ABSTRACT

The character and reservoir quality of six different Mississippian (Chesterian) sandstone units in frontier areas of the Black Warrior Basin of Mississippi have been determined by inspection of cores, examination of thin-sections, and X-ray diffractometry. A total of 113 samples from ten wells was taken from cores of the following sandstones: the Lewis, the sandy Lewis Limestone, the Evans, the Rea, the Sanders, and the Carter. Hydrocarbon production from the basin, which is located in NE Mississippi and NW Alabama, is mostly shallow gas (with minor gas-condensate and oil) from these units. Sample depths range from about 2,500 ft in northern Chickasaw County to about 5,500 ft in Monroe and Lowndes Counties, with the deepest samples coming from almost 11,000 ft in northern Clay County.

The sandstones are very fine- to fine-grained quartz arenites having framework grains that are, on average, 98.3% monocrystalline quartz, 0.8% polycrystalline quartz, 0.2% feldspar, 0.1% metamorphic rock fragments, 0.3% sedimentary rock fragments, and 0.2% muscovite. Remnant clay rims under secondary overgrowths generally outline sub-rounded to well-rounded grains. There is little variation in the character of the framework grains between the different sandstone units.

Differences in degree of compaction and the percentage and type of cement lend some individuality to the sandstones, however. The most common cement is silica precipitated as secondary overgrowths on detrital quartz. Less common is a carbonate (dominantly dolomite with some calcite) cement, but in one highly oolitic Lewis Limestone sample, 54.8% is carbonate. In many samples, the carbonate cement has corroded adjacent framework grains. Authigenic clays, primarily kaolinite, and pyrite occur as post-gramework grain dissolution cements.

Except where cemented by carbonate, the sandstones are highly compacted. This is indicated by sutured, concavo-convex, and long grain contacts as well as occasional microstylolites. Megascopic inspection of cores revealed numerous stylolites in four wells; this includes all three producing wells sampled. Petrographic porosity ranges from 0.0 to 9.3%, averaging 1.6%. Most porosity is secondary, resulting from the dissolution of carbonate and silica cement, and possibly some of the few original unstable framework grains. The Lewis Sand from a depth of about 11,000 ft has no porosity in the samples examined, all pore space being occluded by pyrite.

Relative percentages of framework grains indicate that the provenance terrane was likely a stable craton interior, presumably to the north, having a sedimentary rock cover with very mature quartz sandstones as a major component. This is in contrast to the overlying Pottsville (Pennsylvanian) rocks of the Black Warrior Basin, which contain 10-18% metamorphic rock fragments, and which probably represent detritus shed from a rising Ouachita terrane to the south and southwest. Based on the very fine-grained nature of the sand and the presence of marine fossils, oolites, and thin shale interbeds, the Chesterian sandstones were likely deposited offshore from deltas recognized to the north and east of the study area.