Kerogen

The distinction between conventional and unconventional hydrocarbon accumulations depends on whether oil or gas are hosted within a well-defined trap and whether they can be produced economically by wells. The unconventional oil and gas resources cannot be extracted economically by using conventional methods and technologies, while the conventional accumulations refer to technically and economically recoverable hydrocarbons. The unconventional accumulations are characterized by large resources but poor reservoir properties. Conventional hydrocarbons only account for less than 20% of the world’s fossil fuel resources, whereas unconventional hydrocarbons account for at least 80%. On the Romanian territory, 10 petroleum basins with different hydrocarbon richness have been identified: Moesian Platform, Transylvanian Basin, Eastern part of Pannonian Basin, Eastern Carpathians Flysch, Moldavian Platform, Carpathian Foredeep, Schythian Platform, Maramures Basin, North-Dobrogean Promontory and the Romanian shelf of the Black Sea. In these basins more than 18 petroleum systems have been identified. Almost all these petroleum basins contain unconventional hydrocarbon resources like shale gas, shale oil, heavy oil, tar sands, tight sands gas, gas hydrates and coal bed methane (CBM).

X-ray absorption near-edge structure (XANES) methodology has been employed to quantify
the different sulfur structures present in three Type I and three Type II kerogens. Kerogens
from the Green River (3), Bakken (1), Woodford (1), and Indiana limestone (1) formations were
studied. Both aliphatic (sulfide) and aromatic (thiophene) forms of sulfur exist in all these kerogen
samples. Except for Woodford, all of the kerogens contain oxidized functional groups. Sulfur in
Types I and II kerogens mimics the carbon chemistry in that the sulfur structures are more
aromatic in Type II than in Type I. It was impossible to differentiate elemental sulfur from pyrite
in these samples by using K-edge XANES.

Carbonaceous material (CM) is thought to be a key reductant contributing to the formation of large Au deposits, but there has been much speculation about its source, molecular composition and reactivity. The first successful analytical retrieval of organic compounds from a thermally over-mature (> 550 °C) Paleoproterozoic Cosmo-Howley Orogenic Au deposit was recently achieved by Robert et al. (2016). Here, we have evaluated the nature of the CM associated with this high temperature Au mineralisation via an integrated analytical approach which combined high-resolution in situ laser Raman spectroscopy, micro to nano-scale imaging (e.g., EELS, HAADF-STEM, and HRTEM) and molecular and isotopic geochemistry. We identified two distinct CM types: CM ker – an ubiquitous highly graphitic kerogen typical of high-grade metamorphic conditions formed by regional meta-morphism; and CM fd – small sub-microscopic inclusion-like nodules of highly disordered carbon rich in poly-cyclic aromatic hydrocarbons (PAHs), coincident within the Au-bearing sulfide minerals in hydrothermal vein regions. The paragenetic emplacement and molecular characteristics of CM fd suggests a formation by metaso-matic processes and introduction by a hydrothermal fluid which might also have co-transported Au. CM ker and CM fd gave different Raman spectra indicative of their contrasting origin and structural response to regional and contact metamorphic history and subsequent metasomatism of the Cosmo-Howley deposit. Raman signals indicated CM ker had a graphitic like structure whereas CM fd comprised high concentrations or clusters of PAHs. The broad range of Raman spectra detected here (and by others in similar studies) was likely due to the mixed signals of these two types of CM. The δ13C values of PAH products released via the HyPy treatment of the parent and sequentially demineralised kerogen fractions were measured to be in the range of −20 to −30‰, indicative of an organic biopolymeric origin. The δ13C values of PAHs products decreased with demineralisation, con-comitant with an increase in their concentrations and affinity to the sulfide-minerals (and associated CM fd) suggesting a close relationship. The localised (within 20 mm) co-occurrence of different CM types and apparent abundance correlation of CM fd with Au and sulfides suggests Au mineralisation might be supported by specific CM types, and these relationships should be evaluated further including on a wider Au deposit scale.

A methodology to determine the chemistry and kinetics of the multiple reactions during geological maturation was developed, with a special emphasis on the representation of diagenesis and oil formation processes. The methodology combines a unique macromolecular and kinetic model for hydrocarbon pyrolysis, the FG-DVC (functional group devolatilization, vaporization, cross-linking) model, with a method of analysis based on thermogravimetric analysis with Fourier transform infrared spectroscopy (TG-FTIR). TG-FTIR pyrolysis data from several natural maturation series of coals and kerogens were measured, systematic trends with the degree of maturation were identified, and empirical processes and reaction kinetics during maturation necessary to induce these trends were estimated. This approach eliminates potential inaccuracies when extrapolating kinetic parameters obtained from laboratory experiments to geological conditions. The FG-DVC pyrolysis model was modified to include these maturation processes, with aqueous chemistry providing a guide for such modifications. The resulting FG-DVC maturation model was then used to predict the maturation of several immature samples through the well-known time/temperature history of the basin. The FG-DVC pyrolysis model was subsequently used to predict the open-system pyrolysis decomposition of the predicted maturation residues, and the predictions were compared to TG-FTIR data of the corresponding naturally matured samples. For most of the series investigated, the model gave good predictions of the variations in oxygenated gas precursors, tar Tmax, and extractable yield with maturation. Kinetics derived from open-system pyrolysis for bridge breaking were found to be applicable during maturation. However, faster kinetics were necessary to describe the removal of oxygenated gas precursors. In addition, the removal of methane and tar was found to be too slow during maturation when using open-system pyrolysis kinetics. Artificial maturation experiments using confined pyrolysis were also performed for comparison. While the evolution rates, during subsequent pyrolysis of the maturation residues, of oxygenated gas species are different from those obtained from samples naturally matured, the yields compare favorably with model predictions. The trends for pyrolysis tar and methane from artificially matured samples are similar to those of natural samples but suggest different kinetics.