EASTERN GULF OF MEXICO-2: Here is habitat of oil and gas under W. Florida shelf, slope

The province terminates to the north and east where the Norphlet-Smackover petroleum system onlaps updip on basement and to the south and west where the system grades downdip to lithologies unfavorable for petroleum generation and accumulation (Fig. 1).

The Norphlet-Smackover system is buried to more than 20,000 ft off Alabama and the western Florida Panhandle and ascends from there to the north to less than 14,000 ft (Conecuh embayment)²⁸ and east to less than 11,500 ft (Apalachicola embayment).²⁹ The system is buried to more than 19,000 ft in the Tampa half-basin and terminates apparently in the deep West Florida basin west of the Sarasota arch (Fig. 1).

Discovery of the Destin Dome Block 56 dry gas field, of minor gas-condensate, and light to medium-grade oil shows in wells on Pensacola Blocks 948 and 996 and Destin Dome Blocks 110 and 160, and of good oil shows in onshore wells on Permit Nos. 746 and 814 (Fig. 1)^{29 30} establishes that a significant, regional petroleum source exists on the northern DeSoto Salt basin margin. The lower part of the Smackover formation was identified in Southwest Alabama as that petroleum source.^{31 32 33} It can be presumed that the source continues to the central West Florida shelf and slope with the same petroleum-generating potential.

The oil-gas threshold occurs off the Florida Panhandle where the Smackover formation is buried to around 19,000 ft (Fig. 1). In the gas fields off Alabama, the Smackover top occurs at 20,100 ft,³⁴ and in Destin Dome Block 56 gas field that top is observed at 21,500 ft.¹

On the other hand, in the gas-condensate or medium-grade oil-bearing wells to the east on Pensacola Block 948 and Destin Dome Block 111, the same top is found at 18,280 ft and 17,695 ft, respectively (MMS Office of Resource Evaluation, written communication). The oil-gas threshold is projected here into the Tampa half-basin and identified with the anticipated isobath of the Smackover formation at 19,000 ft.

Norphlet play

Light-grade, 41-51° gravity oil (GOR 950-1,750 scf/stock-tank bbl (stb)³⁵ and 50° gravity gas-condensate (GOR 14,680 scf/stb)^{33 34} are being produced from the Norphlet play (see Fig. 2, Part 1) on land on the Jay-Flomaton trend (Fig. 1) at a depth of 14,600 ft to 15,500 ft and dry, sour gas (NGL yield less than 0.45 stb/MMscf)³⁵ in the sea around the mouth of Mobile Bay from 20,400 ft to 22,600 ft.

Other, sour petroleum occurrences:

• Dry gas had been discovered in Destin Dome Block 56 field on the northern West Florida shelf at around 22,300 ft.
• On Pensacola Block 948, where the top of the Norphlet play occurs at 18,672 ft, 54-56° gravity gas-condensate was recovered on a drillstem test.
• On Destin Dome Block 111, where the top of the play occurs at 18,363 ft, medium-grade (37°, 41°, and 42° gravity) oil was retorted from two conventional cores, and the cores were also described as being gas-condensate-bearing(?).
• On Pensacola Block 996, where the top of that play appears at 17,460 ft, light-grade (45-48° gravity) oil was retorted from conventional cores.
• On Destin Dome Block 160, where the top of the play is as shallow as 16,702 ft, light to medium-grade oil (39-45° gravity) was retorted from conventional cores (MMS Office of Resource Evaluation, written communication).
• Well Permit No. 746 in the onshore Apalachicola embayment encountered oil stains in a 7 ft core at 12,646 ft.^{29 30}

More than 18 million bbl of oil and gas-condensate and 244 bcf of gas have been produced on land around Flomaton. At the mouth of Mobile Bay, gas production in state waters since 1988 from eight fields amounts to 1.732 tcf and 227,000 bbl of associated NGL.³⁵ The latter fields currently undergo full production. Several more dry gas fields of even larger size are located on the adjacent OCS.

A recent assessment³⁶ lists the discovered gas in place in combined Alabama and OCS waters to be more than 10.5 tcf (equals recoverable reserves of 7.87 tcf at an assumed recovery rate of 75%). The recoverable dry gas reserves of Destin Dome Block 56 field are estimated to be as much as 2.6 tcf.³⁷

The attractive Norphlet sandstone belt^{18 38} that had accumulated over Louann salt^{39 40} is made up of an inner zone of alluvial and fluvial deposits and an outer, seaward zone of eolian reservoir development (long, narrow dunes and playa sheet-sands).^{18 40 41} The Norphlet sandstone loses its favorable reservoir properties where it grades into marine deposits.⁴⁰

The sandstone can become 1,000 ft thick in the nearshore dune facies (well Destin Dome Block 160-1).^{1 10 41} That thickness decreases in other wells on the northern West Florida shelf to 200-400 ft. The dune facies is the primary Norphlet exploration target. The preservation or even enhancement of effective porosity at the great depth of burial was differently interpreted^{42 43 44} (W. Frost, written communication).

The sandstone is sealed on top in the Mobile Bay region by a tight, cemented sandstone, considered to be eolian Norphlet sand that was reworked and cemented by the subsequent Smackover transgression,⁴⁰ or by mudstone of the Lower Smackover formation. It was also observed that the uppermost Norphlet porosity is choked with carbon derived from a once-existing oil field (W. Frost, written communication). A lateral seal is provided by the drape of overlying Smackover carbonates over Norphlet dunes.

Commercial traps of the Norphlet sandstone are primarily the combination of dune facies with superimposed Louann salt-cored anticlines.¹ The anticlines are large structures, highly compartmentalized by faulting.² Pre-Louann basement paleotopography and post-depositional faulting can also contribute to petroleum trapping.^{38 45}

Smackover play

More than 586 million bbl of 43-57° gravity oil (GOR 340-1,320 scf/stb)^{46 53} and 47° gravity gas-condensate (13,950 scf/ stb),³³ and 1.8 tcf of gas have been produced on land from the upper Smackover formation (see Fig. 2, Part 1) on the Jay-Flomaton trend (Fig. 1)^{35 46} at the depth from 14,000 to 16,100 ft.

Off Alabama, the Smackover play has not yielded any petroleum accumulations.^{20 47}

Off Florida, the determination of borderline light-grade oil (42° gravity) in only one well on Pensacola Block 996 at the depth of 17,025 ft is based on retorting of sidewall cores (MMS Office of Resource Evaluation, written communication). Good oil stains were recorded on land in the Apalachicola embayment from the lower(!) Smackover formation in wells Permit Nos. 746 and 814.^{29 30}

Smackover carbonates were laid down on a ramp that had risen to the northeast and east. The simple ramp model¹⁹ was modified for the Alabama-Florida offshore region by separating an inner ramp setting from an outer ramp setting.²⁰ The Smackover carbonate had accumulated according to that distinction on the Mississippi-Alabama and northern West Florida shelves in an outer ramp setting. In the Apalachicola embayment, sedimentation took place on a tidal flat⁴⁸ under a strong terrestrial influx.³⁰

Stromatolite mounds (bioherms) were recognized on land in the Smackover play.^{49 50} Production of wet gas and gas-condensate from single wells has been linked to those features in Hico Knowles (Louisiana) and Vocation (Alabama) fields. The identification of two such mounds in state waters in the Apalachicola embayment is based on their seismic signature.⁵¹ In the same region occur apparently more of these features (B. Faulkner, written communication).

On the northern West Florida shelf, well Destin Dome Block 56-1 penetrated three porosity zones of a total of 340 ft, probably in dolomite, and well Destin Dome Block 31 lacks any porosity.²⁰ In that region, early cementation of carbonate allochems seems to have dominated over their dissolution. The Smackover carbonate attains a total thickness of 400-700 ft, with reservoir potential in the upper 250-400 ft.

The regional top seal for the play is created by Buckner anhydrites or impervious Haynesville shales (see Fig. 2, Part 1).

Onshore traps are associated with basement horst blocks⁵² or with Louann salt-induced anticlinal structures with a stratigraphic cornponent. The onshore Jay field^{39 53} is formed by carbonate dissolution and dolomitization,⁵⁴ location on a salt-induced structural nose that is sealed updip by a cross-cutting permeability barrier and bounded to the east by a regional fault.

Southern Province

The Southern petroleum province is made up of four early Cretaceous petroleum systems, yielding five petroleum plays (see Fig. 2, Part 1).

The province embraces the established Sunniland petroleum system, probably also a Dollar Bay and an upper Pumpkin Bay-Brown Dolomite petroleum system, and possibly a Wood River petroleum system, each one with its internal petroleum source.

An Upper Able source rock⁵⁵ cannot be linked to any reservoir. In well Permit No. 942 (Fig. 2), oil was recovered on test from the Lake Trafford formation (B. Faulkner, written communication). Due to the lack of more information, this possible petroleum system's resource potential cannot currently be addressed.

In the absence of well control on the offshore center of this petroleum province, the information obtained from wells on the offshore buried Sarasota arch, on the southern Florida Peninsula, and on the Florida Keys serves here as an analog for the deeper portion of the offshore South Florida basin.

The depocenter of the Southern petroleum province (South Florida basin) underlies the southern West Florida shelf northwest of the Florida Keys, under present-day Florida Bay (Fig. 2).^{4 56} Well Permit No. 778 on the margin of the deep basin bottomed at 18,641 ft in the lower Wood River formation,23 close to basement.

From late Tithonian(?) to Barremian time, the basin was bounded in the north by the ancient divide of the Sarasota arch. By Aptian time, the Sarasota arch became submerged and the basin had expanded north to the latitude of Tampa.⁵ There, the South Florida basin rock sequence had graded into beds typical of the North-Central petroleum province, subsiding northwestward along the DeSoto hinge line (Fig. 2).

The basin terminates in the west at the outcrop in the Florida escarpment, extends from the shelf eastward to the south-dipping Peninsular arch on the eastern Florida Peninsula, and continues to the south beyond the Florida Keys.

The northern limit of oil generation on the southern West Florida shelf coincides with the transition from the South Florida basin, with significant Cenozoic overburden, to the buried Sarasota arch and the platform north of it, with lesser Cenozoic overburden (Fig. 2).

The threshold for oil generation on the southern Florida Peninsula was thought to be as deep as 11,500 ft.⁵⁷ Today's oil window was calculated for the South Florida basin's offshore center to lie between 9,500 ft and 15,000 ft.⁵ Only heavy-grade oil is produced onshore.

The petroleum sources are buried offshore about 2,500 ft deeper than onshore (G. Lore, personal communication) and are, therefore, expected to be more thermally mature. The quality of generated oils would improve accordingly to medium-grade (Sunniland and upper Pumpkin Bay-Brown Dolomite systems) with higher GORs than onshore, and to heavy to medium-grade (Dollar Bay system) (Fig. 2).

Wood River system, play

The present well control is too sparse to assess the regional significance of one gas and one gas-condensate occurrence in the Wood River formation (see Fig. 2, Part 1)^{2 4} with much confidence. The two shows are, however, encouraging for the possible discovery of a commercial play in the offshore Wood River formation. The shows suggest that a not yet identified gas source rock can exist.

U. Pumpkin Bay-Brown Dolomite system

Petroleum source beds of good to excellent quality occur in the upper half of the Pumpkin Bay formation in argillaceous limestone. They are separated from the Brown Dolomite by only a thin section, across which petroleum can have migrated. A locally developed lateral petroleum source can contribute to Brown Dolomite petroleum accumulation (see Fig. 2, Part 1).⁵⁵

U. Pumpkin Bay play

Minor live oil shows were observed in the Pumpkin Bay play (see Fig. 2, Part 1) in well Permit No. 778 and in other wells.

It remains to be substantiated that the upper Pumpkin Bay formation yields a reservoir of regional significance. However, with the favorable petroleum source indications and the occurrence of oil shows, it is accepted here that the possibility of a play development can well exist.⁵

Brown Dolomite play

Oil staining occurs in the Brown Dolomite play (see Fig. 2, Part 1) primarily in the western Florida Keys in wells Permits Nos. 22, 275, and 284.^{5 56 58} Little oil staining was noticed on the northern onshore margin of the oil-generating basin.

Good development of the Brown Dolomite was observed onshore at the northern portion of the oil-generating South Florida basin⁵⁸ and was confirmed offshore in three wells on the Sarasota arch. The Brown Dolomite zone re-appears in the Florida Keys in wells Permit Nos. 275 and 284 and grades to the east (well Permit No. 22) to tight dolomite or limestone. It is presumed that the Brown Dolomite is widely distributed beneath the southern West Florida shelf.⁵

The Brown Dolomite attains a thickness of 100 ft in the northern, onshore portion of the oil-generating South Florida basin, half of which is usually porous. It increases to 400 ft in the western Florida Keys. It is buried to a depth of 12,000 ft in the northern South Florida basin and is separated from the overlying Sunniland play by a 1,000 ft-thick section⁵⁸ that includes the Able and Punta Gorda anhydrites. It is probable that the Brown Dolomite develops into an exploration target on the southern West Florida shelf.⁵

The Brown Dolomite is sealed by impermeable limestones of the surrounding Twelve Mile formation.

Sunniland system, play

Comprehensive geochemical analyses have proven that the lower, dark-colored limestone (micrite) that makes up 30-60% of the Sunniland formation is the source of oil accumulations in the overlying limestone reservoir (see Fig. 2, Part 1).^{59 60}

The petroleum-generating potential varies from poor north of the Sunniland trend to excellent at the trend itself. In addition, the source rock thickness increases to more than 150 ft at the place of the Sunniland fields, creating there an optimum kerogen volume.

Due to the low geothermal gradient of the region,⁶¹ the source rock is onshore thermally immature to moderately mature,^{57 62} resulting in fields of heavy-grade, undersaturated oil. The internal petroleum source had charged the play vertically and laterally.

The system loses its petroleum-generating potential offshore near the southern flank of the underlying Sarasota arch (Fig. 2). Sunniland carbonates are replaced in part by anhydrite east of the South Florida basin center^{5 63} where that anhydrite separates largely the onshore oil-producing Sunniland carbonates from the offshore region in a northwest-southeast direction.⁴ The Sunniland system is sandwiched between two regionally sealing anhydrite complexes, the Lake Trafford anhydrite above and the Punta Gorda anhydrite below.^{4 57}

Onshore production from the Upper Sunniland play amounts to 110.1 million bbl of heavy-grade oil (23-28° gravity,⁵³ GOR 79-120 scf/stb⁴⁶) and 9.7 bcf of associated gas. The one-well Lake Trafford field has produced 283,000 bbl of 26° gravity oil from a second pool in the Lower Sunniland formation (fracture, "rubble" zone). The fields are located at a depth of 11,300 to 11,900 ft.

In the western Florida Keys, well Permit No. 275 tested heavy-grade oil (22° gravity) at a depth of 12,474 ft.⁶⁴ The record of oil shows in a well on Charlotte Harbor Block 672 (Fig. 1)¹ is controversial.

Effective porosity development in the Sunniland play is linked to patchy, bioclastic limestone mounds (bioherms), interpreted to be "barrier tidal-shoal bars,"^{65 66} of a fabric of rudists, platy algae, foraminifera, gastropods, and pellets.^{67 68 69} The mounds can attain a thickness of 40-100 ft. The porosity is locally enhanced by leaching⁶⁹ and to a lesser degree by dolomitization.^{66 68} Dolomite and anhydrite are interbedded with the mounds.⁵⁷

The Sunniland fields are stratigraphic traps with structural overprints.⁵ The mounds bear oil where they are associated with anticlinal structures.^{53 64} The structural relief above the oil-water contact ranges from 20 ft to 75 ft. The structures contain fields that are full to the spill-point.

The Sunniland system attains onshore a thickness of 200-300 ft.⁵⁷ Its thickness is anticipated to reach 800 ft offshore.⁵

With its established onshore production record, the Sunniland play is the primary exploration target in the offshore Southern petroleum province.⁵

Dollar Bay system, play

Dark limestone which makes up 10-75% of the Dollar Bay section, principally in its 260-420 ft-thick middle portion, represent that system's own oil source (see Fig. 2, Part 1).⁷⁰ That limestone is the richest petroleum source rock in the whole province.⁵⁹

The Dollar Bay source is thermally immature on land and has lost its petroleum-generating potential at the Sarasota arch (Fig. 2). The Dollar Bay play is sandwiched between anhydrites at the bottom of the Dollar Bay section and anhydrites at the base of the overlying Panther Camp formation (see Fig. 2, Part 1).⁴

No commercial petroleum production has yet been established in a Dollar Bay carbonate reservoir. It is of significance, however, that some very heavy-grade oil (17° gravity) was recovered in well Permit No. 222 (Fig. 2).^{57 71} In other wells, oil-cut mud was recovered and good oil shows were logged on cores or cuttings.⁷⁰

The Dollar Bay formation possesses considerable potential of developing into another exploration target in the offshore Southern petroleum province if bioclastic buildups with effective porosity, as in the Sunniland play, can be detected there. Structures offshore are considerably larger than onshore.⁵