Midrikah Field: A Major Paleozoic Gas Discovery in a Fault-Bound Play in Eastern Saudi Arabia

Al-Shammery, Hafiz J., Ali Y. Al-Hauwaj, Saudi Aramco, Dhahran, Saudi Arabia

The Midrikah area is the southern nose of the Ghawar anticline, which is formed largely due to compressional tectonics. This stress regime formed the gently dipping eastern flank and the steeply dipping and reverse faulted western flank in Midrikah. The Midrikah field flowed gas from the down thrown block of the reverse fault. This fault is a major feature which originates from the basement and extends up to the Triassic section. The fault, also, merges with the major reverse fault system present in west of the Ghawar field and extends more than 70 km north of the prospect. The Midrikah prospect was mapped using high qual­ity 480 fold 3-D seismic data. Due to the heterogeneity of the Unayzah Reservoir the data was processed to preserve relative amplitude to enable stratigraphic interpretation of the seismic events in the target time level. Depth migration was deemed necessary for proper lateral positioning of the bounding faults of the prospect due to the high dip exhibited by the overburden formations. In addition to depthing of the key geological markers, fault mapping was vital for defining this prospect since fault trapping was proposed as the lateral sealing mechanism. Coherence and dip attributes were instrumental in defining faults and fracture zones in the area. Hydrodynamic data was used to assess the sealing potential of the bound­ing fault in Midrikah. Seismic attributes such as acoustic impedance, frequency and trace classes were utilized to investigate the presence of the Unayzah reservoirs in the prospect. Interpretation of these attributes gave consistent results and aided in delineating areas of favorable reservoir development. The accurate startigraphic and structural models devel­oped from the interpretation of the 3-D seismic data contributed significantly to the success of this wildcat.