Direct Indications & Exploration
Natural seepages of this substance at the surface were among the earliest clues that hydrocarbons existed below ground.
What is oil?
Lines on a map that connect points of equal elevation or depth.
What are contour lines?
The type of rock in which almost all oil and gas accumulations occur.
What are sedimentary rocks?
This term refers to the percentage of empty space within a rock.
What is porosity?
In hydrocarbon pools, this fluid is found at the top because it is the least dense.
What is gas?
These gas-related surface features, including mud volcanoes, can hint at subsurface hydrocarbons.
What are gas seepages?
These maps illustrate the thickness variations of stratigraphic units.
What are isopachous maps?
These rocks form from sediments such as sand, silt, clay, and gravel.
What are clastic rocks?
This property describes how easily fluids can move through rock pores.
What is permeability?
Hydrocarbons accumulate only if this type of barrier prevents upward escape.
What is a trap or impervious seal?
This type of geologist interprets subsurface stratigraphy and structure to pick drilling locations.
What is a petroleum geologist?
Maps that show elevation of subsurface layers relative to sea level are called this.
What are structural contour maps?
Limestone, dolomite, and salt belong to this group of sedimentary rocks.
What are nonclastic rocks?
An impermeable rock layer that prevents upward hydrocarbon migration is known as this.
What is a seal or cap rock?
This upward-arched fold is one of the most common structural traps for oil and gas.
What is an anticline?
These deep holes drilled primarily for information, not production, provide subsurface cores for evaluation.
What are stratigraphic test wells (strat tests)?
These diagrams represent a vertical slice through the earth showing the arrangement and thickness of strata.
What are cross sections?
These rocks, altered by heat or pressure, generally cannot store oil.
What are metamorphic rocks?
The rate of flow through porous rock depends on permeability, fluid viscosity, pressure differential, and this physical dimension.
What is cross-sectional area or flow length?
These structures form when salt intrudes upward, deforming surrounding strata and creating traps at the flanks.
What are salt domes?
Although useful, these direct surface clues do not confirm hydrocarbons in commercial quantity.
What are natural seepages and outcrops?
These maps depict topographic trends of a single stratum using contour lines.
What are stratigraphic maps?
These are the two primary rock types involved in hydrocarbon generation and storage: one generates hydrocarbons, the other stores them.
What are source rocks (shale/limestone) and reservoir rocks (sandstone/limestone/dolomite)?
This combination of reservoir characteristics ultimately determines a reservoir’s ability to contain and produce hydrocarbons.
What are porosity and permeability?
This trap type occurs when reservoir rock loses porosity or changes lithology up-dip, causing hydrocarbons to terminate against nonporous rock.
What are stratigraphic traps?