Lithofacies classification and sequence stratigraphic description as a guide for the prediction and distribution of carbonate reservoir quality: A case study of the Upper Cretaceous Khasib Formation (East Baghdad oilfield, central Iraq)

Abstract

In the East Baghdad oilfield of central Iraq, the Upper Cretaceous Khasib Formation is the largest producing carbonate reservoir. The basic architecture of the “Khasib reservoirs” is heavily impacted by the lithofacies classification and sequence stratigraphic interpretation at East Baghdad field. A description of the Khasib Formation in terms of depositional systems, environments and lithofacies was obtained by studying the vertical distribution of the lithofacies, defined using well-log values and their mutual statistical relationships and constrained by the core data (thin sections). In this paper, a lithofacies classification and sequence stratigraphic interpretation are used to define reservoir geometry, lateral continuity and petrophysical characteristics of the Khasib reservoirs. The “Khasib” carbonate ramp stratigraphy exhibits different lithofacies in different depositional systems at the East Baghdad field. In addition, the log signatures and petrophysical properties of the same lithofacies vary. Multivariate statistical methods (principal component analysis and clustering) have been applied to separate stratigraphic packages and different carbonate lithofacies. Khasib carbonate lithofacies can be better discriminated from cluster analysis based on principal components rather than from arbitrary well log observations alone. The geometry of the strata in the ‘Khasib’ outer ramp is retrogradational, becoming aggradational during the lowstand systems tract (LST) and transgressive systems tract (TST). The highstand systems tract (HST) of the Khasib Formation is typically distinguished by a relatively thick aggradational-to-progradational geometry. This parasequence set is widespread upon the “Khasib” mid- and inner ramp. Petrographic analysis reveals the relative abundance and distribution of characteristic porosity types of the “Khasib” reservoirs. The HST (mid- and inner ramp) of “Khasib” Formation reveals the best reservoir characteristics (thickness, porosity, and hydrocarbon saturation) in the East Baghdad field. This enhancement of the knowledge of the lithofacies within a sequence stratigraphic framework through the application of principal component and cluster analysis is especially important to -optimally delineate and define the potential reservoir intervals of Iraq's Khasib Formation. Such multivariate statistical techniques may have similar beneficial application to carbonate reservoirs elsewhere in the world.