PRECIPITATION OF VATERITE (CACO3) DURING OIL-FIELD DRILLING

Vaterite, a CaCO3 polymorph, is a rare mineral that is said to be metastable under all known conditions. According to the literature, vaterite precipitated from carbonate solution recrystallizes spontaneously to calcite or aragonite. Yet vaterite has been identified in hard tissues of organisms, in gallstones, in contact metamorphic aureoles, in zones of thermal metamorphism, in a meteorite, and in cone-in-cone concretions. Newly precipitated vaterite has formed at the expense of carbonate rock in drilling fluids in wells of New York, Michigan, Nevada, Texas, and New Zealand. Petrographic examination reveals a light brown core of Ca3SiO5 surrounded by a colourless rim of vaterite. The deltaO-18 PDB of New York vaterite is -12.4% and that of the Michigan vaterite is -17.6 parts per thousand, which reflect the oxygen isotopic composition of meteoric freshwater used in drilling. The deltaC-13 PDB value of -19.2 parts per thousand for New York vaterite and that of -17.6 parts per thousand for Michigan vaterite suggest that natural gas dissolved original carbonate in the subsurface. Drilling records from both wells indicate that natural gas was released into the drilling muds from the formations exposed at the time vaterite was encountered. Crossplots of the oxygen and carbon isotopic ratios overlap those of spurrite rocks in thermal metamorphic zones in Israel. A C-14 radiocarbon analysis of the Michigan vaterite gave an age of 953 +/- 39 yr. BP. 88.8 +/- 0.6% is modern carbon and 11.2% is dead carbon. Hence this carbon, and therefore the vaterite, is essentially modern. A sample of the New York vaterite yielded a modern age.