Abstract

The Natuna gas field is one of the world's largest natural gas accumulations with an estimated 222 TCF gas in-place (71% CO₂). The reservoir is a porous carbonate buildup within the Miocene Terumbu Formation covering approximately 310 km² with a maximum column height of 1638 m (crest at 2658 m subsea). The primary goal of the Natuna field geologic study was to provide a sound basis for construction of 3-D volumetric and production simulation models.

Five wells and 3500 line km of 2-D seismic form the primary data base for the geologic study of the Natuna reservoir. The framework for the project was a sequence stratigraphic analysis which defined 9 major time-stratigraphic units within the Terumbu carbonate at the field. Prediction of reservoir facies and quality away from well control required integration of a host of geologic and geophysical data. Seismic facies were mapped and calibrated at well control. Calibration involved tying the seismic facies to lithofacies derived from core description, thin-section petrography and petrophysical analysis of well logs. A strong link between depositional facies and porosity was established. Sequence-based depositional facies maps were constructed as guides for final porosity mapping. An independent geophysical analysis of reservoir porosity was also obtained using a velocity - porosity transform. Integrated porosity maps combined the geological and geophysical data using a contouring style drawn from company experience with Miocene and Holocene analogues. The porosity maps, together with data on permeability, saturation and net-to-gross were used to build the Natuna field volumetric and reservoir simulation models. Close linkages between the reservoir engineering and geoscience teams, together with strong central coordination by Esso Exploration and Production Natuna Incorporated (EEPNI), proved to be critical to project success.