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The dolomite reservoir of the Middle Permian Qixia Formation in the Shuangyushi structure in northwestern Sichuan Basin is deeply buried, thin, and highly heterogeneous. With low-resolution seismic data, it is not easy to identify the reservoir signal and it is difficult to accurately describe the plane distribution of thin reservoirs. Based on high-resolution processing of seismic data, well-seismic combination technology, forward simulation technology, and cross-well profiles, in this study, we defined the waveform characteristics that correspond to different modes of reservoir development, summarized the seismic response modes of reservoirs in the area, optimized sensitive attributes, and finely depicted their plane distribution. The ability of seismic data to identify thin reservoirs was significantly improved after high-resolution processing. According to a comprehensive analysis, the reservoirs of the Qixia Formation in the study area had two response modes: "weak peak" and "complex wave." The waveform clustering attribute was optimized to describe the plane distribution characteristics of the Qixia Formation reservoirs in the study area. Type-I and type-II waveforms were favorable waveforms for reservoir development, and they mainly were distributed in the west of the Devonian denudation line, in a strip along the north–south direction, and mostly along faults. The predictions made with this method agreed well with the actual reservoir development, which provided a reference for the prediction of deep and thin reservoirs in this area. [ABSTRACT FROM AUTHOR]

Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Triassic Leikoupo Formation of Sichuan Basin, Southwest China. Stratigraphic incompleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites. [ABSTRACT FROM AUTHOR]

The '4.20' Lushan earthquake in Sichuan province, China has induced a large amount of geological hazards and produced abundant loose materials which are prone to post-earthquake rainfall-triggered landslides. A detailed landslide inventory was acquired through post-earthquake emergent field investigation and high resolution remote sensing interpretation. The rainfall analysis was conducted using historical rainfall records during the period from 1951 to 2010. Results indicate that the average annual rainfall distribution is heterogeneous and the largest average annual rainfall occurs in Yucheng district. The Stability Index MAPping (SINMAP) model was adopted to assess and analyze the post-earthquake slope stability under different rainfall scenarios (light rainfall, moderate rainfall, heavy rainfall, and rainstorm). The model parameters were calibrated to reflect the significant influence of strong earthquakes on geological settings. The slope stability maps triggered by different rainfall scenarios were produced at a regional scale. The effect of different rainfall conditions on the slope stability is discussed. The expanding trend of the unstable area was quantitatively assessed with the different critical rainfall intensity. They provide a new insight into the spatial distribution and characteristics of post-earthquake rainfall-triggered landslides in the Lushan seismic area. An increase of rainfall intensity results in a significant increase of unstable area. The heterogeneous distribution of slope instability is strongly correlated with the distribution of earthquake intensity in spite of different rainfall conditions. The results suggest that the both seismic intensity and rainfall are two crucial factors for post-earthquake slope stability. This study provides important references for landslide prevention and mitigation in the Lushan area after earthquake. [ABSTRACT FROM AUTHOR]

The Upper Triassic Xujiahe (须家河) Formation in the Sichuan (四川) Basin, Southwest China is distinctive for the basin-scale sand deposition. This relatively rare sedimentary phenomenon has not been well interpreted. Here we addressed this issue by discussing sedimentary framework and conceptual model. Analysis of sedimentary setting implied that the basin received transgression during the deposition. It had multiple provenance supplies and river networks, as being surrounded by oldlands in multiple directions including the north, east and south. Thus, the basin was generally characterized by coastal and widely open and shallow lacustrine deposition during the Late Triassic Xujiahe period. This is similar to the modern well-known Poyang (鄱阳) Lake. Therefore, we investigated the framework and conceptual model of the Sichuan Basin during the Xujiahe period with an analogue to the Poyang Lake. Results show that the conceptual model of the deposition can be divided into transgressive and regressive stages. The first, third and fifth members of the formation are in transgressive stage and the deposits are dominated by shore and shallow lacustrine mud. In contrast, the deposition is mainly of braided river channel sand deposits during the regressive stage, mainly including the second, fourth and sixth members of the formation. The sand deposited in almost the entire basin because of the lateral migration and forward moving of the cross networks of the braided rivers. The multiple alternations of short and rapid transgression and relatively long regression are beneficial to the basin-scale sand deposition. Thus, the main channel of the braided river and its extensional areas are favorable for the development of hydrocarbon reservoir. This provides practical significance to the reservoir evaluation and exploration. In addition, the results also justify the relatively distinctive sedimentary phenomenon in the study area and may also have implications for understanding the large-scale sand deposition elsewhere. [ABSTRACT FROM AUTHOR]