ABSTRACT

The Hackberry subsurface wedge of the middle part of the Frio Formation of southwestern Louisiana is one of the four major deeper water shale wedges in the post-Vicksburg, Gulf Coast Tertiary section. The Hackberry can be divided into two parts. The upper section ranges in thickness from essentially zero to more than 3,000 feet, and consists predominantly of shale containing an outer-neritic (deep-water) faunal assemblage. Some thin and erratically distributed sandstone bodies are present. The lower section ranges from zero to 700 feet and consists essentially of sandstone.

In order to understand the geological history of the Hackberry wedge, a discussion of the stratigraphy and structure of the Frio section has been included.

The complicated Frio stratigraphy of northern Jefferson Davis and Calcasieu Parishes is caused in part by a complex early (Frio) tectonic history and in part by depositional variation of the Hackberry section. The geological history of the area is summarized in a sequence of eight stages. It should be emphasized that these eight stages are a sequence of events, that probably overlap one another, and may have occurred at slightly different times in different areas.

1. Deposition of Vicksburg and lower Frio Textularia seligi zone.
2. Development of lower unconformity (this may be a local unconformity).
3. Deposition of the lower Frio and Hartburg sequences.
4. Uplift, folding, erosion, and development of "pre-Hackberry unconformity."
5. Tilting of the unconformity surface and renewed erosion which formed channels.
6. Filling of the channels with basal Hackberry sandstones, forming a flat surface.
7. Deposition of Hackberry shale sequence with "arenaceous" fauna at its base.
8. Deposition of the remainder of the Frio and the early Anahuac.

The earlier structural movement caused folds and faults which were then truncated by regional erosion. Large channels (600 feet or more deep) were cut into this erosional surface and later filled. The manner in which these channels were cut and filled is uncertain, but may involve turbidity flows. The structural movement, history of erosion, and the complex stratigraphy of the Hackberry together make exploration for Hackberry sandstone reservoirs a high-risk economic venture, but one which may pay high dividends.