Origins and accumulation processes of the Ordovician gas condensate in the Tazhong area, Tarim Basin, northwest China

Abundant gas condensates have been proven in the Ordovician carbonate reservoirs in the Tazhong area, the centre of the Tarim Basin, where complicated geological evolution and multiple hydrocarbon accumulations have occurred. Property, geochemistry, and stable carbon isotopes of the Ordovician condensate are characterized to identify the oil and gas origins in the Tazhong area. Fluid inclusion data, combined with numerical modelling methods was used to determine petroleum accumulation processes. Our results suggest that oils are characterized by mixed sources, with 64% of contributions from the Middle‐Upper Ordovician (O2+3) source rocks and 36% of contributions from the Lower‐Middle Cambrian (Є1+2) source rocks. Gases are primarily generated from the thermal cracking of pre‐existing oils in the underlying strata, with a small amount derived from kerogen cracking accompanied with oil generation. Three petroleum filling stages are determined, including the filling of the Є1+2‐derived oils during the Late Hercynian period, filling of the O2+3‐derived oils during the Yanshan period and oil‐cracking‐gas charge during the Himalayan period. The accumulation processes and relative contribution ratios of the two source rocks vary among the reservoirs and are mainly related to the transport system. Due to the lack of faults in the regions away from the No. 1 Fault Belt, the Є1+2‐derived oils are difficult to fill into the Ordovician reservoirs through the gypsolyte, and thus the accumulated oils are mainly from the O2+3 source rocks. The percentage of the O2+3‐derived oils is high in the southeast and northwest segments of the No. 1 Fault Belt, but relatively low in the middle segment and the vicinity of the No. 10 Structure Belt. Likewise, the late gas charge intensity is controlled by regionally varying conduit systems. Gas condensate formed in reservoirs with high gas/oil ratios. Otherwise, light oil retains with respect to low gas/oil ratios.

Dike distribution density: Method for quantitative mine targets prediction in the South Alatao Mountains area, NW China

The correlation between dike density and regional‐scale mineralization indicates a fundamental criterion for ore‐forming process. Here, a novel dike distribution density method is formulated for evaluating this correlation and exploring mine targets quantitatively. Three parameters (dike density, dike orientation scatter degree and dike fractal dimension) are proposed to express the degree of irregularity and complexity of dike distribution patterns. This method is applied to the South Alatao Mountains area (China), where the dike swarms show regionally well‐developed density gradients and the mineral deposits are spatially associated with abundant dike swarms. On the basis of this quantitative dike distribution density method, 60% of the deposit targets are delineated in this area. This result indicates that the method is an effective quantification tool for prospecting mine targets. The dike distribution density method is applicable for areas where abundant regional‐scale dike swarms and mineralization occur. It should be considered as an effective and complimentary technique for the common mine prospectivity analysis.