Dolomitization of the Jurassic Arab formation gas reservoirs, Shah Field, Abu Dhabi

Abstract

Dolomitization is an important diagenetic process in oil and gas reservoirs of the Jurassic Arab Formation. However, the timing and conditions of dolomitization in the Arab Formation gas reservoir, Shah Field, Abu Dhabi have not been fully explored yet. This study aims to constrain the distribution, timing and conditions of dolomitization and of its impact on reservoir-quality evolution, using petrographic examinations, stable carbon, oxygen and strontium isotope analyses of carbonates, and sulfur and strontium isotope analyses of anhydrite. Diagenetic processes in the Arab Formation include cementation by calcite (circumgranular grain rims, syntaxial calcite overgrowths and equant spar), dolomitization and dolomite cementation, anhydrite cementation and dissolution, micritization, dissolution of allochems, mechanical compaction and stylolitization. Most dolomitization occurred during eodiagenesis and to smaller extent during mesodiagenesis. Sabkha evaporative dolomitization of peritidal sediments and cementation by nodular anhydrite mostly resulted in micro- to fine-crystalline dolomite. Sabkha dolomite may formed during oxidation of organic matter in some cases. Seepage reflux dolomitization of subtidal sediments resulted in medium-crystalline euhedral-subhedral dolomite. The formation of saddle dolomite is attributed to the flux of hot basinal brines. Partial dolomitization of micrite in peloidal wackestone-packstones was presumably initiated during early diagenesis and continued during burial diagenesis. Mesogenetic dolomite cementation occurred in moldic pores of dolopackstones. Mesogenetic precipitation of blocky anhydrite cements in intercrytalline, moldic, and intergranular pores was subjected to partial to extensive dissolution, which may have resulted from thermochemical sulfate reduction. Coarse blocky anhydrite cement was formed in fractures and along stylolites during burial diagenesis. Dolomitization has variable extent of impact on reservoir quality. The best reservoirs are (1) dolopackstones and undifferentiated medium-crystalline dolostones with abundant intercrystalline porosity and dissolved anhydrite cements; and (2) peloidal grainstones cemented by scattered sparry calcite cements.

Date of Award	2016
Original language	American English
Supervisor	Sadoon Morad (Supervisor)