Microfractures in Quartz Grains as a Measurement of Maximum Effective Stress in Sandstone Reservoirs

Abstract:

Effective stress, defined as the load transmitted from particle to particle in the solid framework of a rock, plays a significant role in controlling mechanical compaction and thus reservoir quality in sandstones.

Mechanical compaction in sandstones takes place through rearrangement and ductile/ brittle deformation of framework grains during progressive burial. It is primarily dependent on the magnitude and evolution of effective stress during burial, and on the nature and textural properties of framework grains (mineralogy, grain size/shape, sorting…) and pore-filing solid cements when present.

Here, we propose a method to directly evaluate maximum effective stress in sandstone reservoirs by quantifying the brittle deformation of quartz grains evidenced through the development of microfractures. Quartz microfracturing is documented and quantified by examining thin sections of core samples under SEM CL microscopy.

Previous published experimental studies and observations made on natural samples indicate that quartz burial-induced microfracturing in sandstones is mainly affected by effective stress, but also reflects other factors such as grain size, sorting and proportion of ductile grains (clays, micas…).

In order to investigate the quantitative impact of such factors altogether, we have conducted compaction experiments (>30 tests) on 10 types of sands at 25°C, under dry conditions and pressures up to 55 Mpa. The resulting compressed sands were studied by optical microscopy to quantify fractured quartz grains. Results were processed using R statistical computing language via a multi input model to define a simple equation that provides correction constants for each influencing factor. The resulting equation will then be used to calculate the maximum effective stress recorded by a sandstone reservoir during its burial history, based on the petrographic/mineralogical characteristics (thin section point-counting) and the fractured-grain ratio (obtained by SEM CL) of the sandstone.

The validity of the relationship between fractured-grain ratio and maximum paleo-effective stress will be tested on North Sea reservoirs covering a wide range of present pore pressure conditions.