Lithologic and paleontologic variations within the Oligo-Miocene strata of southwestern Louisiana suggest a time of highly fluctuating relative sea level, such that transgressive-regressive cycles occurred in three orders of magnitude. The second and third order cycles probably represent depocenter shifts and glacial eustatic sea level fluctuations, while the first order pattern reflects geotectonic and geosynclinal fill-compaction relationships.

Within this cyclic regime, migration of the massive sandstone facies, alternating sandstone and shale facies and massive shale facies of southwestern Louisiana, produce rather complex lateral facies relationships. To facilitate environmental reconstruction within this system, a new coefficient termed the facies index is introduced. The usefulness of this coefficient, expressed by the equation: Facies Index = net sand/(number of sands x shale thickness) is based on the concept that a more proximal location to a fluvio-deltaic depocenter is not only characterized by higher percentage sandstone but also by less sandstone-shale interbeddedness for a given percentage sandstone (resulting in a higher facies index).

By superimposing facies index maps on structure and isopach maps, contemporaneous structural and depositional frameworks are derived. More localized depositional and structural models, which are essential in establishing the hydrocarbon source and migration pattern as well as the reservoir characteristics and trapping mechanisms, can then be reconstructed as natural components of the regional system.

During the time of the Heterostagina and lower Discorbis zones of the Anahuac Formation (Oligocene), regressive east-west oriented delta front sands fluctuated (third order cycles) within an overall transgressive regime (second order). These deposits were then overlain by an extensive progradational fluvio-deltaic system which is represented by the upper Discorbis zone and lowermost Fleming Formation (Miocene). This regression was then interrupted by a large transgression which deposited a shale wedge that contained Siphonina davisi (intermediate neritic), Planulina palmerae and the upper bathyal Abbeville Assemblage (southern portion of study area). This transgression was followed by another regression and growth fault episode which prograded the shoreline to the southern extreme of the study area (Rockefeller Refuge of eastern Cameron Parish), where extremely thick lower Miocene sands were deposited within a structural embayment.

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