Lower Paleozoic salt diapirs in the northern part of the West Siberian Basin and the Enisey-Khatanga Trough: structural setting and petroleum habitat

Abstract

Seismic data from the northern part of the West Siberian Basin and the Enisey-Khatanga Trough indicates the presence of up to 15 km-thick Paleozoic sediments underlying the Mesozoic overburden. This finding challenges the traditional notion that folded Paleozoic rocks constitute the economic basement of the basin. The Paleozoic section comprises evaporitic units, most likely of Ordovician age, which were deposited in troughs formed during the Early Paleozoic Uralian rifting. A reinterpretation of the geological structure suggests that salt tectonics has significantly influenced the subsurface architecture of these regions. Seismic evidence for salt diapirs includes (1) significant heights of salt structures (5 km or more), (2) seismic transparency, (3) minibasin tectonostratigraphic successions flanking the salt structures, and (4) radial fault systems in the overlying deposits. Additional evidence for the occurrence of salt diapirs includes magnetic, gravimetric, thermal, electrical, and topographic anomalies. Minibasin tectonostratigraphic successions suggest that diapirism began soon after salt deposition, preceding the onset of the Late Paleozoic compression. Far-field intracratonic deformations modified salt structures during the Mesozoic-Cenozoic, including extensional episodes in the Early Triassic and Early-Middle Jurassic and contractional episodes in the Late Triassic, Early Cretaceous, and the Late Cenozoic. These events led to folding of post-salt strata, which was accompanied by continuous differential compaction and Quaternary post-glacial rebound of diapirs. These processes amplified anticlines in post-salt Cretaceous reservoirs, which host giant gas accumulations.

The thick and thermally mature Paleozoic deposits include source rocks, which likely significantly contributed to the region's hydrocarbon potential. The upward migration of hydrocarbons through salt structures, from the deeply buried Paleozoic section into the Mesozoic overburden, provides a plausible explanation for the long-debated origin of the abundant gas content in Cretaceous deposits.