ABSTRACT:

Scaled physical models have been combined with numerical models in order to integrate knowledge of the evolution of a gravity driven fold-and-thrust belt with its contemporaneous state of stress. The models presented are of gravitydriven non- to undercompacted sediments in a passive margin setting. The application of the contemporaneous stress field to drilling specific problems is presented.

1.INTRODUCTION

Numerous studies exist for tectonically driven fold-andthrust belts above frictional and viscous substrates, e.g. Zagros Mountains (Iran) [1], Betic Cordillera (Spain) [2], Potwar Plateau and Salt Range (Pakistan) [3]. Gravity driven fold-and-thrust belts above viscous substrates, commonly found on passive margins, have not been studied as extensively, despite their hydrocarbon potential and numerous examples that exist on passive margins and their association with significant hydrocarbon reserves (e.g. Lower Kwanza Basin (Angola) [4], Champion and Baram deltas (offshore Brunai, [5]), Perdido fold belt (Gulf of Mexico, [6]) and Cascadia (USA, [7]). Generally, tectonically driven models are either of the indentor [8] or subduction type [9] and focus on the confirmation and/or modification of the critical-taper Coulomb wedge model of [10], [11] and [12] (Figure 1). Both model types use mechanically imposed displacements to generate a growing wedge above a frictional substrate by foreland propagation, where foreland-vergent thrusts accommodate more shortening with respect to hinterland-vergent thrusts (e.g. [13]; Figure 1). The width of the fold-and-thrust belt is a function of basal friction and the amount ofdisplacement imposed (Fig. 1).

Fig. 1. Boundary conditions of Coulomb wedges in cross - sections of thrust models. (a) Indentor driven (model by B. C. Vendeville ; cf. [8]) and (b) subduction driven (image courtesy of Ken McClay ; see [14] for more details).(available in full paper)

Viscous substrates, e.g. evaporites, can behave in a similar fashion, i.e. as weak brittle detachments, if their thickness is very low, i.e. just enough to act as lubrication. Due to a near absence of shear resistance at the basal detachment, a viscous layer with a significant thickness facilitates extensive translation of the shortened overburden over large distances and a broad fold-and-thrust belt is the result. Additionally the low shear strength favors neither back- nor forethrust formation and thus symmetric pop-up structures are typical. Other, external, factors affect the structural style of the fold-and-thrust belt in these settings, e.g. thickness of the salt detachment, dip of the salt layer, bathymetric dip of the continental slope, sedimentation and erosion rates, location and style of salt pinch-out and the temporal and spatial variation of all of the above (e.g. [15], [8]). In this paper we use recently published scaled physical models [16] and compliment these with numerical model analyses that predict the in-situ stress in a complex gravitationally driven fold-and-thrust systems in a passive margin setting.

2.PHYSICAL MODELS

In their 2007 study, [16] built scaled physical models to simulate a linked extension-translation-compression system to examine the kinematic complexities introduced into a gravity driven fold-and-thrust belt by three variables: 1) the gravitational potential of a basinward dip of salt and its overburden, 2) an intra-salt perturbation in the form of a basinward-flattening hinge and 3) the buttressing effect of a distal salt pinch-out.

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