Name: Erekosima, Sarah Okorite.
Matric No: 12/Sci14/011.
Level: 400l.
Dept: Geology.
Course: GEY 402- Micropaleontology
QUESTION: Write short notes on the following
fossils and their application to hydrocarbon
exploration.
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SOLUTION:
CALCAREOUS NANOFOSSIL:
These are extremely small objects less than 25 microns produced by
planktonic unicellular algae. They are made of calcium carbonate. Nanofossils
were the first to occur in the Mesozoic era and have persisted and evolved
through time. The function of the calcareous plates even in living forms is
uncertain. One of the groups that produces Nanofossils are Coccolithophorans
i.e. planktonic golden-brown algae that are very abundant in the world’s oceans.
They accumulate on the ocean floor, become buried beneath layers and are
preserved at nanofossils.
Calcareous nanofossils are an excellent bio stratigraphic tool because of their
rapid evolution and geographic dispersal (I.e. their entire life cycle is in photic
zone of the ocean) as well as their varied and distinct morphologies. The oldest
known calcareous nanofossils are late Triassic; they are crucial microfossil
group in calibrating the Jurassic-Holocene marine record. Relatively little has
been published about the paleo-geographic distributions of calcareous
nanofossils; less is known about their exact paleo-environmental preferences,
although they have been shown occasionally to penetrate into shallow marine
environments. Their main industrial application is their calibration to published
time scales and sequence stratigraphic records, especially the association of
high abundance with condensed marine sections.
Calcareous nanofossils are fossil remains of golden-brown, single-celled algae
that live in the
oceans. Because they are plants, they require sunlight, so
they float near the surface of the water. There are billions of them living in the
oceans today, and they are eaten by anything that is bigger than they are. They
are one of the primary organisms at the base of the food chain. These algae
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make tiny calcite platelets inside their cells, and these platelets (the calcareous
nanofossils or nannos for short) move to the surface of the cell. The platelets
fall off the cell and slowly drift down to the bottom of the ocean. These platelets
are replaced by new ones that constantly are forming within each cell. As these
platelets land on the bottom of the ocean, they are slowly covered up with
remains of other plants and animals and bits of mud and sand that have washed
out with the rivers of the world.
CALCAREOUS MICROFOSSIL
These are fossil that have shells composed of calcite or aragonite. These
organisms are present in the most marine and in some non-marine environment.
T the great oceanic depths characterized by low temperature and high
hydrostatic pressure, however, calcareous remains are largely or completely
dissolved. The depth below which this occurs, which varies in different depth
oceanographic setting is termed “CARBONATE COMPENSATION DEPTH”.
Three types include: calcareous foraminifera, ostracods and calcareous
nanofossils.
CALCEROUS FORAMINIFERA: calcareous foraminifera are a group f
unicellular organism that secrete a riid calcite or aragonite shell. Fossils of these
forms are found in sediments of brackish to marine origin from Silurian to
Holocene in age. Most are benthic but a significant group in the late Mesozoic
and Cenozoic are planktonic forms.
Some stratigraphically important foraminifera developed complex internal
structures and these include the fusulinids ( Pennsylvanian to Permian )n and
several groups of so called larger foraminifera ( Triassic to Holocene) they
occur primarily in carbonate or fine-grained clastic rocks and are excellent time
markers.
Because many species have limited and well-known environmental ranges, they
are excellent paleo-bathymetric and paleo-environmental indicators, especially
in younger Phanerozoic rocks.
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OSTRACODS
Ostracods are microscopic crustaceans whose fossils are found in Cambrian to
Holocene rocks. They occur in most marine and non-marine depositional
environments and are generally excellent environmental indicators. The
paleontological application of ostracods is limited;
 They are rare in many sections
 Many species are endemic to local basins, so their age and
environmental range are poorly understood. Ostracods typically
have rapid evolutionary rates and are useful bio stratigraphic tools
in some situation:
 In Paleozoic sequences
 In marine environments where wide-ranging species are present
 For local stratigraphy in basins of limited extent
SILICEOUS MICROFOSSILS:
These are protists with shells constructed of opaline silica. There is no intense
dissolution of siliceous remains in the deep ocean. They are subject to burial
diagenesis and become rare at great well depths except when recrystallized,
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preserved in nodules or concretions or replaced by pyrite or calcite. Three major
types are: radiolarians, diatoms and silicoflagellates.
There are three major groups of siliceous microfossils: radiolarians, diatoms,
and silicoflagellates.
Typical radiolarians.
Radiolarians are planktonic protists that occur primarily in open marine, deepwater settings. They are useful time indicators and are found in rocks of
Cambrian to Holocene age. They may be the only common microfossils in
abyssal environments, commonly forming radiolarian oozes. Radiolarian chert,
the product of silica diagenesis, is fairly widespread in the geologic record.
Radiolarians are common in some marine source rocks.
DIATOMS
Diatoms are photosynthesizing protists that occur in both marine and nonmarine environments. Marine diatoms range from Upper Jurassic or Lower
Cretaceous to Holocene and are particularly useful for age and environmental
determinations in the upper Cenozoic. Non-marine diatoms range from Eocene
to Holocene and also are useful in the upper Cenozoic. These microfossils can
be a major rock-forming group, forming sedimentary rock (diatomite)
consisting primarily of diatoms. Diatomaceous sediments, when altered by
burial diagenesis, are converted to siliceous shale, porcellanite, and chert. Such
rocks can serve as sources and fractured reservoirs for hydrocarbons (e.g.,
Monterey Formation of California). The changes in rock properties associated
with silica diagenesis permit seismic definition of silica phase transformation
zones in the subsurface (e.g., bottom-simulating reflector).
Typical diatoms.
SILICOFLAGELLATES
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Silicoflagellates are another group of planktonic photosynthesizing marine
protists; they commonly occur with diatoms. Silicoflagellates range in age from
Cretaceous to Holocene. Although not as common as diatoms, they are useful
time indicators, particularly in the upper Cenozoic. As a group, they were much
more abundant during the early and middle Cenozoic than today. They have
been used to estimate marine paleo-temperatures in the late Tertiary and
Quaternary.
PHOSPHATIC NANOFOSSILS:
They are notable conodonts and are composed of crystallites of calcium
phosphate (apalite) embedded in an organic matrix. There is one type i.e.
Conodonts; but fish teeth of less practical utility are found in some marine
strata.
Typical conodonts.
Conodonts are extinct tooth-like microfossils composed of calcium phosphate
whose biological affinities, while poorly understood, lie with chordates.
Conodonts are widely distributed in marine rocks of Cambrian through Triassic
age. They are excellent indicators of time and thermal maturity especially in
carbonates, where other methods of evaluating organic thermal maturity are less
successful. Conodonts are commonly used as zonal indices for the latest
Cambrian through Triassic because they were abundant, evolved rapidly, and
were widespread geographically. Although found in most marine rocks,
conodonts are most efficiently recovered from the insoluble residues of
carbonates dissolved in weak acids or from easily disaggregated shales.
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APPLICATION TO HYDROCARBON EXPLORATION.
The fundamental principal of stratigraphy is that the sedimentary rocks in the
Erath’s surface accumulated in layers, with the oldest at the bottom and the
youngest at the top which explains the principle of superposition. Life’s history
involves one creature appearing, evolving and later going extinct. Thus, with all
these put together, we can say that different layers of sedimentary rocks contain
different fossils.
1. In essence, when wells are being drilled in search of hydrocarbons, different
fossils are encountered in a predictable sequence till the layer they become
extinct. Other wells are drilled as well and after which correlations are done on
the two wells and most likely, a well should penetrate another one but at
different depths. LADs (LAST APPEARANCE DATUM) & FADs (FIRST
APPEARANCE DATUM) i.e. the first and last layers where fossils are found.
Biostratigraphy is very useful in this area.
2. This involves studying the cuttings critically and most times when they can
be viewed under fluoroscope with base oil to get the cutting percentage and to
view grain sizes.
3. This helps to give the depositional environment of that area an also recognise
the faults, folds and fractures that occurred in that layer or well.
4. They are useful in correlating time equivalent horizons from one side to
another it helps in hydrocarbon prospect and trend delineation stratigraphic and
geologic studies i.e. It can be useful in predicting over-pressured zones in
advance of the drill bit.
5. They help indicate hydrocarbon maturity level.
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