Abstract

It is known that economic production of shale gas is currently possible only where shale is naturally fractured. The exploration problem is therefore to locate zones of fracture. In principle, extensive zones of natural fracture in a thick shale can be identified by the measurement of seismic velocity from the surface. The technique of reflection velocity analysis uses differential reflection time of reflections observed at the surface (with various source-geophone distances) to infer the extent of the gas saturated fracture zones. This technique has been applied along an available seismic line in Southeastern Ohio. A continuous velocity analysis of the shale interval along this line was prepared, and areas of depressed velocity have been identified. These results, and the results of drilling in the first area of low velocity will be discussed.

Introduction

Large volumes of shale gas are contained within the Devonian shales of the Appalachian Basin. In general, however, the economics of exploring for and producing this gas are adverse; although the shale is producing this gas are adverse; although the shale is widespread over the basin, only a few fields are currently producing significant gas. Over the years, it has become clear what distinguished the viable shalegas location from the uneconomic location; a good shale-gas well requires that the shale be fractured so that paths exist for the gas to flow from a large volume of shale to the well bore. A thick section of black gas-source shale is also desirable. By now, the variations of thickness of the source shale over the Appalachian Basin are well documented; maps showing these variations have been prepared under other DOE contracts, and are publicly available. The exploration problem therefore reduces simply to this: How can we problem therefore reduces simply to this: How can we locate, from the surface, zones of extensive fracture in the shale?

A current technique for locating zones of fracture includes the use of aerial photographs and satellite images to find linear alignments of surface features, such as faults, linear valleys, or other surface changes. It is hoped that fractured zones far below the surface can be identified using this technique. One weakness of this approach is its assumption that the faults or fractures are vertical. In general, fractures caused by earth tides can probably be assumed to be vertical, but faults associated with tectonic movements are often not so.

Another weakness of the approach is that fault and fracture planes represent escape paths for gas; if the planes extend to the surface, the inference may be that all the gas has gone. This had led to the suggestion that the best place to drill is not on the lineament itself, but some distance from it; the hope is that the forces which produced a fault or fracture large enough to show at the surface are likely to have produced minor fracturing for some distance from the major feature. There is still no assurance that these minor fractures do not connect to the vent. Nor is anyone quite sure of the "best" distance from the lineament.

A second approach to locating zones of fracture is to use knowledge of faults obtained from subsurface well control or seismics. Since these methods identify a fault only by its displacement, fractures without throw are not revealed. The merit of this approach, however, is that faults can be identified which do not reach the surface.

The age of the faulting is important. Young shales, of course, tend to be plastic; a shale as old as the Devonian becomes brittle and capable of fracture. Only faults much younger than the Devonian are likely to have engendered fracturing; the ideal is probably repeated rejuvenation of deep basement movement. Some feeling for this can be obtained, from well control and seismics, by studying the faulted intervals at different levels.

Further, simple flexure of the whole geologic section, without faulting, may be sufficient to cause fracture in the shale.

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