Organic petrology and geochemistry of Triassic and Jurassic coals of the Tabas Basin, Northeastern/Central Iran

Abstract

Jurassic and Triassic coals and organic matter-rich shales of the Tabas Basin, Iran, were investigated with respect to their thermal maturation and petrographic composition. The Triassic coals of the Parvadeh coal field range in vitrinite reflectance between 1.17% and 1.37% and the Jurassic coals of the Mazino coal field between 2.08% and 2.29% VR r . Maceral analysis revealed a predominance of vitrinite in all samples, with slightly higher percentages of inertinite in the Jurassic samples. Rock-Eval analyses confirm the presence of type III kerogen in the Triassic coals, while the Jurassic coals contain strongly carbonized residual kerogen. The range of sulfur contents for coals from the Parvadeh coal field (0.37% to 4.64%) and Mazino coal field (0.45% to 2.92%) is related to the effect of marine water in peat. The studied samples are characterized by the predominance of short-over long-chained n-alkanes. The relatively high Pr/Ph ratios indicate predominance of terrestrial organic matter whereas Pr/n-C 17 and Ph/n-C 18 ratios prove oxic conditions during deposition. In addition, the DBT/Phen ratio shows that these coals formed in fluvial/deltaic environments. Molecular geochemical parameters such as carbon preference index (CPI, 0.99-1.04), methyl phenanthrene index (MPI, 1.20-1.60), methyl naphthalene ratio (MNR, 1.61-3.45), and ethyl naphthalene ratio (ENR, 4-6.78) confirm the high maturity of the samples. Burial and thermal history reconstruction indicates necessity of an erosional thickness of about 4000 m in the Parvadeh area. Towards the Mazino area, a higher basal heat flow up to 80 mW m -2 is assumed for the Paleogene leading to higher maturities.

Depositional and thermal history of a continental, coal-bearing Middle Jurassic succession from Iran: Hojedk Formation, northern Tabas Block

“…The maturity is comparable to that of Middle Jurassic coals from the adjacent Mazino coalfield to the south ranging from 2.08 % to 2.29 % (Zamansani et al . 2019) and is far higher than the reported reflectance for the Upper Triassic coals in the Parvadeh coalfield ranging from 1.19 % to 1.33 %, corresponding to the upper oil window (see Fig. 1b for locations).…”

“…In a recent study by Zamansani et al . (2019), the advanced maturation for the Jurassic coal in the Mazino area is attributed to a post-Jurassic overburden of 4500 to 6000 m according to applied basal heat flow. In the same study, a magmatic effect at 34 Ma and thick sediment deposition due to the Alpine orogeny at 20 Ma has been taken into account.…”

“…report, 2018). Thus, the petrographic study of three coal samples from the lowermost part of the Hojedk Formation is key for the depositional and post-depositional history of the succession in the northern Tabas Block that can be compared to coalgeological data from the Mazinu and Parvadeh areas (central Tabas Block; Zamansani et al 2019;Esmailpour et al 2022). The main purpose of the present contribution is (i) to document a detailed reference section of the lower Middle Jurassic Hojedk Formation by means of careful bed-by-bed logging; (ii) to define its sedimentary facies based on lithology, texture and sedimentary structures as well as on stratal geometries and surface bedding type; (iii) to group the identified facies into facies associations; (iv) to characterize the depositional environment of the Hojedk Formation; (v) to identify the source area and the source rocks of the Hojedk Formation by means of provenance analyses; and (vi) to perform coal petrography on selected samples in order to identify major maceral constituents and to obtain maturity data.…”

“…Thus, the petrographic study of three coal samples from the lowermost part of the Hojedk Formation is key for the depositional and post-depositional history of the succession in the northern Tabas Block that can be compared to coal-geological data from the Mazinu and Parvadeh areas (central Tabas Block; Zamansani et al . 2019; Esmailpour et al . 2022).…”

Distribution model, depositional environment and facies of coal in the AE field, Kutai Kartanegara area, East Kalimantan

“…Another interesting thing is that the coal in the two wells is associated with an inter-distributary bay which is relatively more influenced by the rise and fall of the sea level so that sulfur minerals are found and are interpreted to form in the transitional environment of the lower delta plain. Coal in the lower delta area is generally characterized by continuous and thick seams [8].…”