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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.
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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.
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 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 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.
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