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A petroleum geologist is an earth scientist who works in the field of petroleum geology, which involves all aspects of oil discovery and production. Petroleum geologists are usually linked to the actual discovery of oil and the identification of possible oil deposits, gas caps, or leads. It can be a very labor-intensive task involving several different fields of science and elaborate equipment. Petroleum geologists look at the structural and sedimentary aspects of the stratum/strata to identify possible oil traps or tight shale plays.

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Petroleum geologists make the decision of where to drill for petroleum. This is done by locating prospects within a sedimentary basin. Petroleum geologists determine a prospect's viability looking at seven main aspects in conventional petroleum geology:

These seven key aspects require the petroleum geologist to obtain a 4-dimensional idea of the subsurface (the three spatial dimensions, plus time). Data may be obtained via geophysical methods. Geophysical surveys show the seismology data of elastic waves, mainly seismic reflection. This provides a 3-dimensional look of the trap, and source rock. More data may be obtained from the mudlogger, who analyzes the drill cuttings and the rock formation thicknesses.

Today, there are also unconventional tight plays. Petroleum geologists for these plays work with petroleum engineers and other specialists to make decisions of where to drill for oil. Data is also obtained via geophysical methods (the same as conventional plays, plus fracture data), but these are modernly analyzed with various statistical methods. The geological analysis is done by looking at a combination of geological aspects, with completion analogs. The geological aspects are as follows:

The 'trap' aspect is absent. Tight shale plays, or unconventional plays, do not require a trap to contain hydrocarbons due to the low permeability preventing further migration.

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