Organic Matter Type, Maturity, Depositional Environmental Characteristics, and Liquid Hydrocarbon Potential of Late Carboniferous Kozlu Bituminous Coal and Coaly Shale Beds (Zonguldak-Amasra Basin, NW Anatolia, Turkey): An Application of Biomarker Geochemistry

Abstract

The Upper Carboniferous (Westphalian A) Kozlu Formation consisting chiefly of sandstone, thick coal layers, shale, and conglomerate levels crops out in the Zonguldak-Amasra Basin, NW Anatolia. The Kozlu Formation contains a total of 20 mineable coal layers with the thickness varying from 0.5 to 6 m. The lower level of formation is represented by lacustrine deposits, whereas the upper part is made up of thick flood plain sediments and meandering river deposits bearing laterally continuous coal levels. In this study, coal samples taken from coal layers within the Kozlu Formation at the Kozlu Underground Coal Mining site were evaluated using the data obtained from pyrolysis/total organic matter (TOC), gas chromatography, and gas chromatography–mass spectrometry. The average total organic matter (TOC) value of Kozlu coals is 40.28%. The coals are characterized by relatively high hydrogen index (HI) value (average 262 mg HC/gTOC) and very low oxygen index value (2 mg CO2/gTOC). Pyrolysis data indicate that coals contain dominantly type II and less amount of type III kerogen, and T max values range from 454 to 468 °C. In gas chromatographs, the recorded distribution consists predominantly of low-carbon-numbered n-alkanes and subordinately of high-carbon-numbered n-alkanes and the terrigenous/aquatic ratio value is very low (0.05–0.09). Pristane abundance is greater than that of phytane, and Pr/Ph ratios are in the range of 1.11–1.60. The sterane abundances in the Kozlu coals are in the following order: C29 > C28 > C27. Coals have high C19 and C20 tricyclic terpane concentrations and high (C19 + C20)/C30 ratio, high C30* (diahopane) and C29Ts concentrations, and high C30*/C29Ts ratio and low C31R/C30 and C29/C30 hopane ratios. The dibenzothiophene-to-phenanthrene ratio of Kozlu coals is found to be very low (0.04–0.14). Based on the pyrolysis and biomarker data, the Kozlu coals are interpreted as being deposited in a suboxic–oxic continental environment in which there is effective input of clay and dominantly terrestrial (with significant lipid-rich components) and bacterial organic matter. High T max values, CPI values close to 1, low moretane/hopane (0.23–0.12), equilibrated 22S/(22S + 22R) homopane (for C32), 20S/(20S + 20R) C29 sterane (0.52–0.54) and TA­(I)/TA­(I + II) steroid ratios, high ββ/(ββ + αα) sterane (0.51–0.55), C30*/C29Ts, C30*/(C30* + C30H), MPI-3­(α/β) (1.24–1.41), and MDR and MA­(I)/MA­(I + II) steroid are indicative of mature–late mature organic matter. R o values between 0.9 and 1.25% determined from T max (454–468 °C) values indicate “high-volatile bituminous B–medium-volatile bituminous” rank for Kozlu coals. Kozlu coals having HI values (up to 331 mg HC/gTOC) extremely higher than those of classical coals indicates that these coals have significant oil and gas generation potential, and high S 1 (average 6.04 mg HC/g rock) and S 2 (110.08 mg HC/g rock) values imply that they generate notably high amount of liquid hydrocarbons and still have generation potential.

Maceral and Organic Geochemical Characteristics of the Late Permian Coals from Yueliangtian Mine, Guizhou, Southwestern China

“…Odd-carbon alkanes preferentially degrade under the action of thermal evolution; as a result, the values of OEP and CPI gradually decreased. If the odd-over-even values vary by approximately 1, then the maturity of organic matter is indicated . The mean values of OEP and CPI of YLT coal samples are 1.10 and 1.23, respectively (both close to 1), and there is no obvious odd–even carbon predominance, indicating that the organic matter has high maturity.…”

“…If the odd-over-even values vary by approximately 1, then the maturity of organic matter is indicated. 7 The mean values of OEP and CPI of YLT coal samples are 1.10 and 1.23, respectively (both close to 1), and there is no obvious odd−even carbon predominance, indicating that the organic matter has high maturity. Meanwhile, the values of OEP (0.78−1.25) and CPI (1.13−1.30) were drawn in CPI vs OEP diagram (Figure 6a), which is placed in the mature level.…”

“…The average yield of EOM from YLT coal is 1.01% (Table 3), and this may be related to the 0.98−1.03% R o of coal samples, which falls into the maximal oil window of humic coals. 7 In addition, the proportions of aromatics are much higher than saturates, producing lower ratios of saturated to aromatic hydrocarbons, ranging from 0.14 to 0.76 (av. 0.26).…”

“…10 Some common molecular parameters for saturated hydrocarbons, such as the carbon preference index (CPI) and odd−even predominance (OEP), Pr/Ph and Pr/n-C 17 , Ph/n-C 18 , many ratios of steranes and terpanes, as well as terpenoid biomarkers, are the potential source of valuable information regarding the plant precursors, redox conditions during peatification, and thermal evolution degree of organic matter. 7,9 Similarly, the aromatic molecules containing polycyclic aromatic hydrocarbons (PAHs) and heterocyclic aromatic compounds can also support the geological information. However, due to the thermodynamical dealkylation and methyl rearrangement reactions, they normally produce more maturity parameters such as the methylphenanthrene index (MPI), methylphenanthrene distribution fractions (F), methyl-, dimethyl-, trimethylnaphthalene ratios (MNR, DNR, and TNR), and the methyldibenzothiophene ratio (MDR).…”

“…Organic geochemical characteristics of coal are frequently used to evaluate essential information regarding the coal-forming plant communities, depositional environment, and the maturity level of organic matter. − For instance, the yield of extractable organic matter (EOM) from coal is closely related to the coal rank and organic matter maturity, and EOM is the basic experimental data and may reflect the hydrocarbon generation potential . Some common molecular parameters for saturated hydrocarbons, such as the carbon preference index (CPI) and odd–even predominance (OEP), Pr/Ph and Pr/ n -C 17 , Ph/ n -C 18 , many ratios of steranes and terpanes, as well as terpenoid biomarkers, are the potential source of valuable information regarding the plant precursors, redox conditions during peatification, and thermal evolution degree of organic matter. , Similarly, the aromatic molecules containing polycyclic aromatic hydrocarbons (PAHs) and heterocyclic aromatic compounds can also support the geological information. However, due to the thermodynamical dealkylation and methyl rearrangement reactions, they normally produce more maturity parameters such as the methylphenanthrene index (MPI), methylphenanthrene distribution fractions (F), methyl-, dimethyl-, trimethylnaphthalene ratios (MNR, DNR, and TNR), and the methyldibenzothiophene ratio (MDR). ,− Besides, PACs are classified as persistent organic atmospheric pollutants.…”