FLUID INCLUSION STUDIES OFWELL SAMPLESfrom the hydrocarbon prospective Porcupine Basin, offshore IrelandJ 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 A T I O N 7 8 - 7 9 ( 2 0 0 3 ) 5 5 - 5 9M . F E E L Y , J . P A R N E L LD E P A R T M E N T O F G E O L O G Y , N A T 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 W A Y , I R E L A N DD E P A 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 theestablishment of a chronology for aqueous and oil bearing fluid activity during theevolution of the Irish Porcupine Basin.AIM
OverviewTransmitted light microscope UV light microscope Micro thermometryStudy 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 quartzgrains.Jurassic sandstone samples contain two-phase oil inclusions trapped during syn- andpost-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 moremature 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 therange 50 to 325°C, while salinities are generally <10 eq. wt.% NaCl.
PROCEDURE1. Collection of samplesFig.1: location of porcupine basin Fig.2: The regional setting of the Porcupine Basinshowing the locations of the sampled wells.
Fig.2: The regional setting of the Porcupine Basinshowing the locations of the sampled wells.Sedimentary history of the offshore basinIt starts with pre-rift Upper Carboniferous deltaicsedimentation (including sandstones, shales andthin coals) and ends with deposition of shales abovean end-Eocene to early Oligocene unconformity.Permo-Triassic crustal extension leading to the gen-eration of small rift basins was followed by majorextension in the Middle to Late Jurassic (Shannon,1991).Potential reservoir sandstone intervals which weretargeted for fluid inclusion studies are ofCarboniferous, Jurassic, Cretaceous and Palaeocene-Eocene age. Quartz and calcite cements and dentalquartz and calcite grains in these sandstoneswere studied.
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 ofhomogenization (Th) were measured.• Th measurements of the hydrocarbon bearing inclusions were alsorecorded.• The fluid inclusion wafers were examined under UV light to determine thepresence of oil-bearing inclusions.• The majority of the inclusion were two phase aqueous solutions.• However, hydrocarbon bearing inclusions have also been recorded fromseven wells.• In addition, monophase liquid aqueous inclusions were also observed insome wafers.Steps:
Fluid inclusion petrographyInclusions in all samples display a variety of morphologies ranging from ellipsoidal toirregular 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 indetrital grains. They reflect the incursion of low temperature fluids of an aqueous natureinto the basin.Type 3 through the use of UV light. They are two phase (L+V) hydrocarbonbearing inclusions with a high degree of fill. They were recorded in quartz and calcitecements and as trails in detrital in Jurassic sandstones from seven wells.
Fluid inclusion petrographyFluid inclusiontypesP-inclusions S-inclusionsType 1 (L+V)aqueousQuartz and calcitecements; in veincalcite and quartzIn trails in detritalquartzType 2 (L) aqueous Quartz and calcitecementIn detrital quartzType 3 (L+V)hydrocarbonQuartz and calcitecementIn trails in detritalquartzTable 1: Summary of paragenetic classification for the three types of fluid inclusionrecorded in this study.
UV light microscopy• Yellow to green fluorescence and blue to white fluorescence was displayedby Type 3 inclusions.• In terms of current burial depths, all of the blue/white (lighter more matureoil, e.g. (°API gravity-45-50) fluorescing inclusions are from relatively deeplyburied (>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 colour200300400500600700Lighter/mature oilheavier/immature oil
Fluid inclusion micro thermometryPrimary Type 1 inclusions occur in quartz and calcite cements and homogenise inthe range 50 to 175 °C.Secondary Type 1 inclusions in dental quartz and calcite grains have Th values of50 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 rangefrom 50 to 133 °C.These are trapped in cement and along annealed fractures in detrital grains.FrequencyHomogenization temperature
Discussion• The aqueous inclusion fluids in cements from the Porcupine region are consistently of low tomoderate salinity (< 10 eq. wt.% NaCl).• This probably reflects the paucity of evaporites at depth in these regions, in contrast to partsof the North Sea where inclusion salinities are high.Fig. 3: A Th salinity plot showing fields defined byfluid inclusion microthermometric data a variety ofsettings in the offshore and onshore Irish Massif.Data sources from Feely and Hoegelsberger (1991),Gallagher et al. (1992), OConnor et al. (1993),OReilly et al. (1997, 1998) and this study.
Conclusion• Oil-bearing fluid inclusions occur in cements and detrital quartz in the majority of theJurassic sandstones studied.• The oil-bearing fluid inclusions were trapped during cementation and also postcementation.• UV light studies reveal a range of fluorescence colours, i.e. from yellow through green toblue/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 (Thvalues 50 to 175°C) during cementation.• Detrital grains contain aqueous inclusions reflecting both syn- and pre-basin fluid activity.