Abstract
The key challenge in Concession C97-I is the heterogeneous geology of the reservoir: the oil-bearing fluvial sandstones of the Upper Sarir Sandstone Formation (Lower Cretaceous) being partly substituted by a tight volcanic sequence. Volcanism took place during and
immediately after the deposition of these sandstones with intercalation of volcaniclastic conglomerates and basalt (volcanoes, sills, and dykes) currently at a depth of 12,000 ft. A first volcanic phase of gas-rich eruptions characterized by deposition of pyroclastics was
followed by a second phase consisting of basaltic flows, sills, and dykes. The pyroclastic sediments were affected by lahars, which are matrix-rich debris flows, landslides and mud flows formed when hot pyroclastic debris comes into contact with external water (crater lake, very heavy rainfall) and then slides downward under its own weight at times for tens of kilometers.
Due to the afore-mentioned volcanic events in the area we have the presence of three volcanic occurrences: volcanic complexes with volcaniclastics and basalts, sills and dykes swarms of basalt and volcaniclastic laharic conglomerates.
So far, conventional seismic acquisition and processing (P-wave 3D survey, offset volumes trace inversions, seismic facies and neuronal network analyses) have managed to detect the presence of basalt, but have failed to discriminate volcanoclastic conglomerates from sandstones, i.e. the reservoir. S-wave velocity and resistivity wereselected as physical properties, which allow to differentiate between volcanoclastic and sandstone lithologies. Suitable techniques are magnetotelluric, in association with gravimetric and magnetic for a better integration during processing and interpretation, and 3-component seismic. Propaedeutic feasibility studies confirmed the potential success of the project and defined the most suitable parameters for the geophysical acquisitions.