MORETTI, ISABELLE
Institut Francais du Petrole, Rueil Malmaison, France

Abstract: Abnormal Pressures And Fluid Migration In Foreland

Thrust emplacement induces the fast arrival of consolidated rocks at the top of foredeep deposits which may be very recent and still poorly compacted. Often, the thrust sheet consists of deep pre-compressional series which have already a low porosity due to formal burial compaction. In addition, the decollement level usually has a very low transverse permeability. The pressure field in the forelan is thus strongly influenced by the thrusting since the formation water can not escape vertically. Using numerical modeling we quantify the variation in the pressure field induced by the thrusting, and the resulting hydrocarbon migration pathways in the foreland.

A large number of forelands are continental, as all the American ones from the Andes to the Rocky mountains. In addition when some parts remain marine as in the Apennines, the internal part emerges and then is affected by infiltration of meteoric water. To take into account the variations induced by this water circulation, the potential hydrodynamism has been computed and the resulting abnormal pressure compared to well data in the Andean foreland. Competition between downward migration of the water and upward migration of the HC induces potential hydrodynamic traps. The resulting water pathway is compared to the one deduced from chemical and isotopes studies of samples of springs leaking near the thrusts in the Bolivian Sub Andean Zone.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas