Diagenetic History and Hydrocarbon Potential of Upper Permian Carbonate Buildups, Wegener Halvo Area, Jameson Land Basin, East Greenland (1)

Abstract

The Upper Permian of Jameson Land includes two carbonate sequences, the Karstryggen and Wegener Halvo formations. The Karstryggen Formation contains hypersaline carbonates and localized evaporites that were heavily weathered and dissected prior to deposition of the overlying strata. The overlying Wegener Halvo Formation represents an abrupt and extensive marine inundation over the underlying karstified Karstryggen surface. Bryozoan-brachiopod-algal-cement buildups of the Wegener Halvo Formation are localized on karstic highs, and show up to 150 m of depositional relief. In-situ mound-core deposits are flanked by allodapic limestones, which pass laterally into intermound calcareous shales. The diagenetic histories of the core and flank facies are very different. Core facies porosity was intially obliterated by marine cements, but repeated meteoric exposure altered unstable core facies constituents. This alteration produced extensive secondary porosity through grain and cement leaching with local collapse brecciation. Flank strata, however, underwent little sea-floor diagenesis, and low permeability and mineralogically stable grain composition protected these strata from meteoric alteration. Early cementation and stabilization of core strata led to minimal burial-diagenetic porosity loss. Uncemented flank beds, however, underwent profound mechanical and chemical compaction during this stage. Thus, at the time of hydrocarbon generation, distal flank beds had less than 2% porosity, coarse upper flank beds had 5-10% remnant primary porosity, and core facies deposits had 8-12% secondary pore space. Hydrocarbons generated from surrounding marine shales charged many of the bioherms with oil. Subsequent fracturing and hydrothermal fluid flow, however, flushed hydrocarbons and filled pores with ferroan calcite, barite, fluorite, galena, and baroque dolomite. This heating and flushing is thought to have been especially intense in the Wegener Halvo region; thus, more basinal areas may still have reservoirs containing significant oil in equivalent Upper Permian limestones. If, as is likely, the sea level changes affecting the Greenland Permian were eustatic, then this study may provide significant clues to porosity development throughout the largely unexplored northern Zechstein basin and the Arctic basin of the Barent Sea. This study also provides some important connections to the probably time-equivalent Guadalupian carbonate reservoir rocks of west Texas-New Mexico and Wyoming.