Print this pagePredictive Diagenesis and 
Reservoir
Quality of the Tertiary
System in Nanxiang Basin, Eastern China
DENG, XINHUA
Regional controls on diagenesis and reservoir quality were examined in
Nanxiang non-marine half-graben basin, one of the typical hydrocarbon-bearing
basins in eastern China. Deltaic sandstones at the north slope of the basin were
partially eroded at the end of the Tertiary, and subjected to meteoric water
leaching. This leaching resulted in abundant feldspar dissolution and kaolinite
precipitation, with the maximum porosity and permeability of the sandstones
right below the erosion unconformity. Oil-field water analysis also indicate the
existence of the extensive leaching.
Diagenetic pathways and reservoir quality of fan-deltaic sandstones at the
south part of the basin were mainly influenced by contents of ductile grains and
matrix and texture of rocks. Only sandstones containing less ductile components
and matrix can retain significant porosity and permeability. Burial diagenesis
furthermore modified reservoir quality. A large amount of core porosity
measurements show a pronounced porosity enhancement in the 1400-1800 m interval
of burial. The enhancement was mainly due to the dissolution of feldspar, and
secondary porosity is up to 7-12 % in the interval. The corresponding mean
permeability was also enhanced despite reservoir heterogeneity was exaggerated.
Silica overgrowth and ferrous carbonate cement increase in amount below this
interval. Nonfunctional organic adds in formation water is the highest in the
interval, but no acidic water was detected. Also, this zone is near the oil
window and rapid illite/smectite transition zone. These relationships have
applications both in predicting the occurrence of enhanced reservoir quality
and
in understanding the origins of secondary porosity.