EFFECT OF DEPOSITIONAL ENVIRONMENTS ON
RESERVOIR CAPACITY OF UPPER BAHARIYA FORMATION, MELEIHA OILUIELDS, NORTH WESTERN
DESERT, EGYPT
M. HAMED METWALLI, MOUNIR K. SAAD and TAREK A. ALI
Dept. of Geology. Fac, Sciences, Cairo Univ., Giza. Egypt
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Oil production from the Meleiha area comes from the basal section of the Upper Cretaceous/Lower Cenomanian upper Bahariya Fm. Study of lithofacies, sedimentary structures and wireline log data of some available core samples indicates that the sequence was deposited in tidal flat-near shore shallow marine environments. The sediments were deposited in slightly different water depths with two main fades associations (including 20 types): A tidal flat-tidal channel facies association and a near shore-shallow marine facies association. Herring-bone cross bedding, current ripples, flaser, wavy and lenticular beddings and small scale planner cross-bedding with occasional bioturbation and burrows are characteristic features of the tidal flat and tidal channel sands. The near shore-shallow marine sediments are dominated by medium scale trough cross bedding and current ripples with some horizontal lamination. Although tidal flat environments can be laterally extensive, they are internally complex as a result of numerous channels dissecting them.
The upper pail of the Bahariya Fm is further subdivided into 3 main horizons and subhorizons based on wireline log data (gamma ray), lithology, lateral and vertical variation in facies and facies associations. Depositional processes textures and mineral composition of detritus largely control the reservoir geometry. The reservoir quality appears to be dependent on the depositional facies. The best porosity and permeability (reservoir capacity in short) are observed in the clean, cross-bedded sandstone of the tidal channel facies. The rippled sandstone of the sand flat facies displays a moderate reservoir potential. The laminated heterolithic sandstone of the tidal flat and the green sands of the near shore facies exhibit poor reservoir quality due to significant clay content. Integrated study involving the correlation of various electric log types combined with detailed core description covering greater number of wells is recommended in order to trace the direction of tidal channels for future drilling and prospecting in the area.
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