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This is the unedited abstract of a talk presented in 1999 in Aberdeen. The accompanying OHP /

Power point illustrations for the talk will be available on a different upload. These include my

original constructed creaming curves, and descriptions of some of the fields etc for the Lower

Cretaceous in NW Europe presented at the conference. Some of these were then utilized by

Academics, who were at the conference, in a subsequent Conference themed publication. There is

also a poster on my slide share and an abstract posted on my Linkedin profile.

CONFERENCE: LOWER CRETACEOUS OF THE CENTRAL NORTH SEA:

REGIONAL SETTING AND DEPOSITIONAL ARCHITECTURE; CONFERENCE

ABSTRACTS VOLUME; ABERDEEN, MAY 18th – 19th , 1999. pp 41 – 44

ORAL PRESENTATION

THE LOWER CRETACEOUS OF THE AGAT FIELD AREA; COMPARISONS WITH

THE UK CENTRAL NORTH SEA AND IMPLICATIONS FOR EXPLORATION ON THE

NORWEGIAN CONTINENTAL SHELF

STEPHEN CRITTENDEN1, BRIT E. SAUAR2, FRODI HJALTASON2

1 Independent Geological Consultant, Waye Cottage, Chagford, Devon, UK, TQ13 8HN.

2 Norsk Hydro Exploration and Production, PO Box 200, N-1321, Stabekk, Norway.

Exploration on the continental shelf offshore north west Europe has over the last three decades

progressed from the initial search for large obvious structural traps to the search for more subtle

stratigraphic plays. Within such a strategy the pursuit of the subtle plays, usually stratigraphical

traps, relies upon a thorough understanding of the vast amount of data generated in the preceeding

structural phase of exploration. In other words exploration progresses from the simple to the

complex.

Conventional exploration wisdom indicates empirically that the initial phase of exploration in a

virgin basin/province usually yields the greatest rewards in terms of the volume of hydrocarbons

discovered. Initially, usually after a slow start as a geological / geophysical data-base is developed,

the wells drilled result in the discovery of large volumes of hydrocarbons (either a few large fields

or a number of smaller fields). As time and exploration progresses the size and number of new

discoveries tends to decrease. This concept can be illustrated graphically by a ‘discovery curve’

(colloquially termed a ‘creaming curve’) - volumes of hydrocarbons discovered against time / wells

drilled - which shows that the largest fields are discovered in the initial phases of exploration when

the large structures are drilled and that the subsequent discovered fields size become smaller as the

structures left to drill are smaller. The overall ‘discovery curve’ for the North Sea is now (1999)

considered to indicate a mature petroleum province.

However, other factors which affect the shape of the overall ‘discovery curve’ have to be considered

and include, the division of the exploration region into license areas, technology changes such as

capability of drilling in deeper and deeper water, the progressive increase in infra-structure as fields

are discovered and the development of sophisticated geological models based on sequence

stratigraphy concepts. These factors tend to create a sub-pattern or sub-trend within the overall

‘discovery curve’. The overall discovery curve indicating the ‘maturity’ of the North Sea can

however, be misleading as will be explained.

In the North Sea and adjacent onshore areas there is a great deal of information available from over

30 years of exploration effort which can be examined and evaluated. Soon after exploration has

begun and well data and discovered fields data becomes available the search for prospects becomes

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more refined and the play models more subtle. Large structures are still the usual target (for

economic reasons) but discoveries can be grouped / delineated by either stratigraphic age or play-

type or location (by sub-dividing the region into provinces and sub-basins). These groupings allow,

from a re-evaluation of the data generated to date, the erection of a number of ‘discovery sub-

curves’ for , for example, the Late Jurassic, the Middle Jurassic and the Tertiary, each with its own

initial steep rise and subsequent flattening (Brennand & van Hoorn, 1986; Bain, 1993; Cordey,

1993) as they become mature.

In the Central North Sea the discovery of the large Tertiary play / structure of the Forties Field

provided the impetus for intense exploration of the Tertiary of the Central Graben of the North Sea

by numerous competing oil companies all eager to gain a share of the rewards. The risk had been

reduced by showing that the Early Tertiary was host to giant accumulations of oil and hence oil

companies concentrated their search using a proven play.

Similarly, in the Northern North Sea the Middle Jurassic Brent tilted fault block play epitomised by

the Ninian, Brent and Statfjord fields, became established in the East Shetland Basin.

The proven plays therefore, understandably concentrated the exploration efforts of the oil companies

to, in most cases, the positive disregard in investigating or considering other un-proven plays.

Occasionally other plays which have been developed in other parts of the basin / region, such as the

Upper Jurassic Brae Sands play of the South Viking Graben, prompted some investigation

elsewhere such as, for example, the East Shetland Basin.

The erection of ‘discovery sub-curves’ is important as they reveal that prospects using the tried and

tested plays, such as the rotated and tilted Jurassic Brent fault blocks, are sought and drilled long

after the ‘big ones’ have been found and the onset of the economic law of diminishing returns (see

Cordey, 1993). The ‘discovery sub-curve’ indicates a diminishing return (the curve flattens out) and

that time and effort may have been better employed in the search for large fields (or numerous

smaller fields) of a new play concept: ie in the generation of a steep upward part of a new

‘discovery sub-curve’ where the rewards are greatest. In other words some ‘sleeping giants’ have

been ignored. However, what and where are these ‘sleeping giants’? The new plays, which lead to

the discovery of ‘sleeping giants’, are often found by luck while drilling a prospect of one of the

established plays. This is the case for the Lower Cretaceous as illustrated by, the giant Britannia

Field (1975), the North Glenn accumulation (1975), the Captain Field (1977) and the Agat Field

(1976). These discoveries were ‘concealed’ in the overall ‘discovery curve’ and were found while

pursuing the Jurassic plays, hence the Lower Cretaceous Aptian play was not pursued with vigour

(Crittenden, 1991, 1992, 1993, 1994).

If an objective analysis of the components of the overall ‘discovery curve’ is undertaken the

start of a new Lower Cretaceous ‘discovery sub-curve’ can be seen and which was not pursued.

This is a classic case of exploration effort being concentrated on established plays, particularly the

Jurassic as it represents a perceived reduced risk, which thus suppressed / delayed the innovation of

a riskier new play despite the stimulus of the presence of large discoveries.

It is innovation and free-thinking that results in the generation of new leads and prospects defined

by new play models in a basin. The basin after all has a reduced risk because it has become a proven

hydrocarbon province with an established infrastructure. The innovation of sequence stratigraphy

has helped in the reduction of geological risk. Such sequence stratigraphy modelling has allowed,

for example, an in-depth understanding of the stratigraphical development of the Middle Jurassic

and the distribution of the Brent Sands. These highstand systems tract sands form prolific reservoirs

but the traps are structural. However, where are the coeval lowstand system tract sands which would

have been deposited in the basin lows? Are they in the northern parts of the North Sea and Atlantic

margin in the marine depositional area off the Brent delta front? The Upper Jurassic highstand sands

are the Fulmar Formation and the Piper sands of the Central North Sea graben margin areas while

the coeval lowstand sand fans, the Brae Formation, are found in the grabens.

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Such a sequence stratigraphy classification of play types can be applied to the Lower

Cretaceous discovery data set for the whole of the North Sea and north west Europe and can

be superimposed upon the overall structural trap to subtle trap exploration concept.

The Lower to Middle Cretaceous has been a proven hydrocarbon bearing interval in the North Sea

since the discovery of oil/gas in sandstones in the Moray Firth (1975), offshore the Netherlands

(1978 - Helm Field) and onshore the Netherlands (1938 - De Mient-1 well) and in carbonates

offshore Denmark. In the Saxony basin of the eastern Netherlands and Germany, the Lower

Cretaceous has been exploited for hydrocarbons since the end of the 19th century and is host to the

Schoonebeek Field (in the Dutch section of the Saxony Basin) a giant accumulation of oil (STOIIP

1 billion barrels). Lower to Middle Cretaceous oil and gas accumulations have been discovered in

areas such as, for example, the West of Shetland (Victory Field), the Celtic Sea Basin (Kinsale Head

and Ballycotton gas fields), the Central North Sea of the UK sector (eg. Britannia, Captain,

Goldeneye, Hannay and Scapa fields), the Northern North Sea of the Norwegian sector (Agat Field)

and the Danish sector (Valdemar and Adda fields).

If these discoveries are analysed in detail it is apparent that a number of categories of discovery,

within an overall sequence stratigraphy model, can be recognised which can then be plotted as an

associated set of ‘discovery sub-curves’. These are related both to stratigraphical age within the

Cretaceous and genesis (highstand systems tracts, transgressive systems tracts and lowstand systems

tracts) and can be regionally stratigraphically mapped to generate prospectivity / lead / play models

(see Crittenden, 1982, 1987, 1995 and Crittenden et al., 1991, 1997, 1998).

The Agat Field is a prime example of a deepwater, clastic reservoir - lowstand systems tract - of

Late Aptian-Albian age (Gulbrandsen, 1987; Crittenden et al., 1997, 1998) in the Norwegian sector.

Canyons, initiated in the Jurassic, have provided in the Lower Cretaceous a transportation pathway

feeder system for the reservoir sands which have controlled for example the location of the

Aptian/Albian and Cenomanian Agat Formation sands accumulation (slope-fan channels with up-

dip pinch-out or detached basin-floor fans).

The Lower to Middle Cretaceous stratigraphy of the Agat area is remarkably similar to the UK

sector of the North Sea both in terms of lithologies, log shapes and biostratigraphy. For example, the

same regional Mid/Late Aptian regression is associated with the development of lowstand sytems

tracts in the North Sea region, particularly the Moray Firth ‘Bosun/Kopervik Sands’ fairway (eg.

Captain, Britannia, Hannay and Goldeneye fields) and the West of Shetland exploration area. The

discoveries in the Moray Firth area and the Agat discovery thus provide very useful exploration

analogues for comparison and contrast and for risking purposes for Lower to Middle Cretaceous

exploration in the Norwegian sector of the North Sea.

In conclusion such sequence stratigraphy modelling of the Agat Field and the Lower to Middle

Cretaceous fields in the Moray Firth, West of Shetland exploration area and the fields in other areas

of north west Europe allows similar geological play models to be applied in exploration strategies.

For example, similar structural lineaments / canyon relationships seen in the Agat area can be

sought elsewhere offshore Norway, both North Sea and Norwegian Sea, and west of Britain and

which may be associated with regional Mid-Aptian regression sand development.

The deeper undrilled parts of the North Sea, such as the Viking Graben, the Moere Basin and deeper

parts of the basins of the north east Atlantic frontier margin, particularly offshore Mid Norway are

considered in the light of a sequence stratigraphy model to be favourable sites for Lower to Middle

Cretaceous sand development (as well as in the Late Cretaceous, see Crittenden et al., 1998) and

thus may host ‘sleeping giants’.

References.

Bain, J. S. 1993. Historical overview of exploration of Tertiary plays in the UK North Sea. In:

Petroleum geology of North West Europe: Procedings of the 4th Conference. The Geological

Society, London. 5-13.

Brennand, T. P. & van Hoorn, B. 1986. Historical review of North Sea exploration. In: Introduction

to the Petroleum Geology of the North Sea, Glennie, K. W. (Ed), 1-24.

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Cordey, W. G. 1993. Jurassic exploration history: a look at the past and the future. In: Petroleum

geology of North West Europe: Procedings of the 4th Conference. The Geological Society, London.

195-198.

Crittenden, S. 1982. Lower Cretaceous lithostratigraphy north east of the Sole Pit area in the UK

southern North Sea. J. Pet. geol.,5/2, 191-202.

Crittenden, S. 1987. The Albian Transgression in the southern North Sea Basin. J. Pet. geol., 10/4,

395-414.

Crittenden, S. 1991. Abstract - The Stratigraphy and Reservoir Potential of the Early to Middle

Cretaceous of the UKCS (North of 56N). Petroleum Exploration Society of Great Britain Meeting,

London, May 1991.

Crittenden, S., et al. 1991. The Early to Middle Cretaceous lithostratigraphy of the Central North

Sea (UK Sector). J. Pet. Geol., 14/4, 387-416.

Crittenden, S., et al. 1992. Abstract - Palaeoenvironment, Sequence Stratigraphy and Reservoir

development in the Early to Middle Cretaceous of the North Sea and West of Shetland area, British

Sedimentological Research Group Annual Meeting, University of Reading, June, 1992.

Crittenden, S., et al.1993. Abstract - Palaeoenvironment, Reservoir Potential and Sequence

Stratigraphy modelling of the Early to Middle Cretaceous of the North Sea and West of Shetland

area (UKCS). British Micropalaeontological Society Meeting, University of Aberdeen, March 1993:

Biostratigraphy in Depositional Sequence Stratigraphy.

Crittenden, S., et al. 1994. Abstract - Palaeoenvironment and depositional Sequence Stratigraphy:

the implications for reservoir development in the Early to Middle Cretaceous of the West of

Shetland area. In: Conference, High resolution Sequence Stratigraphy; Innovations and

Applications, March 1994, University of Liverpool.

Crittenden, S. 1995. Abstract - The Reservoir Potential and Hydrocarbon prospectivity of the Early

to Middle Cretaceous of the Southern North Sea. In: Petroleum Geology of the Southern North Sea:

Future Potential, April 1995. Geological Society, London , Petroleum Group Conference.

Gulbrandsen, A. 1987. Agat. In: Geology of the Norwegian Oil and Gas Fields. 363-370.