Sedimentary Fabric

SEDIMENTARY FABRIC

SUBMITTED BY--------

SHIBA SHANKAR ACHARYA ROLL NO-08GG4012

INTRODUCTION Ø Fabric is the mutual arrangement and orientation of the fabric elements . Ø Fabric elements of a sedimentary rock may be a single crystal or sand grain, a shell or any other component. Ø The particles are originally deposited in a gravitationally stable framework. The above assumption has two complications. 1.Not true for fine particles. 2.liquefaction.

TYPES OF FABRIC

BASED ON ORIENTATION OF FABRIC ELEMENTS 1.ISOTROPIC=Orientation of fabric elements is random.

If the particles are highly spherical no preferred fabric will be discernable.

Particles are nonspherical but have no preferred fabric.

ANISOTROPIC=when a preferred orientation is present.

A-axes transverse to flow with b-axes imbricate a(t)b(i . A-axes parallel to flow with a-axes imbricate. a(p)a(i)

3.CRYSTALLOGRAPHIC=fabric shown by alignment of the crystallographic direction. (e.g. c-axes of quartz grains.)

C Of quartz grains axes

BASED ON GENESIS OF FABRIC Ø DEFORMATION=Produced by external stress on the

rock and results from a rotation or movement of the constituent elements under stress or the growth of the new elements in common orientation in the stress field.

Ø APPOSITION =Formed at the time of deposition of the material. This fabric records a response of the linear elements to a force field, such as earth’s gravitational or magnetic fields. Ø GROWTH FABRIC=Orientations resulting from crystal growth and often related to a free surface. The growth of crystals normal to such surfaces as in geodes, veins, & the like, are primary fabric of this type.

MEASUREMENT OF FABRIC The orientation of a fabric element, may be described in terms of two angles. ü STRIKE/AZIMUTH=Angle between some axis of the pebble and median. ü INCLINATION=Angle between axis in question and the horizontal. A diagram that shows both the azimuth and the inclination of the long axis of a fabric element is known as petrofabric diagram. Measure the strike and dip of the a-b plane.

FABRIC

POROSITY ØThe porosity of rock is its property of containing open spaces and can be expressed as the ratio of the total volume of its pore spaces to its total bulk rock volume. Computer microtomograhy image showing the porosity of a sandstone

Shale.

sandstone

Ø Absolute porosity =bulk volume-solid volume x 100 bulk volume Ø Effective porosity =interconnected pore volume x 100 bulk volume on the basis of origin, POROSITY is of two types ü Primary porosity=develops during deposition of sediment. It includes both inter and intra particle porosity. ü Secondary porosity=develops during diagenesis by dissolution & dolomitization & through tectonic movements producing fracture in the rock.

Conditions affecting porosity in a sedimentary deposit 1. The shape and arrangement of its constituent particles 2. The degree of assortment of its particles 3. The cementation and compacting to which a rock is subjected 4. The removal of mineral matter through solution  5. The fracturing of the rock, resulting in joints. source= after lee(1919)

a. Packing density: the arrangement of the particles in the deposit.

b.

grain size:On its own, grain size has no influence on porosity

d = sphere diameter; n = number of grains

c.Sorting :The better sorted the sediment the greater the

.

This figure shows the relationship between sorting and porosity for porosity clay-free sands.

Overall, with increasing burial depth the porosity of sediment decreases.

porosity seems to be a function of depth of burial, according to the expression P=p[e x e x e…..y times] Where p=average porosity of surface clays. y=a x d a=constant

Importance of porosity Ø Especially it allows us to make estimations of the amount of fluid that can be contained in a rock (water, oil, spilled contaminants, etc.). The total volume of oil is the total volume of pore space (VP) in the oil-

ØEffective porosity is a measure of permeability of a rock.

SECONDARY POROSITY can be recognized by vPartially dissolved grains. vUndissolved clay rims around finer grains vOversized pores i.e. large pores of the sizes & shape of grains. Methods of determining porosity ü A common one is to measure the quantity of water required to saturate a known volume of the dry material. ü Another is to compare the specific gravity of a dry sample with that of a saturated sample of the same material.  

PERMEABILITY Ø Permeability is the property of a rock which allows the passage of fluids without impairment of its structure or displacement of its parts. Ø A rock is said to be permeable if it permits an appreciable quantity of fluids to pass through it in a given time . Ø If the rate of passage is negligible the rock is said to be impermeable. More precisely, permeability (k) is an empirically-derived parameter in D’Arcy’s Law, a Law that predicts the discharge 1 darcy is the of fluid through a granular material.

permeability that allows a fluid with 1 centipoise viscosity to flow at a rate of 1 cm/s under a pressure gradient of 1

Ø k is proportional to all sediment properties that influence the flow of fluid through any granular material.

ØTwo major factors controls the permeability of a rock.

1. The diameter of the pathways through which the fluid moves. of the path ways . (tortuosity) 2. Complexity

qAlong the walls of the pathway the velocity is zero (a no slip boundary) and increases away from the boundaries, reaching a maximum towards the middle to the pathway. qAlong the walls of the pathway the velocity is zero (a no slip boundary) and increases away from the boundaries, reaching a maximum towards the middle to the pathway.



Tortuosity is a measure of how much a pathway deviates from a straight line.

The greater the tortuosity the lower the permeability because viscous resistance is cumulative along the length of the pathway. The path that fluid takes through a granular material is governed by how individual pore spaces are connected.

EFFECTIVE POROSITY

Path of fluid in a rock

1.PACKING DENSITY 

Smaller pathways reduce porosity and the size of the pathways so the more tightly packed the sediment the lower the permeability.

2.POROSITY

p o r o s i t y

Decreasing permeability

The larger and more abundant the pore spaces the greater the permeability. permeability

3.GRAIN SIZE Unlike porosity, permeability increases with grain size. ØA ten-fold increase in grain size yields a hundred-fold increase in permeability. 4.SORTING

The better sorted a sediment is the greater its permeability.

5.DEPTH OF BURIAL Like porosity, permeability is changed following burial of a sediment.

In this example permeability is reduced by two orders of magnitude with 3 km of

D E P T H O F B U R I A L

PERMEABILITY

Permeability is not necessarily isotropic . Permeability changes its value with direction. This is illustrated in the following figures. graded bedding

fracture s

THANK YOU