The Impact of Postdepositional Processes on Reservoir Properties: Two Case Studies of Tertiary Carbonate Buildup Gas Fields in Southeast Asia (Malampaya and E11)
Georg Warrlich, Conxita Taberner, Wenche Asyee, Ben Stephenson, Mateu Esteban, Maria Boya-Ferrero, Anna Dombrowski, Jan-Henk Van Konijnenburg, 2010. "The Impact of Postdepositional Processes on Reservoir Properties: Two Case Studies of Tertiary Carbonate Buildup Gas Fields in Southeast Asia (Malampaya and E11)", Cenozoic Carbonate Systems of Australasia, William A. Morgan, Annette D. George, Paul M. (Mitch) Harris, Julie A. Kupecz, J. F. (Rick) Sarg
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This paper investigates the impact of diagenesis and tectonic deformation on the reservoir properties of two Tertiary gas-bearing carbonate-buildup reservoirs that formed in similar depositional environments: E11 of the Luconia Province, offshore Malaysia, and Malampaya, offshore the Philippines. Both buildups have comparable dimensions and ages, contain similar constituents and faunal assemblages, and comprise aggradational zones preceding final drowning. The aggradational zones overlie low-porosity zones, which separate them from earlier shelf carbonates.
The reservoir properties, however, were influenced very strongly by nondepositional reservoir-modifying processes. The diagenetic histories were different for the two reservoirs: secondary porosity formation by early and late diagenetic processes was driven by dolomitization and late leaching in E11 and by exposure as well as burial-related leaching in Malampaya. Low-porosity zones in Malampaya are a result of meteoric diagenesis during exposure and late cementation, whereas in E11 they correspond to nonleached, argillaceous wackestones of deeper-water origin. Connected geobodies of either high or low porosity are the result of these processes and are visible on acoustic-impedance volumes. They follow depositional trends or developed around faults: early diagenetic alterations were found to follow depositional trends; diagenetic overprints occurring in the burial realm can also exploit depositional patterns, but if the diagenetic fluids are guided by faults and fractures, the distribution and orientations of the faults appear be the overriding control. Diagenetic overprint also was found to result in porosity – permeability relationships that are different from those of depositional rock fabrics. Significant postdepositional and syndepositional deformation is recognized only in Malampaya and led to increased fracture development in the low-porosity zones, resulting in very good vertical connectivity across the low-porosity layers. Absence of postdepositional deformation in E11 correlates with absence of conductive fractures and a vertical pressure barrier across the thickest low-porosity zone. The sum of similar depositional processes and different reservoir-modification processes led to overall alike reservoirs with low-porosity and high-porosity layering, but porosity distributions other than expected from depositional processes alone as well as different degrees of fracturing and dynamic behavior.
In this study an improved understanding of the reservoir was found to result from an integrated approach combining all subsurface disciplines. Detailed investigations of the diagenesis incorporating petrography, stable isotopes and fluid inclusions were key to unravel the reservoir formation processes. Understanding those is critical to formulate conceptual geological models that can explain reservoir behavior, constrain the subsurface modeling uncertainties and rank modeled subsurface scenarios.
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The Cenozoic carbonate systems of Australasia are the product of a diverse assortment of depositional and post-depositional processes, reflecting the interplay of eustasy, tectonics (both plate and local scale), climate, and evolutionary trends that influenced their initiation and development. These systems, which comprise both land-attached and isolated platforms, were initiated in a wide variety of tectonic settings (including rift, pas-sive margin, and arc-related) and under warm and cool-water conditions where, locally, siliciclastic input af-fected their development. The lithofacies, biofacies, growth morphology, diagenesis, and hydrocarbon reser-voir potential of these systems are products of these varying influences. The studies reported in this volume range from syntheses of tectonic and depositional factors influencing carbonate deposition and controls on reservoir formation and petroleum system development, to local studies from the South China Sea, Indonesia, Kalimantan, Malaysia, the Marion Plateau, the Philippines, Western Australia, and New Caledonia that incor-porate outcrop and subsurface data, including 3-D seismic imaging of carbonate platforms and facies, to un-derstand the interplay of factors affecting the development of these systems under widely differing circum-stances. This volume will be of importance to geoscientists interested in the variability of Cenozoic carbonate systems and the factors that controlled their formation, and to those wanting to understand the range of po-tential hydrocarbon reservoirs discovered in these carbonates and the events that led to favorable reservoir and trap development.