Mineralogy, petrography, and Rock-Eval pyrolysis of late Oligocene coal seams in the Malkara coal field from the Thrace Basin (NW Turkey)

The Thrace Basin hosts the second-largest natural gas and oil resource of Turkey. Several mineable late Oligocene coal seams are located close to natural gas fields in the basin. This study focuses on the assessment of late Oligocene seams in the Malkara coal field using mineralogical, petrographic, and Rock-Eval pyrolysis data. The variations in coal properties seem to be mainly controlled by common peat-forming vegetation and detrital input ratio into palaeomires. The studied samples are characterized by variable TOC, Tmax and HI values and high H contents. Even though the relatively high H contents seem to be related to high total liptinite contents, the presence of hydrogen-rich telohuminite macerals, particularly ulminite A, may be the main reason for elevated H contents in the studied samples. Furthermore, the Rock-Eval analyses results, which imply the presence of mixed type III-IV kerogens, conflict with coal petrography data. The differences could be a by-product of carbonate minerals breakdown during pyrolysis; however; the presence of hydrogen-rich telohuminite macerals, which could have originated from resinous woody peat-forming plants, could cause suppression S2 peak and elevation of S3 peak due to release additional COx from the breakdown of hydrogen- and oxygen-rich compounds in telohuminite macerals during pyrolysis. Therefore, HI, Tmax, and OI values are not matched with maceral compositions. Moreover, the relatively higher Tmax and %Rr values of studied late Oligocene coal seams in the Malkara coal field than early Oligocene shales and late Oligocene coals in the NE parts of the basin could be related to higher thermal gradient and/or post-Oligocene tectonic movements. Overall, the results imply that the coals are mostly gas-prone; thus, the studied coal seams along with unexploited coal seams near to the study area have a hydrocarbon generation potential for especially natural gas in the Danis , men Formation. The results of this study also show that the Rock-Eval pyrolysis data of low-rank humic coals similar to Malkara coals should be used with caution and cross-checked with results of traditional coal parameters such as ash yields, H contents, maceral, and mineralogical compositions to avoid misinterpretations.

Automated summary

The late Oligocene coal seams in the Malkara coal field are influenced by vegetation and detrital input ratio, exhibiting high TOC and high hydrogen content, mainly generating natural gas. The Rock-Eval pyrolysis data should be used cautiously and verified with traditional coal parameters to avoid misinterpretations.