Diagenesis-porosity evolution and “sweet spot” distribution of low permeability reservoirs: A case study from Oligocene Zhuhai Formation in Wenchang A sag, Pear River Mouth Basin, northern South China Sea

The characteristics of low permeability reservoirs and distribution of sweet spots in the Oligocene Zhuhai Formation of Wenchang A sag, Pearl River Basin were investigated by core observation and thin section analysis. The study results show that there develop the fine, medium and coarse sandstone reservoirs of tidal flat – fan delta facies, which are of mostly low permeability and locally medium permeability. There are two kinds of pore evolution patterns: oil charging first and densification later, the reservoirs featuring this pattern are mainly in the third member of Zhuhai Formation between the south fault zone and the sixth fault zone, and the pattern of densification first and gas charging later is widespread across the study area. Strong compaction and local calcium cementation are the key factors causing low permeability of the reservoirs in the Zhuhai Formation. Thick and coarse grain sand sedimentary body is the precondition to form “sweet spot” reservoirs. Weak compaction and cementation, dissolution, early hydrocarbon filling and authigenic chlorite coating are the main factors controlling formation of “sweet spot” reservoir. It is predicted that there develop between the south fault and sixth fault zones the Class I “sweet spot” in medium compaction zone, Class II “sweet spot” in nearly strong compaction zone, Class III “sweet spot” reservoir in the nearly strong to strong compaction zone with oil charging at early stage, and Class IV “sweet spot” reservoir in the strong compaction and authigenic chlorite coating protection zone in the sixth fault zone.