2004 Vol. 15, No. 1
This paper is a brief review of the tectonic frame and crustal evolution of China and their control over the oil basins. China is subdivided into three regions by the Hercynian Ertix Almantai (EACZ) and Hegenshan (HGCZ) convergent zones in the north, and the Indosinian Muztagh Maqen (MMCZ) and the Fengxiang Shucheng (FSCZ) convergent zones in the south. The northern region represents the southern marginal tract of the Siberian platform. The middle region comprises the Sino Korea (SKP), Tarim (TAP) platforms and surrounding Paleozoic orogenic belts. The southern region includes the Yangtze platform (YZP), the Cathaysia (CTA) paleocontinent and the Caledonides between them in the eastern part, and the Qinghai~Tibet plateau composed of the Gondwana~affiliated massifs and Meso~ and Cenozoic orogenic belts in the western part. The tectonic evolutions of China are described in three stages: Jinningian and pre~Jinningian, Caledonian to Indosinian, and post~Indosinian. Profound changes occurred at the end of Jinningian (ca. 830 Ma) and the Indosinian (ca. 210 Ma) tectonic epochs, which had exerted important influence on the formation of different types of basins. The oil basins distribute in four belts in China, the large superimposed basins ranging from Paleozoic to Cenozoic (Tarim and Junggar) in the western belt, the large superimposed basins ranging from Paleozoic to Mesozoic (Ordos and Sichuan) in the central belt, the extensional rift basins including the Cretaceous rift basins (Songliao) and the Cenozoic basin (Bohaiwan) in the eastern belt, and the Cenozoic marginal basins in the easternmost belt in offshore region. The tectonic control over the oil basins consists mainly in three aspects: the nature of the basin basement, the coupling processes of basin and orogen due to the plates interaction, and the mantle dynamics, notably the mantle upwelling resulting in crustal and lithospheric thinning beneath the oil basins.
As located in the junction of three tectonic plates (the Eurasian plate, the Indian plate and the west Pacific plate), the China continent shows complex regional metallogenic features due to tectonic evolution of “micro~plates, polycycle tectonic movements”. Well developed superimposed metallogenic systems have constituted one of the regional metallogenic features in China. Through the study on superimposed metallogenic system of the middle and lower reaches of the Yangtze River and of the Yuebei basin (northern Guangdong Province), the authors put forward some basic combination pattern of sedimentary magmatic superimposed metallogenic system and summarize its forming conditions (controlling factors).
The analyses of different sulfur forms, the trace elements in pyrites using electron microprobe and the trace elements in coal using INAA (instrumental neutron activation analysis) of the Late Paleozoic coals from the Taozao coalfield in Shandong Province, China, conclude that most sulfur (>75 %) in high~sulfur coal of Taiyuan Formation occurred as pyrite, in which many hazardous elements coexisted and their concentrations varied with their geological origin. The concentrations of hazardous elements in high~sulfur coals from Taiyuan Formation, composing mainly of Cu, As, U, Pb, Mo and Co, are much higher than those in the low~sulfur coals from Shanxi Formation and Shihezi Formation, because the influence of seawater during and after coal accumulation in Taiyuan Formation is stronger than those in Shanxi and Shihezi formations. Moreover, the element As is related to Fe, and both elements exist mainly in the form of pyrite. The element U is richer in the coal influenced by seawater. In addition, the coal affected by the magmatism contains more U, too. When high~sulfur coals are processed with heavy media washing to remove sulfur and minerals, the majority of hazardous elements will also be removed from the coals.
An Upper Cretaceous black~gray~red bed sequence was deposited in the Tethys~Himalayan Sea where abundant foraminifera, especially planktons, were yielded. In the shallow shelf to the upper slope on the north margin of Indian plate was recorded an extinction~recovery~radiation cycle of foraminiferal fauna highly sensitive to paleoceanographical changes. The black unit, consisting of the Late Cenomanian earliest Turonian beds, displays a major extinction, with keeled planktonic and many benthic species as the principal victims at the end of the Cenomanian when existed only low diversity, surface water~dwelling foraminifera. The gray unit spans a long~term recovery interval from the Turonian to the early Santonian with keeled planktonic foraminifera returning stepwise to the water column. The planktonic biota in the red unit, extremely abundant, indicate a biotic radiation during the Late Santonian and the Early Campanian, implying that the high oxygen levels had returned to all the oceanic depth levels, and that the water stratification disappeared, followed by the radiation of all depth~dwellers. The variation on foraminiferal faunas from the whole sequence refers to the extreme warm climate that appeared in the Middle Cretaceous and to the declined temperature toward the late epoch. Substantial deposits for this warming and cooling zones represent the black shales in the Middle Cretaceous and the red beds in the later period of the southern Tibet. The change in the foraminiferal composition corresponded to the formation of dysaerobic facies and to the development of high~oxidized circumstances.
In order to determine the age of the sedimentary hiatus and its geological significance, a study of the calcareous nannofossil biostratigraphy was carried out. Detailed stratigraphical data of the Late Oligocene Early Miocene diagnostic species thus obtained. The nannofossil zonation of this interval was subdivided and the Oligocene Miocene boundary was further determined. Several last Late Oligocene events were recognized, indicating a long~term sedimentary hiatus in the uppermost Upper Oligocene. The time span of the hiatus was estimated for about 2.2 Ma, at least from 23.9 to 26.1 Ma. The lithological and geophysical data from Site 1148 indicate some abrupt sedimentary changes that occurred below and above the hiatus. This hiatus at Site 1148 was probably related to the tectonic change, a major ridge jump during the seafloor spreading in the Late Oligocene South China Sea
The distribution and the concentrations of various chemical elements in street sediments were investigated along a rural urban boundary in Beijing, China. The statistical factor analysis of the data concerned identifies two anthropogenic sources responsible for the contamination of Beijing air. The first source is a steel factory in the western part of Beijing. From this source, Mn, Fe and Ti were emitted into the atmosphere through chimneys and by wind from coal heaps used as the primary energy source for the factory. The second source is a combination of traffic, domestic heating and some small factories in the center of Beijing urban area discharging Cu, Pb, Zn and Sn. Grain size analyses show that most of the metals in the road dust have higher concentrations in the small grain size fraction <0.125 mm, which is the severest case because these small particles with larger specific surface area and high heavy metal contents fly up easily and float in the air for a long time. Besides the anthropogenic contamination, such elements as Y, Zr, Nb, Ce and Rb are derived mainly both from natural soils and from the deserts. This is supported by mineral phase analysis, which shows a clear imprint of materials in road dusts coming from the west China deserts. Our results clearly show that the chemical compositions of the urban road dusts can be used to identify distinctive sources responsible for the contamination mentioned above. The study shows that the chemistry of road dusts is an important monitor to assess the contamination in the urban environment
This paper is mainly concentrated on the geochemical characteristics and origin of gas of Kekeya field in the Tarim basin, NW China. This study shows that Permian mudstone is the main source rock of oil and gas. Based on the carbon isotopes of C1~C4, the carbon isotope of gas in Kekeya field is a little heavier than that in the typical marine derived gas. The relationship between carbon isotopes of methane and ethane is coincident with Faber equation of gas derived from organic matter Ⅰ/Ⅱ. The majority of gas maturity is estimated, based on the formula, at 1.8 % ~ 2.2 % besides K2 and K18 wells. In addition, the gas derived from 0.9 % ~ 1.2 % Ro source rocks may also be mixture. 40Ar/36Ar and 3He/4He ratios from the gas samples also support the mixing process. Moreover, the gas in this region is mainly generated from more mature source rocks although the low mature gas exists.
Thermal groundwater resources were found to have occurred in deepseated bedrock aquifers in the northeastern North China plain near Tianjin, China. Meso~to Neo~Proterozoic and Paleozoic carbonate rocks on the Cangxian uplift are capable of yielding 960~4 200 m3/d of 60 to 96 ℃ water from the wells ranging in depth between 1 000 and 4 000 m. Conductive heat flow of 0.063 to 0.144 2 W/m2 from the deep crust is responsible for this anomalous geothermal field. The water in the Ordovician aquifer is characterized by relatively high TDS, high concentrations of SO4 and SO4?ClNa?Ca type, but the waters from the Meso~ to Neo~Proterozoic and Cambrian aquifers, by relatively low TDS, low concentrations of SO4 and predominantly Cl?SO4Na type. It is noted that when the temperature of the waters increases at a rate of 10 ℃ in the range of 30~100 ℃, the content of SiO2 increases at a rate of 12 to 15 mg/L, and fluoride concentration increases at a rate of 2.3 to 2.5 mg/L. Hydrochemical and isotopic data suggest that the thermal water in the bedrock aquifers is of meteoric origin and recharged in the northern mountain area to the north of the Baodi~Ninghe fault, and then flows laterally for a long distance from the north to the south to the city of Tianjin. Temperature of the waters increases because of heat exchange with the rocks and recharge by conductive heat flow from beneath.
The effect of hazard was determined by the dangerous degree of hazard factor—environment and the vulnerable degree of sustaining body. The research into the latter is of importance for the hazard theory and the formation of laws on the mitigation of natural hazards. The way to evaluate the vulnerable degree is the foundation of and the key to the research. In this paper, the extenics model is established to do this job.
The Daqing oilfield is a multilayered heterogeneous oil field where the pressure are different in the same vertical profile causing many troubles to the adjustment well drillings. The approximately~balanced drilling technique has been developed and proved to be efficient and successful in Daqing oilfield. This paper discusses the application of approximately balanced drilling technique under the condition of multilayered pressure in Daqing oilfield, including the prediction of formation pressure, the pressure discharge technique for the drilling well and the control of the density of drilling fluid.
Tectonic dynamic system transition, one of the main factors in metallogenesis, controls metallogenic fluid movement and ore body location in orefields and on an ore deposit scale (mainly in the continental tectonic setting), and even the formation and distribution of large~scale deposit clusters. Tectonic dynamic system transition can be classified as the spacious difference of the tectonic dynamic system in various geological units and the temporal alteration of different tectonic dynamic systems. The former results in outburst of mineralization, while the latter leads to the metallogenic diversity. Both of them are the main contents of metallogenic effect of tectonic dynamic system transition, that is, the alteration of dynamic system, the occurrence of mineralization, and the difference of regional tectonic dynamic system and metallogenic diversity. Generally speaking, the coupling of spatial difference of tectonic dynamic system and its successive alternation controlled the tempo~spatial evolution regularity of mineralization on a larger scale. In addition, the analysis of mineralization factors and processes of typical ore deposits proved that the changes of tectonic stress field, the direct appearance of tectonic dynamic system transition, may lead to the accident of mineralization physicalchemical field and the corresponding accidental interfaces were always located at ore bodies.
The central Fujian Province, situated on the juncture of paleo~uplift of Wuyishan, Yongmei Late Paleozoic depression and the eastern volcanic rift~faulting zone, is mainly composed of the outcropped metamorphic basements in the Middle~Late and Early Proterozoic, which constitute two upper and lower giant thick formations of Precambrian volcanic~sedimentary cycles, respectively. The formation of Dongyan Group is an important Middle~Upper Proterozoic component, and the Dongyan Group is directly related to massive sulfide deposit in this area. In recent years, plenty of lead, zinc, copper, silver and gold deposits have been found and explored. The Precambrian paleorift setting of the central Fujian Province served as a favorite metallogenic background for the formation of large and superlarge scale volcanic massive sulfide (VMS) lead and zinc polymetal deposits. The Dongyan Group consists chiefly of a set of ancient volcanic sedimentary formations that are composed mainly of greenschist. Its major lithologic types comprise greenschist, marble, quartzite and granofels class including various components. The metamorphic rocks of Dongyan Group are the main composition of Middle and Upper Proterozoic volcanic~sedimentary cycle. The original rock of Dongyan Group, a stable rock association, is volcanic sedimentation and normal marine sedimentation. But the original volcanic rocks, basic and acid, are bimodal. The volcanic rocks were formed in the extensional continental rift setting.
Some pyrrhotite~chalcopyrite~bearing amphibole megacrysts (including pyroxene megacrysts) were discovered in Mesozoic augite diorite~porphyrite at Caoshan in Tongling area, Anhui Province. The amphibole megacrysts, belonging mainly to pargasite and magnesiohastingsite, are characteristic of the amphibole composition derived from mantle and crystallized in lower crust. In general, the aggregates of pyrrhotite~chalcopyrite take the shapes of cylinder and sphere. Three occurrences have been recognized in the amphibole megacrysts: parallel linear, bunchy and scattered. The unique cylinder~like shape of the aggregates and remarkable Ni~poor sulfides in Caoshan are distinctively different from the spherical Ni~rich sulfides in pyroxene megacrysts and any other kinds of megacrysts. In terms of composition, the amphibole megacrysts and their sulfides in Caoshan are similar to those in the pyroxenite xenoliths in Qilin, Guangdong Province. In terms of origin, the pyrrhotite chalcopyrites as exsolution products resulted from the subsolidus re~equilibration of sulfide solid solution within amphibole megacrysts. Such pyrrhotite chalcopyrite bearing amphibole megacrysts were first discovered inside and outside China. This discovery is important for the study of regional magma evolution and its associated mineralizations and ore sources as well.
The formation process of the Dianqiangui basin, a special basin, occurred after the Caledonian orogeny,in the south of Guizhou, the west of Guangxi and the southeast of Yunnan, experienced three periods: it began in the Devonian, persisted in the Carboniferous, and became fiercer in the Permian. Controlled by syndepositional fault~zones, varieties of isolated carbonate platforms, large and small, were developed in the background of a deep~water basin, namely, an inter~platform ditch. And a special paleogeographical Late Paleozoic pattern marked by “platform~basin~hill~trough” was produced in both the Dianqiangui basin and its adjacent areas. Affected by regional tectonic activities and the global changes in the sea level, the platform carbonates and coal measures superimposed each other cyclically on the attached platform. The reef~building on the isolated platform and the margin of the attached platform corresponds to the development of the shale succession in the deep~water basin. All of these elementary characteristics reflect a regular and sophisticated filling succession of the Dianqiangui basin, a result of the dual controls of the regionally tectonic activities and the eustacy. Based on the two elementary features of the third~order sequences, i.e. the regularity of sedimentary facies succession in space and the simultaneity of environmental changes in time, 25 third~order sequences could be discerned in the Upper Paleozoic strata in the Dianqiangui basin and its adjacent areas. On the basis of the two kinds of facies~changing surfaces and the two kinds of diachronisms in stratigraphic records, the regional Late Paleozoic sequence~stratigraphic framework in the Dianqiangui basin and its adjacent areas can be established. There are two types of facies~changing surfaces and two types of diachronisms in stratigraphic records: the static type, a result of the change in sedimentary facies in space, and the dynamic type, a result of the change in time. These two types of facies~changing surfaces led to the generation of the two types of diachronisms: the diachronism of facies~changing surfaces that was formed by the static facies~changing surfaces, and the diachronism of punctuated surfaces that was formed by the dynamic facies~changing surfaces. The two types of facies~changing surfaces and the two types of diachronisms in stratigraphic records are the key to the establishment of the sequence~stratigraphic framework. The sequence boundaries could be divided geologically into four types: tectonic unconformity, sedimentary unconformity, drowned unconformity and their correlative surfaces. All of these four types can be further grouped into exposed punctuated surfaces and deepened punctuated surfaces. The tectonic unconformity is similar to Type Ⅰ sequence boundary, and the sedimentary unconformity is similar to Type Ⅱ sequence boundary defined by Vail et al.. In terms of sequence stratigraphy, the tectonic unconformities of the Ziyun movement, the Qiangui epeirogeny and the Dongwu revolution as well as the drowned unconformity in the transitional period from the Permian to the Triassic can be systematically defined and their geological characteristics are briefly presented.
Some extraditional types—black rock series types of platinum group element (PGE), gold and silver mineralization occurrences were found in the Lower Cambrian in Guizhou and Hunan provinces of southwest China where PGE concentration reaches more than 800×10-6. Sea floor hydrothermal fluid eruption was suggested to have been the main origin of the ore forming materials. The whole process from the sedimentation to the redistribution of the ore~forming elements occurred on the conditions of intermediate to weak alkaline, weak reduction to weak oxidation. The temperature for the sedimentation and redistribution of the ore~forming elements was lower than 210 ℃. At such a low temperature, inert elements such as PGE, Au and Ag could quite easily be remobilized.
Source~contacting gas, which is also called basin~center gas, deep basin gas, is the tight~sand gas accumulation contacting closely to its source rocks. Having different accumulation mechanisms from conventional gas reservoirs that are formed by replacement way, the typical source~contacting gas reservoirs are formed by piston~typed migration forward way. Source~contacting gas accumulations exhibit a series of distinctly mechanic characteristics. According to the valid combination of these characteristics, the estimation for the type of discovered gas reservoirs or distributions of source~contacting gas reservoirs can be forecasted. The source~contacting gas is special for having no edge water or bottom water for gas and complicated gas water relationships, which emphasizes the intimate association of reservoir rocks with source rocks, which is called the root of the gas reservoir. There are many basins having the mechanic conditions for source~contacting gas accumulations in China, which can be divided into three regions. Most of the basins with favorable accumulation conditions are located mainly in the central and western China. According to the present data, basins having source~contacting gas accumulations in China can be divided into three types, accumulation conditions and configuration relationships are the best in type A basins and they are the larger basins in central China. Type Ｂ basins with plain accumulation conditions exist primarily in eastern China and also the basins in western China. Accumulation conditions and exploration futures are worse in type C basins, which refer mainly to the small basins in southern China and China Sea basins. Main source~contacting gas basins in China are thoroughly discussed in this paper and the distribution patterns of source~contacting gas in five huge basins are discussed and forecasted.
Two superwide bands of frequency magnetotelluric (MT) profiles (Yadong~Xuegula, Jilong~Cuoqin) across the Yaluzangbu suture were deployed along the west~east direction, for the research into the electrical conductivity structure in the shallow and deep crust along the west~east and north~south directions in the southern part of Tibet plateau. The main characters of the electrical conductivity structure in this region are: (1) large~scale high resistive bodies exist near the Yaluzangbu suture surface, which extends to the maximum depth of more than 30 km. They are the reflection of the Gangdise granite; (2) small~scale conductive bodies exist in the southern part of the Yaluzangbu suture, and large~scale ones under the suture and in the northern part; (3) conductive bodies widely spread in the crust along the profiles. They are discontinuous, mainly decline to the north and become larger in scale, steeper near the suture, deeper gradually from south to north; (4) under the Yaluzangbu suture, the conductive bodies become larger in scale, more conductive gradually from west to east. These important electrical characters are caused possibly by the India plate subduction to the north. The variation in characters of the large~scale conductive bodies from west to east may be the proof that the plate collision might cause substantial movement along the west east direction.
Ultrasonic imaging logging provides continuous and oriented images of structures vs. depth. In the Chinese Continental Scientific Drilling (CCSD) Project, acoustic borehole images were recorded in the second pre~pilot drillhole which penetrates the metamorphic rocks. This paper focuses on fracture evaluation of the drillhole with these images. Both least square fit and a modified Hough transform are used for fracture extraction, and 269 fractures were mapped in the interval from 69.5 to 1 020 m. Most fractures dip steeply, with an average angle of 54°. Fracture dip directions are dominantly in the range of 220° ~ 280° above the depth of 267 m, but 80° ~ 120° in the lower zones. These observations may indicate the differences in structural movements or in situ stress fields between the upper and lower zones in the drillhole.