A Novel Reservoir Forming Mechanism with Wax-Out Cryo Trapping
Authors: Gan Yunyan , Chen Li , Zhang Jinqing , Mullins Oliver C. , Yan Zhenghe , Tian Ji , Gao Xiaofei , Chen Weihua , Yang Haizhang , Hao Jianrong 
Published: February 2022

In South China Sea, about one half of the producing oilfields contain a type of strange and enigmatic oil. First, these waxy crude oils contain almost no solution gas, with bubble points of few atmospheres only. The formation has never been uplifted to very shallow depths to be degassed to that low-level of solution gas. Second, these crude oils also lack of other lighter alkane components up to C12, which, in any event, cannot be removed by a degas process. These crude oils are also not biodegraded with the full complement of n-alkanes heavier than C12, so the lighter n-alkanes have not been removed by this process. There is no credible explanation of these crude oils from maturity considerations either. This kind of reservoir fluid has long been known, however, has not been understood in terms its mechanism of formation. Petroleum system modeling in corresponding basins has not been accurate with regard to these reservoir fluid properties; thereby yielding significant uncertainty in both basin modeling and understanding the oil.

This paper introduces a newly identified reservoir forming mechanism, for the first time, to account for these seemingly discordant fluid properties. Central to this new mechanism is "Wax-Out Cryo Trapping". When a crude oil in migration encounters its Wax Appearance Temperature (WAT) along the migration path, the wax can crystallize out of the oil yielding a large, localized wax deposit with associated trapped liquids. It is indeed trapped by the temperature. The mobile, dewaxed fluid phase can continue in migration up either becoming trapped in a separate conventional reservoir or lost via a seep. After continued subsidence, the formation holding the solid wax can heat above WAT thereby remobilizing this formerly frozen oil. This waxy oil can then proceed in migration, possibly along a migration path altered by subsidence. This waxy oil can then fill traps associated with the migration path and remain trapped until present day. The Wax-Out Cryo Trapping process is acting as a compositional fractionator spatially separating the initial waxy oil into its light liquid and gas fractions in one fraction and its waxy and occluded heavier liquids in the other fraction. It is routine to have crude oils undergo the phase transition of gas evolution in migration, but in that case, both phases migrate together. In the Wax-Out Cryo Trapping, there is a solid liquid phase transition with spatial separation based on very different mobilities of solid wax and the dewaxed fluid phase.

The paper summarizes the distribution of the waxy "zero-GOR" crude oil in South China Sea and the source rock characteristics. It describes a systematic approach to identify the wax-out process and to rule out other possibilities. This analysis integrated many fluid properties, including PVT, GC, GOR, WAT, and thermal maturity, with hydrocarbon migration and formation bury history from basin modeling. The understanding of this process opens a new window to understand origin of the different fluids and their distribution within the basin, providing the time constraints and guidance for hydrocarbon generation and migration in petroleum system modeling.

Wax-Out Cryo Trapping can happen when waxy oil from source rock migrates along paths with temperatures lower than the WAT temperature. This process is shown to be very common in the South China Sea, and it should also be somewhat common globally for the high wax content oilfields. But it is until now, this new mechanism just got discovered. It is proved to be an innovated model, introduce a new migration pattern for basin modeling and eventually provides the important guidance for oilfield exploration.