During the Miocene and continuing through the Pliocene, great volumes of sediment were delivered rapidly to an eastward-shifting depocenter in southernmost Louisiana. Sediments were introduced to the basin by rivers and were distributed farther by marine currents gulfward and laterally across persistently broad shelf areas; the amount and grain size of the

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sediments decreased with distance from the deltas. At any depositional moment, three gradational facies (sand, sand-shale, and shale) were being developed in bands mainly parallel with the shoreline. This depositional pattern persisted through the late Miocene and Pliocene so that in the total geometry of this net-regressive sedimentary wedge, the facies may be grossly viewed as having developed a sand, a sand-shale, and a shale magnafacies.

The sand magnafacies has been productive. However, the majority of hydrocarbons have been found in the inner- and middle-neritic zones of the sand-shale magnafacies in structural traps formed by salt diapirs and growth faulting. The gulfward limits of their exploitation can be determined reasonably from present data and should be confined largely to water depths up to 600 ft. Any significant discoveries are likely to be in the offshore. Louisiana is approaching maturity in its development of these facies but there is room for limited extension. Offshore Texas is largely unexplored but results of drilling through 1969 appear to have been disappointing. The volumes of sediments in the sand and sand-shale magnafacies remaining to be exploited are estimated as follows:

Table

A possible future source exists in the shale magnafacies, where turbidite sands can reasonably be expected on the updip flanks of salt structures and in the lows between them. The search for reservoirs of this type, particularly in the younger sections, will involve operations beyond the edge of the continental shelf in water exceeding 600 ft in depth. Exploration in the older sections can be accomplished in shallower waters, but will require increasingly deeper drilling. Turbidite exploration demands a considerable sophistication of seismic techniques and the best efforts of geologists, paleontologists, and geophysicists. No reasonable estimate of favorable sediment volume in the shale magnafacies can be made at this time.

Present economics barely have justified the oil industry's endeavor in the prime sand and sand-shale magnafacies in water less than 600 ft deep. The same economics hardly can be expected to support operations beyond the edge of the continental shelf, particularly in the face of deteriorating or conjectural objectives. Increased incentives are necessary to encourage the costly risk-taking that will be required to find and develop any future trends in the upper Miocene and Pliocene shale magnafacies. Instead, it appears that current political attacks on existing incentives will be successful to some degree, and incentives will be lessened. Without substantial compensating factors, economic considerations may jeopardize all future explorations in the Gulf.

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