ABSTRACT:

The relationship between fault geometries and fault deformation behaviors, as well as its controls on fluid flow in reservoirs during extensional reactivation, is explored using a series of coupled deformation and fluid flow numerical models. The results show that the initial length of faults in a fault population is the primary control in determining strain distribution between faults and in transporting fluids through the top seal from the reservoir horizon. During extensional reactivation, longer faults tend to accommodate greater shear strain and accumulate greater throws than smaller faults. Longer faults are also more important fluid upflow conduits, because their greater shear strain results in greater dilation and better connectivity.

1. INTRODUCTION

Fault reactivation and associated rock deformation represent a major risk to trap integrity and hydrocarbon preservation. Data from the Timor Sea indicate that most of the faults in the region have been reactivated during the Late Tertiary extensional event [1, 2]. The distribution of fault movement and rock deformation on the fault populations of the region are inhomogeneous, and this critically controls the breach or preservation of oil traps. To advance our understanding of the relationship between fault attributes (length and orientation) and fault deformation and their impact on fluid flow in reservoirs and faults, we have constructed a group of coupled mechanical deformation and fluid flow numerical models containing simple fault geometries. This paper presents the results of these models. Extensive previous studies have been conducted on rock deformation and fluid flow associated with faults in the fields of petroleum geology and economic geology. For example, Connolly and Cosgrove [3, 4] investigated stress patterns around dilatant fault jogs under strike-slip conditions and inferred fluid flow patterns based on mean stress patterns from their photoelastic analogue modelling experiments. Using a numerical modeling approach, Zhang et al. [5] simulated rock dilation and fluid flow localization around fault dilation jogs under compressional and strike slip conditions. McLellan et al. [6] numerically simulated deformation and fluid flow in an extensional basin model containing one shallow dipping fault, a structural scenario for the Hamersley Basin. Gartrell et al. [7] numerically modeled extensional reactivation of a triple fault intersection, one of structure types in the Timor Sea, and further explored fluid flow patterns for such fault architecture. Walsh et al. [8] showed that fault displacement rates correlate with fault size, based on the results of their deformationonly, particle-material models on normal faults. All these efforts illustrated strain localization or fluid flow focusing into fault zones (in particular, fault intersections, jog structures or longer faults), under contractional, strike slip or extensional conditions. However, the mechanism by which the fault length factor can affect strain partitioning and fault movement (e.g. down throw under extensional settings) in a fault population still needs further work. This is particularly important for the evaluation of integrity and hydrocarbon preservation, and is thus the focus of the present study. It also needs to be mentioned that this work is only concerned with the extensional reactivation of pre-existing faults, rather than fault initiation and growth [9].

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