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James A. Baird (1)
Unconformities in the GulfCoast Province too often have been regarded only as a vague concept and are seldom used as an exploration tool. Their identification and reconstruction can be used to locate relic local and regional structures and to determine past and present locations of hydrocarbons and their economic potential. Late structural deformation can cause secondary migration and/or alter dip rate and dip direction, thus concealing the present locations of trapped hydrocarbons. Structural-stratigraphic relationships can be modified by fault systems, piercement domes and younger unconformities which may intersect older unconformities. The present locations of trapped hydrocarbons can be found on an original buried relic structure or on a later structure, or both, depending upon the nature of the trap and subsequent structural movement and migratory history of the hydrocarbons. Structure and stratigraphy related to this time of movement, as dated by unconformities and paleontology, can be used on a regional and local basis to find embayments, pro-delta fronts, wedges, turbidite deposits and relic structures. By constructing paleogeomorphic maps on each significant horizon, and superimposing maps of successively younger units on those of older units, it is possible to reconstruct the sequence of deposition, structural uplift, erosion and burial. North Chacahoula field, Lafourche Parish, Louisiana, is an example to which these techniques can be applied. Paleogeomorphic maps and stratigraphic cross-sections reveal that six periods of emergence and erosion occurred on top of a tectonic structure which has since been buried. During and after each period of erosion, structural movement tilted the unconformities. The net result is that unconformities intersect each other where earlier unconformities became exposed to later surfaces of erosion, and the unconformities climb in the section away from the axis of the relic structure.
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