Modeling the Interactions of Fluids and Rocks to Predict Reservoir Quality: Applications to the Gulf Coast Frio Formation

POTTORF, R. J., and L. L. SUMMA, Exxon Production Research Company, Houston TX, T. F. SCHWARZER, Exxon Company USA, Houston, TX, and W. J. HARRISON, Colorado School of Mines, Golden, CO

This study evaluates the extent to which computer models of regional paleohydrology and fluid-rock interaction can be used to predict hydrocarbon migration and the evolution of sandstone reservoir quality in the Gulf Coast Frio Formation. An integrated model was developed showing variations in fluid flow and timing of compactional overpressure along three regional cross sections in eastern Texas, southern Texas, and central Louisiana. The model suggests that the onset of extensive overpressures occurred at different times: during the early Oligocene in southern Texas, the earliest Miocene in Louisiana, and the Plio-Pleistocene in eastern Texas. These variations in overpressure timing could have resulted in significant variations in porosity evolution, since overpressured sediments exp rienced decreased fluid movement and consequently less fluid-rock interaction.

In order to test this hypothesis and develop better methods for constraining these types of models with observational data, the mineralogy and geochemistry of authigenic cements from recent exploration wells in the Frio Formation of southern Louisiana and Texas are being analyzed. For example, the fluid-flow model suggests that in South Louisiana, the middle Frio was exposed to meteoric water from 28.4 to 25.2 Ma at burial depths of <2 km. Since 25.2 Ma, the rocks have been in transitional to hard overpressure, and subjected only to compactional fluids moving updip at <0.1 cm/yr. Mineralogic observations are directionally consistent with early meteoric incursion followed by minor fluid flow during burial. The sandstones contain <10% cement precipitated as follows: (1) early k olinite that appears to be dissolving under present conditions, (2) quartz, chlorite, illite, and calcite, and (3) albite and analcime. Modeling of fluid-rock interactions indicates that this assemblage could be precipitated by increasing pore-fluid salinity and temperature in response to burial diagenesis. Porosity predictions based on these calculations will be tested by future drilling of exploration wells.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)