Organic geochemistry of the Middle-Upper Jurassic Naokelekan Formation in the Ajil and Balad oil fields, northern Iraq

Abstract

Sixteen organic-rich argillaceous limestone and black shale samples from three wells with up to 16 wt% TOC (average 9.32 wt%) from the Middle–Upper Jurassic Naokelekan Formation in the west Zagros Basin of northern Iraq were evaluated as source rocks. Both organic geochemical and molecular data indicate an open marine environment at the time of deposition, leading to accumulation of Type-II and Type-III kerogen with varying oil and gas generation potential. The predominance of short-chain n-alkanes (n-C13–n-C19), and C27 and C29 steranes, and absence of oleanane confirm the dominantly planktonic/bacterial and algal origin of the organic matter. Organic richness, low Pr/Ph, and high C35/C34 homohopane ratios suggest alternating anoxic to suboxic depositional conditions. Biomarker distributions are consistent with sporadic anoxia, with unequivocal evidence for stratification of the water column and hypersalinity. The 22S/(22S + 22R) ratios for C32 homohopanes, moretane/hopane, and C29 sterane 20S/(20S + 20R) and ββ/(ββ+αα) ratios vary irregularly, whereas the C27 diasteranes, Ts/(Tm + Ts), triaromatic cracking and MDR ratios vary substantially and increase with burial depth among different homologues within the same sample set, indicating the threshold to peak of the oil-generation window. Based on hierarchical cluster analysis (HCA) and principal component analysis (PCA) of 13 selected source-related biomarker ratios, the source rock extracts have been divided into three groups: Group-1 from the Ba-1 well, Balad oil field, and Groups-2 and -3 from the Aj-8 and Aj-12 wells, respectively, in the Ajil oil field.