Fluvial sedimentology and chronostratigraphy improve reservoir characterization and resolve complex fluid distribution, Krasnoleninskoye area, Russian Federation

Jurassic-age strata from the Sherkalinskaya, Tyumenskaya, and Abalakskaya Formations in the Krasnoleninskoye area of the West Siberian Basin, Russia, form an aggregational, but overall transgressive continental to estuarine to shelf and basin succession filling alluvial valleys. The Interdepartmental Stratigraphic Committee of Russia lithostratigraphically indexes and correlates reservoirs in these strata in ascending order as UK11–UK1a. Core (435 m; 1427 ft) reveal the sedimentology and stratigraphy of this basin-fill. Early development showed gas production structurally deeper than oil in UK11–UK8 reservoirs. Oil over gas production was not resolved by the structural interpretation or lithostratigraphic correlation. Core description at the bed-to-strataset scale identified floodplain-soil horizons and channel diastems as time-stratigraphic markers. Core-calibrated wire-line–log correlation of chronostratigraphic surfaces grouped genetically related facies associations dividing UK11–UK1a reservoir zones into chronostratigraphic packages revealing flow baffles and barriers in all UK zones. For example, UK8 reservoirs produce oil in structurally high locations. Gas is produced in structurally deeper, but stratigraphically equivalent, reservoirs. Well-test data indicate reservoir discontinuity between UK8 chronostratigraphic packages. Facies-association recognition, chronostratigraphic correlation, and principles of channel-migration, channel-avulsion, and alluvial-aggradation styles related to alluvial-valley physiography improved UK8 facies-association mapping and fluid-contact resolution. Channel bathymetry and migration patterns controlled sand versus mud deposition, creating an intra-UK8 barrier isolating structurally deeper gas. Testing geostatistical facies-association mapping methods showed that realizations using multiple-point statistics with geologically biased facies-association maps best represented UK8 facies heterogeneity and stratigraphic isolation of oil and gas reservoirs and the most likely positions and sizes of high net-to-gross areas in alluvial valleys.