Glyn Roberts, Kjell Finstad GGS-Spectrum
The eastern Gulf of Mexico is one of the US’s last remaining petroleum frontier areas. GGS-Spectrum recently conducted a 2D seismic survey (West Florida WF-05) in the eastern GoM which indicates numerous plays, including substantial transitional/continental crust in deeper waters. This marks a significant change in the perception of the area, which was previously thought to be characterized by oceanic crust.
Phase 1 of the survey lies east and southeast of the original Sale 181 area, running SSE along the Florida Escarpment towards the Dry Tortugas and the EEZ boundary and covering OCS protraction areas: De Soto Canyon, Florida Middle Ground, Lloyd Ridge, The Elbow, Vernon Basin and Howell Hook.
The Eastern extent of the survey is approximately 125 mi from the west Florida coast. Water depths vary from 200 m to 3,200 m and the present day bathymetry is dominated by the Florida Escarpment. The survey is being extended Southwards (phase 2) and Westwards (phase 3) into the Henderson and Florida Plains OCS protraction areas, to give total coverage of approximately 27, 000 line km of seismic data.
With the exception of the Sale 181 area, very little modern seismic has been acquired in the eastern GoM prior to this survey due to a drilling moratorium.
The results of this survey will allow companies and authorities to more accurately identify potentially commercial volumes of oil and gas and to consider their future development. What follows is a description of the region and what we have concluded thus far regarding its petroleum potential.
Geological history
The structural history of the GoM began with the break-up of the super-continent Pangea in early Jurassic times (roughly 200 million years ago), creating grabens and half grabens and providing the environment for the deposition of large amounts of sediment. Initially, these would have been lacustrine and red bed clastics followed by marine transgression and the formation of salt (e.g Louann salt), anhydrite deposits and marine sedimentation. The Louann salt was probably deposited during late Middle Jurassic, just after the rifting ended, and was mobilized later as diapirs into strata of Upper Cretaceous to Lower Oligocene age in most of the survey area.
Later, during the Upper Jurassic, the northern part of the eastern GoM was characterized by aeolian Norphlet sandstone deposition in terrestrial sub-basins which could be up to 1,000 m thick in part of the area. In addition, marine shales and clastic turbidites were most likely deposited to the south and west. This was followed by deposition of what is known as the Smackover Carbonates on a ramp surface dipping to the west and southwest. The Smackover is organic rich and represents one of the main source rock intervals in the area; it also contains a number of reservoir intervals.
During this time, the GoM exhibited initial sea floor spreading and volcanism, probably represented today by oceanic crust in the center of the GoM, (west of the area of study); and by volcanics at a pre-salt level in the basin.
From the early to late Cretaceous period (144 to 65 million years ago), there were a series of clastic and carbonate regressive sequences, resulting in the seaward progradation of the basin shelf and the development of a shelf edge reef complex. This regressive sequence produced a thick sequence of carbonate rocks, which interfinger with clastics in the back-reef area.
Between the late Cretaceous and Oligocene periods, deepwater sediment gravity flow carbonates interbedded with pelagic shales were deposited.
A major change in the level of sediment input occurred in the Oligocene, resulting in a thick sequence of clastic gravity flows and pelagic/hemipelagic shales being deposited from late Oligocene to the present day.
Potential plays
In light of this rich geological history, a variety of plays are found both on the Florida Platform (shallow water) and the present day deepwater area (Fig. 1):
Fig. 1: Geological provinces in the eastern GoM.
A) Platform
- Cretaceous Shelf Edge reefs and fans
- Inner Platform plays e.g Norphlet sandstone and James Limestone plays.
B) Deepwater
- Jurassic horst/graben (buried hill) plays
- Jurassic/Cretaceous plays associated with salt tectonics
- Plays associated with the Cretaceous/Ter- tiary Boundary
- Oligocene and Miocene clastic plays.
Potential source rocks are postulated to be present in both the Upper Jurassic (Oxfordian to Kimmeridgian carbonates; and possibly Tithonian shales and carbonates as in Mexico) and the Lower Cretaceous (Aptian to Albian carbonates and shales). The Upper Jurassic source is expected to be gas and condensate prone, while the Lower Cretaceous source is expected to be oil prone.
The Upper Jurassic Norphlet-Smackover petroleum system has been proven by drilling to the North in the Destin Dome area and can be assumed to be present in the Central West Florida Shelf. Petroleum seeps have also been reported in the area.
Some of the play examples are summarized in Fig. 2.
A) Platform Area
1) Cretaceous Shelf Edge Reefs and fans
The present day Florida Escarpment broadly coincides with the Early-Late Cretaceous shelf edge reef, which is made up of a series of stacked carbonate platforms. It has been reported elsewhere that reservoirs are expected within this reef complex, which developed from karsting and dissolution during frequent lowstand episodes.
The nature of the reef is seen in Fig 3a and 3b, which shows a Pre Stack Depth Migration (PSDM) of a typical line across the Florida Escarpment. The PSDM section, displayed in depth, allows the geophysical interpreter to more easily understand the relationship between the platform and the deepwater area. The PSDM data facilitates easier mapping of the base Louann salt from the deepwater area onto the platform (where there also appears to be a salt swell underneath the escarpment) and onto the top of the basement (which is of Palaeozoic/Triassic age).
Fig. 3b: PSDM section displayed in depth.
In addition to the shelf edge play described above, a carbonate slope apron play of Upper Cretaceous to Lower Oligocene age is seen at the foot of the escarpment on many lines. The periodic failure of the upper carbonate slope and the shedding of large volumes of sediment to the basin have created these apron fans.
2) Inner Platform plays
Possible plays landward of the shelf edge are expected to be either of Jurassic age e.g., Norphlet Sandstone stratigraphic plays or Cretaceous in age (e.g., the James Limestone play). Both plays have been successfully exploited in the Viosca Knoll area to the NW of the survey area. The James Limestone play is an extension of an onshore trend through east Texas and south Mississippi and is characterized by patch reef debris and grainstone facies.
Also of note is the possibility of a Thrombolitic reef play as seen in Fig 3b on the basement at a depth of 8,000 m and shallower at 7,000 m indicating that these microbial mounds may have existed over a long period of time in this part of the inner Platform. The algal facies is noted for good porosity and very good permeability when dolomitized. To date, major development of this play has been onshore with over 50 Smackover age fields of this type (e.g. Appleton) developed in the NE GoM area. They are characterized by being located on palaeohighs.
The Cretaceous sequence also exhibits evidence of channels and possible gas effects. These channels are seen on NW-SE strike lines, indicating possible run-off from rivers perpendicular to the escarpment. These rivers would be a source of siliclastics to the basin.
B) Deepwater plays
1) Jurassic 'buried hill' plays
The paleotopographic basement features which characterize ‘buried hills’ plays can act as reservoirs themselves (as in the buried hill structural play), or due to erosion they can give rise to detrital plays, or alternatively as a result of differential compaction they can produce overlying drape plays.
Fig. 4: Buried hill plays.
An example of the buried hills play in the deepwater GoM is shown in Fig 4. The play is analogous to the Brent tilted fault block plays in the UK North Sea.
2) Jurassic/Cretaceous Plays associated with Salt tectonics
Salt tectonics are generally autochthonous and are represented as salt pillows and diapirs. The growth of these pillows and diapirs has been controlled by the weight of overlying sediment and by tectonics. Consequently ‘mini-basins’ are formed between domes. Here we expect that hydrocarbons were sourced in the grabens and migrated to the salt dome areas, where the salt and marls provided seals. The presence of salt extends into the southern part of the phase 1 area of the survey (phase 2 has not yet been collected). Consequently the salt is more extensive in area than has been previously reported in some of the literature.
3) Plays associated with the Cretaceous/Tertiary (K/T) Boundary
In the deepwater area, the K/T boundary represents a transgressive event which deposited marine shales and siliclastics over older relief or possibly providing both good seal and reservoir. As PJ Post states in his 2005 paper, this good seismic marker has for a long time been identified as the Mid Cretaceous Sequence Boundary but is now recognized as the top of the Cretaceous. This change is due to paleontological evidence from modern deepwater wells. Reefs or mounds are also found at this level. It is thought that these mounds could be barrier sands and thus a prospective play.
4) Oligocene/Miocene plays
An extensive basin floor depositional system has been recognized in the deepwater area. Many of these sands have been shown to be ‘bright’ and are seen to onlap onto older sediments. The areas of onlap cover an extensive area and, as seen in Fig 5, possibly indicate different directions of sediment provenance through time.
Salt tectonics have contributed to the structural evolution of the basin, giving drapeover and onlap plays in the younger sediments.
The Miocene has been shown to be productive in the Sale 181 area to the NW of the survey. An example is the newly discovered Spiderman (DC 620/621) and Jubilee (AT 349) gas fields where Anadarko have interpreted a basin floor setting incorporating a variety of deep water depositional processes, including turbidites and channel deposits.
Conclusion
The new WF-05 seismic survey shows a large number of different plays with excellent potential for hydrocarbon accumulation both in the deep and shallow water parts of offshore West Florida. The seismic data shows the presence of continental/transitional crust in the deep water with large rifted grabens/horsts and extensive salt tectonics adding to trapping mechanisms. Further geophysical studies are planned in the near future using AVO and PSDM technology. The gravity and magnetic data that was collected along with the seismic survey will also be integrated with the seismic data. In addition this work may also help answer some of the outstanding questions concerning the nature of the continental/transitional/oceanic crust which have been put forward by geoscientists who have studied the GoM area.
