Effects of Primary Sedimentary Processes on Reservoir Quality of "Deep Wilcox" (Eocene) Sandstones in Fordoche Field, Louisiana

Rowdy C. Lemoine, Thomas F. Moslow, Philip Lowry

The initial discovery and subsequent development of Fordoche field established a "Deep Wilcox" production trend in south-central Louisiana. Production in the field occurs within five intervals: the W4, W5, W8, W12, and W15 sandstones. The W8 and W12 sandstones are the most prolific. They represent shoreface sequences which formed at or near the shelf margin. Cumulative hydrocarbon production from these sandstones is in excess of 22 million bbl of oil and 151 bcf of gas.

Over 1,100 ft (335 m) of conventional core from the W8 and W12 sandstones in six wells were analyzed and compared to petrophysical data to effectively characterized reservoir quality. Primary sedimentation patterns (i.e., physical and biogenic sedimentary structures and sequences) are the dominant control on reservoir quality throughout the field. Burrowed to bioturbated (> 75% burrowed) lower shoreface sandstones possess relatively high average porosity values (16.7%); however, permeability values are extremely low (< 0.2 md). Clay-lined burrow walls create numerous permeability barriers which significantly alter fluid flow patterns within this facies. In contrast, massive to burrowed middle shoreface sandstones, although possessing similar average porosity values (19.8%), have substantially higher average permeability values (8.6 md). Within the middle shoreface facies, higher wave energies winnow the fine-grained silts and clays, resulting in the deposition of a clean, well-sorted sandstone. The entire sequence coarsens upward in both grain size and percent sand and is overlain by a thin (3.5 ft; 1.0 m), tightly cemented calcareous sandstone. Average thickness of the middle shoreface facies in Fordoche field is 23 ft (7.0 m) with stacked sequences within the W12 sandstone as thick as 57 ft (17.4 m) of high reservoir quality sandstones.

Detailed sedimentologic studies in Fordoche field emphasize the importance of separating the reservoir into a sequence of depositional facies with distinct fluid flow characteristics. By defining and mapping these flow units, inhomogeneities within the reservoir interval are readily recognized and resolved during development of the field.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.