![]() ![]() However, as important as these reactions are, other mineral phases, notable carbonate minerals, are also widespread ( Finley & Lorenz 1988 Laubach 2003). Understanding of quartz cement growth has provided insight into fracture development and permeability evolution of fracture networks ( Beach 1977 Fisher & Byrne 1990 Laubach et al. Fractures are commonly either lined with quartz and thus partially open for fluid flow, or completely cemented and thus hydraulically inactive relative to the host sandstone. In tight gas sandstone reservoirs, quartz cement is widespread in fractures. Understanding how and why cement accumulates in fractures is therefore essential for predicting the flow and storage behaviour of these rocks. 2018) and interfere with engineering operations such as hydraulic fracture stimulation. The type and amount of fracture cement can influence fracture spacing ( Hooker & Katz 2015) and, among other factors, cement in pre-existing fractures can affect subsequent fracture growth ( Virgo et al. On a larger scale, the diagenetic state of fractures may influence basin-scale fluid flow ( Fischer et al. Whether fractures are cemented or cement-lined, and the topology of fracture cement and porosity can significantly influence fracture permeability, with consequences for well producibility and reservoir performance ( Laubach 2003 Nollet et al. They provide pathways between organic-rich source layers and matrix pores during hydrocarbon charge, and between matrix pores, hydraulic fractures and the wellbore during production ( Laubach 2003 Cumella & Scheevel 2008 Warpinski & Lorenz 2008). Natural fractures can significantly control flow of formation and production fluids, especially in low-permeability rock units. Thematic collection: This article is part of the Naturally Fractured Reservoirs collection available at: We find that the rock-mass calcite cement content correlates with fracture degradation and occlusion, and can be used to accurately predict where wide fractures are sealed or open. Although the cement sequence Fe-dolomite → ankerite → calcite is widespread, Fe-dolomite and ankerite occur as host-rock cements only, with detrital dolomite as preferred precipitation substrate. In host rock, both albite and calcite content decreases with depth along with greater fracture porosity preservation. 87Sr/ 86Sr ratios of calcite and the presence of porous albite suggest that detrital feldspar albitization released Ca 2+, driving carbonate cement precipitation. Fluid-inclusion analyses indicate calcite cement precipitation at 135–165☌. Using pore and fracture cement petrography, fluid inclusions, and isotopic and elemental analysis, we show that host-rock calcite distribution and remobilization govern porosity degradation and occlusion of fractures >1 mm wide by calcite. In the Piceance and in other basins, processes that control the distribution of these calcite cements have been uncertain. Fracture occlusion by calcite is highly heterogeneous, with open and calcite-sealed fractures found at adjacent depths. Fracture occlusion by quartz is controlled primarily by fracture size, age and thermal history. Cretaceous Mesaverde Group sandstones contain opening-mode fractures lined or filled by quartz and, locally, calcite cement. ![]()
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