Fawad Ahmad Chuhan¹, Knut Bjørlykke²

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

ABSTRACT: Experimental Compaction of Loose Sand-Relationships to Porosity Reduction in Reservoir Sandstones During Burial

In order to understand mechanical compaction, 150 uni-axial consolidation tests were performed on loose sands at high stresses (50 MPa). At low stress level (<5 MPa), the porosity loss is less than 5% in well sorted mono- and poly-quartz/lithic sands, whereas it is rapid (>7%) in clay rich and carbonate sands due to large porosities associated with clay framework and angular carbonate sands. At intermediate stress level (5-25 MPa) coarse sands loose more porosity than the fine grain sands. In the same stress interval compaction (porosity loss) is highest in carbonate sand and lowest in sand rich in mono-quartz with few lithic fragments. This is due to higher degree of grain fracturing in well-sorted medium to coarse, lithic and carbonates sands. At higher stress level (25-50 MPa) porosity loss is almost independent of grain size and lithology in medium to coarse grain sands. Thin sections analysis from compacted sands shows that significant grain crushing is initiated between 10-15 MPa and increases gradually with higher stresses. The initial porosities are between 41 and 55%, and at 25 MPa (2-2.5 km burial) the porosities are reduced to 28 to 39%, which are the values at the onset of quartz cementation at 2-2.5 km depths. Fractures in sand grains similar to those produced experimentally are also found in reservoir sandstones from the North Sea and Haltenbanken areas. The porosity reduction due to mechanical compaction is an important factor, which combined with chemical compaction determine the final porosity at greater depths.

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