This study investigates the seismic velocity anisotropy of two organic-rich shales from the Norwegian Continental Shelf. The tested organic-rich shale samples were from the Upper Jurassic Draupne and Hekkingen formations collected from two wells (16/ 8-3S and 7125/1-1) drilled in the central North Sea and western Barents Sea, respectively. The two tested shales are different in organic matter richness and thermal maturation, and they have experienced different burial histories. The shale core plugs were tested in a triaxial cell under controlled pore pressure. Seismic velocities (V P and V S ) were measured along different orientations with respect to layering to identify the complete tensor of the rock elastic moduli, and to investigate the velocity anisotropy as a function of increasing effective stress. The measured velocity values exhibit strong anisotropy for the two tested organic-rich shales. The anisotropy for both shales is strongest for V S . Seismic velocities follow an increasing trend as the effective stress increases. The anisotropy decreases somewhat with increasing consolidation, probably due to the closing of preexisting fractures and microcracks. The reduction of anisotropy is more evident for the P-wave because it decreases from 0.32 to 0.25 for the Draupne sample and from 0.28 to 0.24 for the Hekkingen sample when the vertical effective stress increases from 26 to 50 MPa. In general, the Hekkingen sample indicates slightly higher velocity values than the Draupne sample due to more compaction and lower porosity. In spite of major differences between the two shale formations in terms of organic matter content, maturity and burial history, they indicate almost the same degree of velocity anisotropy. The outcomes of this study can contribute to better imaging of organic-rich Draupne and Hekkingen shales by constraining the rock-physics properties.
Development of rock physical properties in well‐sorted and poorly‐sorted unconsolidated mono‐quartz sands and sand–clay mixtures as a function of effective stress in both dry and brine‐saturated conditions is assessed in this study. The tested samples were prepared with full control on their mineralogy, grain size, grain shape, sorting, and fabric. The experiments were performed in a high‐stress uniaxial oedometer up to a maximum of 30 MPa vertical effective stress. Sand–clay samples were a mixture of sand grains and clay particles (kaolinite or smectite) in different proportions. The maximum clay volume fraction used in the experiments was at most 30%. The initial bulk density of the tested sand‐dominated samples was adjusted to be close to the maximum index density expected for natural sediments (sand–clay mixtures) during deposition. In pure sand samples, finer grained sand show higher initial porosity than relatively coarser grained sands. Moreover, sand–clay mixtures have lower initial porosity than pure sands. Porosity decreases as a function of increasing clay content. The poorly‐sorted sand samples are less compaction prone than the well‐sorted sand samples. Among well‐sorted sand samples, coarser grained sands are more compressible than finer grained sands. At a given effective stress level, sand–clay mixtures are more compaction prone compared with their sand component alone. Pure sands and clay‐poor sand–clay mixtures (either sand–kaolinite or sand–smectite) show almost the same degree of compaction when tested in both dry and brine‐saturated conditions. In contrast, clay‐rich sand–kaolinite and sand–smectite mixtures (clay volume >20%) are significantly more compact in brine‐saturated condition. The Vp values of brine‐saturated sand–kaolinite mixtures shows a positive correlation with the kaolinite content, whereas Vp starts to decrease substantially when the volume fraction of smectite exceeds 10% of the whole sand–smectite samples. Saturated bulk moduli estimated by Gassmann's fluid substitution agree with measurements for brine‐saturated clay‐poor sand samples. However, the model does not yield proper predictions for sand–clay samples containing 20% clay volume and above, particularly when the clay is mainly smectite. The acoustic and physical properties derived from experimental compaction of pure sands and sand–clay mixtures show a good agreement with rock properties derived from well logs of mechanically compacted pure sands and shaly sands in progressively subsided basins such as Viking Graben in the North Sea. Thus, the outcome of this study can provide reliable constraints for rock physical properties of sands and shaly sands within the mechanical compaction domain and contribute to improved basin modelling and identification of hydrocarbon presence, overconsolidation, and/or undercompaction.
Pore pressure in fine-grained rocks is important with respect to drilling problems such as kicks, blowouts, borehole instability, stuck pipe and lost circulation. In this study, a succession of overpressured, fine-grained, sedimentary rocks located in the Egersund Basin, Central North Sea, was analysed with respect to mineralogical composition, source-rock maturation and log-derived petrophysical properties to highlight the effect of diagenetic processes on the pore pressure. Petrographic and geochemical analyses showed that the overpressure in the study area is largely linked to disequilibrium compaction, illitisation and source-rock maturation shown by log-derived physical properties. Pore-pressure prediction based on the difference of log-derived sonic transit time compared to the normal compaction trend (NCT) of fine-grained rocks can be used to infer the general trends of pore-pressure changes. However, during such pore-pressure prediction (e.g., using Eaton's approach), one should note that with regard to sonic response of the above-mentioned processes, the sonic log-derived, predicted pore pressure in the chemically compacted intervals and organicrich thermally mature successions may show either underestimations or overestimations, respectively.
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