It is Never too Late for Seismic, How Geophysics Contributed to Rejuvenating a Mature Field, Tunu Shallow Gas Development

Spindler, Pierre; Rengifo, Rufino; Lenoir, Nicolas; Coulon, David; Ariawan, Savanto
TOTAL, Balikpapan, Indonesia.

Tunu is a giant field delivering about 1000 MMScfd gas and 18000 bpd of condensate with about 80 wells drilled per year (more than 800 wells drilled) and almost 20 years of production from the Main Zone (2500-4500m TVDSS). Tunu Shallow Zone (TSZ) Upper Miocene to Pliocene unconsolidated formations (above 1500 mTVDss), have always been considered as the main drilling hazard until they were recently identified as new production horizons.

The Shallow Zone consists of fluvio-deltaic parasequences vertically stacked according to a regressive-transgressive pattern. Two main reservoir types have been identified: point bars in fluvial channels and side bars in distributary channels.

Shallow gas reservoirs are scattered all over the field and present a limited extension. Then the use of seismic is essential to locate and evaluate them to enable the field development.

Gas reservoir detectability on seismic is due to a specific petro-elastic behavior in the TSZ. They behave as typical AVO Class III sands, corresponding to low impedance layers and a reflection coefficient increasing with angle. The main challenge on Tunu field is to discriminate them from the abundant coal layers.

Like gas sands, Coal layers have low acoustic impedance that will appear as bright reflectors on seismic data. Nevertheless due to different Poisson's ratio values, with high angle of reflection (beyond 45°) gas sand becomes brighter whereas coal is dimming. It is now standard practice to combine very far angle and even seismic gather analysis to prove gas.

The interpretation of the amplitude anomaly will give access to its extension. The pay thickness is also derived from seismic data. Shallow gas reservoirs are below seismic resolution but the gross thickness of tuned layers is linked to the amplitude value. Well statistical information is then used to derive a Net to Gross ratio, in order to correct for the presence of coal, and to derive the petrophysical and fluid parameters. Finally wells are designed and followed-up directly on seismic.

In less than 4 years, shallow gas production has reached 180 MMScfd (20% of Tunu production), incrementing the potential of the field and enabling a major new development of over 80 wells. Based on this experience the reprocessing of vintage geophysical datasets and the acquisition of new seismic data on a very mature field should be systematically evaluated.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012