Basin architecture and tectonic controls on the Early Cretaceous Sarir Sandstone reservoir, eastern Sirt Basin, Libya

Abstract

An integrated study using gravity, 2D seismic, well logs, and geological data in the eastern Sirt Basin is presented in this research focussing on the Lower Cretaceous Sarir Sandstone which is the most important oil reservoir in the study area. Interpretation of basement architecture shows individual blocks are named in the present paper A, B and C, whilst depocentre represents the most pronounced topographic low within the study area and named D-Graben in this paper.The observed onset of rifting phase in the study area took place in the Early Cretaceous and was divided into three active and inactive phases contemporaneous with deposition of the continental clastic Sarir Sandstone. The basin-wide NW-SE syndepositional fault segments played consequently a significant role in the evolution of the irregular paleotopography, sediment infill and the created accommodation space resulting in controlling sedimentary thickness and facies distributions of the Sarir Sandstone. Some faults were reactivated during the Late Cretaceous-Palaeocene with no abrupt changes in stratigraphic thickness in the post Early Cretaceous section, which suggests non-tectonic post-rift (sagging) phase.The Sarir Sandstone reservoir unconformably overlies the basement (pre-rift sequence), and is therefore unconformably overlain by post-rift marine shales and carbonates of Late Cretaceous. The detailed analyses of log signatures variations, constrained by some core and cuttings information revealed that the deposition of Sarir Sandstone resulted essentially from alterative episodes of braided and meandering fluvial rivers. As a result, the Sarir Sandstone has been divided in general into lower and upper units, separated by a Red Shale bed. In turn the lower Sarir unit has been subdivided into two layers, whilst the Upper Sarir unit, which represents the main reservoir in the study area, has been subdivided into nine layers.