Diagenesis as Related to Burial History, Pressure Regimes, and Low-Permeability Zones in the Upper Cretaceous Lower Tuscaloosa Formation, Mississippi and Louisiana

Paula L. Hansley

Sandstone reservoirs in the lower part of the Upper Cretaceous Tuscaloosa Formation in the Gulf Coast contain large petroleum and natural gas reserves and many are presently overpressured at depths greater than 3,350 to 4,900 m. A petrologic study was initiated to determine the types of diagenetic seals present and the timing of cementation. Hydrocarbons stain most authigenic phases including early iron-rich chlorite and quartz overgrowths and later pore-filling kaolinite and albite, and they are present in dissolution vugs in both ferroan and nonferroan carbonate cements from depths of 2,800 to 6,300 m. In sandstones from 4,575 to 6,300 m depths, two-phase hydrocarbon and hydrous inclusions are abundant between detrital quartz grains and their overgrowths and are common ithin the overgrowths. Uncorrected (because of included petroleum and (or) methane?) homogenization temperatures of these inclusions average 125 °C, indicating that they formed in the late oil or early gas window. Present bottom hole temperatures of wells in the lower Tuscaloosa at depths of about 4,600 m average 145 °C and temperatures at depths of about 5,600 m average 170 °C. These values indicate that the quartz overgrowths formed before maximum burial of the Tuscaloosa.

According to published hydrologic models, the Tuscaloosa was under a hydrostatic pressure regime through the early Tertiary. Burial history reconstructions indicate that shales in the Tuscaloosa in central Louisiana reached the gas generation window (about 125 °C) at a depth of 4 km in the Miocene. At this time, much of the lower Tuscaloosa in the downdip gas trend (greater than approximately 4,300 m) became overpressured and silica and hydrocarbon (pyrobitumen?) cements created low-permeability zones in sandstones, preserving overpressured compartments and trapping hydrocarbons.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California