Quartz growth: understanding porosity-preserving microcrystalline quartz through EBSD, TEM, and NanoSIMS examination of low temperature silica

摘要

Deeply buried sandstones in sedimentary basins typically have low porosity due to cementation and compaction. Formation of microcrystalline quartz has proven to be effective at preserving porosity in deeply buried sandstone petroleum reservoirs, typically cemented by syntaxial quartz cement. There remains much uncertainty about what controls the growth of microcrystalline quartz and how it prevents syntaxial quartz overgrowths. Here, the Cretaceous Heidelberg Formation, Germany, and the Oligocene Fontainebleau Formation, Paris Basin, France, provide a natural laboratory to study silica polymorphs and develop an understanding of their crystallography, paragenetic relationships, and growth mechanisms, leading to a new understanding of the growth mechanisms of porosity-preserving microcrystalline quartz. Data from scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) data illustrate that porosity-preserving microcrystalline quartz cement is misoriented with respect to the host grain upon which it grows. In contrast, ordinary quartz cement grows in the same orientation (epitaxially) as the host quartz sand grain, and typically fills pore spaces. EBSD and TEM observations reveal nanofilms of amorphous silica ( 50-100 nm in thickness) between the microcrystalline quartz and the host grain. The microcrystalline quartz is interpreted to be misoriented relative to the host grain, because the amorphous silica nanofilm prevents growth of epitaxial quartz cement. Instead, the microcrystalline quartz is similar to chalcedony with [11-20] perpendicular to the growth surface and c axes parallel with, but randomly distributed (rotated) on, the host quartz grain surface. Development of pore-filling quartz growing into the pore (in the fast-growing c-axis direction) is thus inhibited due to the amorphous silica nanofilm initially and, subsequently, the misoriented microcrystalline quartz that grew on the amorphous silica. High precision, in situ oxygen isotope analyses of Cretaceous Heidelberg Formation detrital grains and quartz cements show three varieties of authigenic silica growing on detrital quartz grains. Interpretation of these data show that quartz overgrowths grew from meteoric water at about 80°C followed by concentric bands of silica cements. A thin layer of chalcedony was first deposited on both detrital quartz grains and quartz overgrowth cements followed by microcrystalline quartz; this cycle was then repeated. If it is assumed that the closely-related chalcedony and microcrystalline quartz grew from the same water, then isotope data suggest that chalcedony grew at approximately 34°C while microcrystalline quartz grew at approximately 60°C from meteoric water. To further understand the role of chalcedony in microcrystalline quartz growth, chalcedony in two agates from the Citronelle Formation in Louisiana and Lake Superior in Michigan, were studied This study concludes that the bands formed as a result of discrete influxes of siliceous fluid. Within these individual bands there is a sequence of minerals; chalcedony-A (with amorphous silica and nanocrystalline quartz)  chalcedony-MQ (with microcrystalline quartz)  quartz. This paragenetic sequence suggests a viable model for the growth of chalcedony in agates, which helps explain the growth of chalcedony and microcrystalline growth in sandstones. Now that we know what controls microcrystalline quartz growth and why it preserves porosity, it can be used to help identify, rank and appraise deeply buried petroleum accumulations.