Role of Maturity in Controlling the Composition of Solid Bitumens in Veins and Vugs from SE Turkey as Revealed by Conventional and Advanced Geochemical Tools

Solid bitumens found in veins and cracks may have formed from once liquid petroleum by thermal chemical alteration such as maturation or by oxidative processes associated with thermochemical sulfate reduction (TSR), biodegradation, or weathering. In this study, we investigated 22 solid bitumen samples from 17 different veins in the Şırnak Province in southeast Turkey using conventional and advanced geochemical tools with respect to their formation. Southeast Turkey is located in an active collision zone from the Alpine orogeny, and compressional pressure occurred with different intensities across the region, generating an overall increase in maturation from west to east. Special focus was paid to the characterization of polar compounds containing nitrogen, sulfur, and oxygen (NSO) of high molecular weight using Fourier transform ion cyclotron resonance mass spectrometry with electrospray ionization in negative ion mode (ESI-FT-ICR-MS). The results indicate that, among possible secondary processes, thermal stress plays the major role in transforming solid bitumen compositions across the region after their initial emplacement in subsurface veins and vugs. Hydrogen Index values and amounts of extractable organic matter correlate well with Tₘₐₓ. A progressive shortening of n-alkyl chains and increasing proportions of aromatic hydrocarbons are observed by pyrolysis-GC. While common maturity ratios based on saturated biomarkers are at equilibrium or saturated biomarkers are below the detection limit for samples with Tₘₐₓ > 450 °C, those based on aromatic hydrocarbons, such as phenanthrenes or alkyldibenzothiophenes, are useful to describe the whole range of thermal chemical alteration. ESI-FT-ICR-MS results reveal a dominance of nitrogen and sulfur compounds (NySz) with 1–2 pyrrolic nitrogen atoms and 0–2 thiophenic sulfur atoms and nitrogen only (Ny) compounds with 1–2 pyrrolic nitrogen atoms. Condensation, aromatization, and side-chain cracking reactions of NSO compounds take place with ongoing maturation, as indicated by double bond equivalent and carbon number distributions. At highest maturation levels, side-chain cracking has proceeded so far that further condensation and aromatization processes are not possible, which can be interpreted as an indication that solid bitumens have formed from migrated, reservoired petroleum and not from source rock kerogen. An increase in oxygen containing compounds indicates oxidation processes which most likely occurred after emplacement.