Biostratigraphy is the science of using fossils present in rocks to determine their age and depositional environment. Biostratigraphy provides information about age, palaeoenvironments, facies, and sea-level changes contributing to a better understanding of subsurface structures, sedimentology, and petrology. It became a powerful tool to locate hydrocarbon reservoirs with precision. Venezuelan exploration and development sometimes occur in structurally complex thrust belts with poor seismic data. Biostratigraphy can be a critical tool pre-drill and during geological operations to reduce operational costs.
Current biostratigraphic studies are manually done by people who have developed their experience through academic and industrial training over many years. The number of such people employed in hydrocarbon exploration has diminished as the quality of seismic data, and the ease of its interpretation in 3D using workstations has improved. New technological tools have been applied to fossil analysis, such as Micro-CT (Görög et al. 2012) and AI applications to identify fossils (Beaufort et al. 2020; Mitra et al. 2019). These techniques remain in their infancy, with the studies illustrate their potential for the future but are not yet ready for routine work. The application of automated microscopes and the transmission of their images to a central laboratory has been demonstrated (Gard et al. 2016) but still largely relies on human intervention for fossil identification. Compounding the problem of training of specialists is the division of biostratigraphic studies into the identification and application of microfossils (foraminifera, ostracods), nannofossils (calcareous marine algae), and palynomorphs (organic-walled fossils including pollen, spores, and various algae). These different fossil groups contribute complementary information about age and depositional environment while covering the continental to deep marine settings.
Venezuelan basins have long been studied, but this information was gathered over a one-hundred-year period by many people and companies and dispersed in many reports and publications. A lot of this information is lost or even not understood in modern terminology, and it needs to be updated and calibrated through new studies on a basin-wide scale.
Presented here are examples of biostratigraphy as a supportive tool for the search for hydrocarbons in Venezuela. Cases presented in this paper from the large amount of work made in the past using biostratigraphy to explore Venezuelan basins to highlight the importance of biostratigraphy for the country. In the near future, biostratigraphy will be needed to support the search for new reserves in Venezuela.
Some remarkable examples of the use of biostratigraphy in Venezuela integrated with other oil industry disciplines will be presented to highlight its importance to the exploration of hydrocarbons.
Early days
At the beginning of the Venezuelan Oil era, the biostratigraphy of foraminifera, mollusks, ammonites, and palynmorphs studies were applied by oil companies. Great geologists including H.H. Renz, H.D. Hedberg, H. Bolli, J. A. Cushman, R. M. Stainforth P.J. Bermúdez, and M. Furrer add developed knowledge of fossils brought a greater understanding of Venezuelan geology. With the encouragement of the Oil companies, some of them helped many of the students at the Universities (UCV, LUZ, ULA, UDO, and USB) by teaching and bringing up the next generation of paleontologists of the country. From them, María de Lourdes de Gamero, María Antonieta Lorente, Omar Linares, Irene Truskowski, Victor Padrón, Francia Galea, Valentín Rull and others became biostratigraphers and continue their tradition by teaching the new generation of biostratigraphers. We are very few in the country but willing to help other students increase our understanding of biostratigraphy.
This paper is looking to show some examples of the biostratigraphic application of microfossils to understand the petroleum systems around Venezuela.
Orinoco Oil Belt
The Orinoco heavy oil belt is the largest rich oil area in the world, with a total area of 54,000 km2, heavy oil reserves of about 200 billion tons, and recoverable reserves of about 50 billion tons (Mu, Han & Xu 2009). One of the earliest works using biostratigraphy and published by Isea (1987) summarizing the information associated with the status, for that time, of the palaeoenvironmental settings, age, and eustatic cycles for the Orinoco Oil belt (figure 2).
The Tertiary in the Orinoco Heavy Oil Belt is represented by a time interval that includes the Oligocene and Miocene, and sediments were deposited in a coastal-influenced continental province of the Eastern Venezuela Basin (Isea 1987). As seen in Figure 2, sedimentary cycles interpreted from biostratigraphic data help to set the geological framework for deposition along the Orinoco Oil Heavy belt in time. A large amount of biostratigraphic data was used to support this interpretation and the seismic profiles and sedimentological analysis. By using palynology correlated with the foraminiferal zonation, three transgressive-regressive cycles with wide regional distributions were recognized.
The biostratigraphic data obtained allowed the interpretation of palaeoenvironments, which varied from delta plain shales, marine bar sands, channels to marine shales.
Moving to the northern part of the basin (center of it) at that time is buried under the advancing accretionary prism emplaced by the collision with the Caribbean Plate ) of the basin, Moreno-Vasquez (1995) proposed four biofacies associated with Pato Oil field (Freites and Oficina Formations) representing shallow-water marine deposits (near shore to 100 m water depth) associated with seismic reflections in a wide platform. She suggested a seaward migration of palaeoenvironments to the east during the deposition of Miocene sediments in the Maturin basin (Figure 3). Her work allowed calibration in an area with a low tectonic effect (I do not understand this term) and poor seismic resolution. Also, the detailed biofacies analysis provided an excellent tool to monitor the progress of wells while they were drilled, lowering the exploration and production risk
Another important work was the reservoir characterization of the Orinoco Heavy Oil Belt of the Oficina Formation by Kopper et al. (2001). They determined that the Oficina Formation was composed of multiple depositional sequences (11), which were modeled using seismic profiles, well logs, biostratigraphy (palynology and calcareous nannofossils), geochemistry, sedimentology, and sequence stratigraphy. Their final result was a model for coastal deposition (figure 4). This integration, including vertical well sections and paleo-geographic reconstructions based on biostratigraphic data, shows that paleo-environments vary from upper delta plain to fully marine, allowing precise placement of boreholes within the stratigraphic complex reservoir, reducing the exploration risk for any well inside the Orinoco Heavy Oil Belt within this area.
All these examples from the Orinoco Oil Belt put biostratigraphy together with sedimentological and seismic data as an important tool for recognizing the geological framework of the Orinoco Oil Belt.
A little further east, at the Pedernales field (Eastern Venezuela), a similar characterization of several wells (using seismic data, well logs, and biostratigraphy) allows us to understand the eustatic cycles, linking the biostratigraphic events with the regional and global sequence stratigraphy as was demonstrated by Jones et al. 1999 (figure 5). By using high-resolution biostratigraphy coupled with log-seismic correlation, the authors recognized eleven events, both within and between the compartments of the field, with a mean resolution of 20,000 years. They also identified condensed sections by studying variations in species diversity, abundance peaks, and troughs plus biofacies bathymetric events. The high-resolution biostratigraphic-biofacies data demonstrate the existence of a bathymetric gradient at the time when the Amacuro and Perdernales reservoirs were being deposited. These results have implications for picking the reservoir top and base and also in the biosteering of multilateral wells during well site operations. This was done by using many samples for biostratigraphic analysis, interpretation of the fossil content, and linking those data with the sedimentological and seismic profiles.
Western Venezuela
High-impact palynology (HIP), defined as the coupling of high-resolution sequence biostratigraphy, multidisciplinary work, and the alignment of palynology with the attainment of business goals (regional planning and strategies, risk reduction, optimal drilling decisions and investment, petroleum-system modeling) can be substantially helpful to the oil business and was demonstrated by Rull (2002) for the Maracaibo basin. Palynology coupled with sequence stratigraphy allows enhanced stratigraphic resolution, improving regional correlations and reservoir track. The HIP could also bring alternative solutions to stratigraphic problems, including the identification of palynoblocks (facies which has the same palynomorph content that can be tracked along a seismic section or well block). Figure 6 illustrates this concept with its graphical application.
Rull (2002) showed that a “multidisciplinary study initially directed at reservoir correlation, yielded discovery after high-resolution palynological analysis and seismic and log reinterpretation. HIP has been demonstrated as a tool necessary to improve both exploration and production achievements in the Maracaibo Basin. Analogous results could be expected in other areas. The maximum efficiency of high-impact procedures is attained when the palynological analysis is carried out in-house, as part of multidisciplinary teams specially formed for each specific task”.
Outcrops, on the other hand, give the most important view of any geological setting because it adds information for other tools like sedimentology, layers thickness and changes, geochemistry, faults, type of contacts, organic matter, vitrinite, active seeps, seals and fossils. This integrated geoscience dataset is an important input for any static model that helps to understand the potential existence of a hydrocarbon system. An example of it was presented by Arminio et al. (2004) at AAPG Conference in Cancun, Mexico. In their work, they highlighted the existence of a rift-related petroleum system of the Jurassic age exposed in outcrops along the Perijá and Mérida Andes (Venezuela).
Detailed fieldwork including structural, sedimentological, geochemical, and palynological analyses were performed. In the study area, the La Quinta is approximately 500m thick, with the lower 240m made of alluvial fan, fluvial channel, and overbank red bed-type deposits followed by 200m of dark, organic-rich lacustrine shales and micrites that contain ostracods and sporomorphs of Tithonian age. This lacustrine section is capped by 60m of shallow marine shales with bryozoans, also of the Tithonian age (Figure 7). This section is inferred to be in angular relation with the Cretaceous of the Maracaibo Basin. The lacustrine and marine shales showed good to very good and fair to good source rock quality, respectively. The whole succession, as well as the overlying Cretaceous, is in the gas preservation window (Hernandez, 2003). In a regional context, these organic-rich La Quinta facies compare with the Cuiza marine shales of the Colombian Guajira (Renz, 1959) and the Manari lacustrine shales of the Takutu Graben (Crawford et al., 1985).
This work explores the presence of source rocks from the Jurassic age that may have formed effective petroleum systems additional to those already known in the Cretaceous and Paleogene successions, extending the exploration models in northern South America to a new frontier.
The study of unconventional oil reservoirs is one of the essential trends in the search for increased reserves in many countries. In La Luna Formation, the search for unconventional gas shale has created up several projects in Universidad de Los Andes, Venezuela (Liborius-Parada and Slatt 2016). This research focuses on the outcrop of the La Luna Formation that has good properties and conditions for exploration, even though it is known for its heterogeneity and complex structure. As demonstrated by Liborius-Parada and Slatt (2016), the integration of outcrop studies together with lithological, mineralogy, age, facies, and sedimentological and geochemical analysis plus core descriptions of wells available created a dataset for the characterization of unconventional resources in the Maracaibo Lake area. Their results show variation in the kerogen at different maturity levels in their relationship with system tracks interpreted from sedimentological/outcrop and core analysis with the correlation to the ages and palaeoenvironments of deposition (Figure 8). Because of the complexity of the La Luna Formation, this approach brings more details about organic matter and its thermal maturity within La Luna, its relationship with third-order sequences (Slatt 2013). Fossiliferous facies identified by using foraminifera plus ichnofossil traces observed in outcrops and cores helped to identify deposits of the Highstand and Transgressive system track for correlation to the worldwide Cretaceous oceanic event.
As the above examples illustrate, a cost-effective exploration and development program requires a geologic framework, a key component of which is biostratigraphic data that identifies correlates and characterizes reservoirs, sources, and seals. Biostratigraphic data can detect through the analysis of depositional environments the processes that result in changing lithofacies, allowing a predictive model to be constructed of reservoir distribution. Although the quality of seismic data and the ease of its interpretation in 3D has significantly improved over the last 50 years, it still can lack resolution, include insufficient data, be misinterpreted, and apply the wrong velocity model. Biostratigraphy remains a powerful tool in structurally complex areas where these shortcomings are most acute. Biostratigraphic study at the well site during operations has a well-documented record of cost reduction, giving early warning of deviations from prognosis and allowing adjustments to drilling objectives in real-time. From my experience, as a wellsite biostratigrapher, in situ analysis of biostratigraphic samples help to refine seismic interpretation by identifying changes in age and palaeoenvironments that seismic data miss because of lack of resolution, poor data, misinterpretation and the application of the wrong velocity model. The biostratigraphic study also helps to identify minor faults, repetitions, or even big structures like domes, that sometimes could be missed during the seismic interpretation process, and of course, is a vital tool to save money during the drilling process by helping to reach the objective with maximal precision with minimal cost.
The large amount of research that has been done with biostratigraphy in Venezuela for the last 50 years cannot be summarized in one report, but there is a huge number of samples (analyzed but not published), publications, and companies reports that support the importance of biostratigraphy for the country and their search for oil reservoirs on and offshore that will need to be incorporated into any initiative to rescue the Venezuelan Oil industry.
References
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