Ultra-deep carbonate basement reservoirs formed by polyphase fracture-related karstification in the Offshore Bohai Bay Basin, China

The Palaeozoic carbonate basement of the Offshore Bohai Bay Basin (OBBB) presents considerable potential for hydrocarbon exploration. However, the multistage tectonism and complex superimposed palaeo-karstification in the area are unclear, which leads to a lack of understanding on the formation mechanism and distribution of the deep carbonate basement reservoirs. In this study, the occurrence of a fracture-vug network and its fillings in carbonate reservoirs were investigated based on borehole cores, thin sections, and image logs from the southwestern slope of the OBBB's Bozhong Sag. Then the diagenetic fluid properties of the carbonate matrix and fillings were analysed via the data of carbon, oxygen, and strontium isotopes, and major, rare elements from coring intervals. The results revealed that fracture-related karst reservoirs have lithologic selectivity inclined toward dolomite strata. The intersecting relationships, widths, and strikes of the fractures and the regional tectonic background indicate three structural fracture families: NW-, NNE-, and NNW- trending, related to the Indosinian, middle Yanshanian, and late Yanshanian orogeny, respectively. The Indosinian NW- and end-Mesozoic NNE-trending fractures produced by compressional tectonic stress mainly contributed to the formation of the basement reservoirs. The geochemistry of the calcite veins filling these fractures suggests two main types of diagenetic fluids. The fluid of autogenic recharge related to the earlier fills is karstification diffuse flow dominated by internal runoff from rainfall in the highland setting of the Indosinian thrusting orogenic belt. The other fluid of allogenic recharge related to the later fills is the main lateral freshwater flow dominated by external runoff from the catchment in the setting of the horst-lowland within the rifting basin, induced by the Yanshanian destruction of the North China Craton. Finally, the relationship between the three fracture families and two kinds of related fluids is revealed. This allows us to propose a model to understand the polyphase-superimposed fracture-related karst reservoir complexes within the deep carbonate basement of tilting fault blocks that neighbour the Bozhong hydrocarbon kitchen and predict the formation of potential plays with high accuracy.

Sedimentary microtopography in sequence stratigraphic framework of Upper Ordovician and its control over penecontemporaneous karstification,No.I slope break,Tazhong,Tarim block

In order to reveal the relationship between the penecontemporaneous karstification and sedimentary microtopography in sequence stratigraphy,the sequence stratigraphic framework of Lianglitage formation in Upper Ordovician is studied according to the well drilling,logging,geophysical data,detailed observations of core and the paleontology.The Lianglitage formation belongs to the sequence Ⅳ of Ordovician.The second member of Lianglitage formation is prograde sedimentation in highstand systems tract,and is favorable for developing reef flat.The development scale and thickness of reef flat are controlled by the variation of secondary sea level.The types and characteristics of karst in the highstand systems tract show that the late highstand systems tract is dissolved and cemented by the meteoric fresh water and mixed water.Penecontemporaneous karstification is developed at the top of parasequence and high place of geomorphology.Atmospheric diagenetic lens is formed.The developing regulations and controlling factors of penecontemporaneous karstification can provide new clues to the prediction and exploration of favorable reservoir in this area.

Impact of Conjugated Shear Joint of“X”Type on Macroscopic Karstification on the Basis of the Analysis of Google Earth Images

As viewed from space remote-sensing images （e.g. Google Earth images） of South Guizhou and North Guangxi, the authors found that macroscopic karst landscape on the Earth＇s surface is strongly controlled by the Conjugated shear joint of ＂X＂ type. Joints of this kind constitute a huge infiltration network and act as channel-ways for the permeation of meteoric waters from the surface, thus, leading to the dissolution of carbonate rocks nearby. As a result, the karst landscape is formed, which is dominated by linear karst valleys. An ＂X＂ karst valley network structure appears in the area where horizontal strata are distributed, and a feather-like network structure appears in the area where vertical strata are distributed, respectively. When the water permeates downwards to the underground-water level, it will flow horizontally along the strike of ＂X＂ joints toward the local base level of erosion to form an ＂X＂ network system of underground conduits in the area where horizontal strata are distributed, but it is relatively complex, because of the joining of other joints. This is the first time we have made use of Google Earth images to study the karst environment. Therefore, it has been successful in research on the Earth＇s geomorphology, which could only rely on aerial photos and satellite photos in the past. Google Earth images provide low-cost and applicable imaging materials for the study of Earth＇s geomorphology and karst rocky desertification and its control.

Geotechnical Evaluation of Sedimentary Rocks and Karstification Phenomena of Carbonates Exposed along New Upper Egypt-Red Sea Road, Eastern Desert, Egypt

Geotechnical properties of sedimentary rocks exposed along upper Egypt-Red Sea road were investigated to evaluate their geotechnical behavior. One of the most important aims of this work was to determine the engineering problems affected on the road due to the geotechnical behavior of the studied sedimentary rocks and to suggest the suitable solution. Specific weight, unconfined compressive strength, slaking durability and free swelling were measured. The results showed that the studied crystalline limestone, marly limestone and Issawia limestone had higher unconfined compressive strength values ranging from 34,184.00 kN/m2 to 42,602.00 kN/m2 and the studied shale, siltstone and coral reef had lower values ranging from 3,689.00 kN/m2 to 6,933.00 kN/m2. The results showed also that the studied shale, siltstone and mudstone had fast to very fast slaking rate and their slaking amount described as low to medium after Gamble＇s slake durability classification. Replacement and/or chemical stabilization （using lime and/or cement kiln dust） of the expansive sedimentary rocks （shale and mudstone） were recommended to reduce the swelling. Geophysical investigations including shallow seismic, microgravity and GPR （ground penetrating radar） were recommended to scan the sub-grade of the road at El Maaza plateau to determine possible other caves and large voids under the road to avoid its collapse.

Karstification Effect on the Stability of Mosul Dam and Its Assessment, North Iraq

Mosul Dam is located on the Tigris river, 50 Km NW of Mosul;it is 113 m in height, 3.4 Km in length, 10 m wide in its crest and has a storage capacity of 11.1 billion cubic meters. It is an earth fill dam, constructed on bedrocks of Fat’ha Formation, which consists of gypsum beds alternated with marl and limestone, in cyclic nature. The thickness of gypsum beds attains 18 m;they are intensely karstified even in foundation rocks. Therefore, continuous grouting Programme was planned during construction, which was completed in June 1984, with planned operation age of 80 years. Due to insufficient grouting in the foundation, during last years of the last century, the Karstification was enlarged in size and quantity, causing serious problems to the stability of the dam. Since late eighties of the last century, the status of the dam and its probable collapse has caused a panic to the people of Mosul city and near surroundings. Therefore, many attempts were carried out for assessment of the dam;all of them concluded that the Karstification is the main problem and recommended continuous grouting, using modern techniques. In addition, the authorities started to build another “Badush Dam” south of Mosul Dam so that it can stop the first wave if Mosul Dam if collapsed. All geophysical and geological work executed on the dam site;it concluded that the existence of many weaknesses zones, faults and large karstified areas, in different parts of the dam site.

Karstification and Tectonic Effects on the Drainage Trend in the Southwestern Part of Iraq

The extreme southwestern part of Iraq forms a flat terrain, but densely dissected by valleys that flow in different directions with different types. The exposed rocks are horizontally lying, mainly of carbonates of Ratga Formation;Eocene in age;however, eastwards Cretaceous rocks are exposed of different formations;also mainly carbonates with some intercalations of fine clastics and marls. The concerned area is intensely karstified;exhibiting different karst forms and sizes. It is also intensely parted by lineaments of different orientations, which have controlled the drainage style and flow direction. Tectonically, the concerned area lies within the Outer Platform (Unstable Shelf) of the Arabian Plate. Structurally, apart from two main normal faults of E-W trend and intense jointing and lineaments;no other features exist in the area. The intense karstification, presence of lineaments and neotectonic activities has controlled the trends of the valleys forming very complex water shades and basins that locally interfere with each other. These abnormal drainage style and different flow directions of the valleys are studied and correlated with the regional tectonic frame of the region. Moreover, we have used subsurface data;such as groundwater flow directions and geophysical data to indicate the reasons for existing of abnormal drainage style and flow directions of surface water.

Safety of Mosul and Haditha Dams, West Iraq as Affected by Karstification

Mosul and Haditha dams are two large earthfill dams in Iraq constructed on the Tigris and Euphrates rivers, respectively. The two dams were constructed almost at the same period in the mid-eighties of the last century. Both dams suffer from karstification within their foundations. In the former, however, the problem is more severe than the latter. Mosul Dam was constructed on karstified gypsum and limestone beds;different grouting techniques were used, but the works in grouting are still going on to keep the dam as safe as possible as the sealing of the foundation is hampered by the type of geology. Haditha Dam was constructed on karstified limestone and gypsum rocks also. To avoid the effect of the karstification on the safety of this dam, an exceptionally long grout curtain was constructed as foundation treatment work. The length of the grout curtain extended under the earthfill dam and the concrete structures in the river channel and extended beyond the abutments forming left and right sides extensions to cut off water percolation around the dam which could cause the formation of sinkholes. The depths of all parts of the curtain varied following the karstification zones and intensities. The details of both dams are discussed using updated data and relying on the experience of the authors. The current status of both dams is also discussed with some recommendations to keep both dams as safe as possible.

Evaluating Rock Mass Properties of Vipingo Coral Limestone Quarry Based on a Modified Geological Strength Index (GSI) and State of Karstification

The process of evaluating rock mass strength requires that major structural features such as joints that influence rock strength are considered. In carbonate rock masses, however, the strength of the rock mass is largely dependent on intact rock strength and structural features play a secondary role. Laboratory experiments on porous rock have shown that intact rock strength reduces with increasing porosity, which has a direct effect on the rock mass strength. Rock porosity has however not been well accounted for in rock mass characterization methods currently in use. This research applies the modified GSI method for carbonate rock masses which is based on a combination of GSI and total porosity. The main aim is to quantify the GSI with respect to rock porosity which is a direct indicator of the state of karstification, as an inherent feature that affects rock mass strength. An empirical equation is proposed whereby the GSI as observed in the field is modified by a natural log of the value of porosity, giving rise to a modified GSI (GSIm). The GSIm together with laboratory properties of rock is used to determine the properties of Vipingo coral limestone from RocLab software. A deterministic parametric slope stability analysis is done using the finite element software Phase 2 with the rock mass properties as input parameters. The analysis results point to a direct dependence of the slope stability on slope angle, slope height and rock mass strength of the lithological unit. The graphs make a useful design guide for slopes engineered in this type of rock mass.

The hydrothermal karstification and its effect on Ordovician carbonate reservoir in Tazhong uplift of Tarim Basin, Northwest China

With a detailed study on petrology, mineralogy and geochemistry of some important Ordovician carbonate well core samples in Tazhong uplift of Tarim Basin, the distinguishing symbols of hydrothermal karstification are first put forward as the phenomena of rock hot depigmentation, hot cataclasm and the appearance of typical hydrothermal minerals such as fluorite, barite, pyrite, quartz and sphalerite. The main homogenization temperatures of primary fluid inclusions in fluorite are from 260 to 310℃, indicating the temperature of hydrothermal fluid. The fluid affected the dissolved rocks and showed typical geochemistry features with low contents of Na and Mg, and high contents of Fe, Mn and Si. The ratio of 3He/4He is 0.02Ra, indicating the fluid from the typical continental crust. The hydrothermal fluid karstification pattern may be described as follows: the hot fluid is from the Permian magma, containing dissolving ingredients of CO2 and H2S, and shifts along fault, ruptures and unconformity, and dissolves the surrounding carbonates while it flows. The mechanism of hydrothermal karstification is that the mixture of two or more fluids, which have different ion intensity and pH values, becomes a new unsaturated fluid to carbonates. The hydrothermal karstification is an important process to form hypo-dissolved pinholes in Ordovician carbonates of Tazhong uplift of Tarim Basin, and the forming of hydrothermal minerals also has favorable influence on carbonate reservoirs.

Characteristics of Subaerial Karstification and Late Reconstruction in the Dengying Formation,Sichuan Basin,Southwestern China

Tectonic uplift at the end of Sinian resulted in the denudation of the Dengying （灯影） Formation on a different scale and the development of a great number of secondary dissolution vugs beneath the unconformity between Sinian and Cambrian.Geological and geochemical characteristics such as the development of caves,sinkholes,karst tubes and weathering crust,abrupt depletion and negative deflection of δ13C and δ18O in country rock and vug filling toward unconformity surface suggest that subaerial karstification is responsible for the development of secondary karst vugs within 150 m or so beneath the unconformity surface.This kind of secondary dissolution vug was partially or completely filled by different stage dolomite,calcite,quartz and bitumen during the later deep burial and uplift.Facts such as the obvious difference in δ13C and δ18O of the filling in the subaerial karst vug from the country rock and the value of δ13C and δ18O of the filling increasing or decreasing with that of the country rocks,suggest that the hydrothermal fluid charged in vugs is allogenic fluid.Subaerial karstification vug acted as a conduit for allogenic corrosive fluid which can create new secondary vugs and enlarge previous karst vugs.The dissolution,precipitant and recrystallization produced by allogenic corrosive fluid introduction and petroleum thermal cracking,caused contraction or enlargement of the subaerial karstification vugs and the development of new secondary vugs during the deep burial and uplift.The ultimate vug of deep burial dolomitite is determined not only by fluid charge,dissolution,mineral precipitant,and petroleum thermal cracking during the deep burial and tectonic uplift,but also by the amount of initial secondary dissolution vugs created by subaerial karstification.

Features and Controlling Factors of Epigenic Karstification of the Ordovician Carbonates in Akekule Arch,Tarim Basin

ABSTRACT： Tabei （塔北） uplift is an area with the highest hydrocarbon enrichment in Tarim basin, and large oilfields have been found on Akekule （阿克库勒） arch at the middle section of Tabei uplift, with Ordovician carbonate reservoirs. Storage space of the Ordovician carbonate reservoirs in the Akekule arch are mainly caves, pores and fractures resulted from dissolution and/or karstification. The Ordovician carbonate reservoirs are affected and modified by the diagenetie process in penecontemporaneous, epigenetic and burial periods and the multi-stage karstification related with deep hydrothermal activities, among which the most significant effect is from the meteoric water karstification related to the tectonic uplift from the end of Middle Ordovician to the end of Late Ordovician （end of O2-end of 03） and at the end of Middle Devonian （end of D2）. Varied palacogeologic settings and tectonic features at different geologic periods lead to different fluid flowing patterns, karstification mechanisms and transformation features in different regions, which further influence the reservoir distribution. The paleo-uplifts at different periods, such as end of O2-end of 03 and end of D2, control the dominant development zones of karstification; the paleogeomorphology and faults resulted from tectonic uplift control the flowing depth of the karst fluid; and the lithology controls the position and extent of karst development.

Karstification characteristics of the Cenomanian-Turonian Mishrif Formation in the Missan Oil Fields,southeastern Iraq,and their effects on reservoirs

The Middle Cretaceous Mishrif Formation of the Cenomanian-Turonian age is an important Middle Eastern reservoir.Previous studies have shown that carbonates in the Mishrif Formation are affected by karstification and are concentrated at the top of the formation.However,there is a lack of systematic research on the effects of karstification on the formation’s reservoir characteristics.Based on core samples,scanning electron microscopy of thin sections,and geochemical analysis of C,O,and Sr isotopes from wells in the Missan Oil Fields,unconformity was identified at the top of the B21 subzone of the Mishrif Formation.Core and cast-thin-section observations indicate that there is a significant lithological difference above and below the unconformity,and dissolution pores and voids are well-developed beneath it.Vadose silt fills the intragranular pores and geopetal fabric is developed inside some mouldic pores beneath the unconformity surface.The isotopic values of ^(87)Sr/^(86)Sr(0.707270-0.707722)and δ^(18)O(−5.94‰—−2.32‰)suggest that the top of the B21 subzone of the Mishrif Formation has been severely affected by syngenetic karst.Karstification is the key to high-quality reservoir formation and the reservoir’s oil-bearing heterogeneity.This study will inform oil and gas exploration in the Missan Oil Fields and in other areas with similar reservoir characteristics.

Physical Modeling of Landslide Mechanism in Oblique Thick-Bedded Rock Slope: A Case Study

The Jiweishan landslide illustrates the failure pattern of an apparent dip slide of an oblique thick-bedded rockslide. Centrifugal modeling was performed using a model slope consisting of four sets of joints to investigate the landslide initiation mechanism. Crack strain gauges pasted between the slide blocks and the base failed in sequence from the rear to the front as the centrifugal acceleration increased. When the acceleration reached 16.3g, the instantaneous failure of the key block in the front triggered the apparent dip slide of all blocks. The physical modeling results and previous studies suggest that the strength reduction in the weak layer and the failure of the key block are the main reasons for the Jiweishan landslide. The centrifuge experiment validated the previously proposed driving-blocks-key-block model of apparent dip slide in oblique with inclined bedding rock slopes. In addition, the results from limit equilibrium method and centrifuge test suggest that even though the failure of the key block in the front is instantaneous, a progressive stable-unstable transition exists.

Control factors of reservoir oil-bearing difference of Cretaceous Mishrif Formation in the H oilfield, Iraq

To reveal the law of oiliness of the Upper Cretaceous carbonate reservoirs in the Middle East and factors controlling the oiliness, the Mishrif Formation of the H oilfield in Iraq was examined comprehensively. Based on core observation and description, casting thin section analysis, and statistics of physical property and pore-throat structure parameter, etc., in combination with previous achievements and understanding, it is pointed out that the coring interval of Mishrif Formation has strong oiliness heterogeneity, and can be divided into 4 grades, oil rich, oil immersed, oil spot, and oil trace; the oil-bearing grade has obvious facies-controlled characteristic in macroscopic view; and in different oil-bearing grades, relatively oil-rich sections present higher karst development intensity, better physical properties and pore-throat structure characteristics. It is concluded through the study that the sedimentary microenvironment and eogenetic karstification determine the macroscopic oiliness of the reservoir in Mishrif Formation of H Oilfield, and the improvement of pore-throat structure caused by eogenetic karstification is the decisive factor of the micro oil-bearing difference of the reservoir. Finally,the genetic model of reservoir development and oil-bearing difference was established.

Is Mosul Dam the Most Dangerous Dam in the World? Review of Previous Work and Possible Solutions

Mosul Dam is an earth fill dam located on the Tigris River in North Western part of Iraq. It is 113 m in height, 3.4 km in length, 10 m wide in its crest and has a storage capacity of 11.11 billion cubic meters. It is, constructed on bedrocks which consist of gypsum beds alternated with marl and limestone, in cyclic nature. The thickness of the gypsum beds attains 18 m;they are intensely karstified even in foundation rocks. This has created number of problems during construction, impounding and operation of the dam. Construction work in Mosul Dam started on January 25th, 1981 and started operating on 24th July, 1986. After impounding in 1986, seepage locations were recognized. The cause of seepage is mainly due to: 1) The karsts prevailing in the dam site and in the reservoir area. 2) The existence of gypsum/anhydrite rock formations in the dam foundation alternating with soft marl layers and weathered and cavernous limestone beddings. 3) The presence of an extensive ground water aquifer called Wadi Malleh aquifer, which affects considerably the ground water regime in the right bank. The dissolution intensity of the gypsum/anhydrite ranged from 42 to 80 t/day which was followed by a noticeable increase in the permeability and leakages through the foundation. Inspection of the dam situation in 2014 and 2015 indicates that the dam is in a state of extreme unprecedentedly high relative risk. In this work, possible solutions to the problem are to be discussed. It is believed that grouting operations will elongate the span life of the dam but do not solve the problem. Building another dam downstream Mosul Dam will be the best protective measures due to the possible failure of Mosul Dam, to secure the safety of the downstream area and its’ population.

Origin of the penecontemporaneous sucrosic dolomite in the Permian Qixia Formation, northwestern Sichuan Basin, SW China

The characteristics,formation time,and origin of the sucrosic dolomite reservoirs in the Permian Qixia Formation of northwestern Sichuan Basin are analyzed.Core and outcrop description and microscopic analysis of the sucrosic dolomite samples are carried out.It is found that the dolomite has typical features different from other kinds of dolomites:(1)This dolomite is generally medium-coarse in crystal size,and often associated with very finely to finely crystalline dolomite and cave-filling dolomite.(2)Typical identification marks of eogenetic karstification are developed at the top of the upward-shallowing sequence.(3)The medium-coarse crystalline sucrosic dolomite is cut by the early diagenetic karst fabric,and is characterized by dolomite with dissolution edge,dolomite vadose silt in pores,and transgression clay filling between the medium-coarse dolomite crystals.The medium-coarse crystalline sucrosic dolomite was formed earlier than the eogenetic karstification.The sucrosic dolomite with occasional cloudy core and clear rim has bright cathodoluminescence,high inclusions temperature,significant negative skewness carbon and oxygen isotopic compositions,and rare-earth element(REE)pattern similar to seawater,indicating it experienced two periods of dolomitization,evaporative concentration reflux-infiltration and penecontemporaneous seawater circulation hydrothermal fluid dolomitization.The study results not only update the understanding on the dolomitization time of Qixia Formation,demonstrate that the sucrosic dolomite can be formed in the penecontemporaneous stage when seawater reflux superimposed with hydrothermal fluid effects,but also show that the taphorogenesis in the Dongwu period began in the Early Permian.Moreover,the dolomite controlled by the grain bank migration and terrain in the slope break appears in bands of large scale,this knowledge provides basis for expanding the exploration field of this type of reservoirs.

Defects in Foundation Design Due to Miss-Interpretation of the Geological Data: A Case Study of Mosul Dam

Existing engineering problems in Mosul Dam and their background are discussed in this paper. A thorough review of the available geological reports was made. These reports covered many decades of investigations from 1953 up to the investigations performed during the construction of the dam. A large volume of geological information was accumulated during these investigations, but it is unfortunate to see that some of the basic facts were not interpreted correctly. This applies to the incorrect correlation of the encountered beds in the exploration boreholes and miss-understanding of the actual stratigraphic succession at the dam site. This misinterpretation contributed to misleading results regarding the true karst zones and the type of rocks and their thicknesses in the foundation zone and surrounding area. As a result, the dam was placed on problematic foundations consisting of brecciated and highly kartsified gypsum/anhydrite rocks and/or conglomerates in which gypsum forms the main constituent as cementing materials. Karstified beds were not recognized in some depths and were described as normal marl and/or breccias. This also added to the use of improper method of foundation treatment by adopting a deep grout curtain as the main anti-seepage measure instead of using a more positive measure by constructing a diaphragm wall. The mentioned misinterpretations are discussed here in details together with their consequences, and a more accurate picture of the geology is presented.

Development of Sulfuric Acid Speleogenetic Deposits within Cavernous Middle Eocene Beds:Inference on Hydrocarbon Gas Seepages,Giza Pyramids Plateau,Egypt

Development of sulfuric acid speleogenetic mineral deposits within cavernous middle Eocene beds of the Pyramids plateau is linked to hydrocarbon gas seepages.The work carried out field observations,binocular,polarizing,scanning and transmitted electron microscopy investigation,X-ray diffraction and X-ray fluorescence analyses.The morphological and petrographic features and chemical composition of the studied mineral deposits reveal a hypogene sulfuric acid speleogenesis.A model comprised of the following stages can be used to explain the presence of these features.(1)Ascent of hydrogen sulfide(H_(2)S)gas associated with hydrocarbon seepages from the Cretaceous reservoirs under reducing conditions followed by oxidation to sulfuric acid(H_(2)SO_(4)),(2)descent of carbonic acid(H_(2)CO_(3))-rich solution generated from surface sources,(3)reaction of H_(2)SO_(4)and H_(2)CO_(3)with the calcareous and argillaceous host rocks,(4)formation of H_(2)SO_(4)speleogenetic by-products represented by natroalunite,aluminium-phosphate-sulfate,hydrated halloysite and Fe/Mn oxides within the replacive gypsum,and(5)subsequent stresses due to the formation of nearby stratiform cavities gave rise to the development of fractures/veinlets filled with displacive fibrous satinspar gypsum.The study sets the paleokarst features of the Giza Pyramids plateau within a hypogene sulfuric acid karst system developed by the action of groundwater containing H_(2)S,H_(2)SO_(4)and H_(2)CO_(3).

Mosul Dam: A Catastrophe Yet to Unfold

Mosul Dam is multipurpose earth fill dam. It is 3.4km long, 113m in height and its storage capacity reaches 11.11 km3 of which 2.95 km3 is dead storage. The dam is located on the River Tigris in the northern part of Iraq about 60 km in north west Mosul city. The dam was built on highly karstified alternating beds of gypsum, marl and limestone. The dam was operating in 1986 and since then, seepage problems started due to the solubility of the gypsum beds, presence of karstification and the effect of the local groundwater aquifer. Insensitive grouting program was put to stop the seepage and ensure the stability of the dam but it did not stop. The situation became worse in 2014 when ISIS occupied the dam area and grouting operations which were halted. Recent evaluation of the conditions indicates that the dam is in its worst conditions. The failure models of the dam indicate that 6 million people will be affected, and 7202 km2 of land will be inundated. To stop this catastrophe, grouting operations should be continued intensively to elongate the span life of the dam. Water level within its reservoir should be kept at a very low level to minimize the damages in case of dam failure. As a permanent solution, another dam should be built downstream Mosul Dam so that it can take the wave of Mosul Dam in case of its failure.

Characteristics and main controlling factors of reservoirs in the fourth member of Sinian Dengying Formation in Yuanba and its peripheral area, northeastern Sichuan Basin, SW China

Based on the abundant outcrops and 3 D seismic data in Yuanba and its surrounding areas in northeastern Sichuan Basin,the characteristics,main controlling factors and development regularities of reservoirs in the fourth member of Sinian Dengying Formation(shortened as Deng 4 Member)were analyzed comprehensively.The results show that the reservoirs were mainly developed in microbial dolomite associated with the microbial mound and dolarenite associated with grain beach,and had intergranular dissolution pores,inter-frame dissolution pores and karst caves as main storage space.The reservoirs were large in scale and strong in heterogeneity.The analysis of the reservoir genetic mechanisms shows that the reservoirs in the fourth member of Dengying Formation were the products of karstification on mound-beach facies deposited in the second episode of Tongwan movement.The karstification had the marks of eogenetic karstification such as granophyric karst system and half-dissociation zone.The reservoirs were controlled by the sedimentary facies belt,the mound beach facies was the decisive factor in reservoir development.Further analysis indicates that the Yuanba area had a geological background of well-developed platform margin mound beach facies,so mound beach bodies can be taken as a favorable exploration target.