Rinse the shells briefly under cold water and drain well. Sweat the shells briefly in the oil, then add the mirepoix (onions, leeks celery, parsnips) and sweat for 2-3 minutes. Add to cold water, add the parsley and sachet (all the herbs placed into a 4" square of cheesecloth and tied into a sack) and bring to a boil. Reduce heat and simmer for 40 minutes. Strain the stock thoroughly in a china cap or strainer layered with cheesecloth. Cool the stock completely in an ice-water bath and use, refrigerate or freeze immediately.