EAS446lec4-ppt - Lecture notes 4 PDF

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Summary

lecture 4...

Description

9/6/2017

Porosity ➢ Porosity is the fraction of a rock or soil that is occupied by voids (pores) ➢ n = 100 Vv/VT ➢ n = porosity (percentage) ➢ Vv = volume of void space (L3, cm3, m3) ➢ VT = unit volume of material including voids and solids

Matrix

Voids

Representative Elementary Volume

A minimum volume to adequately describe an aquifer parameter

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Classification of Rocks METAMORPHIC

Source of material

SEDIMENTARY

Molten materials in deep crust and upper mantle

Weathering and erosion of rocks exposed at surface

Rocks under high temperatures and pressures in deep crust

Rock-forming process

IGNEOUS

Crystallization (Solidification of melt)

Sedimentation, burial and lithification

Recrystallization due to heat, pressure, or chemically active fluids

Sedimentary Rocks ➢Clastics - composed of fragments, or clasts, of pre-existing rock ➢Carbonates ➢Evaporites

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Clastic and Carbonate Rocks Clastic Rocks Consist Primarily of Silicate Minerals Are Classified on the Basis of: - Grain Size - Mineral Composition

Carbonate Rocks Consist Primarily of Carbonate Minerals (i.e. Minerals with a CO32- Anion Group) Limestone - Predominately Calcite (Calcium Carbonate, CaCO3) Dolostone - Predominately Dolomite (Calcium Magnesium Carbonate, CaMg(CO3)2 )

Grain-Size Classification for Clastic Sediments

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Relative Abundances of Sedimentary Rocks Sandstone and conglomerate (clastic) ~11%

Limestone and dolomite ~14%

Siltstone and shale (clastic) ~75%

Average Detrital Mineral Composition of Shale and Sandstone Mineral Composition

Shale

Sandstone

Clay Minerals

60 (%)

Quartz

30

65

4

10-15

can be as high as 87%. ➢Pyroclastic deposits – Tuff – 14% - 40% ➢Ash – 50%.

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Factors Affecting Porosity ➢Primary ➢Particle sphericity and angularity ➢Packing

➢Sorting (variable grain sizes)

➢Secondary (diagenetic) ➢Cementing materials ➢Overburden stress (compaction) ➢Vugs, dissolution, and fractures

Roundness and Sphericity of Clastic Grains

Porosity

High

Low Very Angular Angular

SubSubAngular Rounded

Rounded

WellRounded

ROUNDNESS

Porosity

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Cubic Packing of Spheres Porosity = 48%

Porosity Calculations - Uniform Spheres ➢Bulk volume (8 spheres) = (2  2r)3 = 64r3 ➢Matrix (sphere) volume = 84/3r3 ➢Pore volume = bulk volume - matrix volume Pore Volume Bulk Volume Bulk Volume  Matrix Volume  Bulk Volume

Porosity 

64 r3  8  4 / 3  r3    1  47.6% 3 64 r 2 3

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Rhombic Packing of Spheres Porosity = 27 %

Packing of Two Sizes of Spheres Porosity = 14%

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Grain-Size Sorting in Sandstone

Very Well Sorted

Well Sorted

Moderately Sorted

Poorly Sorted

Very Poorly Sorted

SORTING POROSITY

Dual Porosity in Sandstone FRACTURE

DISSOLUTION PORE

PORE FRAMEWORK

CEMENT

(QUARTZ)

MATRIX

FRAMEWORK (FELDSPAR)

0.25 mm

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Dissolution Porosity

Partially Dissolved Feldspar Pore Quartz Detrital Grain

Dissolution of Framework Grains (Feldspar, for Example) and Cement may Enhance the Interconnected Pore System This is Secondary Porosity

Thin Section Micrograph - Plane Polarized Light Avile Sandstone, Neuquen Basin, Argentina

Photo by R.L. Kugler

Pore-space Classification ➢Total porosity, nt =

Total Pore Volume Bulk Volume

➢Effective porosity, ne =

Interconnected Pore Space Bulk Volume

➢Of great importance; ➢contains the mobile fluid

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Comparison of Total and Effective Porosities ➢Very clean sandstones : ne  nt ➢Poorly to moderately well -cemented intergranular materials: nt  ne ➢Highly cemented materials and most carbonates: ne < nt

Typical Ranges in Porosity Material SEDIMENTARY Gravel, coarse Gravel, fine Sand, coarse Sand, fine Silt Clay

Porosity (%) 24-36 25-38 31-46 26-53 34-61 34-60

SEDIMENTARY ROCKS Sandstone Siltstone Limestone, dolomite Karst limestone Shale

5-30 21-41 0-40 0-40 0-40

CRYSTALLINE ROCKS Fractured crystalline rocks Dense crystalline rocks Basalt Weathered granite Weathered gabbro

0-10 0-5 3-35 34-57 42-45

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Porosity Determination

• Show calculations of porosity of unconsolidated materials on chalkboard

Sediment Classification ➢Sediments are classified on basis of size of individual grains ➢Grain size distribution curve ➢Uniformity coefficient Cu = d60/d10 ➢d60 = grain size that is 60% finer by weight. ➢d10 = grain size that is 10% finer by weight. ➢Cu = 4 => well sorted; Cu > 6 => poorly sorted.

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Grain-size Distribution Curve

Grain-size Distribution Curve

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Specific Yield and Specific Retention • Specific Yield (Sy): Ratio of the volume of water that drains from a saturated rock due to gravity to the total volume of the rock sample. • Specific Retention (Sr): Ratio of the volume of water a rock can retain against gravity drainage to the total volume of the rock. • n = Sy + Sr

Values of Specific Yield (%) Material Clay Sandy clay Silt Fine sand Medium sand Coarse sand Gravelly sand Fine gravel Medium gravel Coarse gravel

Maximum 5 12 19 28 32 35 35 35 26 26

Specific Yield Minimum 0 3 3 10 15 20 20 21 13 12

Average 2 7 18 21 26 27 25 25 23 22

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