The complex reservoir architecture of the Malampaya carbonate buildup offshore Palawan, Philippines, was initially controlled by a rugged clastic basement morphology, which was overgrown by atoll structures during the Oligocene and early Miocene. Additional factors with major impact on reservoir quality are frequent and high-amplitude relative sea level fluctuations, ocean currents, and prevailing wind directions. Primary depositional reservoir-quality distribution has been overprinted by diagenetic events, primarily as a result of repeated platform-top exposure and submarine cementation. Inherent noise within the previous seismic data introduced by the complex overburden and buildup morphology has resulted in inconsistent seismic attribute distribution. Therefore, earlier reservoir modeling efforts used seismic horizon and volume interpretation only, coupled with the sequence- and cyclostratigraphic architecture and the concept of reservoir rock types for field development planning.

Prior to gas-development drilling, another attempt was made to extract direct reservoir-quality information from the reprocessed three-dimensional (3-D) seismic data to validate the earlier deterministic reservoir models. Improved 3-D prestack depth migration based on a new velocity model has been the foundation of the quantitative seismic analysis for reservoir characterization, static modeling, reserves estimation, and optimized gas development and oil appraisal well targeting. High-porosity areas in the upper part of the reservoir were identified using top-reservoir reflection amplitudes. This provided the tool to minimize penetration of low-porosity, fractured zones prone to mud losses in the gas development wells. A series of acoustic-impedance inversions were applied to create reservoir porosity cubes from seismic and to target wells in good reservoir areas. Porosity cubes are also essential for a correct time-depth conversion of the static model, using a linear porosity-velocity relationship in clean carbonates, which was derived from well data. Several static model realizations were created using the porosity cubes from seismic as a backdrop combined with 3-D seismic facies analysis and a depositional model based on well data and analogs. The results of the five gas-development wells have confirmed the modeled reservoir-quality distribution within the lagoonal part of the northern Malampaya accumulation. Early production performance following first gas in October 2001 is indicative of excellent lateral pressure communication in this area of the buildup, in accordance with earlier dynamic models.

Porosity-height realizations created from the different seismic porosity cubes proved valuable to visualize uncertainty in reservoir-quality distribution within the Malampaya oil rim and formed the basis for targeting a horizontal appraisal well. The MA-10 horizontal oil-rim appraisal well drilled at the end of 2001 confirmed the forecasted facies distribution and reservoir properties as derived from the model.

Based on the new quantitative seismic reservoir characterization, additional areas of potentially good reservoir quality have been identified in the southern Malampaya culmination and on the western flank of the northern culmination. Both areas were previously considered to contain low-porosity reservoir caused by pervasive early marine cementation.