| Abstract |
Geological and geophysical evidence from The Geysers reservoir is consistent with both uniaxial tectonic extension and episodes of extension at times of magmatic intrusion. The creation of new fracture volume within an initial low porosity, liquid-dominated reservoir may cause and sustain vapor-dominated conditions. Simple, two-dimensional modeling of a dilating reservoir shows that a super-heated zone may underlie normal vapor-dominated conditions. These conditions form with either episodic or continuous dilation. Rapid venting or a sustained period of net mass loss from the reservoir are not essential. A small amount of meteoric recharge and steam loss at the surface causes the super-heated conditions to locally disappear, and these models are remarkably similar to the lateral transition from normal vapor-dominated to superheated conditions seen at The Geysers. Sustained vapor-dominated conditions still require relatively sealed boundaries, which in an extensional environment implies a self sealing mechanism on boundary fractures, ductile deformation, or movement on low angle faults. Additional modeling is needed to test in more detail the dilation hypothesis against known reservoir characteristics. |