Academic journal article Journal of Geoscience Education

A Simple Experiment to Demonstrate Overpressured Fluids and Soft Sediment Deformation

Academic journal article Journal of Geoscience Education

A Simple Experiment to Demonstrate Overpressured Fluids and Soft Sediment Deformation

Article excerpt

ABSTRACT

The development of overpressured fluids during soft sediment deformation and compaction can be illustrated using an analogue experiment, which is a simple, inexpensive and reusable teaching tool. Sand, mud and water are shaken in a plastic bottle. When the sediments settle and the water has cleared, the sand supports a layer of mud. Gentle squeezing of the bottle causes the sand to pack more closely and liquefy. Water is expelled from the sand, and becomes trapped beneath, and therefore supports, the mud layer. The pressure in the water beneath the mud changes from hydrostatic to hydrostatic plus the pressure exerted by the weight of the mud layer, i.e. it becomes overpressured. The overpressured water causes blistering and eventual cracking of the mud layer, with a plume of muddy water rising from the blister. The muddy water from the plume sinks because it is denser than the clear water. The mud layer also eventually sinks to become supported by the sand, and hydrostatic pressure is restored to the water column. Natural examples that are the result of this behavior include sub-horizontal calcite veins, clastic sills, and mud- and sand- volcanoes.

Keywords: Apparatus, Education - Laboratory, Structural Geology.

INTRODUCTION

This paper demonstrates a simple, inexpensive experiment for the development of overpressured fluids during compaction and subsequent soft-sediment deformation. Overpressured fluid exerts pressure that exceeds that of the hydrostatic gradient at a specific depth (e.g. Thomeer and Bottema, 1961; Osborne and Swarbrick, 1997). Overpressured fluids operate in various geological processes. For example, the crucial role of fluid pressure in controlling thrust faulting was recognized by Hubbert and Rubey (1959), who showed that large thrust sheets can move on gently-dipping fault surfaces only if the fluid pressure is close to, or exceeds, that of the overburden. The overpressurized fluid effectively supports the mass of the overlying thrust sheet. Evidence for overpressured fluids in thrust sheets includes sub-horizontal veins in the walls of thrusts, these having initiated as cracks caused by overpressured fluid (e.g. Sibson, 1989; Teixell et al., 2000). Sub-horizontal veins are common in sedimentary rocks, and indicate that fluid pressure exceeded the hydrostatic pressure plus the pressure exerted by the weight of the overlying rocks (e.g. Cosgrove, 2001). Furthermore, overpressured fluids are of great interest to the petroleum industry because they influence hydrocarbon production. For example, gushers are wells that release overpressured oil that fountains above ground level.

Because overpressured fluids are important in the field of geology, simple experiments demonstrating the nature of their development are beneficial to students and the classroom experience. Osborne and Swarbrick (1997) showed that overpressured fluids can be produced by: (1) an increase in compressive stress, (2) fluid movement or buoyancy, and (3) changes in the volume of the pore fluid or rock matrix. The overpressured fluids described in this paper are produced by a change in volume of the rock matrix caused by compaction of the sediment volume. Osborne and Swarbrick (1997) describe disequilibrium compaction as being caused by rapid burial and the resultant rapid increase in overburden stress. This requires rapid expulsion of fluids, with fluids being overpressured if the fluids cannot escape quickly enough. Osborne and Swarbrick (1997) showed that the increase of vertical loading during burial of sediment causes the rearrangement of the sand grains and some dissolution at grain contacts. This causes sandstones to compact from about 39-49% original porosity at the time of deposition to about 12-25% porosity after burial to depths of 2-3 km. Although the pore pressure generated by disequilibrium compaction never exceeds the lithostatic pressure, it can exceed the tensile strength of the rock, and therefore it exceeds the fluid pressure needed to open up extension fractures (Osborne and Swarbrick, 1997). …

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