Print this pageSequence Stratigraphy, Sedimentology, and Reservoir Quality of the Lower Devonian Roman Trebsa Field, Timan Pechora Basin, CIS
KAUFMAN, JONATHAN, and JEREMY JAMESON
Studies undertaken on behalf of the Timan Pechora Company indicate that the Roman Trebsa Field is a Lower Devonian sub-unconformity trap that is productive from three stacked, dolomitized, shallow subtidal to peritidal reservoir intervals separated by deeper subtidal sealing units. A high-resolution sequence-stratigraphic framework and reservoir zonation was developed from cores and well logs to help predict field performance and economics. The Lower Devonian is divisible into five sequences, within which fourteen zones have been identified, correlated, and mapped. The top of the Lower Devonian is truncated by a regional pre-Upper Devonian unconformity, which led to karstification.
Deposition occurred on a distally-steepened ramp represented by dolomitized grainstone shoals, restricted subtidal wackestones and peritidal algal boundstones (inner ramp) that pass laterally into deeper marine, burrowed, skeletal wackestone/packstone (middle ramp) and laminated mudstone/shale (outer ramp). Reservoir is developed mainly within the inner ramp lithologies. Reservoir quality is controlled by (1) depositional texture; (2) abundance of carbonate cement; (3) dolomitization style; and (4) vug/fracture distribution. The impact of karstification on reservoir creation/quality is variable; two cored wells show no enhancement of porosity below the pre-Upper Devonian unconformity. One cored well contains a vug-fracture network that is believed to be the result of circulation of unconformity-sourced meteoric waters. Dolomite recrystallization near major faults enhances porosity locally.