These are treated as normal publicly held stocks -- i.e. any earnings whether in cash or stock form are to be recognized in the year these are effected. So your automatic re-investment has three consequences -- (a) the dividend amount is to be taxed by the USA each year -- report on your form 1040 schedule B; (b) if Shell is withholding taxes ( for which jurisdiction --Netherlands ? ) these are either foreign taxes ( eligible for credit ) or US taxes which should show up as withholding to the IRS, (c) depending on the actual transaction details these re-invested stocks will affect your basis in the investment and you need to keep good records--- also when you dispose of these you have to use average or block prices as basis, LIFO sales assumed unless you specifically instruct otherwise.
Another thing to remember is that if these stocks are not traded in NYSE or are held in a private account abroad, you may have FBAR ( generally liquid or liquidable-- Treasury form 114 ) and/or FATCA reporting ( IRS form 8938 )
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