Structural Deformation Characteristics Of Lower Yangtze Region And Its Potential For The Preservation Of Shale Gas

Shale gas is one of the most promising unconventional resources that are under exploration, and it has its own characteristics compared with conventional gas resources. In shale gas systems, the shale itself serves as source, seal, and reservoir. Shale gas may be stored as free gas in natural fractures and porosities, as gas sorbed onto kerogen and clay-particle surfaces, or/and as gas dissolved in kerogen and bitumen. There are many parameters and aspects that affect the accumulation of the shale gas system, including the mineral composition of the shale, total organic carbon content(TOC) and the type of organic matter, thermal maturity, depth of burial and thickness of shale, tectonic movement and the degree of fracture development.There are mainly three sets of source rock in lower Yangtze region which are lower Cambrian, upper Ordovician-lower Silurian and upper Permian. These formations are thick, widely spread, high in TOC and have a relatively high maturity which are favorable for the formation of shale gas. The strong tectonic movements had greatly reworked the oil and gas reservoir in this area, and as a result little progresses had been made in the exploration of conventional oil and gas. We drew several cross sections in this area and the results showed that the deformation characteristics of lower Yangtze region vary in different areas featured by zonation in N-S direction and segmentation in E-W direction. Due to the Indosian-Yanshan movement, the early Paleozoic foreland basin of this region formed the thrust-to-thrust structure which was reserved up to now; since the late Cretaceous, this region experienced intensive extension movement resulting in the formation of many extensional rifted basins and that’s the reason why there is little progresses in conventional oil and gas exploration in this area. From the cross sections we also found that the extensional and reversion movements had not greatly destroyed the previous structures in the Jiangyin-Wuxi detachment fold area and the slightly deformed state of strata below the detachment is very positive for the preservation of shale gas.When compared with different shale gas basins in the United States, it could be found out that the characteristics of gas varied from basin to basin, however, for the lower Yangtze region and Appalachian basin, the feature of shale gas are comparable. The middle Devonian Marcellus shale and the upper Ordovician-lower Silurian shale in lower Yangtze region have similar sedimentary environment, and had experienced similar geodynamic processes. In the Appalachian basin, most oil and gas wells are drilled in the slope area between the foredeep and forebulge of the foreland basin. According to the characteristics of the basin structure and source rock distribution of the two basins, we infer that the slope-foredeep area of a foreland basin is favorable for the preservation of shale gas. For the lower Yangtze region, the Jiangyin-Wuxi detachment fold is located just in the slope area and this makes it the very favorable block for the preservation of shale gas.