A combination of stratigraphic and diagenetic events has trapped oil in thin-bedded, clayey sandstones of the Upper Cretaceous Woodbine-Eagle Ford Formations. Five sandstone units occur in Kurten field and are designated from top to bottom as "A" through "E." Foraminifera and nannofossils indicate these units to be late Turonian. The "C" and "D" units are elongate north to south, 4.5 mi (7.2 km) wide, over 10 mi (16 km) long, and 40 ft (12 m) thick. The "B" and "E" units are thinner and trend northeast to southwest. Grain size coarsens upward in the "B," "C," and "D" units, averaging 0.14 mm and ranging from 0.09 to 0.18 mm. Grain size fines upward in the "E" unit. The sandstone's average composition is 66% quartz, 1% feldspar, 2% rock fragments, and 28% matrix. Sedimenta y structures in the "B," "C," and "D" units grade upward from laminated and bioturbated siltstones to clean sandstones with flaser cross-beds. The "E" unit consists of repeated bedsets similar to "cde" turbidite divisions. Sedimentary structures and bioturbation indicate that the units are offshore bars which have been formed by a combination of river mouth bypassing, storm-surge turbidity flows, and longshore currents.

The porosity is largely diagenetic and occurs in the clayey beds. It appears to have been formed by freshwater leaching along an erosional unconformity overlain by the Austin Chalk. Permeability becomes progressively poorer away from the unconformity and a permeability barrier ultimately forms a poorly defined updip limit for the field, making Kurten a combination diagenetic and stratigraphic trap. Relatively widespread occurrences of offshore bars suggest that similar traps may be fairly common in ancient shelf sediments.

End_of_Article - Last_Page 1681------------