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Book Chapter

Stratigraphy and Geochemistry of Upper Jurassic Diyab Formation, Abu Dhabi, U.A.E.

By
Ahmed S. Al-Suwaidi
Ahmed S. Al-Suwaidi
Abu Dhabi National Oil Company, Exploration Division, P.O. Box: 898, Abu Dhabi, U.A.E.
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;
Ahmed K. Taher
Ahmed K. Taher
Faculty of Science, United Arab Emirates University, P.O. Box: 17551, Al Ain, U.A.E.
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;
Abdulrahman S. Alsharhan
Abdulrahman S. Alsharhan
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;
Mohamed G. Salah
Mohamed G. Salah
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Published:
January 01, 2000

Abstract

The Diyab Formation (Oxfordian to mid-Kimmeridgian) is widely distributed in Abu Dhabi. It consists of argillaceous lime mudstones/ wackestones (rich in organic matter) in the west, which change laterally eastward into oolitic packstones and grainstones. Its thickness reaches up to 1300 ft (395 m) towards the southern onshore Abu Dhabi. Geochemical evaluation of the Diyab Formation was carried out using analytical data from five different laboratories. Source rock screening indicated the presence of a rich source-rock interval in the lower part of the formation in western Abu Dhabi, which becomes a lean source-rock in eastern Abu Dhabi. The Diyab source rock contains oil- and gas-prone kerogen, and was deposited in an intrashelf basin that was enclosed by the Diyab shelfal facies sediments, which restricted water circulation, causing anoxic conditions below wave base.

A combination of geological and geochemical information was used to reconstruct burial and thermal histories of the potentially petroliferous intervals in the Diyab source rock. This source rock was sufficiently mature in the southwestern onshore Abu Dhabi to generate hydrocarbons since Late Cretaceous time. Currently, this formation lies within the gas generation window for most of onshore and southern offshore Abu Dhabi.

Comparative analysis of oil and source-rock characteristics, using both bulk and molecular parameters, carbon-isotope analysis, gas chromatography (GC), and gas chromatography-mass spectrometry (GC-MS), was carried out. The oils from both the Thamama Group and the Arab Formation have the characteristic of oils derived from carbonate source rocks. However, Arab oils are less mature and have lower concentration of saturate, and are isotopically lighter than Thamama oils at equivalent reservoir depth.

The secondary migration scheme was based on a series of structure and paleostructure maps at the top of the Arab-D level. The migration direction of the Arab Formation was controlled mainly by the presence of western synclinal structures. East of these synclines, oil migration is predominantly northeastward. The early Tertiary (50–40 Ma) was the major expulsion phase, with more than 75% of the total oils being generated from Diyab source rock, approximately 95% of which comes from the Lower Diyab section.

Crude oil and extract analysis resulted in identification of four main oil families. The Simsima-reservoired oil appears to originate from the Shilaif source rock, whereas the Thamama oils are sourced from the Shuaiba Basinai Facies (Bab Member) and the Diyab Formation. The Arab oils were sourced mainly from the Diyab Formation, and the Araej oil was sourced mainly from the Diyab Formation and possibly from an unknown pre-Diyab source rock.

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Contents

SEPM Special Publication

Middle East Models of Jurassic/Cretaceous Carbonate Systems

Abdulrahman S. Alsharhan
Abdulrahman S. Alsharhan
Faculty of Science United Arab Emirates University, P.O. Box 17551, Al Ain, U.A.E.
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;
Robert W. Scott
Robert W. Scott
Precision Stratigraphy Associates, RR3 Box 103-3, Cleveland, OK 74020, U.S.A.
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SEPM Society for Sedimentary Geology
Volume
69
ISBN electronic:
9781565761919
Publication date:
January 01, 2000

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