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Digital outcrop characterisation of syn-rift structure and stratigraphy: Nukhul half-graben, Suez Rift, Egypt

Rarity, Franklin Franklin

[Thesis]. Manchester, UK: The University of Manchester; 2012.

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Abstract

Syn-rift exposures are a prime source of knowledge on the structure and stratigraphy of rift basins and are vital for the petroleum industry as analogues for subsurface reservoirs. Focusing on superb exposures of the Oligo-Miocene rift initiation Nukhul half-graben in the Suez Rift, Egypt, this study investigates applications of 3D digital survey techniques, particularly terrestrial light detection and ranging (lidar), for (i) the analysis of syn-rift fault and facies architecture, and (ii) the building and testing of outcrop-based reservoir analogue models for early syn-rift settings.Lidar-based digital outcrop mapping of the Nukhul half-graben, combined with conventional fieldwork, facilitated quantification of thickness and facies relationships within syn-rift strata, and variations of throw along normal faults. The results provide new insights into the tectono-sedimentary evolution of the intra-block half-graben. At rift initiation, regional/eustatic sea-level and antecedent drainage are interpreted as the dominant controls on accommodation development and deposition, respectively. However, after just c. 2.5 myr of rifting, the structural template imposed by the propagation and linkage of four initially isolated pre-cursor segments of the Nukhul Fault became the dominant control on accommodation development and basin physiography. Progressive SE-NW back-stepping of facies strike-parallel to the Nukhul Fault suggests hangingwall subsidence was locally sufficient to outpace falls in regional sea-level, resulting in a dynamic transgressive system that progressed from a restricted tidal embayment to shallow marine seaway along fault strike. After c. 4.3 myr of rifting, regional drowning of the tidal system provides evidence for progressive localisation of displacement onto the present-day block-bounding structures and declining activity on the intra-block Nukhul Fault during the transition from rift initiation to rift climax.Development of digital outcrop techniques such as point cloud facies classification, 3D deterministic channel modelling and net-to-gross analysis provided reliable geostatistics on the geometry, distribution and heterogeneity of tide-influenced facies of the syn-rift Nukhul Formation. The high volume, reliability and spatial coverage of data reduced uncertainties related to stochastic facies modelling (in this case sequential indicator simulation and object-based techniques), facilitating building and testing of high-resolution analogue models for the complex facies and sequence architecture of early syn-rift tidal reservoirs. Visual analysis of static reservoir connectivity suggests some of the smallest-scale depositional elements, i.e. thin intercalations of mudstone and sandstone lamina in heterolithic facies, have the biggest impact on both reservoir volume and vertical connectivity in this syn-rift tidal system. Heterogeneities at the sequence stratigraphic scale also have significant impact on vertical reservoir compartmentalisation, whereas tidal channel lag deposits and tidal mud drapes have more localised effects.Use of lidar, when combined with conventional fieldwork, offers a powerful tool for quantitative spatial analysis of fault and facies architecture, tightly constraining 3D structural and stratigraphic interpretations and effectively increasing the statistical significance of outcrop analogues for reservoir characterisation. Future developments in survey technologies alongside geoscience-specific software for the integration and analysis of outcrop datasets will provide more diverse and quantitative information on geological heterogeneity, and promote wider-ranging applications in Earth sciences.

Additional content not available electronically

A DVD is enclosed in the back cover of this thesis. The disc contains: a VRGS data file for the Nukhul half-graben; a 1 metre resolution octree filtered lidar point cloud of the entire Nukhul half-graben; Virtual Reality Geological Studio (VRGS) viewer software; selected virtual outcrop animations; and a pdf file containing a digitised log correlation panel from the Nukhul half-graben study area.