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Abstract: Gas Exploration in Southern Spain: a Successful AVO/DHI Application

Garcia, C.; Echanove, A.*; and Arrieta, A. - Repsol

The onshore Guadalquivir Basin of southern Spain, and its offshore extension, the Gulf of Cadiz Basin, define a gas province with a daily production of 65 MMCF from turbiditic reservoirs of Miocene age.

After the Lower Miocene Betic compressional phase, and extensional period began, affecting the Guadalquivir Valley and the Gulf of Cadiz area, forming a Neogene foreland basin where a thick sedimentary sequence quickly accumulated. The main objectives are turbidite bodies, with their complete suite of seismic facies: channel, leveed-channel and lobes.

The use of AVO has been successfully applied in this area since the 1980s. Last year, Repsol developed a new approach to AVO analysis, which has improved the success ratio for exploratory wells. The main advantage of this new approach is that it allows, thanks to pseudo-3D interpretation techniques, real data to be manipulated and interpreted. It also avoids limitations imposed by using methods based solely on seismic processing, such as approximations to the complete Zoeppritz equations or inefficiency in the evaluation of Class I and Class 11 AVO anomalies. As a first step, the analysis of standard AVO attributes such as Intercept, Gradient, Gradient*lntercept, etc., allows us to select the key zones of the study area. The new methodology applies on these zones previously selected. The start point is a group of CDP's that has been processed in true amplitude and NMO corrected. The CDP's are loaded as a pseudo 3D in a workstation with interpretation software. Here, the interpreter tracks all the horizons of interest, from which amplitudes are extracted and different displays generated. These displays can be in 2 or 3 dimensions and in any direction. 2D displays (CDP-OFFSET ), are commonly amplitude maps. In 3D displays, we combine AVO information with structural relief. Results can be compared with any theoretical model, keeping always control of the error generated when real amplitudes are replaced by modeled ones. Thanks to the careful examination of these displays, we can decide the best locations for future wells.

Encouraged by this success, Repsol is now evaluating other DHIs in the region, like signal distortion below a gas field due to the presence of gas in the reservoir. Three seismic horizons are selected: the horizon associated with the gas reservoir, one above and one below it. The idea is to analyze the possible changes in frequency (and/or phase) characteristics of the deepest horizon because of the influence of the gas reservoir, represented by the middle horizon. We try to measure how signal changes vertically, from the shallowest horizon to the deep one, and laterally, from the gas reservoir to out of it.

The first objective is to reduce even more the number of dry wells within the basin, but also to use signal distortion, combined with AVO, as an exploratory tool in other areas.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil