Sunday, November 4, 2007

Fracture Factory

With the pressures exerted on the core sediments during drilling, transport and splitting, it is not surprising that there are many cracks and breaks in it. Fractures caused by these processess are classified as drilling induced. There is a team of scientists at the drill site logging (recording) all the fractures in the core, but they are primarily interested in natural fractures. Natural fractures are those created by tectonic movement within the earth's crust or stress on the rocks caused by changes in loading (something heavy above the rock pressing it down - in this area it is usually a volcanic or glacial "something").

Cristina Millan helped me understand some of the different kinds of fractures I have been seeing in the core. She is one of the scientists logging fractures in order to understand the pressures acting on the crust in this area over time.

All of the open fractures are recent in origin - induced fractures. One common type is a tensile fracture, where the pressure experienced by the rock causes two parts to separate. This usually causes a crack straight across the core. We see LOTS of this type of fracture. A second type of induced fracture is a petal centerline fracture. It is caused by the drilling process. This type of fracture has a curved shape coming in from the edge of the core and then makes a straight line down the core. Another common fracture we see happens when rock that is not strong is split in half, creating breaks along weak points and a puzzle or mosaic appearance.

Natural fractures are generally not open or fresh, they are often filled in or mineralized. The most typical natural fractures are veins and faults. Although veins can be broad, in the sediments we are seeing in this core, veins are quite small. The material that fills the fracture to create the vein is usually a different color than the surrounding sediments, making its angled line travelling through the core more visible. A vein usually has a very characteristic angle (or dip) in which they incline through the sediment. Tensile fractures may sometimes occur on vein fractures as the mineralized vein is a weaker point in the core.

Faults are also natural fractures. They are identifiable because some sections of layered sediment will have been shifted up or down or around and appear out of place with the surrounding pattern of layering. The process of fracturing within the rock doesn't remove sediment, it just moves it around or creates spaces for mineralization.

Look at the pictures on this section and see how many of the different types of fractures you can identify.

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