ORGANIC-MATTER IN THE SOULTZ HDR GRANITIC THERMAL EXCHANGER (FRANCE) - NATURAL TRACER OF FLUID CIRCULATIONS BETWEEN THE BASEMENT AND ITS SEDIMENTARY COVER

Organic matter has been observed in cores of the Soultz-sous-Forets gr anite (Alsace, France) at depths between 2158 and 2160 m, in a highly fractured and altered zone. The granite is overlain by a 1400-m-thick sedimentary cover containing petroleum (Pechelbronn oil field). The So ultz area is devoted to Hot Dry Rock geothermics thanks to a high geot hermal gradient (up to 100 degrees C/km). During drilling operations, an artesian source produced oil in a fractured zone of the sedimentary cover (Buntsandstein). Its gas-chromatography and mass-spectrometry ( GC-MS) analysis revealed the presence of normal alkanes (n-alkanes, i. e. linear hydrocarbons) centered on C-17, branched alkanes with a majo r C-19 peak, and few unsaturated hydrocarbons. The aromatic fraction i s present in small amounts. In the Soultz granite, where it is altered , organic matter is exclusively associated with tosudite (interstratif ied clay mineral) which crystallized in plagioclase sites during a hyd rothermal alteration episode. Organic matter has been later displaced and concentrated along veinlets in which illite and carbonates have cr ystallized during another hydrothermal alteration stage. The soluble o rganic matter analyzed by GC-MS is composed of aliphatic acids, n-alka nes with a bimodal C-18 and C-24-C-25 centered distribution, alkylbenz enes and aromatic acids. Organic compounds in the granite would either originate from a single source (immature sediments) or from two sourc es (immature sediments and migration of the Pechelbronn oil). No real evidence was found to prove which hypothesis is the best one. The pres ence of organic matter in the granite shows the importance of fluid fl ows between the sedimentary cover and the granitic basement through ma jor fractures. In addition, the impregnation of plagioclase pseudomorp hs with organic matter is made possible due to their high interconnect ion degree and to the intergranular microfracturation of the granite. The succession of several hydrothermal events with different physico-c hemical characteristics may also be inferred from the occurrence of or ganic matter found in association with neoformed clay minerals in the granite.