Abstract: Focused Fluid Migration along Faults in the Miocene Monterey Formation, California
EICHHUBL, PETER, Dept. of Geological and Environmental Sciences, Stanford University; and JAMES R. BOLES, Dept. of Geological Sciences, University of California, Santa Barbara
The effect of faults on focusing basinal fluid flow in the
Monterey Formation may be quantified by combination of strontium
isotope anaylses of carbonate cement and mass balance calculations
of fluid required to precipitate observed volumes of fracture
cement. Strontium isotopes may serve as natural tracer for
cross
-stratigraphic mass transport, and mass balance calculations
may provide estimates of formation-parallel fluid flow.
A fault zone and connected veins at Jalama Beach, Santa Barbara
County, contain dolomite cement of about 8 m cumulative thickness.
The 87Sr/86Sr composition of dolomite vein
and fault cement averages 0.70882, as compared to 0.70897 for the
country rock dolostone. Other studies have found a good correlation
between strontium isotopic ratio and biostratigraphic age of
Monterey dolostone. Assuming that abundant carbonate in the
dolomite-rich section
at Jalama Beach buffers possible diagenetic
changes in 87Sr/86Sr fluid composition, the
difference in 87Sr/86Sr composition of
dolomite vein and fault cement and host dolostone suggests 260 m of
upward
cross
-stratigraphic fluid flow. If it is assumed that
strontium enters the fault zone at uniform concentration over the
whole depth interval transected by the fault rather than at a
single depth, the inferred distance of upward
cross
-stratigraphic
flow is about 640 m, close to the total thickness of the
section
of
about 700 m at Jalama Beach.
A volume estimate of fluid required for vein and fault
cementation is obtained from the inferred change in CO2
partial pressure as fluid moves upsection. Based on a
CO2(gas) mole fraction of 0.07 in Monterey hydrocarbon
reservoirs. and Henry's Law, the drop in CO2(aqu)
during upward flow is calculated. Upward flow by 640 m and an
assumed pressure drop from an initial 1.27 MPa/km to a final
hydrostatic pressure gradient would precipitate about 0.12
cm3/L dolomite. The initial pressure corresponds to
measured pressure gradients in the formation. For the total
dolomite cement thickness of 8 m, a minimum of 6.7x104
m3 fluid per meter fault length and meter fault depth is
required. Based on the required fluid volume, likely minimum
distances of radial flow towards the fault zone and parallel to the
formation are 3-10 km or about 10 times the cross
-stratigraphic
flow distance. The faults are thus effective in focusing fluid
laterally and expelling fluid towards the surface.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah