Subsidence Mechanisms and Tectonic Control on Seismic Stratigraphy and Reservoir Characterisation of the Kotla Graben and Adjacent Highs, West Central Sirt Basin, Libya

Abstract

Kotla Graben, located in the west central Sirt Basin, Libya, remains a poorly investigated region and this thesis presents an integrated analysis of the tectonic evolution and its control on stratigraphy and depositional facies. The interpretation of an integrated dataset of 2D and 3D seismic and well data has allowed the recognition of the complex evolution of an oblique graben that developed within the larger Sirt rift basin. The interplay between extensional and wrench tectonics, that has an important control on sedimentation patterns, is documented. Tectonic subsidence analysis of the Kotla Graben and adjacent highs reveals three phases of subsidence, developing three second-order sequences from the Cenomanian to Lutetian. Two major fault zones can be mapped; the Dahra-Kotla Fault Zone (DKFZ) and the Amin-Kotla Fault Zone (AKFZ), trending NNE-SSW, forming the boundary of the graben that was initiated by extension at least by the Early Cretaceous. During the Santonian, sinistral strike-slip movements are documented, resulting in uplift and the development of a regional unconformity, with erosion or non-deposition over the highs and the formation of small isoclinal folds over the Dahra Platform. The Palaeocene-Middle Eocene period was characterised by the highest recorded subsidence rates. NW trending en echelon faults can be mapped, producing negative and positive flower type structures, interpreted to be due to sinistral wrenching that occurred in the Early Eocene. A wide variety of geometries and fault segment patterns have been mapped across the DKFZ. These reflect fault linkage between the extensional and contractional zones. The post-Middle Eocene section is absent in the study area and across a large part of western Libya, attributed to regional exhumation and uplift that continued during the Oligocene-Neogene. The dynamic tectonic history developed a platform and graben morphology, with deposition of carbonates on an east-west trending rimmed-shelf and north-south trending ramp. During the Campanian an aggradational geometry was established, dipping basinward, and 40 previously unrecognised carbonate buildups have been mapped, cut by a later channel system filled by Maastrichtian sediments. 3D facies attribute mapping, supported by interpretation of seismic profiles and tied well data, demonstrate that sedimentation in the area was strongly controlled by tectonic subsidence. The results provide a model to predict facies type and distribution within the study area, a valuable assessment tool for future exploration for undrilled hydrocarbon reservoirs.

Date of Award	31 Dec 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Redfern (Supervisor) & Mads Huuse (Supervisor)