U-Pb Dating of Diagenetic Carbonates in Petroleum Reservoirs: Recent Advances and Perspectives

Abstract

The comprehension of the diagenetic evolution of petroleum reservoirs usually relies on relative chronology sometimes derived from equivocal petrographical evidence. This may lead to misinterpretation of the thermal or fluid circulation history of the reservoir. Radiometric dating has long been recognized as an efficient way to place robust time constraints on diagenetic processes, and significant work has been done on K/Ar and Ar/Ar dating to diagenetic illite and K-fedspar in the 80s and 90s. In the recent years, interest has developed for the application of U-Pb dating to sedimentary carbonates. U-Pb dating is traditionally used for dating zircons and other U-rich accessory minerals. Less attention has been paid to its application to carbonates because of their low U content and in many cases their overwhelmingly high levels of common Pb making them impossible to date. The latest generation of MC-ICP-MS mass spectrometer offers the possibility to measure very low U and Pb amounts by isotopic dilution (ID), and Laser Ablation coupled to (MC)-ICP-MS allows in situ isotopic measurements directly on carbonates at thin section scale. In this study we performed U-Pb dating on carbonates and silica collected from different geological settings and compared results obtained by ID-MC-ICP-MS and LA-ICP-MS on the same samples. U-Pb analysis by ID-MC-ICPMS provides high accuracy and reproducibility, but does not permit to investigate the wide range of U-Pb ratio variation that could be documented by in-situ laser analyses. The Laser Ablation approach does constitute a very attractive method. However, major hurdles remain to be overcome, in particular regarding the insufficient understanding of the U-Pb system behavior during in-situ lasing and the lack of well-calibrated, matrix-matched standards for data correction. We carried out a detailed investigation of a natural calcite commonly used for carbonate dating that revealed the existence of small intra-crystal diagenetic domains, impairing the use of this material for accurate data correction. We propose to use an in-house-developed homogeneous synthetic calcite standard, which isotopic composition has been precisely established. Overall, our results show that U-Pb dating by LA-ICP-MS is a promising approach but the ID-MC-ICPMS technique, although more time-consuming, remains necessary to crosscheck and validate ages in many settings.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017