Paleo Environment Analysis Using Geochemical Data of the Intershelf Basin in Cenomanian, Cretaceous of Abu Dhabi

Abstract

The central part of the Cenomanian intrashelf basin in Abu Dhabi is composed of mudstones with high TOC concentration. The mudstone with high TOC in the Cenomanian interval is divided into three zones as named of OAE1d, MCE, and OAE2 of OAE (Ocean Anoxic Events). In this study, the two wells (Well A and Well B) were re-evaluated to confirm carbon isotope curve (chemostratigraphy) and geochemical/biomarker analysis including TOC measurement and to consider paleo environment developing these mudstones within the intrashelf basin. Regarding this intershelf basin, there are several papers to study of the relationship between TOC (Total Organic carbon) and OAE (Ocean Anoxic event) using geochemical analysis. We conducted core observations, GR log correlation, carbon and oxygen isotope analysis, trace elements analysis, mineral mapping, TOC analysis and biomarker analysis of two wells. Sampling was taken at 3ft for both wells. The results of carbon isotope were almost the same as the OAE zone (three OAE zone from lower to upper OAE1d, MCE and OAE2) within previous studies. Flat ammonite/inoceramus in OAE1d mudstone were recognized in both wells. It is highlighted that the ammonite/inoceramus indicates a good example to understand the productivity and preservation of the OAE. The cross lamination of mudstone contained ammonite/inoceramus had not received any bioturbation, therefore, the OAE1d interval is considered to have been buried earlier than biodegradation. This mudstone shows the alternation of gray- and dark-colored (each thickness is 10-30cm) and high TOC contents around 11-13%. However, mineral mapping suggested that illite and pyrite were only partially confirmed in well B. Same samples were increasing C35 Homohopane. Regarding paleo topography, well B was located near hinterland area, which provided carbonate debris with clay minerals. In addition, the results of two wells are plotted with variation of C28 sterane in the ternary diagram. However, oil samples in the Afield located on east side of intershelf basin are plotted in the almost same area in the ternary diagram. The differential was dependent on seawater circulation between intrashelf basin and open marine of Eastern Tethys. By combining the newest technologies like carbon Isotope, ICP-MS, mineral mapping analysis and biomarker analysis, this allows an in-depth understanding of regional geological interpretation. In addition, core and thin section observation is an essential part to the geological understanding.