Oil and Gas Reservoir Formation

Introduction to Reservoirs

• Reservoirs evolve from ancient environments:
  • Rivers, deltas, lakes, etc.
  • Example environments include:
    • Alufiel Fan
    • Braided River
    • Fan Delta
    • Continental Shelf
    • Submarine Fan
    • Continental Slope
• These environments have distinct properties due to their depositional backgrounds.

Importance of Understanding Depositional Environments

• Different origins lead to different reservoir properties:
  • River vs. lake reservoirs exhibit different characteristics.
• Collaboration with specialists is essential:
  • Reservoir engineers must discuss with geologists, petrophysicists, and geophysicists to understand reservoir character.

Reservoir Formation Process

Accumulation and Transformation

• Formation begins with:
  • Accumulation of mud, sand, land plants, and animals over time in a basin.
• Organic deposits accumulate, eventually becoming potential source rocks.
• Geological processes contribute to:
  • Overlying layers of sand that compact the organic material.

Hydrocarbon Generation

• Source rock transforms under pressure and heat:
  • Organic material "cooks" to form oil and gas.
• Migration process:
  • Oil and gas migrate upward due to permeability through channels.
• Trapping mechanism:
  • Oil and gas are trapped by seal rocks, which have low permeability.

Reservoir Structure

Components of a Reservoir

• Seal Rock: Prevents escape of hydrocarbons, leading to accumulation.
• Gas Cap: The top layer of gas above the oil layer.
• Oil Lake: The layer containing the oil.
• Water Lake / Aquifer: The bottom layer containing water.
• Important terms:
  • Gas Oil Contact: Boundary between gas and oil.
  • Oil Water Contact: Boundary between oil and water.

Density and Pressure Relationships

• Different substances have different densities which affect:
  • Vertical arrangement:
    • Gas at the top, followed by oil, then water at the bottom.
  • Pressure gradients:
    • Varying gradients due to density differences:
      • Gas has a steeper gradient.
      • Oil has a moderate gradient.
      • Water has a mild gradient.
• Hydrocarbon migration:
  • Oil rises due to being lighter.
  • Water is pushed down due to being heavier.

Summary

• Understanding the formation and structure of oil and gas reservoirs is critical for effective reservoir engineering and management.