Abstract: Petroleum Geochemistry of the Marib-Jawf Basin, Yemen

G. Isaksen, G. Harriman, D. Taylor, R. Webster

The Marib graben is Yemen's principal petroleum producing region. The main oil discovery is the Alif field, made by Yemen Hunt Oil Company in 1984. Here, light, low-sulfur oil is produced from Jurassic sandstones. The Marib basin formed as a half-graben during the Late Jurassic with deposition of the Meem and Lam Formation marine shales, siltstones and sandstones, typically as turbidite units, followed by delta-front and braided stream fluvial sandstones of the Alif and Sean Formations. Tectonic quiescence during the Portlandian saw the development of restricted marine conditions with high evaporation rates and accumulation of the hypersaline Safer Formation.

This study investigates the petroleum potential of organic-rich rocks, the geochemistry of the oils, as well as oil-source correlations. Through integration of bulk, molecular (biomarker) and isotopic compositions, two oil families have been identified. Family A oils are generated from shales within the Portlandian Safer Formation. These source rocks have TOC contents from 1-35%, hydrogen indices of 700-1000 and predominatly algal organic matter. Family B oils are generated from the Kimmeridgian-Portlandian Meem, Lam, and Alif formations, containing marine algal and terrigenous higher plant organic matter. TOC contents vary from 1% to 15%. Hydrogen indices range from 200 to 800, giving these strata a mixed gas and oil potential.

Shales of the Lam and Meem Formations are considered the main source rocks for the oils found in the basin. Safer Formation rocks have greater oil potential, but are immature over large parts of the basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994