ABSTRACT

Parameters measured by well logs define rock properties and seismic reflections at lithology interfaces. Parameters from standard embedding claystones are normally used when calculating the top sand amplitude variation with offset (AVO) response, but this might give erroneous results when the real claystone rock properties deviate from the standard. Using cuttings and high-quality wireline logs from well 34/8-A-33 H above the Visund field in the northern North Sea, we evaluated rock properties of the Pleistocene glaciomarine claystones, Lower Miocene and Upper Oligocene oozy claystones, Lower Oligocene and Eocene smectite-rich claystones, and two interbedded sands. Glaciomarine claystones fit best with the Greenberg-Castagna equation and have the highest measured velocities even though they are the shallowest buried sediments. Environmental scanning electron microscope analysis proves the Lower Miocene and Upper Oligocene claystones to be oozy. The amount of low-density oozy material causes significant shifts in the log curves and makes the ooze-rich claystones plot far off the trend given by the Greenberg-Castagna equation. We, therefore, developed a new equation for S-wave velocity prediction for ooze-rich claystones with average densities between 1.7 and 1.85g/cm3. The VP/VS ratios increase with depth in the Lower Oligocene and Eocene claystones of the Hordaland Group, and we interpreted this to reflect a downward increase in the amount of smectite, which existence was proven by X-ray diffraction analysis. We modeled how the seismic response at the top of a sand changes with embedding claystone type, saturation fluid, and offset. In glaciomarine claystones, the top of a brine-saturated sand corresponds to a negative trough reflection, in ooze-rich claystones to a positive peak reflection, and in smectite-rich claystones the reflection amplitude is close to zero. The predicted AVO response of sands in oozy claystones is highly dependent on whether the measured or calculated S-wave velocity has been used in the modeling.

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