Estimation of the Relative Contributions of Different Hydrocarbon Charges to the SW Qaidam Basin Accumulations Using Micro-Spectroscopy Analysis of Petroleum Inclusions

In a series of paper, the characteristics of petroleum inclusions described by λmax from the UV and CH2/CH3 ratio from the FT-IR spectra. On the basis of optical microscopic, microthermometric, fluorescence spectroscopic and FTIR spectroscopic analyses three types of hydrocarbon inclusions are identified in the study area: namely (1) yellow fluorescencing oil inclusions, (2) blue fluorescencing oil inclusions and (3) gas inclusions, representing two episodes of oil charges and one gas charge possibly related to readjustment of the associated gases down dip. The first episode of oil charge is represented by the predominantly yellow fluorescencing oil inclusions trapped prior to the quartz overgrowth, whereas the second episode is marked by the blue fluorescencing fluid inclusions occurred after the precipitation of dolomite. Both the UV fluorescence and the FT-IR spectra show two distinct oil inclusion groups (yellow and blue) with λmax at 540 nm and 475 nm, respectively and corresponding CH2/CH3 ratios of 1.2 and 2.3, respectively. Microthermometric data indicate that the two groups of oil inclusions have different homogenisation temperatures (Th), corresponding to oil charge around 25 Ma and 10 Ma, respectively for the Gasi and Hongliuquan oilfields, 10 Ma and 5 Ma, respectively for the Yingdong Oilfield, SW Qaidam Basin. The yellow fluorescencing oil inclusions have relatively low maturity and API gravity compared with the blue fluorescencing ones. The current accumulations in the oilfields have maturity and API gravity similar to that of the yellow fluorescencing inclusions. It is concluded that the earlier hydrocarbon charge is thus the predominant contribution to the current accumulations.