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

Ali I Al-Juboury,Adrian Hutton,Mohammed A Al-Haj,Brian Jones

doi:10.24996/ijs.2020.61.11.23

Ali I Al-Juboury, Adrian Hutton + Show 2 more

Open Access

https://doi.org/10.24996/ijs.2020.61.11.23

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Abstract

The present work is conducted on the Paleozoic (Ordovician) Khabour and the (Silurian) Akkas shales in the Akkas-1 well of western Iraq. The study is aiming to determine the implications of clay mineral transformation, organic mineral distribution and maturity of hydrocarbon generation, using X-ray diffraction (XRD), scanning electron microscopy (SEM) in addition to organic matter concentrations. In the shale of the Khabour Formation, amorphous organic matter is common and includes various Tasmanite-type organic matter, vitrinite, inertinite, and bituminite. The main clay minerals observed include illite, chlorite, kaolinite, in addition to mixed-layer illite-smectite and rare smectite. In Silurian shale, high content of organic matter is recorded in addition to abundant vitrinite and low content of grainy organic matter (Tasmanites) and pyrite. Illite and kaolinite are commonly found in addition to chlorite and illite-smectite clay minerals. Conversion of smectite to mixed-layer illite-smectite (I-S) and an increase in vitrinite reflectance are commonly observed below 2500 m depth in the studied formations, which coincides with oil and gas generation. These results could be used as an indication of higher maturity and hydrocarbon generation in the deeply buried shale of the Khabour and Akkas formations in western Iraq.

Highlights

Clay minerals and organic matter usually coexist in clastic rocks and are used as a tool to identify potential hydrocarbon generation and expulsion due to their high susceptibility to temperature changes that control related mineral conversions and organic maturity [1,2,3,4,5]
To assess the thermal maturity of sedimentary rocks, several paleothermometers are used as thermal maturity indicators to reconstruct palaeotemperature histories; a technique that is widely applied in petroleum exploration [3, 7, 8]
Coexisting clay minerals and organic matter in the reservoir sandstones and shale source rocks are sensitive to temperature changes that accompany hydrocarbon generation and expulsion processes [45]

Summary

Introduction

Clay minerals and organic matter usually coexist in clastic rocks and are used as a tool to identify potential hydrocarbon generation and expulsion due to their high susceptibility to temperature changes that control related mineral conversions and organic maturity [1,2,3,4,5]. It is interesting to note that an increase in illite and chlorite and a decrease in kaolinite clay minerals is observed in the upper hot shale units of the Silurian Akkas Formation (samples 6-7 at depth 2200-2225m, see Figure). The enrichment with mica and the common presence of illite plates and thick hexagonal platy kaolinite (dickite) in these deeper shales is clearly observed (Figures-3 F and 5), while disc-shaped chlorite together with hexagonal kaolinite (Figure-5 B) is present as pore fillings This gradual conversion of smectite into mixed-layer I-S and mica with increasing burial depth and increasing temperature is common in shales deposited in subsiding basins [48].

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Discussion