Mineralogy, geochemistry and source potential of shale in the Cambro-Ordovician Khabour Formation, Iraq

Abstract

Mineralogical, geochemical and the potentiality of source shale rocks in the Khabour Formation from the Akkas-1 well, western Iraq and the Chia Zinnar section exposed in extreme northern Iraq were studied. Whole rock minerals observed are quartz, feldspars (labradorite, albite and orthoclase), pyrite and rare calcite. The main clay minerals observed in the studied shales are illite, chlorite, kaolinite, and mixed layer illite. SEM analysis shows that kaolinite is typically present as hexagonal plates and booklets, some of which are degraded, illite is commonly present as fibers and fine white flakes. Mixed layer illite is commonly observed as framboidal shapes while chlorite is observed as disc-shaped chlorite. Two samples from the upper, Hirnantian, part of the Akkas-1 well exhibit great differences from post Archean Australian shale (PAAS) in both major and trace element geochemistry. This is in contrast to the 13 other samples from the Akkas-1 well which have a trace element composition very close to PAAS. These differences are hard to interpret but they may record the impact of the Hirnantian glaciation. Common redox indicators (Th/U, Cu/Zn, V/Rb, Mn) all suggest an oxic environmental conditions prevailed during deposition of the Khabour Formation in the Akkas-1 well; this is supported by low TOC values. Khabour shales from the Akkas-1 well are organically lean (TOC 0.04–0.84%) and are late to post mature for oil generation. They represent poor sedimentary organic matter (SOM) but the presence of bitumen in the samples suggest source rocks do exist in this part of the succession. The studied samples from the Chia Zinnar section (extreme northern Iraq) are organically lean (TOC <0.2%) and have no source potential. Acritarchs provide new age constraints for the Khabour Formation at outcrop in northern Iraq. Six samples from the Chia Zinnar section contain Tremadocian acritachs and are assigned to the PZ10 palynozone. The surface section is therefore a chronostratigraphical equivalent to the basal shale unit observed in the Akkas-1 well.