Print this pageSequence Stratigraphically Driven Development of Incised Valley Reservoirs, Lower Cretaceous, Western Siberia, Russia
Hydrocarbon production in the Lower Cretaceous Vikulov Formation of western Siberia is mainly from shallow-marine to paralic sands. Although the sands are widespread and show only minor variations in thickness, there are some areas, which were thought to show “rapid, lateral facies changes” as indicated by well-log correlations. Subsequent examination of 2D seismic lines in these areas revealed anomalous reflections, which were interpreted as a “channel” of unknown origin.
In 2006, a sequence-stratigraphic approach to this problem using logs, cores and 2D seismic data revealed that the “channel” was actually a 50-70 m-deep, 2-12 km wide incised valley system, which extended at least 100 km across the western part of the Western Siberian basin. Thus, the “lateral facies changes” were actually attributable to miscorrelations of Vikulov strata across the incised valley walls.
The Vikulov incised valley formed during an Aptian sea-level fall, which led to fluvial downcutting and truncation of several highstand parasequences of underlying wave-influenced deltaic deposits. Recently obtained 3D seismic data clearly show stratal truncation at the valley walls. Amplitude and time-structure maps of the sequence boundary display erosional patterns created by fluvial channels as they meandered across the valley floor. Sharp-based, fining-upward fluvial deposits overlie the valley-floor sequence boundary. These are followed above by coarsening-upward cycles of estuarine-prodelta silty shales and distributary mouth-bar and channel-fill sands deposited in bay-head delta systems. These sands comprise the valley-fill reservoirs. Estuarine-prodelta silty shales cap the valley-fill.
Exploitation of the valley-fill began in 2007 using only 2D seismic and well-log data to guide well locations. 3D seismic data were not obtained until 2008. The production team drilled ~100 wells of which ~70 were successful in a 35 km2 area. Successful wells encountered high net-to-gross bayhead-delta sands and unsuccessful wells encountered prodelta silts. Some unsuccessful wells were mistakenly drilled outside the valley due to inexact 2D seismic delineation of the incised valley boundaries. Production rates have ranged between 375-1125 bbls per day. Engineering and petrophysical data indicate that that the valley-fill reservoirs have a separate, lower oil/water contact from that of the upper Vikulov reservoirs outside and above the incised valley.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009