Questions: Diagenesis and the Lithification of Sediments

Question 1

A reservoir engineer finds that a sandstone at 2 km depth has much lower porosity than a similar sandstone at 500 m depth in the same basin. Which diagenetic processes most likely explain this difference?

• A. Metamorphic recrystallization at depth destroyed the pore spaces
• B. Compaction from overlying sediment weight and cementation by minerals precipitated from pore fluids progressively reduced porosity with burial
• C. Weathering of quartz grains at depth generated clay minerals that filled pore spaces
• D. Increased temperature at depth caused sand grains to melt and fuse together

Answer: (B)

Diagenetic compaction and cementation progressively reduce porosity with burial depth. Compaction squeezes grains together and expels pore water; pressure solution allows grains to interpenetrate. Cementation fills remaining pores with precipitated minerals like calcite, quartz, or iron oxides. Metamorphic temperatures (~200–300°C) are not typically reached at 2 km under normal geothermal gradients, and weathering occurs at the surface, not at depth.

Question 2

Pressure solution is a diagenetic process that occurs because:

• A. Hot fluids dissolve grains uniformly throughout the rock volume
• B. Minerals dissolve preferentially at grain-to-grain contacts where stress is concentrated, allowing grains to interpenetrate and reducing porosity
• C. Elevated temperature causes silica to migrate upward and reprecipitate near the surface
• D. Acidic pore fluids attack grain surfaces evenly, shrinking all grains simultaneously

Answer: (B)

At grain contacts, stress concentrations lower the chemical potential of the solid, making dissolution thermodynamically favorable at those specific points — even at temperatures where dissolution would not otherwise occur. The dissolved material moves away in solution, and grains physically interpenetrate, creating sutured contacts in sandstones and stylolites in carbonates. This is distinct from cementation, which adds material; pressure solution removes it from stressed locations.

Question 3

Early cementation in a sandstone can preserve higher porosity compared to a sandstone that received no early cement during burial.

• T. True
• F. False

Answer: True

Counterintuitively, early cementation can protect porosity by creating a rigid grain framework that resists mechanical compaction during deeper burial. Without early cement, grains rotate, fracture, and compact progressively under increasing overburden, destroying pore space. A cemented framework transmits stress through grain-cement contacts rather than collapsing pores. This early-cementation preservation effect is critical for predicting reservoir quality.

Question 4

Diagenesis and metamorphism differ primarily in the types of minerals they produce, not in the temperatures and pressures at which they occur.

• T. True
• F. False

Answer: False

The defining distinction is temperature and pressure, not just mineralogy. Diagenesis occurs from surface conditions to roughly 200–300°C and a few kilometers depth. Metamorphism begins where diagenesis ends, at higher temperatures and pressures that produce fundamentally different mineral assemblages (garnet, kyanite, sillimanite). The different mineral products follow from the different physical conditions.

Question 5

How does diagenesis control the economic potential of a sedimentary rock as an oil or groundwater reservoir?

Model answer: Diagenesis controls porosity (the fraction of rock volume that is pore space, determining how much fluid can be stored) and permeability (how easily fluid flows through connected pores, determining extractability). Compaction and cementation reduce both properties; dissolution can create secondary porosity. A rock may store fluids but be uneconomic if permeability is too low to allow extraction at viable rates.

Two sandstones with identical depositional histories can have radically different reservoir qualities if their diagenetic paths diverged — one early-cemented (rigid framework, preserved porosity), the other deeply compacted (porosity destroyed). Predicting these differences from burial history is a central task in petroleum geology and hydrogeology.