Characterisation and modelling of deep fault-related karst reservoirs in the Ordovician Tarim Basin, Northwest China

Abstract

Carbonate reservoirs at ultra-deep depth (> 7000 m) in the Tarim Basin, NW China, exhibit extremely low host-rock permeability and porosity. The development and distribution of these reservoirs are predominantly influenced by strike-slip fault systems, whose complex structures largely control reservoir heterogeneity. The scarcity of wells at these depths compounds the challenge of accurately determining reservoir architecture, increasing uncertainty in reservoir character during exploration. This thesis presents a geological study of an ultra-deep carbonate reservoir in the Shunbei oilfield, north-central Tarim Basin. It aims to characterise the geological heterogeneity controlling porosity and permeability distributions using seismic data, wells and geological analogues. The thesis first describes the seismic-stratigraphic framework of the reservoir that aids in predicting the distribution of high permeability and porosity. It identifies seismic facies indicative of various depositional environments, from lagoonal mudstones to reef-shoal complexes, and investigates the influence of hypogene karst formation driven by hydrothermal fluids. This thesis further characterises the geometry and distribution of Permian igneous rocks in the reservoir overburden, identifying five seismic facies that reveal a complex history of felsic and mafic volcanism. Magmatic activity is evidenced by the presence of volcaniclastic mass transport complexes and intrusive sills, and may be linked to the generation of hydrothermal fluids that impact reservoir quality. Lastly, this thesis assesses the structural and sedimentological heterogeneities within the carbonate reservoir by employing a screening study approach that integrates experimental design with sketch-based, 3D reservoir modelling, and flow diagnostics. This analysis focuses on the influence of fault architectures, properties, fractures, and host-rock variability on reservoir behaviour, providing insights into the mechanisms governing fluid flow in such reservoirs. Overall, this thesis highlights the complex interplay of depositional environments and stratigraphy, magmatism, and post-depositional processes on the development of ultra-deep carbonate reservoirs in the Tarim Basin.