Characteristics of middle Jurassic overpressure and tight gas accumulation in Shengbei sub-sag, Tuha Basin, Xinjiang

This paper is the result of oil and gas exploration engineering.

Objective The tight gas in the Shengbei subsag of the Turpan−Hami Basin has become an important field for expanding new exploration battlefields and discovering new reserves. The development characteristics and accumulation mechanism of tight reservoirs have become one of the key scientific issues that need to be solved urgently.

Methods Using comprehensive research methods such as geochemistry, geophysics and oil and gas geology, the characteristics and accumulation stages of tight gas sources and reservoirs in the Middle Jurassic were systematically analyzed, and the characteristics of overpressure development and the controlling effect on tight gas accumulation were determined.

Results The following conclusions are drawn: (1) The organic matter type of the main source rocks is mainly type III kerogen, and the whole is in the mature stage dominated by gas generation. The Middle Jurassic developed low porosity and low permeability—low porosity and ultra−low permeability tight reservoirs, with an average porosity of 7.1% and an average permeability of 0.074×10⁻³ μm². The pore type is dominated by secondary dissolution pores, while clay mineral interlayer pores, pyrite intercrystalline pores and micro−fractures are developed. (2) The Middle Jurassic developed overpressure caused by pressure conduction and hydrocarbon generation pressurization. The pressure coefficient was mainly distributed between 1.2 and 1.5. The overpressure top interface was located in the middle and upper part of the Qiketai Formation vertically. The overpressure is mainly distributed in the east and southeast of the Shengbei subsag, and the fault system controls the distribution range of the overpressure. (3) The hydrocarbon expulsion from source rocks lasted for a long time. From the Late Triassic to the present, there have been at least two main periods of natural gas charging, and the two main accumulation periods are: Late Jurassic to Early Cretaceous and Paleocene to date.

Conclusions The Middle Jurassic tight gas reservoirs are dominated by the accumulation model of "two−stage accumulation from far−source and near−source, pressure−fault coordinated transport, and fault−overpressure coordinated control". The research results in this paper will provide scientific basis and technical support for tight gas exploration and development in Shengbei Sub−sag.