Implication of seismic and borehole data for the structure, petrophysics and oil entrapment of Cretaceous-Palæocene reservoirs, northern Sirt Basin, Libya

Abstract

The geophysical analysis of 27 seismic sections with well logging data and geological information gathered from 56 deep wells covering the study area of northern Sirt Basin proved to be valuable for evaluating its structural setting and oil potentialities. This study aims to throw more light on the structural, stratigraphical and petrophysical characteristics that control two of the potential hydrocarbon reservoirs of Sirt Basin, the Waha Formation of Upper Cretaceous age and the Zelten Formation of Upper Palæocene age. Also, it presents an exploration method that promotes the existing leads and shows how to continue finding additional reserves and continue developing these prospective formations. This comprehensive study reveals that the overall tectonic setting of the area is controlled by northwest-southeast-trending faults formed since the Early Palæozoic times. These faults gave rise to division of the study area into different horsts and grabens with erratic distribution of the studied formations. This pattern may relate to variations in the relationship between withdrawal of material from the underlying upper mantle and the fault movements. The Palæozoic and Mesozoic rocks show stronger lateral and vertical variations in thickness and lithology than those of overlying Tertiary and Quaternary sediments. In addition, the geophysical studies reflect the mixed lithological nature of the studied formations, which greatly influence the total and effective porosity and permeability index, which consequently controlled the percentages of movable and residual hydrocarbon saturations. Moreover, additional information and results concerning the structure, reservoir petrophysics, hydrocarbon accumulation and entrapment were achieved and represented on a number of cross-sections and contour maps.