Click to view presentation in PDF format.
An Integration of Fault
Rock Properties through Time with Basin Modeling*
Marek Kacewicz1, Russell K. Davies2, Michael Welch3, and Rob J. Knipe3
Search and Discovery Article #40349 (2008)
Posted November 11, 2008
*Adapted from extended abstract prepared for poster presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008
1 Chevron ETC, Sugar Land, TX ([email protected])
2Rock Deformation Research USA. Inc., McKinney, TX
3Rock Deformation Research Ltd., Leeds, United Kingdom
The flow pathways in stratigraphically and structurally complex areas require knowledge of the architecture of these geological systems, but also their flow properties such as the permeability and capillary threshold pressure. Typical basin models in such areas are built based on a series of structural restorations that provide a basic geometric description of the evolving system. However, structural restorations do not address dynamically changing fault
and host rock properties. We address the changes of the properties throughout the burial history of the basin from models described in this paper that provide an important predictive capability as to flow pathways and pressure communication within the system. If combined with structural restorations and classical basin modeling, they may become an integrated part of basin modeling workflow in structurally complex areas.
In this article, we discuss a model for the fault
rock properties or
fault
gouge and the change in the properties over time in siliciclastic sediments of sands and shales. Presented examples demonstrate how the improved
fault
and host rock properties lead to better charge and pressure predictions.
|
Modeling ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() In the model for the prediction of Figure 2 shows the modeled compaction for end-members 100% and 0% clay with experimental data fitted to the trends. The dashed line is the compaction curve for an intermediate clay content of 54% based on the model of the end member curves. In low clay content quartzarenites, an important diagenetic effect on the porosity reduction in addition to compaction is the quartz cementation. Walderhaug (1996) modeled the kinetics of the quartz precipitation process and quantified the increase in the quartz precipitation, which depends on the grain size and clay content. At higher clay contents, the quartz cementation is retarded. The increase in the quartz precipitation further reduces the porosity relative to that in the host shown from the compaction curves in Figure 2. Thus the porosity reduction is modeled as a function of the compaction and diagenesis. The compaction trends are a function of the effective stress, which may reduce the effect of the overburden on the porosity loss for higher pore fluid pressures. Thus a rock buried deeply with high fluid pressures may have a porosity equivalent to a shallower depth of burial. The porosity changes are modeled on the stress history and not the depth.Initial Rock Properties and Consolidation State The deformation and Undeformed host rocks that lie along a normal compaction trend or normal consolidation, as shown by the compaction curves in Figure 2 and Figure 3, will deform with a more ductile distributed deformation and are less likely to develop discrete faults. Faulting, however, occurs in rocks that are overconsolidated. Overconsolidation may occur in rocks that follow a normal consolidation path to some depth and are then uplifted. The rocks do not regain their original porosity at a similar mean effective stress or depth and are more brittle and overconsolidated. The greater the uplift or overconsolidation, the more brittle the rock. Strongly overconsolidated rocks, including sands and shales, will deform by open fractures and are more likely to leak, especially if this deformation is focused along the Overconsolidation may also occur due to cementation. This has been referred to as pseudo-overconsolidation. A sandstone, for example, that is buried to temperatures great enough for quartz cementation will have a porosity lower than the normal compaction trend without the cements (Figure 3). The rocks will, therefore, deform with open dilatants fractures that will have a tendency to enhance flow along their paths. Thus the overconsolidation is an important control on the faulting style and flow parameters. The degree of overconsolidation is a measure of the brittleness. The brittleness can be modeled from the unconfined compressive strength of the rocks, the porosity prediction for a given depth relative to the normal consolidation, or the effective stress history from modeling. A common measure of the magnitude of the overconsolidation is the overconsolidation ratio or ratio between the maximum applied stress to the measured mean effective stress. In the red and black curves in Figure 3, the overconsolidation ratio is similar for both rock types although their porosity is different. The greater the difference between the maximum stress and the measured stress, the greater the brittleness of the rock. The rock that is cemented will have a measured maximum effective stress equivalent to a rock with the porosity at the measured depth. In this study, we consider the overconsolidation effect relative to the ratio of the maximum stress, p*, and the measured mean effective stress, p. This p/p* is a convention described by Fisher et al. (2007) for deformation in clean sandstones. For p/p* greater than 0.5 we expect a more ductile behavior and below 0.5, a more brittle deformation. The lower the ratio, the greater is the brittleness. The![]() ![]() ![]() ![]() For a relatively clean quartzarenite with low clay contents less than 15%, the deformation is controlled by the porosity and grain size. For porous sandstones discrete bands of shear or deformation bands develop by the rearrangement of the quartz grains. At higher mean effective stress these grains crush and reduce the permeability of the rock. For lower porosity quartzarenites, the rock develops open fractures that link to form a through-going At higher average clay contents between 15 and 45%, the clays mix with the quartz grains occluding the pore spaces and reducing the permeability, forming phyllosilicate framework Figure 5, for example, shows a model for porosity reduction with moderate clay content in the host and The porosity may be related to the permeability and capillary entry pressures to provide a model for the flow input to basin models. Similar plots provide the estimates of flow through modeled stratigraphic section with a range of average clay contents. The workflow described provides a predictive capability to apply ![]() ![]() In the model, for a relatively clean sand (clay content <15%) buried and compacted, faulting is shown to occur with a porosity in the faults lower than in the sand reservoir at a similar depth. Quartz cementation occurs in both the reservoir and the The brittleness as a function of the porosity is extended to all grain-supported rocks or sediments, as shown in Figure 6B for a sandstone with 25% clay with similar Gulf of Mexico burial histories used for the quartzarenite. Here the brittleness is greater in the faults at a shallower depth as in the more clay-poor quartzarenite described above. The results for a matrix-supported rock are controlled by the expected unconfined compressive strength of the overconsolidated section. The results in Figure 6C show that at depths greater than 3500 meters the shale is normally consolidated, but with shallower burial the rocks are overconsolidated and brittle with a greater brittleness more shallow in the section. Thus in this case, the brittle-ductile transition occurs at 3500 meters, which is a critical depth for the basin modeling. Determination of the expected cross The calculated ![]() ![]()
Fisher, Q.J., S.D. Harris, M. Casey, and R.J. Knipe, 2007, Influence of grain size and geothermal gradient on the ductile-to-brittle transition in arenaceous sedimentary rocks: implications for Marion, D., A. Nur, H. Yin, and D-H. Han, 1992, Compressional velocity and porosity in sand-clay mixtures, Geophysics, v. 57, p. 554-563. Revil, A., D. Grauls, and O. Brévart, 2002, Mechanical compaction of sand/clay mixtures: Journal Geophysical Research, v. 107(B11), p. 2293. Walderhaug, O., 1996, Kinetic modeling of quartz cementation and porosity loss in deeply buried sandstone reservoirs: AAPG Bulletin, v. 80, p. 731-745. We wish to thank Chevron Corporation for the permission to publish this work.
|