Petrology of Upper Smackover Limestone in North Haynesville Field, Claiborne Parish, Louisiana

Abstract

This study has led to a reconstruction of the environment of deposition of the upper Smackover section (Late Jurassic) at North Haynesville field, Claiborne Parish, Louisiana, and may aid in the prediction of other productive trends. The interval studied includes the oolitic and pelletoid B reservoir which contains about 16 million bbl of oil in place. Petrographic examination of etched core slabs resulted in a grouping of upper Smackover carbonate rocks into environmentally controlled facies, each of which has distinctive characteristics on the electric log. Because of probable hypersalinity in the Smackover sea, allochems are almost entirely nonskeletal. In order of decreasing importance, these include: pellets, oolites, lumps and composite grains, algal pisolites and oncolites, and quartz and anhydrite grains. The Smackover transition facies is gradational into the overlying Buckner shale and includes a shaly subfacies composed largely of pellets and recrystallized oolites in a matrix of shale and/or silt-size anhydrite or dolomite, and a sandy subfacies composed of pellets with abundant quartz sand grains and sparry calcite cement. Below the transition facies, in order of decreasing energy level, are: (1) the reservoir facies composed of oolites and superficially coated pellets with spar cement and primary intergranular porosity; (2) the mixed facies composed of subequal quantities of all grain types with spar cement and mud matrix; and (3) the pellet-mud facies composed of pellets in a mud matrix which commonly is dolomitized. The pellet-mud facies also contains minor amounts of very argi laceous dolomite and disturbed mud. Deposition of nonskeletal particles in the late Smackover sea resembled in some aspects that of the present Bahama Banks. Pellet-mud was deposited in warm, quiet, shallow, probably hypersaline water. Subsequently slight differential uplift caused turbulence in local areas, where the mixed facies was deposited. Later regional uplift and regression brought about conditions favorable for extensive oolite accretion, resulting in reservoir-facies deposition. Continued uplift of land areas on the north supplied increasing amounts of terrigenous detritus to the transition facies, and carbonate deposition eventually ceased. Analysis of dry holes in North Haynesville field indicates that nondeposition or complete cementation of the reservoir facies is more important than present structure in limiting the productive area. However, only those wells favorably located with respect to present structure, as well as to structure during deposition and diagenesis, are productive. Additional porosity trends may be discovered by comparing field dry holes with those in other areas.