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Porosity Preservation in Deep, Hot Sandstone Reservoirs

Abstract

Authigenic quartz cement is the most common pore-occluding mineral in deeply buried (>2500 m) quartzose sandstones. Deeply buried reservoirs in the North Sea contain more porosity than expected when the influence of microcrystalline quartz (microquartz) is ignored. Anomalously high porosity in these North Sea rocks typically correlates to observation of microquartz and sponge detritus (Taylor et al., 2015). However, we know little about the nature and origin of microquartz. We have utilized advanced analytical capabilities to improve our understanding of controls on microquartz development in several examples where porosity is preserved in deeply buried sandstone reservoirs. In this study, several advanced analytical techniques were used to evaluate the crystallographic and compositional controls on the formation of microcrystalline quartz. SEM/Cathodoluminescence (CL) confirms that quartz overgrowths have a complex growth history. Previous workers (Kraishan et al. 2000) suggested that CL patterns in quartz cement are largely due to trace elements rather than defects and that aluminum varies consistently between each cement phase. Electron Backscatter Diffraction (EBSD) combined with Wavelength Dispersive Spectrometry (WDS) confirms that the complex banding visible in CL is not due to changes in crystallographic orientation but more likely variations in quartz composition associated with changes in pore fluid composition and/or reservoir conditions. Secondary Ion Mass Spectrometry (SIMS) analysis provides maps of ultra-trace element distribution that confirm that trace amounts of iron, manganese, and titanium can be used as proxies for defect density and temperature. Additionally, SIMS analysis provides oxygen isotope data providing insight into the initial reservoir conditions and temperature of formation of microcrystalline quartz in several formations. Integrating the results from these advanced analytical techniques has helped us develop our understanding of the processes controlling the formation of quartz cement and improved our ability to reconstruct the reservoir diagenetic history and/or trace element impact on quartz growth leading to a proposed model for predicting porosity preservation in deep, hot sandstone reservoirs.