Advances in the Application of Biostratigraphy to Petroleum Exploration and Production*
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, 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 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.
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 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.
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.
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.
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.
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.
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.
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.