Predicting Fracture Porosity Degradation by Calcite Cement in Mesaverde Group Sandstones, Piceance Basin, Colorado

Natural fractures are ubiquitous in sedimentary rocks, forming from the combined effects of burial and tectonic loading and fluid pressure interacting with rock strength and other mechanical properties. In tight-gas sandstones fractures are commonly either open, partially open or sealed by cement deposited during and after fracture growth. Whether fractures are open or sealed can significantly influence fracture permeability. Where occurrence of sealed fractures is heterogeneous, knowledge of the pattern of sealed and open fractures can be used in exploration and development decisions.

Cretaceous Mesaverde Group sandstones contain a set of opening-mode fractures. The fractures are partially cemented by synkinematic crack-seal quartz cement indicative of repeated opening and cementation, and syn- and postkinematic calcite that generally postdates quartz. The degree to which fractures are sealed with quartz cement depends in part on fracture size, with tiny fractures completely sealed and larger fractures (> ~0.1 mm in aperture) incompletely sealed. Fluid inclusion analyses indicate that quartz fracture cements formed within temperature and pressure ranges of 145 to 187°C, and 40 to 95 MPa, respectively, while calcite cements formed at 135 to 165°C, and 35-60 MPa, respectively. Thus fractures formed under significant overpressure during late stage burial and early stage uplift over a time interval of ~32 m.y.

Carbonate cements, dominated by calcite, completely seal some wide fractures; the pattern of fracture sealing is heterogeneous. Calcite sealed and open fractures are found together in the same cores, at different depths. We show that rock mass cement content (degradation index) accurately identifies depths having sealed or open fractures without samples of rare large fractures. Albitization of detrital feldspar sources Ca²⁺. During albitization the anorthite component is dissolved, with albite remaining as a porous daughter phase. Released Ca²⁺ drives precipitation of carbonate cements. The paragenetic sequence consists of early Fe-dolomite, followed by ankerite, and calcite. However, Fe-dolomite and ankerite occur only as matrix pore cements in the presence of detrital dolomite, which acts as a preferred precipitation substrate. Formation of early Fe-dolomite acts as Ca²⁺ sink, preventing formation of fracture-sealing calcite. Albite content is a proxy for fracture-damaging carbonate cements. Both albite and calcite content decreases with depth.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California