Identifying dominant fracture orientations

1. A computer-implemented method for identifying dominant fracture orientations in a subterranean zone, the method comprising:

receiving microseismic event data associated with a fracture treatment of a subterranean zone;

identifying coplanar subsets of microseismic events in the received microseismic event data, each of the coplanar subsets comprising a non-collinear triplet of microseismic events in the received microseismic event data;

determining a basic plane orientation for each of a plurality of basic planes defined by the coplanar subsets, each basic plane orientation being defined by the non-collinear triplet of microseismic events in a respective one of the coplanar subsets;

calculating, by data processing apparatus, the quantity of the basic plane orientations in each of a plurality of orientation ranges;

identifying a dominant fracture orientation for the subterranean zone based on one or more of the quantities;

identifying a cluster of microseismic events associated with the identified dominant fracture orientation;

generating a fracture plane based on the identified dominant fracture orientation and the identified cluster of microseismic events associated with the identified dominant fracture orientation; and

displaying a histogram of the basic plane orientations, wherein the histogram indicates the quantity of basic plane orientations in each of the orientation ranges.

2. The method of claim 1 , wherein the quantity of the basic plane orientations includes a probability value.

3. The method of claim 1 , wherein each basic plane orientation includes a strike angle and a dip angle for one of the basic planes.

4. The method of claim 3 , further comprising identifying the plurality of orientation ranges based on the basic plane orientations by:

sorting the strike angles;

identifying clusters of the sorted strike angles;

sorting the dip angles;

identifying clusters of the sorted dip angles; and

defining the orientation ranges based on the clusters of sorted strike angles and the clusters of sorted dip angles.

5. The method of claim 1 , wherein the plurality of orientation ranges are fixed values determined independent of the basic plane orientations.

6. The method of claim 1 , further comprising:

identifying each coplanar subset of microseismic events from the microseismic event data; and

computing a normal vector for the basic plane defined by each coplanar subset, wherein the basic plane orientations are computed based on the normal vectors.

7. The method of claim 1 , wherein identifying a dominant fracture orientation includes identifying a plurality of dominant fracture orientations, and identifying the plurality of dominant fracture orientations includes identifying the orientation ranges corresponding to local maxima in a histogram of the basic plane orientations.

8. The method of claim 1 , wherein identifying a dominant fracture orientation includes identifying a plurality of dominant fracture orientations, and the method further comprises:

identifying a cluster of microseismic events associated with each of the dominant fracture orientations; and

generating a dominant fracture plane for each dominant fracture orientation based on fitting the respective cluster of microseismic events.

9. The method of claim 1 , wherein identifying coplanar subsets of microseismic events in the received microseismic event data comprises identifying all coplanar subsets of microseismic events in the received microseismic event data.

10. A non-transitory computer-readable medium encoded with instructions that, when executed by data processing apparatus, perform operations comprising:

receiving microseismic event data associated with a fracture treatment of a subterranean zone;

identifying coplanar subsets of microseismic events in the received microseismic event data, each of the coplanar subsets comprising a non-collinear triplet of microseismic events in the received microseismic event data;

determining a basic plane orientation for each of a plurality of basic planes defined by the coplanar subsets, each basic plane orientation being defined by the non-collinear triplet of microseismic events in a respective one of the coplanar subsets;

calculating the quantity of the basic plane orientations in each of a plurality of orientation ranges;

identifying a dominant fracture orientation for the subterranean zone based on one or more of the quantities;

identifying a cluster of microseismic events associated with the identified dominant fracture orientation;

generating a fracture plane based on the identified dominant fracture orientation and the identified cluster of microseismic events associated with the identified dominant fracture orientation; and

displaying a two-dimensional histogram of the basic plane orientations, wherein the histogram indicates the quantity of basic plane orientations in each of the orientation ranges.

11. The computer-readable medium of claim 10 , wherein each basic plane orientation includes a strike angle and a dip angle for one of the basic planes, and the operations further comprise identifying the plurality of orientation ranges based on the basic plane orientations by:

sorting the strike angles;

identifying clusters of the sorted strike angles;

sorting the dip angles;

identifying clusters of the sorted dip angles; and

defining the orientation ranges based on the clusters of sorted strike angles and the clusters of sorted dip angles.

12. The computer-readable medium of claim 10 , wherein the plurality of orientation ranges are fixed values determined independent of the basic plane orientations.

13. The computer-readable medium of claim 10 , wherein the operations further comprise:

identifying each coplanar subset of microseismic events from the microseismic event data; and

computing a normal vector for the basic plane defined by each coplanar subset, wherein the basic plane orientations are computed based on the normal vectors.

14. The computer-readable medium of claim 10 , wherein identifying a dominant fracture orientation includes identifying a plurality of dominant fracture orientations, and the operations further comprise:

identifying a cluster of microseismic events associated with each of the dominant fracture orientations; and

generating a dominant fracture plane for each dominant fracture orientation based on fitting the respective cluster of microseismic events.

15. The computer-readable medium of claim 10 , wherein identifying coplanar subsets of microseismic events in the received microseismic event data comprises identifying all coplanar subsets of microseismic events in the received microseismic event data.

16. A system comprising:

a computer-readable medium that stores microseismic event data associated with a fracture treatment of a subterranean zone; and

data processing apparatus operable to:

receive microseismic event data associated with the fracture treatment of the subterranean zone;

identify coplanar subsets of microseismic events in the received microseismic event data, each of the coplanar subsets comprising a non-collinear triplet of microseismic events in the received microseismic event data;

determine a basic plane orientation for each of a plurality of basic planes defined by the coplanar subsets, each basic plane orientation being defined by the non-collinear triplet of microseismic events in a respective one of the coplanar subsets;

calculate the quantity of the basic plane orientations in each of a plurality of orientation ranges;

identify a dominant fracture orientation for the subterranean zone based on one or more of the quantities;

identify a cluster of microseismic events associated with the identified dominant fracture orientation;

generate a fracture plane based on the identified dominant fracture orientation and the identified cluster of microseismic events associated with the identified dominant fracture orientation; and

display a histogram of the basic plane orientations, wherein the histogram indicates the quantity of basic plane orientations in each of the orientation ranges.

17. The system of claim 16 , wherein each basic plane orientation includes a strike angle and a dip angle for one of the basic planes, and the data processing apparatus is further operable to identify the plurality of orientation ranges based on the basic plane orientations by:

sorting the strike angles;

identifying clusters of the sorted strike angles;

sorting the dip angles;

identifying clusters of the sorted dip angles; and

defining the orientation ranges based on the clusters of sorted strike angles and the clusters of sorted dip angles.

18. The system of claim 16 , wherein the plurality of orientation ranges are fixed values determined independent of the basic plane orientations.

19. The system of claim 16 , the data processing apparatus is further operable to:

identify each coplanar subset of microseismic events from the microseismic event data; and

compute a normal vector for the basic plane defined by each coplanar subset, wherein the basic plane orientations are computed based on the normal vectors.

20. The system of claim 16 , the data processing apparatus is operable to identify all coplanar subsets of microseismic events in the received microseismic event data.