Petroleum Geology of the Gulf of Aden

ALLEN, R. B., Earth Science and Resources Institute, University of South Carolina, Columbia, SC, A. G. ABOUZAKHM, Consultant, Zamalek, Cairo, Egypt, and A. H. SIKANDER, The World Bank, Washington, DC

Evaluation of eight wells (seven offshore and one onshore) in Yemen and nine wells (two offshore and seven onshore) in Somalia, and a regional interpretation of all geophysical data available from ministry files in Yemen and Somalia has been carried out under the auspices of the World Bank-executed Red Sea/Gulf of Aden Regional Hydrocarbon Study Project.

A northwest-southeast pre-Oligocene structural trend affecting the onshore pre-rift Mesozoic and early Tertiary rocks has been overprinted in the offshore by an east-west-trending extensional fault system forming the Gulf of Aden, and segmented by a series of northeast-southwest-trending left-lateral sets of transform faults. The development of the Gulf of Aden is related to extensional faulting and subsidence beginning in the Oligocene with deposition of relatively thin clastics, marl, minor limestones and anhydrite comprising syn-rift and post-rift sequences that generally lack thin evaporites and halite. The pre-rift sequence, on the other hand, contains several important reservoir developments, including clastic units in the Lower to Middle Jurassic and Upper Cretaceous, and carbo ates in the Eocene.

Free oil of 42-44 degrees API was recovered from Sharmah-1 well from the Eocene Habshiya limestone in Yemen and from the Late Jurassic Wanderer limestone (10-35 degree API) at Dagah Shabei-1 well in Somalia. Geochemical analyses performed on well cuttings and a review of company data identify oil-sourcing potential in the pre-rift Upper Jurassic, Lower to Upper Cretaceous, and Paleocene to Eocene in Yemen, and Upper Jurassic and Upper Cretaceous in Somalia. In addition, upper Oligocene to Miocene syn-rift shales and marls offer poor to fair source possibility in the Yemen and Somali offshore. A review of the oil/source correlations on oil recovered from Eocene reservoir in the Sharmah and Ghashwah wells suggest a Paleocene to Eocene source, and an adjoining Mukalia source for the oil n the Upper Cretaceous sand in the Sarar well. No oil/source correlation has been performed on the Somali oil, but it is suspected to have been sourced in the enclosing Kimmeridgian shales.

The geothermal gradients (3.5 degrees C/100 m) and maturation data (vitrinite reflectance, thermal alteration index, and Tmax) in the Gulf of Aden wells in the Yemen and Somali offshore imply that the oil window is relatively shallow (1750-3500 m). Relatively thin Paleogene in the Gulf of Aden area suggests that the Mesozoic and Lower Tertiary sediments which contain multiple source and reservoir sequences are situated within the oil window, and thus are highly prospective for oil and gas.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)