Formation-parallel focusing of fluid flow along faults in the Miocene Monterey Formation, coastal California.

EICHHUBL, Peter, and BOLES, James. R.
Geological Sciences, University of California, Santa Barbara, CA 93106,

Strontium isotopic analyses of carbonate cement and mass balance calculations of fluid required to precipitate observed volumes of fracture cement provide an estimate of fluid focusing along two faults exposed at Jalama Beach, Santa Barbara County. Fault zones and connected veins contain dolomite cement of about 8 m cumulative thickness. Strike-slip displacement along the high-angle faults is inferred to be 100-200 m.

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 total thickness of the section of about 700m 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 600 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.