oa Time-Depth Conversion Challenges in the Overpressure Environment, a Case Study from Caspian Sea.

Turkmenistan’s Block 1 and AB is an asymmetrical ESE-WNW trending anticlinal gas field with small oil rim accumulation and it is located in the South Caspian basin. More than 20 exploration and development wells have been drilled to appraise and develop the main RS8 reservoir of Lower Pliocene age. Two 3D seismic vintages (1997 and 2003) have been PSDM processed and merged to create a single volume. Full suite of wireline logs, with check-shot and VSP have also been acquired in most of the wells in the study area. The formation interval velocity is affected by among other factors – porosity, mineral composition, pore fluid and also effective stress. In overpressure areas, where the sediment are undercompacted, high pore pressure caused the effective stress to be lower which in turn causes the formation velocity to decrease. The normal velocity-depth trend expresses the increase of velocity as porosity is reduced during normal compaction, where pore pressure is hydrostatic and the burial depth of the rock is not reduced. Lower than normal velocity anomalies have been observed in the overpressured wells. On the other hand, in the northeastern most portion of the block where significant uplifts have taken place, loss of porosity due to earlier compaction process are not restored. Therefore, porosity reduction due to earlier compaction processes express itself in higher than normal interval velocity. Several different methods of depth conversion utilizing well and seismic velocity or both has been tested and compared. Seismic to well tie for major formation tops and corresponding horizons has been carried out for all the well penetrations in order to have optimal average velocity at well locations. The velocity trends were also constrained by knowledge about the velocity of the rock at the mudline and at infinite depth. The results shown that Vo-K approach plus high density picked seismic velocity calibrated to well velocity and constrained by other geological information such as the formation isopachs and isochrons give the best result.