The upper 'Unayzah sands constitute a gas reservoir in the south Haradh region of the giant Ghawar field in Saudi Arabia. This part of the 'Unayzah reservoir was deposited as incised channel-fill clastics and associated lowstand delta deposits in a shallow to marginal marine environment with facies varying from siltstone to reservoir quality sandstones. Mapping the reservoir sands posed an exploration/delineation challenge. We investigated the applicability of various seismic attributes in mapping the upper 'Unayzah reservoir in this region.

The existing well control indicates that acoustic impedance is a discriminating attribute between reservoir and non-reservoir facies. Inverting high quality 3D seismic data to acoustic impedance is the initial basis on which the reservoir is mapped. 1D modeling is used to study the effect of reservoir presence on the surface seismic signature. It was discovered that reservoir development causes detectable stretching in the seismic reflections. These features are displayed areally to identify good reservoir development utilizing neural networks and second norm similarity attributes. Both methods show similar reservoir distribution to that mapped by acoustic impedance. Increasing the bandwidth during processing transforms the stretched wavelet to a doublet, which is mapped by attributes as elementary as the mean amplitude of the seismic data. Modeling also shows that the reservoir presence and the subsequent waveform change causes reduction in the frequency content which is displayed using frequency attributes such as the centroid frequency. The different attributes converge, when viewed on a common reservoir distribution map, verifying the presence of high fidelity seismic data and increasing the confidence in identifying the 'Unayzah reservoir distribution.


Hawiyah Hawiyah Haradh Haradh Well-Well-LL Harmaliyah Harmaliyah Minjurah Minjurah Well-Well-K K Well-Well-C C Well Well-B -B Well-Well-A A Sahba Sahba Well Well- -D D Well-Well-EE Well-Well-FF Well-Well-H H Tinat Tinat Well-Well-G Well-Well-J G J Well Well- -II 20 Km 20 Km Figure 1 - Location map of the Haradh region.

The black rectangle represents the 3D seismic survey area.

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The 'Unayzah reservoir is one of the major Paleozoic reservoirs in Saudi Arabia. Significant gas accumulations have been discovered in this reservoir in the Haradh region of the Ghawar field in eastern Arabia. The reservoir shows strong stratigraphic and sedimentologic heterogeneity which rendered 2-D seismic data useless for purposes other than mapping the reservoir structure. A 3D seismic survey is acquired to try to help characterize the heterogeneity of the 'Unayz

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