Petrophysical and seismic properties of lower Eocene clastic rocks in the central Maracaibo Basin

Abstract

High-resolution, well-log cross sections are used to map the continuity of hydrocarbon-bearing sandstone reservoirs in the Centro Lago fields, central Maracaibo Basin, Venezuela. Because most sandstone reservoirs are below the limit of seismic resolution, reservoir mapping relies on detailed well-log correlations. Seismic data are used mainly to define the structural framework. A feasibility analysis based on the correlation of seismic data and petrophysical properties of Eocene clastic rocks was conducted to demonstrate that lithology and fluid content can be differentiated by acoustic impedance known from well-log data. However, low vertical resolution of the three-dimensional seismic data made available for this study (25 Hz dominant frequency) is not adequate to constrain the lateral and vertical continuity of the Eocene reservoirs. The main sandstone reservoirs in lower Eocene clastic rocks of the Centro Lago fields are found in distributary channels and tidal sand-bar facies. These reservoirs provide good-quality fluid-flow units. Channel complexes and sand bars are concentrated along the crest of a north-northeast–trending anticline bounded by parallel left-lateral strike-slip faults. Reservoirs are generally compartmentalized by transverse northwest-southeast–striking faults that intersect the main anticlinal axis. Detailed high-resolution well-log cross sections and mapping, integrated with the structural framework interpreted from the seismic data, indicate the presence of untested oil plays in the Centro Lago fields. These new plays are located along the flanks of the main anticline and are found in Eocene tidal bars in the central and northern areas of the Centro Lago fields.