Geologic Map

Key Takeaways

• The construction of a geologic map requires systematic field work following a standard procedure: the geologist traverses the mapped area along planned transects that cross the major rock units and structural boundaries, records the location and lithology of each exposure (outcrop) encountered, measures the dip angle and azimuth of bedding or foliation using a Brunton compass or digital inclinometer-compass, records the contact between different rock units wherever the contact is exposed, and collects samples for laboratory analysis (petrography, geochemistry, geochronology); these field observations are plotted on a base map (typically a topographic map at 1:50,000 to 1:25,000 scale for regional mapping, or 1:10,000 to 1:5,000 for detailed mapping) and the geological contacts are traced between exposed control points by applying the rule of Vs (geological contacts curve uphill in a V shape that points in the dip direction of the beds, allowing the subsurface geometry to be inferred from the map pattern of contacts crossing topographic ridges and valleys); the result is a two-dimensional representation of the three-dimensional rock structure that would be exposed at the surface by stripping away all soil and vegetation, with the geological cross-section (a vertical slice through the mapped area drawn perpendicular to the dominant strike direction) used to display the subsurface geometry inferred from the surface map pattern and the dip measurements.
• Petroleum-relevant information extracted from geologic maps includes the identification of surface anticlines (which may indicate subsurface structural traps at depth if the surface fold extends to the reservoir level), the mapping of fault traces (which may define the lateral seals or structural boundaries of petroleum accumulations at depth), the identification of oil seeps and bitumen-impregnated outcrops (which indicate that a petroleum system is active in the basin and that hydrocarbons have migrated to the surface along faults or unconformity surfaces), the distribution of potential source rock formations at the surface (from which the subsurface source rock distribution can be predicted by following the map units down into the basin interior where they are buried), and the confirmation of regional structural trends (fold axis orientations, fault strike directions) that guide the placement of seismic survey lines and the interpretation of subsurface structure from the seismic data; in frontier basins where no wells have been drilled and no seismic data is available, the geologic map of the surface geology is often the primary data source for the first petroleum exploration assessment of the basin's potential.
• Subsurface geologic maps are produced from well log, seismic, and formation pressure data rather than from surface observations, and represent the distribution of rock types or structural configuration at a specific subsurface depth or on a specific stratigraphic surface: a structure contour map (the most common subsurface geologic map in petroleum engineering) shows the elevation of a picked seismic reflector or log marker horizon as contours of equal depth, with the map pattern of closed contours identifying structural highs (anticlines, domes) that are potential petroleum traps; an isopach map shows the thickness of a formation or reservoir unit between two correlation markers, revealing thinning and thickening patterns that reflect depositional geometry (paleochannels, delta lobes, clinoforms) or differential compaction; a facies map shows the lateral distribution of different facies (rock types with characteristic depositional environments and reservoir properties) within a formation, identifying which parts of the mapped area have reservoir-quality rock versus non-reservoir-quality rock; these subsurface geologic maps are the primary products of the interpretation of subsurface data and form the basis for reserve estimation, well location planning, and development strategy decisions.
• Digital geologic mapping using GIS (Geographic Information Systems) software has transformed the production and use of geologic maps since the 1990s: ArcGIS, QGIS, and specialized geological mapping software (GeoMapper, MOVE, FieldMove) allow geologists to capture field observations as geo-referenced digital points and lines, automatically compute strike and dip statistics for each mapped unit, trace geological contacts using satellite imagery and LiDAR data layers, and generate map outputs at any scale and in any projection on demand; the digital map database stores all field observations with their geographic coordinates and attributes (lithology, age, dip, contact type, sample number) in relational database tables that can be queried, analyzed, and updated as new data is collected; the integration of digital geologic maps with subsurface well log and seismic databases in petroleum industry GIS platforms (Petrel, Kingdom, Landmark) allows direct comparison of surface geological observations with subsurface data in a common geographic reference frame, enabling consistent structural and stratigraphic interpretation from surface to reservoir depth in a single integrated workflow.
• National and regional geological survey organizations produce and maintain geologic maps of their jurisdictions at standardized scales and with defined geological classification systems: the United States Geological Survey (USGS) maintains the National Geologic Map Database (NGMDB) and has produced digital 1:24,000 scale geologic maps for much of the United States through the STATEMAP and FEDMAP programs; the Geological Survey of Canada (GSC) maintains national and provincial scale maps; the British Geological Survey (BGS), the Bundesanstalt fur Geowissenschaften und Rohstoffe (BGR) in Germany, and the Bureau de Recherches Geologiques et Minières (BRGM) in France perform equivalent functions for their national territories; the compilation of these national geological maps into unified continental-scale databases (the One Geology initiative, which has assembled geologic maps from 130 countries into a web-accessible global geologic map) provides the regional geological context for petroleum basin assessments, carbon capture and sequestration site characterization, mineral resource assessments, and geotechnical hazard mapping on a global scale.