Expulsion and Migration Associated with Unconventional Petroleum Systems

Abstract

Exploitation of unconventional resources has revived questions on the overall efficiency of hydrocarbon expulsion (or primary migration) and has provided an impetus for renewed study of this issue. Much of the literature has suggested that expulsion is quite efficient, with most of the generated products exiting the source. This, however, appears to contradict the estimated volumes from unconventional resources, which often represent a self-sourced petroleum system and require generated hydrocarbons be retained within the source rock itself. This conundrum may have its root-cause in semantics. Expulsion has been defined as either the movement of hydrocarbons out of the kerogen or out of the source rock. These two processes are distinctly different. The former is discussed without consideration of rock properties, while the later takes into consideration rock fabric and is more directly tied to both conventional and unconventional resource assessment. Examination of unconventional plays suggests that there are three primary models for expulsion. The first model is represented by a “massive” source, where hydrocarbons “bleed” from the source rock's edge. This system is represented by such units as the Marcellus Formation (Appalachian basin). Within such systems the generated hydrocarbons are largely retained in the source rock, and the overall expulsion efficiency is limited. As there is little potential for secondary migration, this leads to development of excellent shale plays and somewhat limited conventional resources. The effectiveness of such systems is controlled by the thickness of the source interval. The second model is when a reservoir, conventional or unconventional, is sandwiched between organic-rich source intervals. The expulsion efficiency of such a system is greater than the “massive” system, with the upper and lower source intervals feeding into a common reservoir, but still may be limited. This type of system is represented by plays such as the Bakken Formation (Williston basin). Depending on the nature of the reservoir/carrier, secondary migration may occur and the maturity of the reservoir may be less than that of the produced product. The third model is where the source and reservoir (or carrier) are interbedded. This model leads to the highest expulsion efficiency and may be represented by the Wolfcamp Formation (Permian basin). Secondary migration may occur in this system and it may be difficult to define the net reservoir interval.

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