Magnetostratigraphy of cores from two wells through non-marine Lower Cretaceous sequences in the North Sea area is shown. The diagram illustrates the method used by COREMAGNETICS for presenting magnetostratigraphic data. Each horizontal bar on the colour chart represents one sample. Demagnetising temperature runs from left to right across this chart. The magnetic polarity after each demagnetisation step is colour-coded, with red = normal, green = intermediate and blue = reverse polarity. Thus, the interval 205-225m in well A carries a normal polarity overprint, which is removed by demagnetisation at about 300°C, to reveal a reverse polarity high temperature component, which represents the primary magnetism of the formation. The interpreted magnetic polarity sequence is shown at the right of each demagnetisation plot.

Correlating and dating cores by MAGNETOSTRATIGRAPHY

Magnetic
polarity

Well A
Depth + 2200m

Well B
Depth + 2150

Three available palynological datums constrain correlation with the standard Lower Cretaceous magnetic polarity timescale (right column). This correlation allows absolute ages to be assigned to each polarity reversal in the two cores, thereby providing a robust correlation framework.

Magnetostratigraphy offers a valuable complement to biostratigraphy in developing reservoir zonations.