Reed, Robert M. and Laubach, Stephen E. (1998) Density and distribution of microfractures in sandstones: importance to diagenesis.

an abstract from the 1998 AAPG meeting

Summary

Systematic, quantitative measurements of quartz-lined microfractures in more that 20 siliciclastic units show that these structures are ubiquitous in quartz-cemented rocks. Theories of quartz cement development need to take these features into account.

Introduction

Quartz cement and natural fractures are two of the most critical controls on reservoir quality. Accurate prediction of reservoir quality would be improved if mechanisms controlling the development of quartz cement and fractures were better understood. Our research shows that in a large number of hydrocarbon-producing, low-porosity sandstones, the processes of quartz cementation and fracturing are spatially, temporally, and probably genetically, linked. These results provide an important constraint on models of reservoir quality.

Observations

Quartz-lined microfractures visible using photomultiplier-based cathodoluminescence systems (scanned CL) occur in high but variable densities and account for significant cross-sectional areas of 22 different siliciclastic units that are primarily flat-lying and macroscopically undeformed (i.e., no nearby faults). Lithologies vary widely but in general most units are quartzose, and have low porosity (<15%) and some quartz cement. Some samples contain macrofractures and some do not, and some contain stylolites and some do not.

Observed microfractures range in width from less than a micron to tens of microns. Lengths range from microns to hundreds of microns, grading into quartz-lined macrofractures. Microfracture densities range from 34 to 151 microfractures per mm2. Using measured lengths and widths of microfractures, minimum areas were calculated as half of length times maximum width (double wedge approximation of fracture shape). In the samples studied, microfractures make up from 0.53 to 2.15 areal percent of the rock. Fracture length, width and density are the principal controls. High microfracture densities alone do not necessarily translate to large microfracture areas since many fractures are small.

Textural evidence shows that quartz-lined microfractures formed over the duration of diagenesis, allowing them to have a persistent influence on the process. During the early compactional phase of the diagenetic history, while primary porosity is being reduced by grain rearrangement, many short, wide, intragranular fractures form. Grain-scale areas of high microfracture densities are created. Later in the diagenetic history (as measured by cement zoning and mineral precipitation sequence), longer, narrower, transgranular microfractures are common. These can also locally form areas of high microfracture density.

Conclusions

Quartz-lined microfractures are widespread and locally abundant in a large number of siliciclastic units with a wide range of burial histories from a variety of tectonic settings. Given measured high densities and areas, quartz-lined microfractures cannot be ignored when interpreting quartz cement budgets and fluid flow during diagenesis. Fractures provide conduits for fluid access in otherwise low-permeability rock. For example, high fracture density adjacent to stylolites may allow fluid transfer into and away from dissolution sites. Also, in already well-cemented rocks, microfractures can play a prominent role in porosity reduction by allowing fluid access to otherwise isolated pores.

Acknowledgments

Supported by The University of Texas industry-sponsored natural fracture project and the U.S. Department of Energy.