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Sequence Stratigraphic Framework From Base Ahmadi to Top Mishrif Formation of Cretaceous Cenomanian age in Southern Iraq

Abstract

Mishrif is the most important formation within Cretaceous in Southern Iraq which contains more than one trillion barrels of oil. Mishrif is deposited in a very low angle ramp environment at the edge of intrashelf basin within the Arabian plate. During the past ten years, more than ten thousand meters of cores, numerous core analysis data and new producing wells data have been restudied. It is found that the historically called Mishrif formation is heavily diachronous between different oil fields. Hence, a new sequence stratigraphic framework defines the regional correlation scheme is needed.Based on core, thin section and well logs from 5 oil fields in southern Iraq,a new sequence stratigraphic framework have been established. The regressive cycle from base Ahmadi to top Mishrif which corresponding to the Cenomanian age are selected to be the study formations. Within this regressive 2nd order sequence, five 3rd order sequence has been recognized, those maximum surfaces corresponding to the intrashelf basin deposition are placed at the stable horizontal laminated black shale intervals. The sequence boundaries between those maximum surfaces can be easily identified due to the early dissolution by fresh water filtration, which is also the key reason of thief zones developed in many carbonate reservoirs in the Middle East. The historically called Mishrif formation starts from the top unconformity between Late Cenomanian and early Turonian, ends at the base of shallow water grainy carbonate. A new sequence stratigraphic framework defines five 3rd order sequence called Cen1 to Cen5 from base upward is proposed. The vertical and lateral variation of facies show good consistence with the new sequence framework. Inner ramp grainstone shoal which represent the best reservoir quality rocks shows clear vertical and lateral migration trends. The shoal bodies migrated southwestern ward from Ahmadi to Mishrif formation, and its scale also shrinks from north to south because the depositional dip angle increases. Within this sequence framework, the barrier and baffle intervals are recognized, and the potential early dissolution origin vuggy thief zones are predicted.A new sequence framework containing five 3rd order sequence of Cenomanian age is proposed. This sequence model can better explain the depositional trend and diagenesis process of those reservoirs. It also can provider better understanding about the vuggy thief zones, barrier/baffle zones.