Oman basin

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South Oman Salt Basin (SOSB):

[1]South Oman Salt Basin location.Cite error: The opening <ref> tag is malformed or has a bad name

Location: 

The South Oman Salt Basin (SOSB) is located in Oman on the southeastern edge of the Arabian Peninsula.

This image depicts a Stratigraphic column, lithostratigraphic, and the sequences of carbonate and evaporite cycles in the SOSB. Highlighting the many different hydrocarbon formations present in this basin. [2]

Petroleum System:

Basin Overview:[3]

This basin was developed on a Pan-African basement and contains late Precambrian-Cambrian Ara Formation shales within the Huqf Supergroup. The SOSB is composed of the Northern Carbonate Domain, Southern Carbonate Domain, Athel Sub-basin, Eastern Flank, and the Western Margin. All these domains have unique geologic and stratigraphic features that impact the hydrocarbons formation, resulting in a unique abundance of petroleum sourced rocks distributed across the basin. In the Precambrian-Cambrian Huqf Group formations of the SOSB, transgressions of sandstone and carbonates are present, as well as a regression of carbonates with salt. These sequences of carbonates and evaporates being deposited result in the formation of an Ara Salt Group layer that contains rich hydrocarbon source rocks and large accumulations of oil overlying a crystalline basement. During the deposition of the Ara Group the SOSB is now divided into three palaeogeological domains: Southern Carbonate Domain, Athel Sub-basin, Northern Carbonate Domain. The SOSB reaches depths greater than 30,000 ft in the basin center, with depths then shortening towards the Eastern Flank to about 15,000 ft.

Source Rock Formation:[4]

The oil occurrences in the Huqf Supergroup source rocks located in the Ara carbonate stringers within the Athel Formation are called Huqf oils. Huqf oils are also prominent in the Eastern Flank of the SOSB. Another oil occurrence in the SOSB is Q oils, this oil type is found in Central Oman as well as the northernmost part of the SOSB. Huqf Supergroup starts with a thick siliclastic unit consisting of the Ghadir Manqil Formation of the Abu Mahara Group, then is followed by two clastic-carbonate cycles that are the Masirah Bay/Khufai Formation and the Shuram/Buah Formation of the Nafun Group. This is then ended by the succession of chert-carbon evaporate in the Ara Group. These domains within the Ara Group contain isolate carbonate platforms within the salt that are filled with Sulfate and Halite evaporates that are classified as intra-salt stingers, which are rich shale source rocks. The Ara carbonate stringers hold at least six third-order cycles of carbonate/evaporite sedimentation allowing for a variety of rock formations holding different hydrocarbon reserves. The transgression of the isolated carbonate platforms have shown that majority of these reservoirs tend to be over pressured and contain porous dolomites trapped in the salt at depths of 3km to 8km. The Athel sub-basin, located in the center of SOSB, holds organic rich source rock from the Ara A4 Formation of the Huqf Group. These one-of-a-kind shales include Thuleilat Shale, Athel (Al Shomou) Silicilyte, and U Shale.

Gives insight on depositional environment of SOSB by showing anticlinal and synclinal traps present in Western Margin, as well as Strike-slip faults in the Southern [5]Carbonate Domain.

Trap Styles: 

Reservoirs prominent in the SOSB exhibit structural trap styles to ensure hydrocarbons are staying within the Ara Salt Group and not migrating outside the reservoirs. Anticlinal and synclinal traps are present in the Western Margin, which is northwest of the three domains, normal faults are located in the Eastern Flank due to the extension of diverging plates, thrust faults are seen in the Southern Carbonate Domain due to converging plates, and lastly strike-slip faults are present in the Northern Carbonate Domain due to subduction zones from the Gulf of Oman. There are also stratigraphic traps present in the SOSB basin, the Base Tertiary Unconformity and Late Carboniferous Unconformity are located just northwest of the Athel formation.

Seals:

When analyzing the geologic features of the SOSB, it is evident that the reservoirs have impermeable barriers sealing the hydrocarbons from migrating out of the reservoir. When looking at the rock formations on the SOSB Cross Section, a facies seal is present that consist of Ara Salt amongst various types of shales. These seals are extremely important for exploration and production in the South Oman Salt Basin because majority of the wells drilled require hydraulic fracturing, therefore, these seals allow the for fracking to be contained by not reaching the water bearing formations accumulated in the well.

[6]Cross Section map of the SOSB, displaying the distance and depth of the of the reservoir. Gives great insight on the geologic formations of the Huqf Source Rock, Ara Salt formations, the Athel Stringers, and the migration path of Huqf Oil. 

Drive Mechanisms:

Beginning drive mechanisms in the SOSB consisted of Water Drives, which is a mechanism that uses an aquifer that stimulates the well with fresh water resulting in an increase of the reservoirs drive energy. When looking at the current production rates of the SOSB today, there has been a huge increase of barrels produced daily due to the discovery and implementation of a Combination Drive mechanism that involves Enhanced Oil Recovery Systems (EOR's)[7] consisting of miscible gas injections and flooding. EOR’s are extremely beneficial as they maintain or increase the ability for oil to flow to a well by injecting water, gas, or chemicals. The three main EOR techniques used in reservoirs across the SOSB are polymer injections, miscible gas injections, and/or steam injections. These gas flooding techniques allow for gas to mix with the oil which then reduces its viscosity and tension within the rock. The EOR technology then uses pressure to drive the oil within the reservoir allowing oil to be recovered that was formerly trapped. These advancements have accounted for current production rates doubling to around 50,000 bbls/day or greater.

Completion Method (Frac Design):

The first completion method used was hydraulic fracturing, which is the process of injecting drilling fluid into the reservoir that breaks up the source rock allowing for better extraction of the hydrocarbons. This process is costly, however is essential for efficient extraction of reserves to enhance the production of the well. The South Oman Salt Basin saw their daily production rates double after the implementation of the hydraulic fracturing completion method.

Historical Production/Production Methods:[8]

Beginning Exploration Phase:

The first exploration phase of the SOSB was from 1977-1986 in which two dolomite stringers were drilled to test the movability of hydrocarbons. The first well, drilled in 1977, was found oil-bearing and produced over 9,000 bbls/day at an API of 27, this discovery was the beginning of intra-salt exploration. In 1980 a 54-meter-thick dolomite stringer was discovered with an average porosity of 10% and had production rates greater than 3,600 bbls/day with an API of 29 degrees. However, at this time it was difficult to comprehend the full extent of reserves in the SOSB, more specifically the Ara Salt Formations, due to poor quality of seismic mapping. This transitions into the second exploration phase (1986-1997) where more information was established regarding the depositional environments that formed the Ara Salt Formations. Approximately 274 million barrels of oil-in-place were discovered during the first exploration phase.

Second Exploration Phase (Athel Exploration):

[9]Graph showing the total cumulative production of oil over time in the SOSB. Provides great representation of how technological advancements throughout the SOSB history have benefited overall production.

With the use of 3-D seismic mapping, new stratigraphic insight was formulated showing that thick high-pressured carbonate stringers were accessible all around the SOSB. This advancement allowed for the introduction of the Athel Formation which was home to hydrocarbon-bearing carbonate stringers (Athel Silicilyte) as thick as 500m. While conducting exploration in the Athel Formation three, 4-5km deep, zones were found that produce 300 bbls/day producing light but sour oil that consisted of an API of 48 degrees, porosity of 30%, permeability matrix of 0.02mD, and reservoir pressures of 19.8 kPa/m. During the second exploration phase there was minimal extraction of reserves but gaining knowledge on the depositional environments in the basin assisted in minimizing the complexity of drilling into the Ara Salt Formation. Phase two exploration also accounts for the implementation of a few drive mechanisms and production methods that would be used to improve the reservoirs production rates in the Athel Formation. The first method used was hydraulic fracturing, which is the process of injecting drilling fluid into the reservoir that breaks up the source rock allowing for better extraction of the hydrocarbons. Another beneficial addition to the wells was the idea of a Water Drive, which is a mechanism that uses an aquifer that stimulates the well with fresh water resulting in an increase of the reservoirs drive energy. Both of these additions to the wells in the SOSB have accounted for production rates doubling. Despite the slow production rates displayed in the second phase of exploration, the advancement in seismic mapping helped discover two major oil fields in the Athel Formation that contain over 2.3 billion barrels of oil-in-place.

Third Exploration Phase:

In the third exploration phase (1997-2004) drilling continued into the Ara Salt Formation, but now targeted stringers that had larger gross thickness (around 400-500m), these stringers accounted for the discovery of 1.6 billion barrels of oil-in-place and would consistently produce 3,000-6000 bbls/day. When looking at the current production rates of the SOSB today, there has been a huge increase of barrels produced daily due to the discovery and implementation of a Combination Drive mechanism that involves miscible gas injections and flooding. This advancement has allowed for production rate of around 50,000 bbls/day or greater.  

Information about Drilling:

Geologic Features of Athel Sub-basin: 

Rock Eval Analyses:[10]Thuleilat Shale located at depths of 5,000-14,000ft with a net organic-rich thickness of 293ft, it contains Type I/II kerogen and has an average TOC of 6.8%. The thermal maturity is around 1.0%, showing the shale is a wet gas condensate. The U-Shale shows similar geologic features to the Thuleilat Shale and also produced wet gas and condensate. The Athel (Al Shomou) Silicilyte is a siliceous sedimentary rock containing fine-grained microcrystalline with clastic matter up to 25%. This chert formation is located at depths of 6,500-15,000ft with a net organic-rich thickness of 445ft, it contains Type I/II kerogen and has an average TOC of 3.2%. The Silicilyte has a high porosity reaching up to 30% with a TOC of around 1.0% and a high hydrocarbon saturation of around 80%. These attributes allow Athel Silicilyte to produce light, 45-degree API gravity, oil and wet gas.

[11]Location and depth of the Athel Sub-basin in the SOSB basin.

Reservoir Characteristics (Averages):

Reserves Depths: 3km-8km

Cost: around 3.5 million dollars per well

Porosity: 8%-30%

API: 30-50 degrees

TOC: 1.0%-1.3%

Permeability Matrix: 0.02-0.05mD

Reservoir Pressure: 18-22 kPa/mD

Source Rock/Stringer Thickness: 50m-500m

Production Rate per Well: over 50,000 bbls/day

Active Wells: 400+

Reservoir Depths: contains one of the world's oldest known commercial oil and gas deposits that reach depths of 30,000 feet in the basin center and 15,000 feet along the basins eastern flank.

Geologic Risk/Uncertainties:

[12]This is a 3-D seismic grid image of the SOSB. It shows the Pre-Stack Imaging (PSI) and the Pre-Stack Depth Migration (PSDM) techniques.

Seismic Imaging:[13]

The South Oman Salt Basin still remains a very challenging play due to the complexity of the Ara Group intrasalt stringers. Throughout the advancements of exploration within this basin, geologic uncertainties arise when conducting and analyzing seismic imaging on the Ara sequences. Two common techniques used to enhance the seismic data has been to reprocess the seismic images with Pre-Stack Imaging (PSI) and Pre-Stack Depth Migration (PSDM). PSI technique gives insight on internal connectivity, number of stringers, and size of stringers, while the PSDM technique allows for improved imaging of stringer position, lateral sizing, and internal faulting. These stacking techniques are applied to 3-D seismic surveys that are then merged into seismic grids, allowing for the grids to highlight risk factors involving reservoir development. Even with the advancements of seismic imaging, the SOSB basin still faces challenges regarding proper imaging. In recent years they have experienced many altering discoveries in rock formations and depositional environments that counter original propositions on where reservoir and source rocks are located within the basin due to inaccurate seismic imaging. With the increased implementation of pre-stack imaging, the SOSB has recently been able to formulate more accurate assumptions regarding geologic features of the basin.

Plugging:

One of the greatest unknown risks for both exploration and production in the South Oman Salt Basin is the plugging of halite and bitumen. Halite and bitumen are present in other stringers, however recent discoveries have shown that plugging is affecting the flow rate of reserves in the reservoir. This plugging has appeared before and after hydrocarbon migration, therefore researchers are unsure on the origin of the substance. This geologic uncertainty results in unprecedented migration of hydrocarbons within the reservoir that affect overall production rates in the SOSB.

Petrophysical Uncertainty:

When conducting research on the geologic features on a well, it is important to ensure log readings are accurate. For example, incorrect assumptions on matrix density result in wrong porosity estimates and water saturation calculations. Minimal mistakes regarding log reading can cause drilling into a dry hole creating unwanted expenses. This miscommunication of log reading can pose a costly risk in exploration and production within the SOSB due to the deep drilling depths of the basin. Inaccurate calculations of a well's depth can create huge economic and cost risks due to an uncertainty in time to depth conversion.

Current and Future Assessment:

Resource Assessment:

Oman has the third largest “yet to find volumes” in the world with large quantities of reserves remaining untouched in the Athel Formation of the SOSB. Through Rock-Eval Analysis[14]it is predicted that the three Ara 4 shales (Thuleilat Shale, Athel Silicilyte, U-Shale) and other source rocks hold 179 Tcf of shale gas in-place and 59 billion barrels of shale oil in-place.

Future Exploration & Production:

[15]Laiyyan Al Kharusi - The Enigma of Oman's South Oman Salt Basin - Unlocking its Depths (aapg.org)

This presentation given by Laiyyan Al Kharusi was published through the American Association of Petroleum Geologist and gives a great breakdown of the source rock and hydrocarbon formations prevalent in the South Oman Salt Basin. It gives great interpretation on future exploration and production ventures that companies should be intrigued by due to the richness of the source rocks. The speaker in this video does a great job of emphasizing the extent of the basin and how vastly accumulated it is with reserves giving a positive outlook on the future production and exploration.

References:

  • Nakhle, C. (2020, November 22). The Technological Revolution in Oman's oil and gas industry. Crystol Energy. Retrieved December 6, 2022, from https://www.crystolenergy.com/technological-revolution-omans-oil-gas-industry/
  • Al-Siyabi, H. A. (2005, October 1). Exploration history of the ara intrasalt carbonate stringers in the South Oman Salt Basin. GeoArabia. Retrieved December 6, 2022, from https://pubs.geoscienceworld.org/geoarabia/article/10/4/39/566904/Exploration-history-of-the-Ara-intrasalt-carbonate
  • Oman - Oil & Gas. International Trade Administration | Trade.gov. (2022, September 14). Retrieved December 6, 2022, from https://www.trade.gov/country-commercial-guides/oman-oil-gas
  • panelE.GrosjeanaPersonEnvelopeG.D.Lovea1C.StalviesbD.A.FikeaR.E.Summonsa, A. links open overlay, E.GrosjeanaPersonEnvelope, a, G.D.Lovea1, 1, C.Stalviesb, b, D.A.Fikea, R.E.Summonsa, & AbstractThe South Oman Salt Basin (SOSB) is host to the world’s oldest known commercial deposits. Most of the South Oman oils have been proven to be associated with the source rocks of the Neoproterozoic to Cambrian Huqf Supergroup. (2008, October 10). Origin of petroleum in the neoproterozoic–cambrian south oman salt basin. Organic Geochemistry. Retrieved December 6, 2022, from https://www.sciencedirect.com/science/article/abs/pii/S0146638008003021#bib38
  • Plummer, P. (2021, July). (A) tectono-stratigraphic development of lowstand evaporites and ... Stratigraphic Development. Retrieved December 6, 2022, from https://www.researchgate.net/figure/a-Tectono-stratigraphic-development-of-lowstand-evaporites-and-highstand-platform_fig3_352831431
  • U.S. Energy Information Administration - EIA - independent statistics and analysis. EIA. (2015, September). Retrieved December 6, 2022, from https://www.eia.gov/analysis/studies/worldshalegas/
  • Pollastro, R. M. (1999). U.S. Geological Survey Publications Warehouse. U.S. Department of the Interior  U.S. Geological Survey. Retrieved December 6, 2022, from https://pubs.usgs.gov/of/1999/0050d/report.pdf
  • Schroder, S. (n.d.). Summary log of the A4 carbonate, showing the vertical succession of ... ResearchGate. Retrieved December 6, 2022, from https://www.researchgate.net/figure/Summary-log-of-the-A4-carbonate-showing-the-vertical-succession-of-lithofacies-on-the_fig10_223897030
  • South Oman Salt Basin. SPE. (n.d.). Retrieved December 6, 2022, from https://search.spe.org/i2kweb/SPE/class/Oilfield%20Places/Asia/Middle%20East/Oman/Central%20Oman/South%20Oman%20Salt%20Basin
  • The Enigma of Omans South Oman Salt Basin- Unlocking its Depths. (2021). AAPG. Retrieved December 6, 2022, from https://www.aapg.org/videos/super-basins/articleid/56514/laiyyan-al-kharusi-the-enigma-of-omans-south-oman-salt-basin-unlocking-its-depths.
  • Eshbaugh, M. J. (2013). Enhanced Oil Recovery Techniques helped Oman reverse recent production declines. Homepage - U.S. Energy Information Administration (EIA). Retrieved December 6, 2022, from https://www.eia.gov/todayinenergy/detail.php?id=13631
  • Dourian, K., Chay, C., Daga, G., Vidal, A., & Ipek, A. B. and F. (2018, August 30). Cutting-edge technology boosts Oman's oil and gas production. Arab Gulf States Institute in Washington. Retrieved December 6, 2022, from https://agsiw.org/cutting-edge-technology-boosts-omans-oil-and-gas-production/
  1. U.S. Energy Information Administration - EIA - independent statistics and analysis. EIA. (2015, September). Retrieved December 6, 2022, from https://www.eia.gov/analysis/studies/worldshalegas/
  2. Al-Siyabi, H. A.-S. A. (n.d.). Figure 2. Exploration history of the Ara intrasalt carbonate stringers in the South Oman Salt Basin . Retrieved from https://pubs.geoscienceworld.org/.
  3. Pollastro, R. M. (1999). U.S. Geological Survey Publications Warehouse. U.S. Department of the Interior  U.S. Geological Survey. Retrieved December 6, 2022, from https://pubs.usgs.gov/of/1999/0050d/report.pdf
  4. Al-Siyabi, H. A. (2005, October 1). Exploration history of the ara intrasalt carbonate stringers in the South Oman Salt Basin. GeoArabia. Retrieved December 6, 2022, from https://pubs.geoscienceworld.org/geoarabia/article/10/4/39/566904/Exploration-history-of-the-Ara-intrasalt-carbonate
  5. Al-Siyabi, H. A.-S. A. (n.d.). Figure 1. Exploration history of the Ara intrasalt carbonate stringers in the South Oman Salt Basin . Retrieved from https://pubs.geoscienceworld.org/.
  6. U.S. Energy Information Administration - EIA - independent statistics and analysis. EIA. (2015, September). Retrieved December 6, 2022, from https://www.eia.gov/analysis/studies/worldshalegas/
  7. Eshbaugh, M. J. (2013). Enhanced Oil Recovery Techniques helped Oman reverse recent production declines. Homepage - U.S. Energy Information Administration (EIA). Retrieved December 6, 2022, from https://www.eia.gov/todayinenergy/detail.php?id=13631
  8. •Al-Siyabi, H. A. (2005, October 1). Exploration history of the ara intrasalt carbonate stringers in the South Oman Salt Basin. GeoArabia. Retrieved December 6, 2022, from https://pubs.geoscienceworld.org/geoarabia/article/10/4/39/566904/Exploration-history-of-the-Ara-intrasalt-carbonate
  9. Al-Siyabi, H. A.-S. A. (n.d.). Figure 22. Exploration history of the Ara intrasalt carbonate stringers in the South Oman Salt Basin . Retrieved from https://pubs.geoscienceworld.org/.
  10. U.S. Energy Information Administration - EIA - independent statistics and analysis. EIA. (2015, September). Retrieved December 6, 2022, from https://www.eia.gov/analysis/studies/worldshalegas/
  11. U.S. Energy Information Administration - EIA - independent statistics and analysis. EIA. (2015, September). Retrieved December 6, 2022, from https://www.eia.gov/analysis/studies/worldshalegas/
  12. Al-Siyabi, H. A.-S. A. (n.d.). Figure 23. Exploration history of the Ara intrasalt carbonate stringers in the South Oman Salt Basin . Retrieved from https://pubs.geoscienceworld.org/.
  13. Al-Siyabi, H. A. (2005, October 1). Exploration history of the ara intrasalt carbonate stringers in the South Oman Salt Basin. GeoArabia. Retrieved December 6, 2022, from https://pubs.geoscienceworld.org/geoarabia/article/10/4/39/566904/Exploration-history-of-the-Ara-intrasalt-carbonate
  14. U.S. Energy Information Administration - EIA - independent statistics and analysis. EIA. (2015, September). Retrieved December 6, 2022, from https://www.eia.gov/analysis/studies/worldshalegas/
  15. Laiyyan Al Kharusi - The Enigma of Oman's South Oman Salt Basin - Unlocking its Depths (aapg.org)