Seventeen Ordovician oil samples from Halahatang Depression are investigated for their thermal maturation. On the basis of GC analysis of the alkane fraction as well as GC-MS analysis of the demethylated hopanes, it is assumed that the oils are mixtures of severely biodegraded and nonbiodegraded oils. Therefore, their maturation is assessed on the basis of the distribution and abundance of compounds strongly resistant to biodegradation. The P 1 together with the P 2 values of C 7 light hydrocarbons suggest that the late filled nonbiodegraded oils for Halahatang are super mature. Triterpane and sterane isomerization maturation parameters indicate they barely reached the peak oil generation stage, consistent with the results obtained from the equivalent vitrinite reflectance Rc 1 (0.80–0.95%), while the Rc 2 (1.28–1.60%) delineate a high maturity of Halahatang oils. However these conventional triterpane and sterane isomerization maturation parameters gave only general information and a comparatively narrow range of maturity. Correlation between the maturation parameters of Pr/nC 17 and Ph/nC 18 , Ts/(Ts + Tm) and C 29 Ts/(C 29 Ts + C 29 H), MPI 1 and MPI 3 , 4-/1-MDBT and 4, 6-/1,4-DMDBT are rather good, confirming the validity to assess and differentiate the maturity of the severely biodegraded oils examined, with the exception of naphthalene maturation parameters(DNR 1 and TNR 2 ). According to the maturation classification assembled with selected effective maturity parameters, such as the MPI 3 together with the corresponding [P], the thermal maturities of Halahatang oils are moderate, while Ts/(Ts + Tm) together with 4-/1-MDBT demonstrate a moderate-high maturity.
Natural gas of organic origin is primarily biogenic or thermogenic; however, the formation of natural gas is occasionally attributed to hydrothermal activity. The Precambrian dolomite reservoir of the Anyue gas field is divided into three stages. Dolomite-quartz veins were precipitated after two earlier stages of dolomite deposition. Fluid inclusions in the dolomite and quartz are divided into pure methane (P-type), methane-bearing (M-type), aqueous (W-type), and solid bitumen-bearing (S-type) inclusions. The W-type inclusions within the quartz and buried dolomite homogenized between 107 °C and 223 °C. Furthermore, the trapping temperatures and pressures of the fluid (249 °C to 319 °C and 1619 bar to 2300 bar, respectively) are obtained from the intersections of the isochores of the P-type and the coeval W-type inclusions in the quartz. However, the burial history of the reservoir indicates that the maximum burial temperature did not exceed 230 °C. Thus, the generation of the natural gas was not caused solely by the burial of the dolomite reservoir. The results are also supported by the presence of paragenetic pyrobitumen and MVT lead-zinc ore. A coupled system of occasional invasion by hydrothermal fluids and burial of the reservoir may represent a new genetic model for natural gas accumulation in this gas field.
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