THE ROLE OF OIL MIGRATION IN THE DIAGENESIS OF UPPER CRETACEOUS EUTAW FORMATION SANDSTONES FROM THE SUBSURFACE OF CENTRAL MISSISSIPPI

The Upper Cretaceous Eutaw Formation is among the prolific hydrocarbon reservoirs in Mississippi. In the Heidelberg Field near Laurel the Eutaw occurs as upward coarsening paralic sandstones and siltstones. Sandstone layers at the bottom of the studied core are only inches thick and very-fine grained; these relatively clay-poor rocks are surrounded by heavily bioturbated siltstones and shales. In the upper portion of the 150 ft core the relative amount of usually fine- to medium-grained laminated sandstone increases and the units can be as thick as 5 to 6 ft, again separated by siltstones and shales. The framework composition of the entire section is subarkosic, and glauconite, chert, and muscovite are abundant.

In the upper portion of the core oil occurs almost exclusively in sandstones thicker than 3 or 4 ft. In the sandstone-poor lower section of the core oil occurs in sandstones just a few inches thick. The first significant burial diagenetic minerals present in all the sandstones are quartz overgrowths, which are more abundant in sandstones without oil than those with oil. Oyster shells are found in many of the sandstones, and calcite cement associated with those shells can completely occlude porosity, however, calcite cement is never present in sandstones with oil. These relations imply that the migration of oil into the sandstones stopped porosity-occluding diagenesis.

Alteration of glauconite in the burial diagenetic environment is relatively minor and siderite is the most common authigenic iron-rich phase present in the oilfield core. This is in contrast to alteration of Eutaw Formation outcrops in the northern part of the state in which glauconite is extensively altered and iron oxides and hydroxides are the most common cements.