Organic geochemistry of the middle Paleozoic Ora Formation in Iraq: Implications for source rock assessment and hydrocarbon potentiality

Al-Auqadi Rahma S; Mohamed W Alkhafaji; Ali I Al-Juboury; Alex Zumberge; Nasir Alarifi; Dan Jarvie; Giovanni Zanoni; Harry Rowe

doi:10.1016/j.heliyon.2024.e29782

Organic geochemistry of the middle Paleozoic Ora Formation in Iraq: Implications for source rock assessment and hydrocarbon potentiality

Heliyon. 2024 Apr 18;10(8):e29782. doi: 10.1016/j.heliyon.2024.e29782. eCollection 2024 Apr 30.

Authors

Al-Auqadi Rahma S¹, Mohamed W Alkhafaji², Ali I Al-Juboury³, Alex Zumberge⁴, Nasir Alarifi⁵, Dan Jarvie⁶, Giovanni Zanoni⁷, Harry Rowe⁸

Affiliations

¹ College of Petroleum and Mining Engineering, University of Mosul, Iraq.
² Department of Applied Geology, College of Science, University of Tikrit, Iraq.
³ Al-Kitab University, Kirkuk, 36015, Iraq.
⁴ GeoMark Research, Houston, TX, USA.
⁵ Geology and Geophysics Department, College of Science, King Saud University, Riyadh, Saudi Arabia.
⁶ Worldwide Geochemistry, LLC, Houston, TX, USA.
⁷ RohmTek, Houston, TX, 77024, USA.
⁸ Premier Corex Laboratories, Houston, TX, 77041, USA.

Abstract

The Ora Formation (late Devonian-early Carboniferous) is thought to be a potential source rocks for the Paleozoic petroleum system of Iraq. The source potential from the Ora Formation is evaluated for the first time ever in this study from western and northern Iraq which integrates data from organic geochemistry including Total Organic Carbon (TOC) analysis, HAWK pyrolysis, gas chromatography (GC), and gas chromatography mass spectrometry (GC-MS) and mineralogical X-ray diffraction and scanning electron microscopy. The shale and muddy carbonate succession within the Ora Formation from surface section in northernmost Iraq and subsurface section from two wells (Akkas-1 and Akkas -3) from western Iraq have been employed to assess the source rock potentiality, thermal maturity, kerogen type, organic content, and depositional environment. In addition to organic geochemical analyses, mineralogical XRD and SEM-EDS were used to support the paleoenvironmental interpretation of the Ora Formation. The results from TOC and HAWK analyses reveal that the Ora Formation ranges from poor to good as a source rock. However, the HAWK data suggests that the surface samples from northernmost Iraq are highly mature, highly weathered, or both. Kerogen analysis revealed that the Ora Formation contains immature type III and mixed II-III kerogens. Low TOC values were attributed to factors such as significant clastic input, weathering effects, and the prevailing oxic environment during deposition. The presence of detrital influx of quartz and feldspars, along with the occurrence of illite and kaolinite clay minerals, suggest a detrital input with weathering influence under hot arid and warm humid conditions. Biomarker analysis of the light hydrocarbons using GC and GC-MS revealed that these light hydrocarbons were generated from marine planktonic algae sources, possibly with some contributions from terrestrial and/or microbially reworked organic matter. These high mature light hydrocarbons in subsurface section were originated from anoxic marine shale source rocks. They were most likely from the Cambro-Ordovician Khabour Formation and were contaminated from another source.

Keywords: Iraq; Light hydrocarbon; Middle Paleozoic; Mineralogy; Molecular geochemistry; Ora formation; Source rock potentiality.