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Discriminating Gas Bearing Sands from Shale Using Rock Physics Guided Inversion

Daghistani, Ahmed W.*1; Bakhorji, Aiman 2; Al-Mustafa, Husam 3
(1) Saudi Aramco, Dhahran, Saudi Arabia. (2) Saudi Aramco, Dhahran, Saudi Arabia. (3) Saudi Aramco, Dhahran, Saudi Arabia.

Seismic properties such as compressional and shear wave velocities, bulk density, impedance and Vp/Vs ratio are key elements in seismic reservoir characterization. It is very important to understand the physical link between seismic properties and reservoir properties (e.g, lithology, porosity, pore type, clay content, fluid type and saturation), and rock physics provides such a link. Rock physics, along with the inverted seismic data, can be used to detect the presence of hydrocarbon bearing rocks and forecast their performance during production. Rock physics models and template can be used to predict reservoir properties from the observed seismic properties, to interpret the seismic response away from the well.

This study is carried out over a field in Saudi Arabia that has a gas bearing clastic reservoir that ranges in age from the Late to Middle Triassic. Two wells “A” and “B” have been drilled targeting this reservoir. Well “A” has an average porosity greater than 10% and produced gas; whereas the equivalent formation in well “B” proved to be tight shale with an average porosity of approximately 1%. The main objective of the study was to predict the gas bearing sands using seismic inversion with the help of rock physics.

A feasibility study was conducted to determine the type of seismic attributes that would be suitable for discriminating between gas bearing sands and wet shale. Rock physics cross-plots between various elastic moduli showed that the Vp/Vs ratio versus acoustic impedance (AI) plane showed the best discrimination between the two lithofacies. Moreover, AVO analysis suggested that the gas bearing sand in well “A” exhibits a class 4 AVO response. The tight shale in well “B” shows a class 1 AVO response. These observations indicated that pre-stack inversion and AVO analysis must be jointly conducted to predict changes in lithofacies from the seismic data between the two wells. The elastic properties inverted from seismic data were interpreted using the modeled rock physics template, which in turn was used to predict the lithology and fluid saturation between the two wells.


AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain