Gulf of Mexico earth sciences applied to Nigerian deltas
Integration of geology, geophysics, paleontology
Plio/Pleistocene chronostratigraphic chart for the Gulf of Mexico and Niger Delta area. Under biostratigraphic datums, the nannofossil marker is on the left, the planktonic foram marker is on the right.
A sound basis for exploration can only be obtained by the integration of all the available data: geological, geophysical, and paleontological. In the early years of exploration, identifying a dome, a high, or observing a seepage was generally adequate to convince management and/or get financial support for the drilling of a well.
The above approach worked well and did lead to many large discoveries because there were initially many undiscovered fields. Geologists used paper and pencil to create subsurface maps in order to delineate locations for drilling. After the well was drilled, the pioneer paleontologists looked at the samples in order to establish a zonation, resulting in correlation and age dating. Stratigraphy appeared simple. What was feared was the "gouge" - an older section of higher pressure generally associated with salt domes or "shale diapirs."
After correlations were established and the exploration moved downdip, paleontologists were asked about the environment of deposition of the explored section, and they came up with answers. Paleontologists were sent to the wellsite to pick gouge, casing point, or abnormal pressure zones resulting from sudden environmental changes or faulting, a practice still followed by companies drilling in deepwater and/or in structurally complex areas.
Later on, the geophysicist was added to the geologist/paleontologist team, resulting in a sound tripod for exploration and development. The secret of success was in having the tripod stand on all three legs:
- Geology provided the ideas on where to drill
- Geophysics provided the means to visualize what was being drilled
- Paleontology provided the correlations tying together the idea and the picture.
The planktonic foraminifer Globoquadrina altispira is an important marker horizon.
Integrating disciplines is the key for the ongoing exploratory activities in frontier areas and in the deepwater. Adding a reservoir engineer creates a fourth leg; adding more legs creates added stability. Taking one leg out of a tripod (micropaleontology), however, will make it topple. In the long term, disturbing the balance of the tripod costs a lot more than it saves.
Foraminifers (benthics [bottom dwellers] as well as the planktonics [floaters]) were the fossils first used for correlation and age determination. Calcareous nannoplankton in deep marine environments and palynology (seeds and spores) in nonmarine and shallow marine environments are now part of the biostratigraphic investigation. Each discipline has index markers, local or global, to assist the paleontologist in age determination. As benthic foraminifers are bottom dwellers, they are still the most useful fossil for environmental interpretations. Understanding the relationship between the updip benthic zones and the downdip planktonic zones is important for the following reason.
In the early days of drilling, looking at 10-ft or 30-ft sample intervals, from the youngest to the oldest, and recording the first downhole occurrence of the taxa (fossils) including the markers (mostly benthics) resulted in realistic correlations in closely related basins and fault blocks.
As drilling moved downdip, it was realized that "top picking" based only on benthic markers was not adequate, as the benthic markers either started climbing in the rock section or did not occur in a facies different from which they were originally found and described. Planktonic foraminifers and calcareous nannoplankton fossils are now the most used means for global correlation, local correlation, and age determination because they are floaters and not affected by the substrate.
The benthonic foraminifer Hyalinea balthica is an important marker approximately equivalent to the nannofossil H. selli.
With the advent of sequence stratigraphy, it is now understood that "top picking" can no longer be used with certainty. The focus of paleontological analyses is the recognition and categorizing of condensed sections. Total abundance and diversity counts and plots are particularly useful in delineating condensed sections.
Within a condensed section, the highest occurrence of the planktonic foraminifers and calcareous nannoplankton is the age determinate for the cycle. In addition to containing considerable amounts of biostratigraphic information, condensed sections can be recognized from wireline log and seismic data. They often have a high gamma ray count and form an impedance contrast between over-and-underlying sediments, resulting in a bright seismic reflector.
This bright seismic reflector always lacks sands. Condensed sections are usually widespread at the time of maximum flooding, often the only physical stratigraphic link between deepwater and shelf environments and are considered to be one of the most fundamental stratigraphic units within a depositional sequence.
Back to basics
The deepwater Gulf of Mexico has several play styles including turtle structures, submarine fans, subsalt structures, and pinchouts against salt bodies. Significant sand deposits lie in submarine fans.
There is a mistaken belief that with the advent of sequence stratigraphy and 3D seismic, biostratigraphy and geology are no longer needed. This belief has lead to the drilling of many dry holes. Having a beautiful, colorful, high-resolution picture of something we do not understand should not be the basis for drilling. Combining this high-resolution picture with stratigraphic and geologic knowledge, however, will re-establish the legs of the tripod for a more sound and stable stand.
We need to look at the rocks and understand their reservoir characteristics, their environment of deposition, as well as their age. As oil and gas are found in rocks, we must understand rocks, not just look at images of them.
Integrated studies allow us to understand major depositional systems and allow comparison of different basins having similar depositional characters in different parts of the world. This allows the use of tested ideas in new areas.
Gulf of Mexico, Nigeria
The rate of significant hydrocarbon discoveries in the deepwater areas of the world continues to increase, with the Gulf of Mexico and West Africa leading the way. Of the 27 discoveries reported, 12 are in the Gulf of Mexico, while nine are in West Africa. Of these, three occurred in Nigeria's deepwater areas. The rate of discovery in the Gulf of Mexico and Nigeria is related to:
- The increasing knowledge of the complex depositional models for deepwater reservoirs on passive margins
- Their relationship to penecontemporaneous structural events and the use of 3D seismic data to recognize these models.
Deepwater Nigeria is dominated by thrusting which sets up pinchout, subthrust structures, and onlap exploration plays. Most significant submarine fan reservoirs are off-flank leaving the crests with thin sand zones.
These models were developed for exploration of the Tertiary deltaic-wedge in the Gulf of Mexico. In the early 1980s and throughout the 1990s, they were applied to a similar geologic setting of the same age in the Niger delta. Although the structure in the deepwater GOM basin is dominated by salt kinematics and those in the deepwater Niger Delta by shale-cored toe-thrusts, both developed contemporaneously with gravity-driven clastic sediments. The local structure in both basins separates them into mini-basins, which control the depositional history of the sediments and the migration history of the hydrocarbons.
These sediments, generally called "turbidites" by explorationists, represent the entire menagerie of deepwater deposits, from large-scale slumps and debris flows to grain-flows and thin-bedded turbidites, which may be channelized. Lack of areal continuity is a major problem with these sand reservoirs. The sands are usually laminated, have a substantial thickness of shale, and an areal extent confined by the local basin, as well as the amount of sand available.
This suggests that the best reservoir sands are located in the relatively undisturbed basins and lap onto the basin flanks. In the deepwater Gulf of Mexico, major discoveries have occurred in these reservoirs farther off-structure than discoveries made earlier in deltaic deposits on the shelf.
The best and most continuous sands commonly overlie a structural or stratigraphic discontinuity and grade vertically from thin-bedded channelized deposits to mud-rich sediments capped by fossil-rich, calcareous mudstones. This establishes a reservoir at the base, which in turn is overlain by a good shale seal. Since the sands are laterally discontinuous, facies changes also provide the lateral seal. This is considered a complete sequence or stacking pattern that represents a viable play.
The sequence may be repeated, but whether they are or not, and how often, is dependent on salt withdrawal and basin development. If repeated, the pattern presents the possibility of multiple reservoirs. These relationships are enhanced by eustatic sea level fluctuations. The best reservoirs seem to be coincident with major sea level falls. Although this pattern is well known from the Plio-Pleistocene in the Gulf of Mexico, it also occurs throughout the Miocene and older Tertiary strata. As exploration is extended basinward in the US Gulf, major reserves are being found in the Miocene and may be found in similar settings in the Oligocene.
Recent discoveries in deepwater Nigeria also indicate the highly variable nature of the reservoir and variable timing of structural events in relation to depositional history. The major reservoirs in this area are deepwater sands, which occur in the Upper Tertiary from Lower Miocene through Pliocene.
The Oligocene may also contain significant reservoir sands overlying major stratigraphic discontinuities related to drops in sea level. The strata follow a similar stacking pattern to those in the Gulf of Mexico and occur in similar structural settings. The sands occur in mini-basins defined by toe-thrust structures and onlap those structures. Sub-thrust plays similar to subsalt plays in the US Gulf are also possible here.
Since the stratigraphy in these mini-basins is only roughly predictable, precise age dating is required. It is necessary to understand the sequence of depositional and structural to events relating to potential reservoirs in order minimize the risk of incomplete stacking patterns, and to relate prospects to producing fields or discoveries. Since these are marine sediments, this has been accomplished using detailed analyses of the foraminiferal and nannofossil content of the sediments.
With this fossil biota, resolution is generally less than 1 million years and can be as little as 200,000 years in the Tertiary. Usually, the biota can be used to time-bracket the stacking pattern cycles or sequences which can then be identified, traced on 3D seismic data, and mapped throughout any one mini-basin. The age-sensitive fossils are then used to correlate the sequences from one mini-basin to another throughout the large depositional basin.
The rapid increase in computational power associated with a dramatic drop in cost in computational time during the 1980s resulted in the economic development of numerous (and powerful) geophysical techniques for hydrocarbon exploration. Rather than using these new techniques as adjuncts to the exploration method, management viewed them as a means to cut costs. As a result, one leg (micropaleontology) of the exploration tripod was cut off, and another, geology, was made as small as possible. After a brief flurry of success, the exploration tripod has toppled because of lack of support.
Like it or not, the search for hydrocarbons in the future will cost more - realistically, there is less to be found. Cutting costs by cutting ideas will not work in the long term. Using new techniques and technologies supported by a firm foundation of geology, geophysics, and micropaleontology will prove to be the long-term, low-cost means of exploration.