Mineralogical and geochemical characteristics of the Paleozoic source rocks, Akkas gas field, Western Desert of Iraq: Implications for their origin, maturation and Ordovician-Silurian transition

Abstract

Ordovician-Silurian black shales are the source rocks in the Palaeozoic petroleum system in Iraq. However, their source and depositional conditions from their mineralogy and inorganic geochemistry are quite lacking in previous works. These shales also provide an opportunity to examine the changes in paleoenvironment, paleoclimate and paleoredox conditions across the Ordovician-Silurian transition.The Ordovician black shales comprise of kaolinite, illite, chlorite, quartz, dolomite, calcite, and traces of feldspars and siderite, while the Silurian black shales comprise of illite, quartz, calcite, dolomite and pyrite. Calculated SiO2 (wt %) and Al2O3/TiO2 ratios suggest felsic source rocks for the Ordovician black shales and intermediate source rocks for Silurian hot shales. The shallow marine setting for the Ordovician black shales that changed to a deeper environment during the Silurian time was interpreted from the V, P2O5, and Al2O3 diagrams is consistent with the abundance of trace fossils Monocraterion sp., Cruziana problematica, Diplichnites sp., and Skolithos sp. which refer to proximal shallow marine conditions and abundance of graptolites monograptus in the Silurian black shales, which refer to an open-marine environment. Paleoredox indicators such as TOC and Mn contents as well as Ni/Co, V/Ni, U/Th, and V/Cr ratios prove fluctuation of redox conditions from oxic in the Ordovician shales to anoxic in the Silurian black shales. Late Ordovician glaciation in Iraq can be indicated from the low values of chemical index of weathering (CIA = 52-58) in the samples from the upper part of the upper part of the Khabour Formation that refer to cold and arid climate conditions that prevailed during the late Ordovician time. Abundance of extrabasinal and poor-sorted mud clasts in this interval might support the glaciogenic origin of this part. The black shales in the Khabour Formation is considered as fair source rock (average TOC = 0.9 wt%), while that in the Akkas Formation is an excellent source rock (average TOC = 13.2 wt%) for hydrocarbon generation. Kerogen in the Ordovician and Silurian black shales were derived from marine organic matter as indicated by the binary plot and total C1-C15-n-C6-n-C14-n-C15+ ternary diagram. Organic matter in the Ordovician black shales is highly matured compared to that in the Silurian black shales due to higher illite crystallinity (IC) values in the Ordovician (0.61–0.75° D2θ) relative to that in the Silurian (0.45–0.55° D2θ) black shales.