Structure and formation conditions of the Yarakta productive horizon of the Late Vendian in the southern part of the Nepsko-Botuobinskaya Anticlise

Abstract

The relevance. Eastern Siberia is a region with large raw material resources that plays a crucial role in development of Russian oil and gas complex. Since the 1970s, active exploration for hydrocarbons has been underway in the Nepsko-Botuobinskaya Anteclise. One of the main targets for development are terrigenous Vendian deposits, in which up to half of the region hydrocarbon reserves are concentrated. Therefore, for optimal development of the fields confined to them, a detailed study of lithological factors of formation of terrigenous reservoirs is required. The main aim. Lithological and facies analysis is used to reconstruct and trace the evolution of sedimentation conditions of the Yarakta productive horizon of the Nepa Formation. Objects. Terrigenous deposits of the Yarakta productive horizon of the Upper Nepa subformation of the Upper Vendian Formation, uncovered by deep drilling in the area of the Yarakta field. Methods. Mineralogical-petrographic study of core material, facial analysis of lithological rock types, correlation. Results. The Yarakta productive horizon was formed during Late Nepa time. At that time, there was a partly subaerial and to a greater extent subaerial delta plain (lower delta plain) within the tidal plain. It is assumed to be a delta of advancement on a coast dominated by tidal activity and to a lesser extent by wave activity (mixed type of coast). In the subsequent transgression, in the late Nepean time (upper part of the strata), a bar system began to form in place of the delta plain with the bars advancing toward the coastline in the northwestern direction. The bars had an elongated lentil-shaped shape along the conditional coastline. During the formation of the bars, due to the active wave and tidal activity, the sediments carried out by the delta were redeposited and re-sorted. At the Late Nepa and Early Tertiary boundary, the entire area was a shallow intertidal and terrigenous sebha (lagoon) with clay-siltstone sediments and sulfate-carbonate inclusions in them; sometimes, sandy sediments of small intertidal channels or sandbanks are noted in them. Improved filtration-capacitative properties are associated with the facies of distributary subaquatic channels and large (active) tidal channels.