Quartz Cementation in Cretaceous and Jurassic Reservoir Sandstones from the Salam Oil Field, Western Desert, Egypt: Constraints on Temperature and Timing of Formation from Fluid Inclusions

Abstract

Middle Jurassic and Lower Cretaceous reservoir sandstones in the Salam oil field (in Egypt's Western Desert), are mostly quartz-arenites with abundant quartz overgrowths. Most overgrowths precipitated relatively early in the diagenetic history, only pre-dated by mechanical- and minor chemical compaction. Integration of the diagenetic sequence with the geological and maturation history of the basin, indicates that quartz cementation occurred prior to oil filling the reservoirs during the Eocene, probably during the Late Cretaceous, related to high subsidence and heating rates. Homogenization temperatures (Th) of aqueous fluid inclusions that occur along the boundary between the detrital quartz grains and their overgrowths indicate that the quartz cements started to grow at temperatures from about 116°C to 135°C. These palaeotemperatures are similar to, or slightly higher than, present-day formation temperatures, and show a positive correlation with present depth of burial. The palaeotemperatures obtained from fluid inclusions are higher than predicted from thermal history models. There are several possible explanations for these relatively high palaeotemperatures: (i) the Late Cretaceous burial depths were higher than initially presumed; (ii) the Late Cretaceous palaeogeothermal gradients were considerably higher than present gradients; (iii) migrating hot fluids were responsible for the growth of quartz cements, at temperatures above those resulting from the contemporary conductive geothermal gradient; or (iv) fluid inclusions were trapped at lower temperatures and re-equilibrated during subsequent burial. Although thermal re-equilibration of the grain-boundary fluid inclusions is hypothetically possible, and would resolve the inconsistencies between Th data and the thermal and migration histories, there is no clear evidence supporting re-equilibration, given the narrow ranges of Th in each sample and the lack of a correlation between Th and parameters such as inclusion salinity or size.