A Bit of Geology from Seismic

When a geologist or geophysicist wants to interpret the geological setting of a sedimentary basin being studied on seismic, what he/she first observes is the nature and behavior of the reflections through the whole seismic survey, and how they interrelate.

From the sedimentological point of view, sedimentary reflections on seismic can tell us about the depositional environment and regimes, because they mainly represent bedding planes corresponding to conformable changes in these aspects, meaning that we can extract information on:

  • Energy Level during deposition
  • Sedimentation Rates
  • Environment of Deposition
  • Input Source
  • Degree in diagenesis
  • Pore Contents (fluid fill and cement)

This is based on the assumption of a sufficient difference on acoustic impedance (velocity x density) causing the reflections to show us the difference in order to be able to detect them at the seismic scale. Let’s remember that the difference in acoustic impedance occurs when a reflection changes its level of energy because the sedimentary layers being represented by them,  are located next to layers formed under different circumstances and from different materials and therefore, will reflect the variations on the travel velocities of the acoustic waves and the densities (change on materials).

We can observe and  study features like the Reflection Configuration (geometrical relationships between reflections), the Reflection Continuity (which describes continuity of layers and therefore helps to synthetize a depositional environment), the Reflection Amplitude (provides measures for reflection strenghth which is directly related to lithology contrasts, bedding spacing and fluid contents), and the Reflection Frequency (for estimations of bed thickness and possibly fluid contents).

By doing this, we can interpret and “divide” the seismic reflections in a wider context, into external and internal form of groups of reflections, many times referred as Seismic Facies.

When we define the seismic facies, it is much easier to have a better idea and be more certain of the possible choice of a depositional environment that could have created all these layers being reflected, and therefore, a spatial idea of there the input source and the complete geological setting for a better definition of the possible parts of the petroleum system being recorded.

facies

The essential elements of a petroleum system include the following:

• Source rock
• Reservoir rock
• Seal rock
• Overburden rock

Petroleum systems have two processes:
• Trap formation
• Generation–migration–accumulation of hydrocarbons

These essential elements and processes must be correctly placed in time and space so that organic matter included in a source rock can be converted into a petroleum accumulation.

A petroleum system exists wherever all these essential elements and processes are known to occur or are thought to have a reasonable chance or probability to occur.

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