Salem G. Aljuhani¹

(1) Saudi Aramco, Dhahran, Saudi Arabia

ABSTRACT: Overprint: Cross-Talk of Depositional Patterns in 3-D Data Volumes

Time and stratal slices from a 3-D seismic data volume from Saudi Arabia showed a pattern of meandering channels within Unayzah reservoir interval. These channels were thought to be related to the depositional system of Unayzah. But, in fact, they belong to the overlying Khuff Formation. They are incised into an anhydrite layer (180 ft, 27 ms) above the reservoir. The high impedance anhydrite was partially or completely eroded and replaced by relatively lower impedance siliciclastic rocks. The reservoir is a complex multilateral and multistory interbedded unit of siltstone and sandstone of variable quality with strong lateral and vertical heterogeneity.

The overprint problem in the seismic data refers to the presence of amplitude variations related to strata above and/or below the stratal layer of interest. The overprint can result from multiple reflections, locally mode converted waves, processing artifact, wavelet tail, time delay and amplitude attenuation related to lateral variations in velocity and other rock properties in the overburden.

The overprint phenomenon was investigated on synthetic seismograms generated from well logs. The overprint in this particular seismic volume is mainly related to multiples, wavelet tail, time delay and transmission loss. The overprint phenomenon in the seismic volume is a serious problem. It causes severe real to synthetic seismograms misties. It interferes with the observation of the fluid signal and hinders the mapping of the reservoir properties. This study emphasizes the importance of the comprehensive interpretation of the seismic data rather than the interpretation over a window around the zone of interest.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado