3-D Seismic interpretation and lateral prediction of the Amposta Marino field (Spanish Mediterranean Sea)

Abstract

The objectives of a seismic study of the Amposta Marino field, offshore Spain, were threefold: firstly, to map the top of the main karstified reservoir (C horizon) accurately in order to identify any possibly remaining attic oil; secondly, to map and interpret a sub-horizontal, semi-continuous anomalous reflection (C2 horizon) within the karstified Montsia carbonate reservoir; thirdly, to assess the feasibility of predicting porosity in karstified reservoirs. 1. The acoustic impedance at the interface between cap rock and top reservoir can be increasing or decreasing, depending on the degree of karstification. Hence, the top reservoir reflection can be positive or negative. This interpretation problem, once recognised, could be solved and led to improvements in the structural map. 2. The synthetic seismogram, logs and drilling records of well AMB-10 indicate that the C2 horizon represents a porous karstic layer. It occurs as a semi-continuous, sub-horizontal feature around the flanks of the structure. Distribution of the C2 marker and also the reservoir performance data and geological history of the Amposta Marino field indicate that the actual porosity system in the Montsia reservoir could be the product of an originally compartmentalised porosity network (resulting from karstification) with greatly increased lateral communication as a result of subsequent mixing-zone corrosion. The C2 event is interpreted as a high-porosity layer caused by a sea level stillstand which allowed intense mixing-zone corrosion to occur. 3. The areal distribution of collapse breccia and cave development can be mapped by displaying the root mean square of the amplitude measured in a window corresponding to the karstified reservoir. Qualitatively, areas with high amplitudes correspond to the most enhanced porosity zones. 4. Although very thin compared to total reservoir thickness, enhanced porosity zones, created as a result of mixing-zone corrosion, could contain the bulk of the oil and may therefore represent an important exploration/production target.