The fluid regime during fracturing of the Embla field, Central Trough, North Sea

Marine and Petroleum Geology 04 "0887# 640Ð657

The ~uid regime during fracturing of the Embla _eld\ Central Trough\ North Sea I[A[ Munza\\ K[ Idena\ H[ Johansena\ K[ Vagleb a

b

Institutt for ener`iteknikk\ P[O[ Box 39\ N!1996 Kjeller\ Norway Phillips Petroleum Company Norway\ P[O[ Box 119\ N!3945 Tanan`er\ Norway Received 09 September 0886^ revised 11 July 0887^ accepted 16 July 0887

Abstract The deep Embla _eld\ located in the Central Trough\ North Sea\ represents the _rst oil reservoir in Paleozoic rocks to be developed on the Norwegian shelf[ The reservoir consists of braided ~uvial and alluvial fan sandstones interlayered with ~oodplain:lacustrine mudstones and volcanics and intrusives[ Extensive fracturing of the reservoir has led to the formation of numerous mineralised veins[ Three main types of mineralised fractures can be identi_ed] "0# early diagenetic veins and veins which occur in and around ma_c igneous rocks^ "1# carbonate!dominated veins^ "2# barite\ clay minerals and bitumen veins[ This article presents ~uid inclusion evidence from fracture!_lling minerals\ suggesting that the fractures were important ~uid conduits of both petroleum and brine migration[ The petroleum inclusion data are consistent with an undersaturated oil composition[ A range in salinity between 12 and 01 wt ) NaCleq is found for the aqueous inclusions[ The variation in salinity indicates mixing between a high saline ~uid\ related to evaporites\ and a ~uid with lower salinity[ Microthermometry data suggest that the fracturing events occurred at deep basinal levels\ at temperatures in excess of 099>C[ Þ 0887 Elsevier Science Ltd[ All rights reserved[

0[ Introduction Fracturing of sedimentary sequences may involve sig! ni_cant changes in the ~uid dynamics of a basin[ At the time of their formation and as long as they stay open\ fractures may signi_cantly enhance permeability and ~uid ~ow properties[ Fracture!enhanced permeability may assist in both petroleum _lling and drainage of a reservoir[ Aqueous ~uids introduced from fractures may cause diagenetic reactions\ which in turn may lead to changes in the porosity distribution[ Once sealed by min! eralisation fractures may act as barriers for further ~uid ~ow[ To establish the relative timing between fracturing and petroleum emplacement is therefore important for understanding the ~ow properties of a reservoir[ Fluid inclusions in fracture _lling minerals may give evidence for ~uid migration through fractures "Burruss\ Cerone + Harris\ 0872^ Bodnar\ 0889#\ and help to con! strain the prevailing physical and chemical conditions during and subsequent to fracturing[ Primary ~uid inclusions represent samples of the ~uids present during precipitation of fracture minerals[ In this study ~uid inclusion data from fracture _lling minerals are used to establish the relationship between fracturing and pet!  Corresponding author[ E!mail] ingridmÝife[no

roleum and brine migration in the Embla _eld\ Norweg! ian North Sea[ The deep\ Paleozoic Embla _eld shows an exceptionally high abundance of mineralised fractures[ Petrography and temperature data from ~uid inclusions give evidence for multiple episodes of fracturing at deep basinal levels[ Compositional data of inclusion ~uids in the fracture _llings give valuable constraints on the ~uid transport in the fractures[

1[ Geological setting The Embla _eld "Fig[ 0# is located in the Central Trough[ It represents the _rst _eld to be developed in Paleozoic rocks on the Norwegian shelf[ The structural history of the Central Trough since the Zechstein has been outlined by Gowers and S%bo e "0874#[ They sub! divide the area into 06 structural zones[ One of these\ the Grensen Nose\ represented a part of the Mid North Sea High until the Central Trough subsidence was established in Late Cretaceous times "Gowers and S%bo e\ 0874#[ The Embla _eld\ located on the Grensen Nose\ is bounded to the NE by the Skrubbe fault\ which marks the boundary between the Grensen Nose and the Feda Graben[ The Embla _eld is a westward dipping Paleozoic horst[ The development history and the seismic and geological

S9153Ð7061:87:,*see front matter Þ 0887 Elsevier Science Ltd[ All rights reserved PII] S 9 1 5 3 Ð 7 0 6 1 " 8 7 # 9 9 9 5 1 Ð 1

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Fig[ 0[ Map of the Embla _eld\ Central Trough\ North Sea[

The reservoir section in the Embla _eld\ resting directly on the Upper Devonian mudstones\ consists of inter! layered sandstone and mudstone units[ The depositional environment has been interpreted as a braided ~uvial and alluvial fan setting of the sandstones and a ~ood! plain:lacustrine setting of the mudstones "Knight et al[\ 0882#[ Intrusive and extrusive igneous rocks are found within the reservoir section[ Both acid and ma_c igneous rock types are present[ The igneous rocks are however severely altered by secondary processes\ which have

characteristics of the Embla _eld have been described by Knight\ Allen\ Coipel\ Jacobs and Scanlan "0882#[ The oldest rock so far identi_ed within the Embla _eld is a strongly altered rhyolite in the wells 1:6Ð10S and 1:6Ð 15S[ Very _ne!grained to _ne!grained micaceous sand! stones and silty mudstones rest upon the rhyolite[ An Upper Devonian "Frasnian# age has been indicated by palynological dating\ and a deposition in a ~ood! plain:lacustrine environment was suggested "Knight et al[\ 0882#[

CMYK Page 641

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destroyed the igneous mineralogy and geochemistry[ Dat! ing of the reservoir is di.cult\ due to the lack of mic! rofossils[ An early Permian\ Carboniferous and:or Upper Devonian age has been suggested "Knight et al[\ 0882#[ The petroleum trap is a combination of stratigraphical and structural[ Residual bitumen occurs within parts of the reservoir\ and this bitumen predates the mobile oil "Knight et al[\ 0882#[ Unconformably above the reservoir section\ the Upper Jurassic Tyne Group is present except near the crest where the Upper Jurassic rocks are missing[ Mudstones and argillaceous limestones of the Lower Cretaceous Cromer Knoll Group overlies the Upper Jurassic[ At the crest of the Embla structure the Cromer Knoll Group occurs directly upon the reservoir section\ and forms the top seal of the structure "Knight et al[\ 0882#[ Above the Cromer Knoll Group approximately 899 m of chalk of the Upper Cretaceous:Lower Paleocene Shetland Group\ 2999 m of Hordaland Group mudstone and Nordland Group mudstone\ clay and sand of Cenozoic age are present[ 2[ Sandstone diagenesis The dominating diagenetic reactions in the Embla res! ervoir sandstones are dolomite\ quartz and kaolinite pre! cipitation[ Feldspars and evidence for feldspar dissolution are scarce[ The diagenetic sequence "Fig[ 1#

642

shows an early phase of red dolomite precipitation fol! lowed by quartz overgrowths\ precipitation of ankerite\ siderite\ kaolinite\ barite and bitumen[ A spatial variation in the abundance of diagenetic minerals exists[ The sand! stones are either extensively dolomite cemented or domi! nated by quartz:kaolinite cementation[ The carbonate cemented zones mainly consist of a red dolomite\ which often occurs in rhombs with an oscil! latory zoning[ The red colour is probably caused by small grains of iron oxides[ This red dolomite is most probably formed during early diagenesis[ Later stages in the diag! enetic evolution involve precipitation of minor amounts of ankerite and siderite[ The ankerite either occurs as rims on the red dolomite or as separate grains[ Minor siderite cement mostly occurs in the vicinity of the frac! tures\ especially in well 1:6Ð12S[ Quartz and kaolinite are the dominating authigenic minerals in zones where the early diagenetic dolomite cement is reduced or lacking[ Euhedral quartz over! growths with well developed dust rims are common[ Con! tacts between quartz overgrowths and carbonates are not common[ But in rare cases\ the quartz overgrowths can be observed to predate the ankerite[ Kaolinite occurs both as a coarse!grained alteration product from mica and as a more _ne!grained\ blocky type _lling the pores[ Minor amounts of berthierine occur in association with the blocky\ pore!_lling kaolinite[ The zones with quartz: kaolinite dominated cementation show a better reservoir

Fig[ 1[ Diagenetic sequence of the Embla sandstones[

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quality than the zones dominated by early diagenetic dolomite[ Petroleum inclusions are common in the sandstones in large parts of the reservoir[ They occur both as secondary inclusions within the detrital part of the quartz grains and as primary inclusions in the dust rims of the quartz overgrowths[

main types of veins at this stage are barite and kao! linite:berthierine!rich veins[ Bitumen occurs with the clay mineral veins[

3[ Fracturing of the Embla reservoir

The abundance of the early red dolomite veins is low[ Fluid inclusions are not present[ These veins will there! fore not be further treated in this article[

The cored reservoir section of the Embla _eld shows a large number of fractures\ partly _lled with miner! alisations[ The main orientations of the fractures are NWÐSE with moderate to steep dips "Knight et al[\ 0882#[ A wide variety of fracture minerals occurs\ and the tex! ture of the fractures varies from simple monomineralic types to concentrically zoned or irregular polymineralic types[ Reconstruction of a relative time sequence of the fracturing is hampered by two problems] "0# The di}erent types of mineralised fractures partly occur at di}erent locations[ "1# The variability of the mineralogy of the fractures complicates a rigorous division of the fractures into di}erent types[ A rough outline of three main stages of fracturing can however be made "Table 0#] "0# The _rst stage is probably formed during early diagenetic conditions[ The veins consist of red dolomite\ and have a similar appearance as the early dolomite cement in the sandstones[ Some early veins also occur in and around the ma_c igneous rocks in well 1:6Ð15S[ These are quartz and dolomite veins\ and they are cut by veins of stage 2[ "1# Wide!spread formation of complex carbonate veins[ The carbonate minerals comprise calcite\ dolomite and siderite\ and they occur often together with pyrite[ Some phosphate formation also occurs at this stage[ "2# The

4[ Petrography 4[0[ Stage "0a#] early diagenetic veins

4[1[ Stage "0b#] early veins in and around ma_c igneous rocks Rhyolites occur within the reservoir section in well 1:6Ð10S[ They are extremely _ne!grained\ and represent undoubtedly volcanic horizons[ Any veins occurring uniquely in the rhyolites have not been identi_ed[ Two horizons of ma_c igneous rocks are present in well 1:6Ð15S[ Extensive secondary alteration has led to a complete break!down of the igneous mineralogy[ Relics of pseudomorphic phenocrysts of plagioclase are still pre! sent[ The rocks probably represent severely altered basalts[ Zoned veins of quartz and dolomite occur within these ma_c rocks and the neighbouring mudstones[ The veins are concentrically zoned with idiomorphic dolomite crystals at the borders and quartz in the middle zone "Fig[ 2A#[ A younger vein system which belongs to stage "2# often intersects these veins[ 4[1[0[ Quartz!dolomite veins The older vein system\ the quartz!dolomite veins\ con! tains only aqueous inclusions[ The dolomite is zoned

Table 0 Petrographic summary Vein type

Stage 0 Stage 0 Stage 1 Stage 1 Stage 1 Stage 1 Stage 1 Stage 1 Stage 2 Stage 2 Stage 2 Stage 2 Stage 2 Stage 2

Well

1:6Ð15s 1:6Ð15s 1:6Ð10s 1:6Ð10s 1:6Ð10s 1:6Ð10s 1:6Ð10s 1:6Ð10s 1:6Ð10s 1:6Ð15s 1:6Ð15s 1:6Ð15s 1:6Ð15s 1:6Ð14s

Core depth

Host rock

Vein

feet RKB

m RKB

Lithology

Diagenesis

03491 03599 04140 04155 04200 04248 04255 04260 03110 03608 03491 03599 03503 05615

3319[1 3349[0 3537[4 3542[0 3555[7 3570[3 3572[5 3574[0 3223[5 3375[3 3319[1 3349[0 3343[2 4987[0

Ma_c volcanics Ma_c volcanics Acid volcanics Acid volcanics Sandstone Sandstone Sandstone Sandstone Sandstone Sandstone Ma_c volcanics Ma_c volcanics Mudstone Breccia

Extensive alteration\ very little porosity Extensive alteration\ very little porosity Extensive alteration\ very little porosity Extensive alteration\ very little porosity Red dolomite\ very little porosity Red dolomite\ very little porosity Red dolomite\ very little porosity Red dolomite\ very little porosity Quartz\ kaolinite\ porous Quartz\ kaolinite\ porous Extensive alteration\ very little porosity Extensive alteration\ very little porosity Red dolomite\ very little porosity Extensive alteration\ very little porosity

Quartz\ dolomite Quartz\ dolomite Calcite\ pyrite Calcite\ pyrite Calcite\ pyrite\ kaolinite Calcite\ pyrite\ kaolinite Calcite Calcite\ pyrite Barite Barite Barite\ bitumen Chlorite\ barite\ bitumen Chlorite\ bitumen Barite\ bitumen

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Fig[ 2[ Textural relationships of mineralised fractures in the Embla _eld[ "A#] Quartz!dolomite vein cut by a younger berthierine!bitumen vein[ The quartz!dolomite vein show a zoned structure with quartz in the middle and euhedral dolomite crystals along the walls[ "Sample 1:6Ð15S 03599?#[ Scale of bar  9[4 mm[ "B#] Aqueous inclusions in quartz from the quartz!dolomite veins[ Small crystals of carbonate are often present in the aqueous inclusions[ "Sample 1:6Ð15S 03491?#[ Scale of bar  9[0 mm[ "C#] Trails of petroleum inclusions occurring around the berthierine!bitumen veins when they cross!cut the quartz!dolomite veins[ "Sample 1:6Ð15S 03599?#[ Scale of bar  9[0 mm[

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Fig[ 2[ Textural relationships of mineralised fractures in the Embla _eld[ "D#] Calcite!pyrite fracture[ Pyrite "black# occurs along the walls and zoned calcite in the middle[ "Sample 1:6Ð10S 04155?#[ Scale of bar  4 mm[ "E#] Primary zoning of the calcite\ showing a dark core and multiple growth zones in the outer parts of the crystals[ Mostly aqueous inclusions in the growth zones\ but primary petroleum inclusions also occur[ "Sample 1:6Ð 10S 04155?#[ Scale of bar  0 mm[ "F#] Bitumen!barite vein[ The barite crystals show dark cores with petroleum inclusions[ "Sample 1:6Ð14S 05615?#[ Scale of bar  9[4 mm[

I[A[ Munz et al[:Marine and Petroleum Geolo`y 04 "0887# 640Ð657

with a dark\ dusty core and with a transparent rim[ This zonation is due mainly to very small solid inclusions\ but some very small aqueous inclusions have also been observed[ These aqueous inclusions are however mostly too small for analysis[ The quartz contains scattered aqueous inclusions "Fig[ 2B#[ These aqueous inclusions often contain a solid\ which is birefringent and most likely is a carbonate[ The quartz!dolomite veins do not contain petroleum inclusions\ except at the intersection with stage "2# veins "Fig[ 2C\ described below#[ Petroleum inclusions have neither been found in any of the host rocks[ 4[2[ Stage "1#] carbonate veins Carbonate veins are abundant in most parts of the reservoir[ The mineralogy of these veins shows some vari! ation[ Siderite veins are the most common in the north "well 1:6Ð12S#\ whereas calcite veins dominate in the sou! thern part of the reservoir "well 1:6Ð10S#[ 4[2[0[ Siderite veins The siderite veins show a _ne!grained texture with rhomb!shaped grains[ Very small petroleum inclusions occur in the siderite[ But due to the small sizes of both host minerals and ~uid inclusions\ the samples are not well suited for ~uid inclusion analysis[ 4[2[1[ Calcite veins The calcite dominated veins commonly have a zoned structure with calcite in the middle and pyrite along the rims "Fig[ 2D#[ Six calcite vein samples from well 1:6Ð 10S have been included in this study[ Two of the samples occur in a rhyolite\ whereas four occur in a sandstone with early diagenetic dolomite cement[ The vein min! eralogy and appearance of all samples are similar\ irres! pective of the two di}erent host rocks[ The veins have extremely high abundances of both aqueous and pet! roleum inclusions[ Petroleum inclusions in the host rock\ whether volcanics or sandstone\ have not been observed[ Spectacular primary growth zoning is present in one of the calcite veins "Fig[ 2D and E#[ In this sample\ crystals of calcite appear to have grown inwards from the vein wall in a toothlike texture[ The crystal zoning is due to dust and ~uid inclusions[ The crystals often show a dark inclusions!rich core[ The outer part of the crystals are more transparent\ containing one or a few dark\ inclusion!rich concentric zones "Fig[ 2E#[ The outline of the dark cores is somewhat irregular\ but the inclusions do not occur in obvious trails[ This suggests a relatively early trapping of the inclusions in the dark core\ possibly a pseudosecondary trapping[ The concentric texture of the dark zones in the crystal rim and the orientation of the inclusions within these zones suggest a primary origin of these inclusions[ The inclusions within the growth zones are mainly aqueous[ Occasionally petroleum inclusions also occur in these growth zones[ The other

646

calcite samples have a more granular texture\ lacking primary growth zoning[ Although secondary trails of inclusions\ both aqueous and petroleums\ are found\ a large number of inclusions are not related to obvious secondary textures[ A high number of primary inclusions may therefore also be present in these samples[ 4[3[ Stage "2#] barite\ clay mineral and bitumen veins This stage mainly consists of precipitation of various amounts of barite\ kaolinite\ berthierine and bitumen in fractures[ Cross!cutting relationships between these clay mineral dominated veins and the carbonate veins estab! lish the relative timing between the two di}erent stages at several locations[ Most commonly the veins are domi! nated by clay minerals\ barite is only occasionally present\ and they often do not contain suitable host minerals for ~uid inclusions[ 4[3[0[ Barite veins These veins consist only of barite\ lacking the charac! teristic association with clay minerals and bitumen[ They are hosted by sandstones with relatively good porosity\ showing quartz:kaolinite dominated diagenesis[ The tex! ture of the two samples\ represented in this study\ shows some di}erences[ The barite in sample 1:6Ð10S 03110? shows a _brous\ featherlike texture[ The crystals appear to have grown from the wall into the vein[ In sample 15S 03608? the crystals are more elongated and more oriented along the direction of the vein[ The barite veins contain a very high number of two! phase "L¦V# aqueous inclusions[ Solids are rarely pre! sent in the aqueous inclusions[ No growth zoning is present\ but the aqueous inclusions do not occur in trails[ This could suggest a primary origin of the aqueous inclusions[ Petroleum inclusions are present in both samples[ In sample 1:6Ð15S 03608? the petroleum inclusions are clearly secondary\ occurring in widely spaced trails which cross!cut the fracture[ Sample 1:6Ð10S 03110? contains inclusions of both secondary and possible primary character[ Petroleum inclusions are also common in the host sandstone[ Determination of their relative age in the sequence of fractures is di.cult\ since the barite veins have not been observed at the same location as other vein types[ The obvious secondary character of petroleum inclusions in sample 1:6Ð15S 03608? could possibly suggest that pre! cipitation of the monomineralic barite veins happened at an earlier stage than the barite associated with bitumen and clay minerals[ 4[3[1[ Bitumen!barite veins Composite fracture _llings of barite and bitumen occur in well 1:6Ð14S\ forming a net!veining system through the rock[ The host rock is a sandstone\ heavily altered

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and brecciated[ One sample of this type is represented here[ The bitumen occurs in the middle of the vein and barite along the margins[ The two minerals show some coarse!grained intergrowths "Fig[ 2F#[ A few of the barite crystals show a dark core with abundant petroleum inclusions\ which suggest a primary origin of these inclusions[ In addition the barite also contains secondary petroleum inclusions\ occurring along trails[ Petroleum inclusions have not been observed in the host rock[ Aqueous inclusions are especially abun! dant along the walls of the vein[ The aqueous inclusions do not contain solids[ 4[3[2[ Berthierine2bitumen2barite veins In the two samples represented in this study\ the berthi! erine2bitumen2barite veins occur in the same samples as the quartz!dolomite veins\ and show cross!cutting relationships to them[ This younger vein system has mostly too _ne!grained host minerals for inclusions[ However\ in the intersections between the two vein systems\ trails of inclusions are found in the quartz of the older vein system[ These trails follow the borders of the younger vein system\ and are therefore clearly related to the formation of the berthierine2bitumen2barite veins "Fig[ 2C#[ These inclusions consist of both aqueous and petroleum ~uids[ The aqueous inclusions do not contain solids[ Generally the size of these inclusions are smaller than the early\ carbonate!bearing aqueous ~uid inclusions in the quartz[ Fluid inclusions are also found within the some rare barite grains[ 5[ Analytical techniques The microthermometry was done with a Fluid Inc[ "Denver\ U[S[A[# heating!freezing stage[ The stage was calibrated by various standards in the temperature range −45[5 to ¦024>C[ Since {soft| minerals such as car! bonates may be susceptible to heat\ leading to irreversible changes for the ~uid inclusions\ the samples were not heated during preparation of the wafer[ In all samples the homogenisation of the petroleum inclusions\ which show the lowest temperatures\ were measured _rst[ The temperature was not raised above ¦099>C during the petroleum analyses[ No decrepitation or leakage was observed during measurements of the petroleum inclusions[ The homogenisation of the aqueous inclusions was then measured[ Leakage of a minority of the aqueous inclusions hosted by carbonates was observed during this stage of analysis[ However\ only the reproducible results have been registered[ The _nal stage of mic! rothermometry analysis was freezing of the samples in order to measure the salinity of the aqueous inclusions[ Due to expansion of the ice\ severe leakage of the larger inclusions hosted by carbonates was observed during this stage[ Leakage was only a minor problem for inclusions

in quartz and barite[ Only the results of inclusions show! ing no leakage have been used[ It has therefore not been possible to obtain a complete data set for many of the aqueous inclusions[ 6[ Microthermometry A summary of the microthermometry results is shown in Table 1[ For each population of inclusions the number of analyses\ the minimum\ maximum\ mode and average homogenisation or ice melting temperature are represented[ 6[0[ Petroleum inclusions The petroleum inclusions commonly show a brown colour and a blue ~uorescence colour when irradiated with UV light "254 nm#[ A black solid commonly occurs within the inclusions[ The petroleum inclusions consist of two phases "L¦V# at room temperature\ and they always show homogenisation to the liquid phase[ Figures 3Ð6 show the microthermometry results for the petroleum inclusions of each vein type[ The results are characterised by a large range in hom! ogenisation temperatures within each sample\ generally between a minimum of 34Ð49>C and a maximum of 79Ð 89>C "Table 1#[ Most of the samples show an average homogenisation temperature between 59 and 69>C[ For some of the samples there is a di}erence between the mode and the average homogenisation temperature\ which re~ects an asymmetrical distribution[ There is not any signi_cant di}erence between the results for pet! roleum inclusions in stage 1 and stage 2 veins[ For the few samples with primary growth zones and dark cores\ there is not any distinction between the results for pet! roleum inclusions of assumed primary and secondary origin "Figs 4 and 6#[ 6[1[ Aqueous inclusions The microthermometry results of the aqueous inclusions are summarised in histograms and in cross! plots of melting and homogenisation temperatures "Figs 7Ð01#[ Homogenisation of the aqueous inclusions was always to the liquid phase[ With the exception of one or two inclusions\ the freezing behaviour of the aqueous inclusions was the same for inclusions in all samples[ Some early melting\ starting at approximately −49 to −34>C\ occurred[ This indicates the presence of divalent cations\ most likely Ca[ A hydrate\ interpreted as hyd! rohalite\ disappeared at temperatures between −15 and −19>C[ Ice was the last remaining phase\ melting at temperatures between −19 and −8>C[ The salinity was calculated using the equation of Bodnar "0881#[ Primary and secondary inclusions have been distinguished in the

Table 1 Summary of microthermometry results of inclusions in Embla fractures

Stage 0 Stage 0 Stage 1 Stage 1 Stage 1 Stage 1 Stage 1 Stage 1 Stage 2 Stage 2 Stage 2 Stage 2 Stage 2 Stage 2

Well

1:6Ð15s 1:6Ð15s 1:6Ð10s 1:6Ð10s 1:6Ð10s 1:6Ð10s 1:6Ð10s 1:6Ð10s 1:6Ð10s 1:6Ð15s 1:6Ð15s 1:6Ð15s 1:6Ð15s 1:6Ð14s

Core depth

Petroleum inclusions Homogenisation temperature

feet RKB

m RKB

03491 03599 04140 04155 04200 04248 04255 04260 03110 03608 03491 03599 03503 05615

3319[1 3349[0 3537[4 3542[0 3555[7 3570[3 3572[5 3574[0 3223[5 3375[3 3319[1 3349[0 3343[2 4987[0

nnumber of analysed inclusions[ Minminimum value[ Maxmaximum value[ Modemost common value[

n

Min

Max

Mode

Aqueous inclusions Ice melting temperature Average

18 46 45 20 59 62 32 10 18

46 41 42 33 38 36 59 43 43

62 66 61 78 86 60 80 57 71

52 53 50 63 69 54 65 48 53

55 54 51 51 52 51 61 59 57

23 37

36 32

56 80

36 59

43 50

n

Min

19 11

−13[7 −05[2

Max

Aqueous inclusions Homogenisation temperature Mode

Average

n

Min

Max

Mode

Average

−7[3 −01[8 −01[7 −8[9 −04[9 −02[2

11 19

007 013

036 034

022 015

022 017

10

−08[3 − 7[1 −02[8 −02[2

30

66

008

81

83

12 29 12

−13[2 −00[9 −03[4 −03[8 −06[6 −05[3 −05[7 −06[9 −19[3 −08[1 −08[6 −08[7

45 32 16

72 001 015

006 033 036

81 007 026

099 012 027

09

−08[9 −05[0 −05[8 −06[3

00

090

017

bimodal 019

17

−07[7

15

010

030

bimodal 022

−8[3 bimodal −03[7

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Vein type

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of 0[0Ð0[1 wt ) NaCleq for each of the samples[ An average salinity of 19[1Ð11[1 wt ) wt ) NaCleq is found for the stage 2 veins[ Sample 1:6Ð14S 05615? is somewhat di}erent\ in that a bimodal distribution of ice melting temperatures was found "Fig[ 01#[ The main peak occurs at −08 to −07>C\ which corresponds to a salinity of 10Ð 11 wt ) NaCleq[ The range in average homogenisation temperatures is respectively 017Ð022\ 83Ð099 and 019Ð 027>C for stage 0\ 1 and 2 veins "Table 1#[

7[ Discussion 7[0[ Petroleum _lling of the reservoir

Fig[ 3[ Microthermometry results for petroleum inclusions[

histograms of some samples[ However\ for samples with somewhat unclear textural relationships this distinction has not been attempted[ The melting of the hydrate was in many cases di.cult to observe\ and the data set for hydrohalite melting temperatures is therefore smaller than the data set for ice melting[ The cross!plot of hyd! rohalite and ice melting temperatures "Fig[ 7# re~ects variations in total salinity of the inclusions and di}er! ences in Na:Ca ratios[ In contrast to the petroleum inclusions\ clear di}er! ences in salinity and homogenisation temperatures of the aqueous inclusions were found between the three di}erent stages of veining "Table 1#[ The quartz!dolomite veins of stage 0 and the calcite veins of stage 1 are both charac! terised by a large range in ice melting temperatures\ from a minimum of −13[7>C to a maximum of −7[1>C[ This corresponds to a salinity range of 14[4Ð00[8 wt ) NaCleq[ The average ice melting temperatures fall in the range −03[8 to −01[7>C\ which correspond to a salinity range of 05[6Ð07[5 wt ) NaCleq[ Except for sample 1:6Ð14S 05615?\ each of the stage 2 veins are characterised by a higher and much smaller range in salinity[ For the monomineralic barite veins the di}erence between the maximum and minimum ice melting temperature is in the order of 0[2Ð0[1>C\ corresponding to a range in salinity

Petroleum inclusions occur in dark cores and occasion! ally in outer primary growth zones of the carbonates[ Dark cores of petroleum inclusions in barite of the organic matter!barite veins also suggest a primary origin in this sample[ The berthierine2bitumen2 barite veins rarely contain suitable host minerals for ~uid inclusions[ But in the intersections with the quartz!dolomite veins\ trails of petroleum inclusions occur in the quartz along the berthierine2bitumen2barite veins[ Filling of the Embla reservoir was therefore taking place at the time of cementation of stage "1# and stage "2# veins[ The presence of secondary petroleum inclusions may also indicate later episodes of petroleum migration through or across the veins[ In order to resolve the pet! roleum _lling history of the Embla structure\ information on the composition of petroleum inclusions is necessary in addition to their textural occurrence[ Petroleum ~uids of di}erent compositions have been shown to occur as primary inclusions in successive growth zones in fracture minerals in sedimentary basins\ giving evidence for epi! sodically ~uid ~ow through fractures "Bodnar\ 0889#[ Bodnar "0889# used the ~uorescence colour to distinguish the di}erent compositions of petroleum inclusions[ Only small variations in ~uorescence colour are present in the petroleum inclusions in the veins[ This would suggest relatively similar compositions[ Variations in compo! sition\ especially the methane content\ can in~uence the homogenisation temperatures[ Since the homogenisation temperatures also strongly re~ect the prevailing pressure! temperature conditions during trapping\ it is di.cult to evaluate compositional di}erences from the mic! rothermometry data[ The homogenisation temperatures of the primary and the secondary inclusions are relatively similar\ which may indicate the two types to be related to a continuous episode of petroleum migration[ The microthermometry behaviour of the petroleum inclusions\ i[e[ homogenisation to the liquid phase at lower temperatures than the aqueous inclusions\ shows an undersaturated oil composition[ The petroleum inclusions in the fracture _llings show very similar characteristics\ both in ~uorescence colours

I[A[ Munz et al[:Marine and Petroleum Geolo`y 04 "0887# 640Ð657

650

Fig[ 4[ Microthermometry results for petroleum inclusions[

and microthermometry data\ to the widespread and abundant petroleum inclusions in the sandstones[ This strongly suggests that the occurrence of petroleum inclusions in the veins does not represent only a localised phenomenon\ but that fractures were important conduits for petroleum communication on a reservoir scale[ An inverse correlation between petroleum inclusion abun! dance and the amounts of carbonates is present in the

Embla reservoir sandstones[ The highest abundance of petroleum inclusions occur in sandstones with low or no carbonate cement[ In samples with extensive carbonate cementation only very few or no petroleum inclusions occur[ The fractures may thus have ensured a good ver! tical communication through low permeability zones consisting of mudstones\ sandstones with extensive early diagenetic carbonate cementation and igneous rock units[

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member compositions can been identi_ed] "0# High saline Na!Ca!Cl brines with a possibly distinct Na!Ca!Cl!car! bonate ~uid composition present within the quartz!dolo! mite veins^ "1# Moderate:low saline waters[ All ~uid inclusions in the fracture minerals show higher salinities than the present pore water with a salinity of 4[7 wt ) NaCleq "Knight et al[\ 0882#[ The majority of the inclusions in all veins seem to be dominated by the high saline Na!Ca!Cl brines[

Fig[ 5[ Microthermometry results for petroleum inclusions[

Fig[ 6[ Microthermometry results for petroleum inclusions[

7[1[ Aqueous ~uids Although all the investigated samples show high sal! inities of the aqueous inclusions\ distinct characteristics for the di}erent veins are present[ On the basis of the microthermometry data\ two di}erent aqueous ~uid end!

7[1[0[ The high saline Na!Ca!Cl brine end!member The aqueous ~uid inclusions of the two barite veins are of the high saline Na!Ca!Cl type[ The inclusions are characterised by an extremely limited variation in salinity of the di}erent inclusions[ The variation is less than 0 wt ) NaCleq within each sample[ This demonstrates that the composition of the brine was remarkably homo! geneous on a local scale\ thereby indicating only one ~uid source[ Some di}erence in salinity between the two samples indicate however some spatial variation in ~uid composition[ Of the various mechanisms for generating highly saline waters in basins\ processes related to eva! porites are by far the most e.cient "Egeberg + Aagaard\ 0878#[ Present!day formation waters from oil _elds on the Norwegian shelf are mixtures of a brine end!member and meteoric water "Egeberg + Aagaard\ 0878#[ Aqueous geochemistry of formation waters shows that the brine component is formed by evaporation of sea water beyond the point of halite precipitation\ and is probably expelled from Permian evaporites "Egeberg + Aagaard\ 0878#[ The high salinity of the brine end!member observed in this study is di.cult to produce unless it is related to evaporites[ The microthermometry data of sample 15S 03608? can be combined with a NaCl!CaCl1!H1O phase diagram "Oakes\ Bodnar + Simonson\ 0889^ Yanatieva\ 0835# to calculate the ~uid composition[ The results show an approximate composition of the high saline end!mem! ber of 04 wt ) NaCl\ 6 wt ) CaCl1 and 67 wt ) H1O[ This shows a more Ca dominated composition than the Na!Mg!Cl!SO3 ~uid compositions for marine evap! oration\ but is fully consistent with a diagenetic modi_ed water derived from evaporites[ 7[1[1[ The moderate:low saline end!member The ~uid end!member with moderate to low salinity is more di.cult to establish\ due to the ubiquitous presence of the high!saline inclusions[ A range in melting tem! perature is therefore present in all veins\ except for the barite veins[ A salinity less or equal to 02 wt ) NaCleq is therefore the best estimate for the moderate:low saline end!member possible from the microthermometry data[ 7[2[ Diagenetic and thermal history In order to estimate pressure and temperature con! ditions during trapping of ~uid inclusions\ it is important

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652

Fig[ 7[ Microthermometry results for aqueous inclusions[ Crossplot of melting temperatures for hydrohalite and ice[

to establish the relationship between aqueous and pet! roleum inclusions[ In the quartz!dolomite veins in well 1:6Ð15S the early carbonate!bearing aqueous inclusions are not associated with petroleums[ For these veins the aqueous homogenisation temperatures represent mini! mum temperatures of trapping[ Textural evidence for simultaneous trapping of petroleums and aqueous inclusions has been used as an argument for methane saturation of the aqueous phase "Nedkvitne\ Karlsen\ Bjo rlykke + Larter\ 0882#[ If methane saturation occurs\ the aqueous homogenisation temperatures will represent the actual trapping temperature of both petroleums and brines[ Except possibly for the barite veins "i[e[ sample 15S 03608?#\ textural evidence indicates that aqueous inclusions in vein samples from stage "1# and "2# coexist with petroleum inclusions[ The trapping temperature of the aqueous and petroleum inclusions in the calcite veins would therefore be in the order of 89Ð099>C\ and for the inclusions in the barite veins\ the bitumen!barite vein and the berthi! erine2bitumen 2barite veins the trapping temperature is in the order of 004Ð039>C[ The time!temperature relationships are summarised in Fig[ 02[ A temperature in excess of 89Ð099>C indicates that the cementation of the fractures happened at deep basinal levels[ The diagenetic evolution of the Embla sandstones

shows a sequence with early non!ferroan dolomite and late ankerite and siderite[ An evolution from non!ferroan to ferroan carbonate cementation is relatively typical for red beds elsewhere "Bath\ Milodowski + Spiro\ 0876^ Strong and Milodowski\ 0876^ Metcalfe\ Rochelle\ Sav! age + Higgo\ 0883#\ generally explained by a change from an oxidising environment following deposition to a reducing environment at deep basinal conditions[ The temperature conditions of dolomite precipitation in the Embla _eld has not been possible to establish\ due to the lack of ~uid inclusions[ However\ due to the red colour the dolomite is considered to be low!temperature\ early diagenetic\ deposited in an oxidising environment[ The mineralogy of stage "1# veins clearly suggest pre! cipitation in a reducing environment[ Iron occurs in the ferrous state\ incorporated in minerals like pyrite and siderite[ A similar change from ferric to ferrous iron in the matrix cements of the Embla sandstones is seen by ankerite forming overgrowths on dolomite[ An in~ux of external ~uids is not necessarily required to cause a change in redox conditions[ During burial the formation water will become more reducing as the red bed becomes isolated from the near surface[ In the case of Embla\ however\ the onset of petroleum _lling at this stage of burial must have contributed to a more reducing environ!

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Fig[ 8[ Microthermometry results for aqueous inclusions[

ment[ The pore water in deep basins is generally con! sidered to be relatively stagnant\ with no ~uid contribution from the basement and with low com! pactional ~ow rates "Bjo rlykke\ 0882#[ Furthermore\ ~uid transport in fractures is not only dependant on the frac! ture poroperm characteristics and pressure gradient\ but also on the conductivity of the surrounding sediments "Bjo rlykke\ 0882#[ Primary ~uid inclusions in the fracture cements give evidence for the ~uid present at the time of cementation\ not necessarily if and in what direction this ~uid moves[ Although fractures are temporary phenom! ena\ they may stay open for some time\ depending on the stress situation and the shear strength of the rock[ In some cases the cementation of an open fracture could therefore be unrelated to fracturing and ~uid transport\

just re~ecting di}usion from the host rock or change in pressure or temperature conditions[ In order to establish the relationship between fracturing and cementation in the Embla _eld\ comparison between the diagenetic sequence in the host rock and the fracture give valuable information on the timing[ Except for the monomineralic barite\ all fractures of stage "1# and "2# occur in rocks with extreme low initial poroperm characteristics "igneous rock units\ mudstones\ dolomite cemented sand! stones#[ The fracturing\ not only cementation\ of stage "1# and "2# is clearly post!dating the early diagenetic dolomite cementation[ The di}erence in cements between various stages of veining would also suggest that stage "2# frac! tures were not open during cementation of the earlier stages of fractures\ especially since cross!cutting relation!

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654

Fig[ 09[ Microthermometry results for aqueous inclusions[

ships between di}erent mineralised fractures occur[ We therefore suggest a relatively close\ dynamic relationship between fracturing and cementation of the fractures[ The

change towards more saline compositions from stage "1# to stage "2# would also suggest that transport of aqueous ~uids has taken place[ Evaporites do not occur in the

655

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Fig[ 00[ Microthermometry results for aqueous inclusions[

Embla wells\ but the aqueous evaporite component could originate from the close by neighbouring basins to the east or south[ The ~uid transport is envisaged as a com!

bination of formational and cross!formational ~ow\ where the fractures provide pathways through otherwise impermeable rock sequences[

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656

Fig[ 02[ Thermal history of the fracturing in the Embla reservoir[

Fig[ 01[ Microthermometry results for aqueous inclusions[

8[ Conclusions The Paleozoic Embla _eld in the Central Trough have experienced multiple events of fracturing[ Three main stages of fracturing have been outlined] "0a# early brick! red dolomite veins^ "0b# veins in and around ma_c

igneous rocks^ "1# complex carbonate!pyrite veins^ "2# barite\ clay mineral and bitumen veins[ A dynamic relationship between fracturing and cemen! tation is indicated by the di}erences in vein _lling cements and the cross!cutting relationship between the di}erent stages of veins[ A temperature during fracturing in excess of 099>C is indicated by the aqueous microthermometry[ This suggests that the fracturing took place at deep bas! inal levels[ Fluid inclusion evidence show that petroleum _lling of the Embla _eld started during the stage 1 of fracturing[ The petroleum inclusions show a blue ~uorescence\ and homogenisation temperatures between 34 and 89>C\ with the majority of the inclusions clustering around 59Ð69>C[ The data are consistent with an undersaturated oil com! position[ A complex petroleum _lling history of the Embla _eld is suggested\ where the petroleum in_lling of the structure started at temperatures around 099>C and the fracturing of the reservoir ensured a good vertical communication[ The fractures were also important conduits for brines[ Two distinct ~uid compositions from di}erent sources have been identi_ed] a high saline Na!Ca!Cl brine with a composition ranging from 07 to 12 wt ) NaCleq and a moderate:low saline ~uid with a salinity of 02 wt ) NaCleq or less[ An evaporitic source of the high saline Na!Ca!Cl brines is suggested[ Except for the barite veins\ which were entirely sourced by the high saline Na!Ca!Cl brines\ the aqueous ~uid inclusions show evidence of ~uid

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mixing and possible reactivation during the fracturing events[ The ~uid transport is envisaged as a combination of formational and cross!formational ~ow\ where the fractures provide pathways through otherwise imper! meable rock sequences[ Acknowledgements The authors acknowledge permission to publish the article from Phillips Petroleum Company Norway and Co!venturers\ including Fina Exploration Norway S[C[A[\ Norsk Agip A:S\ Elf Petroleum Norge A[S[\ Norsk Hydro Produksjon A[S[\ Statoil A[S[\ TOTAL Norge A[S[ and Saga Petroleum A[S[ The authors further acknowledge that the interpretations and conclusions presented herein do not necessarily re~ect the opinions of the Co!venturers[ We also appreciate the constructive comments of two anonymous reviewers[

References Bath\ A[ H[\ Milodowski\ A[ E[\ + Spiro\ B[ "0876#[ Diagenesis of carbonate cements in Permo!Triassic sandstones in the Wessex and East YorkshireÐLincolnshire basins\ U[K[] a stable isotope study[ In J[ D[ Marshall "Ed[#\ Dia`enesis of sedimentary sequences\ Vol[ 25\ "pp[ 062Ð089#[ Geol[ Soc[ Spec[ Publ[ Bjo rlykke\ K[ "0882#[ Fluid ~ow in sedimentary basins[ Sedimentary Geolo`y\ 75\ 026Ð047[ Bodnar\ R[ J[ "0889#[ Petroleum migration in the Miocene Monterey

Formation\ California\ U[S[A[] constraints from ~uid!inclusion studies[ Mineral[ Ma`[\ 43\ 184Ð293[ Bodnar\ R[ J[ "0881#[ The system H1O!NaCl[ PACROFI IV Pro`ram and abstracts[ 097Ð000[ Burruss[ R[ C[\ Cerone\ K[ R[\ + Harris\ P[ M[ "0872#[ Fluid inclusion petrography and tectonic!burial history of the Al Ali No[1 well] evidence for the timing of diagenesis and oil migration\ northern Oman Foredeep[ Geolo`y\ 00\ 456Ð469[ Egeberg\ P[ K[\ + Aagaard\ P[ "0878#[ Origin and evolution of formation waters from oil _elds on the Norwegian shelf[ Applied Geochem[\ 3\ 020Ð031[ Gowers\ M[ B[\ + S%bo e\ A[ "0874#[ On the structural evolution of the Central Trough in the Norwegian and Danish sectors of the North Sea[ Marine and Petroleum Geolo`y\ 1\ 187Ð207[ Knight\ I[ A[\ Allen\ L[ R[\ Coipel\ J[\ Jacobs\ L[\ + Scanlan\ M[ J[ "0882#[ The Embla Field[ In J[ Parker "Ed[#\ Petroleum Geolo`y of Northwest Europe] Proceedin`s of the 3th Conference "pp[ 0322Ð 0333#[ Metcalfe\ R[\ Rochelle\ C[ A[\ Savage\ D[\ + Higgo\ J[ W[ "0883#[ Fluid! rock interaction during continental red bed diagenesis] implications for theoretical models of mineralization in sedimentary basins[ In J[ Parnell "Ed[#\ Geo~uids] ori`in\ mi`ration and evolution of ~uids in sedimentary basins\ Vol[ 67\ "pp[ 290Ð213#[ Geol[ Soc[ Spec[ Publ[ Nedkvitne\ T[\ Karlsen\ D[ A[\ Bjo rlykke\ K[\ + Larter\ S[ R[ "0882#[ The relationship between diagenetic evolution and petroleum emplacement in the Ula Field\ North Sea[ Marine Petroleum Geolo`y\ 09\ 144Ð169[ Oakes\ C[ S[\ Bodnar\ R[ J[\ + Simonson\ J[ M[ "0889#[ The system NaCl!CaCl1!H1O] I[ The ice liquidus at 0 atm total pressure[ Geochim[ Cosmochim[ Acta\ 43\ 592Ð509[ Strong\ G[ E[\ + Milodowski\ A[ E[ "0876#[ Aspects of the diagenesis of the Sherwood Sandstones of the Wessex Basin and their in~uence on reservoir characteristics[ In J[ D[ Marshall "Ed[#\ Dia`enesis of sedimentary sequences\ Vol[ 25\ "pp[ 214Ð226#[ Geol[ Soc[ Spec[ Publ[ Yanatieva\ O[ K "0835#[ Polythermal solubilities in the systems CaCl1! MgCl1!H1O and CaCl1!NaCl!H1O[ Zhur[ Prikl[ Khim[\ 08\ 698Ð611[