AN ANALYSIS OF RESERVOIR PROPERTIES AND CONTROLS IN THE COTTON VALLEY GROUP OF JONES COUNTY, MS

The Cotton Valley Group is a well-defined oil and gas reservoir rock in the Mississippi Interior Salt Basin and the Gulf coast region. The Cotton Valley Sands are also referred to as the Terryville Sands in some literature. The objective of this investigation is to use thin section and core analysis to determine the controls on reservoir quality in a productive gas field in Jones County, MS. The reservoir properties, such as porosity, permeability, and effective hydraulic conductivity can be inferred from analyses of physical pore scale structures and broader changes in porous media across a unit.

Three sets of rock core samples, obtained from three gas wells at depths exceeding 16000 ft, were analyzed at the core sample scale and in thin sections. These samples are named for their associated wells: A, B, and H. The core was logged to determine the general lithologies and other physical core scale properties of relevance. Selected zones of the core were further studied at the pore scale through the observation and analysis of thin sections.

The analysis of the cores of the Cotton Valley Group from Jones County, MS shows a range of lithologies, from silty shale with bioturbated muds to clean well-sorted reservoir sands. Lithology varies both spatially and with depth. The A well consists of two cored intervals and is composed predominantly of sandstone, with some organic-rich shale zones. The B core sample consists of silty mudstone to shaley siltstone near the top of the cored interval (16586-16601 ft depth), and grades into a poorly-consolidated mudstone with a few small (<1 ft thick), isolated zones of consolidated siltstone to a depth of 16643 ft. Finally, the core samples from the H well feature reservoir quality sands within the upper portion of the cored interval (16682-16713 ft) with occasional clay layers (not exceeding 1 inch thickness). There is an abrupt transition at approximately 16713 ft depth from sandstone to siltstone. With increasing depth in the H well, the siltstone grades to a very poorly-consolidated mudstone. By 16721 ft depth, the H well core samples are dominated by mudstone. However, at approximately 16739 ft depth, the core samples transition to a more well-consolidated siltstone. This siltstone lithology predominates from 16739-16747.5 ft. The terminal depth for the H well is 16747.5 ft.