Abstract: Mixed carbonate - siliciclastic deposits in a storm-influenced tidal environment: facies control on reservoir quality in the lower Bahariya Formation (Cenomanian), Khepri-Sethos fields, Western Desert, Egypt

HEGAZY, M., and J. MELVIN

The lower Bahariya Formation (Cenomanian) at Khepri and Sethos fields (Western Desert, Egypt) was deposited in a marginal marine, tidally dominated environment. Facies analysis identifies a number of subenvironments whose dominance in the study area fluctuated with time due to fluctuating sea-level conditions. Thus subtidal channels focused deposition of glauconitic oolitic grainstones which were swept shoreward by intense storms from offshore shoals, to be trapped in the relatively deep subtidal "sinks" and subsequently reworked by prevailing tidal processes. Closely associated are lenticular, flaser-bedded and ripple cross-laminated glauconitic quartzose sandstones representing lower intertidal sand flats and bars, with random thin interbeds of storm-generated bioclastic oolitic grainstones. A low energy facies association of mudstones, muddy and sandy heterolithites and lenticular-bedded sandstones represents sedimentation in a mixed intertidal flat setting.

The poorest reservoir quality occurs in the subtidal channel oolitic grainstones, due to pervasive early cementation by calcite and ferroan calcite. The mud-rich deposits of the mixed intertidal flats display only moderate reservoir quality (at best) due to the high proportion of associated clay material. Vertical connectivity, in particular is severely inhibited by abundant interbeds of mudstone. The best reservoir quality occurs in the sandstones of the lower intertidal sand flats, although this is downgraded in a vertical dimension by compaction of flasers and ripple cross-laminae rich in ductile glauconite.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria