Porosities and seismic velocities of mudstones from Wairarapa and oil wells of North Island, New Zealand, and their use in determining burial history

Abstract

The seismic velocities and porosities of mudstones and other fine-grained sedimentary rocks can be used to estimate burial history. The porosity of exhumed mudstone sequences of Wairarapa has been derived from density measurements, and can be used to estimate prior depth of greatest burial, to within about 100 m, by the linear regression function, greatest depth of burial = ™111 porosity +4975, or the exponential function, greatest depth of burial=™8636 log porosity + 14384.

The velocity trend of the exhumed mudstones of southeastern Wairarapa is not as good an indicator of burial history, as it has only a poor fit to the regression function, greatest depth=2666 velocity ™3331. However, fully loaded, fine-grained sedimentary rocks (mainly mudstones) still under lithostatic pressure in North Island oil wells have a good fit to the velocity-depth regression functions, depth = 1026 velocity ™1419 or, depth=6617 log velocity ™1403. This relationship can be used to calculate the thickness of sediment missing from the stratigraphic column.

Comparison of the velocity-depth trend of a partially unloaded, fine-grained sedimentary rock sequence in western Wairarapa with that of several other partially unloaded New Zealand sequences with a known history of vertical uplift (as shown by oil wells) indicates that 1100–1500 m of sediment has been unloaded by erosion.

Keywords:

• burial history
• velocity-depth trends
• porosity-d~pth trends
• exhumed mudstone
• Wairarapa
• unconformity
• oil-well sequences