Characterization and evolution of a retrograding-prograding deltaic system, NE Sirte Basin, north-central Libya — Sequence stratigraphic architecture and controlling factors

Abstract

This study presents the first deltaic system interpreted from three-dimensional seismic and wireline log data in the Sirte Basin, the largest sedimentary basin in Libya and most producing petroliferous basin in Africa. The research focuses on examining the internal sedimentary configuration and sequence stratigraphic architecture of an Oligocene deltaic system in the northeastern part of the Sirte Basin. Three major log facies were defined on the basis of wireline log responses: sandstone, siltstone, and shale. The deltaic system was subdivided into three sedimentary facies based on interpreted log facies and their vertical stacking patterns, seismic geometries, and position along the depositional profile. Those facies are distributary channel, delta front, and prodelta. Three major synchronous and regional stratigraphic surfaces were delineated, encompassing the whole study focus and extend beyond the boundaries of the 3D seismic survey. The deltaic system forms a third-order depositional sequence that is bounded by two unconformable sequence boundaries from below and above. It consists of two system tracts separated by a maximum flooding surface: a transgressive systems tract at the base capped by a regressive systems tract. The deltaic sequence is further subdivided into eleven fourth-order sequences. The development of the deltaic system was mainly controlled by sea-level changes and sediment influx. The deltaic system retreated to the south due to a rise in sea level, reduction in sediment input, with a minor contribution of subsidence resulting from sediment loading. While the progradation occurred as a result of increasing sediment influx that exceeded the creation rate of accommodation space. Additionally, the study provides insights onto the sedimentary and sequence stratigraphic structure, depositional mechanisms, and development of deltaic systems during a stable, post-rift stage of a sedimentary basin.