Casey Moore 氏（University of California, Santa Cruz）

「Geological Consequences, Overpressure, and Hydrofracture
Thresholds Due to Unequal Loading of Sedimentary Units,
at Passive, Transform, and Convergent Margins.」　

2006年4月26日

　Unequal loading of aquifers drives fluid flow with significant geological
consequences at transform, convergent, and passive margins. This talk
develops the examples of fluid flow at these three types of margins and
shows that a nearly isotropic state of stress is required to explain the
observations despite the margin’s differing tectonic stress regimes.

　The development of a pull-apart basin along the San Andreas Transform
Fault system resulted in a large difference in sedimentary loading in the
upper Miocene Santa Margarita Sandstone near Santa Cruz CA. The overlying
diatomaceous mudstone shows a variation in thickness from about 3 km at
the basin center to several hundred meters at the basin margin. At the
basin margin the underlying sandstone injects the overlying mudstone as
dikes and sills and also flowed onto the Miocene seafloor as a sand volcano.

　In the Late Cretaceous-Paleocene Great Valley Forearc Basin Sequence,
similar sandstone dikes and sills occur at the basin margin where the
overlying sedimentary cover is minimized. However the lateral variation
in thickness of the overlying mudstone sequence (1100m to ~ 800 m) is less
dramatic than in the San Andreas Fault system example. The abundance of
sills in both the Great Valley and San Andreas Fault system examples
unequivocally indicate that the fluid pressures reached lithostatic values.
In both examples, hydrocarbons reduced the density of fluids and
assisted in reaching fluid pressures equal to the overburden.

　In the Gulf of Mexico IODP Exp. 308 measured fluid pressures of 0.7 of
effective vertical stress in the muds overlying the unequally loaded
permeable sandy “Blue Unit”. Drilling and preliminary investigation of
the seismic and borehole imaging data indicate no evidence of dikes or
sills emanating from the Blue Unit. Yet, when penetrated by industry
wells, Blue Unit sand flows upward around well bores, locally reaching
the seafloor. Therefore, the fluid pressure conditions in the Blue Unit
must be very close to the hydrofracture threshold and this must be greater
than 0.7 of the vertical effective stress. Hydraulic fracture tests in
industry boreholes require ~ lithostatic stresses indicating a nearly
isotropic state of stress in sediments at less than 1 km depth. Therefore
nearly lithostatic fluid pressure would be required to initiate hydrofracture,
even in this passive margin setting.