The Carbon Isotope Stratigraphy of Mid-Cretaceous Shallow Water Carbonates: Improved Chronostratigraphy and other Implications

Volker C. Vahrenkamp
Abu Dhabi Company for Onshore Oil Operations ([email protected])

Carbon isotope profiles fromancient carbonate sequences provide fascinating insight into the dynamics of ocean chemistry and climate fluctuations of the past. In addition, they provide the geologists with an excellent tool to enhance time resolution in shallow-water carbonate rock sequences, which are commonly poorly constrained by biostratigraphy. This is a particular problem for the vast oil-prone shallow water carbonate platforms that covered the Middle East region during the Cretaceous.
In order to overcome this problem a composite carbon isotope curve has been assembled for the Barremian to Turonian time interval from subsurface cores and published outcrop data of Oman. This sequence contains some of the most prolific oil reservoirs in the Middle East: The Barremian Thamama B/Kharaib Fm, the Aptian Shuaiba Fm and the Albian/Cenomanian/Turonian Natih/Mauddud/Mishrif Fms.
Overall the curve has an excellent match with published reference curves from pelagic sections of the tethys and hence provides a much improved time resolution compared to biostratigraphy available for these shallow water carbonate sequences. Several globally occurring Oceanic Anoxic Events (OAE’s) are clearly discernible and hence provide the link to well dated pelagic sections.
The talk will detail fluctuations in carbon isotope ratios, chronostratigraphy, discuss the implication for platform evolution when correlating curves from different localities and finally speculate on how this might all be related to fossil content, climate fluctuations and even the catastrophic release of submarine-methane hydrates.

AAPG Search and Discovery Article #90073 © 2007 AAPG Foundation Distinguished Lecturer Series 2007-2008