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