Andrea Ceriani¹, Carlos Rossi², Robert H. Goldstein³, Rafaela Marfil², Andrea Di Giulio¹
(1) University of Pavia, Pavia, Italy
(2) Complutense University Of Madrid, Madrid, Spain
(3) University Of Kansas, Lawrence, KS

Abstract: Fluid inclusion assemblages in quartz overgrowths constrain diagenetic systems in jurassic-cretaceous sandstone reservoirs, Libya and Egypt

Sandstone reservoirs (Jurassic-Cretaceous) from opposite sides of the Cirenaica platform (Sirt basin, Libya and Shushan Basin, Egypt's Western Desert) show significant similarities in their thermal and fluid histories during precipitation of quartz cements. Quartz overgrowths preserve primary aqueous fluid inclusions during early and later phases of quartz precipitation. Individual events of fluid inclusion entrapment (fluid inclusion assemblages, FIAs) retain consistent homogenization temperatures arguing against thermal reequilibration. The earliest FIAs preserve homogenization temperatures from about 130 to 150°C. These indicate initial precipitation at temperatures consistent with or higher than the present formation temperatures (~128-136°C). Freezing point depressions in the inclusions of these early FIAs indicate fluids of relatively low salinity, with common values in the seawater to freshwater range. These high temperatures and low salinities can best be explained by perturbation of the system by fluid flow. Aqueous FIAs in later phases of quartz overgrowth preserve homogenization temperatures as low as 114°C. The local presence of some coeval petroleum inclusions yielding the same homogenization temperatures as the accompanying aqueous inclusions argue against the need for a pressure correction. These data indicate a significant drop in temperature, which cannot easily be explained by simple unroofing. Freezing point depressions of the aqueous inclusions in these FIAs indicate high to very high salinities (commonly 20 to 22 wt.% NaCl equivalent). In both basins, the record of increased salinity and drop in temperature may in part be controlled by a regional event of brine reflux.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana