Paragenesis of Mineralized Fractures in Organic Rich Shales

Abstract

Mineralized fractures are common in organic-rich shales and are of interest because the mineralogy can influence shale brittleness and porosity/permeability. We have noted similarities in the paragenesis of mineralized fractures from several shale units of different ages and from different basins (Devonian Marcellus Shale [PA], Mississippian Barnett Shale [TX], Devonian/Mississippian Woodford Shale [OK], and Late Jurassic Haynesville Shale [TX]). The shales contain localized vertical/subvertical and some horizontal mineralized fractures that vary in width from thin (~ 0.04 mm), usually filled with calcite, to thick (> 0.2 mm), which have a complex mineralogy. Some fractures or veins are precompactional although most are interpreted to form late in the diagenetic sequence. The Barnett, Haynesville, and Marcellus contain complex fractures with calcite, dolomite, baroque dolomite, quartz, chalcedony, barite, celestine, pyrite, sphalerite, anhydrite, and albite. New work on the Woodford in southern Oklahoma indicates a similar mineralogy. Dissolution events also occur in the paragenetic sequences and some fractures are associated with brecciation. Fluid inclusion studies suggest interaction with multiple fluids, including hydrocarbons and hydrothermal fluids. Variation in cathodoluminescence and compositional variations within individual minerals indicate precipitation from evolving fluids. In some cases the minerals in the fracture extend into the surrounding shale, which could influence brittleness and the likelihood of reactivation. It is also clear that the nature of the fracture can be influenced by the composition of the host shale. The similar and anomalous mineral assemblages in the fractures from the different shales indicate alteration by similar fluids, internal and/or external, and suggest similar sources for the minerals. The results from different shales in different tectonic settings raise fundamental questions about whether the shales are open or closed systems.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015