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Petroleum Development Oman LLC, PO Box 81, Muscat, Sultanate of Oman.
Fine grained shaly sands of the Mahwis Formation (Haima Group) form one of the reservoir intervals of the Marmul field. Six lithofacies can be distinguished from cores: shale clast conglomerates, trough cross-stratified sands, horizontally stratified sands, massive sands, rippled sands and interbedded shales and sands. Horizontally stratified sands make up 45% of the cored reservoir. Two scales of fining upward sequence occur. Small sequences are 0.3-1.5 m thick and are interpreted as sheetflood deposits on the distal portions of semi-arid alluvial fans. The larger sequences are 25-50 m thick, can be correlated on well logs fieldwide, and probably represent periodic tectonic rejuvenation and erosional lowering of the sediment source area.
It is interpreted (from regional correlation, sediment provenance, and a knowledge of possible source areas) that the alluvial fans were predominantly sandy, had radii of several tens of kilometres and formed an extensive apron bordering a highland area which lay to the southeast. The thinly bedded and shaly character of these sheetflood sands causes problems of petrophysical log evaluation and hence oil-in-place estimation. Reservoir quality is related to lithofacies occurrence and variation in grain size. The large-scale fan sequences correspond to an upward decrease in porosity and permeability and form useful subdivisions for field-scale correlation and reservoir prediction.
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