petroleum geology

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72 Terms

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petroleum system

  • Source rock (where oil forms),

  • Reservoir rock(where oil is stored),

  • Seal rock(rock layer that traps oil),

  • Overburden rock (geological structure that holds oil),

  • And the processes like trap formation & generation-migration-accumulation.

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sedimetary basin

accumulation space for sedimentary rocks that can contain oil and gas reservoirs defined by

  • prospect = A specific site (single deposit) where oil might be drilled.

  • play = series of genetically linked accumulations

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burial history chart

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oil window

the depth/temperature range where oil is generated.

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critical moment

the point in time when hydrocarbons reach the trap — vital for assessing whether oil will be present.

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Total Organic Carbon (TOC)

a key measure of how much organic material is in a rock.

  • >0.6% for carbonates.

  • >1.0% for mudstones/shales.

    => Rocks with >2.0% TOC are very promising for oil generation.

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generation process

  1. Accumulation: Trapped in suitable reservoir structures.

  2. Migration: They move through porous rocks.

  3. Generation: Oil/gas form from organic matter.

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reservoir routine analysis

  • Fluid saturation: How much oil/water is in the pores?

  • Oil traces: Detected using fluorescence under UV light.

  • Porosity measurements: How much empty space is in a rock for oil to be stored

    • flooding the sample w/ He, H2O or Hg

  • Permeability measurements: producibility of fluids

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seal

rock formation that is impermeable and laterally continuous. Holds back hydrocarbons from further migration (to the surface).

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petroleum systems classification

  • charge factor → how much can be generated

    • supercharges

    • normally charged

    • undercharged

  • drainage style → direction & range of migration

    • laterally drained

    • vertically drained

  • entrapment style → how much can accumulate

    • high impedence

    • low impedance

  • typicality

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crude oil composition

  • aliphatic HC

    • saturated HC

      • linear: normal paraffin series = n-alkanes

      • branched: iso-paraffin series = iso-alkanes

      • rings: naphthene series = cycle-alkanes

    • unsaturated HC → double/ triple c-c bonds

  • aromatic HC: Organic compounds which consist only of C and H and contain one or more benzene rings

    • pure aromatics

    • cyclo-alkano-aromatics

    • cyclic sulfur

  • heterocompounds (NSO-compunds): organic compounds which consist of one/more atoms other tan HC e.g. resins & asphaltenes

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gas classification

  • composition

    • dry gas → <5% ethane

    • wet gas → >5% ethane

    • sour gas → contains sig. amount of H2S along H2, He & Ar

    • acid gas → contains sig. amount of H2S & CO2

  • isotopy

    • light gas

    • heavy gas

  • origin of HC

    • bacterial: produced by bacteria from organic matter in sediments (incl. coal) or liquids at low temperatures (<80°C) → isotopically light & dry

      • primary: produced by bacteria from organic matter in sediments

      • secondary: produced by anaerobic bacteria from liquid petroleum

        => depending on the degree of anaerobic biodegradation

    • thermogenic: produced by thermal processes from organic matter in sediments or liquid petroleum → isotopically heavy & wet

      • associated: petroleum gas associated with/ oil

      • non-associated: formed at temp. where liquid petroleum is no longer stable

        => depending on the temp of gas formation (thermal maturity)

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oil alteration in reservoir processes

  • biodegradation Produces secondary microbial methane

  • water washing

  • de-asphalting

  • thermal cracking Produces thermal gas by„secondary cracking“

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crude oil biodegradation

  • Aerobic: Fast, usually where oxygenated meteoric water enters the reservoir.

  • Anaerobic: Relatively slow (x-xx my). End product may be methane (secondary bacterial gas). Bacterial sulfate reduction may produce free H2S.

>80°C starting at OWC

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biodegradation effects on oil

  • decrease oil quality

  • reduces API gravity

  • raises viscosity, sulphur content, asphaltenes %

  • Increases acidity (adds carboxylic acids)

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biodegradation effects on thermogenic gas

  • Increase in i-C4/n-C4 ratio (microbes prefer n-alkanes)

  • Increase in C2/C3 ratio (microbes prefer C3 → easier to remove C atom)

  • Gas drying (microbes produce CH4)

  • Produces isotopically light methane (d13C < -40 ‰

  • Produces isotopically heavy CO2 (d13 > +5 ‰ fractionation light (CH4)– heavy (CO2)

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water washing

HC undersaturated meteoric waters dissolve light HCs from reservoired petroleum →density increases

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de-asphalting

Precipitation of heavy asphaltene compounds often resulted from injection of light (C1-C6) HCs

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de-asphalting causes

  • Gravitational segregation from free oil column

  • Biodegradation-induced asphaltene preciptiation

  • In-reservoir maturation-induced asphaltene preciptiation

  • Pressure reduction-induced asphaltene precipitation (migration, uplift)

  • Adsorption onto clay minerals (kaolinite!)

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thermal alteration results

  • „Secondary“ oil cracking produces condensate, wet gas and finally dry gas

  • Increase in volume

    • isolated system pressure increase (seal failure?)

    • open system possibly loss of HCs

  • Produces pyrobitumen (solid residue)

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unconventional HC products

  • Tar Sand (Oil Sand)

  • Tight Gas & Tight Oil: O&G in low permeable res rock → acidizing, fracking, tensides

  • Shale Gas & Shale Oil: O&G produced directly from extremely low permeable source rock (nD-range) → fracking and horizontal drilling

  • Coalbed Methane: gas absorbed to cal substance → de-watering & fracking

  • Gas-hydrate (Clathrate): „ ice“ composed of gas molecules within water cage

  • Oil Shale: Organic matter-rich (but thermally immature!) rock which yields significant amounts of oil and gas during low-temperature pyrolysis

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petroleum data sources

  • boreholes

    • cuttings/ cavings: small rock fragments created during drilling, and transported to the surface by the drilling mud

    • cores

  • geophysical data

    • wireline logs

    • seismics

    • gravimetry

    • magnetics

  • outcrop analogues

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cuttings preparation

  1. retrieve drill cuttings from the mud system

  2. cloth bag the bulk, unwashed wet cut samples

  3. paper bag washed & sieved dry cut samples → examined under binoculars

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cuttings benefits

  • continus visuels record

  • shows evaluation of HC

  • describes reservoir and lithological

  • Geological correlation and formation identification

    • rock type

    • lithology

    • grain shape/size/ sorting

    • porosity

    • hardness

    • cementation/ matrix

  • Verification of wireline log response

  • Design of strip logs (lithology vs. depth).

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cuttings problems

  • Excess weight on bit powder

  • Insufficient mud viscosity → cuttings are not transported to surface (deviated wells!)

  • Improper mud chemistry high % cavings, selective mineral dissolution, contamination

  • Contamination by cavings

  • Exact depth a ign ent “lag time” ->accuracy 2-5 m at best!

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cores benefits

  • in physical contact w/ the res

  • allows direct measurement of physical properties

  • allows direct observation of grain size/ sorting & sedimentary structures → depositional environment interpretation

  • allows calibration to logs

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routine core analysis

  • Lithologic Description

  • Fluid Saturation

  • Porosity

  • Permeability

  • Grain Density

  • Core Gamma Log

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special core analysis

  • Electrical Properties

  • Acoustic Properties

  • Compressibility

  • Wettability

  • Capillary Pressure

  • Two-phase flow tests

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porosity

the amount of void space in a sample capable of holding fluid

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porosity factors

  • packing

  • sorting

  • grain shape/ size

  • degree of cementation

  • growth of clay minerals

  • mineral dissolution

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porosity measurements

  • volumetric

    • mercury intrusion (MICP)

    • water imbibition

    • gas expansion

  • imaging

    • optical microscopy

    • µCT

    • FIB/BIB-SEM

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permeability factors

  • grain size/shape

  • sorting

  • distribution of pore channels

  • pore occlusion

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wettability

the contact angle Q between a droplet of liquid and a horizontal surface

  • wetting: 0-90°

  • non-wetting: 90-180°

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relative permeability

the percentage (or decimal) of the total permeability of the rock to that one of the multiple fluids

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effective permeability

the permeability of the rock to any one of the fluids

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EOR usage

  • mature fields

  • tight reservoirs

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wireline logs

  • Correlation logs

    • Spontaneous Potential (SP)

    • Gamma Ray

  • Resistivity logs

  • Porosity logs

    • Sonic Log

    • Density Log

    • Neutron Log

  • Dipmeter

  • Cased hole logs

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caliber log usage

  • Borehole condition

  • Formation properties (mud cake permeable zone, fracture zones)

  • Borehole volume cementation

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reasons of diff logging resolutions

  • Fundamental physicochemical processes that cause logging signal

  • Geometry of the logging device

  • Logging speed

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spontaneous potential

measures the natural battery effect that occurs at the interface where foreign ions (drilling fluid) enter porous zones and are juxtaposed against normal fluids in non-porous zones.

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gamma ray

Measures natural radioactivity (U, Th, 40K)

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spectral gamma ray interpretation

  • High Th values:

    • increase in terrigenous clays (smectite, kaolinite)

    • heavy minerals

  • High U values:

    • increased organic carbon source rocks „hot shale“

    • hot dolomite (U enrichment)

  • High K values:

    • Glauconite (mineral in green sandstones, shallow marine deposition)

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resistivity logs usage

  • determine HC vs. water-bearing zones

  • indicate permeable zones

  • determine resistivity porosity

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resistivity logs classification

  • induction logs → measures conductivity

  • electrode logs → measure resistivity

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sonic log

measures interval transit time (∆t →DT) of acoustic waves over discrete distances. t is a function of lithology and F (∆detf).

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density log process

  1. Emits medium-energy gamma rays and measures returning gamma ray energies

  2. Gamma rays are scattered (Compton scattering) as a result of collisions with electrons in the formation

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cased hole logs

  • GR

  • neutron log

  • pulsed neutrinos log

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bright spot

amplitude anomaly caused by a large gas reservoir

  • increase in velocity and density contrast

  • high amplitude

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thermal conductivity relations

  • decreases w/ porosity

  • increases w/ burial depth

  • decreases in GG w/ depth and increases with saturation.

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overpressure generating mechanisms

  • by loading

    • disequilibrium compaction

    • tectonic compression → Murdock cannot dewater fast enough for the pore fluid to remain in hydrostatic equilibrium as it compacts

  • by unloading: load transfer from grain to grain contacts to the pore fluid

    • influx of pore fluid

    • conversion of solid matrix material into fluids = OM conversion/ mineral diagenesis

  • HC buoyancy: the pore water in the water saturated reservoir is at normal hydrostatic pressure

  • hydraulic head: The maximum head (H) due to this mechanism is equal to the elevation of the reservoir at outcrop,

  • osmosis

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shoulder effect

anomalous overpressure zones that occur at the margins of thick, undercompacted, or low-permeability source rock layers.

<p><strong>anomalous overpressure zones</strong><span> that occur at the </span><strong>margins of thick, undercompacted, or low-permeability source rock layers</strong><span>.</span></p><p></p>
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pore overpressure factors

  • storage pores

  • connecting pores

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results of unloading in Mudrock

  • Elastic opening of connecting pores → strongly affects sonic log

  • Only very small increase in total ø → nearly no effect in density log

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underpressure generating mechanisms

  • Reservoir depletion during HC production

  • Relatively isolated sand bodies in the recharge area compared to the discharge area

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plate boundaries

  • divergent → spread away

    • rift basins

    • mid ocean ridges

  • convergent → one plate submerge underneath another

    • subduction

    • continent-continent collision

  • transform → slide past each other

    • strike-slip basin

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divergent setting

  • intercontinental sag → large oval shaped, slowly subsiding, long lived basins resulted from thermal contraction, eclogitization of lowermost crust &/or intraplate stress

  • graben structure = rift basin → Narrow elongate basins which result from crustal stretching

    • syn-rift phase: During early rifting many basins are overfilled with fluvial sediments, but may later become underfilled

    • post-rift phase: After crustal stretching sediments often overlie syn-rift deposits with an unconformity and tend to be

      more uniform

  • passive continental margins → very thick sediment successions

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intercontinental sag characteristics

  • thinning sediments towards the basin margins

  • no major faults

  • continental, lacustrine/ shallow marine sediments

  • controlled by sea lvl variations

  • low GG

  • contain large amount of HCs

e.g. Michigan basin

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graben structure characteristic

  • syn-rift phase:

    • lacustrine shales

    • evaporite deposits

    • numerous normal faults & tilting of fault blocks

    • crustal thinning = high q = high GG

  • post-rift phase

    • gradual heat flow decrease

    • host HC deposits

e.g. North Sea

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passive continental margin characteristics

  • deeper part of syn-rift

    • normal faulting

    • often lacustrine source rocks

    • salt domes

  • post-rift

    • controlled by prograding delta sequences &/or carbonate platforms

    • slope shows historic growth faults, rollover structures & mud diapers

    • show long lasting subsidence trends due t cooling & sediment loading

e.g. margins of Atlantic ocean

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convergent settings

  • subduction

    • fore-arc basin → between island arc & accretionary wedge

    • back-arc basin → on oceanic crust

    • retro-arc foreland basin → on continental crust behind an arc w/ a fold-thrust-belt

  • continent-continenet collision

    • remnant basin → Narrowing ocean basin filled mainly with turbidites

    • peripheral foreland basin → asymmetric basin mainly controlled by the load of the advancing overthrust belt

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fore-arc basin characteristics

  • source rock: organic rich sediments

  • very low GG → bacterial gas

  • deltaic sediment/ shallow marine sediments

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back arc basin

  • may be a good oil region

  • supply sediment from deltaic & shallow marine

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peripheral foreland basin characteristics

  • filled w/ molasses deposits

  • trend form deep marine to continental & from distal fine grained to proximal coarse grained sedimentation

  • low GG

  • compressional structure & over pressuring

e.g. north alpine basin

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piggy back basin

  • deposited on thrust sheets & transported passively on top of the thrusts

  • Sedimentary material mainly from the thrust belt → Low preservation potential (only in young orogens)

e.g. po basin

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strike-slip basin (transform setting)

  • Narrow basins with variable length

  • Extremely fast subsidence

  • Structurally complex

  • Usually short-lived

e.g. Vienna basin

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source rock parameters

  • amount of OM

  • type of OM

    • kerogen → insoluble in organic solvents

    • bitumen → soluble in organic solvents

  • thermal maturity

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kerogen characterisation

  • Elemental analysis → determine C,H, O contents

    • categorised based on H-content from type I = H- rich = high H/C & type III = H-poor = low H/C

  • Pyrolysis (RockEval pyrolysis)

  • Pyrolysis-GC → Investigation of the chemical composition of

    the pyrolysate

  • Microscopy (petrography) → determines macerals

    • reflected light (normal + fluorescent)

    • transmitted light

    • SEM → characterisation of OM in source rocks

    • correlative light & scanning electron microscopy

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bitumen characterisation

  • gas chromatography/ mass spectrometry (GC/MS)

  • isotopy

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