Seismic interpretation, attribute analysis, and illumination study for targets below a volcanic-sedimentary succession, Santos Basin, offshore Brazil

Abstract

During the past decade, a significant exploration effort into deeper water and deeper targets in offshore areas brought more knowledge about the distribution of igneous rocks in the sedimentary basins along many continental margins. Nonhomogeneous illumination effects may occur below shallow, high-impedance igneous rock bodies. The seismic processing, depth imaging, interpretation, and attribute analysis require a special attention when these magmatic bodies affect the illumination of deeper targets. Usually, those structures are not considered in illumination studies, and the salt diapirs govern the analysis. In this work, seismic attributes are quite relevant to constrain the geologic model used to simulate the amplitude maps of the deeper target and to quantify the shadow effects observed on it. These amplitude shadows may either create (false) or hide (true) attribute anomalies. We have modeled a Tertiary volcano-sedimentary succession (VSS) mapped in the northeastern Santos Basin, offshore Brazil, to simulate the effect in the amplitude response of deeper targets. There were multiple magmatic events through the Santos Basin Cretaceous-Tertiary sedimentary succession, intrusive and extrusive. The igneous extrusive rocks intercalated with sediments forming VSS marked by low- and high-amplitude responses with tough lateral discontinuity. The amplitude found well-preserved architectural elements interpreted as lava flows and volcanoes, contrasting with layered sediments. We have defined geobodies constrained by relative impedance and 3D edge detection to build the 3D geologic model of the igneous successions used for seismic simulation. From the geobodies, we modeled two VSS to run the P-waves ray-tracing propagation to simulate the migrated amplitude map of the deeper Lower Albian sequence top. We computed the P-velocity and the density from well logs, and the seismic acquisition geometry was similar to the original. Comparison between real and simulated amplitudes showed the impact of shadow zones caused by shallower igneous bodies.