Special field techniques
| Exploring for Oil and Gas Traps | |
| |
| Series | Treatise in Petroleum Geology |
|---|---|
| Part | Predicting the occurrence of oil and gas traps |
| Chapter | Value of geological fieldwork |
| Author | Denise M. Stone |
| Link | Web page |
| Store | AAPG Store |
Global positioning system positioning
Global positioning system (GPS) devices offer a handy way to find your location with respect to surface maps. Latitude and longitude coordinates are read from a lightweight, hand-held device that determines position by triangulating with satellites overhead. Coordinates are particularly useful when making numerous road stops and can be input into a computer for plotting.
Outcrop gamma-ray logging
Outcrop gamma-ray logging can be a valuable technique for stratigraphic correlation. It allows a quantitative tie between surface and subsurface using a measurement common to both: the gamma-ray curve. This kind of tie can be more credible than “jump correlating” (identifying events on noncontiguous seismic records or lithologic horizons on well logs, separated by distance, as the same or correlative interfaces in the earth), particularly when significant distance is involved.
Two methods of outcrop gamma-ray logging are currently in use:
- Truck-mounted gamma-ray sonde
- Hand-held scintillometer
These tools measure the surface gamma radiation signature emitted by sedimentary outcrops. The data collected can be used to compare and correlate to subsurface sedimentary sections whose radiation signature is measurable only with sophisticated downhole wireline logging tools. (See Basic open hole tools.) Gamma radiation is the most commonly used data set to correlate stratigraphy laterally.
Shallow coring
Surface outcrop samples of a given formation are assumed to be the best available example of that same formation at depth. If this isn't the case, though, another approach exists. A shallow coring program is a cost-effective way to obtain surface geologic data in areas of limited outcrops, severe surface weathering, thick vegetation, and/or thin alluvial cover. A seismic shot hole or water well drilling rig can be strategically positioned for collecting such samples.
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