ZECHSTEIN-2 CARBONATE PLATFORM SUBFACIES AND GRAIN-TYPE DISTRIBUTION (UPPER PERMIAN, NORTHWEST GERMANY)

The Upper Permian Zechstein 2 Carbonate (Stassfurt Carbonate, or Ca2) platform facies of Northwest Germany can be subdivided into twelve sub facies types using slabbed cores from fifteen representative wells. Th in section and scanning microscopic analysis further provide subfacies -specific characteristics, based on distribution, size, shape, and spa tial arrangement of the grains contained in the different subfacies ty pes. Thirteen grain types can be distinguished within the different su bfacies types on the Ca2-platform: 1) one type of oncoid, 2) one type of grapestone, 3) three types of peloids, 4) four types of ooids and 5 ) four types of aggregate grains. Both presence and composition of gra ins are indicative of the different subfacies types. There is also a r elation between grain composition and porosity of the Ca2-subfacies ty pes. The size and quantity of ooids correlate positively with increasi ng porosity, whereas an increasing amount of algal structures (algal-l amination) correlates negatively with porosity. The Ca2-platform carbo nates almost exclusively represent highstand systems tract and lowstan d systems tract deposits. The presence or absence of type-3 aggregate grains within the grainy shoal and algal-laminated shoal subfacies all ows the assignment of these subfacies to highstand (grains absent) or lowstand (grains present) systems tracts deposits. The Ca2-highstand d eposits can be subdivided into four shallowing-upward parasequences (P S3 to PS7) bounded by parasequence boundaries (PSB3 to PSB6) and Zechs tein sequence boundary ZSB4. In contrast to macroscopic core studies, microscopic studies to identify Ca2-subfacies types can utilize cuttin g material. This allows reconstruction of the subfacies distribution o n the Caa-platform, and delineation of potentially porous zones in unc ored Ca2 intervals.