Locating the presence of oil and gas deposits undergorund is a complex process spanning many months of preliminary research followed by exploration and development drilling. Potential sites for exploration are identified from seismic studies but full evaluationcan only be made by drilling wells to see what is actually there.

Formation Evaluation can be grouped into four categories:

Before Drilling
Seismic Interpretation
Offset Data

During Drilling
Mud Logs and Wellsite Geology
Measurement While Drilling (MWD)
Coring

Post-Drilling
Wireline Logs
Production Tests

Petrophysical logging enables large sections of exposed (and sometimes cased) hole to be scanned and variety of geological and reservoir data to be obtained; quantitative analysis can be performed on the data to supplement other information. Historically, petrophysical logging has been called "Wireline Logging", or even "Electric Logging" but neither of these terms adequately describe the current range of logging tools or conveyance methods.

Data Interpretation

Data processing is almost always done by computer, typically in town but increasingly using modern high powered computers at the wellsite. Basic information can be derived by hand using Quick-Look or Shaly Sand methods or by using relatively simple spreadsheets or other processing software.

Types of Logs

Many different types of logs, measurinng various rock properties may be run at each casing point. Generally the first and intermediate logging runs are performed for lithogical evaluation and stratigraphic correlation purposes. Minor hydrocarbon bearing zones may also be identified, together with possible source rock information.

Over the main reservoir section, the amount of information required is much greater and a full suite of logs covering lithology, porosity, permeability and fluid saturations are required.

Additionally there are many other types of tools available for specific purposes, and of helping with the evaluation of cement jobs and other completion operations. The major logs used for routine evaluation of open hole sections are:

Lithology Logs
Gamma Ray
Spontaneous Potential

Resisitivity (Saturation) Logs
Laterologs
Induction Logs
Wave Propagationn Logs

Porosity Logs
Formation Density Log
Neutron Porosity Log
Sonic Log

Miscellaneous
Caliper
Dipmeter
Repeat Formation Tester
Sidewall Cores
Cement Bond Logs