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

https://doi.org/10.1016/j.jafrearsci.2020.103901Get rights and content

Highlights

  • •

    The paper deals with the older known rocks in Iraq.

  • •

    Possible record the impact of the Hirnantian glaciation.

  • •

    Source potential, paleoenvironmental (paleoredox) conditions.

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.

Introduction

The Khabour Formation is the oldest Paleozoic unit known in Iraq. It belongs to Ordovician period and outcropped in northern Iraq and is penetrated by 3 exploration wells, the Akkas-1, Khleisia-1 and Qaim-1 wells (Fig. 1). The Akkas-1 well contains shale intervals that are thought to be prospective source rocks for gas in the southwestern desert of Iraq. Specifically, dry gas recovered from sandstone beds in the K4-K1 members of the Khabour Formation in the Akkas-1 well is thought to be sourced from these shales (Al-Haba et al., 1994; Al-Hadidy, 2007). Aqrawi et al. (2010) mentioned that the shales (assigned to Member 7 of the Khabour Formation, Al-Hadidy, 2007) could be regarded as the principal hydrocarbon source rock in the Paleozoic sequence of western Iraq, extending regionally to Jordan, Syria, Libya, and Saudi Arabia (Lüning et al., 2000; Al-Ameri, 2010; AlKhafaji et al., 2015).

The Khabour Formation commonly are represented by interbedded sandstones and shales that were deposited in a shallow epeiric sea on the northeastern margin of the Arabian Plate, which formed part of the broad passive margin of northern Gondwana (Șengör, 1990; Beydoun, 1991; Loosveld et al., 1996; Al-Sharhan and Nairn, 1997; Al-Juboury and Al-Hadidy, 2009).

In the current study, samples underwent X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and X-Ray Fluorescence (XRF), for mineralogical and geochemical investigations. Potentiality of shale as a good source rock in the Khabour Formation from the Chia Zinnar section exposed in extreme northern Iraq and subsurface samples from the Akkas-1 well, western Iraq were also studied (Fig. 1, Fig. 2) using TOC/pyrolysis analysis and microscopic analysis to assess kerogen types and maturity. Furthermore, selected samples from the studied outcrop section were processed for palynology.

The main objectives of this study are:

  • 1: To use mineralogy, and major and trace element geochemistry of the shales to understand the depositional environment of the Khabour Formation.

  • 2: To indicate the palynostratigraphy of both surface and subsurface layers.

  • 3: To determine the source potential of shales within the Khabour Formation.

Section snippets

Geological setting and stratigraphy

The Khabour Formation crops out in northern Iraq and is encountered in three exploration wells in western Iraq (Fig. 1). The formation base is not exposed at outcrop and has not been penetrated in wells. Its lower age boundary and total thickness is therefore unknown.

In northern Iraq, the Khabour Formation is at least 800 m thick and is unconformably overlain by the Late Devonian Pirispiki Formation (Van Bellen et al., 1959; Blanc et al., 2012). The Akkas-1 well penetrates 1913 m of the

X-ray diffraction analysis

XRD analysis was carried out on 14 samples from the Akkas-1 well. A representative portion of each sample was manually ground to a fine powder and packed into a recessed plastic holder and preferred orientation was minimized. The samples were analyzed using a Philips X-ray diffractometer (PW3710) scanning from 4° to 60° 2θ. Peak identification was enabled using PDF/ICCD database and quantification using Rietveld analysis using Siroquant software (Sietronics, Australia). Analysis was done at the

Mineralogy

The main clay minerals observed in the studied shales are illite, chlorite, kaolinite, and mixed layer illite. Other minerals observed are quartz, feldspars (labradorite, albite and orthoclase), pyrite and rare calcite (Table 1). Representative X-Ray diffractograms illustrating the various mineralogical phases in the bulk rock shales analyzed are provided in Fig. 4.

Fig. 5 illustrates the variation in these clay and non-clay components with depth in the Akkas-1 well. No systematic variation with

Implications of mineralogical and geochemical results

Variations in clay and non-clay mineral composition show no clear pattern in the Khabour shale from the Akkas-1 well. Whereas, increase in illite and mixed layers illite in the deeply buried samples (Table 1) is accompanied with change in illite crystallinity. Conversion of smectite to mixed-layer illite-smectite (I-S) and increase in vitrinite reflectance are commonly observed in the deeply buried shale of the Khabour Formation and this coincides with oil and gas generation (Al-Juboury et al.,

Conclusions

  • 1.

    The studied samples are divided into 2 groups based on major and trace element geochemistry. The groups are

    • a)

      Thirteen samples from the Akkas-1 well (excluding samples AK1_KH_SH2 and AK1_KH_SH3) have a trace element composition very close to PAAS with the exception of uranium that is slightly depleted.

    • b)

      Samples AK1_KH_SH2 and AK1_KH_SH3 from the upper part of the Akkas-1 well exhibit greater variation from PAAS than the other samples with a particularly high Fe2O3, Ba, Zr, Nb, Hf, Ni and Cu content

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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