Low-Temperature Hydrothermal Maturation
Model for
Sedimentary Basins in Geologically Active Areas
Neil S. Summer
An unrecognized mechanism for the maturation
of sediments with respect to
hydrocarbon generation has been operating in certain geologically active areas
of the world. This mechanism is hydrothermal, involving the transport of heat
away from volcanism or deep faulting by laterally flowing aquifers. The
interaction between the influx of heat from such an aquifer and the steady-state
geothermal gradient can result in anomalous temperature fields in the underlying
sediments. The thermal perturbations of these aquifers manifest themselves
vertically as near-constant or inverted hydrocarbon
maturation
profiles.
Horizontally over tens of kilometers, they appear as an aureole of thermally
mature sediments with a regional gradient toward the fluid source. This
maturation
of the or anic matter of the sediment is accompanied with a low-grade
diagenesis of the mineral matrix. In addition, since the thermal fluids may
assist in the migration of hydrocarbons to reservoir rocks, hydrocarbon
accumulations are associated with the model. There are numerous examples of oil
and gas fields associated with volcanism or deep faulting. In some places the
volcanism or thermal fluids have been recognized as the local
maturation
mechanism, but until now no attempt has been made to incorporate all the data
into a model. Therefore, a low-temperature hydrothermal
maturation
model has
been developed for sedimentary basins in geologically active areas. Unusual
diagenetic and
maturation
data from the Pacific Northwest and the San Juan
basin, USA, together with those occurrences of hydroc rbons near volcanic
centers throughout the Mediterranean region can be interpreted using this model.
Overall, these observations indicate that transient thermal events have played a
larger role in the thermal
maturation
of certain sedimentary basins than
expected.
AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.