Solid bitumen, bituminite and thermal maturity of the Upper Jurassic-Lower Cretaceous Chia Gara Formation, Kirkuk Oil Field, Zagros Fold Belt, Kurdistan, Iraq

Abstract

The Upper Jurassic-Lower Cretaceous Chia Gara Formation, a proven marine source rock characterised by predominance of amorphous organic matter (AOM) is known from few outcrops in NE Iraq and a number of exploration wells in the Zagros Fold Belt. This study presents comprehensive organic petrographic and mineralogical investigations of the dispersed organic matter and rock-forming minerals of the Tithonian-Berriasian Chia Gara Formation intersected by the Kirkuk well K-109 drilled in the giant Kirkuk oil field, NE Iraq. The organic petrographic analyses, including incident light microscopy and confocal laser-scanning microscopy (CLSM) were applied to carbonate source rocks with low to moderate organic carbon contents. Our data demonstrate the predominance of bituminite and solid bitumen among the examined dispersed organic matter followed by oil exudates and oil globule inclusions as well as the presence of minor contributions of alginite (i.e., Tasmanites and Leiospheridia), liptodetrinite and vitrinite. Based on optical characteristics and CLSM spectral fluorescence measurements, examinations of bituminite point towards the presence of bituminite type III. The bituminite type III encloses dark orange fluorescent amorphous components suggesting a genetic relationship either with liptodetrinite and/or with solid bitumen. The qualitative assessment of solid bitumen reveals several dissimilar forms identified in the analysed samples. The photometric and spectral fluorescence measurements point to distinct reflectance values ranging between 0.19% and 0.43% SBRr and lambda max values extending from 515nm to 590nm, respectively. The corresponding results of the solid bitumen exhibit (1) a lack of a relationship with depth and, (2) suggest its poor property as a reliable maturity indicator for the examined samples. The presence of oil exudates, oil globule inclusions and their association with solid bitumen suggests a genetic relation with the examined residues of liquid hydrocarbon phase. The performed thermal maturities assessments of dispersed organic matter point towards early-mature to peak-mature oil window with vitrinite reflectance values ranging between 0.52% VRr at 2801m to 0.83% VRr at 3089m depth. Mineralogical analyses revealed the predominance of calcite over dolomite in the examined limestone to marl lithofacies (e.g., clear rimmed dolomite) with localised intensive dolomitisation. In the phyllosilicate-rich matrix, predominance of calcite and siderite is observed, followed by zinc blende and pyrite. The composition of organic matter in the examined Chia Gara Formation with predominance of bituminite and solid bitumen makes the major marine source rock of Iraq's Cretaceous petroleum systems distinctive in terms of hydrocarbon generation and migration at an oil window stage as it provides suitable explanations for its generally low hydrogen index (HI) with only minor terrestrial input. In addition, the results presented herein can be helpful for future evaluation of thermal maturities and characterisation of optical appearances of dispersed organic matter in carbonates by incident light microscopy and especially by CLSM during hydrocarbon prospecting.