Online Journal for E&P Geoscientists

Champagne, Julie ¹; Durlet, Christophe ³; Grélaud, Carine ¹; Razin, Philippe ¹; Schroeder, Stefan ⁴; Masse, Pierre ²; Pabian-Goyheneche, Cécile ^*2
(1) Institut EGID, University of Bordeaux 3, Pessac, France. (2) Total S.A., Pau, France. (3) Biogeosciences, Université de Bourgogne, Dijon, France. (4) University of Manchester, Manchester, United Kingdom.

The onset of obduction in Oman during Early to Middle Turonian caused a broad doming and exposure of the carbonate platform in the eastern part of the southern Arabian Gulf. The associated unconformity is characterised everywhere by exposure and erosion. Locally karstification, coal and paleosoil deposits occurred on top of the Mishrif Formation in Qatar and UAE, and deep incisions developed onto the stratigraphically equivalent Natih Formation in Oman. The diagenetic imprint of this event provides a critical control on the evolution of reservoir properties.

One objective of this study is to test the possibility to build a predictive regional diagenetic framework, in order to better constrain the controls on reservoir quality. Core analysis and interpretation of petrographic and geochemical data for three producing fields in Oman and in offshore Iran allow the comparison of their diagenetic sequence. These investigations suggest similarities in their fluid flow evolution. The earliest paragenetic stage, related to exposure, was dominated by the dissolution of metastable mineralogies, creation of vuggy porosities, matrix stabilization and multistage cementations in a fluctuating meteoric aquifer. It is followed by rapid burial and compaction/cementation before oil emplacement that retarded further diagenetic processes.

In parallel, the diagenetic effects on reservoir properties were studied for an exposure event occurring in the Middle Cenomanian, at the top of a third-order sequence in the Natih Formation. This surface displays incisions on the exposed platform and forced regressive wedges toward the basin, but lacks the 'classical' evidences of exposure. An integrated outcrop-subsurface study supported by petrographical and geochemical investigations shows that dissolution voids created by meteoric fluid circulation were subsequently plugged by phreatic meteoric cements that covered a large part of the platform.

These results show the particularity of each exposure event in terms of diagenetic products and their impact on reservoir properties. They result from complex interrelations between external and intrinsic factors such as the dynamic of meteoric aquifers, geodynamical and paleogeographical context during exposure, chemical reactivity ... These are effective during deposition and subaerial exposure, even if the preservation of exposure-related diagenetic features may subsequently be influenced by burial evolution.