The Formation of Forced Folds and Wing‐Like Sand Intrusions Driven by Pore Fluid Overpressure: Implications From 2‐D Experimental Modeling

Abstract

Sand injections form by intrusion of overpressured, fluidized sand into surrounding low-permeable, fine-grained rocks. Modern 3-D seismic data revealed their abundant occurrence in many sedimentary basins and that their intrusion is typically associated with forced folding and tensile fractures of the sealing cover layer. In order to investigate the kinematic evolution of forced folds in relation to the associated propagation of fractures originating from an overpressured source layer, we performed idealized, quasi-2-D analog experiments. The models consist of noncohesive and cohesive granulates to mimic a sand reservoir and its overburden layer and injected air to produce fluid overpressure in the layered materials. Our results show that forced folding first induces tensile bending fractures at the base of the fold limbs at a certain critical fluid pressure. Due to further increase of the fluid pressure, the apex of these bending fractures serves as origin for branching, conical fractures characterized by shear and tensile failure. Fracture breakthrough is accompanied by a rapid uplift of the breached fold limb and a pressure drop in the reservoir layer followed by a continuous subsidence of the central forced fold. The morphology of the fracture pattern and the forced fold provides helpful implications for understanding formation processes of natural sand injections observed in seismic data and in outcrops.

Patterns and Failure Modes of Fractures Resulting From Forced Folding of Cohesive Caprocks – Comparison of 2D vs. 3D and Single-vs. Multi-Layered Analog Experiments

“…Lohrmann et al, 2003). Physical properties of the used materials, such as frictional shear strength and permeability, were measured and are further described in (Warsitzka et al, 2019). Silicate cenospheres are used to represent the reservoir layer, due to a high permeability (~13 × 10 -11 m 2 ), a small grain density (~750 kg m −3 ) and a small frictional strength (Warsitzka et al, 2019).…”

“…Physical properties of the used materials, such as frictional shear strength and permeability, were measured and are further described in (Warsitzka et al, 2019). Silicate cenospheres are used to represent the reservoir layer, due to a high permeability (~13 × 10 -11 m 2 ), a small grain density (~750 kg m −3 ) and a small frictional strength (Warsitzka et al, 2019). The natural cover of the overpressured reservoir is assumed to be a fine-grained, relatively consolidated sedimentary rocks with a low permeability, a relatively high shear strength and the ability to fail in shear and in tension.…”

“…The experiments are geometrically and dynamically scaled so that governing parameters are proportional between model and nature (Merle, 2015). The scaling procedure of analog models using streaming air to produce a pore fluid overpressure are described in detail in Mourgues et al (2012) and Warsitzka et al (2019). We chose a geometric scaling ratio of γ L = 10 -4 so that 10 cm in the experiment corresponds to 1000 m in nature.…”

“…Brooke et al, 1995;Huuse et al, 2005;Frey-Martnez et al, 2007;Mourgues et al, 2011). At excessive fluid overpressure, the caprock is often uplifted into a forced fold leading to fracturing at the crest and along the hinges of the fold (Cosgrove and Hillier, 2000;Shoulders and Cartwright, 2004;Frey-Martnez et al, 2007;Jackson et al, 2011;Mourgues et al, 2012;Haug et al, 2018;Warsitzka et al, 2019;Meng and Hodgetts, 2020). Depending mainly on the cohesion and the thickness of the cover, these fractures fail in a tensile, shear or hybrid tensile-shear mode (e.g.…”

“…In previous analog modelling studies using brittle granulates to simulate host rocks, 2D experimental (narrow) apparatuses have be applied to monitor fracture patterns and intrusion processes through glass side walls (e.g. Rodrigues et al, 2009;Mourgues et al, 2012;Warsitzka et al, 2019). However, the 3D variability cannot be examined in quasi-2D (Abdelmalak et al, 2012).…”