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GATE Petroleum Engineering Petroleum Exploration Flashcards
50 question-and-answer cards covering Petroleum Exploration as it is examined in GATE Petroleum Engineering. 24 of them are printed below, taken from across the deck — no signup, no paywall on the preview.
24 sample cards from the Petroleum Exploration deck
Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.
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.
What this deck covers
The Petroleum Exploration deck follows the GATE Petroleum Engineering Petroleum Exploration syllabus — 3 chapters and 2 topics — so questions land on material that is genuinely examinable rather than trivia around it. That works out to roughly 16.7 cards per chapter.
Answers are written to be recallable, not just readable — averaging about 194 characters, which is long enough to carry the reasoning and short enough to say out loud.
A deck like this earns its keep on the second and third pass. Read the syllabus first so you know the shape of the subject, then use the cards to find the specific facts that have not stuck.
Petroleum Exploration flashcards FAQ
How many Petroleum Exploration flashcards are in this GATE Petroleum Engineering deck?
50 cards. This page previews 24 of them, sampled evenly across the deck so you can judge the difficulty before installing anything.
Are these GATE Petroleum Engineering flashcards free?
Yes. The preview here is free to read with no signup, and the full 50-card deck is free inside the Examius app.
What do the Petroleum Exploration cards cover?
They follow the GATE Petroleum Engineering Petroleum Exploration syllabus — 3 chapters and 2 topics — so the questions track what is actually examinable.
How should I use these flashcards?
Read the syllabus first so you know the shape of the subject, then drill the deck. Examius schedules each card with spaced repetition, so cards you keep missing come back sooner and ones you know drift further apart.