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2018, Volume 25 Issue 2
15 March 2018 Previous Issue    Next Issue

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Tectonic evolution and hydrocarbon distribution in African basins. ZHANG Guangya,YU Zhaohua,CHEN Zhongmin,ZHANG Diqiu,WEN Zhixin,HUANG Tongfei,WANG Yanqi,LIU Xiaobing,MA Feng,ZHAO Jian 2018, 25(2): 1-14.  DOI: 10.13745/j.esf.2018.02.001 Abstract ( 342 )   PDF (5617KB) ( 3744 )   The African sedimentary basins, with relatively low degree of exploration and huge undiscovered hydrocarbon reserve, are the present hot spots for oil and gas exploration. The African Plate mainly experienced three tectonic evolution stages during Phanerozoic, during which the forming, drifting and breaking up of the Gondwanaland led to the development of several types of African basins. Based on our basin study of tectonic evolution and prototype and petroleum geological analysis, the structural characteristics and rules of hydrocarbon accumulation in different types of rifts are determined as follows. The pericratonic basins in North Africa initiated in the early Paleozoic, in which hydrocarbons mainly distributed in the huge dome structures, formed under the compression of the Hercynian movement, and the Paleozoic petroleum system dominated in the pericratonic basins. The continental margin rift basins in North Africa experienced rifting and subsidence after the Hercynian movement, and the differences in subsidence intensity and depocenter led to diversity in hydrocarbon accumulation and resource potential. The passive margin rift basins in the East and West Africa formed during the Pangea paleoplate breakup and Atlantic and Indian Ocean rifting: in the west, saliferous formations are widely distributed with two petroleum systems developed above and below the saline formations, respectively; in the east, hydrocarbon distribution is heterogenous and controlled primarily by the structural architectures of the basins. The intraplate Central and West Africa rift basins experienced three periods of rifting, i.e, the Early Cretaceous, Late Cretaceous and Paleogene. The EW striking basins were uplifted intensely under compression from the collision of the African and Eurasian Plates in the Late Cretaceous; as a result, hydrocarbons accumulated primarily in the Lower Cretaceous. The NWSE striking basins, slightly affected by the collision, have hydrocarbon distributions mainly in the Upper CretaceousPaleogene. Finally, the eastern African rift basins and the Red Sea basin are newly formed Cenozoic basins with dominant Cenozoic petroleum system. The hydrocarbon in the Cenozoic delta basins is controlled by fan delta sand body distribution and basin architecture. Related Articles | Metrics

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The Precambrian basement play in the Central African Rift System. DOU Lirong,WANG Jingchun,WANG Renchong,WEI Xiaodong 2018, 25(2): 15-23.  DOI: 10.13745/j.esf.2018.02.002 Abstract ( 315 )   PDF (2994KB) ( 393 )   The Central African Rift System (CARS) is a group of MesozoicCenozoic rift basins developed on the Precambrian crystalline basement. Our investigation on the outcrops and drilling data in the CARS indicated that the lithology of the basement rocks are mainly igneous rocks and orthometamorphites with small amount of parametamorphites. Based on the content of dark minerals, the basement rocks can be divided into two types: felsic and mafic rocks. The former is prone to be easily weathered and fractured than the latter, and as a result, the felsic rocks have better poroperm. Field investigation showed that a thick widespread spherical weathered crust formed during the CambrianJurassic; during the Early Cretaceous, the coupling of the strikeslip of the Central African Shear Zone and Precambrian brittle basement resulted in a group of highangle faults, en échelon fractures, microfractures and joints within the basement, while weathering, leaching, epithermal and hydrothermal activities controlled dissolution, cementation and development of the secondary pores. Our data further showed that the weathered crust and fractured reservoirs have layerlike features, where the basement reservoir was vertically zoned, displaying the weathered and leached, fractured, semifilled fractured and tight zones. Moreover, three sets of mudstones formed in the Early Cretaceous, Late Cretaceous and Paleogene, respectively, potentially acting as a top seal for the buried Precambrian hills. The Paleogene mud shales developed immaturely due to shallow burial depth, therefore they could not serve as active source rocks. The Upper Cretaceous shales, deposited in the shallow lake facies, were composed primarily of kerogen type Ⅲ organic matter, although the overall content of organic matters was not high; whereas the Lower Cretaceous shales, deposited in the deep lacustrine facies, were exceptional regional source rocks, providing an excellent top seal for the basement reservoirs. According to the ages of the cap rocks, there are three hydrocarbon plays in the CARS, i.e, the Lower Cretaceous, Upper Cretaceous and Paleogene plays. The exploration of basement reservoir in Chad has led to the discovery of a number of commercial light oil fields in the crystalline basement reservoirs on the northern slope of the Bongor Basin, and opened a new frontier for the hydrocarbon exploration in the CARS. Related Articles | Metrics

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Passive continental margin basin evolution of East Africa and the main controlling factors of giant gas fields: an example from the Rovuma Basin. ZHANG Guangya,LIU Xiaobing,ZHAO Jian,WEN Zhixin,ZHANG Diqiu,WANG Zhaoming,ZHANG Lei,MA Feng,CHEN Xi 2018, 25(2): 24-32.  DOI: 10.13745/j.esf.2018.02.003 Abstract ( 199 )   PDF (4604KB) ( 295 )   Many giant gas fields, concentrated in the Rovuma Basin of the East Africa passive margin basins, were discovered in recent years with a total recoverable reserves up to 3.8 trillion cubic meters. The Rovuma Basin has gone through three stages of tectonic evolution, i.e. the Karoo rift, Madagascar rift and Madagascar drift. The LowerMiddle Jurassic lacustrine and shallow sea source rocks formed in the Madagascar rift period, and came to the oil window, the peak of oil generation and the gas window, in the Early Cretaceous, the Late Cretaceous and the Oligocene, respectively. Affected by the East Africa onshore faults during the Madagascar drift period, massive sandrich sediments accumulated towards the deep water region by way of block transport and deep water sliding. It formed a super giant deepwatergravityflow sandbody, wide and thick, with relatively uniform lithology and favorable physical property within the basin. The marine mudstone during the Madagascar drift period provided a good regional overlying strata. The early uplift onshore East Africa and delta progradation led to the formation of gravity sliding and salt diapir within the Rovuma Basin, and subsequently the East Africa normal fault zone (EANFZ) and the East Africa thrust fault zone (EATFZ), between the Oligocene and Pliocene. As a result, hydrocarbons from the underlying LowerMiddle Jurassic source rocks migrated upward along the normal and deep water thrust faults, accumulating in the structurallithologicalstratigraphy and structural traps to form the giant gas fields. Related Articles | Metrics

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New progress in petroleum geology and new field of exploration in the Muglad Basin, Sudan. WANG Guolin,WANG Wangquan,ZHENG Yonglin 2018, 25(2): 33-41.  DOI: 10.13745/j.esf.2018.02.004 Abstract ( 198 )   PDF (1906KB) ( 315 )   The primary reservoirseal assemblage (RSA) of the BentiuAradeiba formations in the Muglad Basin, Sudan is becoming overexplored, which significantly reduces inventory of drillable prospects and makes further exploration more difficult. Based on our reservoir forming study and recent drilling data, we propose that there are four novel exploration domains in the basin, i.e., the secondary oil reservoir in shallow formations, lithological oil reservoir, buried basement hill oil reservoir, and gas reservoir below source rocks. The four domains are characterized as follows: the fault step zones and palaeouplifts of the Kaikang subbasin are the favorable targets for the secondary oil reservoir in shallow formations; the Fula, Bamboo and Sufyan depressions and Neem South slope are potential candidates for the lithological oil reservoir; the basement hills blanketed by AG2 and/or AG4 source rocks in the center and slope of depressions should be the first choice for buried basement hill oil reservoir; and finally, the AG5 structural prospects distributed in the Neem area, the FulaMoga and SharafAbu Gabra structural zones and the central structural zone of the Sufyan depression, could be selected as favorable targets for the largemiddlescale natural gas reservoir below source rocks. Related Articles | Metrics

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Analysis of hydrocarbon accumulation condition and exploration tactics of the Upper Cretaceous strata in the Termit Basin, Niger. YUAN Shengqiang,MAO Fengjun,ZHENG Fengyun,ZHANG Guangya,LIU Jiguo,CHENG Xiaodao 2018, 25(2): 42-50.  DOI: 10.13745/j.esf.yx.2017-12-1 Abstract ( 162 )   PDF (1693KB) ( 235 )   There are two plays developed in the Termit Basin vertically, namely the upper Paleogene and lower Cretaceous formations. The upper Sokor and lower Cretaceous formations were the dominant and potential plays, respectively. In addition, a continentalmarinecontinental facies cycle, from the Early Cretaceous to Palaeogene, developed an unique marine transgressive sequence in the basin. With increasing degree of exploration on the Paleogene main strata in the Termit Basin, it is desirable to look for a new exploration stratum in the basin. Here, we studied the hydrocarbon accumulation characteristics of a newly explored upper Cretaceous strata in the Termit Basin. We showed that the Lower Cretaceous can be further subdivided into K1, the Donga Formation and Upper Yogou Formation (YS3). Based on the researches of structural modeling, depositional system evolution, and oil and gas accumulation condition, we considered YS3 to be the new prospecting layer. Our study showed that the YS3 group was formed in the fall stage of a high frequency sealevel fluctuation period. It had transitional facies, and developed the interbedded delta front sand and shale system. The YS3 mudstones were the main source rocks of the Termit Basin, extensively matured with good hydrocarbon generation potential. The delta quartz sandstone was a dominant reservoir of high compositional and texture maturity. It was mostly located in the favorable hydrocarbon vertical migration domain, near the source rocks of proper burial depth. Altogether, these features of YS3 afforded it favorable hydrocarbon accumulation conditions. Based on tectonic study and well data, we carried out investigation on the depositional system of the Yogou Formation: we built time and spatial evolution sequences, evaluated provenance system and reservoir features, and selected the optimum exploration zone. In addition, through a paleotopographic comparative study of the depositional period in the eastern, western and southern margins, we discovered that different margins had different topography and depositional features in the Yogou period: the Dinga stepfault in the western margin assumed a steep slope with several deep fractures between uplift and depression, and the belt is narrow; the Yogou slope in the southern margin and Fana lowuplift area are gentle slopes with wide open areas, and steep and gentle slope models were built based on the Yogou slope. Our investigation further reveal that in the Late Cretaceous, the eastern uplift belt was the dominant provenance zone, whereas the western and southern uplifts were minor; as a result, large scale delta front sand, interbedded with mudstone, developed widely in the east of the basin in the later period of transgressive sequence, while the south and west came second and third in development scales, respectively. The source rock study showed that the YS3 mudstone source rock had terrigenous origin and its development was controlled by the amount of sediment supply. The depositional system analysis showed that the gentle slope belts in the southwest and east of the basin are more mature than the western steep slope zone. Compared with the upper Paleogene Sokor layer, the lower layer Cretaceous Yogou Formation YS3 layer lacks substantial cap rock, therefore, in future exploration, we should adopt a different strategy, aiming at the anticline trap, small throw reverse normal fault, consequent normal fault and depositional trap. Related Articles | Metrics

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Quantitative study on growth faults activity and its controlling on hydrocarbon accumulation in the Kaikang Sag, Muglad Basin. LI Juan,CHEN Honghan,ZHANG Guangya,CHEN Guangpo,ZHANG Bin 2018, 25(2): 51-61.  DOI: 10.13745/j.esf.2018.02.006 Abstract ( 163 )   PDF (2399KB) ( 387 )   In this study, ancient fault sliding distance and sliding rate were used in a quantitative study on growth faults in the Kaikang Sag, Muglad Basin, Sudan to quantify variations in fault growth rates and activity intensities in three different rifting phases, i.e., the Early Cretaceous, Late Cretaceous, and Paleogene to Neogene. The results showed that the controlling sag faults developed in a linked multiphase multisection fashion, the controlling structural belts faults underwent linked growth at the beginning and simple growth later, whereas the controlling traps faults experienced simple growth throughout. Fault activities in the Early Cretaceous controlled the distribution of source rocks, while the three riftingsag structural cycles led to differentiation of sand and shale, forming the upper, middle and lower reservoirseal assemblages. Hydrocarbon accumulation was controlled by three main factors: trap type, fault movement intensity in the Darfur group as source rock entered into oilgenerating peak, and fault movement intensity in the NayilTendi formations when paleooil reservoir could be destroyed and readjusted. Transverse anticline in structural transform zone, horst, and antithetic fault block were favorable structures for paleoreservoir or shallow secondary oil reservoir formations, whereas consequent fault block was not favorable. The results also showed that the fault activity ratios of the Darfur group to NayilTendi formations can be used to measure the relative capability of hydrocarbon accumulation in the early period vs. oil reservoir destruction in the late period. This relative capacity, based on actual drilling data, is related to the accumulation layers in the following way: the eastern limb of the Kaikang trough hosts a prolific oil and gas accumulation zone, where under the uplift zone hydrocarbons mainly accumulated in the lower reservoirseal assemblage of the Cretaceous system, forming deposits throughout the eastern fault slope zone. In contrast, accumulation condition in the western limb of the Kaikang trough is poor in its slope and fault step zones, where the northern uplift developed multiple accumulation layers but low production, while the southern uplift faired better with accumulation mainly in the lower reservoirseal assemblage of the Cretaceous system; in the depression zone, secondary oil reservoir might be formed due to strong fault movement in the late period. Related Articles | Metrics

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3D structural modeling and its application in the Termit Basin, Niger. MAO Fengjun,JIANG Hong,OU Yafei,CHENG Xiaodao,YUAN Shengqiang,WANG Yuhua,ZHENG Fengyun,LI Zaohong 2018, 25(2): 62-71.  DOI: 10.13745/j.esf.2018.02.007 Abstract ( 186 )   PDF (1861KB) ( 336 )   3D structural model building is a frontier technique in tectonic research with superior performance in practicability, accuracy and visualization. This technique is normally used for studying local structures, and its application in basinlevel model building has been challenging. Here, we applied 3D structural modeling to the complex structures of the Termit Basin, where two rifts were developed in the Early Cretaceous and Paleogene. Taking the basin (area 30000 km2) as a whole, we built a 3D structural model using 2D and 3D seismic data in addition to more than 100 sets of well logging data, and overcoming technical difficulties such as multifault system, structural complexity and large data volume. Layer and fault grading simulations, automatic fault rename and loop generation, and fault relationship definition, were successfully applied to the Termit Basin. Thus we have developed new tools for studying fine structures at the basin scale. We showed that structural sections in any directions or planes and any welltie sections in the entire basin can be extracted for model building; and 3D model of each zone or local structure can be built to make structural analysis more accurate. Our 3D geological structural model has been widely used in the Termit Basin exploration, in areas such as tectonic units division, explorationzone evaluation, target optimization and well location demonstration. The model showed that the overlapping sequences of the lower Cretaceous sag and upper Paleogene rift formed the big lower and small upper sag structures in the Termit Basin, respectively. It also showed that in the depression period of the Late Cretaceous, marine source rocks distributed widely; and later in the Paleogene, the superposed rifts formed above the marine source rocks, creating favorable loci for hydrocarbon to accumulate in the Paleogene. Based on the newly established basin structural model, the structural characteristics and hydrocarbon accumulation potential of each structural zone can be further defined. Specifically, the Fana low uplift, located between the Moul and Dinga Sags, has relatively well developed fractures for hydrocarbon migration and accumulation, making it the most favorable exploration zone in the Termit Basin. The Araga graben, where fractures were developed, presents good hydrocarbon accumulation conditions. In contrast, the Moul and Dinga Sags, although having good oil deposit, have poor exploration potential due to poor upward migration caused by weak tectonic activities and undeveloped fractures. Moreover, the basin structural model allowed detailed analysis of the tectonic styles and fractures, resulted from the superposed rifts and their controlling effects on hydrocarbon accumulation in the basin, so as to optimize exploration targeting. Application of the structure model has made breakthroughs in the exploration of the upper Paleogene and lower Cretaceous assemblages. Related Articles | Metrics

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Paleogene structure styles and their controls on the hydrocarbon accumulation in the Termit Basin, Niger. ZHENG Fengyun,SHI Buqing,LI Zaohong,YUAN Shengqiang,LIU Bang,WANG Yuhua,JIANG Hong,CHENG Xiaodao 2018, 25(2): 72-82.  DOI: 10.13745/j.esf.2018.02.008 Abstract ( 227 )   PDF (2046KB) ( 202 )   Niger Termit Basin experienced the first and second rifting cycles during the Early Cretaceous and EoceneOligocene, respectively. The two cycles originated through different geodynamic and kinematical mechanisms; as a result, they have different structural styles and basin features. This paper analyzed the geodynamic origin, kinematical features and structural styles of the second rifting cycle, as well as implications of these factors in hydrocarbon accumulation in the Termit Basin, Niger. The findings are the following. During the EoceneOligocene, and under the regional compression stress field caused by the AfricaArab and Eurasia plate collision, the Termit Basin moved as an extrusion from the inner Africa plate in the NEESWW direction and acted as typical strikeslip with extensional characteristics. Building on the NWSE fault system in the first rifting cycle, the second rifting cycle developed the NNWSSE fault system in the Termit Basin. The basin has primarily grabenhorst and half graben structures, with graben style in the north, and Domino half graben style in the south. The EoceneOligocene transtension formed a series of en echelon normal faultsconnected by soft linkageand developed a large number of strike slip transfer structures at both basin and subbasin levels. The Paleogene structure transfer zones could be divided into three basic styles, i.e., parallel, antiparallel and back to back styles, of which, overlapping parallel, antiparallel and back to back overlapping transfer zones are the main structural styles observed in the Termit Basin. The second rifting cycle controlled the EoceneOligocene depositional system of the Termit Basin. During the Eocene initial rifting stage, delta system was formed in Sokor1 Fm.; and during the OligoceneEarly Miocene deeprifting stage, lacustrine mudstone was deposited in the Sokor2 Fm., which generated the most advantage reservoirseal assemblage in the Termit Basin. Meanwhile, structural transfer in the second cycle helped to form many fault blocks and noses and faulted anticline structures, which account for the main types of traps in the Termit Basin. Furthermore, during the OligoceneEarly Miocene deeprifting stage, the main active fault became connected to the Cretaceous source kitchen, this led to extensive vertical and lateral hydrocarbon migrations in the Termit Basin. The growth indices of the main faults implied that the Paleogene hydrocarbon distribution and accumulation were controlled by the fault activity during the OligoceneEarly Miocene deeprifting stage. Based on above analysis, the lithological traps of the Sokor1 and midlower Sokor2 Fms. —located on the strike slope of transfer zones, and new petroleum system in the Upper Sokor2 Fm., all have hydrocarbon potentials and could be further explored. Related Articles | Metrics

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Characteristics and the forming process of the Eocene ultradeepwater gravity flow sandstone reservoir in the Rovuma Basin, East Afirca. ZHAO Jian,ZHANG Guangya,LI Zhi,SHI Buqing,FAN Guozhang,WANG Xiaofeng,WEN Zhixin,FA Guifang,SUN Guotong 2018, 25(2): 83-91.  DOI: 10.13745/j.esf.2018.02.009 Abstract ( 240 )   PDF (3973KB) ( 301 )   A number of PaleogeneNeogene largearea gravity flow sand bodies, bearing super huge volume of gas reserve, were developed in the ultradeepwater region in the Rovuma Basin, East Africa. By studying the Eocene sand body, we investigated the characteristics of the extraordinary sedimentary complex and its corresponding forming mechanisms in different scales, using high quality seabed 3D seismic imaging, well data and nearly one hundred meters of valuable conventional core. From the seismic imaging, it was found that these sand bodies are lenticular and wedgeshaped, enclosed by thick and darkcolored deepsea mud or shale. From the wireline logging data, an abruptcontact exists between sandstone bodies and their surrounding mud. And the sandstones always seem homogenous with excellent porosity, permeability and especially low content of shale in whole interval. However, the detailed observation of bottomhole core indicated these seemingly homogeneous and extraordinarily thick sand bodies belong to gravity flow deposit of mainly sandrich turbidite channel and lobe complex, and are composed of several comparable gravity flow deposit units or sublayers. Each unit or sublayer mainly consists of coarse grained sandstone in the lower part, and fine grained sandstone, silt or clay in the upper part. A clear boundary could be easily identified between any two units, and the common lack of fine grained deposits in upper layers indicates these sandstones were modified to various extents by a certain geological process. After a comprehensive study of the sandstone complexes, together with research on regional sedimentary environment, the formation mechanism of the sandstones can be summarized as the following, the Rovuma river delta directly supplied huge volume of sandrich sediments to the largearea deposit complexes; the deepwater gravity flow deposition and block transportation controlled their grained bedding and internal architecture, one of the most important features of gravity flow deposit. Then the bottom current blew away the upper finegrained suspension of shale and siltsparing the mediumcoarse grained deposits and formed the fascinating deepwater sandstone complexes. Finally, multiple depositional events and channel lateral migration together contributed to the formation of the thick, homogeneous and mudfree sandstone bodies of this unique type of excellent sandstone reservoir. Related Articles | Metrics

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Multiinformation association for interlayer prediction in braided river reservoir: taking Fal Structure of P field, South Sudan as an example. WANG Min,MU Longxin,ZHAO Guoliang,KE Weili,ZOU Quan,KANG Chujuan 2018, 25(2): 92-98.  DOI: 10.13745/j.esf.2018.02.010 Abstract ( 95 )   PDF (3320KB) ( 215 )    Prediction of interlayers in braided river reservoir is a key component in reservoir description. Interlayer predictions have been largely done on the interlayer developmental pattern and channel bar architecture using one single prediction method. Here, based on literature survey, and focusing on the complex interlayers in the braided river reservoir of P filed, South Sudan, we propose an interlayer prediction method on the basis of multiinformation association following genetic analysis of different sand and shale morphologies developed under different depositing conditions. First, in areas where well densities were large, the skeleton profiles and triangular cross sections were established; and scales of different interlayer were confirmed by use of well log and seismic data with high vertical resolution and strong horizontal continuity. Next, based on the lithofacies model, stable barrier model was built by using the deterministic modeling method, constrained by the barrier thickness map. Then, channel and channel bars were simulated in the sand facies by using stochastic method, constrained by sedimentary microfacies. The interlayer model was also established by using stochastic method, constrained by genetic analysis of various types of interlayers. It was established that there were four types of interlayers in the study area, and a fine geological model reflecting their spatial distributions was built on the basis of multiinformation association. Our study found that the scales of interlayers with thickness greater than 2 m can be verified by well and seismic integration. Thus, the scales of interlayers can be quantitatively determined for different layer types and directions, which lays the foundation for predictive model building. Lithofacies probability model based on Kriging interpolation increases lithofacies model accuracy to 94%. The deterministic modeling method, constrained by stable barrier thickness map, can build accurate barrier model for sand layers and sublayers. This objectbased modeling method combines deterministic and stochastic approaches to set parameters such as form, quality, scale and trendfor different interlayers to build a fine geological model of braided river reservoir. Our method may also be applicable for predicting interlayers in other related reservoirs. Related Articles | Metrics

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Genetic mechanism of lowresistivity oil zones and comprehensive identification technology for well logging in the Termit Basin, Niger. LI Zaohong,CHENG Xiaodao,JIANG Hong,ZHENG Fengyun,YUAN Shengqiang,LIU Bang,LIU Jiguo,WANG Yuhua 2018, 25(2): 99-111.  DOI: 10.13745/j.esf.2018.02.011 Abstract ( 111 )   PDF (2398KB) ( 204 )   The Termit Basin of Niger, covered with low salinity stratum water (salinity 200×10-61500×10-6 ), developed a highly complex oilgaswater system of complex fluid type, with several oilgaswater systems present including normal and lowresistivity oil zones. Previous studies demonstrated that it is very difficult to identify the complex fluid type by single data analysis, especially in lowresistivity oil zones. It has also been proven by drilling that, in the Termit Basin, low resistivity oil zones were commonly developed in the Sokor 1 and Yogou formations in the Paleogene and Cretaceous, respectively. The lowresistivity oil zones have two main resistivity features: similar resistivity for the reservoir and adjacent water zone within the same sedimentary period, and similar resistivity for the reservoir and adjacent mudstone. Here, we studied the microscopic mechanism and factors influencing low resistivity oil zones, by using thin section, scanning electron microscopy, Xray diffraction and clay mineral analysis techniques, combining with capillary pressure data and knowledges of rock size, pore structure, clay mineral content type and thickness of oil layer. The results showed that high saturation of bound water due to rocks fine grain size, high clay content and development of micro pores, was the main microscopic cause for the low resistivity oil zone in the basin; while thin reservoir and invasion of saltwater mud filtrate were largely macro impact factors. According to their dominant characteristics and cause, the low resistivity oil zones can be effectively identified by cross plotting resistivity vs. natural gamma, resistivity vs. relative value of spontaneous potential, GEOFI vs. TG, and using fluid density calculated by RFT pressure data. Furthermore, our research revealed that the lowresistivity oil zones are laterally distributed in the oil field of Dinga fault terrace, Fana low uplift and Yogou slope, and vertically distributed in the fluvial sediments of E1 and E2 sand in the Sokor1 Formation and delta, and lacustrine sediments of the Cretaceous. Related Articles | Metrics

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Geochemical characteristics and genesis of natural gas in the Bongor Basin. CHENG Dingsheng,DOU Lirong,WANG Jingchun,LI Wei,WEN Zhigang 2018, 25(2): 112-120.  DOI: 10.13745/j.esf.2018.02.012 Abstract ( 168 )   PDF (1425KB) ( 183 )   The Bongor Basin, located in the northwest part of the Central African Shearing Zone in Chad, is one of the most proliferous basins of the MesozoicCenozoic era. In this study, chemical composition and carbon/hydrogen isotopic analysis were performed on natural gas samples by routine geochemical methods to investigate the origin and maturity difference of natural gas in different structural units of the Bongor Basin. The results showed that the chemical compositions of the natural gas analyzed are mainly hydrocarbon and nonhydrocarbon gases with trace amount of rare gases. Hydrocarbon gases belong to organic gas, such as biogenetic and thermal cracking gases, while inorganic gases included N2, CO2 and H2, etc. Gasgas and gassource correlations were obtained to determine the organic source of the natural gas. It showed that the minor composition dissimilarity in these gases are attributable to facies variations in different structural units, namely the M, P and possibly K group dark clay shale facies; whereas gases generated by organic source rocks in different structural units exhibit large difference in maturity, simply because they were generated in different thermal evolution stages. Related Articles | Metrics

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Geochemical characteristics of crude oil and their implications for hydrocarbon accumulation in the Fula Subbasin, Muglad Basin, Sudan. LI Wei,DOU Lirong,ZHANG Guangya,CHENG Dingsheng,WEN Zhigang,LIU Aixiang，KE Weili 2018, 25(2): 121-129.  DOI: 10.13745/j.esf.2018.02.013 Abstract ( 133 )   PDF (2759KB) ( 227 )   The Fula Subbasin, located in the northeast Muglad Basin, has an area of 5000 km2. The FN oilfield is the biggest oilfield discovered in this subbasin with 100 million tons of oil reserves. Its main hydrocarbon reservoirs included the Aradeiba, Bentiu and Abu Gabra Formations of the Cretaceous. Based on a systematic geochemical analysis of crude oil, the reservoirforming stages of the FN oilfield are discussed. The geochemical characteristics of the crude oil from the three formations are very similar, which indicates that all hydrocarbons originated from the same source rocks, i.e., the Abu Gabra source rocks. The crude oil has both normal oil characteristics, such as full series distribution of nalkanes, hopanes and steranes, as well as biodegraded oil characteristics, such as raised chromatogram baseline and appearance of sharp UCM and 25norhopanes. It revealed, therefore, that the FN oilfield had undergone two oil infilling stages, i.e., the earlier infilling oils had relatively low maturity and was strongly biodegraded, and the later ones had relatively high maturity. The tectonic, burial and thermal evolution histories were used to analyze the hydrocarbon generation and charging and accumulation stages of the FN oilfield. It was shown that the Abu Gabra source rocks experienced two periods of hydrocarbon generation. The first one took place during the Late Cretaceous, with the maturity (Ro) of Abu Gabra source rocks reaching 0.6% within the hydrocarbon generation window; then the crude oil suffered relatively serious biodegradation due to tectonic reversion and uplift by the end of the Cretaceous. The second one happened during the late Paleogene as the Abu Gabra source rocks entered the hydrocarbon generation peak stage. This was the most important process for hydrocarbon generation and oilfield formation in the region. The structural traps related to the second period are the main targets for hydrocarbon exploration in the Fula Subbasin. Related Articles | Metrics

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Characteristics of petroleum genesis and evolution analysis on hydrocarbon generation in the Ronier Oilfield, Bongor Basin, Chad. XU Feng,LI Xianbin,HU Jian,GAN Lin,LIU Mengfei,LI Ming 2018, 25(2): 130-140.  DOI: 10.13745/j.esf.2018.02.014 Abstract ( 184 )   PDF (5620KB) ( 195 )   The Bongor Basin is a MesoCenozoic intracontinental passive rift basin developed under the influence of the Central African shear zone. It is dominated by the Lower Cretaceous sourcereservoircap assemblage. The basin is underexplored overall, and knowledges of its oil/gas resources and exploration prospects are limited. Taking the Ronier Oilfield as an example, through a systematic organic geochemical analysis and combining with regional geological data, we concluded that the Lower Cretaceous source rocks belong to the Lower Cretaceous dark mudstone with TOC content over 3.5%. The characteristics of saturated hydrocarbon gas chromatography, sterane and terpane showed that the source rocks developed in a brackish waterfreshwater and weakly oxidizedweakly reduced lacustrine sedimentary environment, and the organic matter had typical continental lacustrine characteristics in its development and distribution. The organic matter is type Ⅰ, Ⅱ1, Ⅱ2. It has a thick accumulation of hydrocarbon source rock with huge hydrocarbongenerating potential, and is the main source of oil and gas for the Ronier Oilfield. History of the source rock evolution suggests that the central sag differed from the western sag and reached much higher evolutionary maturity in both source rock and oil products. The main oil and gas resources in the Ronier Oilfield on the northern slope came from the central sag, as evidenced in both geological and geochemical aspects. The inverted tectonic zone, along with the trajectory of hydrocarbon evolution and favorable lithofacies zone, controlled the process of oil and gas enrichment. Related Articles | Metrics

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Research progress and core issues in tight reservoir exploration. ZHU Xiaomin,PAN Rong,ZHU Shifa,WEI Wei,YE Lei 2018, 25(2): 141-146.  DOI: 10.13745/j.esf.2018.02.015 Abstract ( 391 )   PDF (929KB) ( 566 )   There are abundance of conventional and unconventional hydrocarbon resources in the Earth, in which the tight oil and gas are considered to be the major types of unconventional hydrocarbon resources in replacing the conventional oil and gas in the next few decades. The characteristics of tight reservoir include high resource quantity, wide distribution area, strong diagenesis, low unit production and high heterogeneity. Therefore, “sweet spots” reservoirs with favorable physical properties are good exploration targets for achieving higher oil and gas production in tight reservoirs. The origin of “sweet spots” reservoirs could be two types: the abnormally high porosity zone or fracture development zone. Recently, much attention has been paid to the tight reservoir researches on reservoir classification, geological characterization, forming mechanism and “sweet spots” prediction, among where genetic mechanism and precise prediction of “sweet spots” have become the focus of scientific investigation. For this reason, reservoir microscopic pore throat structural representation and fracture identification and characterization are the present core issues and future main directions in the tight reservoir exploration and development. Related Articles | Metrics

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Petrologic and diagenetic characteristics of the lacustrine volcanicsiliciclastic tight reservoir: an example from the first member of the Tenggeer Formation in the Lower Cretaceous, Anan Sag, WEI Wei,ZHU Xiaomin,ZHU Shifa,SUN Shuyang,WU Jianping,WANG Mingwei,L Sihan 2018, 25(2): 147-158.  DOI: 10.13745/j.esf.yx.2017-6-5 Abstract ( 101 )   PDF (2814KB) ( 239 )   The Lower Cretaceous source rocks in most Erlian Basin sags developed a set of tight rocks including tuff, tuffite, tuffaceous mud and sandstones. Here, in the first member of the Tenggeer Formation in the Lower Cretaceous of the Anan Sag, and taking tuffaceous rocks as research objects, the petrologic and diagenetic characteristics of mixed rocks were studied using core observation, microscopy, scanning electron microscopy (SEM), cathode luminescence (CL) and Xray diffraction techniques. The results show that the thin lithic tuff, developed in the semideep to deep lakes, exhibits strong devitrification with localized occurrences of microcrystalline feldspar, quartz and dolomite. In comparison, the tuffite and tuffaceous mudstone, developed in the sand sheet of fandelta front and semideep to deep lakes, are mainly in the A2 mesogenetic stage, characterized by dolomite cementation with strong devitrification in authigenic clay minerals. Its reservoir space is relatively small and composed primarily of developing micropores and structural microcracks. Lastly, the tuffaceous sandstone, developed in the sand sheet of fandelta front, is mainly in the A1 mesogenetic stage and characterized by strong compaction and calcite cementation, followed by dolomite and siliceous cementation. Its reservoir space, containing less primary developing pores, is mainly made of dissolution pores formed by dissolved feldspar fragments and carbonate cements. The semideep to deep lacustrine tuff and fandelta front tuffaceous sandstone, therefore, are the favorable reservoir zones in the Anan Sag. Related Articles | Metrics

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Quantitative research on porosity evolution of deep tight reservoir in the Bashijiqike Formation in Kelasu structure zone, Kuqa Depression.#br# PAN Rong,ZHU Xiaomin,TAN Mingxuan,ZHANG Jianfeng，LI Yong,DI Hongli 2018, 25(2): 159-169.  DOI: 10.13745/j.esf.yx.2017-6-6 Abstract ( 137 )   PDF (4407KB) ( 308 )   1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum(Beijing), Beijing 102249, China 2. College of Geosciences, China University of Petroleum(Beijing), Beijing 102249, China 3. Institute of Exploration and Development, Tarim Oil Field, PetroChina, Korla 841000, China Related Articles | Metrics

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Diagenetic differences of tight sandstone of the Lower Jurassic Ahe Formation in the Yiqikelike Area of the Kuqa Depression, Tarim Basin. ZHANG Liqiang，YAN Yiming，LUO Xiaorong，WANG Zhenbiao，ZHANG Haizu 2018, 25(2): 170-178.  DOI: 10.13745/j.esf.yx.2017-6-1 Abstract ( 352 )   PDF (2999KB) ( 275 )    The deep tight sandstone in the Yiqikelike Area of the Kuqa Depression presents both great exploration potential and challenge due to its complex reservoir densification & accumulation process, and strong heterogeneity. Here, the differential diagenetic evolution processes of this tight sandstone were studied through core observation, thin section identification, porosity and permeability measurements, and analyzes of fluid inclusions, fluorescence, carbon and oxygen isotopes, sandstone types and reservoir quality characteristics. Applying the modified Dickinson's triangle principle, the sandstone of the Ahe Formation are divided into four types based on the contents of calcium cement, ductile debris and rigid particles: type C, the calcareous cementitious sandstone; type A, the lithic sandstone with high rigid particle content; type D, the lithic sandstone containing medium level ductile debris (10%25%); and type B, the ductilerich lithic sandstone with more than 25% ductile debris. Types A and D are effective tight gas reservoirs where the blue fluorescence, representing the second oil (gas) filling, is strong as a result of high emposieu and structural fractures. The feldspar and ductile debris contents are somewhat high in type D, in which the porosity is high but permeability is low due to late dissolution. Most type D oil layer samples have yellow and blue fluorescence, and the average porosity of type D is higher at early oil filling but slightly declines after second oil and gas filling start. In comparison, types B and C have stronger compaction and cementation but poor physical properties; only a few samples emit yellow but no blue fluorescence; and they are all effective reservoirs. Notably, there are differences in diagenesis and accumulation between types A and D: the early oil filling of type D slowed the occurrence of structural compaction and carbonate cementation, conducive to the late acid dissolution and oil (gas) filling; whereas the structural fractures affecting the performance of type A was developed by the second rather than the early oil (gas) fillings. Related Articles | Metrics

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Diagenesis and pore evolution of tight sandstone reservoir: a case study from the Upper Triassic reservoir of the southwest Sichuan Basin, China. SHI Zhensheng,LI Xizhe,DONG Dazhong,QIU Zhen,LU Bin,LIANG Pingping 2018, 25(2): 179-190.  DOI: 10.13745/j.esf.yx.2017-6-4 Abstract ( 118 )   PDF (2990KB) ( 287 )   Diagenesis of tight sandstone reservoir dominates pore evolution, and further controls the distribution of relatively highquality reservoirs. The Upper Triassic sandstone reservoir in the southwest Sichuan Basin, with overall porosity less than 6% and permeability less than 0.1×10-3 μm2, is a typical tight sandstone reservoir. In this study, a comprehensive analysis of cores, cast slices and fluid inclusion shows that the tight sandstone reservoirdominated by intragranular dissolved pore, intergranular dissolved pore and moldic poreexperienced compaction, cementation, dissolution and metasomatism. Paragenetic relationship and fluid inclusion homogenization temperatures reveal the succession of the Upper Triassic sandstone: compaction→first phase (sequentially) fracture, illite cementation, quartz overgrowth and calcite cementation→second phase quartz overgrowth and siliceous infill→dissolution of feldspar and debris→chlorite cementation or illite cementation→second phase calcite cementation→dissolution of feldspar and debris→second phase fracture→third phase quartz overgrowth and siliceous infill followed by carbonate cementation→carbonate replacement of feldspar and quartz→third phase fracture→late stage calcite and quartz cementation. The measured vitrinite reflectivity (Ro) value and homogenization temperature of fluid inclusions indicate the sandstone is currently in the A2 to B diagenetic stage, when compaction, the main factor of reservoir densification, causes roughly 27.5% porosity loss; while siliceous and carbonate cementation are responsible for only 5.3% porosity loss. Related Articles | Metrics

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Evaluation and application of porous structure characterization technologies in unconventional tight reservoirs. WU Songtao,ZHU Rukai,LI Xun,JIN Xu,YANG Zhi,MAO Zhiguo 2018, 25(2): 191-203.  DOI: 10.13745/j.esf.yx.2017-6-2 Abstract ( 256 )   PDF (3917KB) ( 401 )   Unconventional tight reservoirs pose a great challenge to researchers because of their complex porous structures and great heterogeneities. In this study, key issues of CT scanning, gas adsorption and MICP application in unconventional tight reservoirs were investigated. Optimization of these techniques led to higher resolution and multidimension representation in porous structure characterization by using a combination of static description and dynamic modeling. Six typical reservoir rocks, including conventional sandstones, tight sandstones, peperites, tuffs, shelly limestones and marine shales were analyzed systematically to understand the structural differences and define appropriate characterization methods for different porous structures. Moreover, the effects of pore size on physical property, resource economic and development technologies are discussed. Future research should be focused on integrating porous structure characterization performed at different structural scales, and evaluating the relationship between porous structure and mobile fluids & reservoir stimulation. Based on our multifield, multidiscipline and multiview study, the reservoir effectiveness can be assessed to provide technological support for the largescale “sweet spot” predictions. Related Articles | Metrics

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High pressure adsorption characteristics of shale gas and their impact on the law of reserveproduction. SHEN Rui,GUO Hekun,HU Zhiming,XIONG Wei,ZUO Luo 2018, 25(2): 204-209.  DOI: 10.13745/j.esf.yx.2017-6-7 Abstract ( 165 )   PDF (1285KB) ( 251 )   Presently, the testing temperature and pressure of the shale adsorption isotherm cannot usually reach that of the reservoir. To solve this problem, the hightemperature/highpressure test methods were designed, and the characteristics of shale adsorption isotherm under reservoir temperature/pressure were analyzed. In addition, variations in free and adsorbed gas with changing reservoir pressure were calculated in accordance with the actual shale core model; the effect of adsorbed gas on the characteristics of gas production was also studied by examining the pressure controlled production of helium & methane using whole diameter shale core. Results show that gas adsorption by visual assessment increases initially with increasing pressure and decreases after peak. It is found that under low pressure conditions, gas adsorption calculated by Langmuir extrapolation is close to that of high pressure experiments; but under high pressure conditions, total gas content calculated by low pressure Langmuir theory is overvalued by about 9.2%. Moreover, below the critical desorption pressure, desorbed gas causes gas production to increase per unit differential pressure; while above the critical desorption pressure, adsorbed gas hardly affects gas production. During the early development, reservoir pressure ranges slightly below the critical desorption pressure so that the adsorbed gas contributes very little to gas production. Therefore making full use of free gas is key to high efficiency reservoir development. Related Articles | Metrics

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Prediction of sandconglomerate reservoirs via seismic facies controlled inversion in the Lower Es-3 of the northern steep slope of the Chexi Sag. ZHANG Sheng,HUANG Handong,WANG Xingxing,ZHANG Jinwei,LI Jinkui 2018, 25(2): 210-220.  DOI: 10.13745/j.esf.yx.2017-6-3 Abstract ( 253 )   PDF (2008KB) ( 316 )   As hydrocarbon exploration focus in the Shengli Oilfield gradually shifted from structural reservoir to hidden reservoir, the glutenite reservoirs have become important exploration targets. Increasing in prospecting efforts and application of new technologies also make the steep slope of the faulted lake basin a target of special interest. Seismic facies controlled inversion has the full advantage that the macroscopic geological bodies and lithology are controlled by seismic facies. It divides the target strata into sedimentary units of different sedimentary characteristics; it constrains the initial value range of each sampling point according to the low frequency background, and controls the window size at the same time, so that the inversion results should have definitive geological significance. The glutenite reservoir of the steep slope belt in the Chexi Sag is a multigenesis deposit with rapid changing lithofacies, making its description difficult. Using seismic facies controlled inversion, where detailed division of sedimentary facies and reservoir physical property analysis were made under the constrains of well log data, the “sweet spot” distribution of the Paleogenes third member of the lower Es3 steep slope reservoir was predicted. As a result, four “sweet spot areas” were identified in the middle and distal end of a braided channel, one of the most favorable reservoir facies. Prediction results show that glutenite identification reached the sand group level with some locations up to the level of small layers in the vertical direction. In the lateral direction, different genetic types of sand body could be distinguished clearly, and the nearshore subaqueous fan and slump fan glutenite reservoir were also better identified, which significantly improved the accuracy of sweet spot prediction for the steep slope of the faulted lake basin glutenite reservoir. Related Articles | Metrics

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Sequence stratigraphy of the carbonate platform of the Upper Carboniferous Chuanshan Formation and relative sea level change control in the Lower Yangtze region: an example from outcrops in Jurong City, Jiangsu Province. ZHANG Zhen,CHENG Rihui,XU Zhongjie,LI Shuanglin 2018, 25(2): 232-245.  DOI: 10.13745/j.esf.yx.2017-5-3 Abstract ( 179 )   PDF (2327KB) ( 247 )   The carbonate rocks of the Upper Carboniferous Chuanshan Formation in the Jurong area consist mainly of cryptite, fine crystalline limestone, bioclastic cryptite, micrite bioclastic limestone, clastizoic cryptite and clastizoic oncolite limestone, in which several platform carbonate depositional facies, including tidal flat, open platform, platformal trough, shoalreef and organic reef, can be identified. The carbonate succession of the Chuanshan Formation is composed of three thirdorder deposition sequences: TypeⅠ sequence S1, and TypeⅡ sequences S2 and S3. Based on lithology, sedimentary facies, paleontology and geochemistry characteristics, the relative sealevel change curve was established, indicating there were three relative sealevel changes during the evolution of the Chuanshan Formation. These relative sea level changes controlled the development of sequences. S1 comprises the lowstand systems tract (LST) of conglomerate of shore limestone and crystalline limestone of tidal flat, transgressive systems tract (TST) of clastizoic cryptite and cryptite developed in platformal trough, and highstand systems tract (HST) of clastizoic oncolite limestone developed in shoal. S2 is made of TST of micrite bioclastic limestone in reef, bioclastic cryptite, clastizoic cryptite and cryptite in open platform, and HST of clastizoic cryptite in open platform. S3 composes TST of cryptite in open platform, HST of clastizoic oncolite limestone in shoal, and crystalline and clumpy limestones in tidal flat. In general, the carbonate platform of the Chuanshan Formation in the Lower Yangtze region developed in a shallow environment with relatively stable tectonic background; there also existed shoal and trough in the platform. Limestones in the open platform and platformal trough are highquality source rocks while shoal reef limestones are good reservoir rocks. Related Articles | Metrics

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Sequence stratigraphy of the carbonate platform of the Upper Carboniferous Chuanshan Formation and relative sea level change control in the Lower Yangtze region: an example from outcrops in Jurong City, Jiangsu Province. WANG Jianmin,ZHANG San 2018, 25(2): 246-253.  DOI: 10.13745/j.esf.yx.2017-12-27 Abstract ( 303 )   PDF (2912KB) ( 280 )   Related Articles | Metrics

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Geochemistry characteristics and Re-Os isotopic dating of Jurassic oil sands in the northwestern margin of the Junggar Basin. HUANG Shaohua,QIN Mingkuan,David SELBY,LIU Yinshan,XU Qiang,HE Zhongbo,LIU Zhangyue,LIU Junjie 2018, 25(2): 254-266.  DOI: 10.13745/j.esf.2018.02.026 Abstract ( 243 )   PDF (3761KB) ( 300 )   The study of hydrocarbon charging history in shallow basin margin is the key to the investigation of the relationship between hydrocarbon fluid and sandstone type uranium deposit. In this paper, the geochemistry, origin and charging history of hydrocarbon from the northwestern margin of the Junggar Basin were investigated by GC-MS, carbon isotope analysis and Re-Os dating. The results showed that the oil sand samples experienced at least 5 levels of biodegradation, which rendered it impossible to characterize hydrocarbon source correctly by sterane parameters. However, the terpane distribution spectra were relatively stable owing to terpanes strong resistance to degradation. In the terpane spectrum, the relative abundance of C20, C21 and C23 showed an increasing trend, and C23/H and G/H ratios were 0.751.34 and 0.270.65, respectively, in keeping with a lacustrine aquatic organism source formed in a brackish and anoxic environment. The consistent light carbon isotope composition (<-28‰) among different samples further indicated that hydrocarbons mainly sourced from the Permian Fengcheng Formation in the Mahu Depression, rather than from the Jiamuhe and Wuerhe Formations. Here, the oil sand age is dated by Re-Os isotope (155±51 Ma) for the first time. The age suggested that the northwest Jurassic strata experienced a largescale hydrocarbon charging during the Late JurassicEarly Cretaceous period. This event covered and protected the paleointerlayer oxidation zone as well as paleouranium ore bodies formed in the middlelate Jurassic era, but it also increased the difficulty in contemporary uranium prospecting. Related Articles | Metrics

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Overview of fine geology. LIU Gang,LIANG Shuneng,YAN Yunpeng,YAN Bokun,WANG Zhe 2018, 25(2): 267-279.  DOI: 10.13745/j.esf.yx.2017-6-10 Abstract ( 236 )   PDF (2583KB) ( 321 )   The necessity of fine geology study is put forward in this paper through requirements analysis of economic development, resource exploration, environmental protection and development of geosciences. The concept and technical system of fine geology are illustrated. Fine geology involves precise and detailed study of geological phenomena based on advanced technology and high sensitivity of the measuring instrument; results obtained from such study can be used for the advancement of scientific research, exploration and economic construction. The technical system of fine geology include remote sensing, geophysical prospecting, geochemical exploration, drilling, isotopic dating, space positioning techniques and computer technology, etc, and an integrated full range airsurfacesubsurface 3dimensional survey usually completes the fine geology study. Taking remote sensing as an example, the feasibility of fine geology is described through analyses of the development and application of geophysics, remote sensing, isotope and geochemistry in fine geology studies at home and abroad. The prospects of applying fine geology in mineral resource exploration, geological research, environmental protection, national defense and geological disaster investigation are discussed in this paper. Related Articles | Metrics

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The Cihai FeCo deposit, Xinjiang: a superimposed hydrothermal mineralization of two mantlederived magmatic series. WANG Yuwang,SHI Yu,TANG Pingzhi,LI Dedong,WANG Jingbin,XIE Hongjing 2018, 25(2): 280-298.  DOI: 10.13745/j.esf.yx.2017-12-24 Abstract ( 160 )   PDF (3275KB) ( 284 )    The Cihai deposit in the Eastern Xinjiang is best known for its special Fe-Co metal assemblage. In this paper, we studied the chronology and geochemistry of maficultramafic rocks, geological characteristics of the symbiotic Co mineral and chemical composition of pyrite at the Cihai ore district, and concluded that the Cihai deposit was a superimposed magmatic hydrothermal deposit of two mantlederived magmatic series, i.e. Cu-Ni and Ti-Fe series. The Cu-Ni series produces plutonic rocks including olivine gabbro, troctolite, gabbro and hornblende gabbro with a zircon U-Pb age of 279.1±1.4 Ma for the gabbros. The rocks of Cu-Ni series have high m/f ratio (mostly higher than 1.5), contain high levels of Co, Ni and Cu, and host Cu-NiCo sulfide mineralized bodies, suggesting their proximity to Co source of the deposit. The Ti-Fe series produces extrusive and hypabyssal rocks including basalt and diabase, and yields a zircon U-Pb age of 276.2±2.2 Ma for basalt. The rocks of TiFe series have low m/f ratio (mostly lower than 1.5), contain low percentage of Ni and Cu but rich in TiO2, and host magnetite ore bodies, all indicating the series is close to Fe source of the deposit. The geological characteristics of orebody and ore suggest that the formation of the main magnetiteCo orebodies were controlled by hydrothermal mineralization, and the deposit may belong to a hydrothermal deposit related basicultrabasic magma. The occurrence and chemical composition of different pyrite types further reveal that the Co mineralization happened after magnetite mineralization, and that the metasomatism of pyrite by volcanicsubvolcanic hydrothermal mineralization formed the Cobearing pyrite and other cobalt minerals. In other words, Fe and Co deposits display complex mineralization from two sources and in two stagesthey are essentially the superposition of CuNi and Ti-Fe magma series. Related Articles | Metrics

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The study on temporal and spatial distribution law and cause of debris flow disaster in China in recent years. ZHANG Nan,FANG Zhiwei,HAN Xiao,CHEN Chunli,QI Xiaobo 2018, 25(2): 299-308.  DOI: 10.13745/j.esf.yx.2017-5-10 Abstract ( 443 )   PDF (5791KB) ( 409 )    China is one of the countries in the world suffering from serious debris flow disasters. The debris flow disaster not only happens frequently, but also causes heavy casualty and property loss. The disaster, meanwhile, is very difficult to prevent and monitor. This study collected data of all debris flow disasters occurred in China from 2005 to 2015, and analyzed the times, places, scales, losses and causes of these debris flow disasters. The following characteristics can be concluded: there was a clear downward trend in debris flow occurrence in recent years; the debris flow disasters were mainly distributed in the western and southeastern coastal provinces; and large or megascale debris flow disasters suffered heavy losses. Through the analysis of specific cases, the main reasons for the serious damage caused by debris flow were found to be the following: inadequate monitoring and early warning systems; improper placement of urban construction sites in the mountainous areas; low standards for disaster prevention and control; poor earthquake disaster management; and weak public awareness of disaster prevention. At the same time, China is actively responding to the threat of debris flow by enhancing the prevention and control standards in earthquake prone regions, inventing a number of new flow blocking technologies, developing monitoring and warning systems with realtime visualization capabilities for better efficiency, increasing the public awareness of disaster prevention, and vigorously developing the mass monitor networks and disaster prevention and warning systems. There is also a paradigm change in the debris flow disaster management strategy involving integrating disaster prevention and control into urbanization planning. Related Articles | Metrics

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Advances in paleowind strength reconstruction techniques: use of transporting capacity analysis. WANG Junhui,JIANG Zaixing,XIAN Benzhong,ZHANG Chunming,LIU Lian 2018, 25(2): 309.  DOI: 10.13745/j.esf.yx.2017-12-26 Abstract ( 323 )   PDF (1980KB) ( 276 )   The paleowind strength is an important component in the study of paleoatmospheric circulation, and is of great significance in the reconstruction of paleoclimate conditions. However, paleowind strength is less well studied in paleoclimatology, mainly because ancient wind has left little trace of its passing, making it difficult to reconstruct paleowind strength from geological records. Thankfully, transport capacities of fluid medium may be derived from sediment properties, which could provide clues for paleowind strength reconstruction. Qualitative restoration of paleowind strength is relatively easy, as particle size and composition of windblown sediments, and thickness of tempestites, directly reflect the wind power during the periods of sediment deposition. By comparison, quantitative restoration of paleowind strength is relatively difficult with scant research publications. In this paper, two independent methods, uncovered from our literature research, are introduced to illustrate how quantitative restoration of paleowind strength can be performed—a technique much needed in paleoclimate studies. Method 1: For eoliandune constituting sediments, the modes of transport including creep, saltation and suspension, are functions of grain properties (such as grain size and density) and wind shear stress. Based on this principle, grain size analysis of eoliandune sediments could be used to decipher paleowind strength. Method 2: Wind blowing over water can effectively transfer energy to form waves, and such process is governed by quantitative windwave relationships; and gravels deposited along shoreline by waves can record the critical wave power. Therefore, particlesize distribution analysis of gravel beach deposits could be used to reconstruct paleowave and subsequently paleowind conditions. Although both methods are limited by application conditions, we believe they are useful new tools in paleoclimate reconstruction and can be optimized. Related Articles | Metrics

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2018, 25(2): 319-319.  Abstract ( 87 )   PDF (195KB) ( 180 )     References | Related Articles | Metrics

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On the dynamic mechanics of global lithosphere plate tectonics. WAN Tianfeng 2018, 25(2): 320-335.  DOI: 10.13745/j.esf.sf.2018.1.1 Abstract ( 385 )   PDF (3408KB) ( 843 )    The dynamic mechanics of global plate tectonics, a hot topic in Earth Science, is a difficult and unsolved problem. This paper reviewed the main plate tectonics hypotheses of the last hundred years as well as critical new knowledges of the past forty years. Drawing inspiration from Rampino & Stothers hypothesis of 1984, which relates periodic comet impacts to global change, the author proposed here a new plate tectonic dynamics hypothesis. As record shows, during the MesozoicCenozoic of the last 200 Ma, the solar system penetrated the Milky Way galactic plane every 33 Ma. The events greatly altered the gravitational field in the solar system, causing destabilization of some asteroids that could then impact the Earth. Based on the seven different orientationmigration models for the global plate tectonics of the last 200 Ma, the author proposed the following hypothesis: giant meteorites impact the lithosphere surface with different impact angles at different locations; such impacts could then sometimes induce mantle diapirs, causing plates to migrate radially or in one orientation depending on the impact angle. Thus, giant meteorite impacts in the periods of 200, 170, 100, 65 and 0.78 Ma were all near vertical impacts to Earths surface, as they induced mantle diapirs and led to radiating extension and migrations of the lithosphere plates. In contrast, the 138 Ma giant meteorite impact event could be an oblique impact to the Indian Plate; and in the period of 35 Ma,the microtektites event exhibited a very low impact angle to the Earths surface. According to this new hypothesis, meteorite impact on Earth is a result of interplanetary gravity change within the solar system. Related Articles | Metrics