Importance of structural studies in location of an oil field.

Structural Features and Hydrocarbon Traps

Structural traps are the most common type of hydrocarbon trap. They are formed by the deformation of rock layers, creating a configuration where hydrocarbons can accumulate. Key structural features analyzed during oil field location include:

• Folds: Anticlines (upward folds) are classic structural traps. The crest of an anticline forms a high point where hydrocarbons, being lighter than water, migrate and accumulate. Synclines (downward folds) are generally not traps but can be important in understanding regional structural trends.
• Faults: Faults are fractures in the Earth’s crust where rocks have moved relative to each other. Faults can create traps in several ways:
  • Fault blocks: Downthrown blocks can be sealed by impermeable fault gouge, trapping hydrocarbons.
  • Hanging wall/Footwall traps: Hydrocarbons can migrate along a fault plane and become trapped against an impermeable layer.
• Unconformities: These represent surfaces of erosion or non-deposition, separating rock layers of different ages. Unconformities can create traps where hydrocarbons migrate upwards and are trapped beneath the unconformity surface.
• Salt Domes: The upward movement of salt creates structural features like anticlines and faults, forming significant hydrocarbon traps.

Methods of Structural Analysis

Modern oil exploration utilizes a range of techniques to analyze subsurface structures:

• Seismic Reflection: This is the primary method. Seismic waves are sent into the Earth, and the reflections from subsurface layers are recorded. These reflections are used to create images of subsurface structures. 3D seismic surveys provide a more detailed and accurate picture than 2D surveys.
• Well Logging: Measurements made in boreholes (wells) provide information about rock properties, including porosity, permeability, and lithology. These logs help to correlate seismic data with actual subsurface conditions.
• Structural Interpretation: Geologists interpret seismic data and well logs to identify and map subsurface structures. This involves identifying faults, folds, and other deformational features.
• Geomechanical Modeling: This involves creating computer models to simulate the stress and strain within the Earth’s crust. This helps to predict how rocks will deform under different conditions, aiding in the interpretation of seismic data.

Case of the Campos Basin, Brazil

The Campos Basin, offshore Brazil, is a prime example of how structural studies led to significant oil discoveries. Pre-salt reservoirs, located beneath a thick layer of salt, are trapped in complex structural settings created by salt tectonics. Detailed seismic surveys and structural interpretation were crucial in identifying these reservoirs, leading to the development of one of the world’s largest oil provinces.

Structural Feature	Impact on Oil Accumulation
Anticlines	Forms a convex upward trap, allowing hydrocarbons to accumulate at the crest.
Faults	Creates impermeable barriers, sealing hydrocarbons in fault blocks or against impermeable layers.
Unconformities	Provides a sealing surface, trapping hydrocarbons migrating upwards.