Facies-Controlled Compositions of Petroleum and Natural Gas in Zechstein Carbonates in Eastern Germany

Abstract

Petroleum and natural gas are produced from the Stassfurt carbonate (Zechstein 2) at the northern and southern margins of the Zechstein basin in eastern Germany. The following facies types can be distinguished at the basin margins: basin, slope, shelf margin (including carbonate sandbars), sabkha, and lagoon. Especially the high-energy carbonates (grainstones and packstones) of the islands on the shelf and, locally, the carbonate sandbars are considered to be excellent pore reservoirs. The paleomorphological traps are formed by Zechstein salt several hundred meters thick. Suprahydrostatic pressures are common in the traps. The oils are characterized by relatively low concentrations of diasteranes, relatively high biomarker concentrations, and a prevalence of even-numbered n-alkanes. This indicates a moderately mature carbonate source rock. The biomarker patterns and isotope ratios of the oil differ, however, providing an indication of the organic precursor material and the sedimentation environment. The Zechstein 2 source rock samples usually contained less than 0.3% total organic carbon; a maximum of 1.65% was measured. The extractable organic material was determined to have a moderate maturity; the [delta][sup 13]C values of the kerogens are between -25.8 and -24.4$, and the biomarkers indicate a carbonate matrix. On the north side of the Zechstein basin, themore » oils show a good correlation with the analyzed source rocks. One the south side, only the oils from the sandbar and slope areas correlate with Zechstein 2 source rocks. Source rocks from lagoonal environments are apparently characterized by another organic facies. The associated gases contain considerable nitrogen (17-68 vol.%); CO[sub 2] concentrations lie below 1%. The carbon and hydrogen isotope ratios of the hydrocarbons in these gases are typical of gases associated with oils from marine source rocks. On the basis of their isotope values, the genesis of the nitrogen and CO[sub 2] will be discussed.« less