ABSTRACT

A combination of stratigraphic and diagenetic events has trapped oil in thin-bedded, clayey sandstones of the Upper Cretaceous Woodbine-Eagleford 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 miles wide, over 10 miles long, and 40 feet 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 mm to 0.18 mm. Grain size fines upward in the "E" unit. The sandstone's average composition is 66 percent quartz, 1 percent feldspar, 2 percent rock fragments, and 28 percent matrix. Sedimentary 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 by-passing, 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 fresh-water 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 field a combination diagenetic and stratigraphic trap. Relatively widespread occurrences of offshore bars suggest that similar traps may be fairly common in ancient shelf sediments.