Petroleum System Analysis: What Have We Learned Since Magoon and Dow's Memoir 60?

Abstract

In Memoir 60, Les Magoon and Wally Dow spelled out the need to understand the genetic relationship between a source kitchen and the resulting volume of accumulated oil and gas, encouraging two steps in the Geoscience thought process. Firstly, the explicit linking of a geoscience study to a business outcome – the oil and gas volumes in fields. Secondly, the wide scope of investigation in a Petroleum System study requires that this is not a specialist but a core team activity. Some organizations now recognize the Petroleum Systems Analyst or Fluid Systems Analyst as a discipline. What has changed since 1994? In a closely coupled system it is true that the sequence of events reservoir-trap-expulsion needs to be honored. However, in many basins oil fields sit above gas-window kitchens. Having since developed the ability to model fluid dynamics over geologic time, we can now understand why. Early attempts to model vertical migration in the Gulf of Mexico Basin showed that charge occurs progressively through mudstones, inducing a time lag in the system. In some cases the migration time lag is so long that the reservoir has not even been deposited at the time of expulsion. So an additional timing factor – time of arrival at the trap, rather than just timing of expulsion – needs to be brought into the timing question. Further, since gas expulsion follows oil, timing can only be used to explain a compositional outcome such as gas instead of oil, not a dry hole. And finally since most of the world's petroleum production is in uplifted and eroded onshore basins, the critical moment – the time of cooling of the source rock and shut-off of expulsion – doesn't seem very critical! Systems seem to be quite robust and not in need of “preservation”. If geochemical tracers such as biomarkers provide the ‘forensic evidence’ for the ‘who dunnit?’ in the System, we could benefit from a strengthening of the linkage between the petroleum and the source rock depositional environment via an Organofacies concept. Via linkage to kinetic compositional models of petroleum expulsion, we can make more use of matching bulk fluid properties such as GOR in validating Petroleum Systems models. Finally, the immense database being amassed from unconventional reservoirs is providing key insights into the mechanisms of the whole system. Rules from the conventional “more cumulative” conventional Petroleum System don't necessarily apply to the “more instantaneous” Unconventional Petroleum System.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016