Report

2D SeisWorks Project

Group C:

Irfan Baig Fernando Afonso Caliki Sissel Grude Marta Lanka

Seismic Interpretation

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Preface This report is part of the subject “seismic interpretation” at NTNU where we have learned the process of seismic data interpretation from some seismic lines outside the west coast of Norway. We will present out work through this report with the different seismic lines and our result after horizon interpretation on these lines. The course in interpretation has given us an insight in the practical ways of doing seismic interpretation and a deeper understanding of the theory of rest of the course.

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Table of contents:

Preface ................................................................................................................................ 2

Introduction ........................................................................................................................ 4

1. Geological history of area .......................................................................................... 5

1.1 Structural Settings .............................................................................................. 5 1.2 Stratigraphy of the Area ....................................................................................... 7

1.2.1 Reservoir Distribution in Northern Viking Graben ...................................... 8 2. Overview of area ...................................................................................................... 10

2.1 Seismic Lines ......................................................................................................... 10 3. Interpretation ............................................................................................................ 12

3.1 Horizons ................................................................................................................. 12 3.2 Terminations ........................................................................................................... 13

3.2.1 Erosional truncation ........................................................................................ 13 3.2.2. Toplap ............................................................................................................. 15 3.2.3 Clinoforms ....................................................................................................... 16 3.2.4 Downlaps ......................................................................................................... 17 3.2.5 Onlap ............................................................................................................... 18 3.2.6. Bowtie ............................................................................................................ 18 3.2.7. Probable mud volcanoes ................................................................................. 19

3.3. Hydrocarbon indicators ......................................................................................... 20 3.3.1 Gas chimney .................................................................................................... 20 3.3.2 Shallow gas ..................................................................................................... 22 3.3.3. Flat spot .......................................................................................................... 22

3.4 Faults ...................................................................................................................... 23 3.4.1 Normal fault .................................................................................................... 23 3.4.2 Listric fault ...................................................................................................... 23 3.4.3 Synthetic fault ................................................................................................. 24 3.4.4 Antithetic fault ................................................................................................. 25

4. Results of interpretation ........................................................................................... 25

5. Epilogue ................................................................................................................... 27

6. References ................................................................................................................ 28

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Introduction This document reports work done on seismic interpretation of North Viking Graben area in the Northern North Sea, Norway. It is based on the interpretation of 2D reflection seismic data with the help of the computer program SeisWorks. For this purpose nine 2D seismic lines in different directions through the prospect area have been interpreted. Results from 2D seismic interpretation clearly show presence of a potential hydrocarbon prospect in the area with all its elements. Objective The objective of this project is to get some understanding of stratigraphic and structural elements of the area, and to identify the different stratigraphic horizons, depositional patterns, faults and direct hydrocarbon indicators on the seismic data.

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1. Geological history of area The northern Viking Graben forms part of the northern North Sea rift system, and is situated between 60° and 62° (Fig. 1.1). It separates the Horda Platform in the east from the East Shetland Basin in the west. The Viking Graben structure is dominated by faults with N-S, NNW-SSE and NE-SW orientations, typically 15-20 km apart defining large tilted fault blocks. The structure is asymmetrical in the north, between the Tampen Spur and the Horda platform, where deformation took place mainly along east-dipping normal faults. To the south between 61° and 60°30’ N, the structure becomes more symmetrical, and returns to an asymmetric pattern further south. The geological framework of the North Viking Graben area is described in two categories, which are structural geological setting and stratigraphical geological setting.

1.1 Structural Settings The northern North Sea is generally believed to have been influenced by at least two main phases of extension after the thinning and regional stretching of the thickened Caledonian crust in the Devonian. The first of these is a Permo-Triassic phase that is poorly defined on seismic data from the northern North Sea. The Viking Graben was established during this phase, and was later overprinted by a roughly E-W-stretching phase in the latest Middle Jurassic to earliest Cretaceous. The faults studied in this area occur in Jurassic rocks, and are thus a product of the latter extension phase only. A regional unconformity separates rotated Triassic and Lower-Middle Jurassic sediments from mainly unfaulted and flat-lying Cretaceous and later deposits. This unconformity represents a time gap of up to 100 Ma on structurally high areas like the Gullfaks Field. The post-Jurassic history of the North Sea is characterized by basin subsidence and continuous sedimentation. The extensional tectonics have controlled the development of North Viking Graben structure resulted in sagging with normal faulting and block rotation in the western part of structure, whereas the eastern part remained as an elevated horst structure. Between the eastern and western region there is a transitional accommodation zone (graben system) which is identified as a modified fold structure. The main faults in the area are formed in mostly consolidated rocks, and only the uppermost part of the fault planes what were poorly consolidated sediments. (Fig. 1.1 & 1.2).

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Fig. (1.1) Structural Map of the area

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Fig. (1.2) Geological Cross Section across the main structure. Picture from millennium atlas.

1.2 Stratigraphy of the Area Stratigraphically the Triassic is the eldest most important age in the area in which Hegre Group (Tiest, Lomvi and Lunde formations) was deposited in a relatively quiet phase and consists of interbedded intervals of shale, claystone and sandstone which is interpreted as continental fluvial deposits, followed by the deposition of Statfjod Formation in a changed climatic condition. Brent Group (Broom, Rannoch, Etive, Ness and Tarbert Formations) was deposited in the middle Jurassic in a marginal marine to deltaic setting followed by the deposition of upper Jurassic Viking Group (a shale sequence of Heather and Draupne Formations) in a transgressive setting. The open marine argillaceous Shetland Group was deposited in Late Cretaceous followed by muddy marine Rogaland Group (Lista, Sele and Balder Formations) deposited in Paleocene to Early Eocene. Smectite clay dominated Hordaland Group was deposited from Early Eocene to Mid Miocene followed by sediment starved, argillaceous Nordland Group in Early middle Miocene (Fig. 1.3).

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Fig (1.3) Stratigraphy of the North Viking Graben. Picture from presentation of Institute of Geology. University of Bergen

1.2.1 Reservoir Distribution in Northern Viking Graben The stratigraphy of the reservoir rocks in this area is shown in Fig. 1.4; reservoirs occur in the Frigg, Cod, Statfjord, and Brent Formations. A significant unconformity occurs at the base of the Cretaceous (Fig. 1.4). Jurassic synrift sediments are unconformably overlain by Cretaceous and Tertiary basin fill deposits. The primary reservoir objectives in the North Viking Graben are Jurassic synrift clastic sediments (lying below the unconformity). Jurassic

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strata in the North Sea area occur, for the most part, in fault-bounded basins related to the development (through regional extension) of the graben system. The Jurassic transgressive system has periods of regression that provided the coarse clastic input that forms the reservoir intervals; these reservoirs are sometimes vertically stacked and are separated by deepwater shales. The Jurassic was a period of active faulting; hydrocarbon traps are usually fault-bounded structures, but some are associated with stratigraphic truncation at the BCU. The depositional environments of the Jurassic reservoirs range from fluvial to deltaic and shallow marine. A second reservoir target in the North Viking Graben is Paleocene deepwater clastics. The Paleocene interval is undisturbed by the rift tectonism, and dips gently into the basin (Fig. 1.4). Over much of the basin, the Paleocene section was deposited in a slope environment (and so contains turbidites or slump sediments); sandstone reservoirs are associated with regressive pulses. Hydrocarbon traps are usually depositionally mounded structures or stratigraphic pinch outs.

Fig (1.4) Stratigraphic column for northern North Sea reservoir rocks (after Parsley, 1990).

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2. Overview of area

2.1 Seismic Lines We have chosen to interpret the strike- and diplines illustrated in Fig. (2.1). The main reasons for our choice of lines are:

• Good coverage of the area. • Many of the lines cross each other in several places; this can be used in the

interpretation to make sure that our horizons are continuous. • The horizons we wanted to interpret were strongest in these lines. • We can see a clear geological trend between the diplines, so one line can be used for

better understanding of other lines.

Fig (2.1) our strike and dip lines are marked red. The seismic acquisition is from outside the west coast of Norway. See fig (2.2). This can be seen from the wells that are on the map view in fig (2.1). The wells from the areas 33/5, 35.9

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and 30/7 are the other limits for our seismic. The area is roughly marked as a big red ring in fig (2.2)

Fig(2.2) Map of project area. The picture is from npd.no.

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3. Interpretation Our method of interpretation was to follow the horizons as far as possible; we looked in multipannel display to make sure that they were continuous. We interpreted in loops, so if we did a mistake, only parts became wrong, and not the entire horizon. Some areas were difficult to interpret because the quality of the data was not good. Then we evaluated the thickness of the layer, and compared it to other layers. We could not interpret in some areas because the data quality was not good, so we could not see the horizons. We have divided our interpretation into three parts, the first part is about seismic stratigraphy and structural features. The second part is related to hydrocarbons indicators. The third part is different fault types.

3.1 Horizons We have chosen to interpret 6 different horizons which are illustrated in fig (3.1).

Fig (3.1) the horizons we have interpreted are marked A to F. This picture is from dipline 1.

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We chose these horizons because we considered them as interesting. They are quite clear and most of them are continuous through out the project area.

Interesting parts of our horizons:

A. This looks like an unconformity. We can see many erosional truncations going up into it. (These erosional truncations are illustrated and described further below). This horizon is terminating towards west, and can only be seen in the eastern part of the diplines, and only in strikeline1.

Part B, C, D and E are similar in many ways. All fill in the big graben and are thicker in the middle part and thinning toward eastern and western parts. See Fig. (3.2) B. Above this layer we can see sequences that are truncating down on it in the

western section, and in the eastern section this horizon is terminating at the top horizon A.

C. This looks like horizontal deposition into the graben. This is top of paleocene. D. Similar to C, but older. This horizon marks the top of upper cretaceous. E. Similar to C and D, but not that horizontal, because it is more affected by

BCU, which has big changes in topography. This horizon marks the top of lower cretaceous.

F. This horizon marks the base cretaceous unconformity (BCU). Below there are many faults, which can be seen in Fig. (3.2). Above is the infill sediments of the graben.

Fig(3.2) Illustrates how layer B,C,D and E fill in the graben. This picture is from dipline 1. This line are horizontally compressed to clarely illustrate the difference in thickness.

3.2 Terminations 3.2.1 Erosional truncation – this is represented by termination of strata against on overlying erosional surface (erosional truncation of reflectors on seismic). Implies the development of erosional relief or an angular unconformity. Toplaps often terminates beneath erosional truncation. The upper line on Fig.(3.3) illustrates toplaps, and Fig.(3.4) is an example of toplaps which we have observed in dipline 2 in our seismic, they are marked with colors in Fig.(3.5)

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Fig (3.3) Erosional truncation. Illustration from”Introduction to Seismic Stratigraphy”. Geological Institute from Krakow.

Fig (3.4) Erosional truncation, from dipline 2

Fig (3.5) Erosional truncation, from dipline 2

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3.2.2. Toplap - Termination of strata against an overlying surface mainly as a result of non deposition with perhaps only minor erosion. Occurs where there is progradation but no aggradation (when relative sea-level is static). Sediments bypass the zone of toplap to be deposited further basinward (successive terminations lie progressively basinward). Toplap is evidence of a non depositional hiatus. An illustration of toplaps are given in Fig. (3.6) and examples of toplaps from our seismic are marked in Fig. (3.7) with arrows. Marginal marine setting: - Represents a change from slope deposition-marine or shallow marine bypass or erosion. – Toplap surface is an unconformity. Deep marine setting – Represents, most likely, a marine erosion surface. – This surface is localized and rarely flat over large areas.

Fig (3.6) Terminations, illustration from Seismic stratigraphy Interpretation – Christopher Juhlin

Fig (3.7) Toplaps from dipline 2

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3.2.3 Clinoforms - A sedimentary deposit that has a sigmoidal (or S) shape. They can range in size from centimetres (like sand dunes) to kilometres (such as entire continental shelves), and can grow horizontally in response to sediment supply and physical limits on sediment accumulation. An illustration of clinoforms are given in Fig.(3.8), with an example from our seismic in Fig.(3.9), where the upper picture is an illustration of the seismic and the clinoforms are painted on the lower picture. This picture only illustrates the lower part of a clinoform, the upper part is eroded.

Fig(3.8) Clinoforms

Fig (3.9) Clinoforms from dipline 2

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3.2.4 Downlaps - A base-discordant relation in which relatively steeply inclined strata terminates downdip against an older surface, which may be horizontal or shallowly inclined. Downlap is associated with progradation. Prograding units may not terminate abruptly at the downlap surface but extend basinwards as a thin veneer. As a result, the downlap surface is associated with a condensed interval. Downlap: - Normally seen at the base of prograding clinoforms. - Usually represents the progradation of a basin-margin slope system into deep water. - A change from slope deposition to condensation or non-deposition. - Very difficult to generate downlap in a nonmarine environment. Marine downlap surfaces: -Important surface is the top lowstand fan. -Occurs at the base of the clinoforms of the lowstand prograding wedge. -Facies below this downlap surface are basinal deposits. Fig.(3.10) below illustrates an downlap and Fig. (3.11) is an example from our seismic, where the downlap is marked with arrows.

Fig (3.10) Downlap, illustration from Schlumberger glossary

Fig (3.11) Downlaps from dipline 2

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3.2.5 Onlap The termination of shallow dipping, younger strata against more steeply dipping, older strata, or the termination of low-angle reflections in seismic data against steeper reflections. Onlap is a particular pattern of reflections in seismic data that, according to principles of sequence stratigraphy, occurs during periods of transgression. This is illustrated in fig.(3.3) Fig.(3.12) is an example from our seismic with the onlap marked on.

Fig (3.12)Onlap is marked on the picture. From dip line 4 3.2.6. Bowtie A concave-upward event in seismic data produced by a buried focus and corrected by proper migration of seismic data. The focusing of the seismic wave produces three reflection points on the event per surface location. The name was coined for the appearance of the event in unmigrated seismic data. Synclines, or sags, commonly generate bow ties. Fig. (3.13) is an illustration of how bow-ties look, and how they are created. Fig. (3.14) is an example from our seismic, where they are inside the red box.

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Fig (3.13) Bowtie from Schlumberger glossary

Fig (3.14) (Dipline 4, SPN=1700, 1700 ms) It could be amplitude anomaly, “bow tie”

3.2.7. Probable mud volcanoes It looks like very unstable material which was going to the top because of overburden pressure. Fig (3.15) and (3.16) may be related to the same unstable deposits.

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Fig (3.15) (Dipline 4, SPN 828, 940ms)

Fig (3.16) it could be many small post- deposition faults which were created inside very unstable deposits under overburden pressure. When we are looking on the map of these faults, they could look like polygons (Dipline 4)

3.3. Hydrocarbon indicators 3.3.1 Gas chimney A subsurface leakage of gas from a poorly sealed hydrocarbon accumulation. The gas can cause overlying rocks to have a low velocity. Gas chimneys are visible in seismic data as areas of poor data quality or push-downs. Fig. (3.18) is an illustration of a gas chimney.

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Fig (3.17) Gas chimneys and their possible effects on gas-hydrate stability, illustration from” www.nature.com/nature/journal”

Fig. (3.18) it could be gas chimneys, and can also illustrate shallow gas. From strike line 1.

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3.3.2 Shallow gas Gas that is trapped in the shallow sediments usually originates from deeper gas reservoirs but can also come from biogenic activity in the shallow sediments. Shallow gas can only be confidently interpreted from high resolution seismic data that has been digitally processed and displayed in true amplitude. Fig.(3.18) can look like shallow gas, but we are not sure. 3.3.3. Flat spot Discordant flat reflector (esp. gas/oil or oil/water contact) . May be large enough to give a fairly strong flat reflection that may stand out on the seismic records.

Fig (3.19) DHI – Direct Hydrocarbons Indicators, illustration from a presentation of direct hydrocarbon indicators.

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Fig (3.20) It could be a flatspot (Dipline 6, SPN 2100.500 – 1800, 1630 ms)

3.4 Faults

3.4.1 Normal fault The main type of fault in our area is the normal fault. This is defined as the hanging wall is moving downwards and the footwall upwards. Most of the faults are truncating at the BCU. Fig.(3.21) illustrates a normal fault, and fig.(3.20) is an example of a normal fault from dip line 6, the fault looks like sealing fault.

Fig.(3.21) illustration of a normal fault from geomaps.

3.4.2 Listric fault One fault type in our area is the listic fault. This is a curved normal faults in which the fault surface is concave upwards; its dip decreases with depth. Hanging wall blocks are rotating and slide along the fault plane. Fig.(3.22) illustrates the listric fault, and fig.(3.23) is a listric fault from our seismic.

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Fig.(3.22) Illustration of listric fault from geomaps.

Fig(3.23) A listric fault from dipline 6 is marked.

3.4.3 Synthetic fault A type of minor fault whose sense of displacement is similar to its associated major fault. Illustrated in fig.(3.24)

Fig (3.24) Illustrates synthetic fault. (dip line 4)

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3.4.4 Antithetic fault One of a set, whose sense of displacement is opposite to its associated major and synthetic faults. Antithetic-synthetic fault sets are typical in areas of normal faulting. Is illustrated in fig(3.25), and fig(3.23) is an example from our seismic.

Fig(3.25) Illustrates synthetic and antithetic faults.

4. Results of interpretation We made some time contour maps which illustrates how our interpretation looks in timedomain. This is illustrated in Fig. (4.1), (4.2) and (4.3)

Fig (4.1) Time contour map of horizon B.

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Fig (4.2) Time contour map of horizon C.

Fig (4.3) Time contour map of horizon D.

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Fig (4.4) Time map of horizon E

5. Epilogue During this project we have learned a lot about the software SeisWorks and about seismic interpretation. Least as important; we have observed how easily mistakes are done, and how one little mistake can lead to big errors. Seismic interpretation is a discipline you learn by doing, and the discipline it is in constant change. It takes time to understand the geology of an area, and see the link between geology and seismic. We learned a lot during this introduction to this discipline, but will need years before we really learn it.

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6. References • http://www.bbm.me.uk/OU/S369/S369_SR_Glossary.pdf • http://www.geofys.uu.se/cj/seis_strat/notes/pdf/seis_strat_interp_notes_web.pdf • http://www.glossary.oilfield.slb.com • http://www.bairdpetro.com/shallow_gas.htm • http://www.npd.no/NR/rdonlyres/D9549B8C-F895-44AD-A1D9-

20375E5446A5/18093/ODsokkelkart2008GullfaksStatfjordSnorre.pdf • Powerpoint presentation ”Introduction to Seismic Stratigraphy”. Geological Institute

in Krakow. • www.nature.com/nature/journal • M. Ter Voorde, R. Ravnas, R. Faerseth and S. Cloetingh (1997). Tectonic modeling of

the Middle Jurassic synrift stratigraphy in the Oseberg-Brage area, northern Viking Graben. Basin Research (1997) 9, 137.

• Gislain B. Madiba and George A. McMechan (2003). Processing, inversion, and interpretation of a 2D seismic data set from the North Viking Graben, North Sea. GEOPHYSICS, VOL. 68, NO. 3 (MAY-JUNE 2003), 838-839

• Millenium Atlas, Petroleum Geology of Central and Northern North Sea. • http://www.geologi.uio.no/for_skolen/jpn-oljerikdom.pdf • http://homepages.see.leeds.ac.uk/~earsro/SOEE5154/All_Slides/SOEE5154_12-

Direct_Hydrocarbon_Indicators.pdf • http://geomaps.wr.usgs.gov/parks/deform/normalfault.gif • http://www.geosci.usyd.edu.au/users/prey/Teaching/ACSGT/EReports/eR.2003/G

roupD/Report2/images/rollover%20anticline.gif • http://www.glossary.oilfield.slb.com/DisplayImage.cfm?ID=197