Natural fracture characterization in tight gas sandstones: Integrating mechanics and diagenesis Jon E. Olson, Stephen E. Laubach, and Robert H. Lander ABSTRACT Accurate predictions of natural fracture flow attributes in sand- stones require an understanding of the underlying mechanisms re- sponsible for fracture growth and aperture preservation. Poroelastic stress calculations combined with fracture mechanics criteria show that it is possible to sustain opening-mode fracture growth with sub- lithostatic pore pressure without associated or preemptive shear failure. Crack-seal textures and fracture aperture to length ratios sug- gest that preserved fracture apertures reflect the loading state that caused propagation. This implies that, for quartz-rich sandstones, the synkinematic cement in the fractures and in the rock mass props fracture apertures open and reduces the possibility of aperture loss on unloading and relaxation. Fracture pattern development caused by subcritical fracture growth for a limited range of strain histories is dem- onstrated to result in widely disparate fracture pattern geometries. Substantial opening-mode growth can be generated by very small ex- tensional strains (on the order of 10 -4 ); consequently, fracture arrays are likely to form in the absence of larger scale structures. The effec- tive permeabilities calculated for these low-strain fracture patterns are considerable. To replicate the lower permeabilities that typify tight gas sandstones requires the superimposition of systematic cement filling that preferentially plugs fracture tips and other narrower parts of the fracture pattern. INTRODUCTION The characterization of naturally fractured reservoirs continues to challenge geoscientists. Although a variety of diagnostic techniques depend on the wellbore for data, ranging from conventional fractured AUTHORS Jon E. Olson  Petroleum and Geosystems En- gineering, Cockrell School of Engineering, Univer- sity of Texas at Austin, 1 University Station C0300, Austin, Texas 78712; jolson@mail.utexas.edu Jon Olson is an associate professor in the Depart- ment of Petroleum and Geosystems Engineering. He joined the faculty in 1995. He has six years of industrial experience. He specializes in the appli- cations of rock fracture and continuum mechanics to fractured reservoir characterization, hydraulic fracturing, and reservoir geomechanics. He was a distinguished lecturer for AAPG in 2007–2008. Stephen E. Laubach  Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Aus- tin, Texas 78713-8924; steve.laubach@beg.utexas.edu Steve Laubach is a senior research scientist at the Bureau Economic Geology where he conducts re- search on unconventional and fractured reservoirs. His interests include fluid inclusion and cathodolu- minescence studies and application of borehole- imaging geophysical logs to stress and fracture evaluation. He was a distinguished lecturer for the Society of Petroleum Engineers in 2003–2004. Robert H. Lander  Geocosm LLC, 3311 San Mateo Drive, Austin, Texas 78738; roblander@geocosm.net Rob Lander develops diagenetic models for Geo- cosm LLC. He obtained his Ph.D. in geology from the University of Illinois in 1991, was a research geologist at Exxon Production Research from 1991 to 1993, and worked for Rogaland Research and Geologica AS from 1993 to 2000. He is also a re- search fellow at the Bureau of Economic Geology. ACKNOWLEDGEMENTS Our research on structural diagenesis is supported by the Chemical Sciences, Geosciences and Bio- sciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy grant DE-FG02-03ER15430 (Predicting fracture porosity evolution in sandstone), by industrial associates of the Fracture Research and Application Consor- tium and by Geocosm’s Consortium for Quantitative Prediction of Sandstone Reservoir Quality. We are grateful to L. M. Bonnell for discussion and to Tricia F. Allwardt and Laird B. Thompson for reviews. The AAPG Editor thanks the following reviewers for their work on this paper: Tricia F. Allwardt and Laird B. Thompson. Copyright ©2009. The American Association of Petroleum Geologists. All rights reserved. Manuscript received June 9, 2009; provisional acceptance July 21, 2009; revised manuscript received August 4, 2009; final acceptance August 11, 2009. DOI:10.1306/08110909100 AAPG Bulletin, v. 93, no. 11 (November 2009), pp. 1535 – 1549 1535