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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.
Biostratigraphy
is the study of rock strata using fossils. Although William Smith’s
principle of faunal and floral succession was to be the cornerstone for
all subsequent work in biostratigraphy, a closer look at fossil
successions was needed. This important step came in 1842 by Alcide
d’Orbigny. His improvement to earlier principles was the recognition
that unique assemblages of fossils might include many formations (lithostratigraphic
units) in one place and only a single formation in another, leading to
the concept of stage.
Albert Oppel
conceived the idea of small-scale units defined by the stratigraphic
ranges of fossil species irrespective of lithology. He noted that some
fossils existed for a short geologic time, hence a short vertical range,
while others were quite long. Each of Oppel’s zones was named after a
particular fossil species, called an index fossil.
In the late
1800’s, a Polish micropaleontologist, Jozef Gryzbowski realized that
rock samples contained fossils that he could recognize from well to
well. In addition, he could predict hydrocarbon reservoirs and even
identify structural features, such as faults and folds. The refinement
of sequence stratigraphy by the Exxon Group led to an increased demand
for biostratigraphy, because high-resolution biostratigraphy was a key
component of this development. All these pave the way for applied
biostratigraphy in exploration and production.
Microfossils Used in E&P
Rock samples from
wells are often limited to ditch cuttings, but may also be sidewall
samples or cores. These are then washed and prepared for picking of
fossil forms in the samples and interpretation. As used in the E&P
industry, three biostratigraphic disciplines are involved, they are,
micropaleontology, nannopaleontology and palynology. The separate
disciplines have arisen due to differences in the size and chemical
composition, which imposes the need for specific preparatory and
analytical procedures. The groups are listed below along with a brief
description.
Micropaleontology
Micropaleontology
involves the study of foraminifera, ostracods, and calpionellids, which
mainly have a calcareous composition; as well as diatoms and
radiolarian, which are composed of silica, and conodonts, which are
phosphatic.
Nannopaleontology
This covers the study of nannofossils,
which are the smallest of the microfossil groups examined routinely.
This group includes coccoliths and nannoliths, and also calpionellids.
Nannofossils are calcareous and examined in transmitted light. They need
polarization techniques for positive identifications to be made.
Palynology
Palynology was
once limited to the study of spores and pollen. However, it has recently
been extended to encompass other organic-walled microfossils,
collectively termed palynomorphs. The groups studied include
dinoflaggelate cysts (dinocysts), acritachs, marine prasinophyceaen
algae, and various freshwater algae, chitinozoa, as well as spores and
pollen. They are examined in transmitted light.
Biostratigraphic Zones
The fundamental unit of biostratigraphy is
the biozone. Biozones are units of stratigraphy that are defined by the
fossil taxa (usually species and subspecies) that they contain. These
biozones are extremely useful in the exploration realm, where basinwide
correlation and large-scale rock units are of interest. But at the
reservoir scale, this coarse resolution leaves much to be desired.
Biostratigraphic Events Concept
The
first and last occurrences of fossils are examples of unique events that
can be used for correlation between stratigraphic sections.
Biostratigraphic events are geologically instantaneous changes to the
stratigraphic distribution (range) of a fossil species. Terminologies
describing these events include Top, extinction horizon, last
occurrences(LO), last appearance datum (LAD), first downhole occurrence
(FDO). Others are acme events, abundance peak, increase, pulse, influx,
flood, coiling change, second occurrence, base, bottom, inception
horizon, first occurrence (FO), first appearance datum (FAD) and last
downhole occurrence (LDO). Figure 1 explains
these terms.
Any
biostratigraphic event that is repeatable with potential field-wide
chronostratigraphic significance is the basis for high-resolution
biostratigraphy. These bioevents can be used to “fingerprint” mudstones,
which can then be integrated with wireline logs and tied around the
field. Central to the application of high resolution biostratigraphy is
thinking at field scale and “pushing the data hard,” but always
integrating and iterating with other geoscience disciplines.
This approach may
call for dropping a formal biozonation in favor of the use of a series
of finer scale bioevents. While it is still possible to relate the local
bioevents back to a broader regional biozonation scheme, strong emphasis
is placed on “anything goes” to develop a localized, field-focused
scheme driven by any repeatable bioevent (Payne et al, 1999).
Well Site Biostratigraphic Support
Timely and
effective wellsite decisions are key to delivering safe, cost-efficient
wells. Wellsite biostratigraphy is a long established tool for the real
time stratigraphic monitoring of drilling, principally used to determine
the stratigraphic position of the drill-bit, and to pick coring and
casing points and total depth (TD). Ditch cutting or sidewall samples
obtained on the drilling rig is prepared and analyzed within the logging
unit and the result tied to a high-resolution biostratigraphic scheme
developed prior to drilling.
Biosteering
Horizontal and
high-angle wells are increasingly being drilled. The need to monitor the
drilling bit and keeping it within the pay section cannot be
over-emphasized. Biosteering is a derivative of the high-resolution
biostratigraphic techniques that attempts to resolve reservoir
penetration challenges. Biosteering is intended to maximize reservoir
penetration by biostratigraphically “fingerprinting” the
reservoir-enveloping, non-pay package during drilling. If the well-bore
encounters non-pay, having passed up through the top of the reservoir or
down through the base, or passes out of the reservoir due to offset by
faults (often of sub-seismic resolution), high resolution
biostratigraphy provides a tool to steer the well bore-back into the
reservoir.
Ecostratigraphic Techniques
The concept of
ecostratigraphy was introduced in 1973. This was a new approach that
encompasses all the ecological (biotic and abiotic) aspects in
stratigraphy. The basic premise is that genetic changes does not proceed
on isolated taxa but in the frame of ecosystems and is, therefore,
intimately associated with the ecological succession. Environmental
factors, far from being distorting signals provide the basis for more
accurate correlations. Events are restricted in space, but if the
geographical domain in which they occur is known, a space-dependent
stratigraphy is possible, as shown in Figure 2.
Quantitative Stratigraphy
Representative
counts and multivariate statistics in conjunction with improved
computing power are part of the recent developments in quantitative
stratigraphy. Biostratigraphic interpretation at present tends to be
subjective, with a lot of emphasis being placed on the interpreters.
Quantitative stratigraphy (QS) is therefore providing an avenue to make
the science of biostratigraphy less subjective. Data sets are subjected
to statistical quantitative analysis to identify those with stronger
potential as correlation tools. Its method is the combination of
mathematical logic with stratigraphic techniques.
Biostratigraphic Workstation
Large
biostratigraphic data are being generated in hydrocarbon-producing
basins. As computing power improves, an alignment with managing and
interpreting biostratigraphic data has resulted in the development of
what we called biostratigraphic workstations. These are specialized
applications, which serve as biostratigraphic databases and
interpretation tools. These biostratigraphic workstations are not mere
stand-alone systems but are getting interactive and integrative. An
example is TACS-IPS. The Technical Alliance on Computational
Stratigraphy (TACS) is a consortium of some major oil companies in
conjunction with the University Of Utah. The team is being sponsored to
develop the Integrated Paleontologic System (IPS). Further plans are
also ongoing to fully develop the analytical, modeling, visualization,
and interpretive functionalities of TACS products.
Andrew Formation, North Sea
In the Joanne
field, biosteering assisted horizontal drilling through thin (10-15ft)
turbidite sands. The reservoir required detailed biostratigraphic
framework that was generated using both in situ and
derived/reworked assemblages and associations (microfacies). The result
was the identification of about 10 discrete and correlative
lithostratigraphic subunits. Using this framework, challenges such as
local dip variations, sub-seismic faults and the thinness of the sand
were better managed. It is noteworthy that lack of knowledge of the 3rd
sub-seismic fault in Figure 3 would have
resulted in the well bore completely missing the reservoir.
The aim of the
study was to identify foraminifera groups and species that are peculiar
to individual reservoir-enveloping shales. The result was that the shale
above reservoir 1 was characterized by some arenaceous forms and
echinoid remains. The second shale interval was characterized by the
consistent occurrence of Spiroplectammina wrightii,
Eggerelloides scabra, Heterolepa pseudoungeriana, and
Lenticulina inornata. They provided a framework for correlating
Maximum Flooding Surfaces and showed that reservoir-scale application of
biostratigraphy in the Niger Delta can be pursued.
The species identified in the study can
serve as proxies where there is a paucity of paleoenvironmental data.
For the species above, a coastal deltaic to middle neritic paleodepth
was suggested and an open-shelf depositional environment, with a
potential for associated proximal turbidites.
The type of
drilling bit, mud composition, and storage condition has been shown to
affect sample recovery. At the Shell Petroleum Development Company, it
was observed that 2 wells drilled in 1991 and 1992 were analyzed in 2003
and found to be nearly barren of nannofossils. However, when a new well
was drilled late 2003 in the same field, cuttings through the same
formation and analyzed as “hotshot” were found to be very rich in
nannofossils. Technically, a major challenge will be the development of
associated competencies and the grooming of more applied
biostratigraphers who are knowledgeable in other geoscience disciplines
to serve as “glues” for putting all these techniques together.
There are
enormous opportunities for growing the science of biostratigraphy and
adding value to the exploration and production value chain.
Geoscientists need to take hold of these opportunities.
Armentrout, J.M. 2000,
http://www.aapg.org/slide_bank/armentrout_john/index.shtml.
Payne,
S.N.J., D.F. Ewen, and M.J. Bowman, 1999, The role and value of
‘high-impact biostratigraphy’ in reservoir appraisal and development,
in R.W. Jones and M.D. Simmons, eds., Biostratigraphy in production
and development geology: Geological Society Special Publication 152,
p.5-22.
Valenti, Rull, 2002, High-impact palynology in petroleum geology:
Applications from Venezuela (northern South America): AAPG Bulletin, v.
86, p. 279-300.
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