Three-dimensional seismic interpretations of Miocene strata in Vermilion and South Marsh Island areas, Gulf of Mexico

The Louisiana continental shelf is the most prolific offshore hydrocarbon province in the nation. Siliciclastic Miocene strata in south Louisiana and the northern Gulf of Mexico continental shelf is the most productive chronostratigraphic unit accounting for 40% of the hydrocarbons produced in the region and 40% of remaining proved reserves.

The majority of active fields on the continental shelf of this region are confined to the upper Miocene and are considered mature. Future production in these fields will, therefore, require interfield and intrafield development. On the other hand only 5% of wells drilled on the Gulf of Mexico shelf have penetrated the lower Miocene strata (below 15,000 fit or 4572 m), in which there is an estimated 10.5 Tcf [297 billion m^3] of deep-gas recoverable resources. With such great potential for new gas resources in the deep Miocene, there is a need for research to delineate a detailed structural and stratigraphic framework to guide future exploration efforts in this region.

The purpose of this study is to identify possible locations of potential undrilled resources in Miocene strata located in the Vermilion and South Marsh Island areas of the Louisiana continental shelf. Previous work on this area by the Bureau of Economic Geology has produced a sequence stratigraphic framework and detailed structural analysis of the upper 3.0 seconds of strata within this seismic survey. Using this work as a frame of reference, we analyze regions in the lower 3.0 seconds of strata to produce a structural interpretation of the lower Miocene.

Identification of structural and stratigraphic features in the lower Miocene provides information as to the formation and trapping of hydrocarbons in the area. Our methodology incorporates the interpretation of seismic attribute, coherency slice, and isochron and isopach maps to identify areas of potential hydrocarbon accumulation. In addition we use sonic logs and synthetics in our interpretation to approximate the depth to which these potential resources may be found.

This project has produced: (1) An interpreted seismic data set that has been correlated to well logs using synthetic seismograms; (2) A geologic description that emphasizes structure below 3.0 seconds (ie. the lower Miocene); (3) Identification of regions in the lower Miocene with structural or stratigraphic features that could hold future reserves and the relationship to regions of current production. Some of these features identified are a delta lobe, a drop graben with rollover features and bright spots, and several traps associated with salt structures and growth faults.