced, the Eagle Ford Shale play became the sole focus for drilling companies in the Western Gulf Basin.
Following the introduction of multi-stage hydraulic fracturing in horizonal wells, production in the Austin Chalk flattened, as attention was directed toward the lower Eagle Ford formation. However, in 2015, production in the Austin Chalk formation grew slightly. Drilling activity grew primarily in Karnes County, as operators were able to finally locate areas with natural fractures. Regarding this area, operators such as Blackbruch Oil & Gas and Gulftex Energy were successful in targeting porous, naturally fractured areas in the formation. In this some parts of this area, porosity levels exceeded 8%, whereas most points within the formation have porosity levels of 2-3%. Additionally, along the Cretaceous Shelf margin in several counties along the formation, there are extremely high reservoir volumes, some of which exceed 15 PHIH-HC. Lastly, operators are re-examining the area due to extremely high flow rates. Typically, high flow rates are associated with high pressures which are generally very appealing to operators.
Eagle Ford Play
The current play in the Western Gulf Basin is the Eagle Ford Formation. The Eagle Ford formation is currently second in the United States in production. The play is located in South Texas and produces from depths varying between 4,000 feet and 14,000 feet. In all, the formation is roughly 400 miles long and 50 miles wide. The formation is unconventional and extremely brittle, thus requiring the process of hydraulic fracturing. The Eagle Ford play is currently the most active shale play in the world, and operators believe that the formation is still developing.
Structural and Tectonic Features
The Eagle Ford formation has a varied thickness. The formation is thickest in the Maverick Basin and thins as it approaches the San Marcos arch region. The primary target of the Eagle Ford play, the organic rich carbonate mudrock in the lower Eagle Ford, grades into silica-rich pepper shale near the northeast of the San Marcos Arch. Within this Pepper Shale, low permeability sandstone layers are the main target for drilling. The extent of the Eagle Ford formations and variations in its thickness are largely determined by regional features including the Maverick Basin, San Marcos Arch, Stuart City and Sligo Shelf Margin, and the East Texas Basin.
The Eagle Ford play is divided into three main production windows: Oil, Natural Gas, and Natural Gas liquids. Initially, the Eagle Ford was drilled primarily for Natural Gas. However, following the price decline of Natural Gas in 2008, E&P companies began exploiting the area for Oil and Natural Gas Liquids. Furthermore, in 2010, just two years after the completion and success of Petrohawk’s Natural Gas well, production in the Eagle Ford reached 1 million barrels per day  and more than 1,000 drilling permits issued. 
Petroleum and Facility Engineering
Drilling operations in the Western Gulf Basin have been going on for hundreds of years with the first well being successfully drilled in 1865.  However, drilling techniques have since improved exponentially and have thus resulted in successful drilling operations throughout the basin. In the current play, the Eagle Ford Formation, operators have developed a unique, methodical, and effective method of drilling for hydrocarbons in the area. After completion of nearly 100 successful wells with undermentioned drilling technique, data shows an overall 52% improvement in drilling time and a 45% improvement in drilling costs. 
After gathering data from previous drilled wells in the area, operators have discovered that a two-string casing design is significantly more effective than the common three-string casing. Following the implementation of this casing design, operators saved 10% of the well time in comparison to the previous three-string design. 
Data taken from wells in the area has shown that water-based mud and mineral-oil OBM with low toxicity and low aromatic content significantly improve costs, fluid handling, and environmental impact. 
Given the unconventional nature of the Eagle Ford Formation, a unique, efficient wellhead design is necessary to effectively drill into the shale. Operators in the area have found that a wellhead with a rotating mandrel hanger resulted in the casing to be rotated throughout the lateral section of the drilling phase. Additionally, the use of a back pressure valve and wellhead pack-off system eliminated the time necessary to wait on cement. 
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- EIA. (2021, January 11). U.S. Energy Information Administration - EIA - independent statistics and analysis. U.S. Crude Oil, Natural Gas, and Natural Gas Proved Reserves, Year-end 2018. Retrieved December 14, 2021, from https://www.eia.gov/naturalgas/crudeoilreserves/.
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- Condon, S., & Dyman, T. (2003). 2003 Geologic Assessment of Undiscovered Conventional Oil and Gas Resources in the Upper Cretaceous Navarro and Taylor Groups, Western Gulf Province, Texas. Pubs.usgs.gov. Retrieved 18 November 2021, from https://pubs.usgs.gov/dds/dds-069/dds-069-h/REPORTS/69_H_CH_2.pdf.
- San Antonio Business Journal. (2021, October 1). Operators Drilling in Eagle Ford. Bizjournals.com. Retrieved December 14, 2021, from https://www.bizjournals.com/sanantonio/subscriber-only/2021/09/17/operators-drilling-in-eagle-ford.html.
- Austin Chalk Oil Gas. (n.d.). Austin Chalk Revivalaustin Chalk Revival. Austin Chalk Revival. Retrieved November 9, 2021, from https://austinchalkoilgas.com/.
- Royalty Company, U. (2013). History of the Eagle Ford Shale. Universalroyaltyco.com. Retrieved 18 November 2021, from https://www.universalroyaltyco.com/resources/history-eagle-ford-shale/.
- Journal of Petroleum Technology. (2016, August 10). Drilling improvements work toward the perfect well in the Eagle ford. Journal of Petroleum Technology . Retrieved December 14, 2021, from https://jpt.spe.org/drilling-systems-automation-and-management-drilling-improvements-work-toward-perfect-well-eagle-ford.