van Dijk, Clemens P.*1; Breuer, Pierre 2
(1) EXPEC Advanced Research Center, Saudi Aramco, Dhahran, Saudi Arabia. (2) Geological Technical Services Division, Saudi Aramco, Dhahran, Saudi Arabia.
The Devonian Jauf sandstones in Ghawar field show highly variable reservoir quality. The best sandstones have well preserved intergranular porosity due to the presence of radial illite coatings which retarded quartz cementation, but some wells are tight due to pervasive authigenic clay matrix. K-Ar dating shows that the radial illite formed between 81-121 Ma, although it could be a recrystallization of an earlier, possibly smectitic clay coat. The clay matrix is composed of illite, kaolinite and chlorite. Although the illite matrix could represent grain-coating illite that has grown into a pore-filling habit, the illite matrix aggregates lack the radial orientation of the clay platelets at the grain surfaces which characterises the late radial illite coatings. Alternatively, the illite and kaolinite matrix could also have formed early as a replacement of feldspars after Hercynian uplift, when the Jauf experienced prolonged exposure at the Permo-Carboniferous land surface.
The objective of this ongoing study is to evaluate the controls on Jauf reservoir quality in central Ghawar using forward diagenetic modelling, which simulates sandstone diagenesis along the reconstructed burial path. Two scenarios of authigenic clay matrix formation were evaluated: early formation below the Hercynian unconformity and late formation during deep burial. Understanding its origin will help predict authigenic clay matrix distribution and associated poor reservoir quality sandstones within the field.
Prolonged Jauf exposure at the Permo-Carboniferous land surface would have started after the tectonic quiescence that followed the Hercynian orogeny, and ended with the deposition of the earliest Unayzah sediments. The intensity of weathering during exposure would have depended on the contemporaneous climate and groundwater flow regime. Specifically, the duration of any ice coverage in Unayzah C times—as a result of the late Paleozoic glaciation—would have had a strong effect on groundwater flow and associated weathering of the near-surface Jauf.
The depth and temperature derived from the burial history form the framework for the simulation of compaction, cementation and porosity at each time step. Specific information about the known or inferred formation conditions (age, temperature or depth) of each cement as well as cement-parent mineral relationships is also needed, which allows testing of different scenarios for the origin of authigenic clay matrix and porosity.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain