Fracture swarms associated with subtle structures greatly enhance production from tight-gas sandstones of the Mesaverde Group and Dakota Formation in the San Juan Basin. The structures include grabens, horsts, and normal faults, and they can be identified using curvature analyses of horizons mapped in three-dimensional seismic data. Their orientations and styles are consistent with the orientation of fractures that have been identified by other authors using outcrop, core, borehole imagery, and production analyses. Integration of production data (rate-versus-time plots) demonstrates the existence of a drainage interference for wells located on some of the structures. This observation further testifies to a positive correlation between the presence of subtle structures, high fracture intensity, and high fracture permeability. The methods and results described herein can be directly applied to other areas. However, because natural fractures can both enhance and retard hydrocarbon production depending on their character, calibration of seismic, engineering, and other data types will be needed to determine whether subtle structures should be considered as drilling targets, or whether they are to be avoided.

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