Faulting, fracturing and in situ stress prediction in the Ahnet Basin, Algeria - a finite element approach

Many low-efficiency hydrocarbon reservoirs are productive largely because e ffective reservoir permeability is controlled by faults and natural fractur es. Accurate and low-cost information on basic fault and fracture propertie s, orientation in particular, is critical in reducing well costs and increa sing well recoveries. This paper describes how we used an advanced numerica l modelling technique, the finite element method (FEM), to compute site-spe cific in situ stresses and rock deformation and to predict fracture attribu tes as a function of material properties, structural position and tectonic stress. Presented are the numerical results of two-dimensional, plane-strai n end-member FEM models of a hydrocarbon-bearing fault-propagation-fold str ucture. Interpretation of the modelling results remains qualitative because of the intrinsic limitations of numerical modelling; however, it still all ows comparisons with (the little available) geological and geophysical data . In all models, the weak mechanical strength and flow properties of a thick shale layer (the main seal) leads to a decoupling of the structural deforma tion of the shallower sediments from the underlying sediments and basement, and results in flexural slip across the shale layer. All models predict ro ck fracturing to initiate at the surface and to expand with depth under inc reasing horizontal tectonic compression. The stress regime for the formatio n of new fractures changes from compressional to shear with depth. If pre-e xisting fractures exist, only (sub)horizontal fractures are predicted to op en, thus defining the principal orientation of effective reservoir permeabi lity. Tn models that do not include a blind thrust fault in the basement, f lexural amplification of the initial fold structure generates additional fr acturing in the crest of the anticline controlled by the material propertie s of the rocks. The folding-induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading. Models incorporat ing a blind thrust fault correctly predict the formation of secondary syn- and anti-thetic mesoscale faults in the basement and sediments of the hangi ng wall. Some of these faults cut reservoir and/or seal layers, and thus ma y influence effective reservoir permeability and affect seal integrity. The predicted faults divide the sediments across the anticline in several comp artments with different stress levels and different rock failure (and proxi mity to failure). These numerical model outcomes can assist classic interpr etation of seismic and well bore data in search of fractured and overpressu red hydrocarbon reservoirs. (C) 2000 Elsevier Science B.V. All rights reser ved.