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Distinct Petrographic Changes Across the Triassic-Jurassic Boundary in the Southwestern Barents Sea — Implications for Predicting Reservoir Quality

Abstract

Distinct petrographic changes across the Triassic-Jurassic boundary in the

southwestern Barents Sea - Implications for predicting reservoir quality

Lina Hedvig Line (UiO), Helge Hellevang (UiO) and Jens Jahren (UiO)

Abstract

A major challenge for reservoir quality predictions in the southwestern Barents Sea is the

complex uplift and erosion history that have complicated the thermal trends of the sediments.

Sandstone reservoirs are currently not located at their maximum burial depths, and

disappointing drilling results are often associated with unexpected poor reservoir quality.

Because seismic and acoustic inversion methods fail to distinguish high-quality from low-

quality sandstone reservoirs, we attempt to attribute reservoir properties to depositional

setting and diagenetic evolution by coupling petrography with sedimentary facies and basin

analyses. Triassic and Jurassic sandstones are the major stratigraphic targets for hydrocarbon

prospecting in the SW Barents Sea, and marked contrasts in texture and mineralogy are

observed between the geological periods. This provides an opportunity to understand how

different initial petrophysical properties change with increasing burial depth and temperature.

Channelized sandstone deposits of Triassic age are expected to have high potential for

preserving porosity with deep burial due to extensive development of chlorite coatings.

However, high concentrations of mechanically weak lithic fragments produce low

intergranular volumes during burial, which often result in poor reservoir quality. Conversely,

textural and mineralogical maturity characterize the Jurassic sandstones in the southwestern

Barents Sea. This petrographic contrast has been related to hinterland rejuvenation and

decreased subsidence rates, followed by extensive sediment reworking in the Jurassic. As

opposed to the Triassic sandstones, the potential for preserving porosity with deep burial is

lower in the Jurassic sandstones due to limited distribution of grain coatings. Jurassic

reservoirs are thus prone to chemical compaction if burial temperatures exceed 70C. Future

exploration campaigns should target Jurassic sandstone reservoirs with maximum burial

depths of less than 2 - 2.5 km. Reworked sediments from both provenance regions have

potential for preserving good reservoir properties.