Online Journal for E&P Geoscientists

Youssef, Abdulkader H.^*1; Kadar, Adi P.²
(1) Exploration, KOC, Ahmadi, Kuwait. (2) Exploration, KOC, Ahmadi, Kuwait.

A bio-based sequence stratigraphic analysis has been done over the Middle Cretaceous Wara Formation and Ahmadi Formation (Tuba Limestone and Ahmadi Shale Members) of Onshore Kuwait.

Two major 3rd order sequences have been identified; one coincides with the Wara Formation and the other corresponds to the Ahmadi Formation and the lower part of the Rumaila Formation.

The Wara Formation consists of alternating sand and shale. It is about 200 feet thick producing clastic oil reservoir. It was deposited during the late Albian, in an inner shelf to marginal marine environment. It represents a 3rd order sequence with the sequence boundary located at the Mauddud-Wara boundary. Samples of the formation are mostly poorly fossiliferous, but samples from green calcareous shale of the formation contains abundant assemblage of high diversity nannofossils, a common planktic foraminifera assemblage and has a marked gamma ray spike on wireline logs indicating maximum flooding surface in the basal part of Wara Formation. The presence of nannofossil markers Nannoconus fragilis and Eiffelithus turriseiffelii indicates late Albian KN31 Zone of Al-Fares (1998). The presence of ascidian spicules in the samples suggests deposition in shallow marine environments.

The Ahmadi Formation is composed of two members, the lower Tuba Limestone Member, considered the main reservoir in the northern part of Kuwait, and the upper Ahmadi Shale Member, considered as a regional seal. The formation is overlain conformably by the limestone of Rumaila Formation, and was deposited during the late Albian-early Cenomanian time, in an inner to middle neritic marine environment. The isopach map of the Ahmadi Formation showed a thickness less than 200 ft., dominated by shale towards the south west, while it increases to more than 400 ft., dominated by limestones of the Tuba Member towards the northern part of Kuwait. Four 4th order marine flooding events have been defined based on the recorded biostratigraphic data within the formations; AH100 MFS at the base of Tuba Limestone is correlatable with the K120 MFS of Sharland et al., (2001) dated at 99.0 Ma (Simmons et al., 2007); AH200 MFS is recognized in the upper part of the Tuba Limestone; AH300 MFS is recognized at the base of the Ahmadi Shale and AH400 MFS is recognized within the topmost Ahmadi Shale.