Fluid Inclusion Isotope Analysis of Vein Cements as a Tool to Reconstruct Basin-Scale Fluid Circulation

Abstract

Understanding the instigators of permeability variations and associated events of fluid flow is essential in assessing the sealing qualities of rock formations. We have documented the first application of a state-of-the-art technique for fluid inclusion isotope analysis on vein minerals to gain better insights into the processes controlling these permeability variations. The analytical technique consists of simultaneously measuring bulk isotope values of δ2H and δ18O of fluid inclusion water in mineral samples of about 1 gram. Fluid inclusion water contained by the mineral samples is released by a crushing mechanism, before being guided to a mass spectrometer for analysis. The fluid inclusions are assumed to contain the remnants of the fluid from which the veins precipitated and, therefore, provide information regarding the provenance of circulating paleo-fluids. Furthermore, absolute precipitation temperatures may be calculated in the case of calcite veins. Altogether, this allows for accurate reconstruction of the timing, patterns and causes of fluid flow on a regional scale, as we show in two case studies. The first case study concerns calcite veins from the Late Cretaceous Jandaíra carbonate platform, which plays as key role in the petroleum system of the Potiguar Basin in northeast Brazil. Fluid flow within these tight carbonate rocks appeared to be a syn-burial process controlled by events of fracturing, which induce increases in permeability. Subsequent drops in permeability are related to calcite vein precipitation, which fills in the open pathways formed by the fractures. Fluid supply to the fracture network happened by upwelling of meteoric waters from an underlying sandstone aquifer. A second case study was performed on vein samples from the Harz Mountains in Germany. Fluid inclusion isotope data was retrieved from a wide array of vein mineral types and suggest the existence of multiple fluid reservoirs in Mesozoic times. Tectonic events drove a fluid pumping mechanism, which caused a variable introduction of fluids from each reservoir to the fracture network. In short, the case studies clearly show the potential of fluid inclusion isotope analysis as a tool for the reconstruction of permeability variations and associated events of fluid flow in rock units.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017