Formation and Preservation conditions of ultra-deep (>6000 m) hydrocarbon systems

For 40 years, the concept of petroleum systems has been well developed and a considerable amount of actual data has been accumulated. However, with the beginning of the development of ultra-large depths (more than 6000 m, although nowadays this boundary has been lowered to 8000 m) it became clear that many processes proceed somewhat differently, and some a priori assumptions cease to be relevant. The article deals with the peculiarities of formation and conditions of preservation of petroleum systems when diving to ultra-large depths. Special attention is paid to numerous examples of HC detection in the liquid phase accumulated over the last decades, when the upper ‘classical’ threshold of formation temperatures, at which, as it was supposed, the transformation of liquid HC into gaseous should occur. Examples of studies of ultra-deep sections, first, well data, considered in the article, allow us to constantly revise upward the maximum temperatures in the pools, leading to oil destruction, previously estimated rather conservatively. This, in turn, causes reassessment of HC potential of many basins. At the same time, the volume of reserves of both oil and gas in ultra-deep sections is constantly increasing, especially in China, where vast experience of direct continental projects has been accumulated. At the same time, there are limited opportunities to prepare any practical recommendations directly, and the development of methods for forecasting such accumulations requires further significant efforts. It can be confidently assumed that the upper limit of the so-called ultra-deep will fall significantly below 8000 m, as the data show that geological constraints are significantly decreasing, technological solutions are continuously emerging, and their costs are steadily decreasing. The development of ultra-deep projects as an alternative to the so-called ‘shale’ projects will inevitably lead to an increase in their efficiency in developing oil and gas deposits, which are not perceived as such today. The article considers examples of ultra-deep hydrocarbon systems in the basins of continental China (Tarim, Dzungarian and Sichuan) and the Gulf of Mexico (Perdido folded zone).