Petrography and diagenetic controls on reservoir characteristics in Unayzah formation, central Saudi Arabia

Abstract EN

-The Unayzah Formation forms the basal part of the Permo- Carboniferous sedimentary succession in central Saudi Arabia and represents a transgressive and regressive unit, comprised of sandstone and shale/claystone sequences situated above the thick Qusaiba Shale (Silurian) and below the Khuff Carbonate (Permian).

The sandstones are fine to medium grained and the majority of the samples are moderately well sorted quartzarenite.

The upper and lower parts of the formation differ in the intensity of their diagenetic changes as well as in their detrital mineralogy and this is probably due to different provenance.

Mechanical compaction due to depth in the range of 9500- 11800 feet reduces the porosity to an average of 25%.

Ductile grains played an important role during compaction as a result of high porosity loss by mechanical compaction.

Silica grains in the form of overgrowth also reduced porosity to some extent.

Important diagenetic changes took place during maturation of the underlying Qusaiba Shale and expulsion of acidic and organic solutions transported into the overlying Unayzah Formation to enhance the development of secondary porosity in the quartz-rich sediments.

The humid and tropical climate and several transgressive and regressive cycles resulting in deep weathering and reworking were also responsible for destruction of unstable minerals which lead to increase in porosity.

The study has its economic importance because of the recent discovery of hydrocarbons in the same formation in wells drilled in the eastern and southeastern parts of the Arabian basin.