--> ABSTRACT: Controls on Porosity of Jurassic Sandstones of Northwest Europe, by Nicholas B. Harris; #91030 (2010)

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Controls on Porosity of Jurassic Sandstones of Northwest Europe

Nicholas B. Harris

The porosities of Jurassic sandstones of the North Sea can be predicted from (1) their thermal maturity and (2) the formation pressure gradient of the reservoir. Thermal maturity measures the time-integrated thermal history; it is expressed in terms of vitrinite reflectance (Ro). Formation pressure is expressed in terms of pressure gradient (psi/foot). At a fixed Ro, porosities are significantly higher if the sandstone is overpressured (pressure gradient >= 0.50 psi/foot); however, increasing the degree of overpressure appears to have little effect on porosity. At a fixed pressure gradient, porosity decreases with increased thermal maturity.

This approach rationalizes much of the observed variation in porosity vs. depth trends of Jurassic sandstones in different parts of northwest Europe. Average sandstone porosities at a given depth typically vary by a factor of three. The approach appears to work because there are only two significant mechanisms of porosity loss that affect the Jurassic sandstones: compaction and formation of silica cements. Compaction requires both increased matrix stress and time. Matrix stress, in turn, depends on burial depth and formation pressure. The variables, burial depth and time, are essentially incorporated into vitrinite reflectance, whereas formation pressure is identified as a separate variable.

Formation of significant amounts of silica cements involves dissolution of quartz grains along stylolite planes and reprecipitation of the silica as cement. Our work in this area suggests that the process of dissolving quartz grains along stylolites depends on time and temperature, variables that directly enter into the calculation of vitrinite reflectance.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.