A structural relationship exists in several areas in the Southwestern region between rock types in the Precambrian and Paleozoic structural trends. The Ouachita geosynclinal belt on the south and east of the Texas Craton which underlies the Southwestern region was the dominant structural feature during the Paleozoic and probably exercised indirect control over all intracratonic structures. Most of the early and middle Paleozoic oil and gas reservoirs are associated with unconformable surfaces. These reservoirs were formed by weathering of the unconformity surface particularly where pre-unconformity folded and faulted structures existed.

Four Paleozoic continent-wide interregional unconformities are present. The approximate dates of the regressional maxima represented are: (1) very late Precambrian, (2) early Middle Ordovician, (3) early Middle Devonian and (4) post-Mississippian. In addition, many regional unconformities are present, the most important in relation to the source, migration, and accumulation of oil and gas being that between the Permian and Pennsylvanian. This regional unconformity is composed of local unconformities associated with structurally positive areas. Early and middle Paleozoic hydrocarbon source beds were probably organic-rich sediments overlying the unconformities. Late Paleozoic Pennsylvanian and Permian sediments were by far the most abundant source of hydrocarbons in the reservoirs in th middle and early Paleozoic formations. Within the Pennsylvanian and the Permian, unconformities are important; however, their importance is overshadowed by lithofacies phenomena in the form of reefs which

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formed the most prolific oil-producing reservoirs of those two periods.

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