--> Abstract: Shale Gas Reservoir Characteristics and Gas Content in the Yanchang Formation, Ordos Basin, China, by Zhenxue Jiang, Zhilong Huang, Zhuo Li, Guoheng Liu, and Jinyan Xing; #90180 (2013)

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Shale Gas Reservoir Characteristics and Gas Content in the Yanchang Formation, Ordos Basin, China

Zhenxue Jiang1,2, Zhilong Huang1,3, Zhuo Li1,2, Guoheng Liu1,3, and Jinyan Xing1,2
1State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China
2Unconventional Nature Gas Institute, China University of Petroleum, Beijing, China
3College of Geoscience, China University of Petroleum, Beijing, China

The Ordos Basin is located in the western part of the Huabei craton, and filled with Yanchang Formation thick dark shale. The mudstone in the Triassic Chang-7 member of the Yanchang Formation deposits in deep-semi deep lacustrine environment and covers an area of 10 x 104 km2. The grey-black mudstone and shale are rich in organic matter with the average thickness of 45m. The total thickness of shale strata ranges from 30m to 90m. The characteristic of shale gas reservoir and gas content are studied using geochemical analysis, porosity and permeability measurement, X-ray diffraction and scanning electron microscope observation.

(1) Shale in Chang-7 member and Chang-9 member are considered to be rich in organic matter after 432 samples were measured, with the minimum TOC value of 0.13%, the maximum value of 10.39%, and the mean value of 4.71%. The occurrence of organic matter is observed to be sarciniform, stratiform, bulk, wrapped in apatite tuberculosis, and distribute between the pyrite particles, using scanning electron microscope.

(2) The content of the clay mineral and detrital mineral in Chang-7 Member is higher than Chang-9 Member, in contrast, the carbonate mineral content in Chang-7 Member is lower. Shale in the Chang-7 Member and Chang-9 Member have less carbonate content and higher quartz content, compared with Barnett shale.

Shale in the Chang-9 Member has higher content of illite-smectite mixed-layer and lower content of kaolinite, chlorite and illite than in the Chang-7 Member. The quartz content is a bit higher, with the mean value of 35% in the Chang-9 Member and 30% in the Chang-7 Member. Antigenic pyrite can be found in almost every sample, with diverse aggregation and distribution patterns, but the content is relatively low. Apatite, feldspar, barite particles also crystallized perfectly.

(3) The primary reservoir space in shale is pores and fractures. Most of the pores are origin of dissolution, with the scale from macro-pore to micro-pore. Also, pores caused by the mineral irregular stack and crystallization can be observed. Diverse fractures, including bedding cracks, pressure cracks, digenetic, clay mineral transformation cracks, etc., can be found.

(4) The porosity ranges from 0.16% to 5.12%, with a mean value of 2.11%. The permeability ranges from 0.00043 to 0.239mD, with a mean value of 0.0133 mD via porosity and permeability measurement of shale. The porosity and permeability of the Chang-9 member are relatively higher than the Chang-7 member, with an average porosity of 3.22% and an average permeability of 0.0016mD.

(5) The porosity of shale has linear correlations with the content of quartz and siderite. The specific area has positive correlation with the porosity, but negative correlation with average diameter of pores, indicating positive correlation between specific surface area and quartz content. The higher the quartz content is the larger specific surface area can be. Also, with the increase of total organic carbon, the specific surface show decrease trend, and then increase and reaches the maximum up to 10m2/g when total organic carbon content is greater than 4%.

(6) The favorable shale gas reservoir is determined to have the specific surface ranges from 6 m2/g to 15 m2/g and the TOC value ranges from 4% to 8%. Lower TOC value (<4%) infers low gas generation amount and low gas content in shale. Higher TOC value (>8%) shale indicates higher gas generation amount but low specific surface area, therefore, decrease the shale gas content to be relatively low.

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