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.