Post Details

18 May 2015

Multi Science

Integrated, multi-science interpretations now represent the state of the art because of the detail and confidence they provide

Geophysics has proven its value in both identifying areas of interest within a broad region and in clarifying structure in specific areas. In some situations the only way to get a reliable picture of a prospective structure is through a  multi-sciences interpretation.

Basement

Sixty-three percent of existing assets in the deepwater Gulf of Mexico are associated with basement structural highs or faults. This indicates 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 is the foundation

IGC believes in the premise that basement is the foundation of a basin. In essence, basement is the bedrock on which much of the dynamics for hydrocarbon accumulation and/or migration rest. Potential Fields have the ability to define basement in a way that seismic data cannot.

The utilization of the Potental Fields methods is becoming even more important because new tectonic models and recent basin restorations indicate that exploration must take into account the Moho discontinuity as well as the basement itself. A complete earth model requires these deeper horizons in order to build viable in-situ interpretations.

Basement structure surface

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 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.

 

Potential Fields show structure where seismic can’t see well

In some places—especially salt/sedimentary and igneous/sedimentary—interfaces, seismic cannot resolve structure well. In these areas, gravity/magnetics interpretations are critical to high-confidence decisions.

 

We come from major energy corporations

IGC professionals come from major energy corporations and understand that clients are looking for interpretations they can use to make decisions. That’s why we deliver actionable advice – definitive interpretations, not academic studies. About 60% of this work is assessing new plays and frontier areas. The 40% is verifying or supporting a seismically-derived interpretation. Here are some of the major applications of our multi-science interpretations.

Regional analysis to identify areas of interest

IGC delivers viable information about basement rock, tectonics, sediment thickness, migration pathways and structure. Clients often use our results to make strategic decisions about where to invest their exploration dollars. Potential Fields data can deliver a 1st pass interpretation of a region and target areas of interest in preparation for seismic acquisition. The cost of regional seismic data is greatly reduced.

Visualize structure when seismic technologies don’t work

Integrated gravity/magnetics interpretations guided by IGC’s extensive experience and proprietary techniques, can help clients reduce risk by verifying deep seismically-derived structure.

  • Resolving structure under low-density section e.g., salt
    • Both magnetic and gravity interpretations contribute to higher-confidence structural mapping, well placement and pay zone depth definition.
  • Resolving structure at the sedimentary/igneous interface
    • Magnetic and gravity interpretations can help identify structures and traps below igneous interfaces such as basalt flows.

Guidance to increase the value of proposed seismic studies

Gravity/Magnetic interpretations can be of enormous value in identifying structures prior to seismic studies. That information can assist clients place survey lines for higher resolution of important structures.

Precise structural models utilizing 3D gravity and tensor analysis

Gravity surveys can now record changes in gravity in all three dimensions. This data is called gravity TENSOR data. Gravity tensor data provides much higher resolution than conventional gravity data. Tensor data can be used to constrain seismic data that has been diffused or absorbed by salt or igneous rock formations (see above). In many geologic environments the result is a much more reliable, higher-resolution structural model.

Outsourcing the interpretation function

Some clients use IGC as their potential fields (gravity and magnetic) department. Some recommended best practices can strongly increase the benefits of this strategy.

What magnetics and gravity see

The two geophysical disciplines that are integrated geologic interpretations for petroleum exploration are the study of variations in the earth’s magnetic field—generally referred to as MAGNETICS; and the study of variations in the earth’s gravity—generally referred to as GRAVITY. (The Glossary link – in the Library and the footer –  explains much of the technical language you may find on this website.)