Simulation of Fault Sealing From Quartz Cementation within Cataclastic Deformation Zones

Robert H. Lander¹, Wilfrido Solano-Acosta², Andrew R. Thomas², Robert M. Reed³, Marek Kacewicz², Linda M. Bonnell¹, and John Hooker³
¹Geocosm, Austin, Texas
²Chevron, Houston, Texas
³Bureau of Economic Geology, Austin, Texas

Although models for fault sealing from juxtaposition and smearing are applied widely (e.g., Yielding et al., 1997), process-oriented models for predicting the sealing potential for cataclastic deformation zones are lacking. Cataclastic deformation zones may have significant sealing potential due to small pore throat sizes and low porosities that result from grain size reduction, compaction, and quartz cementation. Unlike other fault sealing processes, the sealing potential for a cataclastic zone may decline significantly after deformation ceases given that quartz cementation rates are likely much slower than cataclastic deformation rates. The extent of quartz cementation, and thus the sealing capacity of a deformation zone, therefore will be a strong function of the post deformation thermal exposure in addition to the composition and texture of the host rock and the effect that cataclasis has on the grain size distribution and packing.

As a first step toward developing seal models for cataclastic zones we have analyzed the following for deformation zones and host sandstones: (1) porosity, quartz cement abundance, and the abundance of all other significant modal categories, (2) compaction state as indicated by intergranular volume, and (3) the grain size distribution. We then used the grain geometries derived from these images together with reconstructed thermal histories as input for a two dimensional quartz cementation model to evaluate the extent to which cement abundance and pore geometry and sizes can be predicted.

The interpreted images are derived from registered suites of the following image types that were collected with a scanning electron microscope: color cathodoluminescence, backscatter electron, and elemental maps of Si, Al, Na, K, Ca, Mg, and Fe. In addition we collected and registered optical plane light and cross polar images of the studied regions. Cathodoluminescence imaging is a particularly important aspect of the characterization process in that it provides the only reliable means to distinguish between quartz cement and grains. The imaged regions cover areas on the order of 2 mm² and have pixel resolutions of <1 mm, resulting in millions of pixels for analysis. In addition to determining the volume abundances of the primary modal categories, we also used image segmentation techniques to determine the major axis, minor axis, area, and perimeter for each imaged quartz grain.

In the analyzed samples we found the deformation zone to have substantially lower porosity compared to the host sandstone (~11% compared to ~22% for the example shown in Figure 2).

The dominant cause of the lower porosity in the deformation zone is more extensive quartz cementation, although the deformation zones also are somewhat more compacted compared to the host sandstones. As expected, the grain size reduction from cataclasis results in a significant increase in quartz grain surface areas (Figure 2).

Factors that lead to increased quartz cementation rates in cataclastic deformation zones compared to host sandstones include increased nucleation surface area from grain fracturing and faster surface area normalized growth rates due to fewer nucleation impediments on fracture surfaces compared to detrital grain surfaces (Lander et al., 2008). A factor that partially counterbalances these effects is diminished surface area normalized growth rates on finer grained nucleation substrates due to more rapid development of euhedral forms compared to larger equivalents (Lander et al., 2008).

We have applied a two dimensional model of quartz cementation (Prism2D) in an attempt to simulate quartz cementation in the deformation zone. Input for the model includes the geometry of solids prior to quartz cementation, the crystallographic orientations of monocrystalline quartz grains, the post-deformation temperature history, the activation energy for quartz precipitation on non-euhedral c axis surfaces, and relative growth rates on other surface types (a axis non-euhedral and euhedral pyramidal and prismatic faces). The pre-quartz geometrical input is derived from the interpreted images while crystallographic orientations were assigned randomly to the monocrystalline quartz grains. The temperature history was determined by basin modeling and an assumed time of deformation. We derived the activation energy for quartz growth on non-euhedral c axis surfaces by empirically matching the measured quartz cement abundance for the undeformed portion of the modeled sandstone. The relative growth rates for other surface types are based on the experimental data of Lander et al. (2008).

Model results for quartz cement abundance in the deformation zone are within one volume percent of the measured value (Figure 3). The model results also mimic the spatial geometries of the cement and pores. In particular, the simulation accurately predicts that the greatest porosity loss occurs within small pores that are bounded by quartz grain fragments while the largest pores tend to be found adjacent to non-quartz grains.

These preliminary results are highly encouraging and suggest that it is possible to develop accurate quartz cementation models for cataclastic deformation zones that also predict the associated impact on the sizes and spatial arrangements of pores.

References

Lander, R. H., R. E. Larese, and L. M. Bonnell, 2008, Toward more accurate quartz cement models: The importance of euhedral versus noneuhedral growth rates: AAPG Bulletin, 92, 1537–1563

Yielding, G., B. Freeman, and T. Needham, 1997, Quantitative fault seal prediction: AAPG Bulletin, 81, 897–917

Figure 1. Interpreted images from a deformation zone (top) and portions of the undeformed host sandstone (bottom).