Degradation

At depth in the Earth, rocks are geochemically reactive environments. In the presence of hot water, both cement precipitation and dissolution can occur. In many rocks, cement deposits are a principal control on whether or not fractures are open. In sandstone, for example, core observations show that in a given play, field, or bed some large fractures (width > 0.1 mm) are open but lined with cement, usually quartz, whereas others have intermediate degrees of infill and others of comparable size are completely sealed, often with phases other than quartz.

Figure 1 An open quartz-lined fracture from core and a quartz-lined, calcite sealed fracture from outcrop, from Almansour et al. 2019.

An example of the differences in degree and patterns of fill from core and from outcrop is illustrated in figure 1. Qb and Qr mark quartz bridges and rinds, respectively; P is porosity, C is calcite, and F marks fracture walls.

Empirical evidence shows that some cement phases fill large fractures more readily than others. Following Laubach (2003), we infer that there may be two main structural diagenetic processes at work.

The first process, pertaining to the accumulation of quartz, as described by Lander and Laubach (2015), produces bridge-and-rind structure in sandstone fractures and predicts that open fracture pore space can persist for millions of years at high temperature (read the blog post).

A recently accepted paper by Denny et al. advances our understanding of this process. The Denny et al. paper will be summarized in a separate blog post.

The second process, pertaining to the accumulation of calcite and many other phases, results in some large fractures that ‘ought’ to be open being sealed. In a paper from 2003 in AAPG Bulletin, Laubach showed that in some cases, the switch from open to sealed fractures can occur over quite short distances, i.e., well-to-well or bed-to-bed. Laubach (2003) called this phenomenon of large fractures being variably sealed by calcite and other phases ‘degradation’.

The process that causes this variability in the sealing of large fractures is not very well understood, but contrasts in the preserved porosity of wide fractures have been correlated with differences in fluid flow and oil and gas production. If open fractures correspond with more productive zones, their location is potentially vital information for decision makers.

Two new papers address the degradation phenomenon.

Unraveling the degradation process

In research aimed at understanding the processes that seal some large fractures and not others, Weisenberger et al. investigated the geochemistry of calcite and other phases that accumulate in Cretaceous Mesaverde Group sandstone fractures and the fracture host rocks. The opening-mode fractures are lined or filled by quartz and, locally, calcite cement. Fracture occlusion by quartz is controlled primarily by fracture size, age and thermal history, as documented by Fall et al. (2015). Fracture occlusion by calcite is highly heterogeneous, with open and calcite-sealed fractures found at adjacent depths.

In the Piceance basin, where Weisenberger et al.’s samples were obtained, and in other basins, processes that control the distribution of these calcite cements have been uncertain. They might reflect fluid flow patterns, or rock composition, or something else. Using pore and fracture cement petrography, fluid inclusions, and isotopic and elemental analysis, the authors show that porosity degradation and occlusion of fractures >1 mm wide by calcite is governed by host-rock calcite distribution and remobilization.

87Sr/86Sr ratios of calcite and the presence of porous albite suggest that detrital feldspar albitization released Ca2+, driving carbonate cement precipitation. In host rock, both albite and calcite content decreases with depth along with greater fracture porosity preservation. 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. Fluid-inclusion analyses indicate calcite cement precipitated at 135–165°C.

Weisenberger et al. found that the rock-mass calcite cement content correlates with fracture degradation and occlusion, and can be used in this case to accurately predict where wide fractures are sealed or open.

Although clearly much remains to be learned about how the degradation process works, this example shows that accurate predictions of sealing patterns based on a fundamental understanding of calcite accumulation may someday be possible.

Lead author Tobias Weisenberger was a postdoctoral fellow in the Structural Diagenesis Initiative when the work was conducted. He took a faculty position at the University of Oulu and is now a research geochemist for the Iceland GeoSurvey (ÍSOR), in Reykjavik, Iceland.

Degradation consequences

In a study seeking to assess the practical value of predictions of open and sealed fractures, Almansour et al. used a core-based fracture prediction method based on the principles of structural diagenesis and degradation to illustrate a Value of Information (VOI) decision-analysis protocol to inform completion decisions in tight-gas sandstones.

The ratio of late host-rock cement to available pore volume or degradation index uses petrographic observations of cement distributions in core (including sidewall cores) to predict whether nearby but unsampled fractures (widths > 0.5 to 1 mm) are sealed (non-conductive) or open (conductive). It does not rely on process-based understanding such as is represented by the research reported in Weisenberger et al.

Almansour et al. found that measurements from four sandstone plays suggest that the index correctly predicts open vs. sealed fractures with an accuracy in excess of 80%.

The value added is calculated using Bayesian inference in which accuracy of the index serves as the likelihood of the prior distribution of open fractures to assess the posterior probability that data represent a useful predictor of producibility. Software to calculate VOI is provided as part of the paper.

VOI of the prediction method is more than three times the cost to acquire the data. VOI is most sensitive to play-specific geologic and cost parameters including cost to drill, expected revenue from a successful well, cost of completion, cost of acquiring data for the index, and fracture probability distributions.

The VOI approach provides a way to value acquiring fracture data and the example points to a need for zone-specific production data in tight-gas sandstones.

Lead author Abdulaziz Almansour is a graduate of the Energy and Earth Resources (EER) Graduate Program at The University of Texas at Austin and the material reported in this paper formed part of his EER Master’s thesis. He is now a Reserves Geologist working in the Exploration Resource Assessment Department at Saudi Aramco.

Research on all aspects of cement accumulation in fractures is ongoing.

New papers discussed here

Almansour, A., Laubach, S.E., Bickel, J.E., and Schultz, R.A., 2019. Value of Information analysis of a fracture prediction method. SPE Reservoir Evaluation & Engineering. doi: 10.2118/198906-PA | view at publisher | Access VOI software

Weisenberger, T., Eichhubl, P., Laubach, S.E., and Fall, A., 2019. Degradation of fracture porosity by carbonate cement, Piceance basin, Colorado, USA. Petroleum Geoscience. doi:10.1144/petgeo2018-162 | view at publisher

Background reading

Fall, A., Eichhubl, P., Bodnar, R.J., Laubach, S.E., Davis, J.S., 2015. Natural hydraulic fracturing of tight-gas sandstone reservoirs, Piceance Basin, Colorado. Geological Society of America Bulletin 127(1-2), 61-75. doi:10.1130/B31021.1 | view at publisher

Lander R.H., and Laubach, S.E., 2015. Insights into rates of fracture growth and sealing from a model for quartz cementation in fractured sandstones. Geological Society of America Bulletin 127 (3-4), 516-538. doi: 10.1130/B31092.1 | view at publisher | blog post

Laubach, S.E., 2003. Practical approaches to identifying sealed and open fractures. AAPG Bulletin 87(4), 561-579. doi:10 1306/11060201106 | view at publisher

Related forthcoming paper

Denny, A.C., Fall, A., Orland, I.J., Valley, J.W., Eichhubl, P., Laubach, S.E., 2019. A history of pore water oxygen isotope evolution in the Cretaceous Travis Peak Formation in East Texas. Geological Society of America Bulletin, doi:10/1130/B35291.1 | view at publisher | blog post