Hentz, Tucker F., Zeng, Hongliu, and Kilic, Cem O.

Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Integrated 3-D seismic, well-log, and petrophysical analysis of mature gas reservoirs in Vermilion Block 50 and nearby Tiger Shoal fields established a firm genetic context from which reservoir-specific attributes could be derived for the location of additional resources. The 10,000-ft study interval, within which as many as 36 potential gas and oil reservoirs occur, shows minor faulting and represents about 15.5 m.y. of depositionÒnearly the entire Miocene Series.

The overall regressive succession comprises more than 30 fourth-order (~0.5-m.y.) sequences. Paleontologic data indicate that third-order cycle tops coincide with those of published basin-wide coastal-onlap curves. Systems tracts are increasingly dominated by more landward facies upsection. Areal patterns from net-sand maps, 3-D stratal slices of seismic data, and log-facies analysis indicate that the succession represents nearly an entire basal-slope (bottom-lower Miocene) to proximal-shelf (top-upper Miocene) depositional tract. Sequences lower in the section are 200 to 600 ft thick and contain few but thick (200 ft) incised-valleys (IV) filled with shaly estuarine deposits and upper-slope-canyon fills. Local lowstand-wedges onlap the upper slope surface. Conversely, upper sequences, 100 to 200 ft thick, in part comprise blockier fluvial-dominated IV fills and proximal deltaic sands.

Producing and untapped gas reservoirs occur primarily in five facies within the two fields. In Vermilion Block 50, slope-fan, lowstand (LS)-wedge, LS-IV, trangressive-bayhead-deltaic, and late-highstand (HS)-deltaic sands of lower to middle Miocene age predominate. In Tiger Shoal, reservoirs occur in generally stratigraphically and structurally higher mid- to-late-HS-deltaic and LS-fluvial, IV sands in the middle to upper Miocene section.