Burial dissolution of micrite in Middle East carbonate reservoirs (Jurassic–Cretaceous): keys for recognition and timing.
Résumé
This paper discusses burial diagenesis and especially a burial dissolution phase and its effects on micrite microtexture in reservoirs in the Middle East. Three microporous reservoirs were selected: the Cenomanian Mishrif Formation in Iraq; the Kimmeridgian–Tithonian Arab D and the Barremian Kharaib Formations, both in Abu Dhabi. Staining, cathodoluminescence and scanning electron microscopy were used for petrographic observations. In the selected reservoirs, three typical micrite microtextures are distinguished: micro-rhombic, compact anhedral, and rounded. The chronology of diagenesis, based on petrographic observations, shows that mineralogical stabilisation, through high magnesian calcite replacement by low magnesian calcite, led to the formation of micro-rhombic micrites. Non-reservoir facies such as seals present compact anhedral micrites. Correlation between these micrites and high aluminium content suggests alumino-silicate minerals (clays) as a catalyst for pressure dissolution, and thus for reservoir degradation through cementation. Micrites from the upper part of oil reservoirs are dissolved during burial diagenesis. Dissolution is responsible for rounded shape and for reduction of crystal size. It also implies pore diameter and porethroat size enlargement. Comparison of porosity and permeability with scanning electron microscope data confirms that rounded micrites are better reservoirs than micro-rhombic micrites.