Reservoir quality of the Triassic-Jurassic SW Barents Shelf: Diagenesis linked to seismic and sedimentary facies

Sandstones constitute important reservoirs for oil and gas around the globe. The most important reservoir properties are porosity and permeability, which accommodate accumulation and flow of fluids through the reservoir. Reservoir quality is determined by the textural (e.g. grain size, sorting) and mineralogical composition of the sediment (Figure 1). These properties are functions of sedimentary provenance, depositional setting and diagenetic processes during burial.

Figure 1: Examples of reservoir quality as seen in optical microscope. Samples collected from Middle Triassic Kobbe Formation (left) and Upper Triassic Snadd Formation (right). QZ = quartz grains, RF = Rock fragments, MRF = Mudrock fragments, Mica = Mica grain, Chl = Chlorite cement, Ili = Illite, Matx = Matrix. Porosity displayed in blue color.

Prediction of reservoir quality in large frontier hydrocarbon provinces such as the Barents Shelf is presently based on sparse well control and limited seismic coverage of mixed quality. Disappointing results from recent drillings in the region, finding cemented sandstones with low porosity and permeability, have shown that predicting the distribution of high-quality reservoirs poses a great challenge. This project targets methods to predict reservoir quality prior to costly drillings and will build conceptual and regional knowledge important for future prospecting of the southwestern Barents Shelf (Figure 2).

Figure 2: Location of well database in the southwestern Barents Shelf (figure modified from NPD).

Diagenesis refers to all processes affecting the sediment after its deposition and before its complete solidification. Most of the porosity found in deeply buried sandstones is preserved initial void space between framework grains (primary porosity). Mechanical compaction (e.g. grain crushing and packing) and chemical compaction (e.g. cementation) decrease the primary porosity of sandstones during burial. Processes that inhibit or reduce compaction have received special attention during prospecting of deeply buried sandstones. On the Barents Shelf, sediments sourced from the eastern Uralide orogeny are rich in lithic rock fragments that tend to deform easily during mechanical compaction, leading to low intergranular volume and reduced reservoir quality. Development of clay coatings in some sandstone reservoirs has proven efficient in inhibiting quartz overgrowths and significant chemical compaction of the sandstone (Figure 3). An improved understanding of the link between sedimentary provenance, depositional environment and diagenetic processes preserving high reservoir quality is therefore a prerequisite for predicting the distribution of anomalous positive porosity at deeper burial.

Figure 3: Conceptual sketch showing the relative timing of chlorite coatings through burial history of the sediment (fictional burial history curve not to scale). The coating is characterized by two generations of chlorite coatings: The first coating (green) is present at grain-to-grain contacts and fills in embayments of the grain surface. It most likely forms at or close to surface conditions. The second chlorite coating (pink) is absent at grain-to-grain contacts and nucleates on top of the first chlorite coating around temperatures of 80 - 100°C. The coating prevents nucleation of quartz cement

The project will provide updated insight on diagenetic processes in the southwestern Barents Sea and impact future exploration activity in the region by investigating the possibility of developing an assessment model for risk evaluation of reservoir quality during prospect analysis.

Publications:

Line, L. H., 2015. Reservoir characterization of the Middle – Upper Triassic Kobbe and Snadd Formations in the southwestern Barents Sea: The role of chlorite coating (Master thesis).

Line, L.H., Jahren, J. and Hellevang, H., 2018. Mechanical compaction in chlorite-coated sandstone reservoirs–Examples from middle–Late Triassic channels in the southwestern Barents Sea. Marine and Petroleum Geology. https://www.sciencedirect.com/science/article/pii/S0264817218302289?via%3Dihub