Stratigraphy has been recognized progressively as an important factor in the entrapment of Smackover hydrocarbons in the mature Arkansas and Louisiana producing fairway. Although most of the older fields generally were considered to be essentially in structural features, the importance of stratigraphic factors in entrapment has been dramatically focused by the discovery of Walker Creek field, Lafayette and Columbia Counties, Arkansas.

The accumulation in this field is caused by stratigraphic entrapment by the updip termination of porous Smackover beds across a gentle structural nose. Discovery of Walker Creek

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signals the beginning of the second phase of Smackover exploration--the search for combination structural/stratigraphic and wholly stratigraphic traps--and the rebirth of exploration for large reserves in a mature segment of the trend.

The regionally regressive depositional character of the Smackover in this area afforded an excellent setting for the formation of many stratigraphic traps. Porous carbonate zones, successively higher within the Smackover section, were deposited southward across the shelf. The updip terminus of each zone abuts an impermeable seal to form an ideal stratigraphic trap. The sinuous nature of the updip terminus, in many places in conjunction with low relief structural noses or closures, entraps the hydrocarbon accumulation laterally. In addition, many variations in the regional situation, due to the local depositional patterns of individual zones, tend to complicate the simple stratigraphic trap.

Lithologically, the most characteristics reservoir rock is an oolitic-pelletal limestone with intergranular porosity. Porosity up to 30% is not unusual, but average porosity ranges from 10 to 20%. Various degrees of porosity destruction have resulted from the infilling of the primary porosity with sparry calcite cement. Where wave action was not sufficient to winnow out carbonate muds, no primary porosity was developed.

The diverse nature of the stratigraphic traps opens up unlimited exploration opportunity on acreage once considered worthless because it was not located on closed structures. The stratigraphic phase of exploration now promises to be as profitable as was the structural phase in this "old" producing area.

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