Master Thesis Stratigraphic and Structural FactorsInfluencing the Characterization of Tahara Formation, in Gullebi Field,South of Concession NC7A, GhadamesBasin, NW Libya

Abstract

This research presents the interpretation of data from well-logs , core samples and subsurface stratigraphic and facies maps obtained from the Gullebi Oil Field, concession NC7A, Ghadames Basin, NW Libya. The study aims at using well log approach in establishing the sedimentary lithofacies, their successions and environments of deposition in this field, and to find out the possible stratigraphic and structural factors that might influencing the development of sandstone units of the Tahara Formation. The palaeodepositional environments in the field were deduced by combining gamma ray log trends with core data. Lithofacies interpretation was carried out using the gamma ray, effective porosity and resistivity logs obtained from 16 wells. Correlation technique was used to delineate the subsurface trends of these lithofacies. Lithofacies identification shows that the entire well interval consists of sand, silt and shale. Four GR-log motifs representing different lithofacies were recognized in the study area: a funnel log shape representing beach to proximal deltaic lithofacies; a bell log shape representing fluvial channel lithofacies; a serrated to spiky log shape representing transgressive and deep marine slope to basinal shale/silt lithofacies and small scale irregular serrated log shape representing lagoonal-mud flat lithofacies. Three reservoir lithofacies associations in the Tahara Sandstones were defined based on different compositions and diagenetic patterns, represent different reservoir qualities: good quality lithofacies associated with the transitional beach to proximal deltaic sandstones of fine to medium grained with total porosity of 20% and measured log-permeability 61md, medium quality lithofacies associated with fluvial channel sandstones, fine to medium grained, with total porosity 12% and measured log-permeability 7md, and low-quality lithofacies associated with marginal deltaic silty sandstones, silt to very fine grained, with total porosity 10% or even less with measured log-permeability 4md. The porosity and permeability are better developed in areas of sandstone units deposited in beach-deltaic environment. By using the available well control points, isopach maps of the upper sand unit of the lower Tahara sequence and the upper sand unit of the upper Tahara sequence have been constructed and revealed that the accumulated thickness of both sand units is a function of available space to fill and the degree of shallowing or deepening of the associated depositional surface. Also regional log facies maps have been constructed for these two selected sandstone units to easily define the distribution of the representative lithofacies and to delineate the arbitrary boundaries characterizing these studied units of the Tahara Formation. The stratigraphic factors that may influencing the development of the Tahara Formation are better explained by sea level changes reflected on depositional lithofacies types and sedimentation rates (thickness variations), the characteristic mineralogical composition of the studied sandstone units and the differences in diagenetic processes. Other structural factors that may be influencing the deposition of the Tahara Formation related to paleostructural configuration of the Gullebi Field and the major NE-SW striking bounding fault parallel to the axial plane of the Gullebi anticlinal ridge.