Abstract
Our analysis of a comprehensive well log database and complementary mineralogical and geochemical information indicates that the risk for Upper Jurassic shales on the Norwegian Continental Shelf (NCS) to permit severe leakage of hydrocarbons from the reservoir is generally low, even in the case of substantial uplift. The content of brittle minerals, organic content, and compaction are dominant factors that explain the observed discrepancies in acoustic properties of organic-rich caprock shales. In particular, variations in silt-clay content in clay-dominated shales are found to primarily influence sonic velocity and to correlate closely with gamma-ray where the uranium contribution is limited (“grey shales”). Changes in organic content exhibit a stronger density-component and are seen to counteract or mask the compaction effect on velocity and density in Kimmeridgian black shales. The Hekkingen, Draupne and Tau formations are distinctly different from the underlying grey shale formations in acoustic properties, despite that the latter group also contains significant amounts of organic matter. Based on the low permeability and high capillary sealing capacity of clay-dominated shales, we conclude that even for a silty seal, migration through the caprock matrix is highly unlikely. Furthermore, tectonic fracturing is an ineffective leakage mechanism when the seal is poorly consolidated/cemented prior to uplift. Brittleness, related to both mineralogical composition and consolidation, is consequently a crucial parameter for predicting seal integrity in exhumed basins. Our rock physics framework and interpretations relate this rather qualitative parameter to acoustic properties, and thus, to seismic data.
Highlights
A functioning caprock is one of the three vital components conven tionally defining a play within the context of an operating petroleum system
We focus on crossplots of P-wave velocity versus bulk density when discussing rock physics
Hekkingen versus Fuglen and Draupne versus Heather) in conjunction with the rock physics model, we suggest that higher Total Organic Carbon (TOC) explains why a Draupne Formation with similar quartz content can appear less stiff and brittle
Summary
A functioning caprock is one of the three vital components conven tionally defining a play within the context of an operating petroleum system. The most prolific plays on the Norwegian Continental Shelf (NCS; Fig. 1a) rely on Jurassic reservoirs and equivalent age source rock and caprock formations (Hansen et al, 2019, 2020; NPD, 2019). The Nor wegian Petroleum Directorate (NPD) states that the presence of a caprock with sufficient sealing capacity, coupled with Cenozoic uplift and associated erosion, gas expansion and possible fault reactivation, are critical factors for the Jurassic plays in the Barents Sea (Fig. 1b). In the North Sea (Fig. 1c), Jurassic reservoirs and plays rely on the time-equivalent formations for primary seals as in the Barents Sea. An additional, more restricting consideration for the North Sea region is the limited and localized source rock maturation due to shallow burial in regions close to the present-day Norwegian mainland. A functioning caprock with minimal leakage could be essential to compensate for small generated and expelled hydrocarbon volumes, as late Cenozoic uplift is considered to have de-activated the petroleum system relatively quickly after the onset of oil generation (Ritter, 1988; Hermanrud et al, 1990)
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