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

Abstract

We have investigated 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 geologic 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 total organic carbon (TOC) content. We assess the link between depth, acoustic impedance (AI), and TOC in this setting, and we express it as an empirical relation for TOC prediction. In addition, where S-wave information is available, we combine two seismic properties and infer rock-physics trends for semiquantitative prediction of TOC from [Formula: see text] and AI. Furthermore, data from one reference well penetrating mature source rock in the southern Viking Graben indicate 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 are reasonably capturing variations in the actual rock properties. Amplitude variation with offset 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.

Laboratory Evaluation of Mechanical Properties of Draupne Shale Relevant for CO2 Seal Integrity

“…The 9 m of Draupne material sampled from a depth of 2574.5-2583.5 m MD and brought to the laboratory for testing is from the top of the ~85 m-thick Draupne succession in the well. Characteristic index properties of the Draupne shale have been reported from several previous studies [27][28][29]39] and are here supplemented with additional bulk density and porosity measurements in Table 1. The initial bulk densities of all samples for triaxial testing were calculated based on initial mass, diameter and height measurements.…”

“…The clay fraction constitutes roughly 50% of the material, and the main clay minerals are kaolinite, smectite and illite. Amounts of organic content (TOC) have both been measured in the laboratory [27][28][29] and estimated based on resistivity or density logs covering Draupne occurrences over a larger area [39]. With a consistent average TOC of 6-8 wt.%, Draupne is considered an organic-rich shale [41].…”

Modeling maturation, elastic, and geomechanical properties of the Draupne Formation, offshore Norway

“…In order to do this, TOC is first divided into two categories based on the quantity of organic carbon. Since the vast majority of TOC for this region sits between 5 -12% TOC (Hansen et al, 2019), TOC has been broken down into medium and high TOC where 5 wt.% is used as the cutoff for medium TOC. Then, vitrinite reflectance curves were utilized to define the level of maturation.…”

Characterization and Feature Ranking of Well Log Variables Using Data-Driven Algorithms for Total Organic Carbon Estimation of Organic-Rich Shales

“…Vernik and Milovac suggested a range of 70–85 against “ a ”. Hansen et al provided a kerogen density range of 1.1–1.6 g/cm 3 . Schmoker and Hester established a more simplistic relationship between bulk density and TOC TOC = A ( 1 RHOB ) − B where A and B are the two fitting constant parameters, which have the values A = 154.497 and B = 57.261 .…”