When Diagenesis Meets Geomechanics: An Example from a Deeply Buried Tight Sandstone Reservoir

Deeply buried sandstones go through significant diagenetic phases that evolve into tight reservoirs. Successful hydrocarbon production from these sandstones requires drilling long lateral wells and conducting robust geomechanical analysis to guide hydraulic fracturing stimulation. Yet, geomechanics and diagenesis are rarely linked together in reservoir studies. This paper explores the relationship between the geomechanical and diagenetic properties of a deeply buried tight sandstone reservoir. A total of 973 core samples from 28 wells penetrating the target horizon were selected for thin section preparation for detailed petrographic analysis. Point-counting has been performed on each sample to extract semi-quantitative data to characterize the reservoir composition and diagenetic history. X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM) imaging were performed to identify the mineralogy and discern different clay types. Meanwhile, a full geomechanical assessment was conducted, calibrated by extensive geomechanical testing (Triaxial, multistage, micro-rebound and scratch tests) to measure the rock strength and to predict the subsurface stress conditions. The tight sandstones had undergone an extensive level of compaction, demonstrated by welded grain contacts. Mineralogical alteration such as feldspar dissolution took place synchronous with quartz overgrowth cementation and fibrous to leaf-like illite clay mineral crystallization. A positive relationship exists between authigenic illite and porosity confirming the dominance of micro-porosity in the reservoir and also highlighting the reason for the low permeability due to small pore throats and very tortuous flow paths. The Unconfined Compressive Strength (UCS) from the geomechanical analysis showed a large variation with values classifying the sandstone from a strong to very strong rock. The UCS showed a clear negative relationship with porosity and a positive relationship with the percentage of quartz cementation in the set of samples. Therefore, reservoir diagenesis clearly influenced the rock porosity, permeability and its geomechanical characteristics. Integrating various aspects of reservoir geosciences provides a robust understanding of the controlling factors that govern storage, flow capacity and fraccability of tight sandstone reservoirs. The linkage between reservoir diagenesis and geomechanics demonstrates a well-founded integration and characterization of the reservoir's macro to micro scale features. Hence, these relationships can be incorporated into the static and dynamic reservoir models for a more accurate prediction of the field production and for improved decision making.