Abstract

The detailed study of the Cretaceous stratigraphic column and diagenetic studies in the Ku, Maloob and Zaap fields, have identified a low porosity, low permeability rock unit, which acts as a vertical barrier across the Cretaceous reservoir. This semi-permeable rock unit which reduces vertical transmissibility between adjacent strata has been called KSA. This calcareous-clayey and sometimes dolomitized layer has been associated with oil generating rocks and potential seal rocks. The aim of this work is to establish the sedimentological conditions, age and areal distribution of the KSA unit to understand its influence on the dynamic behavior on the Cretaceous reservoir for the Ku, Maloob and Zaap fields.

The interpreted sedimentary model is based on microfacies interpretation. All sedimentological and paleontological data is obtained allowing a reconstruction of depositional environment and origin of the sediments. Subsequently, paleoenvironments are interpreted by correlating stratigraphic sections to establish the lateral and vertical facies changes as well as the construction of the sedimentary model.

To understand the presence of the KSA unit it was necessary to know the Cretaceous sedimentary evolution. Eighteen (18) microfacies were defined. There is an open and shallow platform development. On top of the Cenomanian and Middle Turonian, we found a deepened area due to rising sea level (Oceanic Anoxic Global Event).

Based on sedimentological analysis and the correlation of electrofacies and gamma ray log, we can conclude that the KSA corresponds to a maximum flooding surface with clay peak values in the gamma-ray log averaging 15 to 40 ° API. The relative lack of planktonic and other organic matter suggests that this level is associated with an oceanic event caused by a deficiency of oxygen and possibly an increment in nutrients. Stratigraphic correlations of this interval yield an average thickness of about 35 m in Ku, Maloob and Zaap fields.

The new geological understanding of the semi-permeable KSA unit has been incorporated into a 3D model thus helping to optimize the position of new wells.

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