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.