RATE CONSTRAINTS ON FAULT CEMENTATION BASED ON WELL SCALES

Rapid degassing occurs when pore fluids move from a high pressure reservoir into low pressure production tubing (ÄP up to 31 MPa). The resultant rapid degassing can cause precipitation of carbonate scales at rates up to 5 mm/year. Carbonate well scales from southern California hydrocarbon and geothermal reservoirs have a distinct isotopic signature characterized by (1) positive carbon isotopic values from CO2 loss, (2) oxygen values consistently more positive (up to 10‰) than expected for equilibrium, and (3) positive co-variation ratios of ä18O/ ä13C with slopes generally between 1 and 5. The discrepancy between equilibrium isotopic values and observed oxygen values increases with increasing ÄP and/or temperature. Elevated Mg trace element content and variation (than expected for the crystallization temperature) also occurs in some scales. These geochemical signatures are interpreted as indications of CO2 loss and rapid carbonate crystallization.

The scaling process might be analogous to degassing in fault zones during seismic events. Crustal rupture results in a drop in pore fluid pressure as pore fluids at deep levels in the crust connect with shallow levels. Although cementation in fault zones is similar to the well scaling process, our studies of fault calcite in southern California have shown that the fault zone cements generally lack the characteristic geochemical signatures of well scales described above. We conclude that these fault zone cements are not products of rapid crystallization from fault rupture, but result from infilling of open pore space over a longer time scale than the rupture interval. The volume of fluid available during the short rupture interval is likely insufficient to generate significant cement volume, and the short term, rapid, pressure drops may be relatively small. Nevertheless, the high volume of cement in some fault zones implies that these areas are relatively permeable for considerable time periods following seismic events. The cements probably have growth rates considerably less than 1 mm year, based on a comparison with well scales.