Seismic Attribute Illumination of a Synthetic Transfer Zone

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 Geology of the study area

Transfer zones  a feature where deformational strain is The Scotian Basin is located in the Scotian Shelf, extending transferred from one fault system to another  play an 1200 km southwest to northeast, from the Yarmouth Arch / important role in controlling fluid migration in the United States border to the Avalon Uplift on the Grand subsurface. More specifically, a synthetic transfer zone Banks (Figure 1). Production was first established in 1992, occurs where strain is transferred between two parallel with traps incorporating for stratigraphic and structural normal faults in an extensional system. A previous study components. Nearly all hydrocarbon production on the used surface curvatures derived from a clay model to Scotian Shelf has been limited to the Sable Subbasin near highlight different geological features related to a synthetic Sable Island (NSDE, 2011). 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 Figure 1. Location map of the study area (Google Earth Maps). both dipping in the same direction (Morley et al., 1990). As The Penobscot 3D seismic survey is displayed in the yellow a result, a relay ramp is formed in the overlap zone between rectangle. The approximate location of well L-30 (inside the the two faults (Fossen and Rotevatn, 2015). Relay ramps in seismic survey) is indicated by the black dot. 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 Structural and Stratigraphic history increase sediment accumulation during deposition, and are important elements in controlling fluid migration in the The Scotian Shelf developed as North America rifted and subsurface. Transfer zones also lead to the development of separated from the African continent during the break-up of secondary faults, which are generally sub-seismic (Morley Pangea near the end of the Triassic. It consists of series of et al., 1990). The presence of transfer zones can help platforms and depocenters, the Sable subbasin being one of interpreters delineate secondary features such as fractures, them. Tectonism in the central rift basin during the Early splay shears and Riedel faults (Cahoj and Marfurt, 2014). Jurassic (mid-Sinemurian) resulted in complex faulting and erosion of Late Triassic and Early Jurassic sediments and Paul and Mitra (2013) prepared a clay model to demonstrate older rocks (Figure 2). The first instances of listric faulting the behavior of synthetic transfer zones; using the surface occurred during the Late Jurassic. Middle Jurassic and curvature of this model, Cahoj and Marfurt (2014) illustrated Cretaceous sediment loading of unstable synrift sediments the geometry of these zones. In this paper, we discuss along and to the south of the basement hinge zone initiated intrabasin synthetic transfer zones in the Scotian Shelf near subsidence and development of seaward-dipping growth Sable Island with the help of seismic attributes. Using the faults. These syndepositional faults concentrated Penobscot 3D seismic survey (SEG open data), we sedimentation on the downthrown blocks, resulting in the characterize the two normal faults and overlapping relay local thickening of sediments into the faults. ramp. Further, we analyze coherence and curvature

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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.

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 Figure 3. (a) A transfer zone with left stepping pair of normal faults (modified after Paul and Mitra, 2013), (b) Curvature the Abenaki carbonate bank. The second sequence computed for a synthetic transfer zone on a clay model surface (Berriasian – Turonian) consists of a thick, rapidly deposited (modified after Cahoj and Marfurt, 2014), (c) Coherence and deltaic wedge (Missisauga Formation) and a series of Curvature anomaly representing faults and structureal features. thinner, back-stepping deltaic lobes (Logan Canyon

Formation) separated by the Naskapi Shale (Aptian MFS). en-echelon normal faults. As indicated by Cahoj and Marfurt Growth faulting reached its apex in this sequence. The (2014) in Figure 3b, we expect most positive (K1) and most Petrel, a chalk-rich unit and prominent seismic marker, is negative (K2) seismic curvature anomalies in the up thrown interpreted as the upper boundary of this sequence. and downthrown blocks respectively (Figure 3c). We also

expect that anomalous low seismic coherence values will Seismic attribute analysis delineate the fault plane (Qi et al. 2016), as indicated in Figure 3c. We utilize these seismic attributes to illuminate Figure 3a shows a left stepping pair of normal faults forming the transfer zone in our study area. Downloaded 08/17/17 to 204.158.162.128. Redistribution subject SEG license or copyright; see Terms of Use at http://library.seg.org/ a transfer zone. A relay ramp develops between the two

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.

0 A A’

500

1000 Wyandot Petrel 1500

2000 Seismic Amplitude Positive 2500 0

3000 Negative Time (ms) 2 miles (~3.2 km)

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.

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 Figure 5. (a). Time structure map of Petrel top surface, (b) synthetic transfer zone. 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. Downloaded 08/17/17 to 204.158.162.128. Redistribution subject SEG license or copyright; see Terms of Use at http://library.seg.org/

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 Fault 2 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 Wyandot 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 Petrel 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 Figure 6. Most positive curvature (K1) co-rendered with most visible with curvature anomalies. The relay ramp is also negative (K2) curvature and coherence at the Petrel surface. There is some secondary faulting occurring at shallower depths indicated as an anticlinal shape (K1 anomaly). (Wyandot surface, 1000 ms) due to the presence of a synthetic transfer zone. Discussion and conclusions Acknowledgements Comparison of Figure 3b and Figure 7, indicate that the synthetic transfer zone modeled by Paul and Mitra (2013) is We would like to acknowledge the Nova Scotia Department very similar to the real case scenario of Penobscot. The relay of Energy and Canada Nova Scotia Offshore Petroleum ramp in-between the two faults shows up as a dome Board for keeping the Penobscot 3D seismic survey data (indicated by K1 anomaly), which suggests that the relay

open source, and thank dGB Earth Sciences for providing ramp can act as a potential hydrocarbon trap. Coherence and access to the SEGY files. In addition, we acknowledge SEG curvature attributes delineate different features of the open data for providing easy access to this data. We used the synthetic transfer zone. This enhances the analysis of the Attribute Assisted Processing and Interpretation changing structure, which can help interpreters better consortium’s AASPI software to compute seismic attributes. understand transfer zone geometry. We would also like to thank Schlumberger for providing Petrel licenses to UTPB.

Fault 2 Relay Ramp Fault 1 Coherence High

Low

Opacity K1 and K2 0 1 Positive

K1 K2

2miles Negative

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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.

© 2017 SEG Page 2115 SEG International Exposition and 87th Annual Meeting 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.

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