Clastic diagenesis and reservoir characteristics of Funing Formation in north slope of Gaoyou Depression in Subei Basin

The reservoirs of Funing Formation in the northern slope of Gaoyou Depression are of the fine_grained and moderately sorted sandstones that can be classified as arkoses, lithic arkoses and feldspathic litharenites. Funing reservoir sandstones contain a diagenetic mineral assemblage that recorded the major diagenetic events during burial history. The early calcite was formed at shallow burial, whereas later ferroan carbonates may record a deep diagenetic event. Kaolinite was generated mainly as randomly oriented or vermiform stacks of pseudohexagonal crystals that formed a loosely packed and erratially distributed pore infilling. Chlorite was generated in the deeply buried reservoirs and commonly formed the grain_sized masses. Illite was also generated in the sandstones and formed the grain rims that commonly bridged pores. Authigenic quartz overgrowth and micro crystals contained a variety of organic fluid inclusions that indicated the petroleum filling history. Secondary porosity was created mainly by feldspar, rock fragment and clay matrix dissolution. Based on the diagenetic events related to temperature,the diagenetic stage of reservoirs in Funing Formation was in the period from B_substage of early diagenesis to A_substage of later diagenesis. Reservoir quality was controlled mostly by the sedimentary microfacies and diagenesis. Distributary channel and mouthbar sandbodies commonly have high porosity and permeability, whereas interdistributary and distal sand sheets may have poor petrophisical properties. The reservoir property was controlled by facies in the plane.