Comparison of generative capacities for bitumen and gas between Carboniferous coals from Donets Basin (Ukraine) and a Cretaceous coal from Sabinas–Piedras Negras Basin (Mexico) during artificial maturation in confined pyrolysis system

Abstract

The goal of this work is to study the ability of two immature Carboniferous coals from Donets Basin (Ukraine) to act as source for oil. Heating experiments in confined medium were performed to compare the thermal behavior of these coals, 1l1 Dim (%Rr = 0.55; H/C at. = 0.79) and 2c10YD (%Rr = 0.65; H/C at. = 0.80), relative to a mature Cretaceous coal from Sabinas Basin (Olmos, %Rr = 0.92, H/C at. = 0.77). Macerals analysis carried out on starting materials showed that Olmos is exhausted in liptinite contrary to 2c10YD (20 vol.%) and 1|1Dim (6 vol.%). The vitrinite content is lower for 2c10YD (59 vol.%) than for Olmos (84 vol.%) and 1|1Dim (80 vol.%). Solid bitumen occurs often dispersed in the raw coals. Both petrographic and geochemical analyses on starting materials revealed that the selected Carboniferous Donets coals have better potentialities for bitumen generation than the Cretaceous Sabinas coal. The presence of long chain n-alkanes (> n-C8) in the pyrolysis-GC chromatograms indicates that the two raw Carboniferous coals from Donets Basin can yield non-volatile hydrocarbons under further thermal maturation. It is speculated that some vitrinite macerals present in hydrogen-rich Carboniferous coals from Donets Basin can act as source rocks for oil. As a matter of fact, results showed that the ‘oil window’ occurs between ~ 1.0%Rr and ~ 2.0%Rr for both Cretaceous Sabinas and Carboniferous Donets coals during confined pyrolysis. As expected from geochemical and petrographic analyses of starting samples, the Carboniferous Donets coals yielded more bitumen and hydrocarbons than Cretaceous Sabinas coal during artificial maturation. Low proportions of solid bitumen (< 12 vol.%) are also formed between 1.1%Rr and 1.5%Rr during confined pyrolysis of coals. Two solid bitumen groups have been identified, which correspond to distinct phases of neo-formation. The drop in the solid bitumen content at higher ranks indicates that it contributes to generation of gas during experimental simulation. Moreover, their morphology and porosity change with the level of maturity. The porosity of the vitrinite matrix increases as well as the pore size with increasing maturity. A relationship has been observed between the porosity and the weight loss: the higher pore content of Carboniferous Donets coals is correlated with a higher generation of gas compared to Cretaceous Sabinas coal.