Innovative Methodology Applied to 3-D Exploration in an Emerging Trend; Norphlet (Upper Jurassic) Sandstone, Monroe County, Alabama

William R. Dean, Jozica H. Gabitzsch

Exploration of the Frisco City (lower Haynesville) sand trend of Monroe County, Alabama has lead to the discovery of an updip Norphlet play. The Norphlet (upper Jurassic) sandstone was deposited in the low ravines and valleys that were incised into pre-existing metamorphic basement ridges. Recent 3-D seismic surveys have provided the first look at the areal distribution of these sands. There are, however, certain 3-D acquisition and processing pitfalls that can effect the accuracy of locating these structural and stratigraphic traps. The purpose of this presentation is to illustrate a method for refining a spatially varying velocity field from sonic derived velocities for a more accurate 3-D migration in the North Excel Field area.

Many processors now have one-pass 3-D migration algorithms that can utilize spatially varying velocity fields. The results are generally quite good--especially after 3-D DMO has been applied. in this case, however, over-migration of the Frisco City amplitude was observed. Other events in the Norphlet section below the clastic/carbonate interface were under-migrated and poorly focused. A comparison of the interval velocities extracted from the seismic data to the sonic derived velocities from well control revealed a high correlation before migration and a poor match after. A new spatially varying velocity field was generated by interactively adjusting the migration velocities at 260 locations along a mapped horizon to precisely tie the sonic derived velocity model. Application of the n w velocity field with the appropriate migration algorithm resulted in a product with coherent, well-defined amplitudes and an overall better focused seismic image.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995