Effects of hydrocarbon generation on fluid flow in the Ordos Basin and its relationship to uranium mineralization

The Ordos Basin of North China is not only an important uranium mineralization province, but also a major producer of oil, gas and coal in China. The genetic relationship between uranium mineralization and hydrocarbons has been recognized by a number of previous studies, but it has not been well understood in terms of the hydrodynamics of basin fluid flow. We have demonstrated in a previous study that the preferential localization of Cretaceous uranium mineralization in the upper part of the Ordos Jurassic section may have been related to the interface between an upward flowing, reducing fluid and a downward flowing, oxidizing fluid. This interface may have been controlled by the interplay between fluid overpressure related to disequilibrium sediment compaction and which drove the upward flow, and topographic relief, which drove the downward flow. In this study, we carried out numerical modeling for the contribution of oil and gas generation to the development of fluid overpressure, in addition to sedi- ment compaction and heating. Our results indicate that when hydrocarbon generation is taken into account, fluid overpressure during the Cretaceous was more than doubled in comparison with the simu- lation when hydrocarbon generation was not considered. Furthermore, fluid overpressure dissipation at the end of sedimentation slowed down relative to the no-hydrocarbon generation case. These results suggest that hydrocarbon generation may have played an important role in uranium mineralization, not only in providing reducing agents required for the mineralization, but also in contributing to the driving force to maintain the upward flow.

Effects of Overpressured Fluid Flow on Petroleum Accumulation in the Yinggehai Basin

The Yinggehai Basin is a strongly overpressured Cenozoic basin developed in the northern continental shelf of the South China Sea. The flow of overpressured fluids in this basin has given rise to strong effects on petroleum accumulation. (1) The overpressured fluid flow has enhanced the maturation of shallow-buried source rocks, which has caused the source rocks that would have remained immature under the conduction background to be mature for hydrocarbon generation. As a result, the overpressured fluid flow has increased the volume and interval of mature source rocks. (2) The overpressured fluid flow has strong extraction effects on the immature or low-mature source rocks in the shallow parts. This has increased, to some extent, the expulsion efficiency of the source rocks. More importantly, the extraction effects have strongly limited the effectiveness of biomarker parameters from oil and condensate in reflecting the source and maturity of the oil and gas. (3) The flow has caused the sandstones in the shallow parts to get into the late diagenesis stage, and significantly reduced the porosity and permeability of the sandstones. This study confirms that even in sedimentary basins in which no topography-driven groundwater flow systems have ever developed, the cross-formation migration of overpressured fluids and the resultant energy conduction and material exchange can significantly affect the thermal regime, source rock maturation and sandstone diagenesis. As a result, the effects of overpressured fluid flow must be taken into account in analyzing the mechanism of petroleum accumulation.

Hydrodynamic analysis of clastic injection and hydraulic fracturing structures in the Jinding Zn-Pb deposit,Yunnan,China

The Jinding Zn-Pb deposit has been generally considered to have formed from circulating basinal fluids in a relatively passive way, with fluid flow being controlled by structures and sedimentary facies, similar to many other sediments-hosted base metal deposits. However, several recent studies have revealed the presence of sand injection structures, intrusive breccias, and hydraulic fractures in the open pit of the Jinding deposit and suggested that the deposit was formed from explosive release of overpres- sured fluids. This study reports new observations of fluid overpressure-related structures from under- ground workings （Paomaping and Fengzishan）, which show clearer crosscutting relationships than in the open pit. The observed structures include： 1） sand （--rock fragment） dikes injecting into fractures in solidified rocks; 2） sand （~rock fragment） bodies intruding into unconsolidated or semi-consolidated sediments; 3） disintegrated semi-consolidated sand bodies; and 4） veins and breccias formed from hydraulic fracturing of solidified rocks followed by cementation of hydrothermal minerals. The development of ore minerals （sphalerite） in the cement of the various clastic injection and hydraulic fractures indicate that these structures were formed at the same time as mineralization. The development of hydraulic fractures and breccias with random orientation indicates small differential stress during mineralization, which is different from the stress field with strong horizontal shortening prior to miner- alization. Fluid flow velocity may have been up to more than 11 m/s based on calculations from the size of the fragments in the clastic dikes. The clastic injection and hydraulic fracturing structures are interpreted to have formed from explosive release of overpressured fluids, which may have been related to either magmatic intrusions at depth or seismic activities that episodically tapped an overpressured fluid reservoir. Because the clastic injection and hydraulic structures are genetically linked with the mineralizing fluid source, they can be used as a guide for mineral exploration.

Hydrodynamic regime as a major control on localization of uranium mineralization in sedimentary basins

Uranium deposits in sedimentary basins can be formed at various depths,from near surface to the basement.While many factors may have played a role in controlling the location of mineralization,examination of various examples in the world,coupled with numerical modeling of fluid flow,indicates that the hydrodynamic regime of a basin may have exerted a major control on the localization of uranium deposits.If a basin is strongly overpressured,due to rapid sedimentation,abundance of low-permeability sediments or generation of hydrocarbons,fluid flow is dominantly upward and uranium mineralization is likely limited at shallow depths.If a basin is moderately overpressured,upward moving fluids carrying reducing agents may meet downward moving,oxidizing,uranium-bearing fluids in the middle of the basin,forming uranium deposits at moderate depths.If a basin is weakly or not overpressured,either due to slow sedimentation or dominance of high-permeability lithologies,minor topographic disturbance or density variation may drive oxidizing fluids to the bottom of the basin,leaching uranium either from the basin or the basement,forming unconformity-type uranium deposits.It is therefore important to analyze the hydrodynamic regime of a basin in order to predict the most likely type and location of uranium deposits in the basin.