Hydrocarbon Charge History of the Gasi Oilfield, Qaidam Basin, Western China: Insight from Fluid Inclusion Analysis and Reservoir Diagenesis Investigation
The hydrocarbon charge history of the Gasi Oilfield in the Qaidam Basin, western China was investigated using a suite of fluid inclusion techniques including fluid inclusion petrography, fluorescence spectroscopy, FT-IR spectroscopy and microthermometry. The study was also facilitated by a detailed reservoir diagenesis investigation using optical microscopy, cathodeluminescence microscopy, SEM and XRD. Nineteen samples from the Eocene reservoir interval were selected for detailed investigation, with a particular focus on the YueJian-1 oil producing well which contains abundant large petroleum inclusions up to 50μm in sizes. The reservoir rocks investigated comprise tight fine sandstone and siltstone with an average porosity of 14% (7.4-18.3%) and average permeability of 11 mD (0.04-42.9 mD). The study has found that (1) the reservoir diagenetic sequences are: from an early calcite-->feldspar overgrowth--> quartz overgrowth-->Laumontite-->anhydrite-->Illite/ chlorite-->Late-->calcite-->dolomite; (2) the reservoir fluids evolved from alkalic-->acidic-->alkalic; (3) two episodes of oil charge occurred with the first one represented by predominantly yellow fluorescencing oil inclusions being contemporaneous with or just prior to the quartz overgrowth, and the second episode represented by blue fluorescencing fluid inclusions occurred after the precipitation of dolomite.
Both the UV fluorescence spectrograms and the FT-IR spectra confirm the presence of two distinct groups (yellow and blue) of petroleum inclusions with λ at 540 nm and 475 nm, respectively and a CH3/CH2 ratio of 0.05 and 0.1, respectively. Microthermometric data also indicate that the two groups of petroleum inclusions have different homogenisation temperatures of around 70 οC and 110 οC, corresponding to oil charge around 25 Ma and 2 Ma, respectively. It is concluded that the initial relatively immature oil represented by the yellow fluorescencing petroleum inclusions was charged into the reservoir prior to the major cementation around 25 Ma, whereas the late (2 Ma) charge of the high maturity oil (blue fluorescencing) occurred after the completion of the reservoir diagenesis and the tight cementation of the reservoir. This late oil charge was facilitated by the presence of high angle faults and fractures in the “tight sandstone” reservoir.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California