Overpressure development in sedimentary basins induced by chemo-mechanical compaction of sandstones

Abstract

Mechanisms of overpressure build-up resulting from chemically-induced compaction are investigated by considering that intergranular pressure-solution, controlled by temperature and effective stresses, is the dominant process of chemical deformation in sandstones. The numerical simulations are performed using a thermo-poro-mechanical tool based on the finite element method specifically devised to deal with sedimentary basin modeling. At the material level, purely mechanical and chemo-mechanical deformations are respectively addressed by means of plastic and viscoplastic constitutive components. Porosity data of the Middle Jurassic Garn Formation from the Haltenbanken area of the Mid-Norwegian Continental Shelf are taken as reference for calibration of the sandstone model to be used on synthetic cases of a siliciclastic basin in oedometric conditions. Two situations are proposed involving different depositional sequences of sandy and shaly sediments with the aim to assess the permeability effect in the numerical model. The results have shown that early overpressure development in low permeability formations preserves sandstone porosities by reducing effective stresses and thus retarding pressure-solution compaction, whereas higher effective stresses associated with permeable depositional environments may lead to important chemo-mechanical deformation, resulting in low porosity sandstones and significant overpressure generation at later stages of basin history. An additional case is finally analyzed to investigate the consequences of pressure-solution inhibition due to diagenetic grain coating in a sandstone reservoir. The simulation resulted in porosity preservation and lower overpressure values, showing that a sedimentary basin submitted to an important level of chemo-mechanical compaction can present substantially higher overpressure distribution than basins where this phenomenon did not take place. The paper highlights the importance of integrating stresses, fluid pressure and temperature to represent the porous material evolution in order to describe the geological state of sedimentary basins.

Coupled poro‐mechanical tangent formulation applied to sedimentary basin modeling

“…This section presents the porous material behavior currently implemented in the basin simulator described in Brüch et al 20 The main differences of the constitutive model with respect to its previous version 21 are presented in Sections 2.2 and 2.3. They mainly consist in introducing non-associated formulations for the plastic and viscoplastic models which aim to give more realistic dilating behavior for the sediment material.…”

Formulation of reference solutions for compaction process in sedimentary basins

“…The role of pore pressure and geothermal gradient has been assessed through numerical formulations in previous works. 9,41,44,51,52 The sedimentary basin undergoing compaction is modeled as an infinite layer, perpendicular to the e 3 direction and lying on a rigid substratum along the plane x 3 = 0. Neglecting the tectonic activity, the gravitational field g = −ge 3 stands for the only external loading in the compaction process.…”

“…In order to formulate semianalytical solutions for the compaction process in sedimentary basins, a simplified setting relying upon the following assumptions is adopted: (a) the whole sedimentation process takes place under oedometric conditions; (b) the sediment constitutive material exhibits homogeneous and isotropic mechanical properties in its reference state, that is, at the time it is deposited at the top of the basin; (c) the hydromechanical coupling is disregarded in the subsequent analysis (i.e., the effect of pore pressure is not considered), which amounts to addressing the particular case of highly permeable sediment material (fully drained conditions); and (d) the mechanical evolution of the basin is analyzed under isothermal condition, disregarding the effect of geothermal gradient on the material properties and deformation. The role of pore pressure and geothermal gradient has been assessed through numerical formulations in previous works 9,41,44,51,52 …”

“…A formulation based on limit analysis and micromechanics has been originally proposed in Barthélémy et al 42 for p c . This formulation has been recently modified by Brüch et al 52 using a calibration exponent m p : where p c 0 is the initial consolidation pressure for plasticity.…”

“…As regards the initial consolidation pressure for plasticity, a deliberately high value is adopted for to better visualize the model predictions and thus clearly illustrate each phase of the compaction process described in Section 3. More realistic values of p c 0 could actually be adopted in order to activate plastic deformation at low overburden levels (see, e.g., Brüch et al 52 ). As it will be commented in Section 4.2, the values considered for material properties in the reference state lead to unrealistically overestimating the extent of upper elastic layer.…”