Apalachicola embayment may be continent's last virgin oil basin

Jamil Azad
Azad Exploration Ltd.
Calgary

History has shown that the largest fields in a given basin are usually discovered early in its exploration.

Today in the developed areas of the U.S. and Canada each successful exploratory well yields on the average less than 100,000 bbl of oil. Only one exploratory well in seven is successful.

The odds of discovering a large oil field were much greater in the early days of exploration. For example:

The first well in Pakistan's Sind Province hit giant Sui gas field.
The first wildcat in northeastern India's Assam area turned up Digboi oil field, and the second found Nahorkatiya oil field.
The first two wildcats in Burma (Myanmar) produced Chauk and Yenanyaung fields.

There was a time when it seemed that virtually no wildcat off Southeast Australia, in Ecuador's Oriente basin, or in Alberta's Rainbow Lake area could miss a major discovery.

The lesson is clear: Find a virgin petroliferous basin and be among the first to drill it.

One virgin petroliferous basin remains in North America. The basin is sprinkled with giant structures, all of them undrilled, and their presence is firmly established.

Apalachicola embayment

The Apalachicola embayment since its inception in Jurassic times has been affected by a gradual subsidence and a tilt towards the southwest and deeper part of the basin (Fig. 1 [91,594 bytes]). Otherwise it is free of salt tectonics, folding, and significant faulting.

Fig. 2 [79,730 bytes], showing the configuration of basement in the Apalachicola embayment, has been amended from Dobson and Buffler.¹ Fig. 2 incorporates the results of a seismic survey that were not available to Dobson and Buffler. Also shown on Fig. 2 is Coastal Petroleum Co.'s State of Florida Drilling Lease 224-A, which as will be seen is critical to this play.

The Jurassic is a major petroliferous system in the Gulf of Mexico/Gulf Coast basin, where it hosts more than 150 onshore oilfields and must have contributed greatly to the reserves that have migrated upwards and sideways that are now being produced from Upper Tertiary sediments in the central parts of the basin. The Destin Dome is a salt pillow that has gas in the Norphlet but no Smackover reefs.

Fig. 3 [60,200 bytes] shows the area of Smackover-Norphlet sediments to their pinchout. Note the coincidence between the basement culminations (Fig. 2) and the updip pinchout of the Smackover-Norphlet. Also note that the enduring tilt offers a favorable paleoslope for migrating hydrocarbons and that none of the Apalachicola embayment's large reefs has yet been drilled.

The 420 million bbl Jay (Smackover) oil field in Santa Rosa County, Fla., is similar to these Apalachicola reefs (Fig. 1), but Jay lies outside the Apalachicola embayment proper.

Fig. 4 [63,391 bytes] is a schematic cross-section run between Destin and the Apalachicola embayment highs. The Louann salt pinches out to the west.

The westerly part of the basement surface has low relief and is blanketed by a sheet of basement-derived clastics (Norphlet), which is generally porous and offers a conduit for the Smackover generated hydrocarbons to migrate updip. As the Smackover transgressed on a ramp it developed a number of large reefs.

The eastern part of the section shows the strong relief that results from the erosion of remnants of the Apalachicola orogen. There, the large Smackover reefs that cap the highs are in an ideal position to collect the petroleum that is sourced in the local sediments, organically rich Smackover micrites that surround them, and the petroleum that has migrated over long distances from the deeper part of the basin as well.

The maturation index suggests that petroleum of approximately 50° gravity can be generated and preserved under these circumstances.

Reefs on seismic

Fig. 5 [149,043 bytes] shows the seismic expression of two very large Smackover reefs developed on the Apalachicola ramp. A 1984 study²suggested that these buildups are coral-dominated patch reefs and bioclastic piles. The reefs are approximately 3 miles across and lie at a depth of 16,000-18,000 ft. As indicated above, these traps are well placed to catch locally sourced petroleum as well as petroleum that migrates up the ramp.

Fig. 6 [150,379 bytes] shows the basinal Smackover turning into a reef at contact with a basement high. This true giant is 5 miles across and perfectly placed to collect migrating Smackover petroleum.

The prospects that explorationists would no doubt select first are reefs such as those shown by the two small red crosses on Figs. 2 and 3 because of their optimum location. If any reefs are likely to collect their full load of petroleum, these two should be the best candidates. Fig. 7 [174,659 bytes] shows the more easterly of those two reefs.

A seismic cross-line (not shown) demonstrates four-way closure. Fig. 8 [128,292 bytes] is the westerly buildup. It, too, has a seismic cross line that demonstrates four-way closure. Prominent Norphlet clastics prograde from the right, and a beautiful reef talus drops to the left of the buildup. The framework should be coral-stromatoporoid.

This reef covers roughly 4,500 acres and is about 1,300 ft high. Applying the parameters of the Jay oilfield model (average porosity 15%, salt water 8%), the possible oil in place is 6 billion bbl. The other reef has a potential of 3 billion bbl of oil in place.

It is impossible to tell at this point how many of these very large reefs grew in the Jurassic petroliferous system of the Apalachicola embayment because of the limited amount of seismic available. However, the author has identified 10 very large reefs from the available seismic.

The reefs of the ramp should prove to be alignments of patch reefs, and no one knows how many platform margin and basin margin reefs will be found. A conservative guess would be at least three dozen.

These large reefs are in 35-60 ft of water and at depths of 12,500-18,000 ft. Costs are around $8 million/well. Upside are possible tens of billions of barrels of produced oil, but the catch is that only one opportunity exists to test this entire play.

State Drilling Lease 224-A

The entire offshore area of the Apalachicola embayment is contained within leasing moratoriums.

Leasing in federal waters has been prohibited by executive order of the President of the United States, and it is unknown how long this will remain in effect.

Leasing in Florida state waters has been prohibited by Florida law since 1990. The only exceptions to this moratorium are the existing leases owned and operated by Coastal Petroleum, which are shown in yellow on Fig. 2. These moratoria make Coastal Petroleum's lease a unique drilling island into the Apalachicola embayment Smackover reef play.

These leases date to the 1940s. They granted Coastal Petroleum the exclusive right to explore for and to produce oil, gas, and other minerals on millions of acres of Florida offshore areas. Coastal Petroleum drilled a number of wells, but the only one to test the Jurassic was the last well drilled. That well is located in the Apalachicola embayment. It did not test a Smackover reef, but it did find Smackover and Norphlet present near to the reefs.

Coastal Petroleum filed an application to drill the first test of the Smackover reefs on the reef shown in Fig. 8 in March 1992. The State of Florida and several environmental groups have fought the issuance of this permit, but after two appellate victories and a three week trial a recommended order of an administrative law judge in Florida found that the permit should be issued. Undaunted, the state again denied the permit. Coastal Petroleum expects the final appeal to result in the issuance of its permit to drill in the spring of 1999.

When the drill bit probes these large Smackover reefs we may indeed watch one of this continent's greatest exploration plays unfold.

Jay oil field was the largest discovery of oil east of the Mississippi River. It lifted Florida to the eighth largest U.S. oil producing state.

This Apalachicola Smackover reef play might result in scores of even larger oil fields. Florida would then be doing its part to assure that long term energy supplies exist for this continent.

References

Dobson, Laura M., and Buffler, Richard T., Seismic stratigraphy and geological history of Jurassic rocks, AAPG Bull., Vol. 81, No. 1, January 1997.
Crevello, Paul D., and Harris, Paul M., Depositional models for Jurassic reefal buildups, Third GCS-SEPM Conference, 1984.

The Author

Jamil Azad worked on various international petroleum exploration assignments for Burmah Oil Co. until 1971, when he took up consulting in geoscience in Calgary. He obtained a degree of geologist from Ecole Polytechique Federale in Lausanne, Switzerland.