The Complications in Defining Oip Estimates in Source Rock Reservoirs - Total VS. Producible Oil and Mass Balance Characterization
At the onset of hydrocarbon generation no prolific expulsion (even into the inorganic porosity of the source rock itself) will occur until the absorption potential of the kerogen is first accommodated. This is a direct result of the oleophilic nature of kerogen and the parent-child relationship between the precursor solid kerogen and soluble hydrocarbon byproducts. Once the kerogen is fully saturated a “pre-migration” can finally occur wherein oil is expelled from the kerogen and begins to fill the mineral matrix scaffolding of the source rock. Eventually the entire source rock will become saturated at which point oil pressure will continue to build until either primary migration into adjacent carrier beds occurs, or the oil pressure finally exceeds the mechanical strength of the adjacent beds and fractures them, causing a “burping” event. Even after a source rock has ceased any further hydrocarbon generation or expulsion it is important to recognize that the kerogen itself is still fully saturated to whatever extent the remaining kerogen structure can accommodate absorbed oil. This kerogen associated oil is, for all practical purposes, trapped and not producible due to the extremely strong chemicophysical interactions between the two components. For this reason it might be more appropriate to consider this fraction of oil as nothing more than a soluble fraction of the kerogen’s intrinsic biomass rather than oil that can be exploited and produced. Furthermore, carbonate source rocks are usually composed of mudstones or wackestones which contain pore throats small enough that they can behave like a molecular sieve, resulting in preferential production of the smaller, lower molecular weight hydrocarbon compounds, while the larger, higher molecular weight components are left behind in the rock. Collectively these attributes make it challenging to determine what exactly is being described by a given OIP estimate, which can differ dramatically depending upon what laboratory techniques are employed. These concepts and the various lab-based analytical methods that can be used to derive OIP estimates will be discussed followed by some proposed ideas on how to integrate and interpret the resulting data to facilitate a partitioning between total OIP and producible OIP which we believe is necessary to better characterize these unique reservoirs and make improved predictions of potential recovery.
AAPG Datapages/Search and Discovery Article #90370 ©2020 AAPG Middle East Region Geoscience Technology Workshop, 3rd Edition Carbonate Reservoirs of the Middle East, Abu Dhabi, UAE, January 28-29, 2020