Preservation of Ultradeep Liquid Oil and Its Exploration Limit

Crude oil in reservoirs may crack to gas with increasing buried depth and temperature, resulting in natural gas as the major exploration target in deep strata. Exploration of ultradeep targets in the Tarim Basin, Northwest China, unexpectedly discovered huge accumulations of liquid oil in Ordovician carbonate reservoirs with burial depths of >7000 m. Applying comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry, this paper analyzes the chemical compounds of crude oil in these ultradeep layers and has found that these oils are slightly cracked with low conversion of oil to gas, as evidenced by limited diamondoid compound analogues and low concentrations. Gold tube pyrolysis simulation and kinetic calculation suggest that the cracking temperatures of oil are in the range of 210-220 degrees C. The temperature implies that the deepest liquid oil surviving limit is inferred as 9000 m in this basin with the increase of the temperature. The major controlling factors on preserving abundant liquid oil in ultradeep layers of the Tarim Basin include the insufficient compensation effect of a low geothermal gradient and fast deep burial during a later period. It has potential to find oil in ultradeep (>7000 m) layers.