Evaluation of H2S formation in the Basal Zechstein (Upper Permian of Germany)
This dissertation consists of an introductory chapter (1) and three following chapters (2, 3, and 4) which focus on problems concerning H2S (sourgas) encountered in the Basal Zechstein (Upper Permian) of northwest Germany. ^ Chapter 2 introduces a model of gas souring during migration, an alternative to the commonly reported model of in-place-souring after the gases have accumulated within deep-buried carbonate reservoirs. ^ H2S concentrations of more than 5% in deep carbonate gas reservoirs generally are attributed to thermochemical sulfate reduction (TSR). The Zechstein 2 Carbonate reservoir in NW Germany bears gas fields with H2S concentrations ranging from 0% to 40%. The reservoir is underlain by up to 400 m of sulfate of the Werra Anhydrite. ^ Chapter 3 concentrates on an ancient sulfate depositional system in the Upper Permian Einstein 1 Cycle (Z1), the Werra Anhydrite, underlying the carbonate reservoir from which the sourgases are produced. It acts as a conduit for the migrating gases within the petroleum system of the Germanic Basin. ^ Well cores of seven wells from the Southern Zechstein Basin were examined for their structural and textural features. Facies interpretation was based on cut core material and thin sections. ^ Four main facies types, (I) supratidal, (II) intertidal, (III) shallow subtidal, and (IV) deeper marine, are further subdivided into 10 subfacies types: (1) karst and (2) sabkha within the supratidal, (3) algal tidal-flat, (4) tidal flat and (5) transgressive deposit within the intertidal main facies type, (6) salina, and (7) sulfatic arenites within the shallow subtidal environment. Subfacies types of the (8) slope commonly associated with (9) turbidites and the (10) basin subfacies type subdivide the deeper marine depositional environment. ^ Vertical stacking patterns of these facies and subfacies, types reveal the sequence stratigraphic development of the sulfatic system in response to sea-level fluctuations. ^ Most of the A1 succession is represented by three relatively thick parasequences belonging to the highstand systems tract that shows typical prograding sets. ^ Chapter 4 focuses on pyrites as indicators of BSR and TSR in the Basal Zechstein. ^ The Werra Anhydrite (A1), the sulfate member of the Zechstein 1 Cycle, yields a complex record of diagenetic alteration. Two generations of pyrite reflecting two episodes of sulfate reduction can be reported from the A1: (1) an early pulse of sulfate reduction which led to precipitation of framboidal aggregates of pyrite, up to 70 μm in diameter, during eogenetic diagenesis; and (2) a later prismatic generation of pyrite which commonly replaces secondary anhydrite, with individual pentagon-dodecahedrons up to 40 μm in diameter, it also occurs as thin overgrowth (a few microns) on the older framboidal pyrite. ^ These two generations of pyrite also we found within the overlying carbonate member of the Zechstein 2 Cycle. A third type of blocky-prismatic pyrite comprising aggregates of up to cm-length is found exclusively within the Zechstein 2 Carbonate (Ca2). (Abstract shortened by UMI.)^
Steinhoff, Ingo, "Evaluation of H2S formation in the Basal Zechstein (Upper Permian of Germany)" (1998). ETD Collection for University of Texas, El Paso. AAI9919359.