Attempting to define a subtle trap by seismic attribute mapping of thin sands ? a Timor Sea example

Abstract

The Pituri Prospect was a combined stratigraphic/structural trap on the Puffin Horst in the Vulcan Graben area of the Timor Sea. This paper describes a case study where the problem to be solved was one of mapping the lateral extent of a thin sand defined by well control. The sand is a 7 m thick unit within the uppermost Puffin Formation claystones of Maastrichtian age. The interpreted environment of deposition is submarine which is biostratigraphically calibrated on a basin wide scale with water depths interpreted as Upper Bathyal to Outer Neritic. The fan units have been regionally mapped seismically to define the major feeder systems. The resulting regional sand distribution maps do not however provide a simple explanation for the thin sands which are seen in the Puffin-2, Grebe-1 and Prion-1 wells. Two models were proposed, a submarine fan unit at the very distal end of the Late Maastrichtian lowstand fan system or a shingled turbidite of the Highstand Systems Tract of the Prudhoe deltaics. The sand distribution had to be defined by the seismic data with the calibration of the nearby Puffin and Pascal wells providing the possible northern limits of the sand unit. An innovative approach is called for when using seismic amplitude and attributes to try to describe facies distributions particularly when the units are at the limit of seismic resolution. The amplitude anomalies were analysed for tuning effects, inverted for acoustic impedance and mapped on timeslices datumed on the regional flooding surface at the base of the Palaeocene above the Puffin-2 sand. Modelling for seismic amplitude response was undertaken using nearby well data. The analyses carried out generated a model for the Puffin-2 sand where that unit is a crevasse splay/levee overbank unit which was associated with a major submarine channel system to the south of the well location. The channel system ended at a broad lobate feature which was interpreted to be a submarine fan lobe which was targeted by Pituri-1. The well found no sand in the section interpreted to have the sand unit. What was present, was a 15 metre thick low density, low velocity claystone that is interpreted to be the thickening into the palaeo-low of a minor flooding surface seen at the Puffin-1 and Puffin-2 wells. This then highlights the difficulty in predicting sand distribution from seismic attributes alone. However the ability to predict reservoir distribution from seismic data is of critical importance in future exploration and further work which integrates all seismic attributes and geological/rock physics datasets could provide the means to better reservoir prediction.