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.