Predictive 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. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.