Abstract 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.