Hydrogeological Controls and Indicators for Oil Deposits in Rift Grabens-An Example from the Upper Rhine Graben, France

C. J. Otto, J. Toth

The Pechelbronn petroliferous region of northeast France is located on the west side of the Upper Rhine graben between the Vosges and Black Forest Mountains. Commercial oil occurs in Mesozoic and often Tertiary lenticular, faulted formations at various depths. In areas of oil fields and along fault zones, positive temperature anomalies at shallow and great depths (geothermal gradient: 10°C/100 m), high salinities (TDS > 50,000 mg/L), and anomalously high Sr^-, Br^-, and I^- concentrations in shallow ground waters, flowing wells, and oil/gas seeps have been observed. These phenomena may be explained by topography-induced regionally ascending cross-formational groundwater flow. This interpretation is supported by the isotopic compositio of the formation waters, suggesting a meteoric origin, relatively young ¹⁴C ages (^approx30,000 years), and calculated flow velocities of 0.30 m/year as well as numerical modeling. Ground-water recharges in the topographic highs, converges toward the Graben basin, and discharges under artesian conditions by vertically ascending flow, preferentially along conductive faults. Vertical migration of hydrocarbons is also suggested by high methane concentrations in soils observed across faults with isotope ratios indicative of thermally mature hydrocarbon gases originating at depth. Consequently, a dynamic-genetic relationship appears to exist between basinal ground-water flow, hydrocarbon accumulations, and observed natural phenomena. This relationship may be used for hydrogeologic l-based exploration for hydrocarbons, complementing conventional methods.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.