A primer on West Siberian Basin stratigraphy: Chronostratigraphic cross-references for Cretaceousand Jurassic-age strata, Russian Federation

Stratigraphic nomenclature describing Cretaceousand Jurassicage strata spanning the continental to deep -water depositional realms in the West Siberian Basin is lithostratigraphic, complex, and locally variable. A bureaucratically derived stratigraphic nomenclature system developed during early exploration (1940s) is administered by the Interdepartmental Stratigraphic Committee of Russia. Formation names were assigned on a wellby -well basis according to the geographic location of the well, the facies characteristics of the strata (i.e., facies regions, facies zones), the stratigraphic position, and the hydrocarbon region in which the well was drilled. Although rocks were primarily dated biostratigraphically, little to no consideration was given to the correlation of the strata in time. Thus, basin -scale lithostratigraphic cross sections display a plethora of formation names and lack chronostratigraphic detail. Furthermore, reservoirs are indexed. That is, reservoirs within a formation are given discrete names. Indexed reservoirs are correlated lithostratigraphically across time lines, resulting in incorrect predictions of reservoir continuity and erroneous volumetric estimates. The plethora of formation names and reservoir indices are of little use in relating the geological characteristics of hydrocarbon fields basin wide. Geologic age is the only criterion linking the stratigraphy of one location to the next. Stratigraphic columns are summarized to place formation names and reservoirs into a basin -wide chronostratigraphic context. Until the advent of basin -scale sequence-stratigraphic studies, these summaries offered the only basin -scale method of relating variously named formations and reservoirs chronostratigraphically. Moreover, at the field scale, chronostratigraphic correlation of parasequences, as opposed to lithostratigraphically correlating indexed reservoirs, can resolve field -development problems by yielding more precise estimates of resource volumes and distribution and more ef ficiently placed wells.