Abstract

Whole rock stable isotope records are increasingly used to understand geochemical records within carbonate successions. The carbon isotopic system commonly is used to understand primary ocean productivity and decipher long-term trends in carbon burial because it is relatively cheap, easy to sample from cores or cuttings, and it generally retains it’s original signature through most diagenetic events. Case studies from the Early Cretaceous (Aptian) Shuaiba Formation in Saudi Arabia and Cenomanian-Turonian Eagle Ford Group in south Texas demonstrate how high-resolution chemostratigraphy can be used to resolve correlation problems and chronostratigraphy within a carbonate sequence stratigraphic framework.

The Shuaiba Formation is an extensive carbonate platform that formed adjacent to an intrashelf basin in the Aptian. Within the study area the carbonate platform is covered by an extensive sand sea so high-resolution seismic data does not exist. However, Saudi Aramco has an extensive core and wireline log repository in this field. Almost 2400 stable isotope samples from 21 cores (average 137 m thick) were analyzed to resolve correlation problems across the shelf-to-basin margin of this carbonate platform and provide some level of chronostratigraphy within the proposed sequence stratigraphic framework. The stable isotopic records, in conjunction with available biostratigraphy, indicate the Shuaiba carbonate platform grew up to sea level and filled available accommodation then became exposed during the Early Aptian. While the carbonate platform was exposed, extensive progradation of the carbonate margin into the intra-shelf basin occurred during the late Aptian.

The Cenomanian-Turonian Eagle Ford Group is a prolific carbonate-rich unconventional reservoir in south Texas. This unit contains an isotopic record of Ocean Anoxic Event 2 (OAE2), one of the largest of these events in Earth’s history, characterized by a large (2 to 4 ±) positive carbon isotopic shift. However, detailed isotopic studies and biostratigraphy suggest the outcrops do not record the entire hiatus, and that its upper part was eroded by a regional unconformity. The outcrops of the Eagle Ford and its equivalents are being correlated via the isotopic shift and this correlation, constrained by ash bed geochronology and biostratigraphy will likely indicate differential areas of subsidence that may well relate to reservoir quality.