Upper limit of maturity for hydrocarbon generation in carbonate source rocks in the Tarim Basin Platform, China

Abstract

The upper limit of maturity for hydrocarbon generation (ULMHG) is represented by vitrinite reflectance (VR) that corresponds to the point at which the amount of further hydrocarbon generation is less than 1 % of the total hydrocarbon generation. Six methods were employed to study the ULMHG of the Cambrian and Ordovician carbonate source rocks in the Tarim Basin Platform (TBP), based on the mechanism of hydrocarbon generation, retention, and expulsion. These methods are reduction in atomic ratio for organic elements, variation in hydrocarbon generation potential, optimum simulation of organic materials based on material balance principle, thermal simulation experiment, variation in hydrocarbon retention amount, and variation in hydrocarbon expulsion amount. Studies showed that, although there are some differences between the results obtained using different methods, these differences are small. The ULMHG in the TBP varies from 4.10 to 4.84 % with an average value of 4.55 %, which is in good agreement with the characteristics of hydrocarbon generation under actual geological conditions. The VR of the Cambrian and Ordovician carbonate source rocks in the Tazhong uplift, the Bachu uplift, and the Tabei uplift ranges from 0.64 to 2.34 %, indicating that these areas have not reached the ULMHG. The VR in the Tadong area is very close to the ULMHG, and the VR of most source rocks in the Manjiaer sag has exceeded the ULMHG. In summary, the carbonate source rocks in the TBP have generated large amounts of hydrocarbons and still have high hydrocarbon generation potential to form large-scale oil and gas reservoirs, indicating good hydrocarbon exploration prospects in the deep parts of the TBP. Therefore, the study of ULMHG of carbonate source rocks is of great significance for evaluating the oil and gas resource potential and predicting favorable exploration areas in deep or carbonate rock formations.