40

CHAPTER 3

METHODOLOGY

The sequence analysis for the Late Cretaceous sediments of

Ariyalur-Pondicherry sub-basin has been carried out with an integrated

approach that includes several sub-disciplines of geology and geophysics

(Figure 3.1). The main tool in this work is foraminiferal stratigraphy which

has been used for determination of the paleobathymetry and

paleoenvironment along with biozonations to find out the age control.

Sedimentological analysis has been undertaken for determination of grain

properties, diagenetic processes that the rocks have suffered after post-

deposition and clay mineral analysis for provenance determination. TOC

analysis has been done to correlate the organic content of the rocks to its

faunal data and hence thereby with changes in relative sea level. Well logs,

viz., Spontaneous Potential, Gamma ray and Resistivity logs have been used

to interpret the lithology. The logs are useful to identify the trend of

gradation: coarsening or fining upward sequences, occurrence of cyclic beds

and in identification of the facies stacking patterns, delineating the sequence

boundaries and defining the system tracts in subsurface sections. The

sequence parameters of the subsurface sections are correlated with equivalent

outcrop sections in the area around Ariyalur. Finally, depositional architecture

for the exposed sections of Late Cretaceous sediments of Ariyalur-

Pondicherry sub-basin of Cauvery basin has been proposed.

The detailed procedures for laboratory and field work are

documented.

41

Figure 3.1 Flow chart shows the inter-relationship between different

branches of geology and sequence stratigraphy.

3.1 FIELD DATA DOCUMENTATION

Although sequence stratigraphy was originally designed for seismic

sections, sequence principals can be readily applied to outcrop, core and well

logs. The first step in this approach is to interpret individual beds in terms of

depositional events, including an evaluation of the shear stress in the

environments, the type of flow (currents, waves, tides, combined flow),

42

bioturbation and trace fossils, etc. This information is critical for the next

step, to recognize beds that record similar depositional processes, and to

interpret those bedsets as facies, the records of particular depositional

environments (Figure 3.2 and 3.3). Well-exposed litho sections in the quarry,

mines and river sections have been selected for outcrop studies. The field

geological data like, rock type, sedimentary structures, thickness, mega fossil

contents, grain size, attitudes, rock relationship data were documented in

different outcrop sections. The locations were identified with the Indian

Bureau of Mines.

Figure 3.2 Sequence stratigraphic surfaces binding different systems

tracts. HST is bounded by MFS and SB, LST is bounded by SB

and TS, and TST by TS and MFS.

43

Figure 3.3 An outline of outcrop-based sequence stratigraphic analysis

Describe bedsets (group of beds with similar features and therefore deposited by similar processes during similar depositional events). Bedsets will be treated as facies, the depositional record of particular sedimentary environments.

Describe & interpret beds (record of individual depositional events).

Identify parasequences (based on shallowing –upward successions of bedsets or facies) and flooding surfaces (based on abrupt increases in water depth).

Identify stacking patterns and parasequence sets (based on vertical trends in water depth among successive parasequences).

Recognize potential condensed sections by one or more of the following criteria: Burrowed surfaces Abundant diagenetic

minerals (glauconite, phosphate, siderite, haematite, etc.)

Fossil concentrations Closely spaced

bentonites Radioactive shales

Recognize potential sequence boundaries by one or more of the following criteria: Erosional truncation Evidence of

subaerial exposure Abrupt Basinward

shift of facies

Identify systems tracts (LST, TST, HST) and surfaces (sequence boundary, transgressive surface, maximum flooding surface) based on stacking patterns and significant startal surfaces.

44

3.2 BIOSTRATIGRAPHIC STUDY

Biostratigraphy is the study of the temporal and spatial distribution

of fossil organisms. The advent of sequence stratigraphy has had a major

impetus in furthering biostratigraphic studies (Simmons and Williams 1992;

Armentrout 1996; Emery and Myers 1996). This is for two reasons. Firstly,

the framework of sequence stratigraphy is time. Essentially this means that

sequence stratigraphic studies require a biostratigraphic framework in which

to place the organization of sequence boundaries, maximum flooding surfaces

and systems tracts. Attention has therefore been focused on the development

of biozonation schemes suitable for this purpose. Secondly, the global eustatic

sea level curves published by Haq et al (1988) has led to the desire to locate

synchronous relative sea level changes in stratigraphic sections around the

world, and, of importance to some workers, to relate these to the global

eustatic curve.

Biostratigraphy supports sequence analysis in the following ways.

(i) Development of a constrained time framework.

(ii) Analysis of biofacies.

(iii) Understanding temporal and spatial relationships within

and between systems tracts.

(iv) Predictive modeling of relationships between

depositional systems.

3.2.1 Separation of foraminifers from sediment samples

The collected samples were processed for the separation of

foraminifera using standard microfossil separations technique. Separation of

45

foraminifera from the sample involves the following two steps: (1) Processing

the sample and (2) Sorting of foraminifera from the processed sample.

Processing technique

The main aim of the sample processing technique is to carefully

separate the microfossils from the rock sample without disintegrating the

fossils. The cutting samples are first sieved to separate the larger caving. The

size fraction is very important because most of the foraminifera are greater

then 500-micron size, while the nanoplanktons are less than 65 micron size.

Moreover the different size fractions enable uniform spreading which in turn

will be useful for sorting easily. The processing technique for microfossils

from shale/sandy clay is described below.

Shale/Sandy clay

10 gm of powdered samples is taken in a china bowl with well

name and depth pasted on it. Hydrogen peroxide and water in the ratio of 1:3

is added to the sample. Hydrogen peroxide dissolves the organic wastes.

Sample is soaked in the solution for 6 hours. Then the sample contents in the

china bowl are boiled for about 15 minutes and then cooled. The sample

contents of the bowl is washed and filtered in the ASTM mesh No. 230. The

washed sample is dried and then sieved through ASTM mesh No. 60, 100 and

200 sizes. The sieved samples are preserved in small screw jars (Figure 3.4).

Sorting of foraminifer from the processed sample

The examination of samples begins with coarser samples. It is

spread carefully on sorting tray and observed under microscope. A 24-

chambered slide is taken and water-soluble gum is applied on it. A botanical

46

needle is used to sort and separate fossils from the sediments. The fossil is

picked by wetting a 00 size paintbrush in water and allowing the fossil to get

attached to the brush. The sample is transferred to one of the chambers in the

slide. Usually all benthics are separated from the planktics and put in separate

slides.

10 gms of sample is taken in a china bowl.

Sample is treated with H2O2 : H2O, in the ratio 1:3. Leave for 6 hrs to dissolve organic matter.

The sample is washed over a (ASTM – 230) sieve thoroughly until all the clay particles are removed.

Once dried, the sample is sieved through 60,100 and 200 ASTM standard sieves.

Sample is boiled with water for about 20 minutes.

The washed sample is then transferred to a china dish and dried in the oven for half an hour at 125 °C.

The three fractions are taken in 3 screw jars and labeled with name, depth and sieve no.

Sample is now ready for microscopic observation.

If the sample is hard, add H2O2 and ammonia solution (Quato solution).

Figure 3.4 Flow chart of foraminiferal separation technique

47

3.2.2 Description and identification of foraminifers

The description under microscope involves careful observation of

the following morphological characteristics to identify genus/species: Test,

Chamber shape and size, Coiling pattern, Number of whorls, Apertural

openings, Suture, Ornamentation and pores. Main catalogues used for

identifying genus and species name of foraminifers are Loeblich and Tappan

(1988), Bolli, Saunders and Nielsen (1985).

3.2.3 Foraminiferal biozones

Biozone or zone is the fundamental stratigraphic unit. They do not

have prescribed thickness or geographic extent. Biozonation is the technique

of biostratigraphy to classify rock units on the basis of its unique fossil

content. Biozonation is very useful for correlation and age determination of

rock units. Planktic foraminiferal species were almost entirely ignored as

markers until 1940s because morphologic differences between species were

not appreciated. A change in attitude came after Grimsdale (1951) compared

the ranges of 41 Tertiary planktic species from Gulf of Mexico, Caribbean

and Middle East. After that there was a strong commitment to the zonation of

Cretaceous and Tertiary rock sequences using planktic foraminifers. The

classification and nomenclature of biozones as described by International

Stratigraphic Guide (ISG) has been used here. The following is the brief

description of each biozones as described by ISG (Figure 3.5)

Interval zone (IZ): An interval zone is the body of strata between

documented successive lowest occurrences or successive highest occurrences

of two taxa forms.

48

Taxon range zone (TRZ): It is a body of strata representing the

total range of occurrence of specimens of a particular taxon.

Concurrent range zone (CRZ): an interval zone when the

documented lowest occurrence of one taxon and the documented highest

occurrence of another taxon results in stratigraphic overlap.

Partial range zone (PRZ): When there is no stratigraphic overlap

between the documented lowest occurrence of one taxon and the documented

highest occurrence of another taxon, it is called PRZ.

Oppel zone (OZ): It is defined by ISG as a biozone characterized

by an association or aggregation of selected taxa of restricted and largely

concurrent range, chosen as indicative of approximate contemporarity.

Acme zone (AZ): It is a biozones characterized by quantitatively

distinctive maxima of related abundance of one or more taxa.

INTERVAL ZONE TAXON RANGE ZONE CONCURRENT RANGE ZONE

PARTIAL RANGE ZONE OPPEL ZONE ACME ZONE

Spec

ies

A

Spec

ies

B

Spec

ies

C

Figure 3.5 Schematic diagram illustrating different types of biozones

49

Table 3.1 Standard global foraminiferal zones of Late Cretaceous

(Bolli et al 1985)

Age

(Ma) Stage Planktonic foraminiferal zone

65

70 Maa

stric

htia

n Abathomphalus mayaroensis (TRZ)

Gansserina gansseri (IZ)

Globotruncana aegyptiaca (IZ)

Globotruncanella havanensis (PRZ)

78 Cam

pani

an Globotruncanita calcarata (TRZ)

Globotruncana ventricosa (IZ)

Globotruncanita eleveta (PRZ)

82 Sant

onia

n Dicarinella asymetrica (TRZ)

Dicarinella concavata (IZ)

86 Con

iaci

an

Dicarinella primitiva (IZ)

3.2.4 Temporal and frequency distribution

Depth of ocean water affects the distribution of animals. For

foraminifers diversity drops markedly in waters less than 300m. Classically,

the paleodepth of a site has been determined on the basis of its benthic

foraminiferal assemblages and on the assumption that the criteria derived

from the recent can be applied to the past analogous forms. This assumption

must take into account the up- or down slope migrations of faunas during

times of environmental change. Frequency distribution curves are prepared

for planktic and benthic foraminifers to understand the spatial and temporal

variation, as it gives an idea about paleobathymetry and trends in sea-level

changes.

50

3.2.5 Scanning Electron Microscope analysis

Photographs of the microforaminifers that have been identified

from the studied sections have been taken by Scanning Electron microscope

(SEM). The selected foraminifer specimens were placed in a stud, about 30-

35 in each stud and then coated with gold. The stud is then placed in the

instrument which has been vacuumed. Electron is transmitted to the gold

plated foraminifer samples and the image appears in the screen. After

adjusting the magnification and resolution the selected image is saved in

computer.

Detail specification of the SEM instrument which is used is given

below.

Name of the SEM instrument: PHILIPS PSEM 515

Resolution : 50 Å in SE mode

Magnification : Min- 10x; Max- 1,60,000 (zoom)

Accelerating Voltage : Min- 0.2 kv (continuously variable); Max-

300kv

Spot size : Min- 80 Å; Max- 10,000 Å

Vaccum : Working- 10-6 torr; Ultimate- 2* 10-7torr

Goniometer : Tilt: -15° to + 60° (optional 90)

Z: 22mm; X: 20mm; Y: 20mm;

Rotation: n* 360°

Working distance : 39.5mm with rotation; 62.00mm without

rotation

Specimen exchange time : typically 20mins

51

Maximum specimen size : 85*67*50mm

Image recording : On 120 & 35mm Camera system from

120*90mmCRT screen

Display : 180*135* TV screen

Software : Digital Image Processing

SEM Coating System : BIO-RAD Polar Division

3.3 WELL LOG ANALYSIS

Geophysical studies involve the study of those parts of the earth

hidden from direct view by measuring their physical properties with

appropriate instruments, usually on the surface of the earth. Well log data

have been used to interpret depositional sequences.

The continuous recoding of a geophysical parameter along a

borehole produces a geophysical well log. The value of the measurement is

plotted continuously against depth in the well. Geophysical well logging is

necessary because geological sampling during drilling (cutting samples)

leaves an imprecise record of the formations encountered. Logging is precise

but it needs interpretation to bring a log to the level of geological or

petrophysical experience.

Well log sequence stratigraphic analysis is a methodology that

permits a geologist and geophysicist to subdivide a stratigraphic section into a

series of time related 2nd/ 3rd order depositional sequences and systems tracts.

The best sequence stratigraphic models of the sedimentary fill of basins are

provided by a combination of seismic data, well logs and cores and outcrop

studies in conjunction with biostratigraphy (Figure 3.6). The cores and well

logs and outcrop studies provide access to a detailed vertical resolution of

52

sedimentary sections while seismic and outcrop studies provide the lateral

continuity to the sequence stratigraphic framework and the biostratigraphy

provides the time constraints. All these different sequence stratigraphic

techniques can be used independently of each other to produce accurate

interpretations of the depositional histories of the sedimentary fill of a basin

but the best models come from a combination of all three aspects.

Figure 3.6 Flow chart for an electrosequence analysis

Numbers refer to interpretation steps (Rider, 2002).

Sequence stratigraphy is best determined when well logs are tied to

biostratigraphic markers. Using these two in combination one can:

Identify, match and tie sequence stratigraphic surfaces

Interpret the stacking patterns of the vertical sedimentary

sequences

53

The first step in sequence stratigraphic interpretation is to identify

the predominant sequence stratigraphic surfaces. The most important of these

surfaces are maximum flooding surfaces (MFS) and transgressive surfaces

(TS). These coincide and are correlated with radioactive shales (use of the

gamma log) that are interpreted to have been deposited across relatively flat

surfaces. Once the MFS and TS are established and tied, then the sequence

boundaries (SB) of both carbonate and clastic sedimentary systems are

identified. These will tend to lie directly beneath the sand sized sediment fill

of depressions on eroded and incised surfaces and over the prograding

clinoforms of high stand systems tracts (HST).

In both clastics and carbonates the second and often co-incident

step in the interpretation of well logs and cores is the use of parasequence

stacking patterns (the vertical occurrence of repeated cycles of coarsening or

fining upwards sediment) to identify the lowstand systems tracts (LST),

transgressive systems tracts (TST) and highstand systems tracts (HST) that

are overlain by the TS, MFS and SB respectively (Figure 3.2). These

parasequence cyclic stacking patterns are commonly identified on the basis of

variations in grain size and when these fine upwards are indicated by triangles

whose apex is up while those that coarsen upwards are indicated by inverted

triangles whose apex is down.

3.3.1 Spontaneous Potential Log

Spontaneous Potential or SP log is a measurement of the natural

potential differences or self-potentials between an electrode in the borehole

and a reference electrode at the surface. Artificial currents are not required.

This log measures the electrical current that occurs naturally in boreholes as a

result of salinity differences between the formation water and the borehole

mud filtrate (formation and surface). The principal uses of the SP log are to

54

calculate formation-water resistivity and to indicate permeability. It can also

be used to estimate shale volume, to indicate facies and, in some cases, for

correlation. The SP was first introduced to permit correlation in sand-shale

sequences, principally because certain intervals had typical log shapes. This

shape, in sand-shale sequences, is related to shale abundance, the full SP

occurring over clean intervals, a diminished SP over shaly zones. In so far as

shaliness is related to grain size, the SP is a good facies indicator. For

example (Figure 3.7) shows well marked channel sand; the coarse grained

base is clean while the fine grained top is shaly. The SP is therefore following

grain size change. The SP has now been largely replaced by the Gamma Ray

log for facies identification; the Gamma Ray log has more character and is

more repeatable (Figure 3.8).

Figure 3.7 Facies identification using the SP log

A typical fining upwards channel sandstone giving a bell shaped

SP curve. (After Hawkins, 1972.)

55

3.3.2 Gamma Ray Log

The gamma ray log is a record of a formation’s radioactivity. The

radiation emanates from naturally occurring uranium, thorium and potassium.

The simple gamma ray log gives the radioactivity of the three elements

combined, while the spectral gamma ray log shows the amount of each

individual element contributing to this radioactivity. The geological

significance of radioactivity lies in the distribution of these three elements.

Most rocks are radioactive to some extent. However, amongst sediments shale

have by far the strongest radiation. It is for this reason that the simple gamma

ray log has been called the ‘shale log’. The gamma ray log is still principally

used quantitatively to derive shale volume. Qualitatively, in its simplest form

it can be used to correlate, to suggest facies and sequences and, to identify

lithology (shalyness) (Figure 3.9 and 3.10).

Figure 3.8 The bed definition and character of the SP log compared to

the gamma ray log (Rider, 2002)

56

Figure 3.9 Characteristic gamma ray response for different stacking

patterns

Figure 3.10 The depositional model of sequence stratigraphy

(a) The sequence as a lithological scheme with sequence tracts.

(b) The sequence model with gamma ray log traces and key

surfaces (Rider, 2002).

a

b DS – Depositional sequence (after Exxon) GSS – Genetic stratigraphic sequence (after Galloway)

57

Figure 3.11 Correlation between grain size and gamma ray (Rider, 2002)

Figure 3.12 Model log patterns of systems tracts (After Vail and

Wornardt 1990).

58

3.3.3 Resistivity Log

This log measures the bulk resistivity (the reciprocal of

conductivity) of the formation. Resistivity is defined as the degree to which a

substance resists the flow of electric current. Resistivity is a function of

porosity and pore fluid in a rock. Porous rock containing conductive fluid

(such as saline water) will have low resistivity. A non-porous rock or

hydrocarbon-bearing formation has high resistivity. This log is very useful for

determination of the type of fluids in formations and is frequently used as an

indicator of formation lithology (Figure 3.13 and 3.14).

Figure 3.13 Shale intervals shown on the resistivity logs (Rider, 2002).

59

Figure 3.14 Resistivity logs shows small-scale deltaic cycles (Rider, 2002)

3.4 SEDIMENTOLOGICAL STUDIES

3.4.1 Petrographic studies

Thin section preparation: The representative samples of rocks of

different lithological types collected along the vertical section are taken for

microfacies studies and thin sections are prepared. The thin sections are

studied under a petrological microscope to identify the type of limestone or

sandstone or shale, percentage of matrix and cement, presence of fossils,

leaching and oxidation effect, secondary overgrowth of quartz/calcite,

replacement of feldspar or any other mineral grains, porosity, etc. Microfacies

analysis serves as the basis for describing and classifying the sedimentary

60

rocks. It is used to interpret the environment in which rocks were formed and

to recognize later diagenetic changes and description of reservoir rocks.

Procedure for microfacies analysis

1. General data: (a) Name of the formation, (b) Depth of the location,

(c) Geologic age.

2. Texture : (a) grain size (b) grain contact (c) sorting, (d) bimodal

distribution

3. Fabric : (a) Homogeneous (b) Grain/mud supported (c) Other

features-structures

4. Mineral Composition - Percentage of ferruginous constituents

5. Naming of the rock: (a) Folk’s classification (b) Dunham’s

classification (c) Pettijohn’s classification

6. Depositional Environment: (a) Kinetic energy-based on

presence/absence of micrite/mud/percentage of clay, (b) Faunas

Benthic/Planktic (c) Terrigenous input – grain size, mineral

composition

7. Porosity

8. Diagenesis

3.4.2 XRD analysis

X-ray diffraction (XRD) is a valuable tool to determine the

mineralogy of sedimentary rocks. Besides mineral species identification, a

semi-quantitative determination of mineral constituents can also be made

through this technique. This is especially important for mudrocks where

petrographic methods are of limited utility.

61

Principle

The x-rays are first collimated to produce a sub-parallel beam, the

amount of divergence being controlled by the size of the divergence slit, i.e. a

4° large divergence slit for high angle work to a 0.5° small divergence for low

angle work. The divergent beam is then directed at the sample which is motor

driven to rotate at a regular speed in degrees per minute. When mineral planes

in the sample attain an appropriate angle, they will diffract the X-rays

according to the Bragg’s law.

n=2d sin where, n = an integer, = wavelength of X-ray

d = lattice spacing in Å, = angle of diffraction.

The diffracted beam is passed through a receiving slit and

collimator and, then a scatter slit is introduced to reduce any scattered X-rays

other than the diffracted beams from finally entering the detector. Output

from a diffractometer can be either in analogue or digital form. The

conventional analogue recording is a strip chart whose speed in millimeters

per minute is synchronized with that of the detector in degrees of 2 per

minute so that the x-axis is calibrated in 2. Deflection recorded can be easily

converted into lattice (d) spacings of the minerals present by applying Bragg’s

law.

Instrument

The basic X-ray diffraction system consists of:

i) X-ray generator, including tube for identifying X-ray

ii) Sample holder to hold the specimen and rotate it.

62

iii) Goniometer in which sample holder is mounted.

iv) Detector for measuring the intensity of the diffracted X-rays.

v) Recorder to record the diffraction pattern.

The instrument used for clay mineral analysis is ARL X’TRA. It is

a sophisticated and computer operated instrument of the make of Thermo

Electron Corporation with Beryllium window. It works on 2-2 goniometer

and has scintillation detector. Softwares with all the details of characteristic d’

spacing and 2 values of the minerals are loaded, so that identification can be

done precisely and quickly.

Sample preparation procedure and identification

The method of sample processing is based on sedimentation

principles of Stroke’s law. Following steps are adopted for preparation of

slide using slurry of suspended clay portions.

3.5 TOTAL ORGANIC CARBON (TOC) ANALYSIS

The Total Organic Carbon (TOC) content of sedimentary rocks is a

gross quantitative index of oil generative potential of source beds.

Theoretically, a certain minimum concentration of organic carbon is

necessary to generate enough petroleum during catagenesis to cause expulsion

from the source rocks. TOC values are expressed as weight percent of the dry

rock. A standard scale is used for the interpretation of TOC values and it is

given in Table 3.2.

63

Figure 3.15 Flow chart of sample preparation procedure for XRD

analysis

Powder the raw sample with the help of mortar and pestle.

Disaggregate in distilled water.

Remove the salts by repeated washing with distilled water.

Siphon the clay fraction in suspension and concentrate it into slurry.

Mount the slurry evenly on a glass slide, drop by drop with the help of pipette till a meniscus is formed.

Allow the clay mount to dry at room temperature.

Store remaining clay slurry for future use and reference.

Obtain diffractogram by scanning the oriented slide from 2° 2 to 30° 2 diffraction slides.

Glycolation: Place the mounted slide in a small dessicator containing ethylene glycol for 24 hrs. Solvation is generally done after air drying and before heat treating. Not all clays will react to solvation, but the swelling clays particularly smectite and somewhat vermiculite will expand to specific basal spacings depending somewhat on their cation saturation.

64

Table 3.2 A standard scale of TOC values for source rock evaluation

TOC (wt %) SOURCE ROCK IMPLICATION

< 0.5 Negligible source capacity

0.5 -1.0 Slight source capacity

1.0 - 2.0 Modest source capacity

> 2.0 Good to excellent source capacity

3.5.1 Principle

Carbon is found in sedimentary rocks in the form of inorganic

carbonates and organic carbon. To determine the percentage of total organic

carbon, the crushed sample is first treated with dilute hydrochloric acid to

decompose the inorganic carbon.

The carbonate free rock is then combusted at a high temperature in

the presence of a large excess of oxygen. All the organic carbon is converted

to carbon dioxide that is detected. The amount of carbon dioxide produced is

directly proportional to the organic carbon content of the rock.

3.5.2 Instrument

The most advanced instrument available to study the TOC content

is CR- 412- carbon analyzer. This instrument is a non-dispersive, infrared,

digitally-controlled instrument designed to measure the carbon content in a

wide variety of organic materials including coal, coke and oil as well as some

inorganic materials including soil, cement and limestone.

65

This consists of

a. Measurement unit consisting of furnace assembly, balance,

flow controllers and electric unit

b. Control console with printer.

The sample is placed in a furnace at 1450 in an atmosphere of pure

oxygen at a flow rate of 3.25 lts/min. IR detector detects the evolved CO2.

3.5.3 Methodology for sample preparation

Sample selection must be based on the lithology of the sediments.

Preferably clastics and limestone are to be selected. Ideally the analysis

should be done by picking single lithologies. The process is described by the

following flow chart (Figure 3.16).

3.5.4 Inference from TOC studies

The original concentration of the organic mater in sediment

depends primarily on:

i) Productivity in water above the basin and the amount of

organic matter washed in from sources on land.

ii) Effects of dilution by variable sediment influx, and

iii) Dissolved oxygen content of the water, which depends on

the water depth and circulation.

66

Figure 3.16 Flow chart of sample preparation for TOC analysis

Make the sample homogenous by crushing it with a pestle mortar.

Dry the folded filter paper in an oven at 60C for 10-12 hours and cool it in a

desiccator for 1 hour. Weigh the dried filter paper

The sample, processed is subjected for the instrumentation part.

Transfer the contents of the beaker onto the filter paper with distilled water. Repeat

washing with distilled water until the filtrate is chloride free (test with AgNO3 soln).

Thoroughly wash the cutting sample with water to remove mud contamination.

Crush the sample to 60-100 mesh size

Weigh accurately 5g of sample & transfer it into a 500ml beaker.

Add 50ml of 4N HCl into the beaker slowly with constant stirring of the sample

with a policeman. Heat the contents of the beaker at 80C for 30mins to break up

the dolomite, if present, and leave the sediment acid mixture overnight.

Dry a Whatman No: 1 Filter paper (18-24 cm diameter) in an oven at 50 – 60C

for 4 hours. Cool it in a desiccator for one hour and weigh it accurately.

Fill the beaker containing digested sample with tap water. Let the suspended

particles settle down. Decant the solution into the pre-weighted filter paper in a 4

funnel. Repeat this step 4 times with tap water and 2-3 times with distilled water

67

The criteria used for identifying the different sequence parameters

by study of different branches of geology and geophysics are summarized in

the table below (Table 3.3).

Table 3.3 Methods used for identification of Sequence parameters

Outcrops Well-logs Seismic Biostratigraphy

Sequence boundary (SB)

Facies dislocation, superposition of a relatively proximal on a significantly more distal facies without preservation of intermediate facies.

Same as outcrop

1. Development of high-relief truncation surface, which erodes topsets of older units. 2.Downlap shift in coastal onlap across the boundary

Limited by the resolution of marker fossil available. Minimum faunal abundances are potential candidates of maximum regression and thus can be marked as SB

Transgressive surface (TS)

Marks the boundary between topset-clinoform interval, and an interval of only topsets.

Limited fossil preservation. However abrupt superposition of marine fossil assemblages upon marginal or non-marine ones may infer TS

68

Table 3.3 (Contd…)

Maximum flooding surface(MFS)

Is recognizable by deepest water deposits and marked by farthest landward extent of deep-water facies. Equivalent condensed sections show abundance and diversity of fossils, intense bioturbation, presence of glauconite, phosphate, pyrite, siderite, volcanic ash, hardgrounds.

It is the surface between retrograding unit and an overlying prograding unit. If these are dirtying-up and cleaning-up units respectively, the MFS is surface of gamma- maximum

Downlap surface, where clinoforms downlap onto underlying topset, which may display backstepping and apparent truncation.

Most landward distribution of diverse, open marine, cosmopolitan, often abundant, plankton and deep water benthos.

Lowstand systems tract (LST)

Bounded below by SB and above by TS. Progradational parasequence sets.

Same as outcrop

In essence the LST is recognized in the proximal fossil record by an underlying hiatus, a sudden shallowing up of bio-facies or the superposition of non-marine assemblages on marine. In deep basin it is recognized by increased rates of siliciclastic sediment supply and sediments that contain re-worked fossils and a low abundance of indigenous fossils.

69

Table 3.3 (Contd…) Transgressive systems tract (TST)

Retrogradational parasequence set bounded below by TS and above by MFS

Fining upward sequence in Gamma ray and SP logs (Bell shaped)

Same as outcrop

Have a deepening upward bathymetric signature in the fossil assemblage, usually the superimposition of more distal fossil assemblages upon proximal ones.

Highstand systems tract (HST)

Aggradational to progradational parasequence set bounded below by MFS and above by SB

Coarsening upward sequence in Gamma ray and SP logs (Funnel shaped)

Same as outcrop

Characterized by thick accumulations of fossil assemblages followed by superimposition of proximal fossil assemblages upon more distal ones.