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Recognition of ancient depositional and structural environments within the Oligocene and lower Miocene hydrocarbon production trends of southwestern Louisiana was based on the interpretation of local lithologic and paleontologic variations within the spatial and temporal components of a regional framework. This trend-oriented technique is useful in predicting environmental origins and associated reservoir characteristics.
Spatial relationships were established by generation of lithofacies and structural contour maps produced from approximately 249 well logs. The massive sandstone facies (continental) and the downdip alternating sandstone and shale facies (deltaic) of the area were analyzed with a new coefficient, the facies index. This coefficient, defined as the average sandstone thickness divided by the total shale thickness, combines the effects of percentage of sandstone and amount of sandstone-shale interbeds. These maps were then superimposed on structure and isopach maps to derive simultaneous structural and depositional models.
Paleontologic studies were incorporated to decipher the Oligocene-Miocene depositional history of the area (temporal component), which is directly related to cyclic transgressive-regressive episodes of three orders of magnitude. The second- and third-order cycles probably represent depocenter shifts and glacial eustatic sea level fluctuations, whereas the first-order pattern reflects geotectonic and basin fill-compaction relationships.
With the integration of localized facies analyses, which were based on individual sandstone geometries, log profiles, lithologic relationships, and paleontologic data, an Oligocene-Miocene depositional and structural model was established. It is suggested that the Heterostegina and lower Discorbis zones of the Anahuac Formation (Oligocene) contain regressive east-west oriented (delta front) sands within an overall transgressive regime. The overlying upper Discorbis zone and lowermost Fleming Formation (Miocene) were probably formed by deposits of an extensive progradational delta system. This regressive sequence is overlain by a transgressive shale wedge containing Siphonina davisi (intermediate neritic), Planulina palmerae, and the upper bathyal Abbeville Assemblage which is found in the southern portion of the study area. After this transgression, a renewed regression and growth fault episode began in the north. This system prograded the shoreline to the southern extreme of the study area, where the unusually massive lower Miocene sandstones of eastern Cameron Parish (Rockefeller Refuge) originated from fluvial processes within a structural embayment.
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