--> Abstract: DHI and AVO Analysis in a Frontier Area: an Example from the Trujillo Basin, by J. Machin, C. Garcia, W. Martinez Del Olmo, and M. Blanco; #90933 (1998).
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Abstract: DHI and AVO Analysis in a Frontier Area: an Example from the Trujillo Basin

Machin, J.; Garcia, C.; Martinez Del Olmo, W.; and Blanco, M. - Repsol

The frontier Trujillo Basin is located offshore 200 km south of the prolific Talara Basin, where more than 1400 MBOE have been produced to date. With only two wells positioned on the southeast margin of the basin area available for calibrating seismic data, it is not possible to establish Previous HitdirectNext Hit lithologic or chronostatigraphic control. This, coupled with the interpretation of prospective reservoirs as turbiditic sands in encasing shales has led to a search for Previous HitDirectNext Hit Hydrocarbons Previous HitIndicatorsNext Hit (DHIs) and the use of AVO analysis to aid the evaluation of this poorly explored basin. This approach has been encouraged by the presence of many active Previous HithydrocarbonTop seeps along the basin.

The Trujillo Basin is not a typical forearc basin due to previously three pull-apart basins were created during Paleogene times. Just during Miocene time, the basin has developed as a properly forearc basin. An integrated structural and stratigraphic 2D seismic interpretation has revealed three main depocenters infilled mainly from Eocene sediments in the Trujillo Basin. From seismic lines two main turbiditic episodes, Lower/Middle Eocene and Middle/Upper Eocene sediments have been recognized. They are seismically characterized by basin floor fans and submarine fans in the first episode and are channel, mounds and lobes in the second one. Several turbiditic features associated with possible DHIs such amplitude bright spots, dim spots and flat spots, and frequency loss have been encountered within interpreted turbiditic bodies. Reflection strength, instantaneous phase and frequency have been extracted from selected anomalies to validate our geological model.

Likewise, using a Repsol proprietary method, AVO analyses have been carried out for these selected anomalies. After careful processing, CMPs and traces for different offsets have been loaded into an interpretation workstation as a pseudo 3D project. This has let to pick exactly the most revealing reflectors where a bright spot is taking place. Later, the amplitude values have been extracted and computations of amplitude reflection to offset zero and gradient have been done taking into account Shuey's approximation for each trace. According to this a response AVO model is built for each anomaly taking into account a normalized average intercept to offset zero and average gradient. This model will let us define accurately the class of AVO anomaly and compare among the selected anomalies treating to find out what feature fits better with our assumptions. Finally a simple theoretical fluid replacement model taking into account the collected petrophysical data has been constructed to explain and fit he observations made.

This technique could be used as a reconnaissance AVO tool in the Trujillo Basin and elsewhere. Thanks to the application of this method, the risks associated to certain prospects have been reduced sufficiently to justify drilling.

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