Characterization of an Oligocene Reservoir in Southeast Kurdistan, Iraq

Hsieh, Jean; Begin, Normand; Deutscher, Ross

The Topkhana-1 exploration well was drilled by TALISMAN (BLOCK K39) B.V. in the Kurdistan Region of northern Iraq while the Kurdamir-1 exploration well was drilled by partner Western Zagros and TALISMAN (BLOCK K44) B.V. The wells identified and tested a significant gas/gas condensate resource in the Oligocene Kirkuk Group. This Group is regionally composed of back-reef and reef limestone, dominated by foraminifera and red algae with abundant skeletal fragments. Outcrops of the Kirkuk Group in the nearby mountains reveal tight limestone intervals alternating with porous, partially to completely dolomitized intervals, suggesting a heterogeneous distribution of reservoir quality within the Group. Using the core collected in the two wells, the development of reservoir quality was investigated through petrographic and geochemical studies.

Reservoir quality is highly dependent on dolomitization of the original rock. Limestone intervals are considered poor to fair reservoir with low porosity and permeability. The zones of best reservoir rock occur where there is intense dolomitization along with a dissolution event. A preliminary paragenesis suggests that early stages of diagenesis include micritization of tests followed by dissolution of those tests where they occur below exposure surfaces and then calcite cement precipitation around and within grains. The next stage, dolomitization, is critical for creating reservoir quality. The results of our study suggest this occurs in one event. A final stage includes later anhydrite and calcite cementation.

In order to develop a predictive model of where the dolomitized zones would occur, it is critical to understand the nature of the dolomitizing fluids and the timing of the event. Strontium, oxygen and carbon isotopes were measured on both dolomite and calcite phases. The results suggest that dolomitization occurs shortly after the deposition of the entire Oligocene reservoir in the Early Miocene. This implies that enhanced reservoir quality should occur nearer the top of the reservoir section. The proposed model for dolomitization, reflux dolomitization, suggests that the best dolomite should occur in a belt parallel to the Early Miocene restricted zone. Regional mapping of the carbonate and evaporate formations of the Early Miocene would be useful to assist in outlining the zone of best reservoir quality in the Oligocene.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013