PETROGRAPHY AND DIAGENETIC CHARACTERISTICS OF THE UPPER OLIGOCENE - LOWER MIOCENE GHAR FORMATION IN SE IRAQ

This paper investigates the petrography, sedimentology and diagenetic evolution of the upper Oligocene - lower Miocene Ghar Formation in the subsurface of SE Iraq based on data from well logs, cuttings and core samples from boreholes in the Zubair, Majnoon and nearby oilfields. Lateral facies variations permit the Ghar Formation to be divided into two principal facies, a pebbly sandstone (facies A) and carbonate (dolomite) and calcareous sandstone (facies B), each of which can be divided into subfacies. Facies A rocks, interpreted as alluvial fan deposits, pass into facies B, paralic and shallow- marine sediments in a number of fining- upward successions. This facies change from mainly clastics (facies A) to mainly carbonates (facies B) suggests that the Ghar Formation may have some potential as a stratigraphic trap in SE Iraq. Diagenetic processes include compaction, authigenesis, dolomitization and dedolomitization. In facies A, authigenic minerals were quartz, palygorskite, haematite, Fe-rich dolomite and smectite. In facies B, the main authigenic minerals were kaolinite, illite and illite-smectite. This difference in diagenetic mineralization may indicate variable interstitial waters ranging from fresh to marine. Dolomite was the main carbonate mineral encountered and could be divided into early matrix and pervasive types. The first type was represented by smooth crystals less than 20 microns in size which were associated with illite and illite-smectite mixed layer clays. Pervasive, fabric-destructive dolomite was represented by crystals 30-300 microns in size which were found as void fillings. Dedolomitization was observed where high-Mg calcite filled cavities near the contact with the overlying Fat'ha (Lower Fars) Formation. These diagenetic processes may enhance the reservoir characteristics of the Ghar Formation.