Elastic Inversion in Brazilian Offshore Pre-Salt Carbonates Context

Abstract

This paper presents a 3D seismic deterministic elastic inversion and classification study in an offshore Brazilian pre-salt carbonate field. Usually in this context, we carry out an acoustic impedance inversion studies because the elastic component – Vp/Vs or IS – often does not allow a sufficient lithology discrimination. This is unfortunate as it limits the petrophysical interpretation of the field. Indeed, a 3D elastic information is generally very useful for the reservoir characterization. The studied field is a giant pre-salt field deep offshore Brazil (2100m of water depth). Platforms around the basin are associated to major transform zones and related to magmatic activity. The trap of the field is represented by a combination of the uplifted tilted faulted block, limited by a series of the NNE-SSW faults, on which the carbonate platform (reservoir) is developed. Here the lateral facies change rapidly from reservoir facies of the carbonate platform into muddy and shaly deep lake facies. The carbonate platform is limited by Ariri salt layer at the top and by shales of Picarras formation at the base. The reservoirs are represented by carbonate lacustrine deposits of syn-rift (Itapema formation) and early post rift (Barra Velha formation). The reservoir quality is controlled not only by process of sedimentation but also by strong multi-phase diagenesis and fracturing. Also on the field, important changes of the originally deposited sediments are related to hydrothermal diagenesis which lead to a specific cementation (dawsonite, silicification, dolomitization) and dissolutions resulting in the appearance of vugs, caves, connected conduits and "fractures' corridors" – big scale heterogeneities and secondary media. The complexity of the field is also complemented by the presence of intrusive rocks linked to several pulses of magmatic activity. The objectives of this study were to characterize spatial repartition of the lithology in the reservoir and its porosity to assist to the interpretation and the reservoir characterization of the field. The field also has an intricate structure with four compartments and three different Oil Water Contacts. The challenges on this project are linked to the complex structure of the field with the presence of highly structured magmatic intrusions below the salt and the mix of variable reservoir quality of carbonates difficult to interpret on seismic.