Guest reservoir geology by David Middleton
Saudi Aramco’s recent bond prospectus has generated a lot of media buzz, particularly regarding the production from Ghawar, the largest oil field in the world. Reaction has ranged from “The biggest Saudi oil field is fading faster than anyone guessed,” (not even wrong) to more subdued reactions from Ellen Wald and Robert Rapier, that the prospectus doesn’t really tell us much Ghawar’s decline rate. One thing that the bond prospectus did do, is to paint a picture of the most profitable company in the world and one that is serious when it says it will produce the last barrel of oil ever produced on Earth.
How big is Ghawar? Has it peaked? Is it “fading faster than anyone guessed”? The answer to the first question is: FRACKING YUGE. The answer to the second question was not easily answerable before Saudi Aramco began the process of becoming a publicly traded company. The answer to the third question is: Of course not.
As Saudi Aramco proceeds towards a 2021 IPO, it has had to embrace transparency. This involved an audit of the proved reserves in their largest fields, comprising about 80% of the company’s value. The audit was conducted by the highly respected DeGolyer and MacNaughton firm (D&M). The audit actually determined that the proved reserves are slightly larger than Aramco’s internal estimate.
This is from D&M’s certification letter (Appendix-C in the bond prospectus):
Reserves estimated herein are expressed as net reserves. Gross reserves are defined as the total estimated petroleum remaining to be produced from these properties after December 31, 2017, but before December 31, 2077 (license limit). Net reserves are defined as that portion of the gross reserves attributable to the interests held by Saudi Arabian Oil Company after deducting interests held by others. Saudi Arabian Oil Company has represented that it holds 100 percent of the interests evaluated herein; therefore, net reserves are equivalent to gross reserves for the purposes of this report.
Saudi Arabian Oil Company has represented that it holds interests in certain properties onshore and offshore the Kingdom of Saudi Arabia. Proved reserves have been estimated for 77 reservoirs in 29 fields in this report.
Definition of Reserves
Estimates of proved reserves presented in this report have been prepared in accordance with the PRMS approved in March 2007 by the Society of Petroleum Engineers, the World Petroleum Council, the American Association of Petroleum Geologists, and the Society of Petroleum Evaluation Engineers. Only proved reserves have been evaluated for this report. The petroleum reserves are defined as follows:
Reserves are those quantities of petroleum anticipated to be commercially recoverable by application of development projects to known accumulations from a given date forward under defined conditions. Reserves must further satisfy four criteria: they must be discovered, recoverable, commercial, and remaining (as of the evaluation date) based on the development project(s) applied. Reserves are further categorized in accordance with the level of certainty associated with the estimates and may be sub-classified based on project maturity and/or characterized by development and production status.
Proved Reserves – Proved Reserves are those quantities of petroleum which, by analysis of geoscience and engineering data, can be estimated with reasonable certainty to be commercially recoverable, from a given date forward, from known reservoirs and under defined economic conditions, operating methods, and government regulations. If deterministic methods are used, the term reasonable certainty is intended to express a high degree of confidence that the quantities will be recovered. If probabilistic methods are used, there should be at least a 90-percent probability that the quantities actually recovered will equal or exceed the estimate.
A couple of important clues to Ghawar’s current production rate:
- Gross reserves are defined as the total estimated petroleum remaining to be produced from these properties after December 31, 2017, but before December 31, 2077.
- Proved Reserves are those quantities of petroleum which, by analysis of geoscience and engineering data, can be estimated with reasonable certainty to be commercially recoverable, from a given date forward… If probabilistic methods are used, there should be at least a 90-percent probability that the quantities actually recovered will equal or exceed the estimate.
D&M’s proved reserve number for Ghawar was 48,254 million barrels of liquids (crude oil, condensate and natural gas liquids). That’s just a shade under 50 billion barrels to be produced from 2018-2077.
Ghawar: “The King of Giant Fields”
Discovered in 1948 and located some 200 km east of Riyadh, Ghawar has produced about five million barrels of oil per day in the past three decades. Last year, output from Ghawar accounted for 62.5% of Saudi Arabia’s crude production (about 8 MMbopd) and 6.25% of the world’s total oil production (about 80 MMbopd).
Ghawar is “big”…
Dr. Abdulkader Afifi described the geologic setting in his 2004 AAPG Distinguished Lecture…
Aramco initially discovered oil in Ghawar in 1948, based on surface mapping and shallow structure drilling. Ghawar is a large north-trending anticlinal structure, some 250 kilometers long and 30 kilometers wide. It is a drape fold over a basement horst, which grew initially during the Carboniferous Hercynian deformation and was reactivated episodically, particularly during the Late Cretaceous. In detail, the deep structure consists of several en echelon horst blocks that probably formed in response to right-lateral transpression. The bounding faults have throws exceeding 3000 feet at the Silurian level but terminate within the Triassic section. The episodic structural growth influenced sedimentation of the Permo-Carboniferous sandstone reservoirs, which onlap the structure and the Jurassic and Permian carbonate reservoirs, which accumulated in shoals above structural culminations.
The main oil reservoir is the Upper Jurassic Arab-D limestone, which improves upward from mudstone to skeletal-oolitic grainstone, reflecting successive upward-shoaling cycles. The excellent reservoir quality is due to the preservation of the primary porosity, the enhancement of permeability, and the presence of fractures in the deeper and tighter parts. The oil was sourced exclusively from Jurassic organic-rich mudstones and is effectively sealed beneath massive anhydrite. The general absence of faults at the Arab-D level maintained seal integrity. Current production is almost 5 million barrels per day under peripheral water injection. The southernmost part of the field remains under development, with a final increment of 300,000 barrels per day on stream in 2006.
The structural/stratigraphic setting couldn’t have been better if it was designed for the purpose of becoming a super-giant oil field. The presence of a positive paleo-structure, episodic reactivation of uplift and buried fault system provided for a high-energy depostional environment, critical to the formation and preservation of carbonate porosity and provided pathways from the underlying prolific Silurian source rocks. The Jurassic Arab-D formation is covered by a thick sequence of anhydrite, forming a very effective seal.
The Arab-D carbonate is an incredible reservoir, particularly the skeletal-oolitic grainstone.
Ghawar is subdivided into five segments: Ain Dar, Shedgum, Uthmaniyah, Hawiyah and Haradh.
The 1954 structure map holds up pretty well today.
In 1980, Aramco published all of the data anyone would ever need to calculate the original oil in place (OOIP) for Ghawar:
I planimetered the areas of the five segments and then calculated to OOIP using this equation:
This is what I came up with:
About 183 billion barrels of oil. I also estimated approximate recoverable volumes:
|Primary Water Drive 40%||73,109,450,367||bbl|
|Secondary Waterflood EOR 50%||91,386,812,959||bbl|
|Tertiary CO2 injection EOR 60%||109,664,175,551||bbl|
In order to estimate Ghawar’s current production rate, I needed three numbers:
- Original oil in place.
- Current proved reserves.
- Cumulative production
We have estimates of OOIP and proved reserves, but the cumulative production is a bit “fuzzy”.
Beydoun in his book (The Middle East, 1988) reports that Ghawar had produced 19 Bbo by 1979. According to an article on Ghawar in the AAPG Explorer (January 2005), the cumulative production from the field was 55 Bbo. The International Energy Agency in its 2008 World Energy Outlook states that the oil production from Ghawar reached 66 Bbo in 2007 and that the remaining reserves are 74 Bbo.
Data on Ghawar reported in the past issues of Oil & Gas Journal indicate that when Ghawar came on stream in 1951 it produced 126,000 bopd but production steadily rose with a major boost soon after the 1973 oil shock so that the field’s 1975 output was 4.2 MMbopd; this reached a maximum production of 5.7 MMbopd in 1981. From 1982-1990, the Saudis lowered their oil production for market considerations (most notably the oil crash of 1985) and thus Ghawar’s production was 2.5 to 3 MMbopd during that decade. A senior geologist with Saudi Aramco, A. M. Afifi, in his 2004 AAPG Distinguished Lecture, reported production values of 4.6-5.2 MMbopd for Ghawar from 1993 through 2003. These data indicate that 50-65% of Saudi Aramco’s oil production has traditionally come from Ghawar. Apparently, one half of Ghawar’s production (2.0 to 2.7 MMbopd) comes from the Ain Dar and Shedgum areas, while Uthmaniyah provides 1 MMbopd, and another million barrels or so comes from Hawiyah and Haradh combined.
For my estimate, I used the AAPG number of 55 billion bbl as the cumulative production through 2004. I then used the production data cited in Afifi (2005) as a starting point for a decline curve.
The Aramco bond prospectus noted that Ghawar’s MSC (maximum sustained capacity) was 3.8 million barrels per day (bbl/d) in 2018. Based on Aramco’s definition of MSC, it’s difficult to determine if that is a current value or an average value over the Saudi planning period (which appears to be 50 years). A 2% decline rate, typical of giant oil fields (Höök et al, 2009), fits a current MCS of 3.8 million bbl/d. A 1% decline rate fits a long-term average MCS of 3.8 million bbl/d. Based on the cumulative production and proved reserves, a 2% decline rate seems likely.
A 2% decline rate would lead to Ghawar producing just over 100% of its proved reserves (1p) from 2018-2077 (50.7 billion bbl). Recall that proved reserves (1p) is a >90% probability volume. Proved + probable reserves (2p) is the most likely volume (>50%). 2p is always a little (or a lot) bigger than 1p. As far as I know, Aramco has not published a 2p volume.
A 2% decline rate would lead to a recovery of approximately 65% of the OOIP from 1951-2077.
People have often asked, “How could Saudi Arabia ever replace Ghawar, the largest oil field in the world?” They already have replaced it… and Ghawar is not “fading faster than anyone guessed.” It’s declining as gracefully as befits the world’s super-giant oil field. Aramco plans on being able to produce 12 million bbl/d as for more than 50 years and they have the capacity to do so.
The Peak Oiler’s Nightmare
Almost all petroleum reservoirs exhibit exponential decline curves. They don’t fall off of a Seneca Cliff into the Olduvai Gorge. In aggregate, regional and global oil production has and/or will follow the same pattern, because it is just the sum of the individual reservoirs. Hubbert’s logistic function is an approximation of this basic principle of reservoir depletion.
Peak Oiler Fantasy…
About the author
David Middleton has 38 years of experience as a geophysicist and geologist in the oil & gas industry, including a six-year exile into management. The vast majority of his career has been spent working the Gulf of Mexico. He has been a member of the Society of Exploration Geophysicists since 1981 and the American Association of Petroleum Geologists since 2004.
A note on comments: Abiotic oil aficionados are more than welcome to waste their time posting gibberish, but they won’t waste any of mine. Peak Oiler’s are also welcome to babble about Seneca Cliffs and Olduvai Gorges… And that might just merit wasting some of my time.
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via Watts Up With That?
April 10, 2019 at 12:07PM