The paper presents a study of a Lower Carboniferous (Visean) clastic sequence commonly called Bobrikovsky Formation, deposited in the Volga-Ural Petroleum Province, Orenburg Region. Our investigation included sedimentological description of core samples from hydrocarbon wells and well log correlations. Facies were identified by well log patterns and calibrated by core sedimentology. The Bobrikovsky Formation is proposed to be interpreted as an overall transgressive-regressive succession in a nearshore-tidal environment. Transgressive lagoon-estuary and barrier island facies became regressional lagoon fill-type settings.

Due to the global oil price crisis in 2014, one of the MOL's preventive/reactive measures was to identify geologically or commercially risky elements within their portfolio. This involved reevaluation of all geologic data from Field A in the Volga-Urals Basin. In re-evaluating Field A, several unexpected challenges, problems and pitfalls were faced by the interdisciplinary team performing the task of building a new database, quality checking, and interpreting data dating back to 1947. To overcome these challenges related to this mature field, new approaches and fit-for-purpose methods were required in order to achieve the overall goal of obtaining a reliable estimation of remaining hydrocarbon potential. In the first phase a first-pass 3D geologic model was constructed, along with wrangling, cleaning and interpreting 70 years of subsurface data. This paper focuses on the main challenges involved in evaluating or reevaluating reservoir aspects of a mature field.The primary challenges were related to the estimation of remaining in-place hydrocarbon volumes, the optimization of infill well placement, the identification of primary and secondary well targets, the identification of critical data gaps, and the planning of new data acquisitions. The hands-on experience gained during the development of the geologic model provided invaluable information for the next steps needed in the redevelopment of the field.