Petrophysical evaluation of the Asmari Reservoir Caprock using GR and Sonic Logs, The Parsi Oil field, SW Iran

In the present study, the cap rock of the Asmari reservoir in the Parsi Oil field, located at south west of Iran, were petrophysical evaluated using GR and Sonic logs and its thickness variations investigated. Cutting samples and well logs data from 28 drilled wells of the Asmari reservoir cap rock in this oil field, indicated that the cap rock consisted of anhydrite, halite, marl and a few thin beds limestone. So that it can be classified in the best cap rocks type. Meanwhile, during the drilling process and also on well logs, 6 thin beds with particular properties -named Key Bedsare recognizable. The entrance to the cap rock is clear with increasing in sonic wave velocity because of arrive to the compact anhydrite layer (the first key bed). High pore pressure and elasticity characteristic to gether with considerable thickness of evaporates, made it as a strong seal on the Asmari reservoir. Therefore the intensity of tectonic activities which were affected on the Parsi oil field could not prevent the hydrocarbon escape from the reservoir. Although in some area, these activities have removed upper parts of the cap rock and then the cap rock thickness is not comparable in southern and northern flanks. Very high thickness of the cap rock similar to southern flank of the Parsi oil field, seldom is reported in other oil fields of Iran and around the world. This matter can be related to tectonic events in the Zagros folded and thrust belt, folding mechanism and salt plasticity property.