Organic Content and Maturation Effects on Elastic Properties of Source Rock Shales in the Central North Sea
Peer reviewed, Journal article
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- NGI articles 
Original versionInterpretation. 2019, 7 (2), T477-T497. 10.1190/int-2018-0105.1
We investigate the effects of organic content and maturation on the elastic properties of source rock shales, mainly through integration of a well log database from the Central North Sea and associated geochemical data. Our aim is to improve the understanding of how seismic properties change in source rock shales due to geological variations and how these might manifest on seismic data in deeper, undrilled parts of basins in the area. The Tau and Draupne Formations (Kimmeridge Shale equivalents) in immature to early mature stages exhibit variation mainly related to compaction and TOC content. We assess the link between depth, acoustic impedance (AI) and TOC in this setting, and express it as an empirical relation for TOC prediction. Additionally, where shear wave information is available, we combine two seismic properties and infer rock physics trends for semi-quantitative prediction of TOC from Vp/Vs and AI. Furthermore, data from one reference well penetrating mature source rock in the southern Viking Graben indicates that a notable hydrocarbon-effect can be observed as an addition to the inherently low kerogen-related velocity and density. Published Kimmeridge Shale ultrasonic measurements from 3.85 to 4.02 km depth closely coincide with well log measurements in the mature shale, indicating that upscaled log data is reasonably capturing variations in the actual rock properties. AVO inversion attributes should in theory be interpreted successively in terms of compaction, TOC, and maturation with associated generation of hydrocarbons. Our compaction-consistent decomposition of these effects can be of aid in such interpretations.