Fracture Analysis of the Travis Peak Formation, Western Flank of the Sabine Arch, East Texas

Successful development of low-permeability-sandstone gas reservoirs depends on an understandingof their natural fracture patterns and on predictions of the orientation of horizontal stresses.This report describes the preliminary results of fracture analysis of the Lower Cretaceous TravisPeak Formation, a tabular sandstone and shale unit approximately 2,000 ft (610 m) thick thatproduces gas from low-permeability (less than 1 md) sandstone in much of East Texas. Depth to thetop of the formation ranges between 5,900 and 9,500 ft (1,798 and 2,895 m) in the study area. Thestudy is based on analysis of 565 natural and coring-induced fractures in more than 2,100 ft (640 m) ofwhole core from 8 wells in the Travis Peak Formation, together with borehole televiewer and otherfracture-imaging logs from 3 wells. Natural fractures in the formation strike east-northeast tonortheast and provide potential natural conduits and reservoirs for gas. Quartz cementation andfracture formation were contemporaneous. Microstructures within fracture-filling quartz indicate thatextension fractures in sandstone result from natural hydraulic fracturing. The study also indicatesthat healed microfractures are useful for mapping fracture trends and that the orientation of coring-inducedfractures can be used to infer stress directions and the propagation direction of fracturescreated in hydraulic fracture treatment. The model of natural fracture development predicts thatfractures are most abundant in rocks with the most quartz cement. Natural fractures deserve carefulconsideration in the design of hydraulic fracture treatment for reservoir stimulation.Correlation of the sequence of vein-mineral precipitation with diagenetic history suggests thatnatural extension fractures in sandstone propagated at depths of between 3,000 and 5,000 ft (914 and1,524 m) during the migration of quartz-precipitating fluids. Previous 6180 studies of quartz cement inthe Travis Peak suggest that the fluids may have been deeply circulating meteoric water. Fracturepermeability, which is indicated by results of this study, provides a mechanism for the passage of largefluid volumes recorded by extensive precipitation of quartz cement. Quartz precipitation lowerspermeability and restricts fluid flow, thus raising fluid pressure, resulting in intermittent fracturing andsealing ("crack-seal" deformation). Fractures stopped propagating before maximum burial depth wasachieved. One characteristic of Travis Peak sandstone is preservation of open fractures with widthsof as much as 0.2 inch (5 mm) to depths of as much as 9,950 ft (3,032 m) in rocks that currently havenear-hydrostatic fluid pressures.East-northeast-trending coring-induced petal-centerline fractures are parallel both to fracturescreated in hydraulic fracture treatments and open-hole hydraulic stress tests and to the direction ofmaximum horizontal strain recovery measured with anelastic strain recovery experiments. Coring-inducedfractures are therefore useful indicators of horizontal stress trajectories in this area. Theaverage strike of natural fractures is close to the average strike of coring-induced fractures, butnatural and coring-induced fractures are commonly not parallel. This indicates that stimulation fractures may not propagate exactly parallel to natural fractures in the Travis Peak Formation.
Stephen E. Laubach

Laubach, S. E., 1989, Fracture Analysis of the Travis Peak Formation, Western Flank of the Sabine Arch, East Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 185, 55 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation