GATE Petroleum Engineering Petroleum Exploration Syllabus

Define an arkose.

A feldspar-rich sandstone containing more than $25\%$ feldspar, typically derived from rapid erosion of granitic/gneissic source rocks with limited chemical weathering.

Define a greywacke (wacke).

A poorly sorted, matrix-rich (>$15\%$ clay-silt matrix) sandstone with abundant lithic fragments, typically deposited rapidly by turbidity currents; generally a poorer reservoir due to high matrix.

Define textural maturity of a sandstone.

The degree to which a sediment has approached the end product of clay removal, sorting, and rounding by transport: immature → submature → mature → supermature (clay-free, well sorted, well rounded).

What is mineralogical maturity?

The degree to which unstable minerals (feldspars, lithics, mafics) have been eliminated, leaving stable quartz; high quartz content indicates high mineralogical maturity.

List the stages converting loose sediment into a clastic reservoir rock (diagenesis).

Compaction (grain rearrangement and porosity loss with burial), cementation (precipitation of minerals in pores), dissolution (creating secondary porosity), and replacement/recrystallization.

How does compaction affect sandstone porosity with burial depth?

Increasing overburden pressure rearranges and deforms grains, reducing pore volume; porosity generally decreases approximately exponentially with depth: $\phi = \phi_{0}\,e^{-c z}$ where $z$ is depth.

Name the common cements in sandstones.

Silica (quartz overgrowths), calcite (carbonate), and clay minerals (kaolinite, illite, chlorite). Cementation reduces porosity and permeability by filling pore throats.

How can secondary porosity form in sandstones?

By dissolution of unstable framework grains (e.g., feldspars) or carbonate cement by acidic pore fluids, and by fracturing — creating moldic and intergranular secondary pores.

What depositional environments produce good sandstone reservoirs?

High-energy clean-sand settings: fluvial channels, deltaic distributary/mouth bars, beach/barrier and shoreface sands, aeolian dunes, and submarine fan/turbidite sands.

Why are aeolian (desert dune) sandstones excellent reservoirs?

Wind transport produces very well sorted, well rounded, clay-free, mineralogically mature quartz sand, giving high porosity and permeability (e.g., Rotliegend, Nugget/Navajo).

Define nonclastic (carbonate) reservoir rocks.

Sedimentary rocks formed by chemical precipitation or organic/biochemical activity rather than detrital transport — chiefly limestones ($\ce{CaCO3}$) and dolomites ($\ce{CaMg(CO3)2}$).

Give the chemical compositions of calcite, aragonite, and dolomite.

Calcite and aragonite are both $\ce{CaCO3}$ (different crystal systems: trigonal vs orthorhombic); dolomite is $\ce{CaMg(CO3)2}$.

Write the dolomitization reaction.

$$\ce{2CaCO3 + Mg^{2+} -> CaMg(CO3)2 + Ca^{2+}}$$ Replacement of calcite by dolomite involving magnesium-rich fluids.

Why does dolomitization often improve reservoir quality?

Dolomite has a smaller molar volume than the calcite it replaces; the mole-for-mole replacement causes a volume reduction of about $13\%$, generating intercrystalline secondary porosity.

Compare the origin of clastic vs carbonate sediments by location of formation.

Clastics are largely produced elsewhere and transported to the basin (extrabasinal/allochthonous); carbonates are mostly produced in situ by organisms and chemical precipitation within the basin (intrabasinal/autochthonous).

State Folk's classification basis for limestones.

Based on allochems (grains: ooids, peloids, bioclasts, intraclasts) and the matrix type — micrite (lime mud) versus sparite (sparry calcite cement), giving names like oosparite, biomicrite, etc.

State Dunham's classification of carbonates by depositional texture.

Mudstone, wackestone, packstone, grainstone (by mud vs grain support and grain abundance), plus boundstone (organically bound, e.g., reefs) and crystalline carbonate.

In Dunham's scheme, distinguish a grainstone from a mudstone.

A grainstone is grain-supported and mud-free (high primary porosity, good reservoir); a mudstone is mud-supported with <$10\%$ grains (low porosity/permeability, poor reservoir).

Define micrite and sparite.

Micrite is fine-grained microcrystalline carbonate mud (matrix, low permeability); sparite is coarse, clear sparry calcite cement that precipitates in pore spaces.

List Choquette and Pray's main porosity types in carbonates.

Fabric-selective: interparticle, intraparticle, intercrystalline, moldic, fenestral, shelter. Non-fabric-selective: fracture, vug, channel, cavern. Carbonate porosity is dominantly secondary.

Define vuggy and moldic porosity.

Moldic porosity forms by selective dissolution of grains/fossils leaving a mold of the original shape; vuggy porosity consists of irregular dissolution cavities larger than grains that cut across fabric.

What is a carbonate reef and why is it a good reservoir?

A wave-resistant framework built in situ by organisms (corals, algae, rudists). Its original framework and post-depositional dissolution give high primary and secondary porosity, making reefs prolific reservoirs.

What are ooids and oolitic grainstones?

Ooids are spherical carbonate grains with concentric layers precipitated around a nucleus in agitated shallow water; oolitic grainstones are well sorted, mud-free, and form excellent reservoirs with high interparticle porosity.

Compare typical permeability–porosity relationships in sandstones vs carbonates.

In sandstones permeability correlates fairly well with porosity (intergranular). In carbonates this correlation is weak: fractures/vugs can give high permeability at low matrix porosity, while micrite gives high porosity but low permeability.

Why are carbonate reservoirs more affected by diagenesis than sandstones?

Carbonate minerals (especially aragonite/high-Mg calcite) are chemically reactive and unstable, so dissolution, cementation, recrystallization, and dolomitization strongly and rapidly modify the original pore system.

Define a fractured reservoir and give the dual-porosity concept.

A reservoir where natural fractures provide the main flow paths while the rock matrix provides storage. Dual porosity: $\phi_{total} = \phi_{matrix} + \phi_{fracture}$, with fractures dominating permeability.

Write the relation between bulk, grain, and fluid densities used to estimate porosity from density logs.

$$\phi = \frac{\rho_{ma} - \rho_{b}}{\rho_{ma} - \rho_{f}}$$ where $\rho_{ma}$ is matrix (grain) density, $\rho_{b}$ bulk density, and $\rho_{f}$ fluid density.

Why is carbonate matrix density needed to interpret porosity logs correctly?

Because grain density differs by mineralogy: calcite $\approx 2.71\ \text{g/cm}^{3}$, dolomite $\approx 2.87\ \text{g/cm}^{3}$, quartz (sandstone) $\approx 2.65\ \text{g/cm}^{3}$; using the wrong $\rho_{ma}$ biases the computed porosity.

What role does clay/shale content play in clastic reservoir quality?

Detrital and authigenic clays occupy and bridge pore throats, drastically reducing permeability and increasing irreducible water saturation and log-derived shaliness, lowering net-to-gross reservoir quality.

Summarize the key contrast between clastic and nonclastic reservoirs for an exam.

Clastics: transported terrigenous grains, mostly primary intergranular porosity, porosity-permeability correlate, texture-controlled. Nonclastics (carbonates): in-situ chemical/biogenic origin, mostly secondary porosity (vugs, molds, fractures, dolomitization), diagenesis-controlled, weak porosity-permeability link.