CAROLINA TROUGH POTENTIAL SUMMARIZED

George Carpenter, Roger Amato
Minerals Management Service
Herndon, Va.

The Carolina trough is one of the world's largest undrilled salt basins and may contain important amounts of oil and gas.

The potential for finding large hydrocarbon accumulations in the Carolina trough is believed to be very good, largely because the great majority of salt basins tend to be productive. Salt basins are among the most prolific oil and-as producers of any basin type.

The trough's recoverable resource potential may be as high as 690 million bbl of oil and 16.25 tcf of gas at the 5% confidence level. The figures include resources from postulated small fields.

Kerogen analysis of sediment samples from the closest wells indicates the basin is probably gas prone.

The U.S. Minerals Management Service assessed the trough's undiscovered hydrocarbon resources using a geologic model developed from geophysical data and geological analogs.

Unlike most previously published resource estimates for the trough, the MMS study used a play analysis approach, evaluated stratigraphic plays, and included uneconomic small prospects.1 2 3

The geologic model was used to provide numerical input data to a computer based drilling simulation program called Probabilistic Resource Estimates Offshore (Presto).

Presto uses risk analysis to "explore" user defined prospects.

If the simulated exploration is successful, it generates a probabilistic range of resource estimates.

TROUGH WELLS

The Carolina trough is about 330 miles long and up to 60 miles wide, and water depths are 600-7,000 ft.

Seismic data indicate that sediments exceed 40,000 ft in thickness, more than half of which is probably Jurassic age.4

The trough is unique among U.S. Atlantic basins in that it includes a line of diapirs, probably salt cored, along its sea-ward margin and a related system of growth faults along its landward side that extends for more than 200 miles.

Exploratory drilling for oil or gas has not occurred in the trough, but a number of wells and core holes have been drilled around its periphery (Fig. 1).

The Esso 1 Hatteras Light on Cape Hatteras penetrated more than 10,000 ft of Tertiary and Upper and Lower Cretaceous sandstones, shales, and limestones and bottomed in crystalline basement rock.

At least 18 other wells have been drilled on the North Carolina coastal plain adjacent to the trough.5 They provided limited information about the onshore extension of the trough, and none recorded shows.

The COST GE-1 well in the Southeast Georgia embayment encountered 11,000 ft of Tertiary and Upper and Lower Cretaceous sandstones, shales, chalks, and limestones and bottomed in metamorphosed basement rock.

Shell Oil Co.'s OCS 93-1 penetrated Tertiary, Cretaceous, and Jurassic sandstones, shales, and a few thin limestones.

None of the Deep Sea Drilling Project well sites are close to the trough, but sites 105, 603, and 534 provide information on the nature of Lower Cretaceous and Jurassic rocks that can be extrapolated to the trough's seaward margin.

Shell's 587-1 well penetrated the carbonate platform in the Baltimore Canyon trough through Cretaceous and Jurassic shales, limestones, and dolomite.

None of the offshore wells had oil or gas shows.

STRUCTURE, TECTONICS

The Carolina trough is the most structurally complex basin in the U.S. Atlantic Outer Continental Shelf (Fig. 2).

This is probably because of the suspected accumulation and mobilization of salt into diapirs.6 7

Some of the normal faults identified from seismic data along the trough's landward side are believed to be related to movement of the salt and development of the diapirs. 6

One large fault extends more than 217 miles southwestward from 44 miles east of Cape Lookout to about 230 miles east of Savannah. Seismic lines show increasing throw on the fault with depth from about 10 ft just below the ocean bottom to more than 1,485 ft at 16,500 ft.

Unlike most continental margin growth faults, it does not merge into bedding planes at its lower limit. Instead, the steeply dipping fault plane continues down to a strong seismic reflector that is probably the top of the salt bed.'

At least 26 diapirs have been mapped along the seaward edge of the Carolina trough where the sedimentary section is about 24,000 ft thick.

These appear to be salt domes based on morphology and high chlorinity values measured in short cores over the tops of several domes, which pierce or lie just beneath the ocean bottom. 7

Other structures in the Carolina trough are basement highs, which create sediment drape features, and a large carbonate platform with localized buildups.

GAS, OIL POTENTIAL

Six gas and oil exploration plays in the Carolina trough were identified based on seismic data (Fig. 3, Table 1).

These are:

Anticlines that occur just shoreward of the carbonate platform and could have either carbonate or arenaceous clastic reservoir rocks.
Traps against the larger faults with clastic reservoirs.
Clastic rocks draping over basement highs.
The shelf edge carbonate platform with reefal limestones.
Pinchouts of clastic rocks against the carbonate platform.
Traps in deepwater clastic rocks against the diapirs.

Sandstones serving as reservoirs in the anticlines, basement high, and fault trap plan,s are probably beach bar, delta, or channel in origin based on results of nearby drilling. Porosities of more than 21% were measured in sandstones below 8,000 ft in the Esso 1 Hatteras well.8

Carbonate reservoir rocks in the anticlines could be either oolite grainstones or fractured micrites as in the Baltimore Canyon trough.

Reservoir rocks in the 3,000 ft thick shelf edge carbonate platform are probably subaerially weathered coral-stromatoporoid boundstones or oolite grainstones.

Possible reservoir rocks in the pinchout and diapir plays are turbidite or contourite sandstones deposited in deep water off the edge of the continental shelf. Rocks that could serve as seals include Lower Cretaceous and Jurassic shales and mudstones, dense unfractured carbonate rocks, and salt.

Thick shale beds and at least one thin anhydrite bed occur in the Esso 1 Hatteras Light well.9

SOURCE ROCKS

Possible source rocks are:

The deep basin shales of Jurassic or Lower Cretaceous age in the middle of the trough.
The widespread, organic rich shales of the Lower Cretaceous Hatteras formation that occur in the Atlantic Ocean basin seaward of the continental shelf.
Possible local occurrences of algal limestone in the carbonate platform.

Dark gray and black shales are noted in the Lower Cretaceous section of the Esso 1 Hatteras Light well.9 These shales become thicker and more deeply buried offshore according to seismic data and may serve as source rocks.

The restricted circulation conditions that caused salt accumulation during the early development of the basin may also have created excellent source rocks.11

Basinal shales would be source rocks for anticlines, fault traps, basement drape structures, and possibly even for the carbonate platform.

The Hatteras formation is identified in DSDP core sites 105, 534, and 603 and in numerous other sites in the Atlantic basin.

The Hatteras formation is a dark green to black, highly carbonaceous silty clay. It contains mostly type III (terrestrial) kerogens that are strongly gas prone, and has as much as 20% organic carbon by weight.12

Hatteras is 363 ft thick in DSDP core site 105, but it can be more than 1,650 ft thick. Pelagic facies noted in a few places in the unit suggest that oil prone kerogens could be locally abundant. 12 The formation is believed to be present and perhaps thicker at the base of the continental shelf and could thus serve as the source rock for all prospects in the Carolina trough.

GAS PRONE AREA

The Carolina trough is expected to have similar gas prone Upper Jurassic source rocks and a similar hydrocarbon maturation window as the Baltimore Canyon trough, where kerogen maturation beams between 12,000-14,000 ft below sea level.13

Demaison and others' conclusion is based on a geochemical model of the Atlantic continental margin using well and DSDP core hole data. Wells drilled on the shelf in the Baltimore Canyon trough have an average geothermal gradient of 1.3 F./100 ft, which results in a temperature of about 180 F. at 10,000 ft.

The Esso 1 Hatteras Light recorded a temperature of 170 F. at total depth of 10,044 ft. 14 This is above the threshold temperature range of 154-167 F. for Upper Jurassic and Lower Cretaceous source rocks.15

However, the Shell OCS 93-1 well drilled off Virginia on the continental slope recorded 117 F. at 10,000 ft.16 This reading indicates a lower temperature gradient of .98 F./100 ft on the seaward side of the Baltimore Canyon trough that may also be the case for the Carolina trough.

Significant vertical migration of hydrocarbons could have occurred along some of the large faults on the shelf as well as around the diapirs, which tend to create abundant faults and fractures as they rise.

However, Prather concludes that the lack of hydrocarbons in structures tested thus far in the Baltimore Canyon trough is due to failure of the hydrocarbon charging and migration system in the basin.17

GEOLOGIC MODELING

Developing a quantitative resource assessment required that interpretations of tectonics, structure, geochemistry, stratigraphy and depositional history be converted in part to numerical computer inputs.

Presto is a play analysis computer program that can develop oil and gas resource estimates for prospects within a play under broad ranges of geologic and economic uncertainty.

Presto, which uses the prospect as the basic analytical unit, calculated the resource estimates. Therefore only those geologic and engineering inputs relating directly to potential producing zones within prospects-acreage, pay, oil and gas recoveries-need by synthesized from the data set.

Prospect specific resource estimates are calculated from the zone related input data.

Risk, which quantifies the uncertainty associated with the presence of hydrocarbons, is the single most important element in the Presto model.

Ranges of values for prospect attributes that were used in the analysis of the Carolina trough are shown (Table 2).

Specific values by prospect cannot be reported because they were developed from industry provided proprietary data and are themselves proprietary.

WHAT IT SHOWS

The play and area economic screens reflect additional costs beyond those associated with the minimum economic field size that are required for profitable development of plays and other areas such as the Carolina trough with little or no existing transportation infrastructure.

Economically recoverable resource estimates for the trough were developed using a price path with starting prices in 1987 of $18/bbl for oil and $1.80/Mcf for gas. The path assumes real price growth for 1991 and beyond averaging about 4% for oil and about 5.5% for natural gas.

The high estimate of undiscovered, economically recoverable resources in the Carolina trough assuming current recovery technology is 330 million bbl of oil and 7.76 tcf of gas.

Resources such as clathrates, heavy oil, tight gas, or other unconventional forms are not assessed.

REFERENCES

Dolton, G.L., and others, Estimates of undiscovered recoverable conventional resources of oil and gas in the U.S., USGS Circular 860, 1981, 87 p.
Miller, B. M., and others, Geological estimates of undiscovered recoverable oil and gas resources in the U.S., USGS Circular 725, 1975, 78 p.
Cooke, L.W., Estimates of undiscovered, economically recoverable oil and gas resources for the Outer Continental Shelf as of July 1984, Minerals Management Service OCS Report MMS 85-0012, 1985, 45 P.
Dellagiarino, G., ed., U.S. Outer Continental Shelf basins, maps and descriptions, Minerals Management Service OCS Report MMS 86-0048, 1986, 80 p.
Almy, C.C., Jr., Lithostratigraphic-seismic evaluation of hydrocarbon potential, North Carolina coastal and continental margins, North Carolina Contractual Interim Report, Year 2, 1987, 18 p.
Dillon, W.P., Popenoe, P., Grow, J.A., Klitgord, K.D., Swift, B.A., Paull, C.K., and Cashman, K.V., 1983, Growth faulting and salt diapirism: their relationship and control in the Carolina trough, eastern North America, in Watkins, J.S., and Drake, C. L., eds., Studies in continental margin geology, AAPG Memoir 34, 1983, pp. 21-46.
Dillon, W.P., and Popenoe, P. The Blake plateau basin and Carolina trough, in Sheridan, R.E., and Grow, J.A., eds., The Atlantic continental margin, U : S., Geological Society of America, The Geology of North America, Vol. 1-2, 1988, pp. 291-328.
Brown, P.M., Miller, J.A., and Swain, F.M., Structural and stratigraphic framework and spatial distribution of permeability of the Atlantic coastal plain, North Carolina to New York, U.S. Geological Professional Paper 796, 1972, 79 p.
Swain, F.M., Ostracoda from wells in North Carolina, Part 2, Mesozoic ostracoda, USGS professional paper 234-A, 1932, pp. 59-93.
Arthur, M.A., North Atlantic Cretaceous black shales: The record at site 398 and a brief comparison with other occurrences, in Sibuet, J.C., Ryan, W.B.F., and others, Initial reports of the Deep Sea Drilling Project, Vol. 47, Part R, 1979, pp. 719-738.
Watkins, J.S., Pytte, A.M., and Houtz, R.E., Exploration history and future prospects of the U.S. Atlantic margin, in Halbouty, M.T., ed., Future petroleum provinces of the world, AAPG Memoir 40, 1986, pp. 269-290.
Wise, S.W., and van Hinte, J.E., Mesozoic-Cenozoic depositional environments revealed by Deep Sea Drilling Project Leg 93 drilling on the continental rise off the eastern U.S., cruise summary in van Hinte, J.E., and others, Initial reports of the Deep Sea Drilling Project, Vol. 92, U.S. Government Printing Office, Washington, D.C., 1987, pp. 1,367-1,423.
Demaison, G., Holck, A.J.J., Jones, R.W., and Moore, G.T., Predictive source bed stratigraphy, a guide to regional petroleum occurrence, North Sea basin and eastern North American continental margin, in proceedings of 11th World Petroleum Congress, London, Vol. 2, 1983, pp. 17-29.
Spangler, W.B., Subsurface geology of Atlantic coastal plain of North Carolina, AAPG Bull., Vol. 34, No. 1, 1950, pp. 100-132.
Pigott, J.D., Assessing source rock maturity in frontier basins, importance of time, temperature, and tectonics, AAPG Bull., Vol. 69, No. 8, 1985, pp. 1,269-74.
Amato, R.V., Shell Baltimore Rise 93-1 Well: Geological and operational summary Minerals Management Service OCS Report MMS-86-0128, 1987, 55 p.
Prather, B.E., Petroleum geology of the Upper Jurassic and Lower Cretaceous, Baltimore Canyon trough, western North Atlantic Ocean, AAPG Bull., Vol. 75, No. 2, 1991, pp. 258-277.
Carpenter, G.B., The Presto III resource assessment model, OTC paper 5957, 21st annual Offshore Technology Conference, Houston, 1989, conference proceedings, pp. 9-14.