ABSTRACT

Future lower Miocene-Oligocene discoveries will be found under conditions similar to those controlling present production. Reservoirs are typically sandstone and traps are usually structural, associated with salt domes, fault closures, anticlines, or residual highs. Depositional environment is critical for accumulation of hydrocarbons, with the ideal habitat being a thick section of deltaic or shallow-neritic sandstones interbedded with marine shales. This relationship limits the extent of production downdip where deeper water shales replace sandstones.

The lower Miocene section appears to be prospective both downdip from and on strike with the present producing trend over a 9600 square mile (24,900 km²) area in Louisiana and Texas. The prospective area contains 9200 cubic miles (38,300 km³) of sediments, and lies largely on the sparsely-drilled continental shelf offshore from southwestern Louisiana and south Texas. This future province may be divided into two parts: a probable producing area of 4700 square miles (12,200 km²) containing 4800 cubic miles (20,000 km³) of sediments, and a possible producing area of 4900 square miles (12,700 km²) containing 4400 cubic miles (18,300 km³) of lower Miocene strata. The present lower Miocene producing trend covers 21,000 square miles (54,400 km²) and contains 18,400 cubic miles (76,700 km³) of sediments. In Louisiana, the probable area extends downdip from present productive limits to an estimated depth of - 15,000 feet (4600 m.) on the top of the lower Miocene. Minimum drilling depths of 20,000 feet (6100 m.) may be required to test this sequence which should be composed of from 5 to 15% sandstone. Thus defined, this area is largely offshore from central and western Louisiana.

In western Louisiana, the possible producing area extends downdip from the southern limits of the probable area to a projected depth of - 17,000 feet (5200 m.) on top of the lower Miocene. Eastward, this area continues on land. It ranges from the present producing area on the north to the occurrence of the top of the lower Miocene at an estimated depth of - 17,000 feet (5200 m.) to the south, with the eastern limits defined by the change from a clastic to a carbonate section. A depth of 25,000 feet (7600 m.) or more will be necessary to test the section which should be less than 10% sandstone. None of the wells drilled in the probable or possible prospective areas in Louisiana has completely penetrated the lower Miocene section. Projection of updip well data indicates that lower Miocene beds reach a minimum thickness of 8,000 feet (2400 m.) throughout the potential Louisiana area. In Texas, the probable area was determined by the presence of a favorable sandstone section downdip or along strike from the lower Miocene productive trend. The possible area was outlined by projecting these favorable conditions downdip to where estimated sandstone occurrence appears to be in the 5 to 10% range.

On a trend basis, future Oligocene production will probably not extend appreciably beyond presently defined limits. Depositional environment is the key factor here, as blanket sandstones are unlikely to extend seaward much beyond present control. It is likely, however, that deeper untested Frio sandstones may be present on features downdip from known production, but such occurrences will probably be limited and of local, rather than regional, extent. The Anahuac and Vicksburg sections contain much less sandstone than the Frio in the present producing trends. Thus, the possibility for production outside the present trends in these beds is less than in the Frio.

The lower Miocene-Oligocene section of Alabama and Florida consists mainly of thin, shallow-water carbonates which apparently have little potential for oil and gas production either onshore or offshore. A thicker section of lower Miocene-Oligocene beds is present at greater depths just east of the Mississippi Delta in the offshore Chandeleur Sound, Chandeleur, and Main Pass areas south of Mississippi. Here, carbonates occur in the lower Miocene section and there is a possibility that oil and gas might be found in limestone reefs. Well information in this area has not been encouraging to date, as production is limited to younger Miocene sandstones that overlie the lower Miocene-Oligocene carbonates. Although not completely ruled out, reef production possibilities in this eastern area are too remote to postulate a future trend.

Aside from any production from new provinces, additional reserves will be found in the present lower Miocene and Oligocene producing trends as additional subsurface control and improved drilling and geophysical technology become available. While the total additional reserves from the present producing trend may be quite substantial, the individual fields will generally be smaller and deeper than existing production. Most of these future discoveries should be found on subtler, smaller, deeper traps, such as accumulations on the flanks of salt domes and fault traps. In the future, as in the past, the middle Oligocene Frio Formation will yield appreciable hydrocarbons within the trend in both Texas and Louisiana. Lower Miocene beds will continue to be the most significant source of additional reserves within the present producing trends in Louisiana. Additional production, but in lesser quantities will also be found in the upper Oligocene Anahuac of both Texas and Louisiana. Future discoveries in the lower Oligocene Vicksburg should be minor and confined mostly to Texas.

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Figure 1. Present and future producing trends, lower Miocene and Ogligocene, western Gulf Basin.

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