GAMMA SPECTROMETRY CHARACTERISTICS OF THE ORGANIC-RICH DOUSHANTUO SHALE IN THE MID-YANGTZE AREA

In the western Hubei province of China, the Yangye-1 well was drilled to discover shale gas in the Sinian Doushantuo Formation, which opened the door of a new exploration field for the shale gas. Abundance in organic matter is one of the most important controlling factors for the occurrence of shale-gas reservoirs. The Doushantuo Formation, deposited after the global glacial ablation, is commonly divided into 4 members. Its organic-rich shale is widespread with variant thickness and wide range of maturity in the mid-Yangtze area. Therefore, the Doushantuo Formation is a natural laboratory to elucidate the genesis of the organic-rich Precambrian shale. We established 9 profiles of lithology and gamma spectrometry by outcrop measurement in the western Hubei province and northwestern Hunan province, China. The result show that the 2nd member of the Doushantuo Formation (Z₁d²) is abundant in black shale with high gamma-ray values, especially in the lower part. The natural radioactivity of the rock is closely related to its lithology, and has a good correlation with the content of potassium (K) and thorium (Th). Unlike other rocks, the shale has obviously higher average values of the natural radioactivity with a quite wide range due to the variant uranium (U) content. The Th/U curve of Z₁d² shows that frequent fluctuation of the Th/U ratios between 0.6 and 11 in the lower part follows a decreasing trend with mostly Th/U ratios lower than 2, indicating that turbulent oxidizing-reducing conditions is gradually replaced by anaerobic environment. Besides, those parameters of natural radioactivity like total value, K, Th and U contents, and Th/U ratios are good measures for avail regional correlation and establishing a megascopic sequence framework.