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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 49 (1999), Pages 15-16

EXTENDED ABSTRACT: Late Miocene Mississippi Fan Fold Belt Effect on Deepwater Deposition in the Atwater Valley Area, Offshore Louisiana

Andrew E. Hannan, Norman E. Biles, and George A. Jamieson

Schlumberger Geco-Prakla, 1325 South Dairy Ashford, Houston, TX 77077-2307

ABSTRACT

With well control and regional grids of seismic data now available, can we determine what controls the deposition of reservoir quality sands in the deep water Gulf of Mexico? Sediment gravity flows transport sands derived from the shelf down across the slope. Paleo-topographic features on the slope such as salt domes, growth faults, minibasins, and deepwater fans tend to steer sands into topographic lows. The sands spread out into coalescing sheets at the toe of the slope in abyssal water depths. The Sigsbee Escarpment is an allochthonous salt ridge that is a prominent feature on the offshore Texas and Louisiana slope. The Gulf of Mexico is a passive margin basin. There are several features that can occur on passive margins that control the deposition of gravity driven "turbidite sands". Basement faults exist in the attenuated crust of the passive margin, and the reversal in the slope direction on the far side of the basin may control the location of sandstone deposition in the deep water. Fold belts are also a part of the passive margin.

The Mississippi Fan Fold Belt is located in abyssal water depths offshore Louisiana down slope from the Sigsbee Escarpment. The northeast trending fold belt is characterized by steeply dipping, asymmetrical, southeast thrusting anticlines, and associated overthrust faults. In order to see the effect of the Mississippi Fan Fold Belt on sandstone deposition in the Atwater Valley Area, a portion of a 60,000 mile 2-D seismic data set ("Phase 45") and paleontological control were used to construct structure and isochron maps over the Atwater Valley Area. Regional thinning seen on the seismic data indicates that the Mississippi Fan Fold Belt developed during the Late Oligocene to the Early Pliocene in the Atwater Valley Area. Isopach maps of regional seismic data indicates Late Miocene to Late Pliocene thinning over the fold belt. A well log geological cross-section was constructed across the Mississippi Canyon Area from the Cognac Field south, through the Mars Field, and Mississippi Canyon Block 899, to the Atwater Valley Block 471 Shell #1 well, the southernmost well in the area.

Deepwater slope fan sands were deposited just to the north of the Mississippi Fan Fold Belt in a salt withdrawal basin that contains the Late Miocene to Early Pliocene Mars and Ursa Fields in the Mississippi Canyon Area. Late Miocene and Early Pliocene sands are not present in the Atwater Valley Block 471 Shell #1 well on the crest of a fold belt structure. Seismic data tied to regional well control indicates sand rich Late Miocene and Pliocene section onlapping the north flank of the Fold Belt. Thick wedges of Late Miocene to Pliocene section occur on the north flank of the Fold Belt. Possible hydrocarbon indicators are present on both the northwest and the southeast flanks of the structure away from the well location in (Fig. 1).

The Mississippi Fan Fold Belt may have created some topographic relief on the Oligocene abyssal sea floor. During Late Miocene and Pliocene times, the fold belt was a barrier to deposition for sediments moving down the Louisiana slope from the north because there is an isopach thin to the south of the Fold Belt that would have been in the shadow of the topographic high. Sands may have gone around the northeast flank of the Fold Belt, which plunges to the northeast. Wells have indicated an abundance of Middle Miocene sands in the southern Mississippi Canyon Area. Since there are some Middle Miocene sands in the Atwater Valley 471 Shell #1 well on the crest of a topographic feature in the abyssal plain, there may be more sands to the south of the Atwater Valley Area. The geologic cross-section reveals another sand bypass area around the Mississippi Canyon 455 Union Oil #1 well higher up on the slope.


REFERENCES

Chowdhury, A.N., D.L. Risch, and A.E. Hannan, 1994, Use of sequence stratigraphy in hydrocarbon prospecting: An example from the Green Canyon Area, Offshore Louisiana: Gulf Coast Association of Geological Societies Transactions, v. 44, p. 119-124.

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Figure 1. A northwest to southeast seismic line cutting in a dip direction across the Mississippi Fan Fold Belt through the location of the Atwater Valley 471 Shell #1 well that shows the thinning of the Late Miocene and Early Pliocene section over the crest of the structure. Note the possible hydrocarbon indicators that flank the well location in the Pliocene section.

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