Rock-physics analysis of clay-rich source rocks on the Norwegian Shelf

Abstract

Rock-physics trends and properties of clay-rich source rocks are investigated in selected wells in the North Sea and Norwegian Sea. Properties can vary significantly because of burial compaction, composition, diagenesis, organic richness, and maturation. The many competing effects can be difficult to disentangle in traditional rock-physics crossplots. However, nearly all the sourcerock data in the study are bounded nicely by linear trends that are based on rock-physics models, in acoustic-impedance (AI) versus shear-impedance (SI) crossplots. These reference models serve as a nice screening tool for organic richness and/or maturation level (i.e., hydrocarbon generation and expulsion), regardless of burial depth. The use of rock-physics templates for the early mature to mature stage of clay-rich source rocks is demonstrated by combining a simple basin-modeling approach with a rock-physics model using Backus average in which the organic-rich shale is represented by a transverse isotropic mixture of clays, kerogen, and hydrocarbons. The resulting templates are bounded nicely by reference trends and explain some of the observed trends in the data. However, the local presence of carbonaceous material and diagenesis within the source rock, which have not been accounted for in the rock-physics modeling, might explain why some of the data points have higher impedances and lower V P /V S than the template models. Finally, source rocks with hydrocarbon saturation can cause AVO signatures similar to hydrocarbon-filled sands and therefore represent false positives in exploration.

Organic content and maturation effects on elastic properties of source rock shales in the Central North Sea

“…Changes in elastic properties as a function of increasing burial and compaction can easily be observed in our data (e.g., Figures 6 and 9), and subsequently accounted for when assessing the TOC and hydrocarbon trends. This type of compaction behavior for organicrich and organic-lean shale is previously established and discussed in various papers (e.g., Draege et al, 2006;Løseth et al, 2011;Avseth and Carcione, 2015). We can also observe the potential for ambiguity between shaly sands (closer to shale trend) and well-compacted shales, as well as poorly consolidated sands and organic-rich shales (Figure 9).…”

“…Data from the Tau and Draupne formations in all seven wells are shown as grey points. Our background shale compaction trend obtained locally is seen to closely resemble the published inorganic shale trend (Khadeeva and Vernik, 2014;Avseth and Carcione, 2015). We can see that overlap occurs mainly at greater depth (lower V p /V s and higher AI),…”

“…Also indicated are associated vitrinite reflectance readings and reference lines from Vernik and Landis (1996). (Khadeeva and Vernik, 2014;Avseth and Carcione, 2015). Interpretation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Interpretation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59…”

“…The mineralogical composition was approximated from XRD analysis of the Draupne Formation by Zadeh et al (2017). Mineral moduli and densities were obtained from Mondol et al (2008) and Avseth and Carcione (2015).…”

Effective Attributes Quantification to Bridge Gap between Elastic Properties and Reservoir Parameters in Self-Resource Rocks

“…Løseth et al 12 link acoustic impedance with organic matter to delineate it on seismic data. Per Avseth and Carcione 13 developed rock physics models for screening organic richness. Zhao et al 1 use sum of organic matter and porosity is to delineate organic shale to identify reservoir parameters.…”