ABSTRACT

The carbonate bodies of the basal Swift Formation (Upper Jurassic) occur as anomalous deflections on a relatively uniform mechanical well log section. The areal distribution, stratigraphic relationships and genesis of the carbonate bodies were determined by using the gamma ray log, the spontaneous-potential log, the resistivity log suite, and megascopic and microscopic core analysis.

The carbonate bodies of the basal Swift Formation are coarsening-upward sequences composed of predominantly sand-sized, recrystallized mollusk grains. These grains were transported by strong bottom currents across the irregular sea floor of the shallow epicontinental Jurassic sea, and were deposited under agitated water conditions in the offshore environment. These carbonate sand bodies are analogous to offshore bar deposits of similar age that are recognized across the midcontinent region.

The carbonate sand bodies are up to 22 miles in length and 7 miles in width, and attain a maximum thickness of 155 feet. The bioclastic sand bodies have a lower contact that is apparently gradational with the underlying very fine-grained quartzarenites. The base of the carbonate body is composed of a finely-laminated, recrystallized molluscan packstone that has a low porosity and contains moderate amounts of siliciclastic material in the matrix. This recrystallized molluscan packstone becomes cleaner and coarsens upward into a massive, recrystallized molluscan grainstone with high moldic porosity and good permeability. The upper contact of the carbonate bodies is unconformable with the overlying basal lag deposit and shales.

The convex shape of the carbonate bodies, the high moldic porosity and good permeability of the grainstones, and the overlying shales make these basal Swift carbonate bodies good reservoir rocks. The absence of thermally generated hydrocarbons within the body probably results from the shallow depth of burial and the thermal immaturity of the surrounding shales. The possible presence of biogenic gas derived from the surrounding shales makes these deposits potential sites for exploration.