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, the ideal habitat being a thick section of deltaic or shallow-neritic sandstone interbedded with marine shale. This relation limits the extent of production downdip where deeper shale replaces sandstone.

The lower Miocene section appears to be prospective both downdip from and on strike with the present producing trend over a 9,600 sq mi (24,900 sq km) area in Louisiana and Texas. The prospective area contains 9,200 cu mi (38,300 cu km) of sedimentary rock 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 4,700 sq mi (12,200 sq km) containing 4,800 cu mi (20,000 cu km) of sediments, and a possible producing area of 4,900 sq mi (12,700 sq km) containing 4,400 cu mi (18,300 cu

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km) of lower Miocene strata. The present lower Miocene producing trend covers 21,000 sq mi (54,499 sq km) and contains 18,400 cu mi (76,700 cu km) of sedimentary rock. In Louisiana, the probable area extends downdip from present productive limits to an estimated depth of -- 15,000 ft (4,600 m) at the top of the lower Miocene. Drilling depths of 20,000 ft (6,100 m) may be required to test this sequence, which should be composed of 5-15% 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 ft (5,200 m) on top of the lower Miocene. Eastward, this area continues on land. It ranges from the present producing area on the north to where the top of the lower Miocene is at an estimated depth of --17,000 ft (5,200 m) on the south; the eastern limit is defined by the change from a clastic to a carbonate section. Depths of 23,000 ft (7,600 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 have penetrated completely the lower Miocene section. Projection of updip well data indicates that lower Miocene beds have a m nimum thickness of 8,000 ft (2,400 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 favorable conditions downdip to where estimated sandstone occurrence appears to be in the 5-10% range.

On a trend basis, future Oligocene production probably will not extend appreciably beyond presently defined limits. Depositional environment is the key factor, 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 trend. 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. There, carbonates are present 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 sandstone beds that overlie the lower Miocene-Oligocene carbonates. Although not ruled out completely, reef production possibilities in the east rn area are too remote to postulate a future trend.

Other than 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. Though the total additional reserves from the present producing trend may be quite substantial, individual fields will generally be smaller and deeper than existing production. Most future discoveries should be made on subtler, smaller, deeper traps, such as on the flanks of salt domes and faults. 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 presen producing trends in Louisiana. Additional production, but in lesser quantities, also will 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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