Estimation of Palaeozoic sediments thermal maturity of Northern Pay-Khoy parautochthone on the basis of conodont colour alteration indexes

The Palaeozoic of the norther part of the Pay-Khoy carbonate parautochthone is composed of mainly shallow-water (shoal) limestones and dolostones containing numerous bitumen deposits. Increased concentrations of the bitumen locate in the areas of dolomitisation and tectonized zones with strikes of 20-40°, 60-70°, 110-130°. Conodont colour alteration indexes range from 3.5 up to 5.5 in the Palaeozoic rocks of the parautochthone. This indexes correspond to MK4-MK3 and upper stages of catagenesis (temperature of maturation is about 180-350°С). Clear stratigraphical trend of degree of the maturation exists: mean temperature changes from 300°C in the Ordovician rocks to 170-200°C in the Middle Carboniferous rocks. The trend allows supposing that the Lower Palaeozoic was in the “oil window” in the Middle Carboniferous. In this time mobilization of the organic matter from the Lower Palaeozoic deposits followed by hydrocarbon migration through the jointing zones of northern and north-eastern strike occurred. The migrated hydrocarbon composes the bitumen deposits in the carbonate Palaeozoic rocks.