AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Diagenesis and Fracturing of a Lower Cretaceous Arabian Shallow Water Carbonate Platform: Towards a Better Assessment of Carbonate Reservoirs Heterogeneity
(1) Earth Science and Engineering, Imperial College London, London, United Kingdom.
This study focuses on a Lower Cretaceous Arabian shallow water ramp. Geological models for these units traditionally assume a layer-cake stratigraphy and a high lateral continuity of facies. This is based on the assumption that epeiric platforms have low depositional gradients, broad facies belts and gradual facies transitions. However the petrophysical properties of these units are mainly controlled by diagenesis that can be complex. Understanding how the depositional environment is linked to the early diagenetic history and deriving conceptual models linking the distribution of porosity with primary depositional factors is essential to assess the heterogeneities of these systems that are most of the time largely underestimated.
In this study, part of the Qatar Carbonates and Carbon Storage Research Center, we investigate the relationships between depositional cyclicity and variations in diagenesis of a Barremian-Aptian carbonate ramp in Southern Oman. Outcrops of the Jurf and Qishn formations (time equivalents to the Kharaib and Shu’aiba petroleum reservoirs) are structurally undeformed that allow interpretation of facies, stacking patterns and determination of diagenetic processes in a shallow burial context. A bed-by-bed sampling on two 45 meter thick sections was made. Petrography, mineralogy, carbon and oxygen isotopes and trace elements were obtained for each sample. Facies distribution and the diagenetic history was reconstructed from the analysis of a 5 km long transect that records a transition from a restricted to an open marine depositional environment.
A contrasting diagenetic response from the different platform environments illustrates the role of primary sediment composition in controlling porosity and cement distribution. However, similar facies along a single bed show variations in the amount of dolomitization and porosity. The type of diagenetic fluids and their flow path play a major role on carbonate reservoir heterogeneity. Fractures may be one of the controls on flow path as they act as preferential conduits for diagenetic fluids. It is therefore essential to assess the fracture potential of a facies throughout its diagenetic history. We look at how diagenetic processes impact on the original and present-day mechanical properties of the rocks in order to better predict fracturing in carbonates. The combined analysis of structural and diagenetic features provides the best approach for predicting reservoir heterogeneity.