Hutcheon et al, (1989)  Diagenesis and sedimentology of the Clearwater
Formation at Tucker Lake,  
Bull. Canadian Petroleum Geology Vol 37, 83-97,  give  del/delta 13C values
for calcite as high as +17.5 %o.
The data are in general agreement with the work of Dimitrakopoulos and
Muehlenbachs (1989) .
Accepting the latters' explanation of degradation of bitumen followed by
microbial fermentation 
could in principle generate even isotopically heavier CO2 and in turn
calcite. 
The first reference draws attention to the desirability of determination of
the isotopic compositons
of detrital, pre-bitumen and post-bitumen calcite, since most reported data
involve mixtures thereof.
... a good topic for in situ microscopic isotope analyses!