Thermogenetic degradation of early zeolite cement: An important process for generating anomalously high porosity and permeability in deeply buried sandstone reservoirs?

Abstract

Abstract Diagenetic explanations to anomalously high porosities and permeabilities in deeply buried sandstones have been presented for many reservoirs. For example, grain coatings of microquartz or chlorite can preserve porosity by preventing succeeding quartz overgrowths, and dissolution of grains or cement can generate secondary porosity. Anomalously high porosity and permeability (up to 25% and 500 mD, respectively) characterize the deeply buried (>5 km) Upper Jurassic shoreface sandstones of the Gert Member (Heno Formation) in the Hejre Field in the Danish Central Graben. Optical, cathodoluminescence and scanning electron microscopy were used to characterize the petrography and diagenetic changes that resulted in this high pressure and high temperature (HPHT) reservoir. This was supplemented by chemical analyses (ICP-OES and ICP-MS), mineralogical composition (XRD), and oxygen isotope composition of quartz and calcite cement (SIMS). The petrographic investigation shows that none of the known explanations for anomalously high porosity and permeability can be applied, hence a new hypothesis for deeply buried sandstones in contact with lavas and volcaniclastic lithologies is proposed. The new hypothesis includes reducing mechanical compaction due to early patchy zeolite cement, which was sourced from adjacent lavas and volcaniclastic rocks. Locally, stylolites developed where zeolite cement was less effective, hence the present overpressure formed at a later stage. Increased temperatures during burial resulted in thermal instability of the zeolite cement, which was dissolved and authigenic K-feldspar and quartz precipitated instead. This resulted in a reduction of the cement volume of up to 70% if the zeolite was K-heulandite and a corresponding relative porosity-increase. Thus, a reduction in cement-volume was generated by replacing zeolite of an open crystal structure with mineral phases, K-feldspar and quartz, of denser crystal structures. The thermogenetic breakdown of zeolite was forced by increased temperatures during burial. This process may explain previously enigmatic highly porous and permeable deeply buried sandstone reservoirs.