Origin and impact of authigenic chlorite in the Upper Triassic tight-oil sandstone reserviors of the Ordos Basin, China

The authigenic chlorite is frequently observed in sandstone and may heavily influence the reservoir properties. The seventh member of the Upper Triassic Yanchang Formation (Chang-7) tight oil sandstones are the main tight-oil... [ view full abstract ]

The authigenic chlorite is frequently observed in sandstone and may heavily influence the reservoir properties. The seventh member of the Upper Triassic Yanchang Formation (Chang-7) tight oil sandstones are the main tight-oil sandstone reservoirs of the Ordos Basin, central China, showing high primary intergranular porosity at depths 2000~3000m. These sandstones were deposited in a lacustrine-deltaic setting. Porosity preservation in these tight sandstones is ascribed to the inhibition of quartz cementation and pressure dissolution by authigenic pore-lining chlorite. This research investigates the Origin and impact of authigenic chlorite of the Chang-7 tight oil sandstones. Petrologic features, diagenesis, and diagenetic minerals were determined through petrographic studies including thin section, X-ray diffraction, scanning electron microscope (SEM) and Field-effect scanning electronic microscopy (FESEM) image analysis. The results show that Chang-7 sandstones are mostly fine-grained, feldspathic litharenite and lithic arkose, rich in volcanic rock fragment. Chlorite, the main diagenetic constituent, occurs as coatings, rims, rosettes and replacing grains. Chlorite precipitation was favoured by the presence of biotite and by the abundance of VRF. The chlorite pore-linings were formed by the replacement of biotite and by the direct neoformation of chlorite rims. Ions required for these reactions were derived from biotite and VRF. Fluid flow patterns, burial and thermal history also played a role in chlorite authigenesis and reservoir quality evolution. Sandstones with scarce chlorite often suffered intense quartz cementation and pressure dissolution during deep burial. Discontinuous and thin coatings and rims were not effective in inhibiting quartz cementation, while those too thick severely blocked pore throat and reduced the permeability. The rosette aggregates exerted no inhibition on quartz cementation, while reducing intergranular porosity. Systematic studies are necessary for modelling the diagenetic patterns of Ordos Basin Chang-7 tight sandstones, in order to contribute to the reduction of risks involved in the exploration of these tight oil sandstone reservoirs.