Main characteristics and effectiveness analysis of potential helium source rocks in crust-source helium-rich natural gas reservoirs

The evaluation of potential helium source rocks’ effectiveness is a core issue in the field of helium resource exploration and development. However, previous studies predominantly rely on uranium (U) and thorium (T) contents and the age of the rock for evaluating the effectiveness of potential helium source rocks, which fails to fully characterize the key factors affecting their effectiveness. Therefore, this paper takes four typical potential helium source rocks—granite, mud shale, gneisses, and bauxite, as the research object. Through the establishment of a calculation model for accumulated dissolved helium in pore water, combined with gas reservoir examples, a quantitative analysis of dissolved helium accumulation and exsolution in the pores of each potential helium source rock is carried out. This analysis aims to discuss and summarize the effectiveness of each potential helium source rock and the evaluation method for identifying effective helium source rocks. It is believed that: (1) The exsolution of dissolved helium accumulated in the pores of potential helium source rocks into free helium on a large scale under suitable conditions is the key prerequisite for identifying their effectiveness; (2) In addition to the content of U and Th elements, parameters such as large volume, suitable porosity, and water saturation, the good matching relationship of “sedimentary and burial history, gas accumulation history and tectonic evolution history”, and relatively specific helium generation ability are also the key parameters for judging the effectiveness of potential helium source rocks; (3) By establishing the calculation method of dissolved helium enrichment efficiency (η_He), it is concluded that under similar conditions, free helium enrichment is most easily achieved in granite, followed by mud shale and bauxite, while gneiss poses the greatest difficulty.