petroleum geology

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.

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

burial history chart

oil window

the depth/temperature range where oil is generated.

critical moment

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

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.

generation process

1. Accumulation: Trapped in suitable reservoir structures.

2. Migration: They move through porous rocks.

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

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

seal

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

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

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

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)

oil alteration in reservoir processes

• biodegradation → Produces secondary microbial methane

• water washing

• de-asphalting

• thermal cracking → Produces thermal gas by„secondary cracking“

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

biodegradation effects on oil

• decrease oil quality

• reduces API gravity

• raises viscosity, sulphur content, asphaltenes %

• Increases acidity (adds carboxylic acids)

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)

water washing

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

de-asphalting

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

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!)

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)

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

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

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

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).

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!

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

routine core analysis

• Lithologic Description

• Fluid Saturation

• Porosity

• Permeability

• Grain Density

• Core Gamma Log

special core analysis

• Electrical Properties

• Acoustic Properties

• Compressibility

• Wettability

• Capillary Pressure

• Two-phase flow tests

porosity

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

porosity factors

• packing

• sorting

• grain shape/ size

• degree of cementation

• growth of clay minerals

• mineral dissolution

porosity measurements

• volumetric

  • mercury intrusion (MICP)

  • water imbibition

  • gas expansion

• imaging

  • optical microscopy

  • µCT

  • FIB/BIB-SEM

permeability factors

• grain size/shape

• sorting

• distribution of pore channels

• pore occlusion

wettability

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

• wetting: 0-90°

• non-wetting: 90-180°

relative permeability

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

effective permeability

the permeability of the rock to any one of the fluids

EOR usage

• mature fields

• tight reservoirs

wireline logs

• Correlation logs

  • Spontaneous Potential (SP)

  • Gamma Ray

• Resistivity logs

• Porosity logs

  • Sonic Log

  • Density Log

  • Neutron Log

• Dipmeter

• Cased hole logs

caliber log usage

• Borehole condition

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

• Borehole volume → cementation

reasons of diff logging resolutions

• Fundamental physicochemical processes that cause logging signal

• Geometry of the logging device

• Logging speed

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.

gamma ray

Measures natural radioactivity (U, Th, 40K)

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)

resistivity logs usage

• determine HC vs. water-bearing zones

• indicate permeable zones

• determine resistivity porosity

resistivity logs classification

• induction logs → measures conductivity

• electrode logs → measure resistivity

sonic log

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

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

cased hole logs

• GR

• neutron log

• pulsed neutrinos log

bright spot

amplitude anomaly caused by a large gas reservoir

• increase in velocity and density contrast

• high amplitude

thermal conductivity relations

• decreases w/ porosity

• increases w/ burial depth

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

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

shoulder effect

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

pore overpressure factors

• storage pores

• connecting pores

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

underpressure generating mechanisms

• Reservoir depletion during HC production

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

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

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

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

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

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

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

fore-arc basin characteristics

• source rock: organic rich sediments

• very low GG → bacterial gas

• deltaic sediment/ shallow marine sediments

back arc basin

• may be a good oil region

• supply sediment from deltaic & shallow marine

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

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

strike-slip basin (transform setting)

• Narrow basins with variable length

• Extremely fast subsidence

• Structurally complex

• Usually short-lived

e.g. Vienna basin

source rock parameters

• amount of OM

• type of OM

  • kerogen → insoluble in organic solvents

  • bitumen → soluble in organic solvents

• thermal maturity

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

bitumen characterisation

• gas chromatography/ mass spectrometry (GC/MS)

• isotopy