Petroleum Exploration: Reservoir Rocks, the Petroleum System, Exploration Methods and Resource Classification
1. Rocks, and clastic and non-clastic reservoir rocks
Rocks are igneous (crystallised from a melt: granite, basalt), sedimentary (deposited at the surface and lithified) or metamorphic (recrystallised by heat and pressure: slate, quartzite, marble). Almost all petroleum is found in sedimentary rocks, which divide by origin into two families. Clastic rocks are built of fragments of older rocks transported and deposited as grains: conglomerate (gravel), sandstone (sand, 1/16 to 2 mm), siltstone and shale (mud). Non-clastic rocks are precipitated chemically or biochemically in place: limestone (calcite, CaCO₃), dolomite (CaMg(CO₃)₂), evaporites (halite, anhydrite, gypsum) and chert.
A reservoir rock must have porosity to store fluid and permeability to deliver it. Sandstones hold intergranular primary porosity whose quality is set by sorting, grain size, packing and cementation; they are relatively homogeneous and their porosity and permeability are usually related. Carbonates owe much of their porosity to secondary processes — dissolution into vugs and caverns, fracturing, and dolomitisation, in which the replacement of calcite by the denser mineral dolomite shrinks the solid volume and opens pore space — so their pore systems are heterogeneous and permeability can be dominated by fractures. Shale is the typical source rock and, with evaporites, the typical seal: fine grains give it porosity but almost no permeability.
| Feature | Clastic (sandstone) | Non-clastic (carbonate) |
|---|---|---|
| Origin of grains | Transported fragments, mainly quartz | Precipitated in place, often from shells and reefs |
| Dominant porosity | Primary, intergranular | Secondary: vugs, fractures, dolomitisation |
| Heterogeneity | Moderate; porosity-permeability trend usual | High; permeability often fracture-controlled |
| Diagenetic sensitivity | Compaction and quartz or clay cement reduce quality | Dissolution can create quality; cementation destroys it |
2. Origin, migration and accumulation of petroleum
Petroleum is organic in origin. Organic matter buried with fine sediment in an oxygen-poor setting survives oxidation and is converted, as burial proceeds, into kerogen, the insoluble organic fraction of a source rock. Kerogen type fixes what it will generate: Type I (lacustrine algal, hydrogen-rich) and Type II (marine plankton) are oil-prone; Type III (terrestrial woody plant material, hydrogen-poor) is gas-prone; Type IV (reworked, oxidised) is essentially inert. Maturation proceeds through diagenesis (shallow, low temperature, biogenic methane), catagenesis (the oil window, then wet gas as oil cracks) and metagenesis (dry gas). Temperature and time together control it, which is why a young basin needs to be hotter than an old one to reach the same maturity. Vitrinite reflectance, R₀, is the standard maturity indicator: it rises steadily with maturity.
Primary migration is the expulsion of generated hydrocarbons out of the fine-grained source rock, driven by the pressure of generation and compaction. Secondary migration is their movement through permeable carrier beds and faults, driven mainly by buoyancy (oil and gas are lighter than the formation water) and resisted by capillary pressure at pore throats, until they meet a barrier. Accumulation needs a trap: a reservoir rock shaped or sealed so that buoyant fluid cannot escape upward. The highest point of the trap is its crest; the deepest contour that still closes is the spill point, and the vertical distance between them is the closure. Hydrocarbons that escape a trap by leakage or spilling are said to undergo tertiary migration.
| Class | Examples | What seals it |
|---|---|---|
| Structural | Anticline, fault trap, salt dome flank | Deformation of the beds: folding, faulting, diapirism |
| Stratigraphic | Sand pinch-out, reef, unconformity truncation | A change in the rock itself: lithology, facies, erosion surface |
| Combination | A faulted pinch-out; a folded unconformity | Both a structural and a stratigraphic element |
3. Petroleum exploration methods
Exploration narrows a basin to a drillable prospect in stages. Surface geology and remote sensing map outcrops, structures and seeps. Gravity surveys measure small variations in g; after latitude, free-air, Bouguer and terrain corrections the remaining Bouguer anomaly reflects density contrasts at depth — a low over a sedimentary basin or over a salt dome, since salt is less dense than the compacted sediments around it, and a high over a buried basement ridge. Magnetic surveys respond mainly to magnetite-bearing igneous and metamorphic basement, so they map basement depth and hence sedimentary thickness. Geochemical surveys look for hydrocarbons leaking to the surface. None of these images a trap directly; seismic reflection does.
In seismic reflection a source (dynamite or vibrator on land, air guns at sea) sends elastic waves down; at every interface where the acoustic impedance Z = ρv changes, part of the energy is reflected with reflection coefficient R = (Z₂ − Z₁)/(Z₂ + Z₁) at normal incidence, and geophones (or hydrophones at sea) record the returns. The recorded quantity is two-way travel time, so the depth of a reflector is d = v̄t/2 with v̄ the average velocity above it. 2D lines give cross-sections; 3D surveys give a volume in which faults and closures can be mapped; repeated 3D surveys over a producing field (4D, time-lapse) show fluid movement. A gas sand, with its low velocity and density, can produce a strong negative reflection (a bright spot). The final test is always the exploratory (wildcat) well, whose cuttings, cores, logs and tests prove or disprove the prospect.
4. Petroleum resource classification and evaluation
The industry’s resource framework, the SPE Petroleum Resources Management System (PRMS), classifies all petroleum initially in place by whether it has been discovered and whether its recovery is commercial. Discovered, commercial and recoverable volumes are reserves; discovered but not yet commercial volumes (awaiting a market, a development decision or technology) are contingent resources; undiscovered but estimated recoverable volumes are prospective resources; what can never be recovered is unrecoverable. Each class is then graded by uncertainty. For reserves the categories are proved (1P), proved plus probable (2P) and proved plus probable plus possible (3P). In a probabilistic estimate, 1P corresponds to P90 — at least a 90% probability that the quantity recovered will equal or exceed it — 2P to P50 and 3P to P10.
Evaluation before drilling rests on a volumetric estimate — hydrocarbon in place = gross rock volume × net-to-gross × porosity × hydrocarbon saturation ÷ formation volume factor, times a recovery factor for the recoverable part — run either deterministically with single values or probabilistically, by Monte Carlo sampling of each input’s distribution. The geological chance of success is the product of the independent probabilities that each element works, Pg = P_source × P_reservoir × P_trap × P_seal (with timing and migration folded in). The decision to drill then compares outcomes: expected monetary value EMV = Pg × (value if successful) − (1 − Pg) × (cost of a dry hole). A positive EMV favours drilling. After discovery, reserves are re-estimated by material balance, decline analysis and simulation as production history accumulates — the subjects of the Reservoir Engineering chapter.
| Class | Status | Low / best / high estimate |
|---|---|---|
| Reserves | Discovered, commercial | 1P / 2P / 3P |
| Contingent resources | Discovered, not yet commercial | 1C / 2C / 3C |
| Prospective resources | Undiscovered | Low / best / high |
Key takeaways
- Sandstones are clastic with primary intergranular porosity; carbonates are non-clastic and owe much of theirs to dissolution, fracturing and dolomitisation; shale is source and seal.
- Kerogen Types I and II are oil-prone, Type III gas-prone; maturity runs diagenesis, catagenesis (oil window), metagenesis (dry gas).
- Primary migration is expulsion from the source; secondary migration is buoyant flow in carrier beds to a structural, stratigraphic or combination trap.
- Seismic depth is v̄t/2 from two-way time; R = (Z₂ − Z₁)/(Z₂ + Z₁) with Z = ρv; salt domes give Bouguer lows.
- Reserves are discovered and commercial (1P = P90, 2P = P50, 3P = P10); EMV = Pg × success value − (1 − Pg) × dry-hole cost.
Practice questions (13)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Which of the following is a clastic reservoir rock?
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Answer: A — Sandstone
Sandstone is built of transported sand grains, which is what clastic means. Dolomite and reef limestone are carbonates precipitated in place (non-clastic), and anhydrite is an evaporite that acts as a seal rather than a reservoir.Dolomitisation of a limestone generally
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Answer: A — increases porosity, because dolomite is denser than the calcite it replaces
Replacing calcite by the denser mineral dolomite reduces the volume occupied by solid, so the same bulk volume holds more pore space — secondary porosity. The rock stays a chemical (non-clastic) carbonate; over-dolomitisation with later cementation can reduce porosity, but the general effect is an increase.Type III kerogen, derived mainly from terrestrial higher-plant material, is
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Answer: A — gas-prone
Woody terrestrial matter is hydrogen-poor and oxygen-rich, so on maturation it yields mainly gas. Lacustrine algal kerogen is Type I and oil-prone, marine Type II is also oil-prone, and inert reworked matter is Type IV.Primary migration of petroleum refers to
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Answer: A — expulsion of hydrocarbons from the source rock into adjacent carrier beds
Primary migration is the movement out of the fine-grained, low-permeability source rock. The buoyant journey through carrier beds is secondary migration, and escape from a trap by leakage or spilling is tertiary migration; flow to a well is production, not migration.Which of the following are stratigraphic traps? (More than one option may be correct.)
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Answer: A — A sandstone that pinches out up-dip into shale; B — A buried reef encased in shale; C — A reservoir truncated beneath an unconformity
A pinch-out, a reef and an unconformity truncation all trap oil because the rock itself changes — lithology, facies or an erosion surface — which is the definition of a stratigraphic trap. An anticline traps by folding of the beds, which makes it structural.Which of the following are ELEMENTS (as distinct from processes) of a petroleum system? (More than one option may be correct.)
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Answer: A — Source rock; B — Seal rock; C — Overburden rock
The essential elements are the source, reservoir, seal and overburden rocks. Overburden counts because its burial heats the source rock to maturity. Migration, together with trap formation, generation and accumulation, is one of the processes that act on those elements.A seismic reflection is recorded at a two-way travel time of 2.4 s. If the average velocity above the reflector is 3000 m/s, the depth of the reflector is ______ m.
Numerical answer — type the value.
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Answer: 3600
The wave travels down and back, so depth = v̄ × t/2 = 3000 × 2.4/2 = 3600 m. Equivalently the one-way time is 1.2 s and 3000 × 1.2 = 3600 m; 7200 m is the answer that forgets the return trip.A shale (density 2.3 g/cm³, velocity 2500 m/s) overlies a limestone (density 2.5 g/cm³, velocity 3500 m/s). The normal-incidence reflection coefficient at the interface is ______ (to two decimal places).
Numerical answer — type the value.
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Answer: 0.21
Z₁ = 2.3 × 2500 = 5750 and Z₂ = 2.5 × 3500 = 8750 (in g/cm³ · m/s; the units cancel). R = (8750 − 5750)/(8750 + 5750) = 3000/14 500 = 0.207, which is 0.21. Positive because impedance increases downward.A gravity survey over a salt dome buried in compacted sediments typically shows
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Answer: A — a negative Bouguer anomaly, because salt is less dense than the surrounding sediments
Halite has a density of about 2.2 g/cm³ and does not compact with burial, while the surrounding sediments become denser with depth, so the salt body is a mass deficit and gives a gravity low. Magnetism is irrelevant to a gravity survey, and salt gives no significant magnetic signature.In reserves reporting, 2P means
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Answer: A — proved plus probable reserves
1P is proved, 2P is proved plus probable, and 3P adds possible. In probabilistic terms 2P corresponds to the P50 or best estimate. The categories are cumulative, not multiples.A discovered accumulation that cannot yet be developed commercially because no gas market exists is classified as
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Answer: A — contingent resources
It is discovered, so it is not prospective; it is not commercial, so it cannot be reserves of any category. Discovered-but-not-commercial is exactly the contingent class, and it moves into reserves when the contingency (here, a market) is removed.For a prospect, the independent probabilities are: source 0.8, reservoir 0.7, trap 0.6 and seal 0.5. The geological chance of success is ______ (to three decimal places).
Numerical answer — type the value.
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Answer: 0.168
All four must work, so Pg = 0.8 × 0.7 × 0.6 × 0.5. Working in pairs: 0.8 × 0.7 = 0.56 and 0.6 × 0.5 = 0.30, and 0.56 × 0.30 = 0.168. A prospect whose every element is individually likely can still be a long shot overall.An exploration well has a chance of success of 0.25. If successful, the discovery is worth 80 million dollars (net present value); a dry hole costs 12 million dollars. The expected monetary value of drilling is ______ million dollars.
Numerical answer — type the value.
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Answer: 11
EMV = Pg × success value − (1 − Pg) × dry-hole cost = 0.25 × 80 − 0.75 × 12 = 20 − 9 = 11 million dollars. It is positive, so on this criterion the well should be drilled.