FLUID INCLUSION STUDIES OF WELL SAMPLES from the hydrocarbon prospective Porcupine Basin, offshore Ireland

J O U R N A L O F G E O C H E M I C A L E X P L O R AT I O N 7 8 - 7 9 ( 2 0 0 3 ) 5 5 - 5 9

M . F E E LY, J . PA R N E L L

D E PA R T M E N T O F G E O L O G Y, N AT I O N A L U N I V E R S I T Y O F I R E L A N D , G A L WAY, I R E L A N D

D E PA R T M E N T O F G E O L O G Y A N D P E T R O L E U M G E O L O G Y, U N I V E R S I T Y O F A B E R D E E N , A B E R D E E N , U K

To acquire fluid inclusion petrographic and micro thermometric data to facilitate the establishment of a chronology for aqueous and oil bearing fluid activity during the evolution of the Irish Porcupine Basin.

AIM

Overview

Transmitted light microscope UV light microscope Micro thermometry

Study of fluid inclusion samples in sandstones from 10 wells in the Irish Porcupine Basinwhich reveals the presence of aqueous and oil-bearing fluids in cements and detrital quartz grains.Jurassic sandstone samples contain two-phase oil inclusions trapped during syn- and post-cementation and homogenise to the liquid state at 50 to 125 °C.Early trapping in cement of less mature relatively heavy oil followed by ingress of lighter more mature oil is indicated.Primary two-phase aqueous fluid inclusions trapped in cements are generally of low salinity.Secondary aqueous inclusions in dental quartz and calcite grains homogenise to the liquid state in the range 50 to 325°C, while salinities are generally <10 eq. wt.% NaCl.

PROCEDURE1. Collection of samples

Fig.1: location of porcupine basin Fig.2: The regional setting of the Porcupine Basin showing the locations of the sampled wells.

Fig.2: The regional setting of the Porcupine Basin showing the locations of the sampled wells.

Sedimentary history of the offshore basin

It starts with pre-rift Upper Carboniferous deltaic sedimentation (including sandstones, shales and thin coals) and ends with deposition of shales above an end-Eocene to early Oligocene unconformity.

Permo-Triassic crustal extension leading to the gen- eration of small rift basins was followed by major extension in the Middle to Late Jurassic (Shannon, 1991).

Potential reservoir sandstone intervals which weretargeted for fluid inclusion studies are of Carboniferous, Jurassic, Cretaceous and Palaeocene-Eocene age. Quartz and calcite cements and dentalquartz and calcite grains in these sandstones were studied.

PROCEDURE2. Fluid Inclusion studies

Transmitted light microscope UV light microscope Micro thermometry

PROCEDURE2. Fluid Inclusion studies

• Doubly polished fluid inclusion wafers were prepared.• Fluid inclusion micro thermometry was carried out. • The temperature of last ice melting (Tm) and the temperature of

homogenization (Th) were measured.• Th measurements of the hydrocarbon bearing inclusions were also

recorded.• The fluid inclusion wafers were examined under UV light to determine the

presence of oil-bearing inclusions.• The majority of the inclusion were two phase aqueous solutions. • However, hydrocarbon bearing inclusions have also been recorded from

seven wells.• In addition, monophase liquid aqueous inclusions were also observed in

some wafers.

Steps:

Fluid inclusion petrographyInclusions in all samples display a variety of morphologies ranging from ellipsoidal to irregular and negative crystal shapes. Three principal types of inclusions were recorded during this study.Their recognition is based upon their phase relations at room temperature .Type 1 inclusion are two phase (L+V) aq. Solution with high degree of fill (F≈0.90). Most abundantThey occur as isolated individuals, in random groupings and as trails along post-crystallization annealed fractures. Type 2 monophase aq. (L) inclusions and are found throughout in cements and in detrital grains. They reflect the incursion of low temperature fluids of an aqueous nature into the basin. Type 3 through the use of UV light. They are two phase (L+V) hydrocarbon bearing inclusions with a high degree of fill. They were recorded in quartz and calcite cements and as trails in detrital in Jurassic sandstones from seven wells.

Fluid inclusion petrography

Fluid inclusion types

P-inclusions S-inclusions

Type 1 (L+V) aqueous

Quartz and calcite cements; in vein calcite and quartz

In trails in detrital quartz

Type 2 (L) aqueous Quartz and calcite cement

In detrital quartz

Type 3 (L+V) hydrocarbon

Quartz and calcite cement

In trails in detrital quartz

Table 1: Summary of paragenetic classification for the three types of fluid inclusion recorded in this study.

UV light microscopy

• Yellow to green fluorescence and blue to white fluorescence was displayed by Type 3 inclusions.

• In terms of current burial depths, all of the blue/white (lighter more mature oil, e.g. (°API gravity-45-50) fluorescing inclusions are from relatively deeply buried (>3000 m) Jurassic samples.

• While samples from shallower depths have only yellow to green fluorescing (heavy less mature oil, e.g. °API gravity-25-35) inclusions.

Burial depth(metres)

Fluorescence colour

200

300

40

0

500

600

7

00

Lighter/mature oil

heavier/immature oil

Fluid inclusion micro thermometry

Primary Type 1 inclusions occur in quartz and calcite cements and homogenise in the range 50 to 175 °C.Secondary Type 1 inclusions in dental quartz and calcite grains have Th values of 50 to 325 °C.The salinity of Type 1 primary and secondary inclusions is typically < 10 eq. wt.% NaCl.Th values (to the liquid phase) of Type 3 inclusions recorded from three wells range from 50 to 133 °C.These are trapped in cement and along annealed fractures in detrital grains.

Frequency

Homogenization temperature

Discussion

• The aqueous inclusion fluids in cements from the Porcupine region are consistently of low to moderate salinity (< 10 eq. wt.% NaCl).

• This probably reflects the paucity of evaporites at depth in these regions, in contrast to parts of the North Sea where inclusion salinities are high.

Fig. 3: A Th salinity plot showing fields defined by fluid inclusion microthermometric data a variety of settings in the offshore and onshore Irish Massif. Data sources from Feely and Hoegelsberger (1991), Gallagher et al. (1992), O'Connor et al. (1993), O'Reilly et al. (1997, 1998) and this study.

Conclusion

• Oil-bearing fluid inclusions occur in cements and detrital quartz in the majority of the Jurassic sandstones studied.

• The oil-bearing fluid inclusions were trapped during cementation and also post cementation.

• UV light studies reveal a range of fluorescence colours, i.e. from yellow through green to blue/white indicating an °API gravity range from 25-30 (yellow) to 45-50 (blue/white). Thus evidence presented here indicates the trapping of medium to light oils.

• Aqueous fluids with low to moderate salinity (<10 eq. wt.% NaCl) were trapped (Th values 50 to 175°C) during cementation.

• Detrital grains contain aqueous inclusions reflecting both syn- and pre-basin fluid activity.

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