Verlag des Forschungszentrums Jülich
JUEL-3082
Zwach, Christian
Diagenesis and temperature history of the Cadotte Sandstone Alberta Deep Basin, Canada: integration of reservoir quality analysis and basin modeling
234 S., 1995
ABSTRACT
An integrated approach was applied to study how reservoir quality has evolved in one of the
major gas reservoirs in the Alberta Deep Basin, the Albian Cadotte Sandstone. The study
area lies in the northern part of the basin close to the Elmworth gas field. Porosity and
permeability of the Cadotte Sandstone is very low and decreases generally with the burial
depth. However, detailed analyses show that formation porosity decreases much more
regularly with the estimated maximum burial depth than with the present burial depth.
Geostatistical analysis exhibits a spatial anisotropy of formation porosity in that porosity
varies more in strike direction than in dip direction ofthe formation.
Porosity and permeabilityofthe Cadotte is mainly affected by early formation of pore filling
kaolinite, and later grain-surface-dissolution of rock fragments, and quartz cementation.
Kaolinite formation resulted from the breakdown of muscovite and K-feldspar under the
influence of meteoric water with low [I1]/[K+] ratio. Rock fragments such as chert grains
were affected by grain-surface-dissolution due to surface reactions between illitic clay
minerals and microcrystalline quartz. This process resulted in the formation of stylolitic
grain-to-grain contacts and thick solution seams containing mainly illite. The released silica
precipitated selectively on detrital monocrystalline quartz grains with lower specific surface
area. The overall process of silica redistribution can be regarded as grain coarsening,
analogous to Ostwald ripening. Best reservoir quality is therefore found in well-sorted,
coarse-grained sediments with low amounts of pore filling kaolinite. Critical factors for the
redistribution of silica, besides the maximum burial depth of the formation, are the amount
of detrital quartz grains, the illite content of the rock fragments, the crystal size of the
quartz in the fragments, and the presence of kaolinite between the grains.
Basin modeling applied to the study area shows that the maximum temperature in the
Cadotte Sandstone was reached in Early Tertiary times when deepest burial occurred. A
comparison of quartz cementation temperatures derived from fluid inclusion studies of the
Cadotte with the reconstructed temperature history of the formation showed a close match
of the maximum burial temperature with the maximum cementation temperatures. Fluid
inclusion data of quartz cements may therefore be used to some extent for further
calibration of basin models. Coupled chemical reaction and fluid flow modeling using the
reconstructed temperature history of the Cadotte Sandstone as input showed that advective
fluid flow can not account for the widespread occurrence of quartz cement in the formation.
This indicates the importance of internal redistribution processes during deep burial
diagenesis.
Sensitivity analyses of the basin model showed that the effect of heat insulation of the gas
emplacement in the Alberta Deep Basin is a critical factor in determining the accurate
temperature and heat flow history in addition to lithological parameters such as coal
percentages in formations. Gas filling of pore space may therefore result in a feed back
effect, where increased heating of source rocks due to heat insulation causes additional
hydrocarbon generation.
An enhanced understanding of the history of reservoir quality results from the integration of
reservoir quality analysis and basin modeling and is thus important for reservoir quality
prediction in the exploration and the production of gas, oil and water from sedimentary
formations.
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