--> Abstract: Geological Controls On the Unique Simsima Oil Occurrence of South-Central Abu Dhabi: An Integrated Approach, by A. A. Mohamed; #90942 (1997).

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Abstract: Geological Controls On the Unique Simsima Oil Occurrence of South-Central Abu Dhabi: An Integrated Approach

MOHAMED, AWAD A

In the United Arab Emirates, oil is produced commercially from the Late Cretaceous Simsima Formation only in Shah field of south-central Abu Dhabi. The source of this oil is controversial. Many workers have correlated the Simsima oil to the Middle Cretaceous Shilaif source directly underneath. This study provides evidence for a source other than the Shilaif and offers a comprehensive explanation for the unique occurrence and origin of the Simsima oil in Shah field.

The Simsima oil is found to be highly mature (vitrinite reflectance equivalent 1.00%); whereas the Shilaif source rock in Shah field has not yet reached maturity. However in the basinal area to the west of Shah field the Shilaif has just reached maturity (vitrinite reflectance equivalent .6 - .7%). Therefore, it is not possible to match the highly mature Shah - Simsima oil to the immature or just mature Shilaif Formation directly below.

A good common oil-source and oil-oil correlation is established among Simsima Oil, Dukhan source rock (Jurassic) and oils from other reservoirs stacked below the Shilaif Formation in Shah field. This correlation is confirmed by data from other fields and source rocks across Abu Dhabi.

Shah field witnessed the most vigorous structural growth in Abu Dhabi, resulting in fractures and faults commonly crossing the stratigraphic sequences. At early to middle oil generation time the biggest pulses of oil migration occurred while Shah faults and fractures were still open all the way between the Dukhan source and the Simsima reservoirs. Moreover, the onset of oil generation in Shah area was initiated five to ten million years earlier than the rest of Abu Dhabi due to a considerably higher geothermal gradient. At later stages, with increasing depth of burial, the Nahr Umr shale and the intraformational seals became progressively effective, and minor oil quantities, with advanced maturities, were generated and trapped in the deeper horizons.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria