A discovery of Silurian tight shale gas in Jianshi, Hubei, China

Zhi Zhou, Shu-jing Bao, Xiang-lin Chen, Shu-fang Yu, Ke Chen, Tian-xu Guo, Qiu-feng Xu

Oil and Gas Survey, China Geological Survey, Beijing 100083, China

1. Objective

The study area is located in the north of Jianshi area,Hubei province. According to the geologic survey, the Silurian black shale in the area is characterized by high abundance of organic matter and high hydrocarbon potential. From 1980s, Jianghan Oilfield has drilled 7 wells in Jiannan gas field located in the south of study area, which showed different degree of oil and gas and revealed a good exploration prospect for the Silurian strata.

The purpose of this study is to find out the stratigraphic prospecting of Silurian strata in northern Jianshi area, the characteristics of organic rich shale within Longmaxi Formation (S1l) obtain reservoir physical parameter of the Xintan Formation (S1x) tight sandstone, learn oil-gas reservoir-forming assemblages of this area, and provide data for evaluating potentials of shale gas and tight sandstone gas.

2. Methods

Through geological field survey, sample collection and test analysis to the Silurian strata in northern Jianshi and its surrounding areas, and combined with regional tectonics and lithofacies palaeogeography, CGS oil & gas survey drilled geological survey well JD-1 in northern Jianshi area. The drilling depth of JD-1 is 1810 m, core length is 1784 m, and the whole well section is performed the logging.

In order to obtain the abundance and type of shale organic matter, thermal maturity, gas bearing, reservoir porosity and permeability, core samples collected from the Longmaxi Formation, the Xintan Formation and the Luoreping Formation are analyzed on organic geochemistry and Reservoir physical property. The data mentioned above can combined with gas logging data to evaluate the potential of oil and gas resources in the study area.

3. Results

JD-1 shows that the Silurian strata consists of the Shamao Formation, the Luoreping Formation, Xintan Formation and Longmaxi Formation. The Longmaxi Formation and overlying the Upper Ordovician Wufeng Formation (O3w) is composed of graptolite rich black shale, whose thickness is 43.7 m. The TOC content of shale is 0.2% to 11.24%, with the average value is 4.39%, in which the thickness of shale of TOC>2% is 25 m, mainly in the bottom of the Langmaxi formation and the Wufeng Formation (Fig. 1). The type of organic matter is I-II1type, and the thermal maturity (Ro) is 2.5% to 2.9%, which means that it is in the early stage of over maturity, showing good hydrocarbon generation ability. The Xintan Formation is composed of deep gray argillaceous siltstone with porosity ranging from 5.6% to 11.24% and air permeability ranging from 0.0455 mD to 1.6712 mD, with an average value of 0.777 mD, which is a typical tight sandstone reservoir. The Luoreping formation and Shamao group is composed of thick layer of green-gray green mudstone with permeability generally lower than 10-6mD. Based on the above,the Silurian strata in JD-1 wells drilling area developed the tight sandstone gas source reservoir caprock combinations,which is consist of Wufeng-Longmaxi Formation (source rock) - Xintan Formation (reservoir)-Luoreping Formation(caprock), with the Wufeng-Longmaxi Formation shale gas and Xintan Formation tight sandstone gas reservoir-forming assemblages.

The gas logging abnormal is shown in the Silurian strata during JD-1 well drilling. The total gas logging hydrocarbon in 1190-1738 m well section of the Xintan Formation is more than 1% of thickness of 193 m, and more than 2% of thickness of 93 m (Fig. 1), which is interpreted as gas bearing reservoir. Gas logging total hydrocarbon and methane is increased from 0.09% to 21.5% and 0.087% to 19.89% with gas out of mud ignition success respectively when drilling to 1345 m deep gray argillaceous siltstone. This is the first acquisition of tight sandstone gas in the Silurian strata in the complex tectonic area of west Hubei to northeast Chongqing, which is expected to open a new area of oil and gas exploration and de-velopment in this area. Gas logging total hydrocarbon in 1738.55-1782.25 m well section of the Wufeng-Longmaxi Formation is 0.93% to 11.07%. The black shale is graptolitebearing and the bubble is obvious when the core of the rock inundated. The field desorption gas content of 21 core samples is increased to 0.92 m3/t on average (without loss gas and residual gas). Drilling shows that the Silurian strata in north Jianshi area has the prospect of exploration and development for tight sandstone gas and shale gas.

Fig. 1. Comprehensive evaluation columnar section of the silurian in JD-1 well.

4. Conclusions

JD-1 well drilling in North Jianshi area shows that the Upper Ordovician Wufeng Formation organic-rich shale is characteristed by large thickness, high abundance of organic matter, moderate thermal maturity and good hydrocarbon generation ability, and the Xintan Formation is a typical tight sandstone reservoir. The source-reservoir-cap assemblages for Wufeng-Longmaxi Formation shale gas and Xintan Formation tight sandstone gas are existed in the Silurian strata.

The Xintan Formation tight sandstone gas reservoir and the Wufeng-Longmaxi Formation shale gas reservoir in JD-1 well encountered good oil and gas shows, showing that the Silurian strata in North Jianshi area has the prospect of exploration and development fortight sandstone gas and shale gas.

Acknowledgement

The work was funded by the Project of China Geological Survey (12120121000172121) "Strategic Investigation of the Expansion Area of the West Hubei Shale Gas Demonstration Base" .