Sedimentology of a Visean Fluvio-Deltaic System through Integration of BHI and Core, Dnipro-Donetsk Basin

Summary The Visean deposits of the Semyrenky area comprise fluvio-deltaic sediments transported generally towards the SE, along the Dnipro-Donetsk graben in Ukraine. Seismic sections of the study interval show a ‘layer-cake’ stratigraphy and planar surface morphology during the late Visean period, where minor sea level changes could significantly impact the depositional environment across a wide area. Reservoir properties, however, can vary between wells, with production data revealing the presence of lateral barriers within even the most apparently extensive and continuous of these sands. To gain a better understanding of the nature of the main reservoirs of interest, and to test whether lateral variations in facies/depositional environment could account for recorded variations in production data, detailed sedimentological analysis of available core and borehole image (BHI) data was undertaken through the main reservoir intervals. Stacked fluvial channels, tidal influenced distributary channels, mouth bars, chute channels and less common hyperpycnal lobes have been interpreted. Additionally, BHI analysis has indicated local deviation of geobody orientations from the regional SE trend in some layers. These factors aid in explaining the observed reservoir compartmentalization and the presence of isolated sand bodies.