Seismic attribute illumination of a synthetic transfer zone Paritosh Bhatnagar*, Craig Bennett, Rustam Khoudaiberdiev, Sterling Lepard and Sumit Verma

The University of Texas of the Permian Basin

Summary

Transfer zones a feature where deformational strain is

transferred from one fault system to another play an important role in controlling fluid migration in the

subsurface. More specifically, a synthetic transfer zone occurs where strain is transferred between two parallel normal faults in an extensional system. A previous study used surface curvatures derived from a clay model to highlight different geological features related to a synthetic transfer zone, including fault planes and relay ramps. We follow the same approach, applying our understanding to a 3D seismic survey to identify geological features related to

a synthetic transfer zone. This study discusses the effect of synthetic transfer zones on an intrabasin extensional system, and describes listric normal faults and a relay ramp using the curvature and coherence seismic attributes. Our research area focuses on Penobscot, an offshore potential field in the Scotian Basin.

Introduction

A synthetic transfer zone is a structural area where one normal fault dies out and another normal fault begins, with both dipping in the same direction (Morley et al., 1990). As a result, a relay ramp is formed in the overlap zone between the two faults (Fossen and Rotevatn, 2015). Relay ramps in an intrabasin extensional system play a major role as a conduit for sedimentation, but have minimal effect on basin-

scale stratigraphy (Welsink et al., 1989). Transfer zones can increase sediment accumulation during deposition, and are important elements in controlling fluid migration in the subsurface. Transfer zones also lead to the development of secondary faults, which are generally sub-seismic (Morley et al., 1990). The presence of transfer zones can help interpreters delineate secondary features such as fractures, splay shears and Riedel faults (Cahoj and Marfurt, 2014).

Paul and Mitra (2013) prepared a clay model to demonstrate the behavior of synthetic transfer zones; using the surface curvature of this model, Cahoj and Marfurt (2014) illustrated the geometry of these zones. In this paper, we discuss intrabasin synthetic transfer zones in the Scotian Shelf near Sable Island with the help of seismic attributes. Using the Penobscot 3D seismic survey (SEG open data), we characterize the two normal faults and overlapping relay

ramp. Further, we analyze coherence and curvature attributes to understand the geometry of synthetic transfer zones.

Geology of the study area

The Scotian Basin is located in the Scotian Shelf, extending 1200 km southwest to northeast, from the Yarmouth Arch / United States border to the Avalon Uplift on the Grand Banks (Figure 1). Production was first established in 1992, with traps incorporating for stratigraphic and structural components. Nearly all hydrocarbon production on the

Scotian Shelf has been limited to the Sable Subbasin near Sable Island (NSDE, 2011).

Structural and Stratigraphic history The Scotian Shelf developed as North America rifted and separated from the African continent during the break-up of Pangea near the end of the Triassic. It consists of series of

platforms and depocenters, the Sable subbasin being one of them. Tectonism in the central rift basin during the Early Jurassic (mid-Sinemurian) resulted in complex faulting and erosion of Late Triassic and Early Jurassic sediments and older rocks (Figure 2). The first instances of listric faulting occurred during the Late Jurassic. Middle Jurassic and Cretaceous sediment loading of unstable synrift sediments along and to the south of the basement hinge zone initiated

subsidence and development of seaward-dipping growth faults. These syndepositional faults concentrated sedimentation on the downthrown blocks, resulting in the local thickening of sediments into the faults.

Figure 1. Location map of the study area (Google Earth Maps).

The Penobscot 3D seismic survey is displayed in the yellow

rectangle. The approximate location of well L-30 (inside the

seismic survey) is indicated by the black dot.

© 2017 SEG SEG International Exposition and 87th Annual Meeting

Page 2112

Dow

nloa

ded

08/1

7/17

to 2

04.1

58.1

62.1

28. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Synthetic transfer zone

Early synrift deposition, which was not mapped in the study area, (Anisian to Taorcian) is characterized by a transition from terrestrial rift sediments to shallow marine carbonates and clastics (Welsink et al, 1989). Of note is the Argo Salt Formation, an unstable mobile salt analogous to the Louann

Salt in the Gulf of Mexico. Following the final rifting of North America from Africa, Mesozoic deposition is characterized by two major postrift sequences, one Late to Mid-Jurassic and one Early to Mid-Cretaceous. The first (Aalenian – Tithonian) is a mixed carbonate-clastic sequence characterized by the widespread development of the Abenaki carbonate bank. The second sequence

(Berriasian – Turonian) consists of a thick, rapidly deposited deltaic wedge (Missisauga Formation) and a series of thinner, back-stepping deltaic lobes (Logan Canyon Formation) separated by the Naskapi Shale (Aptian MFS). Growth faulting reached its apex in this sequence. The Petrel, a chalk-rich unit and prominent seismic marker, is interpreted as the upper boundary of this sequence.

Seismic attribute analysis Figure 3a shows a left stepping pair of normal faults forming a transfer zone. A relay ramp develops between the two

en-echelon normal faults. As indicated by Cahoj and Marfurt

(2014) in Figure 3b, we expect most positive (K1) and most negative (K2) seismic curvature anomalies in the up thrown and downthrown blocks respectively (Figure 3c). We also expect that anomalous low seismic coherence values will delineate the fault plane (Qi et al. 2016), as indicated in Figure 3c. We utilize these seismic attributes to illuminate the transfer zone in our study area.

Figure 2. Stratigraphic column of the study area. The first track

indicates measured depth. The second track shows the gamma ray

log for well L-30, the darker color signifies high gamma ray values,

which indicates shaly facies, whereas the light colors are for low

gamma ray values, indicating sand rich facies. The third track

shows the name of the formation and geological age.

Figure 3. (a) A transfer zone with left stepping pair of normal

faults (modified after Paul and Mitra, 2013), (b) Curvature

computed for a synthetic transfer zone on a clay model surface

(modified after Cahoj and Marfurt, 2014), (c) Coherence and

Curvature anomaly representing faults and structureal features.

© 2017 SEG SEG International Exposition and 87th Annual Meeting

Page 2113

Dow

nloa

ded

08/1

7/17

to 2

04.1

58.1

62.1

28. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Synthetic transfer zone

Penobscot 3D survey Penobscot 3D seismic survey was acquired in 1992 with hydrophones and a 4 ms sample rate. The 3D seismic survey area is approximately 33.4 mi2. The data were processed

with a bin size of 39ft x 82 ft (12.5m x 25m). Figure 4 shows a N-S vertical slice of seismic amplitude. There are two major faults in the seismic survey area, which we identified as Fault 1 and Fault 2 (Figure 4 and Figure 5). We chose the Late Cretaceous Petrel formation top for our attribute analysis, as it is an excellent seismic marker.

Time structure and thickness map

Figure 5a is a time structure map of the Petrel surface and shows the features of interest. Shallower depths are indicated by orange, and deeper depths are indicated by purple. We used the coherence attribute to highlight discontinuous features such as the two faults (Figure 5b). They are represented by low coherence values. The seismic cross section AA’, seen in Figure 4, intersects the two faults. Figure 5c shows a time thickness map of the Wyandot

formation (Figure 4), showing thicker segmentation on the down-thrown side of the faults. Fault 1 is dying off and Fault 2, which is dipping in the same direction, is beginning; a relay ramp has formed between them, thus completing the synthetic transfer zone.

Figure 5. (a). Time structure map of Petrel top surface, (b)

Coherence extracted on the Petrel surface, (c) time thickness map of

Wyandot (light colored fissures represent secondary faults). Notice

the two faults (Fault 1 and Fault 2). They dip in the same direction, forming a synthetic transfer zone, indicated by relay ramp.

Figure 4. Seismic amplitude vertical slice through AA’ (NS)

(indicated in Figure 5b). The solid cyan line is the Wyandot

surface and solid yellow line is the Petrel surface.

500

0

1000

2000

1500

2500

3000

Time (ms)

Petrel

2 miles (~3.2 km)

A A’

Wyandot

Positive

Negative

0

Seismic

Amplitude

© 2017 SEG SEG International Exposition and 87th Annual Meeting

Page 2114

Dow

nloa

ded

08/1

7/17

to 2

04.1

58.1

62.1

28. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Synthetic transfer zone

Coherence and Structural curvature The coherence attribute identifies the discontinuity, based on the change in seismic amplitude and waveform shape. The structural curvature attribute calculates the curvedness of the

folding and bending of seismic reflectors (Marfurt, 2015). The K1 anomaly represents the most positive curvature, such as around the peak of an anticline, whereas the K2 anomaly shows the most negative curvature, such as around the trough of a syncline. Figures 6 and 7 show the structural curvature of the Petrel surface co-rendered with coherence; both fault planes are delineated by low coherence anomalies(black color). On the updip side of the two faults,

a strong K1 anomaly (red color) can be seen. Similarly, on the downthrown side, a strong K2 anomaly (blue color) appears. An extension of Fault 2 towards the east, where throw is too small to be displayed by seismic amplitude, is visible with curvature anomalies. The relay ramp is also indicated as an anticlinal shape (K1 anomaly).

Discussion and conclusions

Comparison of Figure 3b and Figure 7, indicate that the synthetic transfer zone modeled by Paul and Mitra (2013) is very similar to the real case scenario of Penobscot. The relay ramp in-between the two faults shows up as a dome (indicated by K1 anomaly), which suggests that the relay ramp can act as a potential hydrocarbon trap. Coherence and curvature attributes delineate different features of the

synthetic transfer zone. This enhances the analysis of the changing structure, which can help interpreters better understand transfer zone geometry.

Acknowledgements We would like to acknowledge the Nova Scotia Department of Energy and Canada Nova Scotia Offshore Petroleum Board for keeping the Penobscot 3D seismic survey data open source, and thank dGB Earth Sciences for providing access to the SEGY files. In addition, we acknowledge SEG open data for providing easy access to this data. We used the

Attribute Assisted Processing and Interpretation consortium’s AASPI software to compute seismic attributes. We would also like to thank Schlumberger for providing Petrel licenses to UTPB.

Figure 7. Most positive curvature (K1) co-rendered with most negative (K2) curvature and coherence at the Petrel surface. Notice the

similarity between Figure 3b and 7.

K1 and K2

Positive

Negative

Opacity

K1K2

0 1

High

Low

CoherenceFault 1Fault 2 Relay Ramp

2miles

Figure 6. Most positive curvature (K1) co-rendered with most

negative (K2) curvature and coherence at the Petrel surface.

There is some secondary faulting occurring at shallower depths

(Wyandot surface, 1000 ms) due to the presence of a synthetic

transfer zone.

Fault 2

Petrel

Wyandot

© 2017 SEG SEG International Exposition and 87th Annual Meeting

Page 2115

Dow

nloa

ded

08/1

7/17

to 2

04.1

58.1

62.1

28. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

EDITED REFERENCES

Note: This reference list is a copyedited version of the reference list submitted by the author. Reference lists for the 2017

SEG Technical Program Expanded Abstracts have been copyedited so that references provided with the online

metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web.

REFERENCES

Cahoj, M., and K. J. Marfurt, 2014, Correlating curvature to fault propogation in clay modeled transfer

zones: 84th Annual International Meeting, SEG, Expanded Abstracts, 1434–1438,

http://doi.org/10.1190/segam2014-1338.1.

Fossen, H., and A. Rotevatn, 2016, Fault linkage and relay structures in extensional settings—a review:

Earth-Science Reviews, 154, 14–28, http://doi.org/10.1016/j.earscirev.2015.11.014.

Google Earth Maps, https://www.google.com/earth/, accessed 15 March 2017.

Marfurt, K. J., 2015, Techniques and best practices in multiattribute display: Interpretation, 3, B1–B23,

http://doi.org/10.1190/INT-2014-0133.1.

Morley, C. K., R. A. Nelson, T. L. Patton, and S. G. Munn, 1990, Transfer zones in the East African rift

system and their relevance to hydrocarbon exploration in rifts: AAPG Bulletin, 74, 1274–1253.

[NSDE] Nova Scotia Department of Energy, 2011, Play Fairway Alanysis Atlas – Offshore Nova Scotia,

Canada: http://energy.novascotia.ca/oil-and-gas/offshore/play-fairway-analysis/analysis, browsed

on March 15, 2017.

Paul, D., and S. Mitra, 2013, Experimental models of transfer zones in rift systems: AAPG Bulletin, 97,

759–780, http://doi.org/10.1306/10161212105.

Qi, J., F. Li, B. Lyu, O. Olorunsola, K. Marfurt, and B. Zhang, 2016, Seismic fault enhancement and

skeletonization: 84th Annual International Meeting, SEG, Expanded Abstracts, 1966–1970,

https://doi.org/10.1190/segam2016-13876567.1.

Welsink., H. J., J. D. Dwyer, and R. J. Knight, 1989, Tectono-stratigraphy of the passive margin off Nova

Scotia, in A. J. Tankard, and H. R. Balkwill, eds., Extensional tectonics and stratigraphy of the

North Atlantic Margins: AAPG Memoir, 46, 215–231.

© 2017 SEG SEG International Exposition and 87th Annual Meeting

Page 2116

Dow

nloa

ded

08/1

7/17

to 2

04.1

58.1

62.1

28. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/