A preliminary study on fluid migration pathways along the Rechnitz detachment fault – a tribute to Prof. Csaba Szabó

Abstract

The fully cored, 2150-m-deep Szombathely-II (abbreviated as Szh-II) well was drilled at the western margin of the Hungarian Danube Basin. Beneath a circa 2-km-thick Neogene post-to syn-rift basin fill, the well not only reached the pre-rift Upper Austroalpine (UAA) basement with its slightly metamorphosed Paleozoic units but also the underlying greenschists of the Lower Cretaceous Penninic unit. The Alpine basement units are separated by a large low-angle normal fault, which is the regional-scale Rechnitz (Rohonc) detachment fault. This fault was responsible for the formation of the Rechnitz–Eisenberg metamorphic core complex (MCC) straddling the Austrian–Hungarian border. Since the large-strain extensional Rechnitz detachment fault has no outcrops in the entire region, the core material was studied to charac­ter­ize the fault penetrated in the borehole from a microtectonic, diagenetic, fluid migration and fractured reservoir point of view. This study presents the preliminary results on core samples with various lithology from the Szh-II well to better understand the Rechnitz detachment system and the corresponding fluid migration pathways. 

The Penninic greenschist below the main brittle deformation zone exhibits a well-developed foliation defined mainly by the oriented growth of actinolite and chlorite; however, the lack of a high-strain mylonitic foliation suggests that duc­tile shearing was not fully localized beneath the brittle detachment fault at the well location. The subsequent dominantly brittle deformation affected both the Penninic greenschists and the overlying UAA units. The Devonian carbonates and schists may represent a thin (only 20-m-thick) extensional allochthon on the top of the Rechnitz detachment fault, but beneath the basal conglomerates of the Neogene basin fill.

Fluid inclusion studies targeted the Penninic greenschists in three samples beneath the detachment fault. Four types of fluid inclusions with variable chemistry and origin were distinguished in several host minerals. Apatite hosts primary CO₂±CH₄+H₂O fluid inclusions (Type IIa, b, c) indicating significant partial fluid loss. Secondary fluid inclusions along healed fractures were entrapped in apatite (Type IId) and clinozoisite (Type I) with a chemistry of CO₂+H₂O and CH₄+H₂O fluid, respectively. Vein-filling calcite, crosscutting the foliation of the rock, contains primary aqueous fluid inclusions (Type III) also containing CO₂ and indication of saturated hydrocarbons in the vapor phase, which were trapped along growth zones of the host mineral. Aqueous CO₂-CH₄-N₂-bearing two-phase fluid inclusions, referred to as Type IV inclusions, were found along subgrain boundaries of quartz veins. Textural analyses showing significantly dif­ferent phase ratios within the same cluster of Type IId and Type III inclusions indicate that aqueous fluids were in or near to conditions of boiling during entrapment. Microstructural observations also indicate that carbonic fluid inclusions along grain-subgrain boundaries of quartz (Type IV inclusions) likely formed during ductile deformation and quartz re­crystal­lization. Improved understanding of the architecture of this fault zone may provide insights into the energetics of subsurface migration of geo-fluids (water and light oil) and various natural gases (CO₂, CH₄, N₂ and H₂) at circa 2 km depth.