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