Geologic time, basins and the petroleum geologist

Nadia Petrov has a new assignment. Prairie Crest wants to know whether its acreage in the Williston Basin still holds undrilled targets above and below the Bakken, and the managers have also asked the geoscience team to screen two other basins, the Permian Basin of West Texas and the Gulf of Mexico, before the company bids on new leases. Ethan Cardinal, the engineer-in-training who started with Lena in PET-200, has been loaned to the project. On his first morning Nadia unrolls a cross-section of the Williston Basin that runs from Montana into North Dakota and says, 'Everything we will do this term starts here. Before you can find oil, you have to know how old the rocks are, how they got where they are, and what the basin did to them.' This course follows that study from the rocks themselves to whole basins.

Petroleum geology is the branch of geology that deals with the origin, migration, accumulation and recovery of oil and natural gas. PET-200 introduced the petroleum system in one lesson; this course takes each part of it in turn. Engineers need this knowledge because every number they use, from porosity and permeability to the size of a trap and the pressure in a reservoir, comes from rocks that were deposited, buried, heated and deformed over tens or hundreds of millions of years. An engineer who understands why a reservoir is shaped the way it is will place wells better, predict surprises sooner and talk to geologists as a colleague. CIV-241 Engineering Geology covers rocks and minerals for civil works; here the focus is on sedimentary rocks and the processes that make and keep petroleum.

Deep time

The Earth is about 4.54 billion years old. Geologists divide that time into eons, eras, periods and epochs, a scale built first from the order of rock layers and their fossils and later fixed in absolute years by radiometric dating. Most of the world's oil and gas comes from rocks of the last 550 million years, the Phanerozoic eon, when complex life flourished and left abundant organic matter. The Phanerozoic is split into three eras: the Paleozoic (about 539 to 252 million years ago), the Mesozoic (252 to 66 million years ago) and the Cenozoic (66 million years ago to the present). Each era is divided into periods, such as the Devonian, the Cretaceous and the Paleogene, whose names appear constantly in petroleum work.

In the Williston Basin, the most important petroleum rocks are Paleozoic. The Bakken, which straddles the boundary between the Devonian and Mississippian periods about 360 million years ago, is both source and reservoir. The Ordovician Red River, the Mississippian Madison and the Devonian Three Forks carry other oil. In the Permian Basin the main targets are, as the name says, mostly Permian, from about 299 to 252 million years old, with Pennsylvanian and older rocks below. In the Gulf of Mexico most oil comes from much younger rocks: Jurassic to Pleistocene sandstones deposited from about 160 million to under one million years ago.

Two kinds of age matter. Relative age tells which rock came first, using a few simple principles: in undisturbed layers, younger rocks lie on top of older ones (superposition); layers were laid down horizontally and later tilted (original horizontality); a fault or intrusion is younger than the rocks it cuts (cross-cutting relationships); and fossils change in a known order through time (faunal succession). Absolute age, in years, comes from radioactive decay. Volcanic ash beds within sedimentary sequences are especially valuable because their minerals can be dated precisely, pinning the age of the sediments above and below.

Petroleum geologists use fossils every day. Microfossils, such as foraminifera, pollen, spores and tiny algae called dinoflagellates, survive drilling and appear in the rock cuttings that come up with the mud. A biostratigrapher at the well site can identify them within hours and tell the drilling team which formation the bit has reached, often before the logs are run. Index fossils, species that lived for a short time over a wide area, let geologists match rocks of the same age between wells hundreds of kilometers apart.

Sedimentary basins

Almost all petroleum is found in sedimentary basins, regions where the Earth's crust subsided over long periods and filled with sediment, often several kilometers thick. Subsidence creates the space for sediment, called accommodation, and burial provides the heat that turns organic matter into oil. Basins form in a few main tectonic settings, and each setting tends to produce a recognizable pattern of rocks, structures and petroleum.

The Williston Basin is a classic intracratonic basin, a broad bowl centerd near Williston, North Dakota, that subsided slowly for most of the Paleozoic and collected layers of carbonate, shale and salt. The Permian Basin began as part of a much larger basin and was broken into sub-basins and platforms by faulting in the late Paleozoic. The Gulf of Mexico is a passive margin: it opened in the Jurassic as North America and South America separated, and rivers have been building deltas into it ever since.

Sea level, subsidence and sediment supply

The rocks that fill a basin record a running contest between three things: how fast the floor subsides, how much sediment rivers and the sea deliver, and how the sea level rises and falls. When sea level rises faster than sediment arrives, the shoreline moves inland, a transgression, and marine shales spread over older beach and river sands. When sediment arrives faster, or sea level falls, the shoreline builds seaward, a regression, and sands advance over muds. Sequence stratigraphy, introduced in Lesson 8, uses these cycles to predict where reservoirs and seals will be.

Burial rates vary enormously. Some deep-sea muds accumulate at less than a millimeter per thousand years; the front of a major delta can receive tens of meters in the same time. As sediment is buried, it compacts: water is squeezed out, grains pack more tightly, and porosity falls. Fresh mud may be 60 to 80 percent water by volume; buried shale at 3,000 meters may have less than 10 percent porosity. Sand compacts less but also loses porosity with depth through compaction and cementation, as Lesson 3 explains. Geologists correct present-day thicknesses for compaction, a process called decompaction, to reconstruct how deep each layer was in the past.

This simple exponential law, often attributed to Athy, describes how porosity decreases with burial depth z. The surface porosity φ₀ and the compaction coefficient c are fitted for each rock type; for shale, typical values are φ₀ about 0.6 and c about 0.5 per kilometer, and for sandstone φ₀ about 0.45 and c about 0.3 per kilometer. The law is approximate, but it is the basis of the burial history models in Lesson 11.

# Porosity loss with depth (Athy's law) and decompaction of a buried shale bed
import math

PHI0, C = 0.60, 0.50          # shale: surface porosity, compaction coefficient per km

def phi(z_km):
    return PHI0 * math.exp(-C * z_km)

def solid(top, base):
    """Thickness of solid grains (km) in a layer between two depths, integrating 1 - phi."""
    return (base - top) - (PHI0 / C) * (math.exp(-C * top) - math.exp(-C * base))

print("depth (km)   porosity")
for z in (0, 0.5, 1, 2, 3, 4):
    print(f"{z:6.1f}       {phi(z):.2f}")

# A shale 100 m thick today, from 2.95 to 3.05 km
top, base = 2.95, 3.05
hs = solid(top, base)
# find the thickness the same grains had at the surface (top = 0), by bisection
lo, hi = hs, 1.0
for _ in range(60):
    mid = (lo + hi) / 2
    if solid(0, mid) < hs: lo = mid
    else: hi = mid
print(f"\nPresent thickness {1000 * (base - top):.0f} m, solid grains {1000 * hs:.1f} m")
print(f"Thickness when deposited: {1000 * lo:.0f} m ({lo / (base - top):.1f} times today)")

depth (km)   porosity
   0.0       0.60
   0.5       0.47
   1.0       0.36
   2.0       0.22
   3.0       0.13
   4.0       0.08

Present thickness 100 m, solid grains 86.6 m
Thickness when deposited: 202 m (2.0 times today)

The program shows that a shale bed 100 meters thick today, buried at 3 kilometers, was about twice as thick when it was first deposited. Burial history models apply the same correction to every layer, which is why the present-day thicknesses on a cross-section are not the thicknesses the basin received.

How petroleum geologists work

A petroleum geologist builds a picture of rocks that no one can see directly. The data come from outcrops where the same formations reach the surface, from seismic surveys, and from wells: cuttings, cores, wireline logs, pressure tests and samples of oil, gas and water. Each source has a different scale. A core shows grains a fraction of a millimeter across; a seismic survey resolves layers tens of meters thick over hundreds of square kilometers. Good interpretation joins the scales, using cores to calibrate logs and logs to calibrate seismic.

The work moves from regional to local. A basin study maps the major rock units, the source rocks and their maturity, and the main structural trends. A play is a group of prospects that share a source, reservoir, seal and trap style; the Montney and Bakken plays are examples. A prospect is a specific trap, mapped well enough to drill. Each stage narrows the area and raises the level of detail, and each produces maps and estimates that engineers use to plan wells and facilities.

Nadia lays out the plan for the Prairie Crest study. First, the team will review the rock types that make reservoirs and seals, using cores from the company's own wells and public cores held at the state core library in Grand Forks, where North Dakota keeps samples from wells across the state. Next, it will map the source rocks and estimate how mature they are. Then it will map traps and sweet spots using well logs and licensed seismic surveys, and finally it will place everything in time on a basin model. The same steps, on a coarser scale, will be used to screen the Permian Basin and the Gulf of Mexico. Ethan's job is to turn each step into numbers that the engineers can use: thicknesses, porosities, volumes and risks. 'Every map we make is a bet,' Nadia tells him, 'and your job is to know how much the bet is worth.'

Geologists also work with uncertainty. A prospect map is a model, not a photograph. The same seismic and well data can often be explained by several geological stories, and each story implies a different volume and risk. Good teams make their assumptions explicit, test them against new wells, and update quickly when the drill bit proves them wrong. Lesson 11 shows how these ideas come together in a basin model and an events chart.