AAPG Middle East Region GTW, Regional Variations in Charge Systems and the Impact on Hydrocarbon Fluid Properties in Exploration

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Estimating the Ultimate Expellable Potential of Source Rocks: Defining “World Class” for Aquatic Organofacies with Examples from the Arabian, West Siberian, Bohai and Williston Basins


An important step in the evaluation of a play or prospect is to consider the potential supply of petroleum charge, which is ultimately constrained by masses and volumes supplied by the source bed. The two factors limiting the mass of petroleum expelled from the organic matter in the source bed are: 1) its initial expulsion potential; and 2) the cumulative fraction of its potential that has been expelled up to its maximum state of maturation. To evaluate the initial expulsion potential, we introduce a workflow to estimate the Ultimate Expellable Potential (UEP), which represents the cumulative mass of oil and gas that can be expelled upon complete maturation of the source rock. For use in resource estimation, these masses can be converted to surface volumes of oil and gas per unit area (mmstb/km2 and bscf/km2 or mmboe/km2, respectively). UEP can be mapped across the depositional extent of the source bed, just as a reservoir depositional system can be mapped. We show examples of UEP mapping based on available public data. Three of the example source rocks are aquatic Organofacies that have charged major conventional petroleum systems: the marine Organofacies A Middle to Upper Jurassic of the Arabian Basin, Saudi Arabia; the marine Organofacies A/B Volgian Bazhenov Formation of the West Siberian Basin, Russia; and the lacustrine Organofacies C Eocene-Oligocene Shahejie Formation of the Bohai Basin, China. We also include an unconventional system: the marine Organofacies B latest Devonian-earliest Mississippian Bakken Formation of North Dakota, USA. The UEPs of the studied source rocks in the Arabian Basin and West Siberian Basin define "World Class" in marine source rocks since these basins are ranked number one and number two in the world by oil endowment. Until more data is available on other lacustrine basins, we offer the UEP of the studied Bohai Basin source rock as an example. In contrast, the UEP of the Bakken Formation source rocks (combined Upper and Lower Members) is relatively small despite its "World Class" unconventional oil endowment. The Bakken's effectiveness, despite its relatively low UEP, reflects the negligible migration losses involved in charging the Middle reservoir member. This illustrates that the often-touted term "World Class" can be rather meaningless. It needs to be considered in context given the task in hand: the greater the (vertical or lateral) migration losses incurred in charging reservoirs, the higher the UEP will need to be to overcome them.