Geochemical and Clay Mineralogical Characteristics of the Black Shale and Constrains on Diagenesis and Maturation, Chia Gara Formation, Iraqi Kurdistan Region, Iraq

Faraj H Tobia,Rezhin K Mustafa

doi:10.46717/igj.55.1b.3ms-2022-02-19

Abstract

Black shale of Tithonian (Late Jurassic)-Berriasian (Early Cretaceous) Chia Gara Formation have been investigated by clay minerals and geochemistry (total organic carbon, major and trace elements) to determine the chemical associations and the relation between their diagenesis and maturity. The X-ray diffraction data show that kaolinite and illite are predominant clay minerals. The samples show low illite crystallinity index (0.56ºΔ2θ), and most of the illite-smectite mixed layers are altered to the illite mineral. Analyzed black shales are recognized by high total organic carbon (TOC) content (1.47- 5.87wt%) and rich in SiO2 (33.19%), followed by CaO (20.54%) and Al2O3 (12.08%). Comparison between the obtained data and the Post Archean Australian Shale values indicate that CaO, Sr, U, V, Ni, Zn and Mo were present in higher concentrations; P2O5, Nb and Hf were nearly similar, while Na2O, MnO, Rb and Ba was much lower. Also, the behavior of the trace elements can be inferred from the enrichment factor. The studied black shales are enriched in Zn, U, V, Ni and Mo. Correlation between elements predict their association and origin. U, V, Co, Ni, Cu, Zn and Mo are related to the phosphate minerals, also the organic matter played a part in the enrichment of V and Ni elements. Al2O3 significantly correlates with Fe2O3, MnO, TiO2, Sc, Hf, Nb, Zr, Th, Ba, Rb in addition to REE indicating their associations with clay minerals. Dependent upon the predominance of illite clay mineral and illite crystallinity index, in addition to the Tmax (426 and 442ºC), the plurality of the studied shales is over mature and anchi-metamorphic. The shales are related to the deep diagenetic zone with a paleo-temperature between 150 and 200°C.

Highlights

Black shales are dark-colored, usually laminated, organic-rich sediments which are related with Upper Jurassic-Lower Cretaceous claystone, clayey sandstone and limestone that are enriched in organic carbon (Wignall, 1994; Pancost et al 2004)
The negative correlation of Sr with total organic carbon (TOC) could be assigning to the adsorption of Sr on clay minerals and/or may enter the calcium lattice defects (Aziz et al 2008) or the black shale was subjected to severe diagenetic processes
The black shale samples are characterized by high TOC contents (1.47- 5.87%)

Summary

Introduction

Black shales are dark-colored, usually laminated, organic-rich sediments which are related with Upper Jurassic-Lower Cretaceous claystone, clayey sandstone and limestone that are enriched in organic carbon (Wignall, 1994; Pancost et al 2004). 2022, 55 (1B), 23-37 potential of the hydrocarbon (regards as source rock) and likewise are regarded as host rocks for metal concentrations (Wignall, 1994; Schultz, 2004). Schultz (2004) pointed out some of these black shales are mostly enriched in V, U, Mo, Zn, Ni, and Cu. Tourtelot (1979) suggests that geological processes factors controlled the accumulation of the black shale not geological settings. Geological processes include the accumulation of the shale and the diagenesis that increased with the burial depth. Anthogenesis processes such as transformation and recrystallization, impact mineralogy, geochemistry and texture of the features of the rocks. Trace and REE composition of the clay minerals have been employed to study the provenance and sedimentary processes (Condie, 1991; Tobia et al 2018) and to determine the diagenetic grades (Ohr et al 1994)

Methods

Results

Discussion

Conclusion