A new type of unconventional natural gas resources: Overpressure-dissolved gas

In overpressure reservoirs, natural gas often coexists in a three-phase mixed form of continuous free state, dispersed free state and water-saturated dissolved state. However, the latter two have not received sufficient attention. In response to this situation, based on detailed characterization of typical overpressure dissolved gas in the Yinggehai-Qiongdongnan basin and the experiment results of natural gas dissolution with high-temperature and overpressure, the concept of “overpressure-dissolved gas” was proposed and its basic features, formation conditions and resource potential were summarized. It refers to the natural gas present in the gas-water transitional zone and the saturated dissolved gas zone within the overpressure reservoirs. The formation of overpressure-dissolved gas requires two basic conditions: the pressure coefficient typically greater than 1.5, and a relatively high gas saturation in the reservoir (10%–35%). Overpressure-dissolved gas exists in the strata from shallow to deep with a multi-stage superimposed pattern; there are at least four combination types: overpressure- dissolved gas with multiple gas caps, overpressure-dissolved gas with single gas cap, gas-bearing water layer without gas cap, and dissolved gas-bearing water layer without gas cap. The basic geological elements required for the formation of overpressure-dissolved gas include the gas source, reservoir, cap rock, gas-water transitional zone and overpressure body. The conditions of gas source, reservoir and cap rock determine the scale of the overpressure-dissolved gas zone. High temperature, high pressure and low-permeability reservoirs control the solubility of natural gas and the thickness of the gas-water transitional zone. The physical properties of sandstone determine the combination types of overpressure-dissolved gas. Changes in pressure control the transformation of different existing states of overpressure-dissolved gas. The overpressure-dissolved gas in the Yinggehai-Qiongdongnan Basin has considerable huge resource potential. Once breakthrough is achieved in this area, it will usher in a new era of natural gas exploration in the overpressured basin.