Organic and inorganic geochemical and mineralogical assessments of the Silurian Akkas formation, western Iraq

The Silurian Akkas Formation has been reported and described only in the subsurface of western Iraq. The formation is divided into the lower Hoseiba Member, which contains two high-TOC “hot” shale intervals that together are around 60 m thick, and the overlying Qaim Member that is composed of lower-TOC “cold” shales. This study investigates the source rock potential of Akkas Formation shales from the Akkas-1and Akkas-3 wells in western Iraq and assesses the relationship between their mineral and elemental contents and their redox depositional conditions and thermal maturity. Twenty-six shale samples from both members of the Akkas Formation from the Akkas-1and Akkas-3 wells were analysed. The results showed that the upper, ~20 m thick “hot” shale interval in the lower Hoseiba Member has good source rock characteristics with an average TOC content of 5.5 wt% and a mean Rock-Eval S₂ of 10 kg/tonne. Taken together, the two “hot” shale intervals and the intervening “cold” shale of the Hoseiba Member are ~125-150 m thick and have an average TOC of 3.3 wt% and mean S₂ of 6.2 kg/tonne. The samples from the Hoseiba Member contain mixed Type II / III or Type III kerogen with an HI of up to 296 mgS₂/gTOC. Visual organic-matter analysis showed that the samples contain dark brown, opaque amorphous organic matter with minor amounts of vitrinite-like and algal (Tasmanites) material. Pyrolysis – gas chromatography undertaken on a single sample indicated a mature (or higher) algal-dominated Type II kerogen. High spore and acritarch colour index values and weak or absent fluorescence similarly suggest that the lower part of the Akkas Formation is late mature to early post-mature for oil generation. “Cold” shales from the Qaim Member in the Akkas-3 well may locally have good source rock potential, while samples from the upper part of the Qaim Member from the Akkas-1 well have little source rock potential. Varied results from this interval may reflect source rock heterogeneity and limited sample coverage.
Mineralogically, all the shale samples studied were dominated by clay minerals – illite and kaolinite with minor amounts of chlorite and illite mixed layers. Non-clay minerals included quartz, carbonates, feldspars and pyrite along with rare apatite and anatase. Palaeoredox proxies confirmed the general link between anoxia and “hot” shale deposition; however, there was no clear relationship between TOC and U suggesting that another carrier of U could be present. Rare Earth Element (REE) contents suggested a slight change in sediment provenance during the deposition of the Akkas Formation. The presence of common micropores and fractures identified under SEM indicates that these shales could become potential unconventional reservoirs following hydraulic fracturing. Evidence for the dissolution of carbonate minerals was present along fractures, suggesting the possible passage of diagenetic fluids.
Palynological analysis combined with existing graptolite studies support a Wenlock – Pridoli/Ludlow age for the Akkas “hot”shales. This is younger than many other regional “hot shale” age estimates and warrants further detailed investigation.