Pressure Solution in Sedimentary Basins: Effect of Temperature Gradient

arXiv:1003.4970v1 [physics.geo-ph] 25 Mar 2010 Pressure Solution in Sedimentary Basins: Effect of Temperature Gradient Xin-She Yang Department of Fuel and Energy and Department of Applied Mathematics University of Leeds, LEEDS LS2 9JT, UK Abstract Pressure solution is an important process in sedimentary basins, and its behaviour depends mainly on the sediment rheology and temperature distribution. The compaction relation of pressure solution is typically assumed to be a viscous one and is often written as a relationship between effective stress and strain rate. A new derivation of viscous compaction relation is formulated based on more realistic boundary conditions at grain contacts. A nonlinear diffusion problem with a moving boundary is solved numerically and a simple asymptotic solution is given to compare with numerical simulations. Pressure solution is significantly influenced by the temperature gradient. Porosity reduction due to pressure solution is enhanced in an environment with a higher thermal gradient, while porosity decreases much slowly in the region where the thermal gradient is small. Pressure solution tends to complete more quickly at shallower depths and earlier time in higher temperature environment than that in a low one. These features of pressure solution in porous sediments are analysed using a perturbation method to get a solution for the steady state. Comparison with real data shows a reasonably very good agreement. Key Words: viscous compaction, pressure solution, asymptotic analysis, temperature gradient. Citation detail: X. S. Yang, Pressure solu- tion in sedimentary basins: effect of temper- ature gradient, Earth and Planetary Science Letters, 176, 233-243 (2000). 1 INTRODUCTION Pressure solution is a very common and im- portant deformation process in porous media and granular materials such as sediments and soils. Pressure solution also occurs in sedimen- tary basins where hydrocarbons and oil are pri- marily formed. The modelling of such com- pactional flow is thus important in the oil in- dustry as well as in civil engineering. One par- ticular problem which affects drilling process is the occasional occurrence of abnormally high pore fluid pressures, which, if encountered sud- denly, can cause drill hole collapse and conse- quent failure of the drilling operation. There- fore, an industrially important objective is to predict overpressuring before drilling and to identify its precursors during drilling. An es- sential step to achieve such objectives is the sci- entific understanding of their mechanisms and the evolutionary history of post-depositional sediments such as shales. Compaction is the process of volume reduc- tion via pore-water expulsion within sediments due to the increasing weight of overburden load. The requirement of its occurrence is not only the application of an overburden load but also the expulsion of pore water. The extent of compaction is strongly influenced by sedimen- tation history and the lithology of sediments. The freshly deposited loosely packed sediments tend to evolve, like an open system, towards a closely packed grain framework during the ini- tial stages of burial compaction and this is ac- complished by the processes of grain slippage, rotation, bending and brittle fracturing. Such reorientation processes are collectively referred to as mechanical compaction, which generally takes place in the first 1 - 2 km of burial. Af- ter this initial porosity loss, further porosity reduction is accomplished by the process of 1 chemical compaction such as pressure solution at grain contacts [1,2,3]. Pressure solution has been considered as an important process in deformation and poros- ity change during compaction in sedimentary rocks [4,5]. Pressure solution refers to a pro- cess by which grains dissolve at intergranular contacts under non-hydrostatic stress and re- precipitate in pore spaces, thus resulting in compaction. The solubility of minerals in- creases with increasing effective stress at grain contacts. Pressure dissolution at grain con- tacts is therefore a compactional response of the sediment during burial in an attempt to in- crease the grain contact area so as to distribute the effective stress over a larger surface. Such a compaction process is typically assumed to viscous [5,6,7] and it is usually referred to as viscous compaction, viscous creep or pressure solution creep. Its rheological constitutive re- lation (or compaction relation) is often written as a relationship between effective stress and strain rate. A typical form of pressure solution is inter- granular pressure solution (IPS) which occurs at individual grain contacts and free face pres- sure solution (FFPS) which occurs at the face in contact with the pore fluid, but most stud- ies have concentrated on the former one (IPS). Extensive studies [1,5,6,7,8] on pressure solutin have been carried out in the last two decades, and a comprehensive literature review on these models was given by

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