Central Arabia basin
The Central Arabia basin is located throughout the west to middle region of Saudi Arabia, and is found in Qatar as well. Petroleum exploration in Central Arabia started over 100 years ago, until oil was first discovered in 1938 by the Arabian American Oil Company (ARAMCO). The main formation in Central Arabia is the Qalibah formation, which contains the source rock where the oil is formed. Hydrocarbons found are typically light oils, which have relatively high Gas-Oil ratio (GOR). Light oils are characterized by having a higher percentage of lighter materials such as Methane (c1) and Ethane (c2).
History of the Basin
The Central Arabian basin contains the large Pre-khuff formation which houses the Qusaiba group. The basin can be found in Saudi Arabia and western part of Qatar. (Fig. 1) shows the formation and members found in the Central Arabia basin. The Qusaiba-Plaozoic petroleum system contains the Ghawar field , which is a very large oil and gas play located in Saudi Arabia. The field contains an estimated 2/3 of the world’s known recoverable oil which is about 48 million barrels. And it was discovered in 1948 and began production in 1951. The central Arabia basin also contains the Safaniya field on the off shore of the Arabian gulf, it is the largest offshore field with 1.2 million barrles of oil production per day. (Pollastro)
Primary Geologic Risks
An in-depth study of the challenges faced when drilling in shallow gas areas in the North Safaniya field in the Arabian peninsula. The study focused on the challenges found when drilling as well as some solutions. The north area of the field contains gas with high amount of abrasive sand production. the sand particles contain different sizes and shapes. The combination of large sand particles and the tendency of gas bubbling through the sea floor can lead to dangerous and life threat accidents. The drilling and production team also faced issues with gas migration through pipe annulus due to low hydrostatic pressure (Alghamdi, 2016)
The study suggests a few solutions to reduce the risk of accident by using the following practices and tubing sizes. The study recommends the use of 16” liners, 118 PCF class “G” cement which helps gas migration during the loss of hydrostatic pressure with the use of a Liquid Latex additive. the paper suggests using 38-inch conductors instead of 30-inch and adding DV tool to provide an open hole isolation for cement placement. The addition of drilling vertical evaluation wells near the actual drilling platforms instead of mud line suspension wells.
Major sandstone reservoirs apart of the Central Arabian Basin, these sandstones are the Ordovician of the Sarah formation and the upper Qasim formation both of which are sealed by Shale. This shale is specifically in the Quasaiba formation which is estimated to be of the Silurian time period. This shale specifically does the job of sealing the upper part of the sandstone reservoir. These sandstones have a unique cap that is very distinctive due to its high resistivities and high velocities on well logs. While these shales make for an impermeable seal and the primary seal. There are intraformational seals within the Khuff which are a big deal in the upper part of the Central Arabian Basin.
The Central Arabia Qusaiba-Paleozoic TPS (Total Petroleum System) is one of two petroleum systems in the Central Arabia basin. The main formation in this system is the Qalibah formation, which includes the important member, Qusaiba. The organic-rich shale facies of the lower Qusaiba member of the Qalibah formation is the source rock for this particular system in the Central Arabia basin.
The Qalibah formation consists of the lower Qusaiba shale, upper sandstones, siltstones, and shale of the Sharawra formation. The lower Qusaiba member contains coarsening upwards sequence due to the deglaciation in the early Silurian, while the upper member has a fining upwards sequence. The basal hot shale of the lower Qusaiba member is the source rock of this system as shown by the well logs on (Fig.2).
The lower Qusaiba hot shale is a dark-grey, micro-laminated, euxinic shale with a TOC that can reach up to 20 wt%. While sandstone facies in the formation have a thickness range of 1-30cm, the hot shale bed thickness is around 75cm. A survey done by Jones and Stump  showed that the richest and thickest source rock occurs on the flanks of the depocenter, rather than within it as described by other researches.
The other petroleum system in the central Arabia basin is the Arabian sub-basin Tuwaiq/Hanifa-Arab TPS. Two organic-rich intervals or marine sandstones and marls make up the source rock of one of the world’s largest single-oil fields.
There are four main tectonic stages that are associated with the major traps in the Central Arabian Basin. The four major tectonic traps can be found in (Table. 1) and they are the “(1) Carboniferous Hercynian Orogeny; (2) Early Triassic Zagros rifting and opening of the Neo-Tethys Sea (3) the First Alpine Orogeny during the Late Cretaceous (Coniacian-Campanian), associated with the collision event that led to the emplacement of the Semail Ophiolite Complex in Oman; and (4) the Second Alpine Orogeny during the middle to late Tertiary.” In the Central Arabian Basin, there are stratigraphic traps as well but often secondary to the tectonic traps.
Primary and Secondary Reservoirs
The primary reservoirs of the Central Arabia Paleozoic are sandstones of the both the Permian Unayzah and Devonian Jauf or Qusaiba Formation. In addition, the primary reservoir includes marine sandstones, cyclic dolomitic shelf carbonates of late Permian Khuff Formation and marine sandstones
Other reservoirs also have clastics of the Pre-Qusaiba Formation with faults bounding and lateral sourcing of shale member. These reservoirs include Cambrian-Ordovician Saq Sandstones, shallow marine sandstones and upper Ordovician glacial. When it comes to the Unayzah group its composed of several fine to course grained sandstones and some silt and clay stones with thin beds of argillaceous limestones. This formation fines upward with the lower part having larger and more sorted grains. In Central Saudi Arabia, the lowest point at which the Unayzah group lies contains mostly debris-flow gravels that have large diameter in length. The reoccurrences that happen within the Unayzah formation suggest sea levels that fluctuates. The upper part of the Unayzah group has about 20% porosity while the lower part has porosity values of 25% which shows an increase in porosity as we go down in depth (Pollastro).
HC type / Maturation
The types of Hydrocarbon found in the Central Arabia Formation can differ based on the production period and type of reservoir in that area. This is linked to several different groups like the Kuff, Unayzah, Qusaiba and Jauf groups. Hydrocarbon types include volatile oil, gas condensate and dry gas across both the Central and Eastern parts of the Arabia basin (Abu-Ali & Littke, 2005).
We can see from (Fig.3) the geographic zones of oil and gas production across the Paleozoic Arabian basin. The Qusiaba hot shale member which is the most dominant in the central part of the formation consist mostly of type II kerogen with an abundance of chitinozoans and graptolites (Pollastro).
Thermal maturation model of the hot shale Qusaiba member is summarized in (Fig. 4) showing the phases of hydrocarbon maturity and generation.
A study was conducted by Bishop about the statistics of maturation and geohistory curves for the Qusaiba source rock of present-day levels of thermal maturity. The study found that present-day thermal mature ranged from immature to over mature and maturation stopped mostly in the early Tertiary period. However, maturation started to happen again because of the Zagros collision in the Miocene era and could possibly be still happening up to this day. In addition, another study conducted by Milner (1998) about the thermal maturity and source rock evaluation, to create a burial history model shown in (Fig. 4) Revealed the location of Paleozoic oil fields and gas fields such as Ghawar field.
The accumulation of oil in the source rock because of lateral migration reached its expulsion in the end of the Miocene and Pliocene eras (Atashbari et al. 2013)
Analysis of any Geologic uncertainties
The Major reservoir formation for the Central Arabia basin include Sarvak, Fahliyan, Gadvan and Asmari Formation. However, Major uncertainties are related to those formations. One of which is related drilling issues such as the difficulty to know how much heat is going to generate with increase drilling activity. Another uncertainty is the difficulty to estimate how the reservoir will act when the well is stimulated and the pore pressure reaching high magnitudes. All these uncertainties need to be addressed to ensure that the well reaches its maximum productivity potential (Astashbari et al. 2013)
Current and Future Assessment of the basin
Enhanced Oil Recovery (EOR).
EOR using CO2 is currently being implemented in Saudi Arabia to help increase the amount of oil recovered. The project uses around 40 MMscf of CO2 to be injected from a gas plant about 85km away from the field. The project was first introduced in 2006, while setting a plan that the project would start in the late 2010’s. The execution relies on four injectors and four producers that are placed up-dip around 600m away from the injectors, with the wells being between them (Fig.5). The CO2 from the gas plant is planned to be captured, compressed, dehydrated, and transported to the injector wells through pipelines as shown in the schematic below (Fig.6). the four injectors and producers are designed specifically for this project. The injection has started in 2016, but no data about it is available now.
- Abu-Ali, M., & Littke, R. (2005, July 1). Paleozoic petroleum systems of Saudi Arabia: a basin modeling approach. Retrieved from https://pubs.geoscienceworld.org/geoarabia/article/10/3/131/566897/Paleozoic-petroleum-systems-of-Saudi-Arabia-a.
- Alghamdi, & A., M. (2016, April 25). Shallow Gas Drilling in North Safaniya, Saudi Arabia: Challenges, Lessons Learned & Recommendations. Retrieved from [https://www.onepetro.org/download/conference-paper/SPE-182841-MS?id=conference-paper/SPE-182841-MS
- Al-Saad, H. & Sadooni, F. (2016, September 9). STRATIGRAPHY AND PETROLEUM SYSTEMS OF THE PALAEOZOIC (PRE‐KHUFF) SUCCESSION, QATAR. Retrieved from https://onlinelibrary.wiley.com/doi/full/10.1111/jpg.12657 .
- Atashbari, Vahid, Tingay, Mark, Amrouch, & Khalid. (2018, December 17). Stratigraphy, Tectonics and Hydrocarbon Habitat of the Abadan Plain Basin: A Geological Review of a Prolific Middle Eastern Hydrocarbon Province. Retrieved from https://www.mdpi.com/2076-3263/8/12/496 .
- Jones, P.J., and Stump, T.E., 1999, Depositional and tectonic setting of the Lower Silurian hydrocarbon source rock facies, Central Saudi Arabia: American Association of Petroleum Geologists Bulletin [https://pubs.usgs.gov/fs/fs-0008-02/fs-0008-02.pdf] , v. 83, p. 314–332 .
- Kokal, Sunil, Sanni, & Almohannad. (2016, September 26). Design and Implementation of the First CO2-EOR Demonstration Project in Saudi Arabia. Retrieved from [https://www.onepetro.org/conference-paper/SPE-181729-MS].
- Pollastro, R. (n.d). Total Petroleum Systems of the Paleozoic and Jurassic, Greater Ghawar Uplift and Adjoining Provinces of Central Saudi Arabia and Northern Arabian-Persian Gulf. Retrieved from [https://pubs.usgs.gov/bul/b2202-h/b2202-h-508.pdf]
- Abu-Ali, M. and Littke, R. GeoScienceWorld. Paleozoic petroleum systems of Saudi Arabia: a basin modeling approach , https://pubs.geoscienceworld.org/geoarabia/article/10/3/131/566897/Paleozoic-petroleum-systems-of-Saudi-Arabia-a