Shale gas reservoir characteristics of Ordovician-Silurian formations in the central Yangtze area, China

doi:10.1007/s11707-016-0565-4

Front. Earth Sci.

2017, Vol. 11

Issue (1) : 184-201 https://doi.org/10.1007/s11707-016-0565-4

RESEARCH ARTICLE

Shale gas reservoir characteristics of Ordovician-Silurian formations in the central Yangtze area, China

Chang’an SHAN^1,²,Tingshan ZHANG^2,³(

),Yong WEI⁴,Zhao ZHANG⁵

¹. School of Earth Sciences and Engineering, Xi’an Shiyou University, Xi’an 710065, China
². State Key Laboratory of Oil and Gas Reservoir Geology and Exploration, Southwest Petroleum University, Chengdu 610500, China
³. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
⁴. Exploration and Development Research Institute, Southwest Oil & Gas field Company, CNPC, Chengdu 610041, China
⁵. Exploration and Development department, Zhejiang Oilfield Company, CNPC, Hangzhou 310023, China

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Abstract

The characteristics of a shale gas reservoir and the potential of a shale gas resource of Ordovician–Silurian age in the north of the central Yangtze area were determined. Core samples from three wells in the study area were subjected to thin-section examination, scanning electron microscopy, nuclear magnetic resonance testing, X-ray diffraction mineral analysis, total organic carbon (TOC) testing, maturity testing, gas-bearing analysis, and gas component and isothermal adsorption experiments. A favorable segment of the gas shale reservoir was found in both the Wufeng Formation and the lower part of the Longmaxi Formation; these formations were formed from the late Katian to early Rhuddanian. The high–quality shale layers in wells J1, J2, and J3 featured thicknesses of 54.88 m, 48.49 m, and 52.00 m, respectively, and mainly comprised carbonaceous and siliceous shales. Clay and brittle minerals showed average contents of 37.5% and 62.5% (48.9% quartz), respectively. The shale exhibited type II₁ kerogens with a vitrinite reflectance ranging from 1.94% to 3.51%. TOC contents of 0.22%–6.05% (average, 2.39%) were also observed. The reservoir spaces mainly included micropores and microfractures and were characterized by low porosity and permeability. Well J3 showed generally high gas contents, i.e., 1.12–3.16 m³/t (average 2.15 m³/t), and its gas was primarily methane. The relatively thick black shale reservoir featured high TOC content, high organic material maturity, high brittle mineral content, high gas content, low porosity, and low permeability. Shale gas adsorption was positively correlated with TOC content and organic maturity, weakly positive correlated with quartz content, and weakly negatively correlated with clay content. Therefore, the Wufeng and Longmaxi formations in the north of the central Yangtze area have a good potential for shale gas exploration.

Keywords reservoir characteristic shale gas Upper Ordovician Lower Silurian central Yangtze area

Corresponding Author(s): Tingshan ZHANG

Online First Date: 26 July 2016 Issue Date: 23 January 2017

Cite this article:

Chang’an SHAN,Tingshan ZHANG,Yong WEI, et al. Shale gas reservoir characteristics of Ordovician-Silurian formations in the central Yangtze area, China[J]. Front. Earth Sci., 2017, 11(1): 184-201.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-016-0565-4
https://academic.hep.com.cn/fesci/EN/Y2017/V11/I1/184

Fig.1 Location of wells and regional tectonic structure of the middle Yangtze area: (a) location of the study area; (b) regional tectonic structure of middle Yangtze area (modified after Fu et al., 2006); (c) location of wells in the study area.

Fig.2 (a) Cross section of three wells showing the stratigraphy in the Wufeng and Longmaxi Formations. (b) Lithologic characteristics of the cores in J1. (c) Core photo of J3, showing a fault at the bottom of Longmaxi Formation.

Tab.1 Mineral content chart based on XRD analysis in the Wufeng–Longmaxi Formations in the J1, J2, and J3 wells

Fig.3 Bar chart and triangular diagram of mineral components: (a) mineral composition bar chart of shale in J1, J2, and J3; (b) mineral composition triangular chart of marine shale in J1, J2, and J3 and other wells abroad; (c) clay?mineral composition triangular chart of marine shale in J1, J2 and J3 as well as wells from the Dianqianbei area.

Fig.4 Photomacrographs and photomicrographs of core lithologic characteristics: (a) Graptolite?fossils are well-developed in black shale, Wufeng Formation, 329.0 m; core photo, J1; (b) Muddy shale, 285.3 m, J1; (c) Microscopic photo of muddy shale, mud>90%,?arenite (5%–8%), pyrite>1%, Longmaxi Formation, 285.3 m, (-) 50×, J1; (d) Siliceous shale, high-angle fractures filled by calcite, 263.2 m, J2; (e) Microscopic photo of siliceous shale, containing numerous radiolaria and sponge spicule fossils, (-) 50×, 263.2 m, J2; (f) Silty shale, 135.4?m, J2; (g) Microscopic photo of silty shale and quartz grains floating among clay minerals, (-) 50×, 135.4 m, J2; (h) Carbonaceous shale, 4064.5 m, J3; (i) Microscopic photo of carbonaceous shale, rich in organic matters, 4064.5 m, J3.

Fig.5 Kerogen macerals of 6 shale samples in the Longmaxi Formation in the J3 well: [a(t)] Photo by transmitted light microscopy, 4061.91 m; [a(f)] Photo by fluorescence microscopy, 4061.91 m; [b(t)] Photo by transmitted light microscopy, 4071.17 m; [b(f)] Photo by fluorescence microscopy, 4071.17 m; [c(t)] Photo by transmitted light microscopy, 4077.30 m; [c(f)] Photo by fluorescence microscopy, 4077.30 m; [d(t)] Photo by transmitted light microscopy, 4083.04 m; [d(f)] Photo by fluorescence microscopy, 4083.04 m; [e(t)] Photo by transmitted light microscopy, 4088.60 m; [e(f)] Photo by fluorescence microscopy, 4088.60 m; [f(t)] Photo by transmitted light microscopy, 4096.00 m; [f(f)] Photo by fluorescence microscopy, 4096.00 m.

Tab.2 Organic geochemistry characteristics of the Wufeng–Longmaxi Formations in the J1 and J3 wells

Tab.3 NMR results of the six?core samples from the J1, J2, and J3 wells

Fig.6 Types of microscopic pores of rocks in the Wufeng and Longmaxi formations in J1: (a)?clay mineral pores, Longmaxi Formation, 297.4 m; (b) clay mineral pores, Wufeng Formation, 329.4 m; (c) clay mineral pores, Longmaxi Formation, 283.2 m; (d) clay mineral pores, Longmaxi Formation, 311.2 m; (e) pyrite?intercrystalline pores, Longmaxi Formation, 127.7 m; (f) mineral moldic?pores, Longmaxi Formation, 325.1 m; (g) dissolution pores, Longmaxi Formation, 301.3 m; (h) organic?matter pores, Wufeng Formation, 330.2 m; (i) microfractures, Wufeng Formation, 331.8 m.

Fig.7 NMR T₂ relaxation?time spectrum before and after centrifugation of core samples.

Fig.8 Relationship of NMR permeability?and?NMR porosity?of core samples.

Tab.4 Separation result of the shale gas content in the J1 and J3 wells

Tab.5 Component analysis of shale gas in the J1 and J3 wells

Fig.9 Relationships among clay minerals, quartz, and TOC of black shale in J1 and J3. Relationship between (a) clay mineral content and TOC and (b) quartz content and TOC.

Fig.10 Correlation of total gas content and TOC in the J1 and J3 wells.

Tab.6 Sample data from the isothermal adsorption test of shale methane (30°C)

Fig.11 Isothermal curves for 10 black shale samples from the J1 and J3 wells.

Fig.12 Relationships among TOC, VRr, and methane adsorption capability of black shale in J1 and J3: (a) TOC and V_L and (b) VRr and V_L.

Fig.13 Relationships among quartz, clay, and methane adsorption capability of black shale in J1 and J3: (a) Quartz content and adsorption capability and (b) clay content and adsorption capability.

Tab.7 The comparison of gas shale reservoir characteristics among the study area, gas shale basins in America, and domestic commercial shale gas producing areas

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