Knut Bjørlykke¹, Fawad Chuhan², Olav Lauvrak², Arild Kjeldstad¹, Kaare Høeg¹

(1) Department of Geology, University of Oslo, Norway, Oslo, Norway
(2) University of Oslo

ABSTRACT: Porosity Loss in Sand and Mud Due to Mechanical and Chemical Compaction - Integration of Experiments, Observations and Models

The porosity of sandstones at a given burial depth reflects their initial composition and the effects of mechanical and chemical compaction during burial. We have undertaken 150 compaction tests on different sands, measuring the porosity loss due to mechanical compaction. We find that in clean, quartz rich sands, porosity loss due to grain crushing is greatest in coarse-grained sands. At stresses between 20 to 25 MPa (2.0-2.5 km depth) porosity in coarse sands may be reduced from about 43% to about 30% whereas, under the same conditions, fine-grained sands can retain nearly 40% porosity. Chemical compaction involves dissolution and precipitation of minerals, and quartz cementation is largely controlled by temperature. Both the mechanical and chemical compaction of mudstones depend very much on their mineralogy. Where smectitic clays are dominant then compaction is controlled principally by the thermal transformation of smectite to mixed layer minerals and finally to illite. These changes are reflected in both log and seismic velocities and are particularly evident in Tertiary sediments in the North Sea basin. In more deeply buried mudstones, kaolinite may be replaced by illite causing additional chemical compaction and quartz cementation. The paper presents a compaction model that utilises the principles of soil mechanics at shallow depth, and temperature dependent Arrhenius type kinetics for chemical compaction at greater depth. This model reproduces the porosity-depth trends estimated from density logs in the North Sea, whereas models based purely on mechanical compaction tend to significantly overestimate the porosity at depths greater than 2 km.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado