Longmenshan Mountain Research Articles

Structural characteristics and implications on oil/gas accumulation in north segment of the Longmenshan piedmont, northwestern Sichuan Basin, SW China

By integrating surface geology, seismic data, resistivity sections, and drilling data, the structural deformation characteristics of the frontier fault of thrust nappes were delineated in detail. The frontier fault of thrust nappes in northwest Scihuan Basin is a buried thrust fault with partial exposure in the Xiangshuichang—Jiangyou area, forming fault propagation folds in the hanging-wall and without presenting large-scale basin-ward displacement along the gypsum-salt layer of the Triassic Jialingjiang Formation to the Triassic Leikoupo Formation. The southwestern portion of the frontier fault of thrust nappes (southwest of Houba) forms fault bend folds with multiple ramps and flats, giving rise to the Zhongba anticline due to hanging-wall slip along the upper flat of the Jialingjiang Formation. In contrast, the northeastern portion of the frontier fault of thrust nappes (northeast of Houba) presents upward steepening geometry, leading to surface exposure of Cambrian in its hanging-wall. With the frontier fault of thrust nappes as the boundary between the Longmenshan Mountain and the Sichuan Basin, the imbricated structural belt in the hanging-wall thrusted strongly in the Indosinian orogeny and was reactivated in the Himalayan orogeny, while the piedmont buried structural belt in the footwall was formed in the Himalayan orogeny. In the footwall of the frontier fault of thrust nappes, the piedmont buried structural belt has good configuration of source rocks, reservoir rocks and cap rocks, presenting good potential to form large gas reservoirs. In comparison, the hanging-wall of the frontier fault of thrust nappes north of Chonghua has poor condition of oil/gas preservation due to the surface exposure of Triassic and deeper strata, while the fault blocks in the hanging-wall from Chonghua to Wudu, with Jurassic cover and thicker gypsum-salt layer of the Jialingjiang formation, has relative better oil/gas preservation conditions and thus potential of oil/gas accumulation. The frontier fault of thrust nappes is not only the boundary between the Longmenshan Mountain and the Sichuan Basin, but also the boundary of the oil/gas accumulation system in northwestern Sichuan Basin.

Middle Jurassic climate oscillations from paleosol records of the Sichuan Basin, SW China

The Middle Jurassic is characterized by major plate tectonic changes, variable atmospheric CO2 concentrations (pCO2), and climate oscillations in both marine and terrestrial realms. However, the limited evidence from the terrestrial sedimentary records hampers a comprehensive understanding of global climate state in this period. Well-exposed strata of the Shaximiao Formation in the Sichuan Basin, Southwest China, preserve successions of paleosols that offer the possibility of evaluating the terrestrial paleoclimatic changes during the Middle Jurassic (Bajocian to Callovian). A total of 151 paleosol profiles belonging to four paleosol types (Entisol-like, Inceptisol-like, Aridisol-like and argillic Inceptisol-like paleosols) were identified and characterized. The quantitative paleoclimate reconstructions based on the bulk geochemistry of paleosol horizons and depths of carbonate nodules indicate a generally semiarid–subhumid climate alternating with arid–humid and cool/warm–temperate climates during this time interval. This dynamic climate terminated in a dry, subhumid, humid/perhumid and superhumid moisture regime characterized by steppe/wet or rain forest floral provinces. We suggest that these climatic fluctuations may have been related to global geodynamic (e.g., the fragmentation of Pangea, “megamonsoonal” circulation, and bolide impacts) and regional tectonic uplift (e.g., the uplift of the Longmenshan Mountains in the South China Block) during the Middle Jurassic. The pCO2 estimates suggest relatively low pCO2 levels within the range of ∼100 ppm–∼890 ppm during the Bajocian to Callovian. Our results coincide with the threshold for the initiation of glaciation and may help to explain the occurrence of cool intervals in the generally warm Jurassic. These pCO2 fluctuations, which coincide with paleotemperature variations in the low paleolatitudes of the Northern Hemisphere, are possibly attributable to global geological events (e.g., wildfire events, eustatic sea-level oscillations, and the transient development of glaciations) in the Middle Jurassic.

Natural gas accumulation in the basin–mountain transition zone, northwestern Sichuan Basin, China

The basin–mountain transition zone is an important area for hydrocarbon exploration. To better understand the process and mechanism of hydrocarbon accumulation in this area, the middle Permian Qixia Formation at the piedmont of the Longmenshan Mountains, northwestern Sichuan Basin, China, was investigated. It is showed that natural gas in the Qixia Formation in the thrust belt (i.e., the Kuangshanliang and Hewanchang areas) was derived from lower Cambrian strata by oil cracking. The reservoirs are filled with amounts of bitumen, which reflects strong oil charging. However, the gas accumulation is not rich, which is due to the later destruction of hydrocarbon accumulations by the invasion of meteoric waters. In contrast, in the thrust front (i.e., the Shuangyushi area) and forebulge belt (i.e., the Jiulongshan area), the reservoirs contain few bitumens and large amounts of natural gas, which is indicative of good preservation for hydrocarbon accumulation. The natural gas was mainly derived from kerogen cracking in middle Permian source rocks, although it was mixed with a small amount of lower Cambrian-derived oil-cracking gases. The thrust front and forebulge belts occur in areas of lesser deformation, and thus are favorable for gas accumulation, due to the strata containing both sapropel- and humic-type source rocks and reservoirs. It is generalized that hydrocarbon resources in (ultra-)deep strata in the basin–mountain transition zone may be an important target for exploration worldwide.

Mechanism of ultra-deep gas accumulation at thrust fronts in the Longmenshan Mountains, lower Permian Sichuan Basin, China

Giant gas has been discovered in the lower Permian Qixia Formation of the Shuangyushi area in the Longmenshan fold-and-thrust front within dolomite reservoir at depth >6000 m. To improve the understanding of this unique ultra-deep reservoir, we address the mechanism of hydrocarbon accumulation in this area. Gas geochemistry implies that most of the gas is derived from mixing humic and sapropelic kerogens, whereas some gases are derived solely from humic kerogens. The gas reservoirs experienced two main stages of hydrocarbon charging corresponding to oil and gas generation peaks of first the lower Cambrian and second lower–middle Permian source rocks, i.e., Late Triassic and Middle Jurassic, respectively. The weak tectonism and the resistance of the ultra-deep structures to tectonic deformation at thrust fronts provided an ideal preservation environment for hydrocarbons. Our data suggest that deep to ultra-deep oil and gas accumulations in mountain front–foreland basin transition zones is a target of future exploration, providing complement to the traditional focus in shallow formations.

On the dolomite reservoirs formed by dissolution: Differential eogenetic versus hydrothermal in the lower Permian Sichuan Basin, southwestern China

Dolomite reservoirs formed by dissolution are significant for hydrocarbon exploration, but their origins are poorly understood because of their complexity. This study investigated the lower Permian Qixia Formation of the Sichuan Basin (northern Longmenshan Mountains), southwestern China, which has recently been the location of giant natural gas discoveries. Petrological and geochemical analyses were undertaken, including trace elements and C, O, and Sr isotopes. The results indicate that the reservoirs were formed by differential eogenetic karstification and hydrothermal dissolution of grain dolomites. Core and thin-section observations identified three types of dolomite based on grain size: fine- to medium-crystalline dolomite (Dol-1), medium- to coarse-crystalline dolomite (Dol-2), and coarse- to megacrystalline saddle dolomite (Dol-3). The Dol-2 types can be further divided into three subtypes according to the grain structure and degree of pore development: euhedral medium- to coarse-crystalline dolomite with pores (Dol-2a), porous subhedral–anhedral medium- to coarse-crystalline dolomite (Dol-2b), and tight subhedral–anhedral medium- to coarse-crystalline dolomite (Dol-2c). The Dol-1 types appear to have formed because of the release of and subsequent metasomatism by Mg2+ in trapped seawater along stylolites at shallow burial conditions (500–1000 m). The Dol-2a types resulted mainly from eogenetic karstification of original limestone and subsequent dolomitization by marine-related fluids. These dolomites are geochemically characterized by relatively flat rare-earth element patterns and relatively low Y/Ho values (mean e 37.1). The Dol-2b and Dol-3 types were formed mainly by hydrothermal fluids and have similar rare-earth element patterns with large positive Eu anomalies (mean e 5.7). The Dol-2c types experienced metasomatism by marine-related fluids mixed with minor hydrothermal fluids. All these formation mechanisms reflect the influence of multistage diagenetic and hydrothermal fluids. The Dol-2a types form the most favorable reservoirs in the study area, followed by the Dol-2b types. The distribution of the Dol-2a types was controlled by eogenetic karstification of grain shoals, whereas that of the Dol-2b types reflects the influence of tectonically derived hydrothermal fluid dissolution of grain shoals. As such, both primary sedimentary features and later differential dissolution (eogenetic and hydrothermal) contributed to reservoir development, which involved factors such as the sedimentary facies (grain shoal), sea-level fluctuations (eogenetic karst development), and faulting (hydrothermal dissolution). Therefore, the formation of dolomite and associated reservoirs is complex. The results of this study provide an improved understanding of the origins and complexity of dolomite reservoirs formed by dissolution, which is relevant to exploration for such reservoirs.

Characteristics of buried structures in northern Longmenshan mountains and its significance to oil and gas exploration in the Sichuan Basin

Since 2014, great gas discoveries have been achieved in the Upper Paleozoic Shuangyushi zone in the piedmont zone of the Longmenshan mountains, northwestern Sichuan Basin, where multiple gas wells with industrial flows have been drilled and completed successively in the Middle Permian Qixia Fm and Middle Devonian Guanwushan Fm, etc. Along with the steady progress of exploration there, to make a further in-depth study on the characteristics of the frontal–piedmont zones in the Longmenshan thrust–nappe structures will be of great significance to both a better understanding of Upper Paleozoic gas reservoirs in this study area and the expansion of oil and gas exploration field. In view of this, based on the newly deployed 3D seismic surveys, gravity–magnetic–electronic data, and practical drilling information from exploration wells like Well ST 9, the characteristics of the frontal–piedmont zones in the Longmenshan thrust–nappe structures are analyzed as well as the features of the buried structures, gas reservoir forming conditions and exploration prospect there. The following findings were obtained: (1) A huge buried structure belt is developed underlying the Longmenshan front fault belt No. 1 (LSFFB 1) and marine strata are thus formed below the Lower Triassic Jialingjiang Fm due to the roles of compressive folding, extrusion and deformation; (2) The detachment horizons from LSFFB 1 go from shallower in the south to deeper in the north, turning from Cambrian slipping in the south to basement-involved in the north. The Paleozoic and its overlying strata were overall uplifted with a higher degree northward and so did the strata between the up-thrown and down-thrown sides of the faults with a greater displacement distance and higher folding degree, resulting in that the Paleozoic strata in the north are 1500 higher uplifted than those in the south; (3) In the western Jiange and Guangyuan areas, the eastern LSFFB 1 presents a tectonic pattern of “three steps” as a whole. In conclusion, there are good gas reservoir forming conditions in LSFFB 1, the down-thrown sides of which cover an area of 1800 km2, where the Guanwushan Fm and Qixia Fm provide good accumulation and preservation conditions for gas reservoirs, which will become favorable gas exploration targets in the deep marine strata in NW Sichuan Basin.

Crustal Uplift in the Longmen Shan Mountains Revealed by Isostatic Gravity Anomalies along the Eastern Margin of the Tibetan Plateau

AbstractThis study examines the relationship between high positive isostatic gravity anomalies (IGA), steep topography and lower crustal extrusion at the eastern margin of the Tibetan Plateau. IGA data has revealed uplift and extrusion of lower crustal flow in the Longmen Shan Mountains (the LMS). Firstly, The high positive IGA zone corresponds to the LMS orogenic belt. It is shown that abrupt changes in IGA correspond to zones of abrupt change of topography, crustal thickness and rock density along the LMS. Secondly, on the basis of the Airy isostasy theory, simulations and inversions of the positive IGA were conducted using three‐dimensional bodies. The results indicated that the LMS lacks a mountain root, and that the top surface of the lower crust has been elevated by 11 km, leading to positive IGA, tectonic load and density load. Thirdly, according to Watts's flexural isostasy model, elastic deflection occurs, suggesting that the limited (i.e. narrow) tectonic and density load driven by lower crustal flow in the LMS have led to asymmetric flexural subsidence in the foreland basin and lifting of the forebulge. Finally, based on the correspondence between zones of extremely high positive IGA and the presence of the Precambrian Pengguan‐Baoxing complexes in the LMS, the first appearance of erosion gravels from the complexes in the Dayi Conglomerate layer of the Chengdu Basin suggest that positive IGA and lower crustal flow in the LMS took place at 3.6 Ma or slightly earlier.

Nappe and klippe structures in the middle segment of the Longmenshan Mountains, Southwest China

Nappe and klippe structures in the middle segment of the Longmenshan Mountains, Southwest China

Numerical simulation study on the impact of the Wenchuan earthquake to the seismic environment of the Lushan earthquake

The finite element numerical simulation method was used to investigate the impact of the Wenchuan earthquake on the seismic environment of the southern Longmenshan Mountain. The three-dimensional viscoelastic geological model, including the Longmenshan front-range fault, central fault, back-range fault, the Bayan Kala block, and the South China block, was constrained by the results of the geological survey, coseismic dislocation, and the GPS observation of Wenchuan and Lushan earthquakes. We studied how the calculation results were influenced by gravity potential energy caused by high altitude topography, the low velocity layer in the middle crust and the rupture of the Beichuan-Yingxiu and Jiangyou-Guanxian faults, by modifying the parameters of a model. The Wenchuan earthquake was simulated by reducing the Young’s modulus of the possible generating fault. We calculated the Coulomb stress of the Dayi blind fault, Dachuang-Shuangshi fault, and the “Y”-shaped blind thrust fault between the two faults above. The simulation results show that the Coulomb stress has a similar distribution on the fault planes of the southern Longmenshan faults zone caused by the Wenchuan earthquake, considering the low-velocity layer or the double-rupture surface. The Coulomb stress on the top of the Dachuan-Shuangshi fault surface was about 0 MPa, and it gradually reduced towards the southwest direction. At the source of the Lushan earthquake, the Coulomb stress was about −0.02 MPa. For the west branch of “Y”-shaped fault, the Coulomb stress on the fault surface was significantly reduced and was about −0.02–−0.03 MPa at the source of the Lushan earthquake; after the Wenchuan earthquake, the Coulomb stress on the east branch of the “Y”-shaped fault plane was about −0.01–0.03 MPa; for the Dayi fault, the Coulomb stress was increasingly obvious on the fault surface from the southwest to the northwest after the Wenchuan earthquake. Therefore, the Wenchuan earthquake could reduce the risk of an earthquake on the Dachuan-Shuangshi fault and the west branch of “Y”-shaped fault earthquake, but it increases the risk on the east branch of “Y”-shaped fault and the Dayi fault. The maximum Coulomb stress near the source was located on the Dayi fault. Considering the terrain and the simultaneous failure of the Beichuan-Yingxiu and Jiangyou-Guanxian, the presence of the low velocity layer increases the Coulomb stress on the four faults, and obviously increases towards the east branch of the “Y”-shaped fault and the Dayi fault. If the terrain and the low velocity layer are considered, compared with only the ruptured Beichuan-Yingxiu fault, the Beichuan-Yingxiu and Jiangyou-Guanxian faults’ simultaneous rupture increased the risk of earthquakes on the Dachuan-Shuangshi fault, the east branch of “Y”-shaped fault, and the Dayi hidden fault, but reduced the risk of the west branch of “Y” type fault. In any of the above models, the Coulomb stress changes on the four fault planes of southern Longmenshan Mountains were influenced by terrain. In the case of certain crust, mantle medium properties, and spatial structure, the existence of the low velocity layer on the west of the Longmenshan fault zone, the topographic features of the Qinghai-Tibet Plateau to the Sichuan Basin, and the conditions for the simultaneous rupture of the central fault and the front-range fault during the Wenchuan earthquake, are the important factors that affect the Wenchuan earthquake to the Lushan earthquake. Among them, the terrain and low velocity layer have the most significant influence on the Coulomb failure stress for the area around the Lushan earthquake area caused by the Wenchuan earthquake. The effect of terrain features is more obvious in the north section of the selected section, yet the effect of the Wenchuan earthquake simultaneous rupture planes is smaller.

Characteristics of isostatic gravity anomaly in Sichuan-Yunnan region, China

Sichuan-Yunnan region in China, a tectonic transition belt where earthquakes occurred frequently and intensely, has a distinct variation characteristic of gradient zone of Bouguer gravity anomaly (BGA). Many deep faults and epicenters of severe earthquake scatter along the BGA gradient zones. Here we apply two forward models (Airy model and Vening Meinesz model) of isostatic gravity mechanisms (local versus regional) in this region to calculated the isostatic gravity anomaly (IGA). Afterwards, the relationship between IGA and distribution of faults as well as seismicity is also illustrated. The IGA results show that the two models are similar and most parts of the study area are in an isostatic state. Most featured faults are distributed along the steep anomaly gradient zones; earthquakes tend to occur in the non-isostatic area and steep gradient belt of IGA. The distribution of root thickness based on regional mechanism can be associated with the main trend of BGA variation. The regional mechanism is more plausible and closer to the reality because of its relatively further consideration of the horizontal forces derived from adjacent particles in the crust. Then we analyze the effect of isostasy on the tectonic movements and find that the isostatic adjustment is not the main cause of the continuous uplift process of Longmenshan Mountain fault zone, which is due to the Indian-Eurasian continental collision.

Natural gas exploration prospect in the Upper Paleozoic strata, NW Sichuan Basin

Progress of natural gas exploration has recently been made in the Upper Paleozoic marine-facies reservoirs in the NW Sichuan Basin. It has been proved that the Middle Permian Qixia Fm pore-type dolomite reservoirs may be distributed continuously in a large area, besides, for the first time favorable gas-bearing Devonian dolomite reservoirs in great thickness were discovered in the downhole there. Recent research achievements indicate that: (1) The Lower Cambrian quality source rocks in huge thickness were found in the study sedimentary area with advantageous hydrocarbon source conditions. (2) Influenced by the Caledonian movement, the whole NW Sichuan Basin was lifted up to form a massive box-like NS-trending plunging uplift low steep in the west but high sluggish in the east, laying a robust foundation for the inheritance of Devonian, Carboniferous, and Permian platform margin reef large-scale sedimentation and dolomitization. (3) In the Upper Paleozoic strata, there were multiple planes of unconformity and multi-stage supergene karst occurred, improving the preservation and seeping capacity of reservoirs. (4) Tectonic deformations resulted from the nappe structure in the Longmenshan Mountain provided good hydrocarbon source fractures and sealing conditions for oil and gas reservoirs, which is conductive to the formation of a giant gas pool. It is concluded that the back-thrust anticline belt east to Hidden fault I in this study area has favorable conditions for large-scale structural–stratigraphic and structural–lithologic combination traps. As suggested in this study, relatively strong dolomitization was found in the Qixia Fm in the Shejianhe River–Zhongba higher palaeogeomorphic deposit area, so the most favorable exploration target of the Qixia Fm will be around the central axis of Caledonian paleo-uplift to the east ridgeline, and the most favorable prospect of the Upper Palaeozoic will be the central axis to the west of Hidden fault I where three gas bearing reservoirs will be possibly explored at the same time.

New Plants from the Lower Devonian Pingyipu Group, Jiangyou County, Sichuan Province, China.

Descriptions of Lower Devonian plants from Yunnan, South China, have revolutionized concepts of diversity and disparity in tracheophytes soon after they became established on land. Sichuan assemblages have received little attention since their discovery almost 25 years ago and require revision. With this objective, fieldwork involving detailed logging and collection of fossils was undertaken in the Longmenshan Mountain Region, Jiangyou County and yielded the two new taxa described here. They are preserved as coalified compressions and impressions that allowed morphological but not anatomical analyses. Yanmenia (Zosterophyllum) longa comb nov is based on numerous rarely branching shoots with enations resembling lycophyte microphylls, without evidence for vasculature. The presence of sporangia is equivocal making assignation to the Lycopsida conjectural. The plant was recently described as a zosterophyll, but lacks strobili. These are present in the second plant and comprise bivalved sporangia. The strobili terminate aerial stems which arise from a basal axial complex displaying diversity in branching including H- and K- forms. These features characterise the Zosterophyllopsida, although the plant differs from Zosterophyllum in valve shape. Comparisons indicate greatest similarities to the Lower Devonian Guangnania cuneata, from Yunnan, but differences, particularly in the nature of the sporangium border, require the erection of a new species, G. minor. Superficial examination of specimens already published indicate a high degree of endemism at both species and generic level, while this study shows that Yanmenia is confined to Sichuan and Guangnania is one of the very few genera shared with Yunnan, where assemblages also show a high proportion of further endemic genera. Such provincialism noted in the Chinese Lower Devonian is explained by the palaeogeographic isolation of the South China plate, but this cannot account for differences/endemism between the Sichuan and Yunnan floras. Such an enigma demands further integrated geological, palaeobotanical and palynological studies.

Connectivity of earthquake‐triggered landslides with the fluvial network: Implications for landslide sediment transport after the 2008 Wenchuan earthquake

AbstractEvaluating the influence of earthquakes on erosion, landscape evolution, and sediment‐related hazards requires understanding fluvial transport of material liberated in earthquake‐triggered landslides. The location of landslides relative to river channels is expected to play an important role in postearthquake sediment dynamics. In this study, we assess the position of landslides triggered by the Mw 7.9 Wenchuan earthquake, aiming to understand the relationship between landslides and the fluvial network of the steep Longmen Shan mountain range. Combining a landslide inventory map and geomorphic analysis, we quantify landslide‐channel connectivity in terms of the number of landslides, landslide area, and landslide volume estimated from scaling relationships. We observe a strong spatial variability in landslide‐channel connectivity, with volumetric connectivity (ξ) ranging from ~20% to ~90% for different catchments. This variability is linked to topographic effects that set local channel densities, seismic effects (including seismogenic faulting) that regulate landslide size, and substrate effects that may influence both channelization and landslide size. Altogether, we estimate that the volume of landslides connected to channels comprises 43 + 9/−7% of the total coseismic landslide volume. Following the Wenchuan earthquake, fine‐grained (<~0.25 mm) suspended sediment yield across the Longmen Shan catchments is positively correlated to catchment‐wide landslide density, but this correlation is statistically indistinguishable whether or not connectivity is considered. The weaker‐than‐expected influence of connectivity on suspended sediment yield may be related to mobilization of fine‐grained landslide material that resides in hillslope domains, i.e., not directly connected to river channels. In contrast, transport of the coarser fraction (which makes up >90% of the total landslide volume) may be more significantly affected by landslide locations.

SURFACE PROCESS AND FLUVIAL LANDFORM RESPONSE TO THE MS 8.0 WENCHUAN EARTHQUAKE, LONGMEN SHAN, CHINA

The MS 8.0 Wenchuan earthquake of May 12, 2008, in the Longmen Shan mountain range area in China, led to two roughly parallel NE-trending thrust and strike-slip surface ruptures of the Beichuan, Pengguan, and the Xiaoyudong faults. Coseismic deformation changed the topographical gradient and produced massive landslides and debris flows, causing a corresponding response of the fluvial landforms. In this paper, based on data regarding the surface ruptures and changes to the topography and drainage resulting from the earthquake, the influence of the thrusting and strike-slipping on fluvial landforms and drainage are integrated and analyzed. The results are shown for the following five main aspects: (1) the strike-slipping driven by the earthquake caused the formation of new tectonic diversion points of river channels; (2) the thrusting driven by the earthquake caused the formation of new tectonic slope-break points in the river channel; (3) the strikes of the faults activated by the earthquake controlled the river channel direction; (4) the uplifting driven by the earthquake led to changes of riverbed gradient profiles and their base levels; and (5) exceptionally heavy rainfall after the earthquake initiated landslides, debris flows and floods, and will continue to be a hazard for several decades to come.

Reservoir characteristics and main controlling factors of oil sand in Houba area, northwestern Sichuan Basin, China

Houba oil sand in frontier Longmenshan Mountain is one of the most typically important unconventional resources. The basic reservoir characteristics of oil sand and the main factors affecting reservoir quality were examined in this article based on porosity, permeability, and mercury porosimetry measurements; thin section analyses; SEM observation; and X-ray diffraction analysis. This study shows that the oil-bearing sandstone reservoir is mainly medium−coarse-grained sublitharenite and litharenite. The main pore type is intergranular pores, including residual primary intergranular pores, dissolved intergranular pores, and dissolved intragranular pores; fractures are common in this study area. The quality of sandstone reservoir is of high porosity and high permeability with a high oil saturation of 89.84 %. It is indicated that the main controlling factors of the reservoir in the study area include deposition, diagenesis, and tectonism. Deposition laid a foundation to porosity evolution, and channel sand is the most favorable depositional facies for the reservoir. Diagenetic alterations are the keys to reservoir evolution; dissolution and chlorite coatings cementation play an effective role in the generation and preservation of pores. Compaction, carbonate cementation, and quartz overgrowth cause many damages to the reservoir porosity. Fractures caused by structural breakages can improve the reservoir permeability and they also can provide fluid migration pathways to the late corrosion, which formed a lot of corroded fissures as reservoir and percolation spaces.

Effects from the Wenchuan Earthquake and seismic hazard in the Longmenshan Mountains at the eastern margin of the Tibetan Plateau

Effects from the disastrous Ms 8.0 Wenchuan Earthquake of May 12, 2008 show that the seismic hazard is closely related to the proximity of active faults and elevated areas of unstable rock as well as the recurrence rate of large earthquakes. Dextral-slip thrusting in the Longmenshan Mountains formed large displacement along ~250km of the Central Longmenshan Fault and smaller offsets along sub-parallel faults. The distribution pattern of the major destruction has a strong northeast–southwest orientation closely related to the ruptured faults and decreases noticeably 3–5km away from the Central Longmenshan Fault and ~2km from the other faults. Thousands of landslides, rock falls and debris flows triggered by the earthquake lie chiefly adjacent to the principal fault, and smaller-scale landslides along the other faults as well as in distant areas. Some landslides formed dams whose impounded waters threatened downstream cities. Future landslides, rock falls and debris flows will result from rock loosened by the earthquake during rainy seasons or subsequent earthquakes.The Central Longmenshan Fault had maximum displacements of 4.6m dip-slip and 6.1m dextral-slip and the total dip and dextral-slip displacement for all faults is 5.9m and 7.0m, respectively. These faults apparently converge at depth and account for the crustal shortening in the region. Their displacements coupled with convergence rates determined by GPS measurements provide an estimated recurrence interval for an Ms 8.0 earthquake along the Central Longmenshan Fault of between 4600 and 6600years and that for the entire Longmenshan Mountains between 1000 and 3000years.

Transient water and sediment storage of the decaying landslide dams induced by the 2008 Wenchuan earthquake, China

Earthquake-triggered landslide dams are potentially dangerous disrupters of water and sediment flux in mountain rivers, and capable of releasing catastrophic outburst flows to downstream areas. We analyze an inventory of 828 landslide dams in the Longmen Shan mountains, China, triggered by the Mw 7.9 2008 Wenchuan earthquake. This database is unique in that it is the largest of its kind attributable to a single regional-scale triggering event: 501 of the spatially clustered landslides fully blocked rivers, while the remainder only partially obstructed or diverted channels in steep watersheds of the hanging wall of the Yingxiu-Beichuan Fault Zone. The size distributions of the earthquake-triggered landslides, landslide dams, and associated lakes (a) can be modeled by an inverse gamma distribution; (b) show that moderate-size slope failures caused the majority of blockages; and (c) allow a detailed assessment of seismically induced river-blockage effects on regional water and sediment storage. Monte Carlo simulations based on volumetric scaling relationships for soil and bedrock failures respectively indicate that 14% (18%) of the estimated total coseismic landslide volume of 6.4 (14.6)×109m3 was contained in landslide dams, representing only 1.4% of the >60,000 slope failures attributed to the earthquake. These dams have created storage capacity of ~0.6×109m3 for incoming water and sediment. About 25% of the dams containing 2% of the total river-blocking debris volume failed one week after the earthquake; these figures had risen to 60% (~20%), and >90% (>90%) within one month, and one year, respectively, thus also emptying ~92% of the total potential water and sediment storage behind these dams within one year following the earthquake. Currently only ~0.08×109m3 remain available as natural reservoirs for storing water and sediment, while ~0.19×109m3, i.e. about a third of the total river-blocking debris volume, has been eroded by rivers. Dam volume and upstream catchment area control to first order the longevity of the barriers, and bivariate domain plots are consistent with the observation that most earthquake-triggered landslide dams were ephemeral. We conclude that the river-blocking portion of coseismic slope failures disproportionately modulates the post-seismic sediment flux in the Longmen Shan on annual to decadal timescales.

Rupture of deep faults in the 2008 Wenchuan earthquake and uplift of the Longmen Shan

The Longmen Shan Mountains rise up 6,000 m over a distance of just 100 km, but the mechanisms driving formation of this striking topography are debated. Analyses of crustal movements during the 2008 Wenchuan earthquake suggest that fault-induced uplift plays a role in building the high topography.

Behaviour of Slope Protecting and Retaining Structures during Wenchuan Earthquake on May 12 2008

At 14:28, May 12th 2008, a strong earthquake measured Ms8.0 hits Longmenshan mountain area, west Sichuan province, China. The quake is induced by the thrust of Longmenshan central fault, results in around 50000 rock avalanches and landslides, destroys numerous villages and towns, and causes nearly 90000 fatalities or missing. Based on field reconnaissance, the geology and fault patterns in Longmenshan mountain area are firstly introduced, then the damage or failure phenomena of slope protecting and retaining structures in quake-hit area is described and discussed, including slide-resistant piles, gravity retaining walls, shotcrete, anchored concrete frame, cable bolts and protecting nets. Special discussion is given to the behaviour of cable bolts based on measured data. And finally design issues is discussed in quake-prone areas.

Along-strike topographic variation of the Longmen Shan and its significance for landscape evolution along the eastern Tibetan Plateau

Regional topographic and geomorphic analyses reveal first-order topographic variations from high-elevation and low-relief interior plateau to the relatively low elevation, high-relief marginal plateau in eastern Tibet. Field investigation and slip distribution modeling after 2008 Ms. 8.0 Wenchuan earthquake indicate significant along-strike variability during the rupture that appears to correspond to different segments of a single fault system. This observation motivates a more careful examination of topographic features along the Longmen Shan to explore the connection between the seismic cycle and mountain building. Analyses of topographic relief, hillslope gradient, and channel gradient indices reveal significant differences in the character of topography along the Longmen Shan mountain front. The central portion of the range exhibits the highest slope, relief and steepness of river longitudinal profiles. Whereas the southern Longmen Shan exhibits only subtle differences associated with slightly lower hillslope and channel gradients, the northern Longmen Shan is characterized by topography of significantly lower relief, lessened hillslope gradients, and low-gradient channels. We consider two explanations for these topographic differences; first, that the differences in topographic development along the Longmen Shan reflect different stages of an evolutionary history. Alternatively, these may reflect differences in the rate of differential rock uplift relative to the stable Sichuan Basin.