Print this pageUnconformities: Key to Hydrocarbon 
Migration
and Entrapment
Glenn S. Visher
Analysis of hydrocarbon distribution in the Mid-Continent area and throughout
the world, indicates that unconformities control the depositional history, the
reservoir development pattern, migration pathways, and hydrocarbon-seal
development. Anticlinal and fault traps are most commonly the site for
accumulation of hydrocarbons, but many structures either do not contain
hydrocarbons or contain only minor accumulations. Traps not closely associated
with an unconformity surface cannot segregate hydrocarbons from the many
thousands of pore volumes of fluid that migrate along the unconformity surface
from the compacting source rocks in the basin center.
Unconformities caused by periodic changes in sea level are readily observed
from seismic and log sections and facies models. Unconformities below the
Simpson, Woodford, Morrowan, and the Wolfcampian granite wash are the controls
for the accumulation of hydrocarbons. Patterns of truncation, onlap, topography,
and structural history of these surfaces is the key to discovering new
hydrocarbon accumulations in the Mid-Continent.
Location of subtle traps associated with these unconformities has not been
widely used in the Mid-Continent. Detecting hydrocarbon accumulations requires
isopach mapping of intervals between event markers, both below and above the
unconformity surface. These isopach maps provide information on timing and
pattern of the structural history, topographic control for onlapping reservoir
units, points for fluid migration
into overlying onlapping sequences, and
truncation of underlying reservoir units. Study of these maps indicates areas
for hydrocarbon accumulations in various stratigraphic units, including the
Simpson, Hunton, Misener, Mississippian carbonates, Morrowan, and a succession
of onlapping Atokan and Desmoinesian rocks and in Wolfcampian granite wash.
AAPG Search and Discovery Article #91039©1987 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma, September 27-29, 1987.