Youcai Tang

Crustal structures across the western Weihe Graben, North China: Implications for extrusion tectonics at the northeast margin of Tibetan Plateau

The stable Ordos Plateau, extensional Weihe Graben, and Qinling orogenic belt are located at the northeast margin of the Tibetan Plateau. They have been thought to play different roles in the eastward expanding of the Tibetan Plateau. Peking University deployed a linear seismic array across the western end of the Weihe Graben to investigate the crustal structures of the tectonic provinces of this structure. Receiver function analyses revealed low-to-moderate Poissons ratios and anticorrelations between Poissons ratios and topography beneath the Qinling Orogen. These features may indicate a tectonic thickening of the felsic upper crust by folding and thrusting within the Qinling Orogen. We observed a strong horizontal negative signal at the midcrust beneath the Ordos Plateau which may indicate a low-velocity zone. This observation would suggest the stable cratonic Ordos Plateau had been modified due to the compression between the Tibetan Plateau and the Ordos Plateau. We also observed an abrupt 4 km Moho offset across the Weihe Fault, changing from ~44 km beneath the Ordos Plateau to ~40 km beneath the Qinling Orogen. We conclude that the Weihe Fault is a lithosphere-scale fault/shear zone, which extends into the upper mantle beneath the Weihe Graben. It acts as the major boundary separating the stable Ordos Plateau and the active Qinling Orogen.

The Origin and Mantle Dynamics of Quaternary Intraplate Volcanism in Northeast China From Joint Inversion of Surface Wave and Body Wave

We present a 3-D model of NE China by joint inversion of body and surface waves. The joint inversion significantly improves the resolution at shallow depths compared with body wave tomography alone and provides seismic evidence for the origin of Quaternary volcanism in NE China. Our model reveals that the mantle upwelling beneath the Changbaishan volcano originates from the transition zone and extends up to ~60 km, and spreads at the base of the lithosphere with the upwelling head ~5 times wider than the raising tail in the lower upper mantle. However, low velocities beneath the Halaha and Abaga volcanoes in the Xingmeng belt are confined to depths shallower than 150 km, suggesting that magmatism in the Xingmeng belt is more likely caused by localized asthenospheric upwelling at shallow depths rather than from the common deep source. A small-scale sublithospheric mantle convection may control the spatial and temporal distribution of Quaternary magmatism in NE China; that is, the upwelling beneath the Changbaishan volcano triggers the downwelling beneath the southern Songliao basin, where the high velocity imaged extends to ~300 km. The downwelling may further induce localized upwelling in the surrounding areas, such as the Halaha and Abaga volcanoes. Thanks to the joint constraints from both surface and body waves, we can estimate the dimension of the convection cell. The convection cell is located between 42°N and 45°N, spreads around ~500 km in the W-E direction measured from the distance between centers of downwelling and upwelling, and extends to ~300 km vertically.

Microseismic Monitoring of Stimulating Shale Gas Reservoir in SW China: 2. Spatial Clustering Controlled by the Preexisting Faults and Fractures

Microseismic monitoring is crucial to improving stimulation efficiency of hydraulic fracturing treatment, as well as to mitigating potential induced seismic hazard. We applied an improved matching and locating technique to the downhole microseismic data set during one treatment stage along a horizontal well within the Weiyuan shale gas play inside Sichuan Basin in SW China, resulting in 3,052 well-located microseismic events. We employed this expanded catalog to investigate the spatiotemporal evolution of the microseismicity in order to constrain migration of the injected fluids and the associated dynamic processes. The microseismicity is generally characterized by two distinctly different clusters, both of which are highly correlated with the injection activity spatially and temporarily. The distant and well-confined cluster (cluster A) is featured by relatively large-magnitude events, with ~40 events of M 1 or greater, whereas the cluster in the immediate vicinity of the wellbore (cluster B) includes two apparent lineations of seismicity with a NE-SW trending, consistent with the predominant orientation of natural fractures. We calculated the b-value and D-value, an index of fracture complexity, and found significant differences between the two seismicity clusters. Particularly, the distant cluster showed an extremely low b-value (~0.47) and D-value (~1.35). We speculate that the distant cluster is triggered by reactivation of a preexisting critically stressed fault, whereas the two lineations are induced by shear failures of optimally oriented natural fractures associated with fluid diffusion. In both cases, the spatially clustered microseismicity related to hydraulic stimulation is strongly controlled by the preexisting faults and fractures.

Toward the Origin of Long‐Period Long‐Duration Seismic Events during Hydraulic Fracturing Treatment: A Case Study in the Shale Play of Sichuan Basin, China

Long‐period long‐duration (LPLD) seismic events have been recently observed over several unconventional reservoirs, and are postulated to play a significant role in accommodating deformation within the stimulation zone. However, the origin of LPLD events is still under debate. In this study, we present observations of tremor‐like events during the multistage hydraulic fracturing treatment of a pilot horizontal well within the Weiyuan Shale play in southwestern Sichuan basin, China. These tremor‐like signals recorded by the deep downhole monitoring array (15 Hz geophones) resemble the previously reported LPLD examples in terms of both duration and frequency content. In this particular case, with the concurrent records from the surface broadband array, we however confirm that these signals are located well outside the treatment area, and most probably originate from regional earthquakes that occurred during the treatment course within 250 km radius of the treatment well, instead of local sources directly related to the hydraulic stimulation. This study implies that the attenuated signals of regional seismicity may constitute a particularly deceptive pitfall for identifying and misinterpreting LPLD events during hydraulic fracturing, especially in seismically active regions. Our results also highlight the importance of surface broadband array in monitoring hydraulic fracturing activities.

Microseismic Monitoring of Stimulating Shale Gas Reservoir in SW China: 1. An Improved Matching and Locating Technique for Downhole Monitoring: An improved match and locate technique

We introduce an improved matching and locating technique to detect and locate microseismic events ( 4 < ML < 0) associated with hydraulic fracturing treatment. We employ a set of representative master events to act as template waveforms and detect slave events that strongly resemble master events through stacking cross correlograms of both P and S waves between the template waveforms and the continuous records of the monitoring array. Moreover, the residual moveout in the cross correlograms across the array is used to locate slave events relative to the corresponding master event. In addition, P wave polarization constraint is applied to resolve the lateral extent of slave events in the case of unfavorable array configuration. We first demonstrate the detectability and location accuracy of the proposed approach with a pseudo-synthetic data set. Compared to the matched filter analysis, the proposed approach can significantly enhance detectability at low false alarm rate and yield robust location estimates of very low SNR events, particularly along the vertical direction. Then, we apply themethod to a real microseismic data set acquired in the Weiyuan shale reservoir of China in November of 2014. The expanded microseismic catalog provides more easily interpretable spatiotemporal evolution of microseismicity, which is investigated in detail in a companion paper.

Temporal variations of near‐surface anisotropy induced by hydraulic fracturing at a shale play site in southwest China

Knowledge of the geometric properties of fractures and cracks in a petroleum reservoir is important to reservoir exploitation. When aligned and partially connected, fractures and cracks can act as conduits for fluid flow and thus can significantly increase the permeability of the reservoir. The aligned fractures and cracks, on the other hand, are an effective means to generate seismic anisotropy. In this study, we utilize the seismic data recorded by a vertical array installed in a shallow borehole at a shale play site in southwest China. By applying seismic interferometry to the ambient noise data recorded by 12 threecomponent geophones, we extract P and S waves propagating vertically along the borehole. The S waves show up to 20% velocity variations with respect to their polarization directions. Such large Swave anisotropy can be explained by the horizontal transverse isotropic model and is likely caused by natural fractures that are widely present in the area and align approximately in the NE-SW direction. During the 13-day period of hydraulic fracking treatment, we also observe large and systematic temporal variations in S wave velocity, degree of Swave polarization anisotropy, and fast polarization direction. By comparing our observations with normal strain changes calculated with a half-space elastic model, we speculate that strain changes induced by hydraulic injection and fracturing are likely to be responsible for the observed temporal variations in seismic anisotropy. As such, seismic interferometry with shallow borehole acquisition might provide an alternative means to monitor hydraulic fracturing and wastewater injection in the future.