Mesozoic source rock systems at the junction between Eurasia and Gondwana: Tethyan vs Boreal controls in the Southeastern Caucasus, Northeastern Azerbaijan

Northeastern Azerbaijan occupies a complex geodynamic setting at the convergence of the Scythian-Turanian (Epihercynian) Platform and the Meso-Tethyan accretionary prism, evolving along the Northern Gondwana margin. Functioning as a key segment of the regional North Crimea – Greater Caucasus – Kopetdagh Meso-Tethys marginal sea, the accretionary prism records a pivotal transition from Lower-Middle Jurassic shale-rich aspid facies to Cretaceous flysch sequences. Despite decades of research on Mesozoic source rocks formed within this complex geodynamic setting, the lack of a global realm-based framework has historically led to contradictory evaluations of the stratigraphic attribution of effective source rocks. This study resolves these inconsistencies by integrating geochemical and paleontological data, revealing that the Jurassic-Cretaceous succession reflects a distinct Boreal-Tethyan polarity. This polarity is interpreted to arise from contrasting source-to-sink configurations that controlled the accumulation and preservation of organic-rich sediments.

We identify the massive Middle Jurassic (Aalenian-Bajocian) shale succession as a regional gas-prone-dominated system (max. TOC 2.43%); its mixed Type II/III kerogen signature and paleoenvironmental context mirror those of European epicontinental basins. Consistent with global platformal trends, the Neocomian unconformity lies directly upon Middle Jurassic shales across the northern domain. This confirms the negligible regional role of the Upper Jurassic Callovian-Tithonian succession, globally recognized as the most prolific Mesozoic source rock interval, which is here constrained by uplift-driven erosion and low-latitude (~15°N) aridity, except in restricted zones where preserved biohermal limestones reflect warm Meso-Tethyan influence.

Consequently, the overlying Neocomian succession exhibits marked heterogeneity, with geochemical evidence aligning with regional paleontological records. While predominantly regressive and gas-prone in the Boreal northern sector, southern zones subject to intensive Meso-Tethyan influence record "pulse-like" Tethyan transgressions that deposited discrete, high-quality oil-prone shale intervals, mainly within the Hauterivian-Barremian (max. TOC 5.96%; HI up to 557 mg HC/g TOC). These pulse-like inflows created temporary anoxia in the southern flysch basin, favoring high-quality organic matter preservation comparable to that in the oil-prone Tethyan Zagros Basin (Garau Fm.), characterized by Type II kerogen and HI values reaching 600 mg HC/g TOC.

Crucially, the Upper Cretaceous Cenomanian-Coniacian shale succession (Kemishdagh and Kemchi Fms.) emerges as the region’s primary oil-prone system (TOC 2.92-6.02%; HI 406-479 mg HC/g TOC). Attributed to Oceanic Anoxic Event 2 (OAE 2), its exceptional productivity parallels that of the prolific Tethyan Zagros Basin to the south, while standing in sharp contrast to the organic-lean, Boreal-influenced Terek-Caspian Basin to the north, confirming that Tethyan oceanic events exerted limited influence on the Eurasian interior. Supported by the alternation of thick shale layers with tuff and bentonite interbeds within the Upper Albian-Coniacian succession of the accretionary prism, this OAE 2 signature correlates with the Late Albian-Late Turonian explosive volcanic phase documented in local reports and is biostratigraphically anchored by Tethyan radiolarian colonies. Furthermore, co-occurring cosmopolitan radiolarians, carbonate-shelled planktonic foraminifera, and benthic taxa north of the prism reveal that, despite dominant Tethyan control, the basin maintained biotic connectivity with the European epicontinental province.