Tuesday August 11, 2026  Edition 121

Nearly one-third of conventional oil and gas was generated by Cretaceous-age source rocks. This week, we will go back to the Cretaceous period (143–66 Ma) and look at an unconventional play in Argentina and a conventional reservoir in Kuwait.

Next week (17–20 August), I'll be attending IMAGE '26 in Houston! I hope to see you there! If you have not yet registered, this is a top event for geoscience research and networking. I always enjoy visiting with many former colleagues and peers, and I encourage you to attend! You can register here.

Rasoul Sorkhabi

Editor, Core Elements

A Look Into Argentina's Agrio Shale

Study area of Pilmatué Shale Member in Argentina (AAPG Bulletin, July 2026)

The U.S. shale revolution motivated the development of similar unconventional shale plays in South America, but only the Vaca Muerta shale in Argentina’s Neuquén Basin has been practically developed.

Manuela Zalazar and colleagues published a paper in the AAPG Bulletin that offers insights into the Agrio Formation, an important potential shale play in the Neuquén Basin.

Let’s take a look.

Neuquén Basin tectonics: The Neuquén is a retro-arc foreland basin that evolved in three stages:

1. Late Triassic–Early Jurassic: Extensional stage with several sedimentation depocenters
2. Early Jurassic–Early Cretaceous: Post-rift stage controlled by a steep subduction zone, associated magmatic arc, and basin fill of more than four kilometers of sediments
3. Late Cretaceous–Cenozoic: Compressional stage related to Andean uplift and associated flexural subsidence

Basin stratigraphy: Marine sediments deposited from the Late Jurassic to the Early Cretaceous are within the Mendoza Group, which comprises four formations:

1. The Tordillo Formation (Kimmeridgian, 155–149 Ma): Syn-rift Fluvial and lacustrine sediments
2. The Vaca Muerta Formation (Tithonian to Berriasian, 149–137 Ma): Post-rift deepwater to carbonate ramp black shale
3. The Quintuco/Chachao Formation (Berriasian, 143–137 Ma): shoreface to shallow inner shelf sediments
4. The Mulichinco Formation (Valanginian, 137–132 Ma): shallow marine sediments
5. The Agrio Formation (Valanginian to Barremian, 137–121 Ma): Outer to inner shelf mixed siliciclastic-carbonate ramp sediments

The Agrio Formation is divided into three members:

1. The Pilmatué Shaly Member (Lower)
2. The Avilé Sandy Member (Middle)
3. The Agua de la Mula Shaly Member (Upper)

The researchers focused on the Pilmatué Member, which is upper Valanginian to lower Hauterivian in age.

What they did: The researchers studied a complete stratigraphic section of the Pilmatué Member near the El Portón oil field. Their studies included:

• A Gamma-ray survey on the outcrop section to estimate the clay volume of sediments using a portable gamma spectrometer
• Studies of 15 thin sections by petrographic and scanning electron microscopy
• XRD analyses of 20 samples for mineralogical composition
• Schmidt hammer tests on 89 rock surfaces for hardness measurements
• Scratch tests on 24 samples to estimate rock strength

Sedimentology:

• Pilmatué sediments are dominated by marl (42.8 percent), followed by fine-grained limestones (35.2 percent), tuffs (12 percent), shales (8.7 percent), and coarse sandstone (1.3 percent).
• Eleven facies were identified from Facies 1 (massive marls) through Facies 11 (fine-grained volcaniclastic sediments) based on matrix and cement compositions.

Organic-rich intervals:

• Organic-rich intervals were deposited in an outer carbonate ramp environment under low oxygenation and energy during marine transgression.
• Three organic level (OL) vertical intervals of marl and mudrocks with high total organic carbon (TOC) and kerogen types II and III were identified:
  • OL1 (40 meters thick, TOC 2.84–6.63 percent)
  • OL2 (54 meters, TOC 2.3–9)
  • OL3 (50 meters, TOC 1.8–7.1)

Geomechanical properties:

• Schmidt hammer tests yield variable hardness based on lithology:
  • Massive fine-grained limestones (23–51 R)
  • Laminated fine-grained limestones (33–43 R)
  • Calcareous tuff (36–45 R)
  • Massive marls (24–48 R), shale (28–42)
  • Medium-grained sandstones (19 R)
  • Laminated marls (15 R)
• Scratch tests for marls revealed that strength values range from 22.4 to 97.1 megapascals, with higher values observed in massive marls.

Why it matters:

• The Pilmatué Member sediments extend more than 40,000 square kilometers in Argentina’s Neuquén and Mendoza provinces.
• Its thickness is up to 600 meters in Neuquén and up to 300 meters in Mendoza.
• The Agrio is an important source rock formation for conventional and unconventional plays.

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What Controls Oil Flow in Kuwait’s Mauddud Reservoirs?

Mauddud reservoir study in Kuwait (AAPG Bulletin, July 2026)

Kuwait is home to the Greater Burgan field, the world’s second-largest conventional oil and gas field (after Saudi Arabia’s Ghawar field). There are also several other giant oil fields in Kuwait.

Stephen Ehrenberg and colleagues in AAPG Bulletin describe the reservoir geology of the Mauddud Formation in northern Kuwait’s Raudhatain and Sabiriyah giant oil fields.

What they did:

• The researchers compiled data on core descriptions from 84 wells to assess depositional facies distribution.
• Their analytical methods included description of thin sections, cathodoluminescence (CL), scanning electron microscopy (SEM), and stable isotope analyses.

Mauddud formation stratigraphy:

• The formation was deposited on a gently seaward-sloping carbonate ramp during the Albian age (113.2–100.5 Ma).
• It is about 117 meters thick in the Raudhatain and Sabiriyah oil fields.
• It is sandwiched between the older fluvial-deltaic siliciclastic of the Burgan Formation at the bottom and the marine mud rocks of the Wara Formation on the top.

Estimated Oil Recovery (EUR) in the Mauddud Formation:

• 2.1 billion barrels of oil in Sabiriyah Field
• 1.0 billion barrels of oil in Raudhatain Field

Mauddud petrography:

• Ninety-four percent of the 569 thin sections are limestones. The remaining 6 percent are either dolostones or sandstones.
• The limestones are divided into three groups:
  1. Porous limestone with high permeability and oil staining
  2. Nodules or tightly cemented concretionary masses
  3. Cemented zones or intervals of more than a foot thick with no oil stain

Natural fractures:

• Most fractures are up to centimeters long and narrow (millimeter-scale) and oriented at angles of 70–90 degrees.
• Some fractures are oil-stained, while others are unstained, likely indicating different stages of fracturing.

Why it matters:

• The study shows that the Mauddud reservoir’s heterogeneity is largely due to heavily calcite-cemented nodules and thicker cemented zones.
• The study offers important geologic input for reservoir engineering.

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