Sedimentology, Geochemistry and Reservoir Potential of Sandstones in the Silurian Akkas Formation, Western Iraq

Abstract

The source rock potential of "hot shales" in the Silurian Akkas Formation in Iraq has been investigated by numerous studies, but the reservoir potential of sandstone intervals in the formation has received less attention. This study investigates the sedimentology and geochemistry of sandstones from the Akkas Formation in the Akkas-1, Akkas-3 and KH5/6 wells in western Iraq. The composition of sandstone samples from the Akkas wells is similar; in general they are classified as sub-litharenites, quartz-arenites and sub-arkoses. Scanning electron microscopic analysis identified extensive microporosity and good pore connectivity, suggesting that these sandstones have the potential to form hydrocarbon reservoirs. The sandstones from the KH5/6 well are more lithic-rich than those from the Akkas wells and are classified as sub-litharenites. They have larger, more connected pores and better reservoir potential. Low permeability shale intervals within the Akkas Formation and the conformablyunderlying Ordovician Khabour Formation form barriers to hydrocarbon migration into the Akkas and Khabour sandstones. Hydrocarbon migration from the Akkas "hot shales" in the Akkas field is therefore controlled by faulting and fracturing.Petrographic and whole rock geochemical analyses showed that the composition of sandstones in the Akkas Formation is different from that of sandstones in the Khabour Formation. The chemical alteration index ranges from 77.39 to 87.06%, indicating intense weathering of the provenance area before sandstone deposition. The studied samples are texturally mature which indicates good potential for fluid storage capacity. A decrease in feldspar content in the Akkas Formation is attributed to possible recycling of sediments from the Khabour Formation into the Akkas Formation following the Hirnantian glaciation, or to longer distance transportation from the source area.

Clay Minerals and Organic Matter from Deeply Buried Ordovician-Silurian Shale in Western Iraq: Implications for Maturity and Hydrocarbon Generation

“…Concerning source rock potential of the Paleozoic source rocks in Iraq, Aqrawi et al (2010) [26] revealed that the lower Ordovician shales (assigned to Member 7 of the Khabour Formation, according to Al-Hadidy, 2007, [18]) and the lower Silurian ("hot" shale of the Akkas Formation) could be regarded as principal hydrocarbon source rocks in the Paleozoic sequence of western Iraq, similar to their role across the region in Jordan Syria, Libya, and Saudi Arabia [27,23,25]. The lower "hot" shale unit of the Silurian Akkas Formation, western Iraq, has an average-good source rocks (TOC 1.2-5.25 wt%, mean 2.2 wt%; S2 1.2-8.7 kg/tonne, mean 4.2 kg/tonne) [28]. Whereas, TOC values are 0.17-1.42wt% for the 2750-3000m interval and 0.5-1wt% for the 3570-3650m interval in the Khabour Shale of Akkas-1 well, as suggested by Al-Ameri (2010) [23].…”

Review of the Lower-Palaeozoic Reservoir Prospectivity of W. Iraq With Reference to Jordan

“…upper shale) of later Llandovery age (see Aqrawi et al, 2010 for its definition in Iraq). In terms of Iraqi stratigraphy, this entire interval falls within the Akkas Formation of the Western Desert area, in which there is basal hot shale, Hoseiba Member and uppermost Qaim Member (Al-Hadidy, 2007 andAl-Juboury et al, 2019).…”

“…In the Akkas-1 well, light oil (42° API) was discovered in fractured sandstones and siltstones of the Qaim member, which has been assumed to be sourced from the hot shale of the underlying Hoseiba Member (Al-Habba et al 1994 andAl-Juboury et al 2019). Al-Khafaji et al (2015) concluded that only the Khabour Formation can be regarded as source for the gas and condensate in the Akkas field, whereas the hot shales of the Akkas Formation have not reached a sufficient maturation level in the well investigated.…”

“…Mohammed et al (2020) concluded that the black shales of Khabour Formation are fair source rocks, highly matured and reached the peak of gas generation. Al-Juboury et al (2019) reported visual porosity in the sandstones from the Akkas-1and -3 wells ranging from 1% to 10%, but pore connectivity was considered often poor, which might affect the reservoir potential. In the Akkas-1 well, reservoir potential was assessed as good (of 7-10% porosity) to moderate (of 5-7% porosity).…”

“…The Lower Palaeozoic succession is thus a primary exploration target with some discoveries in Ordovician and Silurian sandstones (e.g. Al-Haba et al 1994;Aqrawi, 1998;Al-Hadidy,2007;Tamar-Agha, 2009;Al-Ameri, 2010;Aqrawi et al 2010;Al-Khafaji et al 2015;Al-Juboury et al 2019;Abdula et al 2020 andMohammed et al 2020) where it is thick in both eastern Jordan and western Iraq ( Figs. 2A & 2B).…”