--> Abstract: Focused Fluid Migration along Faults in the Miocene Monterey Formation, California, by P. Eichhubl and J. R. Boles; #90937 (1998)

Datapages, Inc.Print this page

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