Introduction to Geo-mechanics:

Today we will discuss the topic about tress and Strain in Sedimentary Basins

Stress and Strain in Sedimentary Basins

At shallow depths in sedimentary basins there are soft clays and loose silts and sands, while diagenetic processes have transformed these sediments to claystones, shales, silt- and sandstones at greater depths. Sedimentary rocks continuously undergo physical and chemical changes as a function of burial depth, temperature and time, and important hydro-mechanical parameters change during burial,erosion and uplift.An understanding of these processes is important in order to predict the magnitude and distribution of sediment properties and stresses in the basin. The in-situ stress condition affects the rock response to changes in the stress field due to drilling and petroleum production. Soil and rock mechanics (geo-mechanics) have mainly been developed to solve engineering problems in relation to landslides and surface and underground construction. These are usually at very shallow depths compared to that of a petroleum reservoir.We will here focus on some aspects of geo-mechanics of particular relevance for the petroleum geologist.

1. Subsurface Fluid Pressure and Effective Stress Condition

A distinction should be made between total stress, effective stress and fluid pore pressure. This is not always done in technical reports and publications related to petroleum geology.

1.1 Total and Effective Stress

In general, stress (σ) is defined as force per unit area. The overburden weight of the sediment including the weight of the fluid in the pore space produces a vertical stress (σv). For a sedimentary basin with a fairly horizontal surface, and without major lateral variations in the sediment compressibility, the vertical stress at any point can simply be computed as:
σv = ρbgh where ρb is the average sediment bulk density of the overlying sequence, h is the sediment thickness and g is the acceleration of gravity. This is the vertical total stress or the lithostatic stress.It may be calculated more accurately by integrating the varying density over the depth of the sediment column. The effective vertical stress (σv) is defined as the difference between the vertical total stress (σv) and the pore pressure (u): σv = σv −u (11.2) This is the effective stress which is some times called the average intergranular stresses because it is transmitted through the grain framework. It is the effective stress that governs the mechanical compaction of sediments where little chemical compaction (cementation) has taken place. It should be noted that the local intergranular particle-to-particle contact stress is many times higher than the effective stress as defined here, due to the small area of contact. The total overburden weight is carried by the mineral grain framework and the pore pressure

2. Normally Consolidated Versus

Over consolidated Sediments
A layer in a sediment sequence that never before in its geological history has been subjected to higher vertical effective stress than at present, is called normally
consolidated (NC). If, on the other hand, the sediment has been subjected to higher effective stresses, e.g. by previous glacial loading, by higher overburden that
subsequently has been eroded, and/or by pore pressures in the past that were lower than at present, the sediment is called over consolidated (OC) as it has been
preloaded. The ratio between the past maximum effective vertical stress and the present stress is commonly called the over consolidation ratio (OCR).
At relatively shallow depths in a sedimentary basin(less than 2−3 km, <70−90◦C), the mechanical compaction processes dominate over the chemical
compaction in siliceous sediments. At higher temperature(deeper burial) chemical compaction processes become dominant in controlling the rate of compaction. Carbonate sediments may, however,become cemented and highly overcon solidated at shallow depth.
The hydro-mechanical properties at shallow depths may be very different for a normally consolidated sediment sequence compared with an over consolidated one, depending on the magnitude of the OCR.

For the over consolidated sediment, the compressibility and permeability are usually much lower and the shear strength significantly higher. As discussed below, the lateral stresses in over consolidated sediments may be higher than in normally consolidated

3. Horizontal Stresses in Sedimentary Basins

Knowledge of the magnitude and distribution of horizontal stresses in sedimentary basins is important in relation to petroleum exploration, drilling and production.
Their magnitude is also important in the interpretation of seismic signals used in field exploration and in reservoir production management. In a
sedimentary basin the geo-mechanical properties vary from those of loose cohesionless sediments at shallow depths to dense and cemented sedimentary rocks at greater depth. This affects the horizontal (lateral) stress distribution with depth. While the vertical stresses are determined by vertical equilibrium , the magnitude of lateral
stresses cannot be determined by equilibrium equations and is statically indeterminate. Their magnitudes are governed by a number of factors, including the overburden/erosion (loading/unloading) and uplift history of the basin and the deformation characteristics of the sedimentary rocks. These are a result of gravitational
and tectonic forces, and also of stress changes caused by chemical compaction and the accompanying volume change. Their magnitude is determined based on a understanding of the geological history, theoretical and semi-empirical relationships, and field measurements.

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