Nine Mesozoic and Cenozoic palaeogeographic maps are presented to illustrate the petroleum prospectivity of the South Caucasus from a fresh perspective and as part of the wider Caucasus region. Previously, elements of petroleum systems – reservoir, source and sealing lithologies, and the timing of their formation – have mostly been examined within individual sub‐basins or licence blocks, and regional understanding has been limited. Emphasis is placed here on the onshore prospectivity of Georgia and Azerbaijan; the well‐known Pliocene Productive Series of eastern Azerbaijan and the southern Caspian is not considered. The Great Caucasus Basin (GCB) formed in the Early Jurassic following closure of PalaeoTethys, and remained a significant feature, despite structural modifications, until end‐Eocene underthrusting and uplift converted the basin into the Greater Caucasus mountains. By the Toarcian a major delta system had developed along its northeastern margin, while the Transcaucasus block to the south was mostly covered by a shallow sea with limited sediment supply. Bajocian volcanism across the South Caucasus was accompanied by modification of the structure of the Great Caucasus Basin with the intrusion of tholeiitic dykes, possibly associated with onset of northward NeoTethyan subduction. Rising sea levels led to the abandonment of the GCB delta system. Relative uplift of the South Caucasus in the Bathonian created lowlands surrounded by marginal settings in which paralic deposits and coals were laid down. Jurassic hydrocarbon source rocks include deep‐marine shales deposited within the Great Caucasus Basin together with coals; their potential is confirmed by numerous seeps within both Georgia and Azerbaijan. Various Middle Jurassic sandstones are potential reservoirs. Carbonates dominated by the late Callovian, with widespread development of Oxfordian reefs and of Late Jurassic evaporite basins in the North Caucasus. Bedded anhydrites in Georgia comprise potential seals. Shallow‐marine clastics again became widespread across the Caucasus in the Cretaceous, later replaced by carbonates including chalk‐like limestones. Deeper‐marine conditions persisted in the Great Caucasus Basin, which became less well‐defined and split into separate depocentres. Fractured chalks are known reservoirs in the North Caucasus and prospective reservoirs in the South Caucasus. Uplift of the southern South Caucasus during the Paleogene led to northward transport of sediment into evolving E‐W to ESE‐WSW basins in eastern Georgia and western Azerbaijan. Marine deposits within these basins form reservoirs, including thick fractured volcanogenic turbidites in eastern Georgia. Reduced sediment supply here at the start of the Late Eocene allowed organic‐rich restricted‐marine source rocks to accumulate. Rapid uplift of the GCB associated with underthrusting at the end of the Eocene led to emergence of the Greater Caucasus mountains. The prolific Maikopian source rocks were deposited in restricted‐marine conditions during the Oligocene and Ear...

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The Neogene Rioni and Kura foreland basins in Georgia are located between the converging Greater and Lesser Caucasus fold-and-thrust belts. The Rioni Basin continues westward into the Black Sea whereas the Kura Basin extends eastward into Azerbaijan and the Caspian Sea. "Pre-" and "post-salt" petroleum systems are distinguished in the Rioni Basin separated by an Upper Jurassic evaporite succession of regional extent. The pre-salt petroleum system in the northern Rioni Basin is still poorly understood. Bathonian shales have generated oil which has been recorded in Middle Jurassic sandstones. However, as the origin of the oil in Upper Jurassic sandstones (e.g. at the Okumi oil discovery) is still problematic, the pre-salt petroleum system remains poorly constrained. Gas-rich, high volatile bituminous coals of Bathonian age may represent a CBM play.The post-salt petroleum system in the Rioni Basin is charged by two prolific source rock units: the Middle Eocene Kuma Formation and the Oligo-Miocene Maikop Group. The petroleum potential of the Kuma Formation, which is about 40 m thick, is classified as good to very good. The Oligocene part of the Maikop Group is several hundred metres thick and contains source rocks with up to 5 wt.% TOC in its lower part. Additional source rocks are present in Cretaceous and lower Paleogene levels. Oil is produced from fractured Upper Cretaceous carbonates in anticlinal structures below the Neogene unconformity and from Mio-Pliocene siliciclastics in fault-related anticlines. Trap formation and hydrocarbon accumulation is interpreted to have occurred since Maeotian time. Proven oil reserves are very low (~2 million tons) and suggest low charge efficiency. Several stratigraphic horizons containing potential source rocks are present in the Kura Basin of eastern Georgia. Although oil-source correlations have yielded unsatisfactory results, the Maikop Group is the most likely source rock, despite its relatively poor petroleum potential which is at best "fair" in the Tbilisi area in the west of the basin. Additional potential source rocks include Middle and Upper Eocene shales. Fractured Middle Eocene volcaniclastic rocks are the best producing reservoirs for hydrocarbons, but oil accumulations are also found in fractured Upper Cretaceous carbonates and in Lower and Upper Eocene, Oligocene and Neogene siliciclastics. Biomarker data suggest a Cenozoic (or Upper Cretaceous) source rock containing abundant terrigenous organic matter. Anticlines and positive flower

In the broader Caucasus region, multiple extrusive volcanic units are present within the Jurassic, Cretaceous, Eocene and Miocene sedimentary successions. Partial reworking of volcanic material from various provenance areas into Eocene, Oligocene and Miocene reservoir units is commonly observed in the Eastern Black Sea and in the Rioni, Kartli and Kura Basins of onshore Georgia. Reservoir quality has in general been negatively affected by volcanic rock fragments which may have undergone complex diagenetic alteration. However, despite concerns regarding reservoir quality, oil at the Samgori field, the largest field in Georgia (~200 MM brl recovered), is hosted in altered Middle Eocene volcaniclastic sandstones interbedded with deep-water turbidites. Previous studies of core material from numerous wells in this field showed that most of the oil is contained in altered, microfractured, laumontite-rich tuffs which have fracture and cavernous net porosities averaging 12% and average permeability of 15 mD. The laumontite tuffs comprise only up to 20% of a tuffaceous sandstone section and occur as isolated lenses or pods on a sub-seismic scale (i.e. 5-10 m thick), causing highly variable oil productivity from one well to another.The petrographic analysis of samples of Middle Eocene volcaniclastic sandstones from outcrops in the central part of the Kartli Basin around Tbilisi broadly confirms the main conclusions of studies completed some 30 years ago which were based on the analysis of subsurface samples. However, the surface samples analysed show that zeolitization events typically did not improve, but actually reduced, reservoir quality due to extensive zeolite cementation. The poor reservoir properties of the plug samples, which are age-equivalent to the proven subsurface Middle Eocene reservoir interval, highlight fracturing as a key factor controlling the presence of exceptional producers (up to 9000 b/d) in the Samgori field complex. The study therefore underlines the critical role of fracturing of the Middle Eocene volcaniclastic reservoir sequence in the Kartli Basin.

<p>The Lesser Caucasus (LC) double-wedge orogen accommodates the crustal shortening due to far-field effects of the collision between the Arabian and Eurasian plates. Subsequent convergence of Arabia and Eurasian plates during the late Alpine time caused extensive intracontinental deformation in the LC. Herein we introduce the active deformation structural style of the Georgian part of the LC orogen based on seismic reflection profile, several oil-well, and surface geology data. Seismic reflection data reveals the presence of a Khrami basement thrust sheet, fault-related folds,<em> </em><em>triangle zone,</em><em> </em><em>and duplexes. </em>The rocks involved in the deformation range from Paleozoic basement rocks to Pliocene-Quaternary basaltic lava flows.</p> <p>Pliocene-Quaternary lava flows are involved in compressional deformation and are related to an out-of-thrust sequence of the Khrami basement thrust sheet. Based on the interpreted seismic reflection profile, the crustal-scale duplex was recognized under the basement thrust sheet which propagates northward along the Early Jurassic shale layers.</p> <p>The structural architecture and tectonic evolution will be briefly presented and discussed in the new regional balanced and reconstructed cross-section across the axial zone and retro-wedge of the LC and published fission-track data (Gusmeo et al., 2021, 2022), as well as detailed examples of active tectonics, and seismicity (e.g., Tsereteli et al., 2016).</p> <p><strong>Reference</strong></p> <p>Gusmeo, T., et al. (2022). Tectono-thermal evolution of central Transcaucasia: Thermal modelling, seismic interpretation, and low-temperature thermochronology of the eastern Adjara-Trialeti and western Kura sedimentary basins (Georgia). J. As. Earth Sci. 238, 105355.</p> <p>Gusmeo, T., et al. (2021). Structural inversion of back-arc basins-The Neogene Adjara-Trialeti fold-and-thrust belt (SW Georgia) as a far-field effect of the Arabia-Eurasia collision. Tectonophysics 803, 228702.</p> <p>Tsereteli, N. et al. (2016). Active tectonics of central-western Caucasus, Georgia. Tectonophysics 691, 328-344.</p> <p><em><strong> </strong></em></p> <p> </p> <p> </p>