ABSTRACT: Depositional Environment and Reservoir Properties of the Lower Tuscaloosa "B" Sandstone, Baywood Field, St. Helena Parish, Louisiana

Randall S. Miller, Jack L. Groth

Baywood field was discovered in September 1987 with the drilling of the Keck Partners Clemons No. 1, located in Sec. 45, T4S, R4E, St. Helena Parish, Louisiana. The discovery well encountered a 47 ft thick Lower Tuscaloosa sandstone in the "B" stratigraphic interval at 14,540 ft with a 21 ft gas column on water. The perforated interval 14,544 to 14,554 ft potentialed 2200 MCFGPD, 290 BCPD, and 12 BWPD with a flowing tubing pressure of 3200 psi. Subsequent development drilling to date has led to three gas completions, one oil completion, and three dry holes.

Detailed sedimentological analysis of a conventional core from the discovery well reveals that the reservoir sandstone was deposited in a deltaic channel to point bar environment and not a shoreline as interpreted by others. The sandstone has a sharp (erosional) lower contact that in some wells has an associated basal lag conglomerate. Succeeding sandstone is medium- to fine-grained and exhibits large-scale,

trough cross-stratification that fines upward to very fine-grained, ripple-stratified sandstone. Locally, carbonaceous detritus and mudstone rip-up clasts are present. The sandstone is capped by a root-mottled and burrowed, silty claystone. The entire sequence is overlain by fossiliferous marine sediments marking delta abandonment and transgression.

In terms of composition and diagenesis, the reservoir sandstone is anomalous to most Lower Tuscaloosa sandstones. The sandstone borders on a quartz-arenite (Q = 94%, F = 0%, RF = 6%) and is cemented by a combination of quartz overgrowths (11%), kaolinite clay (6.5%), and dolomite (5.5%). Most Lower Tuscaloosa sandstones in comparison are mineralogically immature with a high lithic fragment content and are cemented by grain-coating chlorite clay.

Core porosity in the reservoir sandstone ranges from only 2% to 17%, and permeability is from 0.1 md to 402 md. Average porosity is 13%, and average permeability is 88 md. Porosity is comprised of both primary intergranular and leached-grain, secondary pores. Microporosity that usually comprises a significant amount of the total porosity in most Lower Tuscaloosa sandstones is minimal, resulting in lower immobile water saturations and no low-resistivity pay problems.

Determination of critical water saturations derived through special core analysis and used in conjunction with detailed log analysis is imperative to recognizing pay within the sandstone. Reservoir fluid studies indicate that the reservoir exhibits retrograde condensation characteristics. The primary drive mechanism is depletion gas with a partial water drive.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990