Assessing sandstone reservoir quality: identifying the reality

Abstract

Quartz cementation is one of the most important cements governing reservoir quality in sandstones. The presence of clay coats plays a crucial role in preserving anomalous high porosity in deeply buried sandstones by inhibiting porosity-occluding macroquartz cementation. More robust and greater grain coat coverage is required for higher temperature reservoirs to preserve significant amounts of porosity.

The first and main component of this research entails a series of hydrothermal experiments simulating quartz cementation and grain coat development, particularly authigenic chlorite, at specific temperature steps to mimic the conditions of deeply buried reservoirs and develop predictive models for clay-coat-controlled reservoir quality in such settings. Naturally-occurring sandstone samples from the Lower Jurassic Cook Formation of the Oseberg Field (Norway) were exposed to a silica supersaturated solution for up to 360 hours at temperatures of 0–250 °C. An array of microscopic, analytical and modelling techniques was employed to track the mineralogical alterations, quantify the temperature-dependent volumetric changes of authigenic chlorite, the thickness and coverage of the clay coats, and capture the evolving reservoir quality attributes in the 2D and 3D domain. Results show that grain coating chlorite is formed through a mixture of the solid-state transformation and dissolution-precipitation mechanisms from berthierine transformation, replacement of siderite and neoformation on precursor-free substrate surfaces. Ceasing of porosity loss and permeability maintenance correlate directly to grain coat volume increase at temperatures higher than 175 °C.

The second component presents a series of hydrothermal experiments performed on laboratory synthesised biofilm-rich, pure sand samples to test the influence of microbial extracellular polymeric substances (EPS) on early mineral precipitation at temperatures up to 120 °C. An artificial solution was used to synthesise and preserve the microbial communities and promote mineral genesis during each experimental run. Textural evaluation of the samples shows that EPS-coated surfaces serve as templates for the nucleation of early mineral precipitates. Poorly-ordered, morphologically clay-like material developed at points of contact between quartz grains and where EPS structures pre-existed.

Future energy and climate change mitigation developments require better characterisation of subsurface reservoirs that can act as energy sources or storage media. This research has important implications for diagenesis studies, providing key insights that can be used to improve predictability of reservoir quality modelling applied in a wide range of geoscience applications.