uAbstract

uFigure Captions

uIntroduction

uBackground

uTechniques

uCase studies

  uGorm field

  uAndrew Formation

  uNiger Delta

uConclusion

uReferences

uAbstract

uFigure Captions

uIntroduction

uBackground

uTechniques

uCase studies

  uGorm field

  uAndrew Formation

  uNiger Delta

uConclusion

uReferences

uAbstract

uFigure Captions

uIntroduction

uBackground

uTechniques

uCase studies

  uGorm field

  uAndrew Formation

  uNiger Delta

uConclusion

uReferences

uAbstract

uFigure Captions

uIntroduction

uBackground

uTechniques

uCase studies

  uGorm field

  uAndrew Formation

  uNiger Delta

uConclusion

uReferences

uAbstract

uFigure Captions

uIntroduction

uBackground

uTechniques

uCase studies

  uGorm field

  uAndrew Formation

  uNiger Delta

uConclusion

uReferences

uAbstract

uFigure Captions

uIntroduction

uBackground

uTechniques

uCase studies

  uGorm field

  uAndrew Formation

  uNiger Delta

uConclusion

uReferences

uAbstract

uFigure Captions

uIntroduction

uBackground

uTechniques

uCase studies

  uGorm field

  uAndrew Formation

  uNiger Delta

uConclusion

uReferences

uAbstract

uFigure Captions

uIntroduction

uBackground

uTechniques

uCase studies

  uGorm field

  uAndrew Formation

  uNiger Delta

uConclusion

uReferences

Figure Captions

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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.

Background

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.

Evolving Techniques 

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.

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 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.

Case Studies 

Gorm Field, North Sea 

Drilling problems were encountered during field development as a result of overpressures. The solution would be to set the casing in a very thin marl layer that is in some cases absent across the field. A biostratigraphic study was carried out to determine casing point. The 7-ft horizon with competent lithology was identified using localized bioevents. The zonation was applied to 12 wells and later to a further 13 wells in a neighboring field, where all were set correctly.

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 3^rd 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 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.

Conclusion 

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.

References

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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