Quartz Cementation History of the Heidelberg Sandstone, Germany: In Situ Microanalysis of δ18O
Abstract - Microcrystalline quartz prevents the growth of ordinary quartz cements and leads to anomalously high porosity in deeply buried petroleum reservoirs. Oxygen isotope analysis of microcrystalline quartz and other quartz cements provide data to help understand the growth mechanisms for porosity preserving microcrystalline quartz. High precision, in situ oxygen isotope analyses of Cretaceous Heidelberg Formation detrital grains and quartz cements show three varieties of authigenic quartz cement growing on detrital quartz grains. This micron-scale data provides evidence that: 1) porosity preserving microcrystalline quartz forms on a chalcedony substrate and 2) that there were two episodes of fluid influx into the Heidelberg Formation. Detrital quartz has an average δ18O composition of +9.40/00 and mesoquartz (syntaxial overgrowth) has an average composition of +19.30/00, microcrystalline quartz has an average composition of 21.70/00. Estimates of the δ18O composition of chalcedonic quartz are complicated by the problem of isolating the microcrystalline quartz from the chalcedony; mixtures of the two give a consistently higher δ18O (27.40/00) than microcrystalline quartz. From oxygen isotope data, the formation of microcrystalline quartz and chalcedonic quartz is interpreted to have taken place in a small temperature range of between 80 and 1400C. Wavelength dispersive spectroscopy (WDS) data supports the paragenetic data and suggests that two episodes of enrichment in aluminum and iron in the microcrystalline quartz and chalcedony. These two distinct layers formed from two episodes of highly concentrated brines, emanating from a hydrothermal source associated with nearby faulting in the Harz Mountains mining district, Germany.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California