Unconventional natural gas extraction via hydraulic fracturing coupled with horizontal drilling (HDHF) has generated disruptive growth in the domestic energy sector. Field analyses of residual HDHF fluids have detected halogenated species, potentially the product of unexplored reactions between authigenic halides and HDHF additives. Utilizing a custom high-pressure reactor system, we simultaneously screened 12 frequently disclosed, functionally diverse HDHF additives to uncover transformation chemistry. One emergent pathway, the halogenation of cinnamaldehyde in the presence of ammonium persulfate, demonstrated the potential for oxidative breakers to react with halides to yield reactive halogen species. Halogenated product formation, product distribution, and kinetics were evaluated with respect to shale well subsurface condition, linking transformation risk to measurable well-dependent characteristics (e.g., halide compositions, well temperatures, and pH). In a representative flowback brine, the brominated product dominated on a molar percent basis (6 ± 2%, as normalized by initial cinnamaldehyde loading) over chlorinated (1.4 ± 0.4%) and iodinated forms (2.5 ± 0.9%), reflecting relative halide abundance and propensity for oxidation. This work demonstrates that relevant subsurface reactions between natural brines and hydraulic fracturing additives can result in the unintended formation of halogenated products.