The Influence of Gas Invasion on the Composition of Crude Oil and the Controlling Factors for the Reservoir Fluid Phase

Oil and gas in deep strata is complex in property and variable in phase type, which imposes great challenges to exploration. This is particularly true in the deep Ordovician of Tarim Basin (NW China) where abundant oil and gas resources were discovered with various phase types including gas, condensate, volatile oil, normal oil, and heavy oil. The prediction of the composition and phase type of deep fluid is a crucial issue in future development. In this study, typical oils from different types of reservoirs from the East Lungu area (north Tarim Basin) were sampled and analyzed with two-dimensional gas chromatography/time-of-flight mass spectrometry (GC x GC-TOFMS) for the reconstruction of gas invasion on its impact oil compositions and reservoir phase type. The concentration of diamondoids in crude oil and the loss of n-alkane decreased gradually along the westward direction of gas invasion. Coupled with several other parameters, the quantitative evaluation of the gas invasion intensity indicated that gas invasion occurs from east to west in the late period (12-2 Ma) in the study site and altered the paleo reservoirs formed in the Late Hercynian (293-255 Ma), and thus formed the condensate (strong gas invasion), volatile oil (weak gas invasion), and normal oil reservoir (no gas invasion), successively. The prediction of the phase type distribution provides a scientific basis for deep oil and gas exploration and resource assessment.