Microscopic pore-fracture configuration and gas-filled mechanism of shale reservoirs in the western Chongqing area, Sichuan Basin, China

FU Yonghong,JIANG Yuqiang,DONG Dazhong,HU Qinhong,LEI Zhi’an,PENG Hao,GU Yifan,MA Shaoguang,WANG Zimeng,YIN Xingping,WANG Zhanlei

Table 2 Characteristics of organic pores and micro-fractures in different types of pore-fracture configurations.

Pore-fracture configuration Causes of micro-fractures Contact with organic matter Organic pore and micro- fracture characteristics Characteristics of oil/water- saturated NMR T2 spectrum Pore-hydrocarbon- generating pressurized fracture contact Hydrocarbon-generating pressure leads to the formation of micro- fractures at the edge of the particles Direct Dominated by bubble-like organic pores with large diameters (20-200 nm); the length of micro-fractures is limited, and the fracture width is 100-500 nm The oil-saturated T2 spectrum is bimodal and its peak area is similar to that of its water-saturated spectrum; the pore relaxation time is 0.01-10.00 ms, and micro-fractures with relaxation time longer than 100 ms are rare Pore-inorganic diagenetic fracture separation Pore-clay fracture separation Mostly interlaminar micro-fractures formed by diagenetic transformation of clay minerals Indirect Locally developed bubble-like organic pores with diameters of 5-100 nm and occasionally larger than 100 nm; elongated micro-fractures with fracture widths of 100-1000 nm The oil-saturated T2 spectrum is bimodal and its peak area is smaller than that of its water-saturated curve; micro-fractures with relaxation time longer than 100 ms are more abundant in the water-saturated T2 spectrum Pore-mineral dissolution fracture separation Mostly formed at edges of soluble materials (e.g. feldspar, calcite, and dolomite) Indirect Bubble-like organic pores are developed far away from micro-fractures, with a pore size of 50-100 nm; the fracture width is uneven The oil-saturated T2 spectrum is bimodal and its peak area is slightly smaller than that of its water- saturated curve; micro-fractures with relaxation time longer than 100 ms are slightly more abundant in the water-saturated T2 spectrum Pore- compacted fracture separation Formed in rigid minerals because the overlying formation pressure exceeds the limit pressure of the mineral Partially direct Organic pores in direct contact with fractures are small and deformed locally, with a diameter of 5-100 nm; the development of micro-fractures is limited surrounding rigid particles, and with a width of 500-1000 nm The oil-saturated T2 spectrum is unimodal with a weak response of 1-10 ms, and its peak area is smaller than that of its water-saturated curve; micro- fractures with relaxation time longer than 100 ms are more abundant in the water-saturated T2 spectrum Structural micro- fracture contact Tectonically influenced, mostly seen between minerals and organic matter Direct Organic pores are basically not developed; micro-fractures run through the entire field of view, and the width is relatively large, usually greater than 1000 nm The oil-saturated T2 spectrum is bimodal and its peak area is slightly smaller than that of its water- saturated curve; micro-fractures with relaxation time longer than 100 ms are similar in both T2 spectra Organic matter edge fracture contact Diagenesis or thermal maturation causes organic matter to shrink or mineral transform and shrink, and microcracks are formed at the edges of organic matter and minerals Direct Organic pores in contact with micro-fractures are not developed; Long micro-fractures surrounding the organic matter, with fracture widths greater than 200 nm The oil-saturated T2 spectrum is bimodal with the main response of 0.01-1.00 ms, and its peak area is smaller than that of its water- saturated curve; micro-fractures with relaxation time longer than 100 ms are more abundant in the oil-saturated T2 spectrum