Exam 2

What contents of field notes are required to draw lithostratigraphic columns

What lithology is this?

Conglomerate

What lithology is this?

Sandstone

What lithology is this?

Limestone

What lithology is this?

Dolomite

What lithology is this?

Shale

Lithostratigraphic Hierarchy

• Super group: largest unit, made of two or more groups or formations that share lithological characteristics

• Group: mode of two or more formations with shared lithological features

• Formation: the primary and most essential unit, defined by its lithological properties and map ability.

• Member: subdivision of formation, with distinct characteristics but not mappable at scale of formation

• Bed: smallest unit, representing distinct depositional layer

Taxon-range Zone

Single taxon

Concurrent-range Zone

Uses two taxa

Assemblage Zone

What does the Caliper sensor measure?

The size of the drilled borehole

Electrical Sensors measure?

Detects porewater amount, permeability, and ionic concentration of sediment and porewater

Gamma-ray sensors measure?

Measure the amount and type of radioactive elements (U, Th, K) in sediment

What is drilling mud composed of?

Functions of Drilling Fluids

• Provide hydrostatic pressure to control the well

• Create a seal between borehole and formation

• Remove rock cutting from the well

• Lubricate and cool the drill string

Types of Drilling Fluids

• water-based muds

• oil-based muds

Properties of Drilling Fluids

• mud weight

• viscosity

• pH

• Fluid Loss

• Salinity

Composition of Drilling Muds

bentonite clay to increase viscosity and suspend cuttings, and barite to add weight.

Bed strength/induration/cemented

Caliper measures this.

Caliper Measurements

• on gauge: well consolidated formations, non-permeable formations

  • massive sandstones

  • calcareous shales

• Larger than bit size: Formation soluble in drilling mud or formations weak and cave in.

  • Salt formations drilled with fresh water

  • unconsolidated sands, gravels, and brittle shales

• Smaller than bit size: formations swell and flow into borehole or development of mudcake for porous and permeable formations

  • swelling shales

  • porous, permeable sand stones

Spontaneous Potential (SP) Logs

• measures natural electric potential

• permeable formation, Low SP

• Impermeable= higher SP

Resistivity Logs

• is function of permeability, porosity and pore fluid salinity in rock

• Frequently used to identify lithology that varies based on permeability

  • High Resistivity=little porewater

  • Low resistivity =more porewater

Remember, the more ionic charge (saline) in sediment (cations on clays or NaCl in porewaters), the greater the electrical conductance and less resistivity

Gamma Ray Logs

• Shales have high gamma radioactivity

Lithology from lowest to highest gamma radiation

Limestone: 5-10

Dolomite: 10-20

Sandstone: 10-30

Shale: 80-140

Gamma Ray Logs Interpretations

Cylindrical GR

• Even block with sharp top and base

  • Eolian

  • Braided Fluvial

  • distributary channel-fill

  • submarine-canyon-fill

  • carbonate-shelf-margin

    • evaporite fill or basin

Funnel GR

• Coarse up and sharp top:

  • crevasse splay

  • river mouth bar

  • delta front

  • shoreface

  • submarine fan lobe

  • change from clastic to carbonates

Bell GR

Fine up and sharp base:

• fluvial point bar

• tidal point par

• deep tidal channel

• tidal flat

• transgressive shelf

Symmetrical GR

Prograding and retrograding

• reworked offshore bar, regressive to transgressive shore face delta

Serrated GR

Saw teeth

• fluvial floodplain

• storm dominated shelf

• distal deep-marine slope

Dotted contact

Diffuse/gradual

Sharp Contact

Straight Line

Angular Unconformity

Tilted or folded, with new horizontal layer on top

Disconformity

Clear erosion between sedimentary layers

Nonconformity

Where sedimentary rock is deposited on top of igneous rock

Paraconformity

Similar to a disconformity, this is a gap between parallel rock layers. The key difference is that there is no obvious erosional surface or discolored contact; it is a gap where beds are parallel and little apparent erosion has occurred. 

Sandstone on shale

Sharp Contact

Sandstone on Conglomerate

Diffuse contact

Bed strength/induration/cemented

Use Caliper

On gauge: Well consolidated

Larger than bit: Formation weak and cave in

Smaller than bit: Formations swelling or development of mud cake for porous and permeable formations

Permeable vs. impermeable beds

Caliper: non permeable formations are on guage, permeable form mudcakes and show smaller than bit size

Spontaneous Potential (SP): permeable= low SP

impermeable= high SP

Resistivity Logs: HLow resistivity =more porewater (permeable)

Low resistivity=little porewater (impermeable)

Swelling lithologies

Caliper: smaller than bit size= swelling

High ionic strength pore fluids

Resistivity Logs: the more ionic charge in the sediment, the less resistivity

Fining/coarsening upward sequences

Gamma Ray Log

Gamma ray logs infer grain size (and so subsequently inferred depositional energy)

Gamma Ray Logs: Shale

• Value: 80-140

• Shale has high gamma radioactive Response

Caliper Log: Shale

• most shales are on gauge

• Larger than bit= brittle shales (caving)

• Smaller than bit= swelling shales (sloughing)

Spontaneous Potential (SP) Log: Shale

Impermeable Shale= Higher SP Value

Give an indication of shaliness (maximum negative deflection is clean porous, permeable sand; minimum/low SP is shale or cemented limestone)

Resistivity Logs: Shale

• Low resistivity

• Deep and shallow are the same

Caliper Logs: Sandstone

• On gauge: Impermeable sandstone

• Smaller than bit size: Permeable sandstone

• Larger than bit size: unconsolidated sands

Spontaneous Potential (SP) Logs: Sandstone

Low SP, because sand is permeable

Resistivity Logs: Sandstone

Tight Sandstone: Higher than shale, higher than porous sandstone

Porous Sandstone: Higher than shale, lower than tight sandstone

Gamma Ray Log: Sandstone

Low gamma ray value, but higher than limestone

Acts as representation of grainsize

Gamma Ray Log: Limestone

Very low

Lower than sandstone and shale

Caliper Log: Limestone

Probably on gauge

Spontaneous Potential (SP) Logs