It is reasonable to conclude that hydrocarbons begin to form soon after the organisms from which they are derived are buried with the sediments that constitute both source rock and reservoir bed. Therefore, structural-mapping methods used to define likely areas of accumulation should be used to reconstruct the geologic history from the time of deposition by analyzing both the initial structural growth and the later structural tilting that influenced the migration and entrapment of the hydrocarbons.

Structural mapping methods, commonly used today, locate "structure" as it presently appears without regard for (1) initial local structural uplift which influenced the movement of hydrocarbons as they began to migrate, or for (2) later structural tilting which tended to breach the original "paleostructural" trap and possibly caused the original accumulation to move to another place. Therefore, it is possible, using customary structural mapping methods, to map a high properly that has no accumulation because no trap existed at the critical time of initial migration. Likewise, it is also possible that the initial structural closure has no accumulation today because it was breached by later structural tilting.

The purpose of this paper are (1) to demonstrate a logical method of paleostructural mapping by use of carefully selected isopachous intervals, and (2) to define later structural tilting and to present a practical method of applying it to understand and define better the likely areas of accumulation.

Examples of application at North Francitas field, Jackson County, Texas, and at Rayne-Bosco-Ossun fields, Acadia and Lafayette Parishes, Louisiana, are presented.

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