Abstract

Three-dimensional (3-D) seismic interpretation and drilling results indicate complex sedimentary geometries of the Malampaya and Camago buildups (offshore Palawan, Philippines) with localized progradation due to unidirectional offbank transport alternating with vertical aggradation. Successive reduction of size during buildup growth and backstepping of the protected landward margin in response to transgression ultimately appear to have triggered the demise of carbonate production and platform drowning. The shallow-water platform top sediments repeatedly show signs of subaerial exposure before reflooding. A modeling functionality was developed to allow development of multiple-scenario 3-D reservoir models in an exploration or appraisal stage. The model enables merging of seismic-scale observations based on 3-D volume and horizon analyses with subseismic scale information from well data; however, inherent noise within the seismic data introduced by the complex buildup morphology has resulted in inconsistent attribute distribution and fault dimming. These difficulties are compounded by erratic velocity distribution within the limestone, nonhyperbolic move out, and a narrow relatively low-frequency spectrum, all of which prevent the use of the 3-D seismic volume as hard data but rather allow its use as a soft constraint for guiding the geological interpretation and ultimately the modeling process. Seismic data quality in such complex morphologic settings and scarcity of well data hamper greatly the use of geostatistically driven modeling approaches; therefore, a new functionality was developed within Shell's proprietary integrated 3-D modeling suite (GEOCAP), which allows deterministic model reservoirs using seismic horizon and volume interpretation, sequence- and cyclo-stratigraphic architecture, and the concept of reservoir rock type. Seismic velocity in clean carbonate formations is predominantly a function of porosity distribution. To assess time-to-depth conversion uncertainty, the reservoir rock type based models were first produced in the time domain. Only after differential 3-D depth conversion of these models could the scenarios be reconstructed in the depth domain. The depth models subsequently were used to derive permeability and saturation 3-D distortions, and thus hydrocarbon volumes for each deterministic scenario. The models were then used for simulation purposes.

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