| Strong heterogeneity of the deep-buried Paleogene clastic rocks in the Zhu Ⅰ Depression of Pearl River
Mouth Basin was noted and the oil well productivity is greatly influenced by permeability of sandstone reservoirs. The
controlling factors of effective reservoirs include the structure position, sedimentary microfacies, hydrodynamic
conditions, grain size, matrix and geothermal gradient. Based on the data of casting thin section, scanning electron
microscope, physical properties, core observation, geothermal and DST, the features of effective reservoirs are studied.
Then permeability limits of effective reservoirs were calculated by using the plane radial flow method. According to the
evolution relationship between physical properties and burial depth of porous clastic reservoirs, the lower limits of burial
depth of sandstone reservoirs with different grain sizes were reviewed, studied, and predicted comparatively. The results
showed that: (1) The rock types of Enping Formation and Wenchang Formation are mainly feldspathic quartz sandstone
and lithic quartz sandstone, which are dominated by primary pores. The sandstones of the Wenchang Formation were
characterized by extra low porosity and ultra low permeability. The sandstones of the Enping Formation are characterized
by extra low-medium porosities and low-medium permeability. (2) Permeability lower limit of effective reservoir varies
in different areas. The permeability lower limit of effective reservoir constrained by economic productivity (40 m3/d) is
close in Xijiang Sag and in Lufeng Sag, which is approximately 5×10-3 μm2, while in Huizhou sag that is 10×10-3 μm2.
(3) For relatively pure sandstone (content of mud and cement is less than 5% respectively), the coarser the particle size,
the greater the lower limit of burial depth. For example, the burial depth corresponding to the permeability lower limit
（10×10-3 μm2）constrained by middle-class productivity of coarse sandstone in LF2 structure is about 500 m deeper than
that of fine sandstone. (4) The lower the geothermal gradient, the greater the lower limit of effective reservoir burial
depth. For example, the lower limit of reservoir burial depth constrained by middle-class productivity of coarse
sandstone with low geothermal gradient in Xijiang area is about 4 500 m, while that in Lufeng and Huizhou areas with
high geothermal gradient is about 4 300 m. (5) Matrix content was used to quantitatively describe the influence of
microfacies and hydrodynamic factors so as to define the lower limit of burial depth close to the actual formation
conditions. It is revealed that when the mud content increased by 5%, the lower limit of the buried depth decreased by
300-500 m. For deep oil and gas exploration, more attention should be paid to sandstone reservoirs with high structural
maturity and low mud content. |
