Delineation of Complex fault network North Slope, Alaska using seismic attributes Sumit Verma*, University of Texas Permian Basin, Texas, USA

Shuvajit Bhattacharya, University of Alaska Anchorage, Alaska, USA

Summary

The North Slope, Alaska has a complex fault system in the

subsurface due to different episodes of tectonics. The most

producing reservoirs are fault controlled. Our study area lies

in the south of the well-known Prudhoe Bay and Kuparuk

River oil fields. The Triassic-aged Shublik Shale, which is

the most prominent source rock, has gone through three

stages of extensional tectonic activities during the Jurassic,

Cretaceous, and Eocene. To understand the complex fault

system, we computed an ensemble of volumetric seismic

attributes, including coherence, curvature and aberrancy,

and studied them along the Shublik Shale surface. In this

study, we have divided the structures into three types based

on seismic signature, 1. significant fault throw on vertical

seismic section, 2. insignificant fault throw but clearly

visible flexure, 3. insignificant fault throw and very weak

flexure. We observed type 1 faults on the vertical seismic

section, and seismic attributes which trends in WNW

direction, these faults have large lateral extent. The type 2

faults have similar orientation as type 1. The type 2 faults are

clearly visible on the curvature and aberrancy attributes.

Although, the type 3 structures have no visible throw on

vertical seismic, but, it can be seen as two fault lineation

(which are orthogonal each other) on curvature and

aberrancy attributes. Based on our attribute analysis and

regional geologic understanding, we believe that, the type 1

and type 2 fault sets are of Jurassic age, whereas the two

faults of the type 3 were formed in Cretaceous and Eocene

with an orientation of nearly east-west and north-south

orientation. These type 1 faults display cross cutting, single-

tip and double-tip abutting relations with the older west-

north-west striking faults.

Introduction

The Northslope Alaska has many producing fields including

the Prudoe Bay, Mline Point, and Kuparuk Oil field. Our

study area, seimic survey- Strorms 3D (Divison of Oil and

Gas, 2017), is located near these fields (Figure 1). Nixon et

al. (2014) studied the fault interactions and recativation at

the Mline point field. They identified two generations of

faults, Cenozoic aged north-northeast oriented faults, and

Jurassic aged west-northwest oriented faults. The two set of

faults have compartmentalized the researvoirs. In this study

area, we obsereved two different episodes of tectonic events.

This paper focusses on illuminating the comlex fault

network in the Storms 3D with the help of seismic attributes.

In the past, reseachers have obsereved multiple fault sets in

the same area, in different locations. These faults may

originate in as congugate faults under the same stress field,

or they can be caused by overprinting of multiple stressfields

during different geologic time (Zhao and Johnson, 1991,

Bhattacharya and Verma, 2019). In general, the angle

between the congugate faults can be defined by Anderson’s

criterion. Whereas, in the case of overprinting of multiple

stress fields, the yournger stress field can cause new faults

with different orientations and can also cause a reactivation

of the pre-existing faults (Kim et al., 2001). Based on the

regional geologic understanding, three fault sets found in

this dataset correspond to the Jurassic, Cretaceous, and

Eocene. This suggests a history of polyphase fault

development in the basin. This is critical in terms of the

geometry of the petroleum system and access to the

reservoir.

Geology of the Study Area

Our study area, which is Storms 3D seismic survey area is

located south of the well-known hydrocarbon producing

field in the North Slope, Alaska. The nearby fields Milne

Point, Kuparuk River, Sag River Oil Pool, have been

producing oil and gas since 1970s from the Lisburne,

Ivishak, Kuparuk, and other deep reservoirs. Most of the

well-known producing oil-fields are related to the Jurassic

aged Beaufortian rifting (Houseknecht and Bird, 2011).

The Barrow Arch, which is an east-west trending rift in the

north, and the Brooks Range, which is an east-west trending

thrust and fold range in the south, structural highs bound the

Colville Basin (Figure 1). The sedimentary rock deposits of

the North Slope has been divided into three major tectono-

stratigraphic events, including Ellesmerian, Beaufortian, and

Brookian (Figure 2). The Ellesmerian rifting sequence

consists of Carboniferous to Triassic passive-margin

deposits. In this the extensional settings normal faulting took

place. The Lisburne Group consisting primarily of

carbonates of and Shublik formation consisting primarily of

Shales are part of Ellesmerian sequence. The Beaufortian

rifting sequence was deposited in Jurassic to Early

Cretaceous (Bird, 1985). Normal faulting took place due to

this rifting the faults were south dipping. The Barrow Arch

came into picture due to this rifting. The Brooks Range

resulted during the Brookian orogeny, when continent-

continent collision took place in Cretaceous and Cenozoic

(Nixon et al., 2014).

Figure 1. The study area indicated by the blue rectangle. The

study area is near the Prudhoe Bay oil field, Alaska (after Nixon et al., 2014).

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Complex fault network North Slope, Alaska

Seismic and Well Log Data

The Storms 3D seismic survey area is approximately

280mi2. This survey was acquired in 2005 with 5s record

length, and with a maximum offset range of 14,500 ft. The

nominal fold of this survey was around 40. The data were

later processed with a bin size of 110x110 ft (Divison of Oil

and Gas, 2017). There is one well inside the survey area.

Figure 3 shows the seismic amplitude section vertical section

and the Shublik Formation time-structure map.

Seismic Attribute Analysis

Structural curvature

Structural curvature of seismic reflectors illuminates its

curvedness of the bending and folding. Opposed to one in

2D, two principal curvatures are required to characterize the

3D structures. One of the two can be, the most positive

principal curvature (k1), which displays anomaly around the

crest of the anticline. Whereas the other one can be most

negative principal curvature (k2) which display anomaly

around the trough of the syncline (Chopra and Marfurt,

2007). In this paper, we have used the most negative

curvature, because it illuminates lineation. Curvature is a

vector, where the magnitude of the curvature measures the

amount of curvedness and the direction of curvature is the

direction of the lineation (Guo et al., 2016).

Aberrancy

Aberrancy is defined as the third derivative of a curve or a

surface (Figure 4). As the curvature measures the lateral

changes in dip, aberrancy measures the lateral changes in

curvature. Aberrancy illuminates the center of the curved

bed. In the case of a fault with very small throw, the

reflectors show a continuous signature without any

significant change in the waveform shape. In such cases,

coherence attributes (e.g. coherency, eigen structure

coherence, Sobel filter similarity) does not identifies such

faults, whereas aberrancy can highlight these faults quite

well (Qi and Marfurt, 2018).

In the case of a fault, the total aberrancy magnitude

anomalies (high values) appear at the fault plane, whereas

the total aberrancy azimuth indicates the direction of the

downthrown side.

0 MD

Figure 2. Simplified stratigraphic column on the North Slope (modified after Garrity et al., 2005) on the right, along with

the gamma-ray log curve on the well X. The Shublik Shale

(highlighted in yellow) is studied in this paper.

GR 150 (m)

8500 ft

9200 ft

Figure 3. (a) Seismic section along SW-NE showing the Kekiktuk

and Shublik formation top surfaces. (b)Time structure maps of the Shublik. Note the black dash line indicates the NW-SE oriented

crossline seismic section in Figure 3a. The magenta arrow shows

basement related fault structure. The faults in the northern part of the survey indicated by yellow arrows.

a)

b)

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Complex fault network North Slope, Alaska

Discussions

The faults in the northern part of the survey has a

predominant strike of WNW, shows significant amount of

throw, displayed as number 1 (type 1) seismic cropped

section on Figure 5b. Coherence (Figure 5a) was able to

detect these faults. For number 2 (type 2) location (on Figure

5b) coherence shows, small incoherency. Curvature (Figure

5b) as well as aberrancy (Figure 6a), display strong anomaly

for this flexure. Notice on the Figure 5b, the strike (of k2) or

the fold axis of this flexure is around N 105o (purple color).

For number 3 location (with type 3 faults), the vertical

seismic section shows, very little flexure, these flexures

remain undetected on the coherence map, whereas strong

lineation can be seen on coherence as well as aberrancy

image. These lineation are at 90o to each other, and they

strike in N175o and N85 o. Nixon et al. (2014) discussed

about single tip and double tip abutting of the younger faults

on the preexisting faults. Figure 7 indicates that the two

younger (type 3) faults single tip/double tip abutting against

the (type 1 and type 2) older basement related structure.

Figure 4. Top- The concept of dip, curvature and aberrancy on a curve (modified after Qi and Marfurt, 2018). The red circle

indicates peak, and the blue circle indicates trough. Bottom–

small offset faults, are seen as a continuous reflector by seismic with a little flexure. So, such faults are not visible on the coherence, but are clearly seen on curvature and aberrancy.

Figure 5. (a) Coherence and (b) the most negative curvature (k2)

strike modulated with most negative curvature magnitude slice along the Shublik surface. In the southern part, impression of

faults are barely visible in coherence, whereas very clearly seen

curvature. The magenta arrow shows basement related fault

structure. Interpreted structures on the Shublik surface with most-

negative curvature (k2) attribute. As per orientation, three

dominant types of faults can be observed on the attributes, including basement-related WNW-oriented Faults and two faults

striking nearly N-S and E-W. Note the picture shows three type of

faults, type 1- significant throw (yellow arrow), and type 2- smaller throw but visible flexure(magenta arrow), and type 3- insignificant throw and very weak flexure.

b)

a)

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Complex fault network North Slope, Alaska

Conclusions

Coherence attributes were not useful in illuminating the

flexures and faults with sub-seismic throw, whereas the

aberrancy and curvature were able to detect the two sets of

faults along NS and EW (striking N170o and N85 o). Since,

these two sets of faults are crosscutting, single tip and double

tip abutting against the long faults with significant throw

(striking N105o), indicates that these faults are younger in

age. Based on the regional geologic understanding, the

WNW-oriented faults are the oldest (Jurassic), followed by

EW-oriented faults (Cretaceous), and NS-oriented are the

youngest (Eocene).

Acknowledgements

We would like to thank the Alaska Department of Natural

Resources, Division of Oil and Gas for making the tax-credit

3D seismic data available. We used Attribute Assisted

Seismic Processing and Interpretation Consortium’s AASPI

software to compute seismic attributes. We would also like

to thank Schlumberger for providing Petrel licenses. Ikon

Science’s RokDoc was used for well-to-seismic tie.

Figure 7. (a) Total aberrancy azimuth modulated with total aberrancy magnitude along the Shublik Shale surface. (b) Cross cutting, single tip abutting, and double tip abutting faults. In Figure 7a, the bright areas indicate high flexure or aberrancy values. The two sets of faults (NWN

and NEN), are cross cutting across the older basement-related WNW faults (white arrow). Also, you can notice, the single tip abutting of

younger set of faults (yellow arrows) (modified after Bhattacharya and Verma, 2019). Relatively high values of aberrancy can be seen near the abutting tips due to localized strain development.

a)

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