Abstract

Petrographic recognition of oil inclusions under ultraviolet excitation is a relatively simple technique that plays an important role in investigations of hydrocarbon migration. We observed brightly fluorescing, mostly two-phase, oil-bearing fluid inclusions in and near microfractures in a sample of a 3.5-billion-year-old hydrothermal barite. The oil inclusions are strikingly similar to those found in conventional Phanerozoic reservoirs. However, careful petrographic observations combined with nondestructive analytical methods and modified sample preparation techniques using colored dyes revealed that these inclusions are artifacts. Most likely the inclusions contain oil-based polishing fluids introduced during sample preparation. Evidently, the fluids infiltrated opened or underpressured inclusion cavities via micropores and microcracks initiated during sample polishing and grinding. They mixed with residual fluids and air, and as the capillary pressure and surface tension reached equilibrium the inclusions came to resemble natural oil-bearing inclusions. We suggest that artifacts can be reliably recognized through careful petrographic observation and sample preparation using dyes. Oil-inclusion artifacts are particularly likely in soft, easily cleavable and oil-wetting minerals, including evaporites and carbonates, which are common components of petroleum reservoirs around the world. Also pseudo-inclusions could form in hard minerals and have aqueous compositions. In addition to pointing out a potential interpretive hazard, the study has implications for oil migration in cold, young reservoirs, because it illustrates that oil inclusions can form by refilling of cavities without the need for cementation.

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