USGS National and Global Oil and Gas Assessment Project - North-Central Montana and Williston Basin Provinces, Heath Formation Assessment Unit Boundaries

The Assessment Unit is the fundamental unit used in the National Assessment Project
for the assessment of undiscovered oil and gas resources. The Assessment Unit is
defined within the context of the higher-level Total Petroleum System. The
Assessment Unit is shown herein as a geographic boundary interpreted, defined, and
mapped by the geologist responsible for the province and incorporates a set of known
or postulated oil and (or) gas accumulations sharing similar geologic, geographic, and
temporal properties within the Total Petroleum System, such as source rock, timing,
migration pathways, trapping mechanism, and hydrocarbon type. The Assessment
Unit boundary is defined geologically as the limits of the geologic elements that
define the Assessment Unit, such as limits of reservoir rock, geologic structures,
source rock, and seal lithologies. The only exceptions to this are Assessment Units
that border the Federal-State water boundary. In these cases, the Federal-State water
boundary forms part of the Assessment Unit boundary.