Diagenesis and reservoir quality in tight gas bearing sandstones of a tidally influenced fan delta deposit: The Oligocene Zhuhai Formation, western Pearl River Mouth Basin, South China Sea

摘要

Deeply buried Oligocene Zhuhai sandstones are important gas exploration targets in the western Pearl River Mouth Basin, South China Sea. The tight sandstones are characterized by low porosity (less than 10%), low permeability (less than 1 mD) and strong microscopic heterogeneity due to diagenesis during their geologic history. Petrographic analysis is undertaken based on cores, thin sections, X-ray diffraction and scanning electron microscope, that is helpful to understand the facies, lithologic characteristics, diagenesis and their impact on reservoir quality. Six lithofacies were defined in the Zhuhai sandstones, namely bedded granule-rich coarse sandstone to granule conglomerate, cross-bedded medium-to coarse-grained sandstone, cross-bedded fine-to coarse-grained sandstone, structureless sandstone, interbedded sandstone and siltstone and claystone and shale. They were accumulated in the fan delta systems and tidal fiat systems. The sandstones are dominated by feldspathic litharenite, litharenite, sublitharenite and quartz arenite. Major portion of the porosity is due to secondary pores and primary pores are relatively less dominant. Primary pores are more important to tight sandstones, well preserved with moderate compaction and weak cementation. Well-sorted, medium-to coarse-grained sandstones with rigid quartz and rock fragment grains commonly have high primary porosity. Early cements such as chlorite rims are helpful for primary porosity preservation. The dissolution of feldspars in the Zhuhai Formation helps to increase the total porosity, whereas it also decreases the permeability and spoils the tight reservoir quality.