The purpose of subsurface mapping in the geology of petroleum Is to find traps that contain oil and gas pools and the information obtained from wells forms the heart of the data upon which subsurface geology depends, other information are obtained from :
- Geophysical surveys.
- Pressure and temperature surveys.
- The production history of producing oil and gas pools.
Structural Maps and Sections
Subsurface structures may mapped on any formation boundary, unconformity, or producing formation that can be identified and correlated by well data. Structure may be shown by contour elevation maps or by cross-sections.
Isopach maps show by means of contours the varying thickness of the rock intervening between two reference planes commonly bedding planes or surfaces of unconformity. Isopach maps offer a simple method of showing the distribution of a geological unit in three-dimensions (3D) thickness of individual formations of reservoir rocks of groups of formations of intervals between unconformities and a normal stratigraphic contact or formation boundary, may be mapped in this manner.
Isopach Maps are used to:
- Determine the time of faulting and folding.
- The time of traps formation in regional studies.
- Development of a pool, especially in showing the thickness of the pay formation.
An isochore and isopach map are the same only when both the top and bottom surfaces of the layer shown are horizontal. When the layer shown is inclined, as is usually the case, the thicknesses displayed in an isochore map of the layer will be greater than the thicknesses displayed in an isopach map of the same layer. Unfortunately the terms isopach and isochore are widely confused, and many times maps of True Vertical Thickness (TVT), which by definition are isochore maps, are incorrectly labeled isopach maps.
There are several kinds of facies maps, but the most common type used in petroleum geology is "Lithofacies Maps" which distinguish the various lithologic types rather than formation.
Paleo-geologic and Sub-crop Maps
Paleogeology may be defined as the science that treats the geology as it was during various geologic periods. A map in the past that shows the paleogeology of an ancient surface is called a paleogeologic map.
A subcrop map is a paleogeologic map in which the overlying formation is still present where as a paleogeologic map shows the formation boundaries projected in part into the area from which the overlying formation has been eroded.
Internal Property Maps
Maps detect the characteristics of a single unit and its shape. It includes many types:
- Iso-porosity maps.
- Iso-volume or iso-vol. maps.
- Iso-concentration maps.
- Iso-hydrocarbon maps.
- Iso-bar maps
- Iso-potential maps.
- Water- encroachment map.
These maps depend on geophysical anomaly (such as local variations or irregularity in the normal pattern) which after correction may be attributed to some geologic phenomena.
These maps are used for mapping various kinds of chemical analysis of rocks and their fluid contents. It may show the surface distribution of hydrocarbons where those hydrocarbons are found at the surface in large amounts than normal indicating that there is a seepage of oil or gas.