Print this pageInfluence of Depositional Sand Quality on Porosity and
Permeability: 
Examples
from Brent Group Reservoirs in the Northern North Sea
S. N. Ehrenberg, P. A. Bjorkum, and P. H. Nadeau
Multivariate statistical analysis was applied to examine correlations between
reservoir quality and petrology in two data sets from the Middle Jurassic Brent
Group. One of the data sets is from relatively shallow depth and has been little
affected by chemical diagenesis (Statfjord Nord & Ost Fields; 2.3-2.6 km below
the sea floor), while the second data set is from a more deeply buried reservoir
having an advanced degree of diagenesis (Huldra Field; 3.6-3.9 km). Much of the
total variation in porosity and permeability (0.02 mD to >7 D in both sets) can
be accounted for by laboratory measurements of parameters mainly related to
depositional sand quality, including "shaliness" (represented by bulk-rock
alumina content), early carbonate cement, feldspar content, and grain size. De
pite major differences in the proportions of different sedimentary facies in the
two data sets, they have similar ranges of depositional sand quality and
therefore probably had similar reservoir quality early in their burial history.
Deeper burial diagenesis at Huldra Field has shifted the average porosity and
permeability lower and produced a bimodal permeability distribution, apparently
reflecting preferential preservation of permeability in the cleaner sandstones.
Based on these examples, a method is outlined for "unmixing" the diagenetic
and lithologic components of variation in regional compilations of sandstone
porosity-permeability data. The procedure and its consequences are illustrated
using a regional compilation of core data
from the Brent Group of northern North
Sea.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California