Can Sediments Generate Helium? Implications from 232Th-238U-40K Concentration Logs from the Northern Arabian Plate

Identification of source rocks bearing helium generation potential is essential to construct a robust play fairway for natural helium exploration. The main source rock for helium generation are widely accepted as granitic or metamorphic basement rocks of cratons while some researchers suggest that hydrocarbon source rocks and sediments might also generate helium. One of the most critical implications for potential zones is the presence of radioactivity as He generation is sourced from the alpha decay of ²³²Th, ²³⁸U, and ²³⁵U. Thus, more He generation means more decay, characterised by increasing radioactive heat. In addition, distinguishing heavy thorium minerals as clay types by ²³²Th-⁴⁰K cross-plots could also indicate potential zones. Therefore, measuring and assessing the ²³²Th-²³⁸U-⁴⁰K levels play a critical role in any region for natural helium exploration.

This study brings forward well log interpretation approach as one of the transferable methods from the oil and gas industry into natural He exploration by examining the ²³²Th-²³⁸U-⁴⁰K concentration logs, known as SGR logs, which are generally neglected or overlooked although they provide numerous benefits for subsurface evaluation.

Based on the methodology 2 main research questions emerge for this study to answer;

• Can sediments and hydrocarbon source rocks might generate He or contribute to the He generation process?
• Can SGR Logs provide a robust methodology for detection of potential He generating intervals in sedimentary successions?

To answer these questions, Early and Mid-Triassic sediments from the Northern Arabian Plate are selected as a case study. Recently unlocked Mid-Triassic hydrocarbon play, including source rocks, and CO₂ / N₂ readings on gas chromatography of nearby wells make the region unique and a perfect study area to test the hypothesis. Radiogenic heat generations (A) have been calculated using the equation below to track radioactivity levels.

A = 0.01 p (9.52 ²³⁸U + 2.56 ²³²Th + 3.48 ⁴⁰K)

A; radiogenic heat (μWm^–3),

p; rock density (g/cm3),

²³⁸U, ²³²Th, ⁴⁰K; Uranium ²³⁸U (ppm); Thorium ²³²Th (ppm); potassium ⁴⁰K (%)

Regarding the observations, a 1-15 m. thick, theoretical He generation zone has been detected in the shales of the Early Triassic succession. A consistent significant peak in radiogenic heat levels reaching 4 μWm^–3 coincide with rapid increases in calculated He log and heavy thorium minerals content. Additionally, shales are represented by as high ²³²Th-²³⁸U levels as granitic basements. A thickness map of potential He generation zone demonstrates that the zone gets thinner towards ESE at where large fault zones dominate the regional geology.

As a conclusion, the findings of this study suggest that sediments might generate natural He and potential zones might be identified by the help of SGR logs. The results can also shed light on the He generation potential of Triassic sediments deposited in the other regions of the Arabian Plate. Moreover, the proposed workflow can be applied for any region or rock type if the interval of interest is covered by ²³²Th-²³⁸U-⁴⁰K concentration logs.