Genetic features of gas-condensate reservoirs in the Kekeya Field,southwest depression in the Tarim Basin,China

Oils, condensates and natural gases in the Kekeya Field, southeast depression of the Tarim Basin were studied for their geochemical characteristics. According to the distribution analysis of the C₂/C₃ values with C₁/C₂ values, C₂/C₃ values with C₁/C₃ values, as well as C₂/C₃ values with dryness index, there are two different types of natural gases in the studied field, which are spatially regularly distributed. One is the oil cracking gas, located on shallow reservoirs over X ₅ ² reservoir, namely Upper oil legs; the other is kerogen cracking gas, located on X ₇ ² reservoirs, X₈ reservoirs and E₂ k reservoirs, namely Lower oil legs. In addition, the distribution patterns of molar concentration of oils and condensates with different carbon numbers of the n-alkanes in the Kekeya Field indicate that the crude oils have experienced several kinds of secondary alterations, which were closely related to the charging of gaseous hydrocarbons after petroleum accumulation. These results indicate that, based on the research of δ ¹³C values of individual hydrocarbons, heptane values and isoheptane values of light hydrocarbons and aromatic maturity parameters for oils, condensates and natural gases, oils and gases were charged at different geological time in the Kekeya Field.     

The effectiveness assessment of gas accumulation processes in Kuqa depression,Tarim Basin,Northwest China

The effectiveness of gas accumulation processes is controlled by several main geological factors in-cluding charging force,features of gas conduit,sealing properties of caprock,etc. Based on the analysis and statistics of the large-medium size gas accumulations in China,the main parameters,in-cluding the excess pressure difference between the source rock and reservoir bed,the area coefficient of the gas conduit,and the thickness or displacement pressure of caprock,and the criteria for the as-sessment of gas accumulation processes have been established. Using the parameters and the criteria above,the effectiveness of gas accumulation processes in the Kuqa depression was quantitatively evaluated. By integrating the parameters of the excess pressure difference between the source rock and reservoir bed,the area coefficient of fault conduit system,and the caprock thickness in gas charging period,a comprehensive assessment of the effectiveness of gas accumulation in the Kuqa depression has been made. The result reveals that the Tubei-Dawan area,the Central Kelasu area and the Dongqiu-Dina area are three highly-effective areas for gas accumulation in the Kuqa depression.     

Geochemical characteristics and accumulation of marine oil and gas around Halahatang depression,Tarim Basin,China

There exists a petroleum system rich of oil and gas around Halahatang depression,where the oil and gas possess obvious local distinctions of properties in different parts.The research proved that the discovered crude oil and natural gas in the region derived mainly from O2 3 source rock,and the differences of its properties were controlled by the oil and gas filling intensity.The comprehensive study result shows the oil and gas reservoirs of the region mainly underwent three important accumulation phases:late Caledonian-Early Hercynian epoch,late Hercynian epoch,and Yanshan-Himalayan epoch. In the first phase,the oil and gas derived mostly from Cambrian source rock,which formed the primary ancient oil reservoirs,then suffered strong degradation and remained a great quantity of pyrobitumen in the high position of Tabei uplift in the present.In the second phase,the O2 3 source rock of Manjia'er depression started its generation of hydrocarbon,which accumulated in the high position of Tabei uplift afterwards,and then biodegradated to heavy oil in the late Hercynian epoch.In the last phase,the O2 3 source rock of southern Halahatang depression and margin of Manjia'er depression started its peak of generating liquid hydrocarbon,which mostly accumulated in the trap formed before the Indo-China and Yanshan epoch,and in somewhere the heavy oil suffered dilutions in various degrees or serious gas invading,to lead to obvious crude oil divergence.     

Geochemical features and genesis of the natural gas and bitumen in paleo-oil reservoirs of Nanpanjiang Basin, China

Bitumen from the Nanpanjiang Basin occurs mainly in the Middle Devonian and Upper Permian reef limestone paleo-oil reservoirs and reserves primarily in holes and fractures and secondarily in minor matrix pores and bio-cavities. N2 is the main component of the natural gas and is often associated with pyrobitumen in paleo-oil reservoirs. The present study shows that the bitumen in paleo-oil reservoirs was sourced from the Middle Devonian argillaceous source rock and belongs to pyrobitumen by crude oil cracking under high temperature and pressure. But the natural gas with high content of N2 is neither an oil-cracked gas nor a coal-formed gas generated from the Upper Permian Longtan Formation source rock, instead it is a kerogen-cracked gas generated at the late stage from the Middle Devonian argilla- ceous source rock. The crude oil in paleo-oil reservoirs completely cracked into pyrobitumen and methane gas by the agency of hugely thick Triassic deposits. After that, the abnormal high pressure of methane gas reservoirs was completely destroyed due to the erosion of 2000--4500-m-thick Triassic strata. But the kerogen-cracked gas with normal pressure was preserved under the relatively sealed condition and became the main body of the gas shows.     

Geochemical characteristics and accumulation of marine oil and gas around Halahatang depression,Tarim Basin,China

There exists a petroleum system rich of oil and gas around Halahatang depression, where the oil and gas possess obvious local distinctions of properties in different parts. The research proved that the discovered crude oil and natural gas in the region derived mainly from O₂₊₃ source rock, and the differences of its properties were controlled by the oil and gas filling intensity. The comprehensive study result shows the oil and gas reservoirs of the region mainly underwent three important accumulation phases: late Caledonian-Early Hercynian epoch, late Hercynian epoch, and Yanshan-Himalayan epoch. In the first phase, the oil and gas derived mostly from Cambrian source rock, which formed the primary ancient oil reservoirs, then suffered strong degradation and remained a great quantity of pyrobitumen in the high position of Tabei uplift in the present. In the second phase, the O₂₊₃ source rock of Manjia’er depression started its generation of hydrocarbon, which accumulated in the high position of Tabei up-lift afterwards, and then biodegradated to heavy oil in the late Hercynian epoch. In the last phase, the O₂₊₃ source rock of southern Halahatang depression and margin of Manjia’er depression started its peak of generating liquid hydrocarbon, which mostly accumulated in the trap formed before the Indo-China and Yanshan epoch, and in somewhere the heavy oil suffered dilutions in various degrees or serious gas invading, to lead to obvious crude oil divergence.

     

Geochemical characteristics and accumulation of marine oil and gas around Halahatang depression,Tarim Basin,China

There exists a petroleum system rich of oil and gas around Halahatang depression, where the oil and gas possess obvious local distinctions of properties in different parts. The research proved that the discovered crude oil and natural gas in the region derived mainly from O₂₊₃ source rock, and the differences of its properties were controlled by the oil and gas filling intensity. The comprehensive study result shows the oil and gas reservoirs of the region mainly underwent three important accumulation phases: late Caledonian-Early Hercynian epoch, late Hercynian epoch, and Yanshan-Himalayan epoch. In the first phase, the oil and gas derived mostly from Cambrian source rock, which formed the primary ancient oil reservoirs, then suffered strong degradation and remained a great quantity of pyrobitumen in the high position of Tabei uplift in the present. In the second phase, the O₂₊₃ source rock of Manjia’er depression started its generation of hydrocarbon, which accumulated in the high position of Tabei up-lift afterwards, and then biodegradated to heavy oil in the late Hercynian epoch. In the last phase, the O₂₊₃ source rock of southern Halahatang depression and margin of Manjia’er depression started its peak of generating liquid hydrocarbon, which mostly accumulated in the trap formed before the Indo-China and Yanshan epoch, and in somewhere the heavy oil suffered dilutions in various degrees or serious gas invading, to lead to obvious crude oil divergence.     

Frasnian-Famennian boundary events in Tarim Basin,Xinjiang, Northwest China

The Frasnian-Famennian Boundary Event (Kellwasser Event) is one of the seven strongest Phanerozoic faunal turnovers. The importance of this first order of global bio-event was taken into consideration when the International Subcommission on Devonian Strat…     

Magnetic and mineralogical characteristics of hydrocarbon microseepage above oil/gas reservoirs of Tuoku region, northern Tarim Basin, China

The Tuoku region in northern Tarim Basin of China is a key area for studying oil/gas reservoir rocks. The magnetic and mineralogical parameters of well cuttings from two wells, well S₇, situated on oil/gas field, and well S₆, at an oil/water interface, were measured. The two wells are located in the same structure with similar strata and types of lithology, but well S₆ is a showing well of oil and gas 5 km northwest of well S₇. The purpose of this paper is to evaluate the possibility and distribution of secondary magnetic alteration that may have occurred due to hydrocarbon migration above an oil/gas accumulation. It is concluded that the magnetism of well cuttings from major strata in well S₇, including source rocks, oil reservoir rocks and cap rocks, and in Quaternary (Q) soil is higher than that from well S₆. The Cambrian oil-bearing strata and cap rocks have even higher magnetism in well S₇. The shape and parameters of magnetic hysteresis loops indicate that soft (H _c<20 mT,H _s<0.3 T) ferrimagnetic components dominate the magnetic carriers within the strongly magnetic strata of well S₇, whereas a mixed paramagnetic and ferrimagnetic distribution occurs in well S₆ (for example, low coecivityH _c and nonsaturating magnetized character). Analysis of heavy minerals shows that the contents of iron oxide (magnetite, maghemite and hematite) in well S₇ are often higher than those in well S₆. The magnetite content in samples of cuttings from Cambrian rocks can reach 9.7% in oil-bearing strata in well S₇, and in strata Ekm and N₁j are 1.215% and1.498%, respectively. Typical spherical magnetite grains are found within the main source rocks and the soils in well S₇. By analysis of surface microtexture and of trace element contents, we infer that the spherical magnetite is composed of aggregates of ultrafine particles that are probably authigenic magnetite formed in a hydrocarbon halo background. Project supported by the National Natural Science Foundation of China and the Geological Industry Foundation of China (Grant No. 49374216) and Foundation of the State Key Laboratory of Oil/Gas Reservoir Geology and Exploitation of China (Grant No. 9608).     

Potential evapotranspiration changes in Lancang River Basin and Yarlung Zangbo River Basin,southwest China

ABSTRACT

Understanding potential evapotranspiration (PET) changes under climate change is of great importance for hydrological research. The trends of PET changes and their driving forces were investigated in the Lancang (LRB) and Yarlung Zangbo (YRB) river basins, southwest China, using diagnosis graphs and the Mann-Kendall test. Analysis of variance was applied to examine the contribution of different climatic variables to PET. The results show that: (i) there was a statistically significant increase in PET in the period of 1957–2015 in the LRB, while it showed a markedly decreasing trend in the YRB; (ii) PET in both basins is fairly sensitive to wind speed, relative humidity, solar radiation and maximum air temperature, and the interactions between wind speed and relative humidity are also important; and (iii) the increase in PET in the LRB is due mainly to the increase in maximum air temperature and decrease in relative humidity, while declines in wind speed and solar radiation are the main reasons for the decrease in PET in the YRB.     

Fission-track-age records of the Mesozoic tectonic-events in the southwest margin of the Ordos Basin, China

Based on the analysis of apatite and zircon fission track (FT), the FT age distribution and the peck-ages of the Mesozoic tectonic events in the southwest margin of the Ordos Basin (OB) were discussed. (1) The early event mainly occurred at 213-194 Ma with a peck-age of 205 Ma, and corresponded to the tectonic uplift and the mega-clastic deposit in the southwest OB during the Late Triassic. (2) The middle event included at least two episodes. One was at 165-141 Ma with a peck-age of 150 Ma, and the other was at 115-113 Ma with a peck-age of 114 Ma, corresponding to the over-thrusting and the mega- clastic deposits in the southwest OB during the Late Jurassic to the Early Cretaceous. (3) The late event mainly demonstrated the regional uplifting and included at least two episodes. One was at 100-81 Ma with a peck-age of 90 Ma, and the other was at 66-59 Ma with a peck-age of 63 Ma. Additionally, the relationship analysis of the tectonic event and the mineralizing chronology revealed that the extreme environment of the peck-age event and the subsequent moderate activity could be the key factors of the multiple resources coupling and coexistence in the OB.     

Fission-track-age records of the Mesozoic tectonic-events in the southwest margin of the Ordos Basin,China

Based on the analysis of apatite and zircon fission track (FT), the FT age distribution and the peck-ages of the Mesozoic tectonic events in the southwest margin of the Ordos Basin (OB) were discussed. (1) The early event mainly occurred at 213–194 Ma with a peck-age of 205 Ma, and corresponded to the tectonic uplift and the mega-clastic deposit in the southwest OB during the Late Triassic. (2) The middle event included at least two episodes. One was at 165–141 Ma with a peck-age of 150 Ma, and the other was at 115–113 Ma with a peck-age of 114 Ma, corresponding to the over-thrusting and the mega-clastic deposits in the southwest OB during the Late Jurassic to the Early Cretaceous. (3) The late event mainly demonstrated the regional uplifting and included at least two episodes. One was at 100–81 Ma with a peck-age of 90 Ma, and the other was at 66–59 Ma with a peck-age of 63 Ma. Additionally, the relationship analysis of the tectonic event and the mineralizing chronology revealed that the extreme environment of the peck-age event and the subsequent moderate activity could be the key factors of the multiple resources coupling and coexistence in the OB.     

Fission-track-age records of the Mesozoic tectonic-events in the southwest margin of the Ordos Basin,China

Based on the analysis of apatite and zircon fission track (FT), the FT age distribution and the peck-ages of the Mesozoic tectonic events in the southwest margin of the Ordos Basin (OB) were discussed. (1) The early event mainly occurred at 213–194 Ma with a peck-age of 205 Ma, and corresponded to the tectonic uplift and the mega-clastic deposit in the southwest OB during the Late Triassic. (2) The middle event included at least two episodes. One was at 165–141 Ma with a peck-age of 150 Ma, and the other was at 115–113 Ma with a peck-age of 114 Ma, corresponding to the over-thrusting and the mega-clastic deposits in the southwest OB during the Late Jurassic to the Early Cretaceous. (3) The late event mainly demonstrated the regional uplifting and included at least two episodes. One was at 100–81 Ma with a peck-age of 90 Ma, and the other was at 66–59 Ma with a peck-age of 63 Ma. Additionally, the relationship analysis of the tectonic event and the mineralizing chronology revealed that the extreme environment of the peck-age event and the subsequent moderate activity could be the key factors of the multiple resources coupling and coexistence in the OB.

     

Paleomagnetic results of the Cretaceous marine sediments in Tongyouluke, southwest Tarim

Southwest Tarim (hereafter SW Tarim) is one of afew areas that well developed Cretaceous marinesedimentary rocks in China [1]. The Cretaceous marinesediments are stretched in front area along the Tian-shan and Kunlun Mountains. Toward the center ofTarim Basin, the Cretaceous sediments are buried bygreat thickness of Tertiary and Quaternary sedimentswith little exposure. Compared with the Cretaceousterrestrial strata of north Tarim, the Cretaceous marinestrata of SW Tarim continue and d…     

Mega-shoaling in carbonate platform of the Middle Triassic Leikoupo Formation,Sichuan Basin,southwest China

Shoaling is a common type of sedimentation in the evolution of carbonate platform,and commonly has poor continuity.This paper presents a newly discovered and rare type of shoaling,i.e.,mega-shoaling in nearly basin scale,which is developed in the Middle Triassic Leikoupo Formation of the Sichuan Basin,southwest China.During the Leikoupo time,the studied Sichuan Basin experienced hot and dry climate conditions and developed a carbonate platform within a restricted epicontinental sea.In B sub-layer of the Lei-1-1 sub-member of the Leikoupo Formation a series of grainstones of shoal facies accumulated throughout almost the entire basin,thereby generating features associated with basin-scale mega-shoaling.By detailed core examination and microscopic observation of thin sections,it is shown that the lithology of this set of grainstones is dominated by doloarenite(calcarenite)followed by oolitic dolomite(limestone).In addition,it contains three types of sedimentary sequences characterized by upward-coarsening and upward-shallowing as the followings:restricted lagoon to platform interior beach;restricted lagoon to platform interior beach and to platform flat;and tidal flat to peritidal beach.Subsequently,a multicyclic stratigraphic division and correlation revealed that this set of grainstones can be well traced and compared horizontally,and is generally isochronous.In addition,a template for logging facies,established based on core calibrations and logging data,was employed to analyze the 235 wells in the basin.The results demonstrate the shoal grainstones to be 10–40 m thick with a15×104km2continuous distribution area.These findings indicate that the carbonate platform developed mega-shoals within a short period of time.The genesis of such a mega-shoaling was investigated by focusing on various shoaling conditions,such as paleo-tectonics,paleo-geomorphology,paleo-climate,sea-level changes,and palaeo-hydrodynamics.A specific combination of independent geological factors creates beneficial geomorphologic conditions for the mega-shoaling including a quiescent paleo-tectonic environment,relatively flat paleo-geomorphology and evaporites filling up and leveling off.In addition,a stably settling carbonate platform underwent sea-level fluctuations through swift transgressions and protracted regressions,which is not only conducive to continuous,multicyclic and superimposed vertical development of grain beaches but also beneficial for the horizontal migration,coalescence and superimposition of individual grain beaches.As a consequence,large-scale and continuously-distributed grain beach sedimentation emerges and mega-shoals develop.     

塔里木盆地新元古代裂陷群结构构造及其形成动力学

塔里木盆地新元古界的构造属性及结构构造长期以来存在争议,也是深层研究的重点及难题.通过区域探井和地震资料联合解释,结合航磁资料综合研究发现,塔里木盆地深层存在近20个大小不等的南华纪-晚震旦世裂陷.裂陷发育在前寒武纪变质结晶基底上,与上覆显生宙盖层构造格局迥异.受正断层控制呈半地堑、不对称地堑及垒堑相间的构造样式,从东北到西南可分为NWW、NEE、NW向展布的三个裂陷群,地层最大厚度可达4100m.从南华纪到震旦纪主要断裂继承性活动,断陷沉降中心沿断裂向东迁移,震旦纪末期至早寒武世断裂活动减弱至停止.断裂走向及沉降中心展布表明,新元古代塔里木陆块不同部位分别处于NNE-SSW、NNW-SSE向拉张古应力场(相对现今),并伴有顺时针旋扭作用.根据同沉积断裂的活动性差异、岩浆活动、裂陷充填沉积物及与航磁异常的协调性分析,裂陷的构造属性多以大陆裂谷及陆内断陷为主.裂陷的主要发育期在0.8～0.61Ga,其形成与南阿尔金-西昆仑洋、南天山洋的初始打开为响应,且是Rodinia超大陆主要裂解期的产物.     

Significance of fossilized microbes from the Cambrian stromatolites in the Tarim Basin,Northwest China

Samples of the Cambrian microbial dolomites were collected from Penglaiba section,a well-exposed stratigraphic section in the northwestern area of the Tarim Basin.This study provides an analogue for mediated dolomites that can precipitate in microbial mats and biofilms.The Cambrian stromatolitic dolomites were studied using high-resolution scanning electron microscopy.The results are as follows:(1)dolomites with 50 nm to 100 nm spherical nanostructures are aggregated into minerals of larger sphericities;(2)nanospherical dolomites of 50 nm to 170 nm diameter are densely arranged as dumbbell-shaped or chained aggregates;(3)silicified filaments,as well as dumbbell-shaped and chain arrangements,are preserved as important microstructures.On the basis of sedimentological,compositional,geochemical,and petrographic data,the microstructures were interpreted as nanoglobules that function as bacteria in the nucleation and filament mineralization stages.The microstructures function as such because they are wrapped in extracellular polymeric substance(EPS)or mucus and mineralized fossils.Silicification accounts for the exceptional preservation of microbial mat structures,including biofilms,as well as filamentous and coccoid microbes.In addition,EPS process is capable of binding different elements,with preference for Si,Mg,and Ca.Such suitable composition favors microbe mineralization and dolomite nucleation on organic substrates.These microscopic structures suggest bacterial mineralization and provide visual evidence for the origin of microbial dolomites.     

Irrigation impact on annual water balance of the oases in Tarim Basin,Northwest China

By taking the sum of annual precipitation and lateral water input (in which irrigation water withdrawal is the main component) for water availability, the Budyko hypothesis and Fu's formula derived from it was extended to the study of oases in the Tarim Basin, Northwest China. For both long‐term (multi‐year) and annual values on water balances in the 26 oases subregions, the extended Fu's formula was confirmed. Regional patterns on water balance on the 26 oases subregions were related to change in land‐use types due to increased area for irrigation. Moreover, an empirical formula for the parameter was established to reflect the influences of change in land use on water balance. The extended Budyko framework was employed to evaluate the impact of irrigation variability on annual water balance. According to the multi‐year mean timescale, variabilities in actual evapotranspiration in the oases were mainly controlled by variability in irrigation water withdrawal rather than potential evapotranspiration. The influences of variability on potential evapotranspiration became increasingly apparent together with increases in irrigation water withdrawal. Copyright © 2010 John Wiley & Sons, Ltd.     

Early dolomitisation of the Lower-Middle Ordovician cyclic carbonates in northern Tarim Basin,NW China

High-frequency metre-scale cycles are present within the Lower-Middle Ordovician carbonate successions in northern Tarim Basin, NW China. These metre-scale cycles were variably dolomitised from top to bottom. Three types of replacive dolomites were recognised, including dololaminite(very finely to finely crystalline, planar-s to nonplanar-a dolomite;type-1), patterned dolomite(finely crystalline, planar-s dolomite; type-2), and mottled dolomite(finely to medium crystalline,nonplanar-a(s) dolomite; type-3). Petrographic evidence indicate these dolomites were primarily deposited in supratidal to restricted subtidal environments, and formed in near-surface to shallow burial realms. Geochemically, all types of dolomites have similar δ13C and 87Sr/86 Sr ratios comparable to calcite precipitated in equilibrium with the Early-Middle Ordovician seawater. These geochemical attributes indicate that these dolomites were genetically associated and likely formed from connate seawater-derived brines. Of these, type-1 dolomite has δ18O values(.4.97‰ to.4.04‰ VPDB) slightly higher than those of normal seawater dolomite of the Early-Middle Ordovician age. Considering the absence of associated evaporites within type-1 dolomite, its parental fluids were likely represented by slightly evaporated(i.e., mesosaline to penesaline) seawater with salinity below that of gypsum precipitation. More depleted δ18O values(.7.74‰ to.5.20‰ VPDB) of type-2 dolomite and its stratigraphic position below type-1 dolomite indicate the generation of this dolomite from mesosaline to penesaline brines at higher temperatures in near-surface to shallow burial domains. Type-3 dolomite yields the most depleted δ18O values(–9.30‰to –7.28‰ VPDB), pointing to that it was most likely formed from coeval seawater-derived brines at highest temperatures in a shallow burial setting. There is a downward decreasing trend in δ18O values from type-1 through type-2 to type-3 dolomites, and in abundance of dolomites, indicating that the dolomitising fluids probably migrated downward from above and persisted into shallow burial conditions.     

Geological characteristics and shale gas distribution of carboniferous mudstones in the Tarim Basin,China

Extant research on Paleozoic mudstone is well developed in the Tarim Basin, while the research on Carboniferous mudstone is relatively weak. Through systematic study of lithology, geochemical characteristics,reservoir characteristics and gas–bearing properties of Carboniferous mudstone in the Tarim Basin, this study aims to provide a geological basis for the Paleozoic shale gas exploration and development, favorable zone optimization, and resource potential evaluation in the Tarim Basin. The results show that the sedimentary environments of organic-rich mudstone in the study area were mainly basin facies and slope facies. Lithology is dominated by black carbonaceous mudstone, followed by calcareous mudstone, siliceous mudstone, and siliceous rocks. Mudstone is mainly developed in the Kalashayi Formation,which is located in the Bachu and Markit slope belt, with the cumulative thickness of 30–200 m. The organic carbon content is commonly more than 0.4%, and the organic matter types are type II and type III. Thermal evolution degree is widely distributed from a low mature to over mature stage, and different tectonic units have a greater difference. The contents of quartz plus feldspar are between 12% and 82.5%, with an average of 45.8%. Thecontent distribution of clay mineral is from 12% to 57%,with an average of 38.2%. Carbonate minerals(mainly siderite) content is below 50%. The brittle mineral content of the mudstone is approximately 65%, with a strong compressibility, and the mudstone has the material basis of forming crack and natural fracture. Microscopic pores in micro–nanometer level are well developed in the mudstone, including micro bedding joint, microcrack,interbedded pores of clay mineral, nanoscale intragranular or edge pores in the massive organic matter, bioclastic micropores, and mineral dissolution pores, etc. According to the standards provided by the Ministry of Land and Resources in China, the Kalashayi Formation in Bamai Area is a favorable area for shale gas development.