Download GEY_402_assignment,_OCHAI__1

NAME: OCHAI ESI JONATHAN
MATRIC NO: 12/SCI14/018
DEPARTMENT: GEOLOGY
LEVEL: 400 L
COURE CODE: GEY 402 ASSIGNMENT( Micropaleontology)
QUESTION
Write short notes on the following and their applications to hydrocarbon
explorations.
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Calcareous microfossils
Calcareous nanofossils
Siliceous microfossils
Phosphatic microfossils
INTRODUCTION
In this ever-changing economic and political climate, petroleum explorationists and field
development geologists are being asked to find more oil and develop older reserves.
Concomitant with this demand comes the array of new computing, drilling and surface
engineering technologies. Therefore, it is a welcoming challenge that geologists should look
inward and rediscover how they can add more value to the exploration and production business.
This has led biostratigraphers, usually niche service providers, to evolve new techniques and
approaches, challenging old ones and aligning the science with the business needs.
Microfossils, as the name implies, are those fossilize remains that require specialized methods of
preparation study. They normally cannot be studied by ‘naked eyes’ and require the use of a
microscope. To give some historical account, the association of micropaleontology to
petroleum exploration is almost a century old. The earliest use was demonstrated by Josef
Gryzbowski of Poland in1890 and many recall his pioneering effort in stratigraphy and
correlation of beds. The commercial aspect of their importance was realized by many geological
surveys, oil and gas and coal companies who employed teams of micropaleontologists to learn
more about the rocks they were handling. These studies have also gained impetus and with
systematic documentation world over by various oil companies, and have proved beyond doubt
their predictiveness in local and geological analyses. Further oil companies also have been a
major stimulus to the growth of micropaleontological studies. Assigning age to the rock is one of
the primary requirements of micropaleontological studies as input to reconstruct stratigraphy. In
marine sedimentary strata, foraminifera are known to occur abundantly (but at times not so
abundant) and hence their usefulness as a tool for dating and correlating sediments in the realm
of exploration. Apart from assigning age to the rocks the other central aspect of
micropaleontological studies is the prediction of water depth and environment of deposition of
the sediments. Such studies are vital to understand the depositional conditions and to prepare a
depositional model with reasonable predictiveness. Conventionally micropaleontological studies
have remained largely a tool for exploration arena. In recent times, change in the mind set
amongst many biostratigraphers is helping the studies to a gain foot hole as predictable means in
the sphere of development geology and reservoir modeling. This is being achieved by wayn
high-resolution biostratigraphy at reservoir scale. Scope of micropaleontology in the
interpretation of paleo-wate depth and paleoecology are also vital points to develop geological
depositional models. Their application to biofacies studies has been proved beyond doubt
especially while dealing at reservoir scale. Besides the role discussed, the importance of
micropaleontology in well sit biosteering for real-time stratigraphic monitoring of drilling
by way of predicting stratigraphic position of drill bit is another central point used by oil
companies. This exercise is cost beneficial by way of maximizing reservoir penetration and
production index in high angle and horizontal wells.
A. CALCAREOUS MICROFOSSILS:
Calcareous microfossils have shells composed of calcite or aragonite. These organisms are
present in most marine and in some non-marine environments. At 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
oceanographic settings, is termed the carbonate compensation depth (CCD). There are three
principal types of calcareous microfossils:
i.
calcareous foraminifera
ii.
ostracods, and
iii.
Calcareous nannofossils.
Calcareous foraminifera
Typical calcareous foraminifera.
Calcareous foraminifera are a group of unicellular organisms (protists) that secrete a rigid calcite
or aragonite shell (or test). Fossils of these forms are found in sediments of brackish to marine
origin from Silurian to Holocene in age. Most are benthic (bottom dwelling), but a significant
group in the late Mesozoic and Cenozoic are planktonic (floating) forms.
Some stratigraphically important foraminifera developed complex internal structures and,
frequently, large test size. Studied primarily in thin section, these include the fusulinids
(Pennsylvanian to Permian) 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
paleobathymetric and paleoenvironmental indicators, especially in younger Phanerozoic rocks.
Ostracods
Typical ostracods.
Ostracods are microscopic crustaceans whose fossils are found in cambrian to Holocene rocks.
They occur in most marine and nonmarine depositional environments and are generally excellent
environmental indicators. The paleontologic application of ostracods is limited because (1) they
are rare in many sections and (2) many species are endemic to local basins, so their age and
environmental ranges are poorly understood. Ostracods typically have rapid evolutionary rates
and are useful biostratigraphic tools in some situations:
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In Paleozoic sequences
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In marine environments where wide-ranging species are present
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For local stratigraphy in basins of limited extent
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In lacustrine environments, where they are frequently one of the few microfossils present
Ostracods may also indicate thermal maturation of source rocks.
Calcareous nannofossils
Typical calcareous nannofossils.
The term calcareous nannofossils includes both fossil coccoliths and nannoliths. Coccoliths are
minute (<25μm) calcite objects produced by unicellular marine plants (golden-brown algae). The
origin of nannoliths is uncertain, but these calcite bodies are associated with fossil coccoliths
assemblages in marine sediments and are also organically derived.
Calcareous nannofossils are an excellent biostratigraphic tool because of their rapid evolution
and geographic dispersal (i.e., their entire life cycle is in the photic zone of the ocean) as well as
their varied and distinct morphologies. The oldest known calcareous nannofossils are Late
Triassic; they are a crucial microfossil group in calibrating the Jurassic-Holocene marine record.
Relatively little has been published about the paleogeographic distributions of calcareous
nannofossils; less is known about their exact paleoenvironmental 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.
APPLICATIONS OF CALCAREOUS MICROFOSSILS TO HYDROCARBON
EXPLORATIONS
Microfossils have many applications to petroleum geology. The two most common uses are:
biostratigraphy and paleo-environmental analysis.
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BIOSTRATIGRAPHY:
Biostrattigraphy is the differentiation of rock unit based upon the fossils which they contain.
Paleoenvironmental analysis is the interpretation of the depositional environment in which the
rock unit formed based upon the fossils found within the unit. There are many other uses of
fossils besides these, including paleoclimatology, biogeography, and thermal maturation.
Recognition of unconformity in the subsurface is undoubtedly being done using geophysical
techniques but they are also being done by biostratigraphic methods viz., absence of biozone(s).
Indirect evidences like nature of preservation of foraminiferal tests i.e. abraded forms that at
times are associated with lateritized material. However, theseevidences have to be verified by
other tools but are nevertheless thought provoking. Through biostratigraphy the hiatuses in
geological history are being estimated routinely by many micropaleontologists. The fundamental
principal in stratigraphy is that the sedimentary rocks in the earth's surface accumulated in layers;
with the oldest on the bottom and the youngest on the top. The history of life on Earth has been
one of creatures appearing, evolving, and becoming extinct. Putting these two concepts together,
we observe that different layers of sedimentary rocks contain different fossils. When drilling a
well into the earth's crust in search of hydrocarbons, we encounter different fossils in a
predictable sequence below the point in time where the organism became extinct.
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PALEOENVIRONMENTAL ANALYSIS:
Through this, the fluctuation in sea level can be reconstructed by initiall inferring the
paleobathymetry and then integrating the same on a regional scale using seismic stratigraphy for
reconstruction of transgressive/ regressive cycles within a time frame. Once this exercise is
completed, based new techniques that have emerged over two decades the depositional
sequences can be inferred. The sequences thus identified, depending on whether they form part
of transgression, regression or a high stand of sea can be used for depositional models to surmise
the disposition of likely reservoir and cap rock facies and thereby giving directions for a
successful exploration campaign.
B. Calcareous Nanofossils
Calcareous nannofossils are extremely small objects (less than 25 microns) produced by
planktonic unicellular algae. As the name implies they are made of calcium carbonate.
Nannofossils first appeared during the Mesozoic Era and have persisted and evolved through
time. The function of the calcareous "plates", even in living forms, is uncertain. One extant group
that produces "nannofossils" is the Coccolithophorans, planktonic golden-brown algae that are
very abundant in the world's oceans. The calcareous plates accumulate on the ocean floor,
become buried beneath later layers, and are preserved as nannofossils. Like the planktonic
foraminifera, the planktonic mode of life and the tremendous abundance of calcareous
nannofossils make them very useful tools for biostratigraphy.
C. SILICEOUS MICROFOSSILS
Siliceous microfossils are protists with shells constructed of opaline (amorphous) silica. There is
no intense dissolution of siliceous remains in the deep ocean. Sediments deposited below the
carbonate composition depth are commonly enriched in silica by removal of the carbonate,
sometimes to the point of forming siliceous oozes. With subsequent remobilization of the silica,
deep-sea cherts may be formed. Siliceous microfossils are subject to burial diagenesis and
become rare at great well depths except when recrystallized, preserved in nodules or concretions,
or replaced by pyrite or calcite. There are three major groups of siliceous microfossils:
radiolarians, diatoms, and silicoflagellates.
Radiolarians
Typical radiolarians.
Radiolarians are planktonic protists that occur primarily in open marine, deep-water 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
Typical diatoms.
Diatoms are photosynthesizing protists that occur in both marine and nonmarine environments.
Marine diatoms range from upper Jurasic or Lower Cretaceous to Holoccene and are particularly
useful for age and environmental determinations in the upper Cenozoic. Nonmarine 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 (diatomites) consisting primarily
of diatoms. Diatomaceous sediments, when altered by burial diagenesis, are converted to
siliceous shale, porcellanite, and cherts. 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 tansformmation zones
in the subsurface (e.g., bottom-simulating-reflector).
Silicoflagellates
Typical silicoflagellates.
Silicoflagellates are another group of plankton photosynthesizing marine protists 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
Cennozoic. As a group, they were much more abundant during the early and middle Cenozoic
than today. They have been used to estimate marine paleotemperatures in the late Tertiary and
Quaternary.
D. PHOSPHATIC MICROFOSSILS:
Phosphatic microfossils, notably conodonts, are composed of crystallites of calcium phosphate
(apatite) embedded in an organic matrix. There is one type of stratigraphically significant
phosphatic microfossils (conodonts); but fish teeth, of less practical utility, are found in some
marine strata.
Conodonts
Conodonts are extinct toothlike microfossils composed of calcium phosphate whose biological
affinities, while poorly understood, lie with Chordate. 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.
Individual conodonts vary greatly in morphology, and taxonomy was originally based on the
morphology of these individual specimens. While conodonts are common, the preserved remains
of the soft-bodied animal that bore them are extremely rare. Based on a few preserved wholeanimal specimens discovered recently (e.g., conodonts appear to have been located in the
cephalic area and may have functioned as teeth. However, the conodont animal apparently bore
many conodonts of differing shapes and morphologies, based on the study of the very rare
whole-animal specimens and rare bedding-plane groupings of conodonts representing individual
animals. This recent information has led to more accurate multielement species concepts.
APPLICATION OF PHOSPHATIC AND SILICEOUS MICROFOSSILS TO
HYDROCARBON EXPLORATION
1) In an operational environment, microfossils can be examined shortly after being brought
to the surface in cuttings.
2) Well-site analysis permits immediate identification of stratigraphic levels and drilling
objectives, minimizing drilling time.
3) Microfossils can also be used to accurately predict over pressured zones in advance of the
drill bit.
4) In offices of hydrocarbon exploration activities, microfossil studies allow precise local,
regional, and global time-stratigraphic correlations that help in hydrocarbon prospect and
trend delineation, regional stratigraphic and geologic studies, and exploitation
evaluations.
5) Analysis of microfossils helps scientists on the field to recognize paleoenvironmental
distributions, which in turn helps them to interpret sequence stratigraphy and reconstruct
the paleogeography and paleoclimate.
6) Some microfossils function as “paleo-thermometers” by undergoing irreversible color
changes with post-burial heating. As such, they indicate hydrocarbon maturity levels.