Facies Control on Diagenesis and Reservoir Quality of Fluvial Sandstones; an
Example from the Middle Jurassic of Yorkshire
Peltonen, Christer
B.1, Knut Bjørlykke1 (1) University of Oslo, Oslo, Norway
Two sandstone facies
from the Middle Jurassic Ravenscar Group (onshore Yorkshire, England) were compared with
respect to mineralogy and diagenesis. Samples from
fluvial channel sandstones contained no K-feldspar, and had relatively high
amounts of kaolin (14.3% avg.), whereas the crevasse-splay sandstones had
significant amounts of K-feldspar (4% avg.) and a low relative amount of authigenic kaolin (7% avg.). Petrographic
evidence suggests that kaolinite was precipitated
prior to quartz cementation. The more permeable channel sandstones would have
served as conduits for meteoric water flow during shallow burial, whereas the
crevasse-splay sandstones had limited hydraulic conductivity and therefore
lower fluxes. This explains the differences, with respect to dissolution of
feldspar and precipitation of kaolin, within these two fluvial sandstone facies. The degree of leaching of feldspar and other
unstable silicate minerals such as mica may therefore be taken as a measure of
the degree of meteoric water flushing and thereby the degree of sandstone
connectivity. Sandstones showing evidence of only small or moderate amounts of
leaching may represent bodies which are poorly connected to the main aquifers.
The absence of K-feldspar in the channel sandstones provides a good basis for
prediction of reservoir quality at greater depth (>130 degC).
Pore-filling illite would not have developed if these
channel sandstones were buried more deeply due to lack of a local potassium
source. The crevasse-splay sandstones on the other hand would have greater
potential for authigenic illite
growth, and the difference in reservoir quality of these two facies would therefore be even greater. This information
may be used during production and evaluation of fluvio-deltaic
and shallow marine reservoir sandstones.