In recent years,high-yield reservoir rocks of cave-accumulated type have been found by drilling some wells in the weathered leaching zone of Lower Palaeozoic in Jiyang depression,Shandong,of which No.10 well in Gubei in particular.The cave accumulations were formed in the Jurassic,the basement rock of the cave is carbonate one to the Majiagou group of Middle Ordovician.The cave is 10.08 metres high,the accumulation is 8.28 metres thick and unfilled cave space remains in 1.8 metres high.The cave accumulations are in disorder,which consist of three components:1)collapsed materials(breakdowns)of the basement rock forming the cave;2)destructive products in chemical sediments of the cave;3)materials from other places in the stage of the formation or post-formation of the cave.Cave accumulations in Gubei No.10 well,from down to up,can be divided into four lithologic formations of cave breccia,cave mudstone,cave mudstone-breccia and cave broken stone,etc.Mesozoic sporopollen fossils were contained and found in the cave mudstone,which may have given evidence for the determination of the time for the accumulation of the cave.Cave accumulations,avoiding the normal pressure because of protecting by the cave wall,have preserved a great many porosities in them.Because the finer accumulations,the higher percentage of primitive porosity,thus,porosity of the cave mudstone is the best,microporosity mainly,up to 37.9%.Cave accumulations in Jiyang depression,from the Early Carboniferous onwards,have been successively buried.Porosities and corrosional corridors of the cave have been enlarged and increased by the corrosion of acid water solusions from the movement of the Tertiary fault zone and through the unconformable structural zone after their buring,thus,improving permeability of the cave accumulations and making them a good high-yield reservoir rock.
