Landmark Papers in Applied Biostratigraphy Best Practices

Partington et al. (1993a), and its intended companion (Partington et al., 1993b), represents an excellent example of best practice in the application of biostratigraphy to sequence stratigraphy at a regional scale, providing a series of maps suitable for mapping petroleum system elements.

The data set is from the hydrocarbon-bearing and -generating Jurassic sediments of the North Sea. Thirty-three regionally correlatable marine condensed sections containing maximum flooding surfaces were recognized using a combination of biostratigraphy, wireline logs (over 500 wells) and seismic data. This subdivided the North Sea Jurassic succession into 32 genetic stratigraphic sequences. Each maximum flooding surface was biostratigraphically calibrated using microfossils (dinoflagellate cysts, radiolaria, ostracods and foraminifera). In particular, the relationship of the sequence scheme to biozones and bioevents defined by dinoflagellates enabled correlation across the entire well set and calibration with the ammonite-standard stratigraphy of the onshore. These correlations provided the basis for a basin-wide stratigraphic framework for the Jurassic of the North Sea Basin that reduced lithostratigraphic uncertainties and allowed for prediction of petroleum system element distribution. The sequence model developed still guides exploration and production thinking today.

The paper has served as an excellent demonstration of how to apply and document biostratigraphy in sequence definition. Attention was focused on maximum flooding surfaces which, following the suggestions of Loutit et al. (1988), and Emery and Myers (1996), have relatively readily recognizable biostratigraphic (abundant and diverse fossil assemblages), well log (high Gamma excursion) and seismic signatures (laterally persistent high-amplitude horizon). Because these surfaces are recognized across the entire basin, they represented continuous deposition as opposed to a break in deposition (i.e., sequence boundaries) and they form a framework between which other sequence stratigraphic surfaces and systems tracts can be placed. They also have excellent biostratigraphic calibration to standard chronostratigraphy. As a result, this approach has been followed in other parts of the world (e.g. Sharland et al., 2001).

Partington et al. (1993b) gave further details of the application of the biostratigraphically-calibrated sequence scheme within the Late Jurassic. This paper presented log correlations, chronostratigraphic charts, and gross depositional environmental maps that helped to elucidate the stratigraphic and geographic distribution of key petroleum system elements such as reservoirs.

Armentrout, J. M., 1991. Paleontologic constraints on depositional modeling: examples of integration of biostratigraphy and seismic stratigraphy, Pliocene-Pleistocene, Gulf of Mexico. In Weimer, P. and Link, M. H. (Eds.), Seismic facies and sedimentary processes of submarine fans and turbidite systems. Springer-Verlag: 137-170.

Armentrout (1991) provides anyone trying to interpret complex stratigraphy in shelf or slope environments with a workflow for integrated analysis. His methodology employs three common data sets; biostratigraphy, well logs, and seismic profiles. Using data from offshore Texas in the Plio-Pleistocene of the Gulf of Mexico, he details the necessary steps to analyze quantitative foraminifera and calcareous nannofossil abundance and diversity, and bioevents (typically extinction datums). Benthic foraminiferal faunal checklists are used to define paleobathymetric zones and biofacies to interpret depositional environments across the shelf-slope profile. Well logs are combined with faunal/floral abundance and diversity plots to identify condensed sections. These are identified by relatively high gamma ray responses and associated abundance and diversity peaks of planktic taxa. Faunal discontinuities are combined with well logs to identify third-order and higher-order sequence boundaries. This approach combines the techniques of Vail (1987) and the Exxon interpretation methodology, which focused on unconformities and relative sea level lows, with the techniques of Galloway (1989) who placed emphasis on times of maximum inundation of the shelf and regionally extensive flooding surfaces and condensed intervals. Potential pitfalls and the criteria for quality-check of biostratigraphic data are also discussed. In particular, the apparent discrepancy of log and core based interpretation of shoreline facies occurring within bathyal biofacies is resolved by recognition of a shelf-margin setting at lowstand and subsequently overlain by transgressive mudstones of deep-water facies.  The interpreted paleo-water-depth of these transgressive mudstones often rapidly deepens due to the progressive compaction of the shelf-to-slope clinoforms.

The third component of the workflow is the use of seismic profiles. Six basic seismic facies are defined and their internal characteristics and lateral continuity are discussed. Armentrout explains how to use synthetic seismograms to relate the stratigraphy interpreted from fossil data and well logs to seismic stratigraphy. He also details techniques for relating biofacies to seismic facies to produce map units for sand-prone and seal-prone facies on both the shelf and adjacent slope.

Integrated stratigraphic analysis, such as documented in this paper, has become the standard technique employed by the petroleum industry in basins such as the Gulf of Mexico, which is rich in biostratigraphic data. Previous work tended to focus on interpretation of seismic character (terminations, lapping relationships, geometries) to interpret sequence stratigraphy with only passing regard to other data sets. This paper combines the techniques used by leading seismic and well log interpreters with high-resolution biostratigraphic techniques to provide the means for a truly integrated interpretation that is internally consistent between the three most common data sets available to petroleum exploration or other stratigraphic analysts.

O'Neill, B. J., Duvernay, A. E. and George, R. A., 1999. Applied palaeontology: a critical stratigraphic tool in Gulf of Mexico exploration and exploitation.  In Jones, R. W. and Simmons, M. D. (Eds.), Biostratigraphy in production and development geology. Geological Society Special Publication 152: 303-308.

O'Neill et al. (1999) use foraminifera and calcareous nannoplankton to resolve issues of 'missing' reservoir facies in two Gulf of Mexico fields where seismic images have poor resolution along high-angle salt flanks and subsalt locations. The examples include two deep-water sand fields, Shell's 'Bonnie', a salt dome field located on the shelf, and 'Mars', a mid-slope salt-rimmed mini-basin field. 

At Bonnie, the play, based on 3D seismic, was to test a down-thrown fault block where seismic amplitude anomalies were projected up-dip along the flank of a salt-dome. The well failed to encounter the projected reservoir sands. The biostratigraphic analysis documented that the objective section had been penetrated, but the predicted sands were missing "most-probably' due to stratigraphic pinch-out.  Recognition of this facies pattern resulted in a down-dip side-track that discovered significant hydrocarbon reserves in gravity-flow sands that pinched-out against a syndepositional structural/topographic high.

In the Mars case history, the 1989 discovery well and subsequent delineation wells, correlated using biostratigraphy, defined a deep-water gravity-flow depositional system within the mini-basin.  The biostratigraphic data included foraminifera, calcareous nannoplankton and palynomorphs.  In 1991, an appraisal well was designed to test interpreted pay horizons beneath a salt-overhang. The well successfully penetrated the salt and bore-hole biostratigraphy documented penetration of the objective section without encountering the predicted reservoir sands.  Further analysis suggested lateral facies thinning of the gravity-flow sands with pinch-out down-dip to the well location rather than salt truncation.  The integration of biostratigraphy and structural analysis enabled the Mars team to model the salt movement through time that controlled reservoir architecture, and thus, constrained volume estimates.  These data were critical to Shell's decision to proceed with the $1.2 billion field development.

In both cases discussed by O'Neill et al. (1999), integration of the high-resolution biostratigraphy, and seismic structural and facies analyses led to a depositional model that resolved lateral pinch-out of reservoir sands, thickest within synclinal axes and thinning against salt-cored highs.  Field development planning, based on the integrated depositional model, led to cost savings from fewer wells placed at optimal locations and the discovery of additional resources with side-tract wells drilling down-structural dip where thick reservoir sands occur.

Neal, J. E., 1996. A summary of Paleogene sequence stratigraphy in northwest Europe and the North Sea. In Knox, R. W. O'B., Corfield, R. M. and Dunay, R. E. (Eds.), Correlation of the Early Palaeogene in northwest Europe. Geological Society Special Publication 101: 15-42.

Neal's (1996) paper presents a high resolution chronostratigraphic correlation framework for the Paleogene of northwest Europe, constructed by integrating subsurface and outcrop data using graphic correlation of cycles interpreted using sequence stratigraphic principles. The graphic correlation technique, using a composite standard database, was first introduced by Shaw (1964, 'Time in Stratigraphy').  In this paper, the methodology was used to facilitate the ordering of depositional cycles where the biostratigraphic record was equivocal due to low species diversity and the lack of regionally reliable index taxa.

Paleogene North Sea sediments record five major regressive-transgressive cycles.   These major cycles overprint nineteen higher frequency cycles that control the distribution of depositional facies.  The cycles are best documented along basin margins in shallow water settings. Once recognized, the cycles can be correlated offshore into deeper-water facies where they can be tied to the geologic time scale using a fully marine stratigraphy.

To establish the regional correlation framework, Neal (1996) integrated the biostratigraphic data into a 'composite standard', ordering the temporal succession of stratigraphically useful fossils.  This composite data, graphically displayed, facilitated correlation of depositional cycles, and helped recognize 'missing' or thin depositional cycles toward basin depocenters where some thin-cycles were previously unrecognized.

Neal (1996), using the graphic correlation methodology, correlated key bounding surfaces across northwest European Paleogene basins, resulting in recognition of 30 depositional sequences within five long-term transgressive-regressive relative sea-level changes.  This stratigraphic framework facilitated mapping depositional systems tracts useful in predicting petroleum system lithofacies elements and the modeling of source rock maturation, and timing of trap formation.

Holmes, N. A., 1999. The Andrew Formation and "biosteering" – different reservoirs, different approaches. In Jones, R. W. and Simmons, M. D. (Eds.), Biostratigraphy in production and development geology. Geological Society Special Publication 152: 155-166.

Technological development of long-reach deviated and horizontal wells, and multi-lateral wells, have resulted in significant increases of borehole-to-reservoir connection.  Keeping the well trajectory within the optimal interval of the reservoir involves both down-hole drill-stem monitoring while drilling and real-time wellsite reservoir-scale biofacies analysis.

Holmes (1999) describes two examples of biosteering in deep-water sands of the Late Paleocene Andrews Formation in the North Sea, United Kingdom sector.  Definition of foraminiferal morpho-groups and microfacies analysis provide high-resolution lithofacies characterization, useful in both the pre-drill well-path planning, and at wellsite during drilling to provide real-time data for steering decisions.

The first example, from the Joanne Field, describes biosteering through 10-15 foot thick turbidite sands.  The reservoir is a succession of deep-water sand and limestone facies. Within the pay intervals, the occurrence of local dip variations, subseismic faults, and lateral thickness variations in the sand reservoirs, challenge well-bore steering.  Both in-place and reworked microfossils are used to characterize up to ten discrete field-wide lithostratigraphic units. Wellsite identification of these correlatable biofacies/lithofacies units allowed for correcting well-path deviation to avoid drilling non-reservoir facies.

The second example is from the Andrews Field.  Here the discussion by Holmes (1999) focuses on identification of the interbedded fossil-rich mudstone seals.  The high-resolution biofacies analysis is used to characterize potential sealing qualities and lateral continuity of adjacent silt and claystone intervals.  Characterization of baffle versus barrier qualities of the non-reservoir facies was used in planning the well path to optimize production through well placement and stand-off from oil-water, and gas-oil contacts.

Identification of high-resolution integrated biofacies and lithofacies correlation datums for biosteering is routinely applied in the North Sea, U.S. Gulf Coast and Gulf of Mexico, and other basins world-wide.

Lowman, S. W., 1949. Sedimentary facies in the Gulf Coast. AAPG Bulletin 33: 1939-1997.

Lowman (1949) provides a detailed analysis of methods developed for interpreting the complexities of the stratigraphy and structure in the Gulf Coast Basin, with special emphasis on developing a paleo-water-depth model. A discussion of regional stratigraphy of the upper Tertiary stressing the stratigraphic variability is followed by a presentation of a study of the benthic foraminifera along three transects in the Gulf of Mexico and how their depth distribution can be referenced against fossil assemblages in transgressive deposits in the subsurface. A discussion is presented concerning the determination of evolutionary extinction of key microfossils versus stratigraphic variability of others reflecting changes in water depth habitat. Correlations in the subsurface are dependent on the microfossil fauna in the transgressive units above and below the non-fossiliferous regressive sands.  Each couplet of coastal plain to shelf and slope sedimentation, most often, thickens basin ward, thereby outbuilding the continental margin. This outbuilding results in flexures of the units and causes structural complexities, typically growth-fault systems, which enhance fault trapping for hydrocarbons. The role of subsidence under depositional load, thereby enhancing the development of Gulf Coast depocenters, is mentioned as well as comparisons to other U.S. sedimentary basins and several overseas basins.

This publication has stood for many years as the definitive work for understanding the lithofacies and biofacies in the Gulf Coast Basin. The paper set a standard for future work, both local and regional, in comprehending the geology of this incredibly petroliferous basin.

Rull, V., 2002. High-impact palynology in petroleum geology: applications from Venezuela (northern South America). AAPG Bulletin 86: 279-300.

Rull (2002) summarized new and traditional approaches to the application of palynology as a high-resolution and high impact tool applied in oil and gas exploration, and production of fluvial to shallow marine facies in Lake Maracaibo, Venezuela. Innovative concepts introduced are high-resolution ecostratigraphy at basin scale and the concept of 'palynoblocks' in structurally complex areas to estimate missing section. Rull's paper is particularly noteworthy for demonstrating the identification of subtle stratigraphic trap potential through detailed quantitative palynostratigraphy integrated with wireline logs and seismic data.

The ecostratigraphic approach (Poumot, 1989) was tested in the Maracaibo Basin allowing identification of up to 21 palynocycles equivalent to 3rd order eustatic cycles in fluvial to coastal sections of Eocene, Oligocene and Miocene age (Rull and Poumot, 1997; Rull and Lorente, 1999). Palynocycles were also recognized in the Paleocene to middle Eocene of the Maracaibo Basin (Rull, 2000). In addition, the palynocycle methodology has been used successfully in tectonic interpretation and high-resolution reservoir correlation. The application of this high-resolution palynology correlation helps define more precisely the stratigraphic interval of interest, and facilitates its identification in seismic profiles and electric logs.

Rull (2002) provides an innovative definition of 'palynoblocks' as a tectonic element limited by faults and characterized by the occurrence of a single palynological zone as its top.  Significantly, this paper introduced palynoblocks as a concept with tectonic, biostratigraphic and chronostratigraphic attributes.

Seismic correlation and mapping of identified palynoblocks provides an immediate detailed chronological picture of the surface below major unconformities. This allows the interpretation of relative movement of the tectonic blocks in time and the quantification of the magnitude of erosion as well as the location and magnitude of missing sections in structurally complex areas.  Rull (2002) demonstrated that detailed history achieves a better understanding of the differences in the petroleum systems of neighboring blocks through a more precise reconstruction of their individual burial history.  In turn, he showed that this allows a more precise estimation of source rock maturation and timing of trap formation.

Van Hinte, J. E., 1978. Geohistory analysis – application of micropaleontology in exploration geology. AAPG Bulletin V. 62, N. 2: 201-222.

Van Hinte (1978) redefined the 60 year-old (at that time) technique of basin subsidence analysis, providing a methodology useful in Petroleum System modeling of what later became known as the Critical Moment of hydrocarbon expulsion (Magoon and Dow, 1994; see Geochemical Landmark Papers).

Van Hinte demonstrated that the use of high-resolution and high-precision biostratigraphic analysis of multiple fossil groups is crucial to unraveling the geologic history of a basin and specific exploration areas. Advances in the correlation of relative age determinations based on biozones to absolute linear time scales provided for a +1 million year precision in the Cenozoic and a +2 million year precision in the Mesozoic. Using this 1978 precision in age determination, combined with direct measurements of interval thickness and interpretive data including paleobathymetry and thermal gradients, it became possible to calculate precise rates of basin fill, subsidence, compaction and uplift. It improved estimation of the amount of time missing at unconformities and the amount of sedimentary section missing. With the use of a high-confidence linear time scale, it became possible to plot other time-related data such as heat flow, porosity change with burial, petroleum maturation and migration, and to confidently extrapolate trend curves to fill gaps. This paper helped move biostratigraphers out of the shadows of a qualitative world into a world where their data could be expressed in numerical forms that were more understandable and relatable for a broad range of geoscientists, engineers and management.

Van Hinte provided the equations, select nomograms and the work-flow to execute a modern geohistory analysis of a well section or sections. The work-flow could be applied to calculate a variety of sediment accumulation rates, and rates of subsidence and uplift. Quantification, even with broad error bars, could establish a trend line of estimated paleo-water-depth that could be used to determine paleobathymetry at any particular time along the age-calibrated trajectory. This led to new ways to map facies distribution (tied to initial subsidence rate), timing of structural events, and separation of basement subsidence from subsidence within the sedimentary section related to compaction, diapirism or faulting. The high-precision age determinations also impacted petroleum system analysis. Fluid migration out of a basin could be predicted through construction of maps of compaction rate. Mature hydrocarbons and other fluids move from areas of high compaction rate to areas of low compaction rate, thus biostratigraphy aided the predictive ability of this methodology. None of the step-change improvements in basin analysis and 3D and 4D fluid-flow modeling, structural analysis or petroleum system analysis would have been possible without the concurrent advances in high-resolution, high-precision, biostratigraphy and age determination.

Hardenbol, J., Vail, P. R., Ferrer, J., Montadert, L. and Blanchet, R., 1981. Interpreting paleoenvironments, subsidence history and sea-level changes of passive margins from seismic and biostratigraphy. Oceanologica Acta, Special Issue (0399-1784): 33-44.

Hardenbol et al. (1981) analyzed basement subsidence, eustatic sea-level change, and sediment supply for the Cenomanian off northwest Africa to illustrate the procedure of geohistory analysis: a quantitative stratigraphic technique that combines stratigraphic and paleobathymetric information into a time-depth framework.  Hardenbol et al. (1981) paper followed on the work of van Hinte (1978) based on U.S. Gulf Coast data, and demonstrated the applicability of the technique to the detailed structural evolution, basin analysis and reservoir potential of petroleum systems of passive margins in general.

The northwest Africa section was chosen, because of the amount and quality of seismic and paleontologic data, and is the same one used to document Mesozoic sequences by Todd and Mitchum (1977, AAPG Memoir 26; see Sequence Stratigraphy Landmark Papers). Hardenbol et al. (1981) demonstrated that a stratigraphic framework which integrates seismic and biostratigraphic datums yields stratigraphic resolution far superior to either method alone. Repeating the procedure for networks of seismic lines defines sequences over a wide area of variable environments.  In turn, this can be used to delineate relative sea-level changes, the relative magnitudes of which can be used to assign tentative ages to sequences by comparing successions to one in which the stratigraphy has been documented. They note that the technique of geohistory analysis in its modern quantified form became feasible with the introduction of reliable time scale (chronostratigraphic) systems based on the careful integration of bio-, magneto-, and seismic stratigraphy and radiometric dating. In order to determine the water-depth of the seafloor (paleobathymetry), the technique also requires detailed paleontology and facies records using methodologies first demonstrated by Natland (1933) and Bandy (1953) for California (see Paleontology Landmark Papers) and Lowman (1949) for the Gulf Coast (described above).

Hughes, G. W., 2000. Bioecostratigraphy of the Shu'aiba Formation, Shaybah Field, Saudi Arabia. GeoArabia 5: 545-578.

Biostratigraphic data has long been used to assist in determining paleoenvironments (e.g. Natland, 1933). This has provided key input into regional facies mapping within basins to help understand the distribution of reservoir and source rock units. Less common, but increasingly important, is the application of biostratigraphic data to provide detailed facies mapping at the reservoir scale within a single oil field. Hughes (2000) documents a dissection of the important Shu'aiba Formation reservoir in one field in Saudi Arabia using foraminiferal, calcareous algae and macrofossil biofacies distribution, as identified from thin-sections and cores of multiple well material, within a chronostratigraphic framework. The result is a series of high-resolution facies maps essential for production well planning. As such, the study forms an excellent case study of this type of application and a model for how carefully collected and evaluated paleontological data can be used to enhance hydrocarbon production from existing assets.

The Shu'aiba Formation and its various lithostratigraphic equivalents form major hydrocarbon reservoirs across the Arabian Plate with a complex internal architecture relating to relative sea-level change controlling the progradation, retrogradation and exposure of a carbonate platform margin into a coeval intrashelf basin (e.g. Witt and Gokdag, 1994; van Buchem et al., 2010). The platform margin includes bioherms or banks composed principally of rudist bivalves and these constitute a key reservoir facies although as this paper shows, platform interior facies can have reservoir character also.

The Shaybah Field lies within the modern gigantic dune fields of the Rub' al Khali desert and this has historically prevented high quality seismic imaging of the reservoir. To overcome this, Hughes (2000) examined vertical and coeval lateral variations in semi-quantitative biofacies character within well core material in 50+ wells to define lagoon, back-bank, bank-crest, fore-bank and upper ramp depositional environments. This required detailed identification of foraminifera, calcareous algae and rudist bivalves (often as fragments) and establishing their likely paleoecological preferences by reference to sedimentological associations, modern-day analogues and photosynthetic associations (e.g. Banner and Simmons, 1994).  By placing the distribution of these paleoenvironmentally sensitive biofacies (integrated with other subsurface data) in a temporal framework (defined by biostratigraphy).  Hughes was able to produce maps interpolating depositional character for seven time slices within the Shu'aiba Formation across the entire field. These maps helped define reservoir architecture in 3D space and hence, better future well placement and a facies-constrained production, and stimulation strategy. By calibration of biofacies character to Fullbore Formation Microimager (FMI) logs, non-cored wells with FMI logs could be incorporated into the analysis.