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Re-Os and the Utility of Sulfides in Hydrocarbon Systems

Abstract

Nearly all petroleum systems contain sulfide occurrences. Non-economic quantities of pyrite (FeS2), sphalerite (ZnS), galena (PbS), and chalcopyrite (CuFeS2) are the most common sulfide minerals, and native metals gold and silver may also be intimately associated with hydrocarbon systems. Notably, Re-Os dating was first developed and perfected for sulfides minerals. Concepts forged in Re-Os studies of ore systems can guide new thinking about fluid sources and migration within hydrocarbon systems. The more recent advent of Re-Os isotope geochemistry to petroleum systems has provided the industry with both a radiometric clock for source rock and a means to examine the timing of maturation and migration of hydrocarbon – even multiple maturation events in non-conventional settings (Stein et al. 2012, AAPG, #1236837). Re-Os dating of black shales has significantly refined period and stage boundaries in the geologic time scale, particularly for the Triassic (Xu et al. in press, PPP). Further, Re-Os geochemistry has presented us with new ways of thinking about episodes of global anoxia and euxinia and the paleoenvironments that led toward and ultimately produced source rock (e.g. Georgiev et al. 2011, EPSL). Remarkably, descriptions of sulfides in many ore-forming environments simply mention unusual abundances of organic material intergrown with ore or fluid inclusions filled with methane. For example, some of the renowned Au mineralization in the Carlin district in western U.S. is breccia with a hydrocarbon matrix. The intimate occurrence of Au and hydrocarbon is a simple and clear record of hydrocarbon migration in tandem with mineralization, and Re-Os constraints can be used to link Carlin hydrocarbon to Paleozoic source rocks (Stein 2013, Treatise on Geochemistry). Sulfides are part of a sedimentary basin's history and give key insight to fluid source and history in petroleum-producing regions. Re-Os isotope geochemistry can be used to distinguish sulfides that form from hydrocarbon-rich fluids in isotopic equilibrium with typical basin sedimentary rocks (e.g. sandstones, carbonates) versus hydrocarbon-rich fluids that were influenced by or partially derived from intrusion of sills. Identifying and characterizing the fluid is a first order criterion for modeling hydrocarbon systems. Re-Os dating of sulfides in this context will help us unravel the construction of petroleum systems. Funded by the CHRONOS project.