Example of an Intra Sandstone Reservoir Seal – From Palaeocene Tyr Member Sandstone, Siri Canyon, Danish North Sea
This study demonstrates the diagenetic evolution glaucony-rich deep water sandstones from the Rau-1A well in the Siri Canyon, Danish North Sea. The major diagenetic phases in the studied well are microquartz, large syntaxial quartz overgrowth, calcite and chlorite. Chlorite forms an intrareservoir hydrocarbon seal, and our study demonstrates the influence of early diagenetic quartz on the formation of the chlorite seal. Early opal and microquartz are precipitated close to shale contacts and prevent the interaction between abundant detrital glaucony and pore-fluid and thus the formation of grain-coating berthierine. It also preserves porosity and permeability in marginal and isolated parts of the sandstone.In other parts of the sandstone grain coating berthierine precipitated. It was transformed to chlorite with increased depth. A second phase chlorite is distinguished by its morphology and slightly different diffraction pattern. It fills the porosity and reduced permeability in the upper marginal part of the reservoir which had preserved porosity and permeability due to microquartz cementation. The pore-filling chlorite acts as an impermeable seal to hydrocarbon migration and prevents the topmost part of the reservoir to be oil filled. On basis of XRD data, it is possible to demonstrate a slight shift of actual peak position in overlapping peaks of chlorite in the studied samples. This variation indicates two facies of chlorite in different time. On basis of the SEM studies, these facies are identified as an early grain coating chlorite and a later pore filling chlorite. The change in diffraction pattern indicates that the relative amount of two types of chlorite varies in relation to reservoir properties. The study demonstrates the conditions for formation of an intraformational chlorite seal and relates its possible formation to high-porosity/high-permeability zones of the reservoir. The study suggests that the formation of an intra-sandstone seal of diagenetic chlorite relates to the distribution of early diagenetic microquartz (which preserve porosity and permeability), and early diagenetic chlorite which reduces the permeability.
Sandstone: Geochemistry, Uses and Environmental Impact, 2013, p. 253-269