Offshore North Sea 1974, Exploration Geology and Geophysics, Technology Conference and Exhibition, Stavanger, Norway

,RLAii0F

EXPLO

GEOLO

GEOPH

Offshore North Sea

Technology Conference and Ex

:tion

Stavanger - Norway Sept. 3rd

-.

Scheepsbouwkunde

s

he Hogeschool

O N S

- 714

COPYRIGHT:

REPRODUCTION IS PROHIBITED WITHOUT WRITTEN

PERMISSION FROM 0 N S -74

AND THE AUTHORS

CONTENTS

-EXPLORATION - GEOLOGY AND GEOPHYSICS SECTION

Code

G -IV/1

Title

NORTH SEA PETROLEUM GEOLOGY

J.P. Hopkinson

Chief Geologist,

E. Nys,Tther

Exploration Manager,

Norsk Hydro a.s, Norway

Pages

1 - 31

G-IV/2

_{NORTH SEA PETROLEUM IN GEOLOGICAL}

PERSPECTIVE

_{1 - 26}

E. Robert Schroeder

Vice President

TerraMar Consultants, U.S.A.

G-IV/3

THE EKOFISK AREA

-DISCOVERY TO DEVELOPMENT

_{1 - 17}

L.M. Rickards

Senior Vice President

Phillips Petroleum Company

Europe-Africa, England

G-IV/4

_{CRETACEOUS-TERTIARY CARBONATE}

RESERVOIRS IN THE NORTH SEA

_{1 - 20}

M.L. Harper

Regional Geologist

Amoco Europe Inc., England

B.E. Shaw

Chief Geologist

Amoco Norway Oil Co., Norway

G-IV/5

_{CONTINENTAL SHELF NORTH OF 620}

_LAT.

4ep4á

A. Moe

reA.

o

te

Senior Geophysicist

Norwegian Petroleum Directorate,

Norway

Code

Title

Pages

G-IV/6

BARENTS SEA

-GEOLOGICAL AND GEOPHYSICAL FRAMEWORK

1 - 18

O. Eldholm

Associate Professor

Department of Geology

University of Oslo, Norway

E. Sundvor

Research Associate

Seismological Observatory

University of Bergen, Norway

G-IV/7

PRODUCING AND PROSPECTIVE AREAS OF

THE NORTH SEA AS INDICATED BY RECENT

SEISMIC TECHNIQUES

N.A. Anstey

Director

Seiscom Delta, England

G-IV/8

GEOLOGICAL PREMISES FOR OIL AND GAS

POSSIBILITIES OF THE WEST ARCTIC

SEAS OF THE USSR

I.S. Gramberg

Director General

Scientific and Technical

Complex "Sevmorgeo", USSR

oo00oo

1 - 12

O N S

74

GIV/1

NORTH SEA PETROLEUM GEOLOGY

Mr. J.P. Hopkinson,

Chief Geologist,

Norsk Hydro a.s

Mr. E. NysTther,

Exploration Manager,

Norsk Hydro a.s

ABSTRACT

Continuous subsidence and sedimentation since early Permian times

resulted in the deposition of thick sedimentary successions of

varied composition in the North Sea Basin.

In places, the total

thickness of the sediments may attain 6000 m or even 7000 m.

In the 10 years that have elapsed since prospecting started in

the offshore parts of the basin, considerable reserves of both

oil and gas have been reported from all geological systems within

its boundaries.

Such a distribution of reservoirs in time is

rather unusual even in the most prolific of the known producing

basins.

This paper will try to reveal the relationship between

the productivity of the basin and its tectonic and

sedimentologi-cal history.

INTRODUCTION

The stratigraphy and structure of the lands bordering the North

Sea have been well knolnn for a considerable time, and the many

geplogical similarities observed obviously indicated a certain

degree of continuity.

All attempts to extend known geology across

the North Sea were however pure conjecture; the only firm evidence

available was from dredged samples and from onshore mining

opera-tions which extended below the sea bed.

It was not until exploration for hydrocarbons in the North

Sea

began in the mid 1960's that concrete information

_{on the geology}

of this significant part of the European continent

_{began to}

be-come available.

Even then, the amount of data released to the

outside world - that is to say, to that part of the world

_not

directly involved in the oil industry - was very small.

Neverthe-less, the flood of data from the hundreds of wells

_{now drilled,}

and from the thousands of kilometers of seismic lines

_{shot, has}

steadily built up, to the extent that the geology of

_{the Southern}

North Sea basin is now quite widely known.

This is not so for the more northerly areas.

A good deal of

information has in fact been made available, but in detailed

reports onlimited areas (with a few exceptions).

In the present paper we intend to paint in more of the detail

of the petroleum geology of this northern part of the North

Sea - with not too fine a brush - and to set it in perspective

to the surrounding areas of Europe.

EXPLORATION HISTORY

Although small quantities of oil and gas have been produced

in North Western Europe for many years, it was not until the

discovery in

1959

of the L. Permian Rotliegendes gas field at

Schlochteren in the Netherlands that serious interest in the

hydrocarbon potential of the North Sea was aroused.

The

Per-mian basin was known to extend across the southern North Sea

between the Netherlands and England, and this area was an

obvious target for further exploration.

Drilling by NAY close to the Dutch coast began in

1961,

but

the main exploration effort was delayed until the

countries

bordering the North Sea set up the legislation necessary

for

the awarding of offshore acreage.

The

first licencing round

took place in the UK in 1964 (leaving aside Denmark, where

the entire sector was granted to the Dansk Undergrunds

Con-sortium in 1962) and drilling began in block

38/29

at the end

of the year; this well, drilled by Amoseas, was the

first to

be drilled in the North Sea proper.

By late

1965,

BP publicly announced the discovery of the first

commercial gas field in the North Sea - West Sole.

This was

followed in

1966

by the Leman, Indefatigable, and Hewitt fields.

In the same year the first oil in the North Sea was discovered

by Burmah, in block 48/22, but was proved to

be uncommercial.

cers, a success ratio of nearly 1

in

5.

By this time drilling had also begun in the Norwegian sector,

but with no significant results, apart from traces of oil

reported by Esso from its

25/8

well.

In the Danish sector,

two wells had been drilled on the "A" feature, both of which

were reported to have "slight traces of hydrocarbons".

In the following year, drilling began in the Netherlands, and

gas finds were reported from two locations.

By this stage, exploration activity was dying down.

Drilling

began in more northerly waters, but no oil

_{was found, and, despite}

the cod gas/condensate field discovery in Norwegian waters in

late

1968,

_{discouragement spread through the industry and several}

international companies began to cut down

_{on their involvment in}

the area.

Exploration was revitalized by Phillips' announcement in 1970 of

a major oil discovery made at Ekofisk in the latter part of 1969,

and the emphasis of activity shifted northwards.

_{In the third}

round of licensing in UK waters in mid 1970, it

_{became apparent}

that the main area of interest was the Northern

_{North Sea, close}

to the median line with Norway.

During 1971 the long lasting territorial dispute

_{between Germany}

on the one hand and Holland and Denmark on the other

_{was resolved,}

Holland and Denmark agreeing to cede

₅₀₀₀

km2

and 6900 km2

respec-tively to Germany.

_{This gives Germany a 30 mile}

_{common offshore}

boundary with the British sector.

-4

The entire German sector has been under concession more or less

continuously since the early 1960's, though exploration has been

effectively at a standstill since 1967, twelve unsuccessful wells

having been drilled at that time.

It is anticipated that the areas ceeded by Holland and Denmark

will eventually be reassigned to the original rights holders

by mid 1974.

Subsequent to the Ekofisk discovery, further oil discoveries

followed in rapid succession - Forties, Auk, Montrose, and, in

1972, the highly significant Brent discovery, by Shell/Esso.

In Norwegian waters other fields had been located near Ekofisk,

which was developing into a major complex of oil discoveries, and

in 1971 the discovery by Petronord of the Frigg gas field in

block 25/1 was announced.

The two wells in the Danish sector which had been reported to

have "slight oil shows", Al and A2, were announced to be

commer-cial, and further oil and gas finds were reported from the M and

L features.

Since 1972 exploration has been concentrated in the North,

in

the Scottish Basin, where the Piper field was discovered in

late

1972, and in the Viking Basin, along the median line between the

UK and Norwegian sectors.

The most exciting area has been that

around Shell/Esso's Brent field, where a series of important

oil

discoveries has been made.

Exploration activity in the northern

North Sea is now largely focussed on pre-Cretaceous reservoirs

in this area.

The North Sea Basin may be classified as an intracontinental

basin.

It includes the sediments covered by water up to the

Shelf-edge at approximately 62°N.

As suggested by T. Sorgenfrei

(1969) the name should also be extended to cover the onshore

sedimentary subbasins in Eastern England, the Netherlands,

Denmark and Northern Germany.

Its history dates back to the end of the Hercynian (Variscan)

orogeny that folded the pre-Permian sedimentary complex in the

central and southern parts of the Basin.

There exists thus a

marked unconformity between the basin sediments and the

pre-Permian subcrop.

In the northern parts of the basin i.e. north

of approximately 59°N very little is known of the pre-Permian

hotory.

The basin has right from its early days had a fairly complex

build-up, a tendency that continued up into late Cretaceous

times.

A very early development could be seen into ridges and

troughs, some of which persisted through a long period of time,

whereas others had a fairly short life geologically speaking.

There is also in places a tendency towards migrating axes of

deposition, although this is more an exception than a general

rule.

It has been suggested by several authors that the creation of

the North Sea Basin came as a result of rifting in the

crust

in association with the breaking apart of the European and North

American continents.

This gave rise to the long and narrow,

bifurcating zones of subsidence or grabens that are so

characte-ristic of the area.

_{The movements along these zones must have}

come to an end upon the final separation of the two continents

which commenced about 120 million years

ago.

Between the subsiding zones there existed stable blocks which

were transgressed by the sea from time to time.

This resulted

in a relatively thin sediment cover and important sedimentary

lacunae are found here.

In late Cretaceous this differentiated sinking between blocks

and basins became less marked and from early Tertiary on the

simple, symmetrical basin that now occupies the central portion

of the North Sea was developped.

The main depositional area for the Permian sediments was the

Zechstein Sea that occupied the general area S of

58°30'N.

Already at this stage sedimentation was influenced by the

FynRingkobing High that was probably emergent during this

period of time and the Mid North Sea High that received no

Rotliegendes and only a thin cover of Zechstein Carbonates.

North of 58°30'N there is a general lack of knowledge, but

it

is highly probable that Permian sediments may be found in

the

deeper parts of the Viking Basin which probably started forming

during this period.

In the Triassic sedimentation was more governed by the zones of

subsidence than it was ir the Permian.

This resulted in thick

sedimentary columns over restricted basinal areas whereas the

wider marginal areas such as the peripheral cratonic platforms

and the interbasinal highs received little or no sedimentation.

The FynRingkobing High received sediments

for the first time

whereas the Mid North Sea High seems to have been

outside the

area of sedimentatbn.

In the latter area the sediments

could,

however, have been eroded during the Kimmerian orogeny.

To the north the Shetland Platform and the Odin High seem to

have been emergent, the latter then separating the Viking Basin

where Triassic sediments have been proven

and the northernmost

part oC the Fisher Bnsin where su oh sectiments probribly

OXAt.

These two basins were probably connected both in the North and

in the South thus making the Odin High an island.

The Jurassic sedimentation was also mainly confined to zones of

rapid subsidence.

These are, however, not necessarily the same

as the Triassic ones.

The Mid North Sea High and the

Fyn-Ringkobing High were again active and either no sediments were

deposited here or they were eroded in connection with the

Kimmerian earth movements.

To the North the Shetland Platform remained outside the

deposi-tional area as before.

The Odin High, however, was almost

com-pletely transgressed and only the southernmost tip remained

outside and above the transgressive sea.

In the basins sedimentation continued but was not contemporaneous

or of the same magnitude everywhere.

_{The maximum Liassic}

sedi-mentation took place in the Danish Embayment where

a total Juras

sic thickness of about 2500 m reflects the importance of

the

subsidence.

In the Fisher Basin, the Central Basin and the Scottish Basin only

sediments from Middle and Upper Jurassic are recorded indicating

either a halt in the subsidence during Lower Jurassic or erosion

during the Kimmerian earth movements.

In the North, however, the Viking Basin seems to have been active

throughout the Jurassic even though the Kimmerian

_{earth movements}

had a strong influence on the geological buildup

_{of the area}

creating tilted faultblocks over which a pronounced unconformity

developed.

-8

The transgression that took place in the Upper Jurassic

(Kimme-ridgian) gave rise to basinwide uniform sedimentation that

persisted until Lower Cretaceous times.

Some of the zones of

subsidence were especially active during this period.

This is

true for the Central Basin where in places more than 1000 m of

sediments accumulated.

At this time the Viking Basin was

split

up into elongated deposenters in each of which an accumulated

thickness of Upper Jurassic

Cretaceous sediments of over

2000 m is recorded.

In the Upper Cretaceous a widespread transgression took place

across the whole of the North Sea Basin area as a result of

epeirogenic movements.

This resulted in the formation

of uniform

and persistant limestone and chalk deposits.

One can still see

the influence of the subsiding zones on

the sedimentation pattern.

The greater sediment thickness

in the Mesozoic basins might in

places, however, be a result of sediment

compaction rather than

of faulting or of flexure

controlled depression of the basin

floor.

With the Tertiary the tectonic

pattern that controlled Mesozoic

deposition changed considerably.

The general subsidence of the

area persisted as in the Cretaceous but with a marked difference

in the rate of subsidence between the central and marginal

parts

of the area.

This subsidence created the North

Sea Tertiary

Basin with a sediment thickness

that possibly exceeds 3500 m

in

its central parts.

STRATIGRAPHY

Permian

At the close of the Hercynian orogeny erosion

took place in the

uplifted areas and debris was transported by rivers and wind

action and deposited in the lowlands.

As most deposits were

con-tinental an increasingly arid climate in Lower and

Middle Permian

gave rise to

It

predominantly red coloration of the sediments.

These consist mainly of sandstones and shales with minor amounts

of evaporetic material and go by the name of Rotliegendes.

The

sediments are widespread, forming the reservoir for most of the

gas fields in the southern North Sea, and are recorded in a

nlamber of wells in the central North Sea.

With the Upper Permian a marine transgression took place in

what is known as the Zechstein Sea.

This sea covered most of

the present North Sea up to approximately 58°30'N and extended

through northern Germany and into Poland.

The deposits were

mainly composed of evaporites and include in the basinal

areas

one or more complete evaporitic cycles from carbonates through

MgK rich salts.

At the basin margins these deposits

inter-tongue with sandstones and shales often of red coloration.

As in such marginal areas there may be no lithological break

between the deposits of the Rotliegendes, Zechstein and Triassic

and there exists no paleontological evidence

_{as to their age,}

these sediments are often refered to

_{as PermoTriassic.}

With an increasing amount of newly deposited

_{material in the}

basinal areas the plastic salts became more and

_{more unstable}

until at last they started moving upwards.

_{This caused the}

overlying sediments to be folded into anticlines

_{or domes}

pro-viding trapping possibilities in the sediments

_{on the flanks of}

or above the moving salt.

_{These movements, which apparently}

started as early as Triassic in places, have made it impossible

to give even an approximate figure for the

_{overall thickness of}

the Permian deposits.

The importance of the trapping possibilities

_{caused by the}

salt-movements is clearly seen in the Ekofisk

_{area where apparently}

all oilfields are closely connected

_{to halokinetic features.}

10

-Triassic

At the close of the Permian period the Zechstein sea dried up

and widespread continental deposits were formed.

The area of

sedimentation apparently extended further north than in the

Permian and Triassic sedimentation took place in the Viking

Basin.

The Fyn-Ringkobing High was also the site of

sedimen-tation though much reduced.

On the other hand it appears that

the Mid North Sea High was outside the area of sedimentation.

The threefold sequence of Bunter, Muschelkalk and Keuper that

is well developed in the East England and German Basins, is

not found complete north of the Mid North Sea High.

Due to

their continental nature the sediments do not give much evidence

as to their age.

It seems clear, however, that the marine

trans-gression of the Muschelkalk Sea never extended so far north.

By a rather broad analogy with what is known from the southern

part of the North Sea, the sediments are sometimes assigned a

Bunter or a Keuper age.

Most often, however, they are

undif-ferentiated and just refered to as Triassic sediments.

With the start of the Triassic period the sedimentation pattern

changed considerably from what it was in the Permian.

Faulting

that was apparently initiated in the Permian continued into the

Triassic creating downthrown blocks over which elongated

depo-senters developed.

In places more than 2000 m of sediments

accumulated.

The predominant lithology is green and red shales,

often marly, and similarly colored sandstones.

A locally

deve-loped lagoonal environment is indicated by the deposition of salt

beds in the southern parts of the North Sea and dolomite beds in

some of the subbasins to the North.

Due to penecontemporaneous faulting that affected the sediment

distribution in the Triassic and later removal of sediments

during the Kimmerian earth movements in the early and

late

Jurassic, the original extension and thickness of the

Triassic

mate of the present thickness and distribution of these beOs due

to correlation problems between wells.

Jurassic

The sediment pattern in the

_{Jurassic did not change very much}

from that in the Triassic, (fig.

_3).

_{One recognizes the same}

depositional axes and only along the basin margins where

rela-tively thin deposits

_{were formed could there be changes in this}

picture.

_{The depositional environment}

also tended to be similar

in the marginal areas of the

_{basin where mainly continental,}

del-taic and near shore deposits are present in the sequence up to

and including the Dogger.

_{The climate, however, became more}

humid as shown by the lack of the typical red coloration of

sediments and the existence of

_{coal seams between the clastic}

beds.

_{The prominent lithology is}

sandstone and shale.

In most other areas the Rhaetian

_{transgression flooded the}

desertic Triassic landscape with

_{a transformation to dark marine}

shales.

_{The water can not have been very deep, however, and}

in places continental sandstones can be found interbedded with

the marine series.

With the Dogger a new episode

_{of regression begun in the North}

Sea area and continental

_{deposits were laid down even in the}

basinal parts of the North

_{Sea Basin.}

Following the Dogger

earth movements tilted the

_{sediments and a pronounced angular}

unconformity was formed in

_{parts of the area.}

The transgression that followed

_{in Upper Jurassic (Oxfordian}

-Kimmeridgian) times resulted

_{in the deposition of widely}

dis-tributed black shales that

_{probably were laid down in an}

euxinic environment.

_{These shales are by many believed to be}

the best source rocks for petroleum in the North Sea

_Basin.

-

12

-kt.t!LH,!eou

The transgression that started in the Upper Jurassic extended

into the Lower Cretaceous and new areas were flooded by the

advancing sea.

This is especially true for the Mid North Sea

High where Lower Cretaceous deposits are found resting directly

on pre-Jurassic sediments most often of Triassic age.

There is evidence of tectonic instability during the Lower

Cretaceous in that numerous lacunae are recorded in the

sedi-mentary succession.

Since most wells are drilled on saltdomes

the lacunae may be interpreted as the results of erosion and/or

nondeposition during periods of growth of the domes.

It is

therefore probable that a complete Lower Cretaceous succession

may be found in the synclines between such domes.

The predominant lithology is clay (shale).

In marginal areas,

however, clean sandstones can also be found.

The sea was

mode-rately deep in the center of the basin, shallowing out toward

its margins.

More shallow marine conditions persisted during the Upper

Cretaceous with chalk and limestone deposition.

The

environ-ment must have been open marine shelf facies with clean water

and little or no supplies of argillaceous material.

Carbonate

sedimentation was not initiated at the same time everywhere,

however, and it was not until the Maastrichtian that the

car-bonate shelf reached its ultimate extent.

At this time the

shelf edge must have had its northernmost boundary at

approxi-mately 59°N with argillaceous sedimentation and only minor

amounts of carbonates deposited in the open sea towards the

north.

The sea now covered more extensive areas than even before

and

the Odin High and most of the Shetland Platform were

trans-gressed.

Sedimentation was still largely controlled by the tectonic ptttern

that was initiated in Permian

_{- Triassic times.}

_{We thus find the}

thickest Cretaceous deposits along thed_d

_{zones of subsidence,}

(fig.

4 ).

Tertiarz

At the end of the Cretaceous period,

_{a profound paleogeographic}

change took place.

_{The Chalk Sea, which at its greatest extent}

in the Senonian and Maastrichtian had

_{covered a good deal of}

Western Europe, became more restricted.

_{In the succeeding Danian,}

chalk deposition was restricted to

_{a relatively narrow belt in}

the southern and central North Sea,

_{extending into Northern}

Europe via Denmark, and divided by

_{the surviving high features of}

the Mid North Sea High and the Fyn-Ringkobing

_{High, (fig.}

_5).

The relatively deep (maximum 100 m),

_{clear, calm, open marine}

shelf waters in which the Danian Chalk was deposited were

sur-rounded by comparatively shallow shelf

_{seas, in which were}

deposited dark marine shales interspersed with sands derived from

the recently rejuvenated land

_{areas of Europe and the British}

Isles.

_{The boundary between the two depositional regimes was}

of course subject to fluctuation,

_{and in a number of cases}

drilling has proved an alternation

_{of the two lithological}

facies.

Chalk deposition as such

_{came to a close at the end of the}

Danian, though a considerable amount of limestone may have been

deposited locally until well into the Eocene.

The paleogeographic features which had a controlling effect

on

sedimentation during the early

_{Tertiary were the Shetland}

Plat-form, the Scottish Basin, and

_{the main land}

_{mass of the British}

Isles on the West; and the Fenno-Scandian Platform to the East.

In the early Paleocene the

_{Fyn-Ringkobing/Odin high trend}

_was,

as mentioned above, still an important

_{factor; but later in the}

Paleogene the Odin High was the only remaining effective

sedi-mentary control within the basin.

14

-After the final disappearance of the "Chalk Sea", clastic

deposition continued throughout the North Sea Basin, the coarse

clastics (i.e. sands) being derived from the surrounding land

masses and possibly at times from the Odin High.

Round the edges of the basin deposition may often have been in

a deltaic environment.

The Paleocene and Eocene deltaic flats

of S.E. England are well documented, and evidence provided by

drilling suggests that similar depositional conditions may well

have existed along much of the coast line of the western land

mass at this time.

Other sand bodies, which are likely to be turbidite, may at the

same time have been deposited along the axes of the basins

existing in the North Sea at the time.

(Fig. 6

).

The early Tertiary was of course the time of the Brito-Icelandic

volcanic episode, which was reflected in the North Sea sediments

by the deposition of tuff horizons.

The early stages of the

Alpine Orogeny resulted in fluctuations of the coastline, and

opposite the Shetlands themselves, the whole width of the

Shet-lands platform was probably above sea level a good deal of the

time.

A coastal plain extended eastwards from the more

moun-tainous regions in the West.

Further to the South and North where the so called "cliff"

bounding the Shetland Platform is less well developed, the area

was undoubtedly a shallow shelf sea at most times, though

occasional periods of emergence are quite probable.

In the south of the North Sea Basin, in for example

S.E. England,

the relief of the land was much lower, resulting in much wider

fluctuations of the coastline.

The shoreline positions shown in

figs. 5 and 6

are therefore very approximate.

By the early Eocene, the relatively high

_{relief of the}

surround-ing land had been reduced by erosion; and rapid clastic

deposi-tion had effectively filled in

_{the marine basins of the North}

Sea - in particular the Viking basin, north of the Odin High,

where the greatest subsidence had

_{taken place.}

_{The supply of}

abundant coarse clastic sediments was drastically reduced, and

with a rise in relative

_{sea level leading to the widespread}

Ypresian transgression, a period

_{of monotonous marine shale}

deposition began over most of the

_{North Sea, and continued}

throughout the rest of the Paleogene.

The effects of the Alpine

_{orogeny on sedimentation in the North}

Sea culminated towards the end of the M. Miocene.

_{The Odin High,}

and probably considerable areas of the Northern North Sea, appear

to have been briefly exposed

_{at this time, resulting in an}

ero-sional unconformity.

_{Sedimentation resumed in the U. Miocene,}

by which time the surrounding

_{land areas of Europe and}

the British

Isles assumed approximately their present outlines.

The area of

the Odin High continued to be

_{an important control, but the main}

subsidence was now in the Central

_{North Sea Basin (a trend which}

had begun with the onset of

_{the Alpine movements in the late}

Paleogene). (Fig. 7).

The sediments were dominantly

_{marine shales, Neogene coarse}

elastic sedimentation being concentrated in the Viking

Basin,

where the total Neogene thickness

_{is much less.}

Once more, the subsiding basins

_{were slowly filled by elastic}

sediments, and only the remodelling thatlook place during the

Pleistocene glaciations remained before the North Sea assumed

its present form.

HYDROCARBON SOURCE ROCKS

The most important

_{source rocks for the gas fields of the}

Southern basin of the North Sea are to be found inthe Upper

Carboniferous Coal Measures

_{which underly the Permian}

and Triassic

16

-reservoirs of the area.

The organic content of these strata is

conducive to the formation of gas rather than liquid

hydrocar-bons.

It is of interest to note that the major gas fields coincide with

an area which has a relatively high geothermal gradient of the

present time.

This suggests that the conditions leading to the

maturation of the hydrocarbons in the Carboniferous source rocks

and their subsequent migration have been very persistant, and

have continued up to the present.

It also suggests that long

distance

secondary migration did not take place - otherwise the

correlation between fields and geothermal gradients would not be

apparent.

In the Central and Northern basins of the North Sea, the major

source of hydrocarbons are shales within the Jurassic system, and

in particular the dark radioactive shales formed during the

trans-gression following the development of the Kimmerian unconformity.

Shales in the L. Cretaceous and Tertiary may have been important

sources for some accumulations, and analytical studies of crudes

show that more than one source of hydrocarbons is involved.

Again, reference to the geothermal gradient map (fig. 9

)

shows

that many of the fields coincide with the areas of highest

geo-thermal gradient at present.

Geochemical studies have shown

that the areas of ancient geothermal maximae were in the same

location.

It is no coincidence that this area is also identical to

that of

the maximum subsidence of the developing basins.

If the fo/mation of the North Sea was indeed connected with

the

intitiation of the Atlantic Ocean, it is to be expected

that the

main areas of subsidence will be the axes of potential

rifting

between ancient cratonic plates.

subsiding basins in which the

potential source rocks

_{were deposited also provide the optimum}

conditions for maturation and primary

_{migration of the resulting}

hydrocarbons; and in which potential

_{reservoir rocks were}

depo-sited within and around the basins.

Carboniferous rocks are absent in the Central and Viking

Basins;

_{and the Permian and Triassic sandstones are of minor}

importance as potential reservoirs,

_{despite their often excellent}

petrophysical characteristics,

_{except in areas where these rocks}

are brought into juxtaposition with

_{source beds of later Mezozoic}

age by block faulting.

DISTRIBUTION OF HYDROCARBONS

In the offshore parts of the North Sea Basin, commercial

quanti-ties of hydrocarbons have been discovered in all geological

sys-tems from Permian through Tertiary.

_{For the North Sea Basin}

as

a whole this picture may he extended to cover the Carboniferous

by including the small onshore oilfield in the Midlands of

Eng-land and some of the gasfields in Northwestern Germany that

are

producing from rocks of this

age.

A distribution of petroleumproducing horizons in

time of this

magnitude is rather exceptional

_{for any sedimentary basin on a}

worldwide scale, and shows

_{the importance of the}

sedimentary

column's possibilities in

the North Sea

area.

The distribution is such that in some parts of the area only one

of these horizons may have interest as an

_{exploration target}

whereas in others nearly all geological systems mentioned

are

regarded as having oil and/or

_{gas potential.}

_{This is especially}

true for the long and narrow zones of subsidence

_{where a thick}

sedimentary column has accumulated

_{almost continuously since}

the

basin started to develop.

18

-With regard to the possibility of petroleum accumulations in

the pre-Permian rock sequence, it has been recorded that both

the Carboniferous and the Devonian posees potential reservoir

rocks in various parts of the basin.

Due to their position

below the Hercynian unconformity, however, it will be

difficult

to predict their possible existence and composition in most

wells to be drilled.

This together with the considerable

depth at which they in most instances will be found will not

encourage any systematic search for hydrocarbon

accumulations

in these rocks, and has not done so in the past.

The Lower Permian, Rotliegendes sandstones, are the most

pro-lific reservoir-rock in the South of the North Sea Basin

and

makes up most of the gasfields of the area.

The source-rocks

for this gas are thought to be coalseams in the underlying

coal measures of the Carboniferous.

The ease with which the gas might have migrated up

through the

unconformity and the source-rock's position in relationship

to

the potential reservoirs has undoubtedly been a

controlling

factor in the distribution of gas deposits.

Another controlling

factor has been the evaporitic sequence of the

Zechstein above

the Rotliegendes.

It is believed that in many cases where there

is no salt the evaporitic cap-rock has been insufficient to hinder

the gas in migrating upwards.

The Hewett field may be taken as

example of this.

The field produces from the Triassic

Bunter

sandstones and the Zechstein Carbonates whereas the

Rotliegendes

is dry.

As the gas here has very much the same

characteristics

as the gas from the other gasfields of the area, this gas is

believed also to originate in the

Carboniferous.

The gas must

then have migrated along fractures or

faults across the evaporitic

barrier

of the Zechstein which here has no

salt.

The Zechstein

earbonatc

may in places aloo contain oirniricant

amounts of hydrocarbons.

_{This occurs in some of the gasfields}

of the south of the North Sea Basin and it has been rumoured that

the Auk and the Argyll field in the North

_{Flank of the Mid North}

Sea High are producing oil from the Zechstein carbonates.

This

has, however, to our knowedge,

_{never been confirmed by the}

com-panies sitting on the fields.

The only significant Triassic discovery offshore in the North Sea

Basin is the Hewett field, which as indicated above has probably

received its gas from the Carboniferous.

_{The general lack of}

petroleum accumulations in potential

_{reservoir-rocks of this age}

is believed to stem from the poor source-rock characteristics

of the Triassic sequence as a whole.

_{Otherwise the potential}

reservoir-rocks i.e. the sandstones of

_{Keuper and Bunter age are}

of fair to good quality and should

_{be regarded an objective in}

most parts of the North Sea Basin.

With the recent discoveries of

_{petroleum in sandstones of probable}

lower and middle Jurassic

_{age in the north of the North Sea Basin,}

this system has turned out to be

_{the most prolific}

_{source of oil}

in the North Sea area.

_{Production from similar levels has long}

been established in Northern

_{Germany and the Netherlands onshore}

and has created a strong incentive

_{for further prospecting}

in

these areas even though the

_{discoveries are of a much smaller}

magnitude than those in the offshore

_ones.

The terrigenous sediments of the

_{Lower Cretaceous on the}

con-trary, have hitherto yielded

_{hydrocarbons only in the onshore}

areas of Germany and the Netherlands

_{and prospects in these}

beds are not highly regarded in

the offshore parts of the North

Sea Basin.

20

The Upper Cretaceous carbonate column offers better

possibili-ties in that its potential reservoirs have a fairly wide

dis-tribution.

Production from the fields in the Ekofisk area and

the Dan field comes partly from the Maastrichtian and partly

from the overlying Damian chalks of similar lithology.

These

rocks have often an excellent porosity whereas the permeability

generally is poor.

The usual very good productivity from wells

producing from these levels is therefore thought to be caused

by an extensive fracturing of the rocks.

The fractures also

probably served as pathways during the migration of petroleum

from source rocks in Paleocene or in the uppermost part of

Jurassic.

In addition to the Danian carbonate production mentioned above,

oil and gas have been found in Lower Tertiary sandstones at

several localities

for example, Frigg, Heimdall, and Forties,

Sandstone developments with good reservoir characteristics and

with hydrocarbon shows have encountered in several wells, but

depths have been too shallow for maturation of hydrocarbons from

associated source rocks, or for retention of hydrocarbons

migra-ting upwards from deeper sources.

References

Birks, J., "The oil potential of the North

_{Sea", North Sea}

Conference, London Sept.

1972

Christian, H.E., Jr., "Some observations on the Initiation of

Salt Structures of the Southern British North Sea", The

exploration for petroleum in Europe and North Africa,

Institute of Petroleum, London

₁₉₆₉

Dunn, W.W., S. Eha, H.H. Heikkila, "North

_{Sea is a tough}

theater for the oil-hungry industry to

_{explore", The Oil}

and Gas Journal, January

8, 1973

Fertl, W.H., R.J. Cavanagh, J.C.

_{Shepler, B.G. Cope, "Precise}

formation evaluation provides key

_{to Rotliegendes",}

Petroleum and Petrochemical International,

_March

₁₉₇₃

Gaskell, T.F., "Finding and Exploiting

_{Gas in the North Sea",}

Offshore, May

1968

Gill, W.D., "The North Sea Basin",

_{Proceedings Seventh World}

Petr. Congress, vol. 2,

₁₉₆₇

Hallam, A., "Mesozoic geology and

_{the opening of the North}

Atlantic", The Journal of Geology,

_March

₁₉₇₁

Harper, M.L., "Approximate Geothermal

_{Gradients in the North}

Sea Basin", Nature Vol 230, March

_{26, 1971}

Heybroek, P., U. Haanstra, D.A.

_Erdman,

_{"Observations on}

the geology of the North Sea

_{Area", Seventh World Petroleum}

Congress, Proceedings, vol.

2, 1967

James, R.A., "Geological promise of

_{the Northern reaches",}

Ocean Industry, February

₁₉₇₄

Kent, P.E., "Outline Geology of the Southern North Sea

Ba8in",

Proceedings Yorkshire Geol.

_{Soc., vol.}

_{36, 1967}

Oftedahl, C., "Miocene Volcanism

_{in the North Sea", Nature,}

vol. 230,

March 12,

1971

Pergram, R.M., G. Rees, "Widening

geological perspectives

on

the N.W. European Shelf",

_{World Oil, March}

₁₉₇₂

Sorgenfrei, T., "A review of

_{Petroleum Development in}

Scandi-navia", The exploration for

_{petroleum in Europe and North}

Africa, Institute of Petroleum,

_London

₁₉₆₉

_a

22

-Sorgenfrei, T., "Geological perspectives in the North Sea area",

Bull. Geol. Soc. Denmark,

1969 b

Statens oljedirektorat, "Oversikt over Nordsjoens

geologi",

Stortingsmelding nr.

30

Thomas, A.N., P.J. Walmsley, D.A.L. Jenkins, "Forties is

multi-billion barrel field", Petroleum & Petrochemical International,

October

1973

62° 6". si 60° 59 56°--52 5. 3° MET AND PLAT RN I° !MID N NTII A MGM

'

vipaulro NORTH SEA

Main structural elements.

144.

141111

-4,4Wasisr,

4° 6° o FENNOSCANDIAN PLATFORM 7°

FIG. 1

6

G -IV/1

1 -60° 53° 0

-

24

NORTH SEA TRIASSIC DEPOSITIONAL REGIME.

FIG. 2

r.73. - I V / 1

59. 57° 5 I. 3° 5 7° NORTH SEA JURASSIC I SOPACH 200 isopach in meters Jurassic absent 50 100 km.

FIG. 3

3-IV/1

V° 3° SS°

\

-

26

-it 6° _?* NORTH SEA CRETACEOUS ISOPACH 400 sopach in meters lcretaceous absent O SO k m

FIG.4

G -IV/1

6e 6" 60° S9° SHETLA kATFORP.1 3° /3 7° LOWER PALEOCENE (DANIAN) PA LE O G EOGR A PH Y Dan. Chatk laippattnia Shatki, Stelf Seas tiarainnt=tail ErtagnMq _LT L NORTH SEA FENNO -SCANDLAN PLATFORM

FIG. 5

e°

G -IV/1

17/7

cautmn4, Km

110...

D SO 100

6.° se 59° 53° 4° -

28

o io ERA PLAT FOR r1./ 1 -4 SOUTHERN BASI4 It NORTH SEA M PALEOCENE (Base Eocene) PALEOGEOGRAPHY 222 CONTINENTAL ss, SHALLOW SHELF SEAS

LOCALLY EXPOSED AT TIMES

ETL, AREAS OF POTENTIAL MARGINAL

SANO DEVELOPMENT

c=2 AREAS OF POTENTIAL

TURBIDITE SANO DEVELOPMENT

j MARINE BASINS 50 100 krn FENNO-SCANDIAN PLATFORM STERN IMIT OF L.T RTIARY D POSITS DA H EMBA ENT

G IV/1

62°4! S. 61. 60° 59. 4° 3° _2° 4 6° 7°

FIG. 7

G -Iv/1

NORTH SEA TOTAL TERTIARY ISOPACHS

CONTOUR INTERVAL SOOm

/

TERTIARY ABSENT

Km 50 100

ma

30

-ODIN HIGH

NORTH SOUTH

VIKING BASI N CENTRAL BASIN

MEZOZOIC and OLDER E MEZOZOIC and OLDER

MEZOZOIC and OLDER

STAGE 1(to END OF PALEOCENE) A considerable amount of coarse elastics was deposited in

the subsiding basin North of the Odin High, while Daman Chalk was deposited in the Central

North Sea f3asin

STAGE 2 ( to END OF E-OCENE) The dominant sedi ment in both V k ing and Central Basins was clay, t hough

some sand, were deposited in the North The Odin High was active, and the main subsidence v still to the North

STAGE 3 (to END OF OUGOCENE Subsidence continued in the North, and began to be important in the

Central North Sea Basin late in the Paleocene Coarse elastics were generally less abundant Activity of the

Odin High wes less marked

U MIOCENE TO RECENT

STAGE I.(to PRESENT) Deposition continued as in stage three,(with the main subsidence to the South,) till the

late M.Mocene,at which time the main effects of the Alpine orogeny were felt, with an erosional unconformity

across the Odin High From U Miocene to Recent, sands are abundant to the North, shales and clays to the South

FIG: 8 TERTIARY DEVELOPMENT OF THE NORTHERN NORTH SEA.

53° 52°

wH

55° se rno

,0

01100.60

ATM.10*°°

_...2111111-

,ggill11040,..10

,

_,--

_Ogifore

1":01 - _ROW% _._

'..

_011.04%.

28

Aar

_{All.nrarlip...:r,}

30 11 111

All,r1 Pi)

:Ill

'llitViikAll

_{.40 / AA,'}

111.".

....N.0.04arik

ir~11¡°00/01,

0,00r0Joweroler.

100

Of/

_a/

fr

_,. 28 28 NORTH SEA APPROXIMATE GEOTHERMAL GRADIENTS (IN °C/ km HACHURE INDICATES GEOTHERMAL HIGHS 0 50 ix km. After M.L. Harper (Amoco) in Nature, March 1971

G -IV/1

ONS

74

G-IV/2

NORTH SEA PETROLEUM

IN GEOLOGICAL PERSPECTIVE E. Robert Schroeder Vice President TerraMar Consultants U.S.A.

Beyond doubt, exploration of the North Sea has been the most highly

publicized and broadly discussed of all the many and colorful exploration ventures in the history of the petroleum industry. _{The Arabo-Persian Gulf} has been a part of the industry _{so long that in spite of its vast reserves,} it does not, nor has it ever, received the style or degree of general

publicity that activities in the North Sea receive routinely. For a brief episode, the North Slope of Alaska was a prime topic of conversation in North America. _{Significant events are regularly occurring in Southeast} Asia. _{These events are certainly a major interest to industry people, but} none have received the attention which has been _{accorded the North Sea} petroleum province. _{There are several reasons why this is so.}

The results to date in terms of total _{ultimate reserves} discovered and the regularity with which new discoveries are made are, by world standards, quite spectacular. The physical environment is such that the marine

technology being applied and the requisite investments are also spectacular.

The scramble for these resources by both private and government entities is in itself fascinating, and finally These reserves are being found _{on the door step of Europe}

where there are ample, if _{not overwhelming, means of}

communication by a highly literate free press.

The trade journals compete furiously _{for new means of analyzing the} significance to Western Europe, _{even the world, of the discoveries being} made in the North Sea. _{There is no newspaper or magazine published in}

Northwest Europe worth its price _{which has not discussed all rational,}

and sometimes irrational, aspects of events in the North Sea. _Occasionally a sober note will be heard suggesting that _{the energy problems of Europe} are not resolved for all time or that the world is not really in the grip of a few oil companies, but this usually occurs in some dry professional or trade journal or on the back page of a more conservative _newspaper.

2

Interest on the part of the public has stimulated unprecedented response of all kinds from members of the petroleum industry. Obviously, it is essential to a company's public image that it be represented, in some manner, in the North Sea in order to assure stockholders that their

management is alert. Responsible representatives of the larger companies are compelled to make quotable public statements in an attempt to inject reason into the printed material being scattered like snow by politicians, economic philosophers and "guardians of the public interest." It is

obvious that in this mix of social and technical communication things do get out of perspective. It seems essential, therefore, to inject

perspective into consideration of the North Sea petroleum province.

However, in this paper we shall restrict ourselves to that scientific

discipline which is the base for all natural resources--geology. CLASSIFICATION

To place something in perspective, that is to classify it, it is necessary to go beyond the object being considered, in this case the North Sea petro-leum province, and compare it with all other objects of similar nature wherever they may be found or whenever they may have occurred. Comparison

in turn requires a system of standards and by applying these standards, a system of classification can be established. Now we all know that human beings love to classify things--one in relation to the other. For example, historians used to speak of the military conflict from 1914 to 1917 as

"The Great War." Thus, this conflict was compared with the previous, less great wars. After 1945, it was necessary to establish a new classification

system, so historians labeled "The Great War" "World War I" and the con-flict during the early 1940's, "World War II." This is a new and open-ended classification system. Let us hope that this unprecedented foresight

on the part of historians is unnecessary.

Scientists are known, compulsive and very systematic classifiers. They have achieved this recognition because, of course, scientists only work with measurable facts. Certainly the supreme accomplishment of scientific classification was achieved when those most scientific of classifiers--the taxonomists--classified all of us in this room Homo sapiens sapiens. That

is a scientific fact.

Among _{scientists, geologists are excellent classifiers--we classify} according to length and width and depth, but we also interject time, and to complicate things, we inject geological processes. _{The geological} literature is replete with erudite discussions and _{dissertations--even} arguments--about the proper classification of sedimentary rocks, the age of formations or the relation of fossils _{on the evolutionary scale--ad} infinitum. _{These debates are apparently essential to the evolution of} our science of geology.

We geologists who work professionally _{on behalf of the petroleum industry}

are blessed with a desire, if not a command, to be pragmatic. _Normally, we do our work--we publish little--and we are primarily concerned with what works best. _{But even petroleum geologists must classify and one of}

the things which we classify is the subject of this paper--Petroleum

Producing Provinces. BASIN CLASSIFICATION

Since geologists first recognized that the _{materials which compose} sedi-mentary rocks are derived primarily from other rocks and _{that sedimentary} units vary in thickness and composition from place to place, geologists have been concerned about the proper classification of the basins in which

sediments occur. _{Sedimentary basins have been classified on the basis} of their location, their shape, their _{thickness, their contents and their} relation to mountain systems. _{Perhaps one of the more learned modern} efforts was by Marshall Kay in his "North American Geosynclines" (1951). Some of the concepts and a few of the terms utilized by Dr. Kay have been adopted by the geological profession, _{but in fact, few pragmatic petroleum} geologists distinguish between _{Zeugogeosynclines and Autogeosynclines.} In 1952 Dr. Lewis Weeks presented

an enlightened analysis of the geological factors which control the occurrence of major oil accumulations in sedimen-tary basins (Figure I). _{He offered a systematic classification of} sedimen-tary basins which he described _{as "simple yet complete; and one that is} useful for all purposes--particularly _{to the oil geologists."}

In 1958 Dr. Weeks further elaborated

on the geological factors which control major oil accumulations. _{On the basis of his work and the work of Knebel and} Rodriguez-Eraso (1956), the real significance of the major oil accumulations to the world petroleum industry became apparent. _{The expression "elephant} hunting," referring to major oil fields, became a popular _{term. An}

FROM L. G. WEEKS,

1952

FIGURE I

CLASSIFICATION OF GEOSYNCLINES

OR

DEPOSITIONAL BASINS

GROUP CLASS AND SUB-CLASS

MOBILE BELT MARGINAL (EXTRA-CONTINENTAL) OPEN MARGINAL CLOSED MARGINAL INTRA-CONTINENTAL INTERMONTANE

SECOND ORDER BASINS DEVELOPED WITHIN THE MOUNTAINS OF A MOBILE BELT MEDIAN MASS STABLE REGIONS FORELAND SHELF NEAR INTERIOR REMOTE INTERIOR GRABEN OR HALF-GRABEN STABLE COASTAL

"elephant" was an accumulation with at least 100 million barrels (14.3 million tons) of reserves. _{It was a prerequisite to gaining the attention}

of management that exploration geologists _{were at all times hunting for} elephant-sized accumulations in the places where elephants _occur.

In 1967 Uspenskaya presented a classification of oil and gas provinces at the World Petroleum Congress in Mexico City (Figure II). This

classification is largely based on regional tectonic _{style. The author} did not attempt to classify all the then known oil and gas provinces much less the North Sea. _{As a matter of fact, so little was known as recently} as 1967 about the subsurface geology of the North Sea _{Basin that few,} if any, geologists would have been willing to express an opinion. In the light of present knowledge, it is _{apparent that in accord with Uspenskaya's}

classification system, the North Sea petroleum _{province properly falls in}

the Group "Platform Provinces"; _{Subgroup "Slopes of Stable Platforms";} Type "Marginal Depressions."

Then in 1968, the American Association of Petroleum Geologists had as the theme for its national convention in Oklahoma City, the "Geology of the Ciants." _{Out of that meeting AAPG Memoir 14 was published in 1970.}

In that volume Halbouty et al in a two part presentation, provided a most

valuable analysis of the giant petroleum _{fields and the geologic provinces}

wherein giant accumulations occur throughout the world.

In that Memoir and in this paper "giant" oil accumulations are defined

as those which contain 500 million barrels (71.4 million _{tons) of ultimately} recoverable oil reserves or oil-equivalent gas converted at 6000 standard cubic feet per barrel.

For the first time, information on accumulations in the USSR was incorporated. For the first time, too, equal attention was given to "giant" gas fields; that is, those gas accumulations with _{3.5 trillion cubic feet (100 million} cubic meters) or more of reserves. _{Petroleum provinces were classified by} their geologic characteristics _{as these characteristics relate to the} origin and preservation of giant _{petroleum accumulations.}

We in the industry are deeply indebted to all the authors who _{contributed to this Memoir.}

Dr. H. D. Klemme is credited with compiling the basin classification on behalf of these authors. _{Klemme (1971) subsequently published}

6

G IV/2

TYPES OF OIL AND GAS PROVINCES FIGURE II

Group Subgroup Type

Platform provinces (of ancient and young platforms) Central (interior) portions of stable platforms Interplatform depressions

Interior graben troughs

Slopes of stable platforms

Marginal depressions Deeply sunk (junction)

depressions Platform flanks of foredeeps Depressions of mobile ancient platforms and interplatform mobile zones Provinces of mobile belts (geo-synclinal and epi-platform) Peripheral portions of geosyncline folded systems

Frontal folding and interior portions of foredeeps Transversal depressior<: of outer uplifts of folded systems Intermontane and interior depressions of geosynclinal folded systers Superposed depressions of median masses Superposed graben troughs in meganti-clinoriums Relic depressions in meganticlinoriums Intermontane and Interior depressions of epiplatform orogenic areas From Uspenskava, 1967

discussion of the interrelation of giant fields and sedimentary basins. Halbouty et al (1970) and Klemme (1971) _{take a very pragmatic approach} to petroleum province classification (Figure _{II1). Under the heading} Basin Geology, two major types of _{provinces are identified: Cratonic and} Intermediate. _{The distinction is based on the known or presumed nature of} the earth's crust underlying the _{sedimentary beds.}

Cratonic basins are underlaid by thick continental crust whereas the intermediate group of basins are underlaid by intermediate type crust as occurs at the borders of present or former continental _{plates. A third major subdivision which} presumably will be named Oceanic is only referred to. _{The oil industry is} just beginning to investigate deep water areas but within this decade, major petroleum accumulations will be found in Oceanic basins.

Within the Cratonic group, Halbouty _{et al identified three types of basins}

termed: _{Interior Simple Basin, Intracontinental Composite or Multicycle} Basin and the Graben, or Rift Basin, respectively. _{An additional five} types of basins are identified under the _{heading of "Intermediate Crust."} The three Cratonic types are different in tectonic style and in _sedimentary history. _{Interior Simple Basins}

occur in the interior of continents. They have received only a single cycle of sedimentation which usually occurred during the Paleozoic period.

Intracontinental Composite or Multicycle Basins are the most loosely defined of all basin types.

Their characteristics will be described in more detail below. _{Both Halbouty et al (1970) and Klemme (1971) place the} North Sea depression in this basin class.

Cratonic Graben or Rift Basins _{generally occur in the more exterior parts} of cratons. _{Many had open access to oceanic}

basins during late Mesozoic and Tertiary deposition. _{An excellent example is the}

Red Sea graben.

_A

semi-rift like character is apparent in many Cratonic basins, _{but other} geologic characteristics usually require that they be classified as "Intracontinental Composite or Multicycle Basins."

Under the collective _{name "Intermediate" Halbouty}

et al (1970) and Klemme (1971) recognize five types of sedimentary basins which _{contain giant} petroleum accumulations. _{The most significant is Intermediate}

BASIN TYPES CHARACTERISTICS DESCRIPTION CRUSTAL TYPE BASIN TYPES CHARACTERISTICS DESCRIPTION Type 1 INTERIOR-SIMPLE saucer shaped Moderately large, flat dish-like, single cycle basins-often with Paleozoic platforms or embay-ment facies-with basement uplifts and

sedimentary struc-tures located on interior portion of craton

INTRACONTINENTAL-in- GRABEN or half gra-eluding composite, ben or RIFT

foreland shelf, re-mote interior to

intermontane

Large(subcontinental Miogeosynclinal) to

small (Intermontane) basins, with two or more cycles of

depo-sition-first cycle often platform or shelf sediments and second cycle orogen-ic clastorogen-ics-located around margin of craton (Requires further subdivision Type 4 EXTRACONTINENTAL down-warp to small ocean basin

Large to medium siz-ed basins locatsiz-ed along margin of small ocean basins. With development along "tethys" of 4a platform and trough 4b trough only-and development along small ocean basins 4e, downwarp into Mediterranean-like sea. Type 5 STABLE COASTAL or pull-apart Linear Coastal basins with down-faulted Mesozoic and Tertiary located along cratonic mar-gins-possibly sepa-rated by "sea floor

spreading."

Small to Medium siz-ed graben or rift faulted basins. Pos-sible incipient or dormant "sea floor spreading."

INTERMEDIATE

Type 6 Type 7

INTERMONTANE (back)-Second stage, STRIKE INTERMONTANE

(back)-Second stage, TRANS-VERSE

Small, second cycle Tertiary clastic basins lying trans-versely upon the

de-formed eugeosyn-clinal troughs which were developed

par-allel to certain coast lines-possibly underthrusting of

"oceanic plates."

Same as 6 with dif-ferent orientation and greater linear extent.

From Rlemme 1971

Type 8

FIGURE III

Upper Tertiary DELTA

Upper Miocene to re-cent 'birdfoot deltas in coastal areas where major conti-nental drainage areas exist.

CRUSTAL TYPE CRATONIC

Extracontinental Downwarp to a Small Ocean Basin. _{The Arabo-Persian Gulf} geosyncline falls in this class. _{Continuing on, Type 5 is the Stable} Coastal or Pull-apart Basin. Type 6 is the Intermontaine Second Stage, Transverse Basin. _{Type 7 is the Intermontaine Second Stage, Strike Basin,} and Type 8 is the Upper Tertiary Delta.

Again, this classification system places the North _{Sea Basin among the}

Cratonic Intracontinental Composite or Multicycle Basins.

What are the geological characteristics which typify Cratonic Intracontin-ental Basins?

Location

Shape

Sediments

Age

Fundamental to the identification of this type basin is that it be located at the periphery of a cratonic mass and be under-laid by acidic basement rocks typical of continental platforms.

These basins may vary in shape, but generally they have broad, relatively flat floors above basement, and a relatively uniform degree of

tectonic style on their peripheries.

There is no minimum thickness of sediments

which characterize Cratonic Intracontinental

Basins. _{However, by definition the sedimentary} sequence reflects two or more cycles of

sedimentation.

Frequently the first episode of sedimentation; that is, the older sedimentary _{sequence, is of} Paleozoic age. _{The overlying sequence or} sequences may be of Paleozoic or younger age.

G-IV/2

Size _{These basins may vary in size from relatively} small isolated depressions as the Uinta Basin in North America to vast regions as in Western Siberia.