Formation mechanism of condensate oil and waxy oil in Structure X of Xihu Depression

Oil in the Structure X of the Xihu Depression can be divided into two groups based on the differences in physical properties. One is the condensate oil, which is characterized by low density and low wax content; and the other is the waxy oil, which is characterized by medium density and high wax content. Geochemical characteristics of crude oil were studied in details in order to define the causes of the differences. It is so obvious that there are great differences between condensate oil and waxy oil in saturated hydrocarbon chromatography characters. However, the characteristics of sterane, terpane, and carbon isotope of condensate oil are as the same as the waxy oil. It means that they come from same source, and secondary alteration is the real cause to the difference in physical properties. Detailed correlation of lighter hydrocarbon chromalogram and fingerprint parameters the two oils suggest the following: (1) There is a mirror image relationship of total hydrocarbon chromatography between condensate oil and waxy oil. (2) Condensate oil is characterized by high index of nC₇/MC_yC₆ and low index of Tol/nC₇, whereas waxy oil shows an opposite pattern. (3) There are covariant relation of the index of nC_m+1/nC_m of N-alkanes between the condensate oil and the waxy oil, suggesting that oil has suffered from gas invasion. The condensate is the results of evaporative fractionation, and the waxy is the remainder.