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WH19: Calcite as an indicator of vertical fluid transport in hydrocarbon systems
J.R. Boles
University of California @ Santa Barbara. Santa Barbara, CA 93106

Calcite associated with faults in petroleum basins can provide evidence of the
timing and magnitude of vertical fluid transport.
A number of examples have been documented in the hydrocarbon basins of Southern California by our research at
UCSB.

The calcite is found within fault zones and the associated country rock.

Field and petrographic evidence, including breccia and twinned crystals, suggests
that calcite is formed at the time of deformation, rather than as a passive filling of
void space long after fault movement.

Uranium series dating of the calcite in some areas allows direct timing estimates of faulting and fluid movement.

Some of these systems were active as recently as a few hundred thousands years ago.

The carbon and calcium components of calcite come from separate sources.

The reaction of pore water with hydrocarbon releases dissolved carbon for calcite. In
cases in which the calcite 13C is particularly light (35-45 per mil), the carbon is
from oxidation of methane to carbon dioxide.

The source of the calcium for the calcite varies depending on whether the system is rich in clastic or biogenic
components.

In general, the calcite is not simply a result of calcite dissolving deep in the basin and precipitating at shallow levels in fault zones.

Several techniques have been used to demonstrate the magnitude of vertical fluid transport.
One is comparing crystallization temperatures of calcite within the fault to the maximum burial temperature for the wall rock.
A second technique is the application of Sr isotope stratigraphy in marine rocks.

Given the fact that there are predictable changes in Sr isotopic ratio of Tertiary marine biogenic carbonate, it is
possible to infer the vertical component of transfer from the Sr isotopic ratio of the
calcite vein-fill.

This has been particularly successful in the biogenic-rich, clasticpoor Miocene Monterey and equivalent strata in the Southern California area, where
interference from clastic components is minimal.

Several examples will be discussed from the Santa Barbara coastal area and the San Joaquin basin of California.

Both areas have undergone transpression during the late Tertiary.
These examples demonstrate that the vertical component of fluid transport associated with faulting is on the order of one to two kilometers.

In some examples, the rupture of an over-pressured section is required to explain the magnitude of vertical transport and the crystallization temperatures.