The Ordovician in Tahe-Lunnan oil area contains buried hill type reservoirs developed during the Paleozoie and finalized in the Mesozoic.Multi-stage tectonic uplift and denudation resulted in well-developed buried hill and karst

forming fracture-vug systems that constitute the Ordovician reservoirs.Weathering crust karstification mainly happened in the west and central areas of Tahe-Lunnan oil area

while deep karstification mainly occurred in the east in the whole Ordovician.Oil occurs in buried hill but only enriches locally to form oil pool

and the distribution of oil and gas is controlled by the development of reservoirs.Multiple stages of oil generation

migration

accumulation

and tectonic adjustment in this oil area caused the Ordovician reservoir characteristics varying with different structural zones:the west part of the area contains heavy oil reservoirs

the west of the central slop hold oil of normal density

Sang-ta-mu fault horst

the south slop of Sang-ta-mu

and the east of Lunnan fault horst

harbor condensate gas reservoirs with oil rim

and the east slope has condensate gas reservoirs.Hanging hydrocarbon reservoir with an inclined oil-water contact was also formed due to the skylight effect at the buried hill top.