Abstract

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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