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Geochemical characteristics and microscopic analysis of the Late Jurassic organic-rich carbonates in the Shabwah depression, southeastern Yemen: implications for organic matter input and depositional settings

This study investigates the Late Jurassic organic-rich Lam Member in Yemen's Shabwah depression, revealing that elevated organic matter and sulfur levels, supported by specific biomarker and microscopic evidence, indicate a marine depositional environment characterized by anoxic, sulfidic conditions and high bioproductivity driven by a warm climate, which facilitated the accumulation of marine-derived organic matter.

Original authors: Adel Mohammad Al-Matary, Mohammed Hail Hakimi, Ali Saleh Naji Ghurab, Bassim S. Al Khirbash, Murad M. Abdulfarraj, Azmi Saleh Ali Alawdi

Published 2026-06-25

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CC BY 4.0

Original authors: Adel Mohammad Al-Matary, Mohammed Hail Hakimi, Ali Saleh Naji Ghurab, Bassim S. Al Khirbash, Murad M. Abdulfarraj, Azmi Saleh Ali Alawdi

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Earth's history as a giant, ancient library. In this library, the Late Jurassic period (about 150 million years ago) was a particularly hot, steamy chapter where the planet was a "greenhouse" with no ice caps.

This paper is like a team of detectives (geologists) investigating a specific, dusty shelf in that library located in southeastern Yemen. They are looking at a layer of rock called the Lam Member, which is made mostly of limestone and dark, muddy shale. Their goal? To figure out why this specific layer is so rich in ancient organic "sludge" (dead plants and animals) and what the world looked like when it was formed.

Here is the story they uncovered, broken down into simple parts:

1. The "Gold Mine" of Ancient Life

The team found that this rock layer is incredibly rich in organic matter. Think of it like a sponge that has soaked up a massive amount of oil and life. Some samples were up to 8.4% pure organic material and even 18.8% sulfur.

The Analogy: Imagine a swimming pool that isn't filled with water, but with thick, oily soup. That's how rich this rock is. This tells us that a huge amount of life was trapped there, but it didn't rot away.

2. The "No-Oxygen" Trap

Why didn't the organic matter rot? Usually, when dead things sink to the bottom of the ocean, bacteria eat them, and they disappear. But in this ancient sea, the bottom was a dead zone.

The Clue: The researchers found tiny, ball-shaped crystals of iron sulfide (called framboidal pyrite) that are smaller than a grain of sand. They also found very low ratios of certain chemicals (Pr/Ph).
The Metaphor: Think of the ocean floor as a room with the windows sealed shut. No fresh air (oxygen) could get in. Because the "air" was gone, the bacteria that usually clean up the mess couldn't survive. The dead plants and animals just piled up, layer by layer, like snow in a room with no wind to blow it away. This created a perfect "time capsule" for organic matter.

3. Who Was the "Guest of Honor"? (The Source of Life)

The team wanted to know: What kind of life was dying and sinking here? Was it trees from a forest, or tiny creatures floating in the water?

The Evidence: They used a "chemical fingerprint" (biomarkers) and looked at the rocks under a microscope. They found glowing, jelly-like blobs called alginite (made from ancient algae) and very few pieces of wood (vitrinite).
The Conclusion: The ocean was teeming with plankton and bacteria, like a massive underwater salad bar. There were very few land plants involved. The "soup" was made almost entirely of marine microorganisms, not trees or leaves.

4. The "Sticky Floor" and the "Hot Bath"

Two main things helped create this massive layer of organic rock:

The Hot Bath: The Earth was in a warm-water climate. Just like a warm bath makes you sweat and feel active, this warm climate made the ocean life reproduce like crazy. More life meant more "food" sinking to the bottom.
The Sticky Floor: The ocean water was salty and layered (stratified). Imagine a layered cocktail where the heavy, salty syrup is at the bottom and the lighter water is on top. The two layers didn't mix. This kept the oxygen trapped at the surface, leaving the bottom completely empty of oxygen. This "stuck" the dead algae in place before it could decompose.

5. The Big Picture

The paper concludes that the Lam Member in Yemen is a perfect example of a "perfect storm" for creating oil source rocks:

Hot weather made life explode in the ocean.
Salty, layered water kept the bottom of the ocean oxygen-free.
No oxygen meant the dead life didn't rot; it got buried and preserved.

In short: The researchers found that millions of years ago, a warm, salty, and oxygen-starved ocean in Yemen acted like a giant, natural freezer, preserving a massive amount of ancient algae and bacteria. This preserved "frozen" organic matter is what makes this rock layer so valuable today as a potential source of energy.

Note: This explanation sticks strictly to the findings in the paper regarding the geological history, the type of organic matter, and the environmental conditions of the Late Jurassic. It does not speculate on future drilling or economic applications not explicitly detailed in the text.

Technical Summary: Geochemical Characteristics and Microscopic Analysis of Late Jurassic Organic-Rich Carbonates in the Shabwah Depression, Yemen

Problem Statement
The Late Jurassic Lam Member (LM) within the Shabwah depression of southeastern Yemen represents a significant organic-rich carbonate unit with high hydrocarbon potential, characterized by Total Organic Carbon (TOC) values reaching up to 14 wt.% and Hydrogen Index (HI) values up to 600 mg HC/g TOC. Despite its economic importance as a source rock, there is a notable scarcity of integrated studies regarding the paleoenvironmental attributes that controlled the deposition and preservation of organic matter (OM) within these sediments. Specifically, previous research has lacked a comprehensive synthesis of biomarker distributions, elemental analysis, and kerogen microscopic data to fully elucidate the sources of organic matter and the specific depositional conditions (e.g., redox state, salinity, and climate) responsible for OM accumulation.

Methodology
To address these gaps, the study analyzed 82 cutting samples from five exploration wells (Al Jamilah-1, Tubba-1, Al Maz-1, Bin Haydar-1, and Misah-1) in the Shabwah depression. The research employed an integrated multi-disciplinary approach:

Organic Geochemistry: Measurements included TOC content, Rock-Eval (RE) pyrolysis (S1, S2, Tmax, HI, PI), and elemental analysis (CHNS) of isolated kerogen.
Biomarker Analysis: Saturated hydrocarbon fractions were analyzed via Gas Chromatography (GC) and Gas Chromatography-Mass Spectrometry (GC-MS) to identify n-alkanes, isoprenoids (pristane/phytane), hopanes, terpanes, and steranes.
Isotope Geochemistry: Stable carbon isotope ( $\delta^{13}$ C) compositions were determined for both aliphatic and aromatic fractions.
Microscopic Examination: Reflected light and UV fluorescence microscopy were used to characterize macerals (e.g., alginite, vitrinite, bituminite) and framboidal pyrite. Transmitted light microscopy of isolated kerogen was utilized for palynofacies analysis (AOM, palynomorphs, phytoclasts) to determine kerogen types.

Key Results

Organic Matter Abundance and Quality: The LM samples exhibit high TOC (up to 8.40 wt.%) and Total Sulfur (up to 18.85 wt.%). Rock-Eval data indicates a predominance of Type II and II/III kerogen, with HI values frequently exceeding 250 mg HC/g TOC, signifying hydrogen-rich organic matter.
Organic Matter Source: Microscopic analysis revealed a dominance of alginite (telalginite and lamalginite) and bituminite, with low vitrinite content. Biomarker data supports a primarily marine origin (planktonic and bacterial) with minor terrestrial input. This is evidenced by:
- Unimodal n-alkane distributions dominated by short-to-intermediate chains ( $C_{15}$ – $C_{26}$ ).
- Low Pristane/Phytane (Pr/Ph) ratios (< 2).
- High $C_{30}$ hopane relative to $C_{29}$ norhopane.
- $\delta^{13}$ C values for aliphatic fractions ranging from -26.60‰ to -29.59‰, consistent with marine organic matter.
Depositional Environment: The data collectively points to deposition under anoxic to dysoxic, sulfidic, and stratified conditions:
- Redox: Low Pr/Ph ratios (0.30–1.26) and the presence of small-sized framboidal pyrite (<5µ) indicate oxygen-depleted bottom waters.
- Sulfidic Conditions: High sulfur content and a strong positive correlation between Fe and S (R² = 0.99) suggest extensive pyritization under sulfidic conditions.
- Salinity Stratification: The presence of gammacerane (G/C30 ratios up to 0.61) indicates water column stratification driven by moderate to high salinity.
Paleoclimate Context: The study links these depositional conditions to the warm, humid greenhouse climate of the Late Jurassic. This climate likely enhanced weathering and nutrient delivery to the basin, stimulating high primary productivity in the photic zone, which, combined with anoxic bottom waters, facilitated OM preservation.

Significance and Claims
The paper claims that the integration of geochemical and microscopic data successfully reconstructs the paleoenvironmental history of the Lam Member. The study concludes that the accumulation of organic-rich carbonates in the Shabwah depression was driven by a synergy of tectonic subsidence (creating restricted basins), a warm-water climate (enhancing productivity), and anoxic/sulfidic depositional conditions (ensuring preservation).

Specifically, the authors assert that:

The LM sediments were deposited in a marine environment characterized by anoxia and sulfidic conditions, which were critical for the effective preservation of organic matter.
The organic matter is predominantly marine-derived (algal and bacterial), forming hydrogen-rich Type II kerogen, rather than being dominated by terrestrial plant input.
The elevated warm-water climate of the Late Jurassic played a pivotal role by promoting eutrophic conditions and nutrient influx, which, when coupled with water column stratification, created an ideal setting for the formation of these organic-rich source rocks.

These findings provide a refined understanding of the depositional controls on the Lam Member, offering insights into the regional petroleum system of the Sab'atayn Basin.

1. The "Gold Mine" of Ancient Life

2. The "No-Oxygen" Trap

3. Who Was the "Guest of Honor"? (The Source of Life)

4. The "Sticky Floor" and the "Hot Bath"

5. The Big Picture

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