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Because large amounts of hydrocarbons have been found in reservoirs of deltaic origin, deltas have been extensively studied in both the modern and rock record. Internal morphologies and geometries of reservoir-potential deposits within most types of deltas are today reasonably well understood. Fan deltas and the geometries of their sandstone reservoir bodies are exceptions.
To provide a better understanding of fan-delta reservoirs, 700 wells and 365 cores from the Utikuma-Nipisi fields of north-central Alberta were studied in detail. The Utikuma-Nipisi fields, which contain 751.4 million bbl of oil reserves in fan deltas, produce from structural-stratigraphic traps in Middle Devonian Gilwood sandstones of the Watt Mountain Formation. These sediments are part of the clastic apron that surrounded the Peace River arch, a positive granitic terrane that had relief of more than 2,300 ft during Middle Devonian time.
In the Utikuma-Nipisi area, arkosic sediments were transported from the Peace River arch by ephemeral braided streams and deposited as a fan delta at the margin of the Elk Point Sea. In upper reaches of the delta, porosities and permeabilities in the coarse alluvial fan-braided stream portions have been occluded by fine-grained sieve deposits. Seaward of the delta front, prodelta sediments act as fine-grained permeability barriers. Only in the delta front have significant reservoir-potential porous deposits accumulated. These primary intergranular porosities and permeabilities are attributable to sorting and reworking by fluvial processes as well as wave and tidal energies in the depositional basin. Discontinuities in these delta-front reservoirs were the result of delta-lobe switches
Results of this analysis suggest hydrocarbon exploration in fan deltas should target delta-front depositional settings.
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