Graham Blackbourn: Blackbourn Geoconsulting In some central and
southern parts of the West Siberian Basin (WSB), oil and gas
shows are widespread and numerous accumulations have been
discovered in weathered and fractured mid and late Palaeozoic
rocks below the Jurassic unconformity, especially on
structurally high areas adjacent to Late Permian and Triassic
grabens, and including Palaeozoic reefs and other carbonate
rocks (Karnyushina, 2005). Most of these rocks are metamorphosed
to varying degrees, but areas of relatively low-grade Palaeozoic
lithologies have been encountered. Lithologies hosting Triassic
and Palaeozoic sub-unconformity reservoirs within the Middle Ob
region include (Myasnikova et al., 2005): » Basic volcanics:
basaltic rocks and associated tuffs (Surgut Arch, Fedorovsk
crest). » Intermediate volcanics: andesite-porphyries, andesite-
basalt-porphyries, brecciated rhyolitic porphyries. » Acid
volcanics: dacites, rhyolites, ignimbrites (North Danilovsk
area) and associated welded tuffs. » Ultra