Regionally the “main dolomite” (Hauptdolomit) of northwestern German Zechstein (shallow water) is made up of 30–100 m thick oncolitic dolostones. In almost every case anhydrite is observed within the sequence suggesting an environment of elevated salinities (FÜCHTBAUER, 1968). Studies on thin sections of oncolitic carbonates of a gasfield in the Emsland area reveal a shallow marine environment, sometimes even intertidal as shown by intraformational flat pebbles of reworked material. Different stages of diagenesis within the carbonates occur. The most interesting fact concerning the detached rims of first-generation carbonate cement is discussed below and shown by several thin-section photomicrographs.