Print this page
Click
to article in PDF format.
Advances in the
Application
of Biostratigraphy to Petroleum Exploration and Production*
By
G.O. Giwa1, A.C. Oyede1, and E.A. Okosun2
Search and Discovery Article #50029 (2006)
Posted January 18, 2006
*Adapted from presentation andAAPG International Conference, Paris, France, September 11-14, 2005
1Shell
Petroleum Development Company, Warri, Nigeria
2Federal University Of Technology, Minna, Nigeria
Petroleum exploration
routinely employs biostratigraphic zones in dating rock units. As the world’s
hydrocarbon basins mature, most of its subsurface uncertainties lie at reservoir
scale, hence the need for a change of strategy in the application of
biostratigraphy. This gave rise to “production biostratigraphy,” where
biostratigraphy can play a role in reservoir characterization, correlation, and
well site operations. To apply this technique, field specific bioevents are
determined and tested for consistency.
North Sea applications resulted in successful re-development of the Gorm field, horizontal drilling of a thin, 12-ft thick Andrew reservoir and the successful appraisal of a Norwegian field. In the Niger Delta, Nigeria, shales within three reservoirs in a swamp field were “fingerprinted.” The results showed consistent occurrence of Spiroplectammina wrightii (Syverstril 1905), Eggerelloides scabra (Williameon 1858), Heterolepa pseudoungeriana (Cush 1922) and Lenticulina inornata’d (Orbigny 1846). They provided a framework for correlating Maximum Flooding Surfaces, determining facies associations and the option of biosteering is being explored. Biosteering (usually in conjunction with geosteering) is intended to maximize reservoir penetration by biostratigraphically “fingerprinting,” the reservoir-enveloping non-pay stratigraphic units during drilling.
Seismic/sequence biostratigraphy, ecostratigraphy, quantitative stratigraphy, and biostratigraphic workstations are part of the recent advances that are highlighted. The effect of sample storage and type of drilling bit and mud on biostratigraphic analysis is also discussed.
|
|
In
this ever-changing economic and political climate,
BackgroundBiostratigraphy 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&PRock 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 ZonesThe 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.
Evolving TechniquesBiostratigraphic Events Concept
The
first and last occurrences of fossils are
High-Resolution Biostratigraphy
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
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 SupportTimely 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.
BiosteeringHorizontal 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 TechniquesThe 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 StratigraphyRepresentative 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
Case Studies
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.
Niger Delta Shale Fingerprinting, Nigeria
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 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.
Conclusion
The type of
drilling bit, mud composition, and storage condition has been shown to
affect sample recovery. At the Shell 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.
ReferencesArmentrout, 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 |
