Mylonitization: A Microscopic Mechanism of Gas Storage in the Lower Cambrian Lujiaping Shale, Northeast in Upper Yangtze Area, China
Yong Ma, Ningning Zhong, and Hui Han
State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing, China
Shale gas may be stored as free gas in pores and fractures, as adsorbed gas in organic matter and in inorganic minerals, or as dissolved gas in oil and water (Curtis, 2002). In shale-gas systems, the nanometer- to micrometer- size pores in matrix, along with nature fractures, form the storage space and flow-path network for natural gas (Slatt and O’Brien, 2011; Loucks et al., 2012). Pores within organic matter are widely recognized as a significant component of pore systems in gas shales (Loucks et al., 2009;Ambrose et al., 2010; Passey et al., 2010; Sondergeld et al., 2010; Curtis et al., 2011; Slatt and O’Brien, 2011; Milliken et al., 2012). Other pores in mineral matrix and microfractures, usually micrometer- or even millimeter-size, have also been identified and discussed recently for their importance in gas storage and migration pathway (Slatt and O’Brien, 2011; Loucks et al, 2012).
The Lower Cambrian Lujiaping Shale in the northeast of upper Yangtze area, deposited in the shelf environment of passive continental margin, is rich in organic and gas. The canister desorbed gas contents of gas shales above 1 m3/t locate from 490 to 825 m in burial depths. And the best gas shales locate at 762 m in depths, which desorbed 8080 ml gas from a 25 cm long shale, equal to 3.64 m3/t gas content. However, the marine gas shales experienced several episodes of intensive tectonic motions after the end of hydrocarbon generation, such as the Indosinian, Yanshanian, and Himalayan orogenies (Ma et al., 2004; Hao et al., 2008, 2011). And now it is adjacent to Chengkou Faults in Dabashan arc-like fold belts. The thermal maturity is above 4.0% Ro. Having a high content gas shale in such a strong tectonic deformation, shallow buried and high maturity area, which obviously exceeds the knowledge of shale gas storage mechanism before.
The Lujiaping rocks consist of dark gray, thin-bedded, organic-rich shale. In addition, tight fold and cleavage are developed in observation of the Lujiaping Shale outcrop. The cleavage domain is 500µm to 2 mm long, 20 to 80 µm wide, straight and parallel to each other, dark grey with brownish under the polarizing microscope. The clay mineral is dark grey with brownish while the organic matter particles only reveals brownish for it is not easily distinguished under the plane polarized light, together they makes up the cleavage domain.
For the detailed observation of cleavage domain under the field emission scanning electron microscopy (FESEM), lots of nanometer-size organic matter particles are filled in the clay mineral stripes, while the directional arrangement of clay mineral particles reveals mylonitic structures (Figure 1). Lots of nanometer-size organic matter particles mix with the clay mineral particles (Figure 1A), forming plenty of nanometer-size contact boundaries. Meanwhile, these nanometer-size contact boundaries produce lots of intergranular gaps, which make up the fundamental storage space of shale gas. Every clay mineral stripe and the organic matter particles filled in it composes a organic-inorganic mixed flake, about 3 to 8μm wide and 10 to 20 μm long (Figure 1B). These nanometer-size gaps are linked with each other by the intergranular boundaries while the microfractures connected these flakes. So the mylonitic shales develop nanometer-size intergranular gaps to aggregate gaps flake to pore network pore system, together they make up a 3D inter-connected pore network.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013