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10 May 2015

The Predictive Value of Basement

Over 70% of existing assets in the deepwater Gulf of Mexico are associated with basement structural highs or faults indicating the predictive value of basement studies.

IGC’s Definition of Basement

The geologic term “basement“ can be defined in several different ways. What is equated to “geologic basement” generally is a specific deep-sequence boundary. This boundary will vary geographically; for example, in the Northern Gulf Province “geologic basement” is considered by some authors to be a mid-Jurassic sequence boundary (MJS) below which lie high-density formations of various lithologies. While that definition has important geologic meaning, it does not fully describe the potentially thick wedge of unaltered, relatively nonmagnetic older sediments overlying magnetic or crystalline “basement.”

“Acoustic basement” is a term used for the surface below which little if any seismic energy can penetrate. Locally, it could be equivalent to geologic basement, magnetic basement, or even some other surface.

IGC includes the following as a part of the definition of “Magnetic Basement”:

  • Top Precambrian metamorphic and igneous rocks
  • Top Paleozoic metamorphic and igneous rocks
  • First deep high-velocity refractor (5.4 to 7.0 km/sec)

 

Basement structure

Importing the basement structure surface into basin modeling techniques has proven to be of consequence in the proper determination of heat flow, maturation analysis and hydrocarbon migration pathways. A basement surface determined from a second science instead of importing a seismically-derived surface also avoids the possibility of circular reasoning.

The basement structure map provides a unique opportunity to interpret structural style of a basin. Correlation made between basement structure and sediment warping has been shown to explain relationships between basement structure and oil and gas distribution in the region.

IGC’s Magnetic Basement Structure Map also remains a seismically, unbiased interpretation providing the explorationist a second opinion to the possible structure underlying his prospect. The integration of the magnetic basement structure and seismically-derived sedimentary structure for a specific area is left to the explorationist.

 

Even deeper structure is proving to be important

New tectonic models and recent basin restorations indicate that exploration must take into account the Moho discontinuity as well as the basement structure itself. Current exploration is investigating the characteristic types of crust: shield, platform, orogenic belt, continental rift, marginal sea, and ocean basin. A complete earth model requires both basement and Moho structural horizons in order to build viable in-situ interpretations.

Gravity and magnetic data “see” much deeper than conventional reflection seismic data, and are used to constrain upper crustal thickness and composition, magnitude of crustal extension, and the upper crustal (radiogenic) contribution to heat-flow for basin modeling. Gravity and magnetic data record lateral variations in subsurface density and magnetic susceptibility, respectively, with wavelength of the observed signal indicating depth of source body.

Crustal types for a locale are identified by the interpretation process based on magnetic basement structure integrated with seismic refraction and gravity data.

 

Basement interpretation in action

You may find it interesting to see basement interpretation in action in this article – – Basement Structural Analysis from Oil & Gas Reporter.