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Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Halite in the Upper Jurassic of the Marib-Jawf Basin, Yemen
(1) ExxonMobil Exploration Company, Houston, TX.
(2) ExxonMobil Production Company, Houston, TX.
The Marib-Jawf Basin is a Kimmeridgian northwest-trending extensional graben in western Yemen. There are 36 discoveries in the basin, with original EUR of almost 1.7 billion barrels of oil and condensate and 18 trillion cubic feet of gas. These occur in sandstone reservoirs with halite seal, in traps formed in part by salt structuring. Evaporite deposits and decolling salt movements are the critical elements in the development of trap, top seal, and foot seal.
Halite occurs in thick beds in the Safer Formation (Upper Tithonian), up to 750 meters net thickness. Meter-scale anhydrite beds occur at the top and base of halite beds. The halite is sub-aqueous salt deposited in restricted-marine basins.
Thinner halite deposits occur in the Alif Formation (Lower Tithonian) in the downdip SE end of the basin, mostly in the lower Yah Member, but also in the middle Sean Member and the upper Alif Member. The Safer and Alif formations merge to form the Sabatayn Formation in the Shabwah Basin SE of the Marib-Jawf.
Intervals of shale, siltstone, sandstone, and thin limestone beds divide the Safer Formation into 5 members. These fluvial-alluvial to paralic- and shelfal-marine deposits formed when the evaporitic basin was desiccated. The fluvial sandstones in the Safer Formation contain some oil and gas. Fluvial to deltaic-marine sandstones of the Alif Formation are the major reservoirs, with halite of the Safer Formation forming the topseal.
Some shale in the Safer Formation is organic-rich (<16% TOC, HI < 955 mg HC/gC). They generated oil in deeply buried portions of the basin, and the oil occurs in sandstone reservoirs of the Safer Formation. The oils are low gravity (14-27 API) and 3-6% sulfur, with biomarkers typical of anoxic hypersaline source rocks (Pr/Ph <1.0, abundant gammacerane, C35 pentacyclic hopanes, and C27 cholestanes).
Halite deformed by gravity sliding on listric faults detached in the basal salt, forming rafts that enclose the Alif Formation. This created gaps in the Alif Formation, where salt is grounded on the underlying Lam Formation. Salt flowed into the lowside of normal faults that moved during the Cretaceous. Salt diapirs are present mainly in the SE end of the basin and the adjacent Shabwah Basin.
Most fields are structural closures formed by salt movement. Halite cementation in Alif sandstone reservoirs on the downdip side of structural closures in the SE end of the basin form stratigraphic traps, as Al-Raja Field.