Petroleum Geology and Rock Properties Review

Comprehensive flashcard set covering rock properties, rock-forming processes, weathering and erosion, the petroleum system framework, and topographic/structure contour map interpretation.

What five key rock attributes should a petroleum engineer identify when presented with a rock's name?

The rock's basic chemical composition, process of origin, potential environment of deposition, key physical properties (such as porosity and permeability), and its role as a source, reservoir, or seal rock.

What is the petroleum engineering significance of halite?

Halite (rock salt) is an evaporite formed in arid environments with high evaporation rates. Because it lacks void space and permeability, it cannot serve as a reservoir rock, but it acts as an excellent reservoir seal.

What percentage of the world's oil and gas fields produce from sandstone reservoirs?

Approximately $80\%$ of the world's oil and gas fields produce from sandstone reservoirs.

How do petroleum engineers typically identify downhole rock types without physical samples?

They use well logs, which measure downhole physical properties such as natural gamma radiation, electrical resistivity, bulk density, and neutron count rates.

Why are igneous and metamorphic rocks generally not of primary interest in petroleum systems?

They generally do not contain organic matter necessary to generate hydrocarbons, nor do they possess the primary void space needed to store and transmit oil and gas.

What is a mineral according to geological definitions?

A naturally occurring, solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure.

What is the geological definition of a rock?

Any naturally occurring solid mass or aggregate of minerals or mineraloid matter, which can also include organic matter.

What is a parent rock in geology?

The original rock from which a new rock, sediment, or soil forms through geological processes.

How does cooling rate affect the crystal size of igneous rocks?

Rapid cooling of magma or lava results in smaller mineral crystals, whereas slow cooling produces larger mineral crystals.

Which figure depicts the step-by-step process of preparing a petrographic thin section from a core sample?

Step 3 attaches the sample billet to a petrographic glass slide, Step 4 removes the excess billet, and Step 5 yields the final polished core thin section.

What sequence of events describes the complete life cycle of a quartz sandstone derived from granite?

Granite undergoes weathering and erosion, the resulting sand sediments are transported by wind or water, deposited (e.g., as beach sand), buried under overburden, and lithified through compaction and cementation.

What two distinct processes convert loose sediments into solid sedimentary rock during lithification?

Compaction (squeezing sediments together by overburden weight) and cementation (gluing sediments together via mineral precipitation from solution).

What is the difference between weathering and erosion?

Weathering is the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at the surface, whereas erosion is the physical removal and transport of sediment away from the site of weathering.

By what percentage does water expand when it freezes during frost wedging?

Water expands by approximately $9\%$ upon freezing, exerting a wedging force that propagates cracks in parent rock.

What physical weathering process creates sheet fractures and exfoliation domes such as Enchanted Rock, TX?

Pressure release (or unburdening), where deep-seated igneous rocks like batholiths expand and fracture along concentric zones of weakness when overlying pressure is eroded away.

What is onion skin weathering, and where is it commonly observed?

A form of thermal expansion and contraction caused by extreme daily temperature swings (e.g., $50^\circ\text{F}$ at night to $120^\circ\text{F}$ by day), common in desert environments where outer rock layers flake off.

What chemical equation represents the dissolution of limestone in carbonic acid?

$$\text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Ca}^{2+} + 2\text{HCO}_3^-$$

How do physical and chemical weathering processes interact in spheroidal weathering?

Physical fracturing breaks rock into blocks and increases the available surface area, allowing chemical weathering to decompose edges and corners faster, transforming angular blocks into spherical boulders.

What is a petroleum system?

A unifying geological concept encompassing all essential elements (source rock, reservoir rock, seal rock, overburden rock) and processes (generation, migration, trapping, accumulation) required for a hydrocarbon accumulation to exist.

How do petroleum province, sedimentary basin, and petroleum system compare in geographic scale?

A petroleum province is the largest geographic scale, a sedimentary basin is an intermediate-scale depression filled with sedimentary rock, and a petroleum system represents the smaller pod scale of active source rock and related petroleum.

What are the core requirements for a rock to serve as an effective source rock?

It must be organic-rich (typically organic content $> 4\%$ by weight), thermally mature, and must have resided within the petroleum temperature/depth window.

What two critical petrophysical properties must a reservoir rock possess?

Porosity (sufficient void space to store hydrocarbons) and permeability (the ability to transmit fluids through interconnected void space).

What distinguishes a conventional petroleum system from an unconventional petroleum system?

In a conventional system, the source rock, reservoir rock, and seal rock are distinct rock units requiring fluid migration. In an unconventional system, a single rock unit (such as the Barnett Shale) serves as source, reservoir, and seal simultaneously.

Which type of petroleum trap is illustrated in this block diagram?

A stratigraphic trap, where hydrocarbon accumulation is sealed above an unconformity by an overlying cap rock.

Why must a trapping mechanism be formed prior to hydrocarbon migration?

If the trapping mechanism forms after migration, the migrating oil and gas will bypass the area and escape rather than accumulating in a localized reservoir.

How does a structure contour map differ from a topographic map?

A topographic map depicts the 3D ground surface elevation above sea level, whereas a structure contour map depicts the elevation or depth of a specific subsurface rock horizon.

What is the difference between elevation and depth in petroleum engineering and geology?

Elevation is the vertical distance above or below a fixed datum (typically sea level), whereas depth is the vertical distance measured from ground level down to a point in the subsurface.

What do V-shaped contour lines indicate when they cross a stream on a topographic map?

The 'V' shape always points uphill or upstream, opposite to the direction of water flow.

What is the contour interval on a topographic or contour map?

The constant vertical elevation difference between adjacent contour lines.

How is gridding used in the automated creation of structure contour maps?

Gridding interpolates elevation values across a regular grid framework from scattered control points (well locations) to generate continuous contour lines.