Reservoir characterization and well production proxy analyses on drill cuttings: Case study from the Flysch play in the Vienna Basin (NE Austria)

Drill cuttings, though rarely used, are crucial subsurface samples to understand petrographic properties affecting reservoir quality. Unlike core material, cuttings are continuously available along the wellbore and can be used during drilling to monitor progress. Therefore, cuttings may allow a semi-quantitative, statistical calibration of rock properties from the subsurface, but they are often underutilized. Although fracture and vein orientations cannot be reconstructed from drill cuttings, the presence of veins and their internal textures (open, partially sealed or sealed) in specific formation sections and depths can be identified and analyzed using e.g., transmitted light microscopy and cathodoluminescence to supplement characterization at the well site and subsequently assess production behavior. Borehole gamma ray logs in combination with handheld portable X-ray fluorescence (pXRF) analyses on cleaned and dried drill cuttings can be used to further improve the depth accuracy of the cutting samples and to geochemically fingerprint the samples, based on the Si/Al ratio, as a proxy for sandstone-rich and mudrock-rich sections of the well. In this study, eighty-three sandstone cutting samples from two wells, covering ~400 m of stratigraphy targeting the Paleocene-Eocene Greifenstein Fm. equivalent (Glauconite Sandstone, GLS) in the Vienna Basin (Austria), were studied. They also cover parts of three different reservoir sections (1. to 3. GLS). The Flysch play in the Vienna Basin hosts several sandstone-mudrock interbeds and is composed of several nappes, forming complex reservoir compartments. The glauconite contents vary between different sections of the GLS, where the highest is observed in the 3. GLS. The sandstones are predominantly cemented by ferroan calcite, resulting in low optical porosity (<5%) in both wells, with only individually elevated porosity, related to partially dissolved K-feldspar grains. A paragenetic sequence solely based on cuttings further highlights that reservoir quality in the studied section is independent of sandstone compaction, but is related to lower optical porosity in finer-grained sandstones and higher carbonate vein cement contents. Furthermore, productive intervals are related to lower Fe + Mg contents. The understanding of reservoir properties, diagenesis, and their influence on fluid flow is crucial for successful exploration and reduction of uncertainty in reservoir production and development. The diagenetic variations from cuttings and the geochemical fingerprint by pXRF are linked to reservoir quality and production performance of individual well perforations. This approach can provide additional information on reservoir quality where core material is unavailable.