Methanogenesis during Shu’aiba diagenesis: examples from Al Shaheen Field, Block 5, offshore Qatar

Evidence has been found for the involvement of bacterial methanogenic processes in the diagenesis of late Early Aptian Shu’aiba Formation sedimentary rocks from Block 5, offshore Qatar. This is in the form of δ13C values significantly higher than the presumed late Early Aptian marine signal. High δ13C values were recorded in early-formed inclusion-rich calcite that represents neomorphosed aragonite/marine cement (+6.6‰) and the latest precipitated blocky non-ferroan calcite cement (+8.9‰). Oxygen-isotope data suggest that the later methanogenic episode occurred during burial. In bulk rock samples from another well, δ13C values of +6.5 to +11.5‰ were determined from a 10 ft thick zone located 4 ft below the top of the Shu’aiba. In other wells for which data were available the equivalent interval of the Shu’aiba was characterised by expected marine δ13C values. Finally, δ13C values up to +9.1‰ were recorded in a localised area of dolomite replacement immediately beneath the Top Shu’aiba surface, which can be dated to after the onset of Nahr Umr Formation deposition. These are the first reported examples of methanogenic reactions affecting the Shu’aiba Formation and provide new insight into the diagenesis of this formation, which is often assumed to be dominated by meteoric processes. The evidence presented for the resetting of the carbon-isotope composition of the bulk rock has implications for the application of carbon-isotope stratigraphy to the Shu’aiba Formation.