Sediment Compaction and the State of Stress in the Earth
As sediment is buried within the earth, the stresses acting on a volume of rock increase because of the weight of overlying sediments and the effect of tectonic forces. Those changes in stress compact the buried sediment and lead to changes in its properties — such as porosity, permeability, elastic wave velocity, and rock strength — that can affect hydrocarbon production, seismic reservoir characterization, and well-log measurements.
This chapter reviews the state of in situ stress in the earth and explores the compaction of sediments. The chapter begins by discussing changes in porosity with depth and calculation of vertical stress and then introduces the concepts of effective stress and overpressure. Next, estimation of horizontal stress is discussed, along with bounds on the horizontal effective stress resulting from shear failure and fault slip. Finally, the chapter considers the impact that tectonic strains in the earth exert on the in situ stress field. The sign convention in this book is that compressive stresses are positive, whereas tensile stresses are negative.
When sediment is buried to greater and greater depths, the weight of overlying sediment increases and compacts that sediment. In regions where lateral variation in stratigraphy is small and where variation in density is also small, stress from the weight of overburden is directed chiefly in the vertical direction. Stress of that kind is referred to as overburden stress. In such regions, the vertical or overburden stress sV at depth z is given, to a good approximation, by
Figures & Tables
This is a recording of the 2010 Distinguished Instructor Short Course (DISC) by Colin Sayers. The state of stress within the earth has a profound effect on the propagation of seismic and borehole acoustic waves, this leads to many important applications of elastic waves for solving problems in petroleum geomechanics. This course provides an overview of the sensitivity of elastic waves in the earth to the insitu stress, pore pressure, and anisotropy of the rock fabric resulting from the depositional and stress history of the rock, and introduces some of the applications of this sensitivity. The course will provide the basis for applying geophysics and rock physics solutions to geomechanical challenges in exploration, drilling, and production. See catalog #233A for the accompanying DISC book.