Cores and Well Logs: Keys to the Sequence Stratigraphy of the First Eocene Reservoir, Wafra Field (PNZ)
Dennis W. Dull and W. Scott Meddaugh
ChevronTexaco Exploration and Production Technology Company, Bellaire, TX
Located in the Partitioned Neutral Zone (PNZ) between Saudi Arabia and Kuwait, the First Eocene Reservoir is the youngest of five producing intervals that range in age from Eocene/Paleocene to Lower Cretaceous at Wafra Field. The First Eocene dolostones were deposited in arid to semi-arid conditions on a shallow, low to moderate energy inner shelf or ramp setting. The presence of evaporites suggests restriction was sufficient for the development of hyper-saline lagoons and sabkha.
The sequence stratigraphic interpretation of the First Eocene is based on five recently cored wells. These wells have numerous hardgrounds that increase in frequency to the north. Intraclastic rudstones occasionally overlay the hardgrounds and sub-aerial exposure surfaces. In some cases brecciation is observed beneath the hardgrounds indicating intermittent sub-aerial exposure and incipient soil formation.
The shallowing upward cycles are capped by mud-dominated rocks, hardgrounds and exposure surfaces which are correlative with gamma ray (GR) highs that help define the cycle tops on well logs. Many of the cycle tops and associated GR highs can be correlated across the entire ~20 km length of the Wafra field. The ability to correlate the fine-scale GR pattern and correlative cycles likely indicates that the First Eocene was deposited in an aggradational/progradational portion of a tectonically stable shelf where subsidence kept pace with carbonate deposition.
The First Eocene has been subdivided into ten interpreted high frequency sequences (HFS) bound by hardgrounds, subaerial exposure surfaces, or lithofacies tract offset. Placement of the ten observed HFS into the regional sequence stratigraphic framework is on-going.