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Application of Foraminiferal Litho-Biofacies in the Interpretation of Deep-Water Depositional Environments and Reservoir Properties [Abstract]
As the 3-D seismic boom of the 1990's and the resulting prospects have faded into recent memory, the quest for more prospects continues. Prospect generation within areas that have seemingly been picked over will yield additional prospects using a more rigorous geologic approach combined with weaving together existing 3-D data sets and collecting new data in intervening areas which have never been shot, and re-shooting data which initially had been poorly acquired. While not a stand alone exploration tool in itself, new approaches in high-resolution biostratigraphy developed in the deep water of the Gulf of Mexico and North Sea combined in a sequence stratigraphic framework and iteratively tied into merged and expanded seismic data sets will yield more prospects. As prospects become deeper and more stratigraphic in nature, seismic frequencies and resolution become much lower. This leaves prospects relying more on geologic and biostratigraphic models to describe the type and distribution of reservoirs and reservoir heterogeneities.
Advances in biostratigraphy over the last twenty years have been driven by exploration in deep water settings. It has been seen that deep-water microfossil assemblages generally yield richer microfossil assemblages than shallow water environments. The specialized biostratigraphic concepts and techniques that were developed greatly improve stratal correlations and explain reservoir and seal heterogeneity. South Louisiana lends itself to this approach because of the existing data set of deep wells.
Wells in South Louisiana are typically dominated by deltaic and fluvial sediments above 12,000 feet subsurface, while below that depth they encounter sediments and microfossils characteristic of continental slope and rise depositional environments. Differences between deep and shallow-water depositional systems in the area are related primarily to the contrasting effects of global glacioeustatic sea level fluctuations. On the paleo-shelf, sea level fluctuations are large compared to the average water depth during a cycle. Relative to the beginning of a cycle, sea levels can range from ~350 feet above to ~60 feet below the initial depositional surface. Therefore, as the amount of accommodation available for sediment deposition on paleo-shelves in South Louisiana fluctuates, depositional environments cycle dramatically through: deep outer-neritic delta front (prodeltaic); shallow marginal marine deltaic; shoreface and non-marine paleosol. In marked contrast to what is observed in paleo-shelf environments, deep-water continental slope and rise sediments are affected by cyclical water depth changes that are small compared to average water depths. The major changes in deep-water fossil assemblages and related depositional environments that are observed at depths below 12,000 feet in South Louisiana wells are therefore driven primarily by the more dramatic bathymetric and environmental changes occurring contemporaneously on neighboring continental shelves. Changing shelf environments control sediment bypassing of the shelf edge and thus modulate sediment deposition on the slope and rise.
Historically, geologists working with biostratigraphic data in South Louisiana rely most heavily on the biostratigraphic "tops" summary of foraminiferal micropaleontological reports, typically placing the names of the biostratigraphic "tops" or stratigraphic "highest occurrences" (HOs) of key microfossils on one-inch well logs to aid in correlation. While benthic foraminiferal data have traditionally been used almost exclusively to correlate South Louisiana wells, exploration in deep water section offshore has shown better correlation results by utilizing planktic foraminifera and nannofossil data combined with traditional benthic data. In addition, much previously overlooked biochronologic and biostratigraphic information is contained in the detail of micropaleontological reports now digitally recorded by most if not all paleontologists working in the region. This extra paleontologic detail can be displayed in well log format and plotted with other correlation logs such as SP, GR, and resistivity. Typical paleontologic curve types include various benthic and planktic index species abundances, benthic and planktic environmental indicator assemblage abundances (e.g., groups of species indicating paleobathymetric zones), and summary curves such as numbers of species per sample (i.e., diversity) and numbers of HOs per sample (i.e., rate of faunal or floral change).
By displaying data that typically does not appear in a tops summary in well-log curve format, it is possible to: (1) make a more precise biochronologic correlation of the drilled section, and (2) interpret in considerable detail successions of depositional environments represented in the drilled section. Displaying biostratigraphic data graphically, in well-log curve format, also makes it easier to normalize the differences between individual paleontologists who may have slightly differing criteria for the identification and biostratigraphic significance of fossil species.
Like deep-water depositional environments, deep-water benthic foraminiferal assemblages observed in biostratigraphic well-log curve format reflect physical and chemical factors other than water depth at the immediate site of deposition. Studies of modern deep-water benthic foraminiferal distributions reveal strong linkages to parameters such as bottom-water oxygen, sedimentary organic matter, grain-size, mineralogy, and bottom current strength. Those lithologic and oceanographic parameters, in turn, are controlled partly by regional water mass properties, e.g., oxygen concentration, and partly by local topographic setting and depositional environment, e.g., within a confined minibasin, in the path of episodic turbidity flows, or on elevated microhabitats that develop on relief created atop abandoned submarine fan lobes. All of these deep-water subenvironments are characterized by sedimentary parameters that are critical to hydrocarbon exploration and reservoir and seal evaluation.
Although biostratigraphic reports do not traditionally relate deep-water benthic foraminiferal assemblages to parameters other than water depth in the American Gulf Coast and Gulf of Mexico, such relationships are now beginning to be exploited by biostratigraphers working in integrated reservoir description teams elsewhere in the world, e.g., in the North Sea and offshore Brazil. The ultimate potential for describing depositional settings and reservoir properties using existing inexpensive biostratigraphic data and available analog studies in exploration and development groups is very great and when realized will have a strong positive impact on petroleum geoscience and on the economic bottom line. By using these new biostratigraphic techniques, better insights into depositional systems, stratal geometries, facies, and lithologies of drilled section can be assessed in a zone where well to well correlations of sands and reservoir heterogeneities are often very difficult to assess.
To understand how foraminiferal species abundance data can be used to determine the lighology and stratal geometry of a drilled deep-water section, the concept of "biofacies" and the close relationship that exists between biofacies and lithofacies must first be appreciated. The nature of this relationship is demonstrated in the definitions of the two terms. A lithofacies is a mappable rock unit differentiated on the basis of its lithologic properties from other strata. A biofacies is also a mappable rock unit, but is differentiated on the basis of its fossil biota. As previously mentioned, deep water benthic foraminifera are lithology dependant, rather than water depth dependant like the traditional understanding of shelf biota. Because deep-water biofacies are primarily lithology dependant they can appropriately be termed a litho-biofacies. A litho-biofacies column for a South Louisiana well, interpreted from graphically displayed foraminiferal biostratigraphic data in well-log curve format is presented in .
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
1Speaker, Dominion Exploration and Production, New Orleans, LA 70112
2 Earth Studies Associates, New Orleans, LA 70131
Copyright © 2003 by NOGS (The New Orleans Geological Society)