--> Abstract: Reservoir Characterization and Origins of High-Quality Shale in Lower Silurian, China, by Qin Zhang, Hongyan Wang, Honglin Liu, Renhe Liu, and Wenhua Bai; #90180 (2013)

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Reservoir Characterization and Origins of High-Quality Shale in Lower Silurian, China

Qin Zhang1,2, Hongyan Wang1,2, Honglin Liu1,2, Renhe Liu1,2, and Wenhua Bai1,2
1Langfang Branch - Research Institute of Petroleum Exploration & Development, PetroChina, Langfang, Hebei, China
2National Energy Shale Gas R&D (Experiment) Centre, Langfang, Hebei, China

Based on geological survey, core description, data of drilling and logging and sample experimentations, geochemistry, mineral composition and pore structure of Lower Silurian shale are characterized and the origins of high-quality shale are also studied and discussed.

1. Reservoir characteristics

Lower Silurian shale developed mainly in Sichuan Basin, and the distribution is stable with the thickness ranging from 100m to 700m. The thickness of the shale increases gradually toward south and southeast. Near uplift and erosion area, the thickness of shale decreases, ranging from 20 to 50. Shale generally contains a certain amount of sand, and quartz-siltstone is enrichment in local section, siliceous composition is enrichment in the bottom. Pyrite and graptolite are massively distributed along the bottom layer. From lower to upper part, the color of shale fades, sand content increases and the organic matter content decreases. The equivalent vitrinite reflectance ranges from 2.79% to 3.98%, staying at thermal cracking stage. Brittle minerals content in shale is high, ranging from 30% to 64%, clay content is relatively low between 31% and 51% and the average carbonate content is below 20%. Illite, chlorite and mixed-layer illite and smectite are the main compositions in clay and no kaolinite and smectite exist in clay. The shale is tight with porosity ranging from 0.84% to 6.48% and permeability from 0.01mD to 17.21mD.Micro and nano pores are the main types and they can be further divided into organic matter pore, Intergranular pore, intercrystalline pore and dissolution pore. Among them, the organic pore increases with maturity increases. Other fissures are also well developed and favorable for both free and adsorbed gas storage.

2. Controlling factors of high-quality shale

The research result indicates that high-quality shale is controlled by sedimentary facies, abnormal high pressure, coupling of bury depth, hydrocarbon charge, time and reservoir development and favorable preservation condition.

(1) Mineral composition and the organic matter content are controlled by sedimentary facies

Brittle mineral content in shale deposited in deep water shelf is much higher than that in both shallow and semi-deep water shelf. Different sedimentary facies impact not only shale composition but also organic matter content. The experiment result shows that TOC in carbonaceous deep water shelf is highest compared with that in other sedimentary facies while TOC in silty deep water shelf, semi-deep water shelf and shallow water shelf is less than that in black shale deep water shelf. Considering composition and organic matter content, deep water shelf is much favorable for high-quality shale development.

(2) Coupling of bury depth, hydrocarbon charge, time and reservoir development

Lower Silurian shale in Sichuan Basin has undergone long time shallow burial process from early Silurian to late Permian which resulted in better preservation of inorganic pores. Between late Permian to early Triassic, the shale entered into hydrocarbon generation peak, in middle Triassic, the burial depth of Lower Silurian shale was 4000m and the reservoir temperature was 120°C, the crude oil began to crack into bitumen and natural gas. With continuous subsidence, burial depth reached 6500m and temperature was 160°C in middle Cretaceous. At this time, all crude oil had cracked into gas and the pressure created by cracking process not only generated some pores and fissures but also displaced formation water and inhibited destructive diagenetic alteration. To some extent, quality of the reservoir was improved.

(3) Abnormal high pressure better preserves pores in shale

Haynesville located in the junction of U.S. east Texas and west Louisiana is deposited in reducing environment and mainly composed of carbon shale. The area with abnormal high pressure is not only the core area for shale gas exploration but also featured by high EUR and high initial production which means abnormal high pressure is beneficial for shale gas. Overpressure is quite common in south Sichuan Basin, and the pressure coefficient is generally over 1.3. Drillings in Changning, Fushun and Yongchuan with pressure coefficient over 2 are all with high initial production of shale gas which demonstrated that abnormal high pressure can improve reservoir fracility and enhance in situ gas content.

(4) Preservation condition is critical for shale gas accumulation

The intensity of tectonic movement directly influences the enrichment of shale gas in the reservoir. Sichuan Basin has undergone multiple tectonic movements and the effect is destructive which leads to finding stable area for shale gas targets. Field drillings have proved the theory we have proposed. In Zhaotong area characterized by intensive tectonic movement and large open faults, almost all drillings are lost and the gas content desorpted in the field is lower than 0.1 m3/t. The homogeneous temperature from fluid inclusion testing and thermal history indicate that destructive movement occurred when shale gas accumulated in Zhaotong area which resulted in low gas content. In order to optimize the favorable shale gas targets, preservation condition should be taken into account.

3. Conclusions

Based on what we discussed above, in order to screen favorable shale gas areas, origins of high-quality shale should be emphasized. Deep water shelf, abnormal high pressure, better preservation condition and well coupling of bury depth, hydrocarbon charge, time and reservoir development should all taken into account.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013