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