A Key Question For Any Subsurface Petroleum Fluid: How Long Has it Been There?

Abstract

Dating petroleum charge times, oil residence times and charge rate in a trap would be a pivotal development and eliminate, at a stroke, much arm‐waving, geopoetry and speculation concerning petroleum charging times and routes. Charge times and rates are key variables in controlling hydrocarbon prospectivity as they define volumes of trapped petroleum and the dynamics of trap integrity, including leakage and spillage phenomena. Such constraints would allow for basin‐scale prospectivity estimates based on an assessment of fraction of total basin charge trapped and more realistic assessment of regional caprock efficiencies when only limited cored cap rock material is available. Yet, a practical method for dating petroleum field charging time from the analysis of reservoired fluids does not exist. Arguably, the most important constraining information in a petroleum system model is currently inaccessible, despite decades of petroleum geochemical and basin modelling research! Ooops! At PRG and Lundin, we have worked towards filling this gap by investigating how chemical changes in oils, effected by the natural nuclear radiation dose in any petroleum reservoir, could work as a proxy for fluid residence time. In theory, the chemical alterations in petroleum reservoir fluids, caused by in‐reservoir radioactivity could be used as a chronometer, although there are great analytical challenges associated with such a strategy. The “Rip van Winkle Project” team has identified both radiodegradation and radiosynthesis proxies that yield measurable signals at realistic natural subsurface radiation dosage, and together with our industrial partners, started to scrutinize the viability and issues relating of the first putative fluid “residence age measurements”, of oils in reservoirs. In this presentation, we highlight (a) why there is a need for a functional fluid residence age dating technology; (b) how the radiolysis proxy approach works in theory and the complex practical challenges evident; (c) what are the current knowledge gaps; and (d) perhaps most importantly, how this research area impacts petroleum system analysis as we currently understand it.

AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019