储层自生方解石碳同位素值应用于油气运移示踪

STABLE CARBON ISOTOPE OF AUTHIGENETIC CALCITE USED IN RESERVOIRS TO TRACING THE HYDROCARBON MIGRATION

以鄂尔多斯盆地上古生界气层为例,探讨了砂岩储层中自生方解石的碳同位素组成特征及变化机制,并剖析了其与天然气成熟度和二氧化碳碳同位素值之间的内在联系,进而认为:方解石的碳同位素组成与天然气的二氧化碳参与胶结物沉淀作用有关;其碳同位素值变化主要取决于含二氧化碳天然气的热演化程度;气源层早期成气演化阶段生成的成熟度较低的天然气中二氧化碳碳同位素值轻,所形成的自生方解石碳同位素相应较轻;而后期生成的成熟度较高的天然气中二氧化碳碳同位素值重,造成方解石碳同位素值相对也较重。因此可据储层自生方解石碳同位素比值与天然气成熟度及相应二氧化碳碳同位素值之间的关系,结合该盆地上古生界天然气的生成和运聚特点,将自生方解石碳同位素值作为油气运移示踪指标,并以此确定出米脂气田石盒子组气层天然气是由南向北运移的。

The characteristics and change mechanism of stable carbon isotopic composition of authigenetic calcite in sandstone reservoirs are studied and their genetic relations with natural gas maturity and the δ13C values of CO2 are analyzed by taking the Upper Paleozoic gas reservoirs in the Ordos Basin as examples. The carbon isotopic composition of calcite is associated with the involvement of CO2 of natural gas in precipitation of cements, and the change of carbon isotope value mainly depends on the thermal maturity of natural gas containing CO2. The natural gas with relatively low maturity generated at the early stage of gas source rock evolution contains CO2 with light carbon isotope, thus the authigenetic calcite correspondingly has light carbon isotope. In contrast, the natural gas with relatively high maturity generated at the late stage of gas source rock evolution contains CO2 with relatively heavy carbon isotope, thus the authigenetic calcite correspondingly has heavy carbon isotope. According to the relationships among the carbon isotope values of authigenetic calcite in reservoirs, gas maturities, and carbon isotopes of CO2, and in combination with the features of generation, migration and accumulation of the Upper Palaeozoic gas in this basin, the carbon isotope value of authigenetic calcite can be used as an indicator for gas migration. The direction of natural gas migration in the Shihezi Formation of Mizhi gas field is believed to be from south to north.