Application of Microfossils in Various Studies | Geology Optional Notes for UPSC PDF Download

Biostratigraphic applications in hydrocarbon exploration

• Biostratigraphy focuses on studying the distribution of fossil organisms throughout geological time periods, emphasizing the concepts of evolution and extinction.
• Index fossils, crucial in biostratigraphy, are easily identifiable, abundant, widely spread, and exist within specific time frames.
• Fossils vary in preservation, ranging from whole organisms to partial remains, influenced by factors like fossil nature, preservation mechanisms, and rock diagenesis.
• Fossils are categorized by size into macrofossils (visible to the naked eye), microfossils (0.001mm to 1mm in size), and nannofossils (5-60 μm in size).
• Biostratigraphy aids in identifying rock formations, correlating time-equivalent units over vast distances, disregarding lithological variations due to environmental changes.
• Its significance in petroleum exploration lies in determining sedimentation ages, reservoir characteristics, correlations, and well site operations through field-specific bioevents testing.
• Additional applications include deducing paleoenvironment, paleobathymetry, evaluating thermal maturity, and identifying self-generating source rock strata.

Factors Controlling the Abundance of Fossils

• Throughout the geological record, fossils serve as indirect indicators of past environments. They offer insights into environmental conditions (paleo-indicators).
• Fossils are sensitive to environmental changes and can only thrive within specific ranges of environmental parameters.
• Recent research has established a significant relationship between the abundance and diversity of fauna and various environmental factors in depositional lines along dip and strike. These factors include:
  • Water depth
  • Temperature
  • Salinity
  • Oxygen levels
  • Nutrient availability
  • Light availability
  • Proximity to land
  • Topographic variations
• Changes in these environmental parameters directly influence the diversity and abundance of fossils, allowing scientists to reconstruct environmental histories based on fossil records.

Applications

• Age: Biostratigraphy is commonly used for relative age dating in the oil industry. This method relies on the presence, overlap, or absence of specific index fossils with short and unique life spans. Dating is typically determined by the first downhole occurrence (FDO), while the last downhole occurrence (LDO) can sometimes be inaccurate due to mechanical factors.
• Identifying Unconformity and Structures: Biostratigraphy plays a crucial role in reconstructing subsurface geology in the oil and gas sector. It helps in identifying missing or repeated geological sections by analyzing the absence of specific biozones. Detailed sampling allows for determining the duration of hiatuses and missing sections, aiding in identifying unconformities critical for hydrocarbon exploration.
• Sequence Stratigraphy: Biostratigraphy is essential for correlating lithofacies of similar or different ages, which helps in understanding changes in sea level and identifying sequence boundaries and systems tracts. This integration with seismic and well logs data aids in stratigraphic trap evaluation.
• Sequence Boundary: This chronostratigraphic surface indicates a relative sea-level fall. Biostratigraphy can help resolve hiatuses and erosional periods caused by significant sea-level drops, but it may face challenges in resolving smaller magnitude falls. Changes in fossil assemblages across this boundary indicate shifts in paleoenvironment.
• Lowstand Systems Tract: Marks a relative decrease in sea level and is characterized by a shift in facies towards the basin. Biostratigraphy records abrupt changes in assemblages from marine to non-marine/shallow marine, such as shifts in pollen to dinoflagellates ratios in palynology.
• Transgressive Systems Tract: Marks a relative sea-level rise, resulting in deepening and basinward shifts. Biofacies transitions from terrestrial to shallow marine to deeper marine assemblages in a complete vertical succession.
• Maximum Flooding Surface: Represents the peak marine inundation resulting from overall deepening. This condensed section in biostratigraphy offers rich fossil records, particularly of planktonic foraminifera and deep-water benthic organisms, facilitating dating and correlation across basins.

Paleoenvironment

• Organisms in different habitats
• Ecological dependence on factors like temperature, salinity, oxygen levels, and water depth
• Calcareous nannofossils in marine settings
• Dinoflagellates in various marine environments
• Palynology importance in defining depositional environment
• Characterizing and predicting facies distribution for hydrocarbon exploration
• Geometry and quality inference in carbonate reservoirs
• Distinguishing fore-reef or back-reef in carbonate models
• Understanding marine vs. continental influence in sandstone reservoirs

Paleobathymetry

• Micro and macro-fossils as paleobathymetry indicators
• Use of benthic foraminifera for paleobathymetry inference
• Planktonic foraminifera as palaeoenvironment indicators
• Ratios between different foraminiferal assemblages for analysis

Understanding Well Site Biostratigraphy

• Wellsite biostratigraphy plays a crucial role in drilling operations by integrating with lithological descriptions, log correlations, and seismic interpretations.
• It aids in decision-making processes such as planning well paths, setting casing points, determining overpressure zones, and establishing total depth requirements.
• Assigning ages to strata is essential for confirming the stratigraphic position, achieved through bioevents like First Downhole Occurrence (FDO) and Last Downhole Occurrence (LDO) calibrated to their age limits.
• Enhancing the number of footage drilled within a reservoir body in horizontally drilled wells is facilitated by characterizing fossil content above and below the reservoir body.
• If the fossil assemblage above the reservoir indicates a specific stratigraphic position, drillers are guided to proceed stratigraphically downwards.

Implications of Source Rock Potential and Thermal Maturity

• Palynology is widely utilized to quantify and evaluate the characteristics and thermal maturity of source rocks.
• Palynofacies analysis involves describing the total kerogen content of palynological residues, identifying individual palynomorphs, plant debris, and amorphous components, and assessing their proportions, size spectra, and preservation states.
• Structured terrestrially-derived organic debris from land plants is linked to gas generation, while amorphous organic matter and algal palynomorphs are associated with oil-prone kerogen.
• Palynology, in conjunction with other disciplines, aids in evaluating the thermal maturity of organic-rich sediments, with color changes in palynomorphs and organic matter serving as indicators.