Source-Rock Potential of Jurassic Deposits on the Northern and South-Western Flanks of the Kosbulak Trough (North Ustyurt Basin, Kazakhstan): Evidence from Rock-Eval Pyrolysis of Two Exploration Wells

The Kosbulak Trough, in the eastern part of the North Ustyurt Basin in western Kazakhstan near the border with Uzbekistan, is one of the deepest Mesozoic depocentres of the basin, yet published organic geochemical data on its source rocks remain scarce. This study evaluates Rock-Eval pyrolysis data from two exploration wells located on opposite flanks of the trough: Well N on the northern flank (11 cuttings samples, 1657–1975 m depth) and Well S on the south-western flank (752 cuttings samples, 450–4688 m depth). Formation tops from well logs and palynology were used to compare the same stratigraphic units in both wells. In Well N, the Middle Jurassic coal-bearing interval (1685–1875 m depth) has median TOC of 5.81 wt.% (up to 32.2 wt.% in coal), S2 of 1.08–31.59 mg HC/g rock and HI of 43–189 mg HC/g TOC, indicating gas-prone Type III organic matter; Tmax of 424–429 °C and measured vitrinite reflectance of 0.43–0.44% show that it is thermally immature. In Well S, the Middle Jurassic (3242–4010 m depth) is leaner (median TOC 1.06 wt.%, median S2 1.16 mg HC/g rock), but its richest coaly beds at 3522–3563 m depth (TOC 3.32–4.93 wt.%, S2 5.41–8.34 mg HC/g rock) have Tmax of 435–441 °C, corresponding to the onset of the oil window. In the Lower Jurassic and Triassic of Well S (below 4010 m depth), Tmax decreases to about 407 °C; this is attributed to low S2, oil-based-mud contamination and bitumen impregnation observed in core, rather than to decreasing maturity. Regional vitrinite reflectance and palaeotemperature data support this interpretation. The results indicate that Middle Jurassic source rocks are richer but immature on the northern flank and leaner but more mature on the deeper south-western flank; the deeper axial part of the trough, not sampled here, is suggested as the most probable gas-prone generation area.