📄 earth science

Genesis of Natural Gas in the Bozhong Sag, Bohai Sea: Constraints from Pyrolysis Experiments

Based on closed-system gold-tube pyrolysis experiments, this study demonstrates that deep natural gas in the Bozhong Sag is primarily generated from highly mature lacustrine sapropelic kerogen rather than coal-derived sources, challenging the assumption that heavy ethane isotopes necessarily indicate a humic origin.

Original authors: Kuo Li, Long Su, Tao Ye, Tongyao Zhang, Peng Hao, Ge Yan, Haidong Wang

Published 2026-09-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Kuo Li, Long Su, Tao Ye, Tongyao Zhang, Peng Hao, Ge Yan, Haidong Wang

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

Deep beneath the waves of the Bohai Sea, off the eastern coast of China, lies a geological puzzle that has challenged scientists for decades. The Bohai Bay Basin is a region rich in oil, but it also holds vast, hidden reserves of natural gas trapped in deep rock layers. For explorers, knowing exactly where this gas comes from is a matter of survival. If the gas was formed by the intense heat cooking ancient plant matter, it behaves one way; if it was formed by the breakdown of liquid oil that was already trapped underground, it behaves another. The difference matters because it tells geologists how deep they must drill and which rocks to target. In the world of energy exploration, the origin story of a gas field determines its future. The question is not just academic; it is the key to unlocking resources that could power cities for years to come.

To solve this mystery in the Bozhong Sag, a specific depression within the Bohai Sea known for its massive gas discoveries, a team of researchers turned to the laboratory. They could not simply wait for nature to cook the rocks over millions of years, so they decided to speed up the process. They took samples of the ancient mud and coal found deep underground, as well as samples of the crude oil trapped in the reservoirs, and placed them inside sealed, heavy-walled metal tubes. These tubes were then heated in a furnace to temperatures far higher than the surface, simulating the intense heat and pressure of the deep Earth. By carefully controlling the heat, the scientists could watch the rocks and oils transform, releasing the gases they contained, and then analyze those gases to see what they were made of.

The experiment was designed to test two main ideas about how the deep gas in this region was formed. One theory suggested the gas came from the cracking of liquid oil that had been trapped in the rocks. The other theory proposed that the gas was generated directly from the solid organic matter in the source rocks, a process that happens when the rocks get extremely hot. The researchers ran these simulations at three different levels of heat intensity, representing different stages of geological maturity. They then measured the composition of the gas produced, looking specifically at how much methane it contained compared to heavier gases, and they analyzed the carbon atoms within the gas molecules to see if they were heavy or light.

The results were clear and decisive. The gas produced by heating the liquid oil in the lab remained relatively "wet," meaning it contained a significant amount of heavier hydrocarbons alongside the methane. It also carried a distinct chemical signature that was lighter than the gas found in the actual deep reservoirs. In contrast, the gas produced by heating the solid source rocks directly was much "drier," consisting almost entirely of methane, and its chemical fingerprint matched the natural gas discovered in the Bozhong Sag almost perfectly. The researchers found that the natural gas in the field is methane-dominated, with dryness coefficients ranging from 0.73 to 0.94, a level of dryness that the oil-cracking simulations simply could not reproduce. This evidence strongly suggests that the gas was not formed by the breakdown of trapped oil, but rather by the direct thermal breakdown of the solid organic matter in the source rocks themselves.

A further layer of complexity arose from the chemical weight of the gas molecules. In the past, scientists often assumed that if the ethane molecules in a gas field were heavy, the gas must have come from coal or plant-rich rocks, known as humic sources. However, this study revealed a different possibility. The experiments showed that when the solid lake-bed rocks were heated to very high temperatures, they also produced gas with heavy ethane molecules. This means that the heavy chemical signature found in the Bozhong Sag does not necessarily point to coal as the source. Instead, it indicates that the lake-bed rocks were cooked to an extreme degree, reaching a level of maturity that is rare but possible in this deep basin.

The study also clarified the timing of these events. The solid rocks began generating significant amounts of gas at a moderate level of heat, while the liquid oil required much higher temperatures to break down into gas. Since the deep reservoirs in the Bozhong Sag have reached those extreme temperatures, the oil that might have been there has likely already been converted or expelled. The gas currently sitting in the deep wells is therefore the product of the rocks themselves, cooked over millions of years until they released their final, dry breath of methane.

These findings provide a new map for the future of exploration in the region. By understanding that the gas comes from the direct cracking of solid lake-bed rocks rather than the breakdown of oil, geologists can better predict where similar gas fields might exist. They now know that the heavy chemical signatures they see are not a sign of coal, but a sign of extreme heat acting on lake sediments. This distinction allows them to look deeper and with more confidence, knowing that the rules of the game in the deep Bohai Sea are different from what was previously assumed. The work confirms that the vast gas reserves in the Bozhong Sag are the result of a specific, intense geological history, offering a clearer path to understanding the energy hidden beneath the waves.

Technical Summary: Genesis of Natural Gas in the Bozhong Sag, Bohai Sea

Problem Statement
The Bohai Bay Basin is a prolific oil province with significant deep natural gas discoveries, such as the BZ19-6 metamorphic buried-hill condensate gas field. However, the genesis of these deep gases remains controversial. While global gas fields are typically sourced from marine or transitional coal-type strata, the Bohai Sea is a continental lacustrine basin dominated by oil. Existing literature offers conflicting interpretations: some studies attribute the gas to humic (coal-derived) kerogen, others to oil-cracking, and some to advanced thermal evolution of oil-prone source rocks. Key uncertainties persist regarding the distinctions between gases of different origins, the distribution of effective source rocks, and the specific formation mechanisms of deep natural gas in this setting. Specifically, it is unclear whether the heavy ethane carbon isotopic values ( $\delta^{13}$ C-C $_2$ H $_6$ ) observed in the Bozhong Sag indicate a coal-derived origin or result from the cracking of highly mature lacustrine sapropelic kerogen.

Methodology
To resolve these uncertainties, the authors conducted closed-system gold-tube pyrolysis experiments coupled with online trace-gas analysis. The study utilized a comprehensive suite of geological samples representing different precursor types:

Sapropelic Source Rocks: Three low-maturity Paleogene lacustrine mudstones and two mature lacustrine source rocks from the Bozhong Sag.
Humic Source Rocks: Low-maturity Paleogene coal and mature-to-highly mature Mesozoic coals.
Crude Oils and Extracts: Representative light crude, extra-heavy oil, and various bitumen fractions (saturates, aromatics, resins, asphaltenes) from different structures.

The experiments simulated thermogenic gas generation at equivalent vitrinite reflectance (EasyRo) values of 1.5%, 2.0%, and 2.5% (corresponding to 420°C, 450°C, and 480°C). A two-stage pyrolysis approach was employed on source rocks: primary pyrolysis to simulate oil generation and secondary pyrolysis of the residual kerogen to isolate kerogen-cracking gas. Gas compositions, dryness coefficients, and stable carbon isotopic compositions ( $\delta^{13}$ C-CH $_4$ , $\delta^{13}$ C-C $_2$ H $_6$ , $\delta^{13}$ C-C $_3$ H $_8$ ) were measured and compared against genetic discrimination diagrams.

Key Results

Differentiation of Gas Types:
- Dryness: Kerogen-cracking gas (from both sapropelic and coal sources) and coal-type gas exhibit significantly higher dryness coefficients (>70–90%) compared to oil-cracking gas (<70%) at equivalent maturity levels.
- Isotopic Composition: Kerogen-cracking gas possesses markedly heavier carbon isotopic compositions than oil-cracking gas. Specifically, $\delta^{13}$ C-CH $_4$ and $\delta^{13}$ C-C $_2$ H $_6$ become progressively enriched in $^{13}$ C with increasing thermal maturity.
- Oil Cracking Characteristics: Oil-cracking gas is relatively wet and isotopically light. The isotopic composition of cracked gas is strongly controlled by the precursor oil; heavier oils generate heavier gases. Notably, as oil cracking proceeds, $\delta^{13}$ C-C $_2$ H $_6$ becomes substantially heavier, while $\delta^{13}$ C-CH $_4$ remains relatively stable.
Generation Windows and Yields:
- Kerogen Cracking: The principal gas-generation interval for lacustrine sapropelic kerogen occurs at moderate to high maturity (EasyRo 0.8%–1.5%).
- Oil Cracking: Extensive cracking of in-source retained oil and reservoir oils occurs at high to overmature stages (EasyRo >1.5%). Light oil fractions exhibit higher cracking thresholds but greater maximum gas yields compared to heavy fractions.
- Coal vs. Sapropelic Kerogen: While coal-type gas and sapropelic kerogen-cracking gas share similar dryness and ln(C $_1$ /C $_2$ ) vs. ln(C $_2$ /C $_3$ ) trends, coal-type gas is distinguished by heavier $\delta^{13}$ C-CH $_4$ values due to the heavier initial isotopic signature of humic kerogen.
Application to Bozhong Sag:
- Comparison of experimental data with natural gas from the Bozhong area reveals that the discovered gas has a substantially higher dryness coefficient than experimental oil-cracking gas.
- The geochemical signature of the Bozhong gas closely resembles kerogen-cracking gas derived from highly to overmature lacustrine sapropelic organic matter.
- The heavy $\delta^{13}$ C-C $_2$ H $_6$ values observed in the field do not necessarily indicate a coal-derived origin; they can be attributed to the high thermal maturity of the sapropelic kerogen and contributions from the isotopically heavy third member of the Shahejie Formation (E2s3).

Significance and Claims
The paper claims to provide critical experimental constraints on the genesis of natural gas in continental lacustrine basins. By systematically comparing the geochemical characteristics of kerogen-cracking, oil-cracking, and coal-type gases, the study refutes the hypothesis that the deep gas in the Bozhong Sag is primarily derived from oil cracking. Instead, it identifies kerogen cracking as the dominant gas-generating process.

The authors assert that their findings offer a geological basis for deep-gas exploration in offshore areas, specifically by clarifying that heavy ethane isotopic values are not exclusive to humic gas. This distinction is vital for accurate genetic interpretation and resource assessment, preventing the misclassification of sapropelic-derived gas as coal-type gas. The study concludes that a refined model of late-stage gas generation from lacustrine source rocks is necessary to account for the complex interplay of precursor type, thermal maturity, and cracking mechanisms in the Bohai Sea region.

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