The role of gravitational collapse in controlling the evolution of crestal fault systems (Espírito Santo Basin, SE Brazil)

Abstract

A high-quality 3D seismic volume from offshore Espírito Santo Basin (SE Brazil) is used to assess the importance of gravitational collapse in the formation of crestal faults above salt structures. A crestal fault system is imaged in detail using seismic attributes such as curvature and variance, which are later complemented by analyses of throw vs. distance (T-D) and throw vs. depth (T-Z). In the study area, crestal faults comprise closely spaced arrays and are bounded by large listric faults, herein called border faults. Two episodes of growth are identified in two opposite-dipping fault families separated by a transverse accommodation zone. Statistical analyses for eighty-four (84) faults show that fault spacing is < 250 m, with border faults showing the larger throw values. Fault throw varies between 8 ms and 90 ms for crestal faults, and 60-90 ms for border faults. Fault length varies between ~410 m and 1750 m, with border faults ranging from 1250 m to 1750 m. This work shows that border faults accommodated most of the strain associated with salt growth and collapse. The growth history of crestal faults favours an isolated fault propagation model with fault segment linkage associated with the lateral propagation of discrete fault segments. Importantly, two episodes of fault growth are identified as synchronous to two phases of seafloor erosion, rendering local unconformities as competent markers of fault reactivation at a local scale. This paper has crucial implications to the understanding of fault growth histories as a means to assess drilling risk and oil and gas migration on continental margins with important salt tectonics. As a corollary, this work demonstrates that: 1) a certain degree of spatial organization occurs in crestal fault systems; 2) transverse accommodation zones can form regions in which fault propagation is enhanced and where regional dips of faults change in 4D.

Corridors of crestal and radial faults linking salt diapirs in the Espírito Santo Basin, SE Brazil

“…Despite salt dissolution and the pressure exerted by overburden units on the Salt Ridge, no evidence of collapse as in Frumkin et al, (2011), Ge and Jackson, (1998) or Mattos et al, (2016), is observed in the 3D seismic volume for this same structure. This finding contrasts with observations for the eastern part of this seismic volume, where salt withdrawal and collapse play a major role in fault evolution (Alves et al, 2009;Ze and Alves, 2016). Throw-depth (T-z) data for the 12 representative faults, show that displacement decreases towards the Mid-Eocene horizon H6 (Fig.…”

“…The reasons for localised fault reactivation are also investigated and discussed in light of published work on different parts of the Espírito Santo Basin (e.g. Baudon and Cartwright, 2008;Omosanya and Alves, 2013;Ze and Alves, 2016). At the end of this paper, our results, and the integrated effects of halokinesis on the trapping and migration of fluids, are compared to areas with similar halokinetic structures.…”

“…The orientation of the principal stresses estimated by inverting the faults on the Salt Ridge is NE-SW (Table 2), agrees with the NNE-trending extension direction suggested by Chang et al (1992) for the SE Brazil. Wherever the orientation of the principal stresses and the strike direction of faults is coincident, there is a greater tendency for faults to reactivate vertically along their length (Baudon and Cartwright, 2008a;Ze and Alves, 2016). The predominant strike direction for the faults in the western part of the Espírito Santo Basin is NW-SE.…”

Structural inheritance and its control on overpressure preservation in mature sedimentary basins (Dongying depression, Bohai Bay Basin, China)

“…Border faults were first identified in rift basins forming prominent fault escarpments on their shoulders; they have large displacements and lengths due to accommodating the bulk of strain and stress in extensional settings (Crossley and Crow, 1980;Ebinger et al, 1987). This structural control occurs at different scales of analysis; in the Espírito Santo Basin, SE Brazil, Ze and Alves (2016) proposed that fault families developed on the crest of salt structures were controlled by border faults, which have the largest maximum throws and accommodate most of the strain and stress on the crest of salt structures. Faults close to border faults are often less active, and are in a less preferential position to propagate and reactivate.…”

Fault interpretation uncertainties using seismic data, and the effects on fault seal analysis: a case study from the Horda Platform, with implications for CO&lt;sub&gt;2&lt;/sub&gt; storage

“…Lindsay et al, 1993;Childs et al, 1997;Fristad et al, 1997;Fulljames et al, 1997;Knipe et al, 1997;Yielding et al, 1997Yielding et al, , 2010Yielding, 2002;Bretan et al, 2003;Faerseth et al, 2006). In general, these methodologies describe a capillary seal, where surface tension forces between the hydrocarbon and water prevent the hydrocarbon phase from entering the water-wet phase; hence, the volume of hydrocarbons that can be contained by the fault is controlled by the capillary entry pressure (Smith, 1980;Jennings, 1987;Watts, 1987). The capillary entry pressure depends on the hydrocarbon-water interface (specifically the wettability, interfacial tension and radius of the hydrocarbon), the difference between the hydrocarbon-phase and water-phase densities and the acceleration of gravity.…”