Layer-bound Faults Research Articles

Control by preexisting morphology on layer-bound faults in the southern Qiongdongnan Basin, NW South China Sea

The ideal development of layer-bound, polygonal faults occurs in an isotropic stress field. However, some populations of layer-bound faults appear to have originated as ‘polygonal’ faults, but display atypical plan-view fault geometries that suggest development under anisotropic stress conditions. High-resolution 2D/3D seismic data in the southern Qiongdongnan Basin displays three tiers of layer-bound faults with distinct strike variations and complex structural patterns that depart from the geometries of isotropic polygonal fault cells. The lower fault tier displays preferred orientations along E-W, NE-SW, NW-SE directions; the middle fault tier shows a primary E-W orientation and a secondary N-S trend; the shallow tier features strong N-S and E-W orientations. Fault patterns in map view include orthogonal, quasi-polygonal and circumferential geometries, among which the orthogonal pattern is dominant in the shallow fault tier. Several factors are inferred to cause these variable geometrical and structural patterns in layer-bound faults, they are: pre-existing tectonic faults, an uplifted region, local slopes, contourite depressions, and large contourite-related channels. The orthogonal fault patterns have longer E-W orientation parallel to the contourite channels and shorter N-S trends perpendicular to the thalwegs. The presence of submarine channels is suggested to locally deflect the stress orientation, which in turn impacts fault patterns.

Using polygonal layer-bound faults as tools to delimit clastic reservoirs in the Levant Basin offshore Lebanon

The Levant Basin offshore Lebanon contains an array of layer-bound normal faults in the Oligocene–Miocene units. The faults are believed to have nucleated in fine-grained sedimentary rocks similar to polygonal fault systems worldwide, and as a result, they may be influenced by lithological heterogeneities in the host-rock unit. Three-dimensional seismic data and amplitude extraction from offshore Lebanon were used to map deep-water channels and fan lobes, demonstrating that the distribution, geometry, and growth of the layer-bound normal faults are affected by these sedimentary bodies, which are impacting fault propagation. Three fault types are identified, differentiated by their growth models and overall geometry. Type I faults in the deep basin indicate that a competent unit is present in the lower Miocene and is affecting their growth. Type II and III faults in the Latakia ridge and the margin, respectively, are smaller because of restriction by upper Miocene and lower Oligocene basin-floor fans. Based on their seismic expression and on analogy with other basins containing similar faults, it is very likely that the overlying and underlying units causing restriction consist of coarse-grained clastic intervals. Therefore, reservoirs offshore Lebanon are probably lower Miocene in the deep basin, whereas in the Latakia ridge and the Levant margin, reservoirs are in upper Miocene and in lower Oligocene units. This study provides an additional evidence that layer-bound, or polygonal faults, can have practical industrial application during exploration of hydrocarbons, even in the absence of well data.

Growth of layer-bound normal faults under a regional anisotropic stress field

Abstract Layer-bound normal faults commonly form polygonal faults with fine-grained sediments early in their burial history. When subject to anisotropic stress conditions, these faults will be preferentially oriented. In this study we investigate how faults grow, evolve and interact within regional-scale layer-bound fault systems characterized by parallel faults. The intention is to understand the geometry and growth of faults by applying qualitative and quantitative fault analysis techniques to a 3D seismic reflection dataset from the Levant Basin, an area containing a unique layer-bound normal fault array. This analysis indicates that the faults were affected by mechanical stratigraphy, causing preferential nucleation sites of fault segments, which were later linked. Our interpretation suggests that growth of layer-bound faults at a basin scale generally follows the isolated model, accumulating length proportional to displacement and, when subject to an anisotropic regional stress field, resembling to a great extent classical tectonic normal faults.

Outcrop examples of soft-sediment deformation associated with normal fault terminations in deepwater, Eocene turbidites: A previously undescribed conjugate fault termination style?

A set of small-scale, layer-bound faults developed in a package of three Eocene turbidite sandstone beds in a wave cut platform, Thandwe Beach, Myanmar display unusual downward terminations between converging conjugate faults. The faults were initially triggered by loading during deposition of the uppermost turbidite sandstone in the fault-affected stratigraphic package. The growth of a small gravity-driven anticline, and subsequent loading by a post-deformation turbidite modified the initial fault geometry, and re-mobilized part of the layer-bound sequence causing thinning of the sequence to zero in the vicinity of the fold crest. Typically conjugate faults display an X-shaped pattern of intersection, while the observed faults before reaching the point of intersection die out in a drastically thinned basal sandstone located in a keystone block, which requires either an underlying detachment or that a volume of sand and fluid was lost from the basal layer. Probably a combination of gravity-driven detachment (primary) movement and volume loss (secondary) permitted development of the faults. The structures described represent yet another aspect of the wide variety of gravity-driven features that form in the deepwater turbidite fan settings. It is thought that such features have not been previously described in the literature, and represent a new style variant within the general theme of conjugate fault development.

The Genesis of Layer-Bound Polygonal Faults in the Sanzhao Sag of Songliao Basin

Through new 3-D seismic data and the technique of coherent slice, the development characteristic of layer-bound polygonal faults (PFs) is summarized in Zhaozhou oilfield in the south of Sanzhao sag, Songliao Basin. the layer-bound faults that show preferential alignment have lengths of 1503000 m, throws of 510 m, most strikes parallel to the slope direction and aligned faults are antithetic to the bedding dip. PFs activity coincided with a late K2qn to early K2y stage tilting event, gravity sliding model is proposed as a possible mechanism for the deformation.

Influence of stratigraphic setting and simple shear on layer-bound compaction faults offshore Mauritania

We have used three-dimensional seismic data to examine the geometry of layer-bound compaction faults on the Mauritanian continental margin. The faults deform a fine-grained Neogene unit which mantles buried canyons. Above the canyons, most layer-bound faults strike perpendicular to the canyon axes, whereas adjacent to the canyons the faults are omni-directional and intersect bedding planes in polygonal patterns. On the flanks of a large structural high, faults also strike perpendicular to the slope direction, displaying a concentric pattern. Faults that show preferential alignment are located in areas with increased bedding dip, and most aligned faults are antithetic to the bedding dip. Where the bedding dip is greater than ∼1°, the synthetic faults dip more steeply than the antithetic faults. This discovery was unexpected because the stress tensor due to the effect of gravity on a dipping margin would cause the antithetic faults to form with steeper dips, which we show theoretically. We hypothesise that the layer hosting the faults has been subjected to simple shear of ∼20° or more, with the top of the layer displaced downdip by ∼100 m relative to its base. A potential application of these, and similar, observations is to assess slope stability on continental margins.

Polygonal faults in the Sanzhao sag of the Songliao basin: their significance in hydrocarbon accumulation

Polygonal faults, generally distributed in fine-grained sediments, are layer-bound faults and are important in hydrocarbon accumulation. Using 3D seismic data, we analyzed the plane and profile features of faults developed in the Qingshankou formation of the Sanzhao sag. We identified these faults as having typical features of polygonal faults: 1) layer-bound; 2) normal faults; 3) slight fault displacements and steep in dip angles; 4) multi-directional in strike and 5) a single fault has a short horizontal extension. In addition, these faults intersect each other and form polygons. These polygonal faults are the result from the combined action of compaction, volume contraction and episodic hydraulic fracturing, conditions favorable for oil/gas accumulation. They are the dominant channels for migration of fluids in the Qingshankou mudstone, forming a large number of fault-lithologic oil traps. Polygonal faults improve reservoirs.

Layer-bound compaction faults in fine-grained sediments

This paper describes examples of a recently recognized type of soft-sediment deformation associated with early compaction of fine-grained sediments. This type of deformation was originally described from the North Sea Basin, where Paleogene slope and basin-floor claystones are deformed over an area of >150 000 km 2 by a layer-bound system of minor extensional faults arranged in polygonal patterns in map view. The development of this regionally extensive polygonal fault system has been attributed to volumetric contraction during early compactional dewatering on the basis of detailed strain measurements carried out using high-resolution three-dimensional seismic data. A comprehensive review of published two-dimensional and three-dimensional seismic data from 27 other layer-bound fault systems from many different sedimentary basins is presented in this paper. The only factors common to all 28 examples of layer-bound faults are that the deformed units are only found in marine depositional settings, are dominantly composed of ultrafine-grained smectitic claystones or carbonate chalks, and are characterized by high porosity and extremely low permeability. Other factors such as sedimentation rate, organic carbon content, age, depth of burial, methane content, and pore-fluid chemistry are not systematically correlated with this deformational response. The correlation between distribution of deformed units and ultrafine grain size suggests that the deformation mechanism is related to colloidal properties as part of this type of compactional response. The restricted distribution of layer-bound fault systems to predominantly pelagic depositional units with often low sedimentation rates is compatible with a recently presented model of volumetric contraction during early burial. We build on this model of fully three-dimensional compaction to propose that layer-bound faulting is an expression of the process of syneresis, whereby pore fluid is expelled from sedimentary gels under the spontaneous action of osmotic or electrochemical forces.

Seismic Expression of Layer-Bound Fault Systems of the Eromanga and North Sea Basins

This paper compares the seismic expression of Lower Tertiary mudstone-dominated sequences of the North Sea basin with Cretaceous siliciclastic sequences of the Eromanga Basin. The paper focuses on the development of layer-bound systems of minor extensional faults that pervasively deform these sequences in both basins. Layer-bound fault systems have only recently been recognised in the North Sea with the availability of high resolution 3D seismic data. The resolution achievable with 3D mapping has revealed that in map-view, these faults are arranged in polygonal networks reminiscent of dessication cracks. This property of polygonality is accounted for in a genetic model based on basin-wide volumetric contraction of muddy sediments during the early stages of compactional dewatering.