C-O isotope composition of the authigenic carbonate in Low Tertiary reservoir sandstones in Tarim Basin and its implication on the fluid origin.

Carbonate is one of the most widely developed authigenic minerals in Low Tertiary reservoir sandstones found in Tarim Basin. Its formation commenced in eodiagenesis and continued to mesodiagenesis. Calcite is the dominant, and dolomite the common component, both with a certain amount of Fe and Mn. Analysis result shows that δ 13 C value of the authigenic carbonate varies in a range of -31‰~ +6‰, but in -12‰~+3‰ for most samples. δ 18 O value is in 12.4‰~28.5‰, but mainly concentrated in 18‰~24‰. The C O isotope composition is much more variable than that of sedimentary carbonate, and in addition, with a different mode. The calculated oxygen isotope composition of the aqueous fluid in equilibrium with the authigenic carbonate possesses a bi model distribution pattern. Two frequency peaks of δ 18 O value are found at -4‰~-2‰ and 4‰~6‰ respectively. The former features buried saline water, while the later reflects the dehydration of argillaceous sediments and diagenetic water rock reaction. The highest frequency of δ 13 C value of CO 2 in equilibrium with the carbonate is -4‰~-8‰. This group of CO 2 most probably comes from the upper mantle. Some other origins for CO 2 are the thermolysis of source rock and diagenetic decomposition of marine carbonate.