AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Quantification of Effects of Cementation on Subcritical Crack Index by Comparison of Outcrop and Subsurface Samples of Williams Fork Formation, Piceance Basin
(1) Shell International Exploration and Production, Houston, TX.
(2) Bureau of Economic Geology, Jackson School of Geological Sciences, University of Texas at Austin, Austin, TX.
(3) Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, TX.
Subcritical crack index (SCI) is a rock mechanical property that influences fracture characteristics including aperture distribution and clustering (Olson et al., 2009). SCI measurements performed on samples yield a value for the present day characteristics of the rock. At the time of fracturing sandstone might have been at a different diagenetic stage (degree of compaction, cementation, porosity, permeability, etc.) linked to its burial and thermal history. As the degree of lithification changes during burial through compaction and cementation, the rock`s response to fracturing may change in concert.
To determine the range of SCI values for sandstone of given framework composition at different diagenetic stages, measurements were made on a set of Upper Cretaceous Williams Fork core samples and their outcrop equivalents. Subsurface and outcrop samples were paired carefully on the basis of their framework grains and textural properties. These core and outcrop samples have contrasting burial and thermal histories related to burial and uplift associated with the Late Cretaceous to Tertiary Laramide orogeny and subsequent orogenic movements. Differences in thermal histories are clearly reflected in the amount of quartz cement that mainly controls the degree of consolidation. Deeply buried core samples represent well-consolidated end-members; whereas, their poorly to moderately consolidated outcrop equivalents are useful as guides to the mechanical properties of the reservoir rock when they were in a less diagenetically altered state by quartz cement. Wide variations between SCI values from outcrop and core samples demonstrate the effects of diagenesis on rock mechanical properties and the need for integrating diagenesis into geomechanical models for more accurate prediction of fracture network characteristics.
Using outcrop samples as analogs or guides to the properties of subsurface rocks may be misleading if the burial histories of the two are very different as duration and amount of burial affects quartz precipitation rates and volumes. In addition, outcrop alterations including kaolinite, Fe-oxide, zeolite, siderite, and calcite can change the diagenetic and mechanical characteristics of the rock.