ABSTRACT

The Frio Formation, a major productive unit in South Louisiana, never has been described petrographically, although conventional cores and thousands of sidewall cores have been collected.

The lower Frio consists of about 1,000 ft of alternating sandstone and shale in the updip stable shelf area, but it thickens to an observed maximum of 7,000 ft in the downdip unstable shelf (Nodosaria embayment) area. The lower Frio is mainly a regressive sequence, and has been divided into 4 units (designated A, B, C, and D from top to bottom) on the basis of electric-log correlations and other characteristics.

Three lithologic sequences characterize each unit: a massive sandstone facies on the north (updip); an interbedded sandstone-shale (deltaic?) facies exhibiting considerable lateral variation; and a downdip sequence of alternating sandstone and shale in which the shale content increases markedly downdip. The sandstone bodies are thin and relatively persistent.

Unit D, at the base, marks the beginning of the regression, has the least amount of sandstone, and has smallest amount of production. Shoreline and deltaic sandstone bodies are poorly developed. Unit C shows an increase in sandstone content and production. The deltaic facies is well developed, and shows a marked increase in volume and areal extent. Most of the production is associated with this facies, although production does occur in marine sandstone facies. Unit B shows only limited southward regression, but exhibits considerable development of marine facies. Numerous fields produce from this unit. Significant production also comes from channel-like bodies with erosional lower contacts. These bodies may be bar fingers or distributary channel deposits. Unit A is the most regressive. It contains the largest amount of massive continental and deltaic plain sandstones, as well as a well-developed delta system. As a result, marine sandstones are limited in number and extent. The distributary sandstone bodies account for major production and several giant fields.

Independent stratigraphic, petrographic, and paleontologic studies demonstrate that the lower Frio contains strata deposited in continental, deltaic, and inner to middle neritic environments. Production, although primarily controlled by structure, is definitely affected by sedimentation and depositional patterns. Because structure in the Frio is now well known, future discoveries will be determined primarily by stratigraphic, petrographic, and paleontologic studies similar to this one.

1. Full text with illustrations to be published in Am. Assoc. Petroleum Geologists Bull.

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