De-Risking a Gas Development using Geophysical Methods

Mannini, Alessandro
HESS OIL & GAS Sdn. Bhd., Kuala Lumpur, Malaysia.

This study investigates the optimal geophysical determination of sand distribution using seismic data for a Tertiary turbidites gas field. A total of 625km2 full-fold new 3D seismic data was acquired and processed in 2001. In 2004 the 3D seismic dataset was reprocessed with the objectives of improving signal-to-noise, removing multiples and generating near/far-trace data for AVO analysis. A further reprocessing campaign was carried out in 2010 in order to further improve the seismic imaging beneath the gas clouds over two different areas and to improve the frequency content of the data whilst reducing noise and multiples. Other objectives were to generate high-quality flattened gathers that can be used for pre-stack Inversion and to sharpen the image of the faults for accurate placement of development wells. It is standard industry practice to quantify reservoir property inverting seismic data usually via a model based approach; unfortunately the area posed several challenges to this standard approach mainly due to the lack of well controls as well as due to the uncertainties associated with wavelet extractions near the gas cloud. In order to account for the above problems it was decided to combine the benefits of an Extended Elastic Impedance (EEI) projection to maximize a particular property, such as the separation between hydrocarbon and brine sand or between sands and shale without the need of building a model or a extracting wavelet with the benefit of a model base simultaneous elastic inversion (MBEI) followed by a neural network (NN) analysis aimed at producing a seismic derived Volume of Shale (Vsh). EEI lithology volume shows an excellent match with the well data as well as a very good continuity of the sand presence both down dip of the well but also beneath the gas cloud. EEI Fluid volume amplitude extraction along the mapped top of Sand 3 show an excellent match between the Gas Water Contact (GWC) and the area characterized by highest probability of hydrocarbon bearing sand occurrence. Seismic derived Vsh volume also shows an excellent match with the well data as well as a very good continuity of the sand presence both outside and beneath the gas cloud. In light of the observations made using the latest geophysical methodologies EEI and MBEI-NN applied to the Field our confidence about the sand presence both far from well control and beneath the gas cloud has broadened

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