Unconformity Analysis

ABSTRACT The shelf or foreland of a geosyncline or basin is an area in which there are commonly many unconformities. As a result of repeated tilting, the strata above and below each unconformity are not strictly parallel. Unconformity analysis is a method of reconstructing the history of each unconformity and determining the relations of each set of strata. Four situations are considered: (1) simple tilt—successive transgressions are essentially parallel, (2) differential tilt—successive transgressions are not parallel, (3) synclinal folding, and (4) anticlinal folding. Southwestern Arkansas Cretaceous and Tertiary strata are examples of (1) and (2), the northeast Texas Cretaceous interval illustrates case (3), and the Pennsylvanian section of the north flank of the Hunton anticline in Oklahoma is an example of case (4). In each of the examples there are three or more important angular unconformities; the distribution of the formations above and below each unconformity constitutes an important phase of the analysis. The foreland shelf is a region conducive to the migration of petroleum and its accumulation, most especially in stratigraphic traps. Associated with each unconformity are two sets of potential stratigraphic traps—one below and one above. By careful unconformity analysis one can determine the subsurface trend and width of potential producing zones. Without such an analysis one may drill for an objective which is not present beneath the well location. The conditions presented here in theory and by example are duplicated in many parts of the United States. A list is appended of those areas in which unconformity analysis might be used to find more oil in stratigraphic traps.