Elemental Geochemistry of Upper Campanian Shales in the Western Pontides (Türkiye): Sedimentary Environment Controls on the Organic Matter Accumulation and Source Rock Potential

Abstract

This study investigates the factors controlling organic matter accumulation in organic-bearing shales around the Bolu area of Western Pontides and their implications for hydrocarbon potential through elemental and organic geochemical analyses. The organic-bearing shales in this area contain both organic matter and various critical trace elements and also attract interest as a hydrocarbon resource. In the end, the paleoceanographic model of the depositional environment was established. The findings indicate that the studied shales were deposited in a shallow-marine shelf environment with a brackish to saline marine setting, under semi-humid to semi-arid paleoclimate conditions, and subject to low to moderate chemical weathering. However, paleoclimate and variability in water column productivity did not significantly influence organic matter enrichment. Redox indicators suggest deposition occurred under oxic to suboxic conditions, with minor oxygen deficiency aiding organic matter preservation. Fluctuations in sedimentation rate likely influenced organic matter by either promoting its degradation in an oxidizing environment or diluting its concentration. While detrital input contributed to organic matter enrichment, clay minerals did not appear to play a significant role in its preservation. These shales exhibit fair organic matter content and are characterized by Type IV kerogen, indicating limited or no gas generation potential, thereby classifying them as spent hydrocarbon sources. Furthermore, prolonged atmospheric exposure of outcrop samples may have led to organic matter degradation, resulting in residual carbon at high thermal maturity. By integrating multiple geochemical proxies, this study presents the first comprehensive assessment of these shales, offering new insights into their hydrocarbon potential.

Keywords

over-mature shales / organic matter enrichments / trace elements / paleoenvironment conditions / source rock potential / western Pontides (Türkiye)

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