Petrographical Features of Organic Matter from Upper Jurassic Naokelekan Formation, Kurdistan-Iraq: A Study on Regional Thermal Maturity Trends

Abstract

Upper Jurassic Naokelekan organic-rich strata are ubiquitous across the Kurdistan Region of Iraq. Laterally, continuous layers, of substantial thickness, outcrop in the highly folded and imbricated zones and in subsurface sections in the Low Folded Zone. In this study, 15 samples from 5 outcrops, viz., Barzinja, Sargelu, Barsarin, Karak, and Bnavya, were examined, which represent the entire lithological alternations of limestones and shales encountered in the Whole Naokelekan Formation. This study assesses the type, the thermal maturity and the petroleum generation potential of the organic matter contained in the upper Jurassic Naokelekan formation throughout the area. The qualitative petrographical evaluation of the studied samples revealed that the main organic constituents are solid hydrocarbons, in the form of microgranular migrabitumens, with minor amounts of pyrobitumens. Equivalent vitrinite reflectance estimations (0.59–1.12% R_o) indicate that the upper Jurassic sequence in the western part of the area is at a mature stage, while the same sequence in the eastern part is at late mature stage.