Reservoir Quality Variations in the Oligocene Kirkuk Group in Southeast Kurdistan, Iraq

Abstract

This abstract is submitted to the session in celebration of the career of Paul “Mitch” Harris. Mitch has taught me that to understand carbonates, you need to investigate all aspects of its depositional and diagenetic history by using core, well logs, seismic interpretation, biostratigraphy, geochemistry, petrography, outcrop analogs and anything else that you can. This project is my attempt to apply his teachings in order to understand the complex reservoir. Significant oil and gas discoveries in the Topkhana and Kurdamir blocks of southeast Kurdistan are primarily contained within the Oligocene Kirkuk group. This group is regionally composed of middle ramp foraminiferal and red algae grainstones as well as coral boundstones/rudstones that likely form small bioherms. Reservoir quality is highly dependent on both the original depositional facies as well as the overprint of dolomitization. Predicting the spatial distribution of the best reservoir quality is challenging. Earlier studies (Hsieh et al., 2013) suggest that the best reservoir rock quality occurs where there is intense dolomitization along with dissolution of the foraminiferal tests. Original intergranular porosity was often occluded by marine cement and thus, non-dolomitized grainstones were considered poorer quality and not the primary target. Based on Sr-isotope geochemistry, dolomitization was shown to occur primarily in the earliest Miocene. This suggested a “top down” dolomitization of the Kirkuk Group where the dolomite geobody would be located beneath the dolomitizing fluid body. Early interpretation of the 3D seismic data supported this hypothesis. However, recently collected cores provided not only a down-dip limit to the dolomitization front, but also added a twist in the reservoir quality story. Dolomitization only improved reservoir quality if the process was complete while partially dolomitized rocks were in fact poorer quality than non-dolomitized rocks. Additionally, coral boundstone and rudstones that seem to preserve the depositional porosity have similar flow properties as the best dolomitized rock. The spatial distribution of the best reservoir quality is therefore a combination of where there is thorough dolomitization as well as where original depositional porosity and permeability is preserved. Isotopic analyses of the new core samples may help to refine the paragenesis and provide a new framework for interpretation of the 3D seismic data.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015