Origin of subsequent condensate pool in Lunnan low uplift, Northwest China

Abstract

Lunnan (轮南) low uplift, where developed amounts of condensate pools in both Ordovician and Carboniferous reservoirs, locates in the north uplift of Tarim basin, Northwest China. Lunnan area has experienced intensive gas invasion since the Himalayan movement, causing multiple types of accumulation coexisting in the Ordovician reservoir. Geochemical evidence shows that these condensate oils are of low maturity (vitrinite reflectance levels near 1.0%) and the dominant component of the condensate gas is dry gas (the content of CH4 over 94%), which reflects that the condensate pool in Lunnan area is not the product of thermal cracking, but the subsequent accumulation derived from the phase fractionation. Furthermore, our observations on the C6-C7 hydrocarbons in Lunnan area are qualitatively similar to the effect of phase fractionation with condensates having higher paraffin and lower aromaticity than residual oils. Thus, two types of the condensates are defined by their phase behavior, which are saturated condensate pool and the nonsaturated one. The saturated condensate pool with an oil ring mainly developed in Ordovician reservoir, with large difference between formation pressure (FMP) and the dew point pressure (DPP). The origin of these condensates is considered as the in-situ segregation triggered by the invasion of excessive dry gas. On the other hand, the nonsaturated condensate pool that almost existed in the Carboniferous reservoir is a hydrocarbon reservoir containing slight difference between FMP and DPP, without oil ring, and evaporative fractionation due to the fault movement controls its generation.