Unlucky basin (Geological history of the Lena-Anabar Basin)

Alexander Egorov

Lena-Anabar depression is located in the north part of the Siberian platform. Two flanks - platform and folded are singled out in its modern structure. Marine Permian, Triassic and Jurassic sediments are exposed in folds on the surface. In Middle Riphean, sediments 'splashed out' from grabens and formed sublatitudinal depression. In the beginning of Middle Riphean in some areas basalt volcanism took place. In the end of Middle, Late Riphean, Vendian and Cambrian sedimentation occurred in vast epicontinental seas which covered the most part of the Siberian platform. Sediments were mostly carbonate; fluctuation of thicknesses (1200-1500 m) is insignificant. During Prepermian time the area of their distribution was much wider. After considerable break, carbonate sedimentation was replaced by terrigene. Permian sediments on platform flank are presented by alluvial facies. Total thickness of the Permian section varies from 100 to 600 m, gradually increasing to the north. Permian sediments within the folded flank are presented by mainly sea facies. Their thickness is about 3 km. Triassic sediments on both flanks are presented by mainly marine facies, though completeness and thicknesses of their sections differ considerably. Induan, Early Olenekian, Norian and Rhaetian sediments 100 m thick are present only on platform flank while on the folded flank they are presented by all stages and their thickness reach 1300 m. Formation of local consedimentary structures occurred in time very dynamically. In Devonian and Carboniferous overall high standing of all northern part of the Siberian continent was marked. In Permian number of small heights were formed along the frame of the modern Olenek high. In the beginning of Early Triassic basalt magmatism took place almost everywhere; in the Middle and in the beginning of Late Triassic, as well as in Late Paleozoic general high standing of the north of Siberia was marked. In Jurassic the Olenek high continued to exist and Ust-Anabar appeared. However the great bulk of small local highs, as well as in general all folding on the northern flank of the depression appeared in Cretaceous as a result of huge left lateral strike-slip fault. Source rocks are known in Vendian, Cambrian and Jurassic. However according to the results of back-strip analysis, Vendian and Cambrian petroparent rocks have fallen into oil window only on boundary between Cretaceous and Paleogene period and only in those parts of the basin where there were no good cap-rocks. Therefore oil migrated to areas of the Olenek high frame and Anabar shield; there it was oxidized and has turned to bitumens, which are widespread in Permian alluvial sediments and to a less degree in Cambrian reef and fissured collectors. Jurassic source rocks in the most bent parts of basin have fallen into the oil window only in the end of Paleogene and are still there now.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013