ABSTRACT

Conventional cores from five wells in the Greensburg-Joseph Branch fields area of St. Helena Parish, Louisiana, were described and sampled to determine the depositional environments, lithology, and diagenesis of the First Tuscaloosa Sand, the producing sand of the Lower Tuscaloosa Formation in this intermediate-depth part of the trend. The cores totalled 172 feet in length and were taken from the upper half of the sand, which is typically about 30 feet thick where productive, or from the overlying finer-grained section. Greensburg and Joseph Branch fields occur in an area of limited low relief structural nosing; neither anticlinal closures greater than 20 feet nor faults were recognized by structure contour mapping. Based on the updip and lateral shaling out of the First Tuscaloosa Sand and the virtual absence of structure, Greensburg field is interpreted to be a stratigraphic trap; Joseph Branch is a one-well field, likely also a stratigraphic trap.

The First Tuscaloosa Sand appears to have been deposited in a distributary channel which crossed the subaerial lowland part of a delta. It is a very fine- to fine-grained sandstone for the most part, with a few thin shale interbeds. Ripple laminations, clay clasts, and deformational structures characterize the channel fill portion of the section. Intense bioturbation of sand tops, unidirectional trough cross-bedding, and flaser structures were recognized in places, indicative of a deltaic environment influenced by tidal currents.

Petrographic analysis of 20 thin-sections from selected intervals revealed that the First Tuscaloosa Sand is primarily a sublitharenite, with scattered quartzarenite intervals. The average sandstone consists of 68.3% framework grains, 11.4% matrix, 12.9% cement, and 7.4% pore space. Quartz, almost all of which is monocrystalline, is by far the dominant framework grain. Cementing by quartz overgrowths appears to represent a significant proportion of the pore filling, but could not be quantified. Calcite (up to 25.5%) and siderite (up to 27.0%) are also important cements. Matrix, particularly as chlorite pore-linings around quartz grains, occupies up to 36.0% of the sandstone volume.

The First Tuscaloosa Sandstone has been extensively modified by diagenesis. Initial (primary) porosity was reduced by compaction and later by formation of quartz overgrowths. Secondary porosity developed as a result of the dissolution of unstable grains and late-stage carbonate cements. Chlorite grain rims appear to have formed after quartz overgrowth precipitation in zones now carbonate cement-poor.