Natural Microfractures in Quartz Grains: A New Tool to Evaluate Maximum Paleo Effective Stress and Overpressure Onset in Sandstone Reservoirs
Mechanical compaction in sandstones increases during burial in response to increasing vertical effective stress with depth but the fluid overpressure (OVP) can hinder this trend. The degree of mechanical compaction constitutes one of the main controls on reservoir quality in sandstone reservoirs and mainly reflects the maximum effective stress reached during the burial history. In overpressured reservoirs, maximum effective stress commonly corresponds to the onset of OVP. Determination of the depth and time of maximum paleo-effective stress, hence OVP onset, constitutes critical information for the understanding and the prediction of sandstone reservoir quality.
We report here a new method to evaluate the maximum paleo-effective stress recorded in a sandstone reservoir based on natural microfracturing in quartz grains. Our approach builds upon Chuhan (2001)’s experimental works and Makowitz and Milliken (2003) studies on the extents of quartz grain microfracturing in sandstones.
In order to quantify the relationship between effective stress and quartz grain microfracturing development we performed oedometric mechanical compaction experiments on sands in dry condition, at vertical pressures up to 500 bars. Sand samples of variable grain size, ductility and sorting were tested to study the impact of these individual lithological factors on the intensity of microfracturing of quartz grains. The results were compared to natural sandstone reservoirs under hydrostatic conditions from the North Sea and constituted a statistically representative database that permitted to formulate a mathematical model describing the extent of microfracturing as a function of effective stress and lithology. This model, yet perfectible, provides a way to back calculate the maximum effective stress corresponding to the Fractured Quartz Grain Ratio (FQGR) measured for a given sample.
The approach was tested to reconstruct maximum paleo-effective stress recorded in the BRENT reservoirs in a range of North Sea wells, and to place constraints on the onset of fluid OVP build-up. The results show important variations in depth and timing of OVP onset depending on the well and/or the panel in a single province. The results bear implications on the processes responsible for OVP development, and are interpreted as reflecting differences in charge history and migration paths. They also illustrate that the earlier the timing of OVP development the better the preservation of reservoir quality.
AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018