Kuche Depression Research Articles

Comparison on Rare Gas Geochemical Characteristics and Gas Originations of Kuche and Southwestern Depressions in Tarim Basin, China

The Kuche Depression is considered as the most important gas resource potential and gas exploring area with great gas resource potential and prospect in the Tarim Basin. Based on geochemical experimental analyses and comprehensive geological studies, the general geochemical characteristics of molecular and isotope compositions of rare gases as well as hydrocarbon gases and nonhydrocarbon gases are comparatively studied in the Kuche and Southwestern Depressions. Then, their genetic types are separately identified and gas originations are comprehensively discussed. The main results are as follows. (1) Gas fields in the Kuche Depression have a higher methane abundance, accompanied with low N2and CO2abundances, but the Akemomu gas field in the Southwestern Depression has a relatively lower average methane abundance, accompanied with high average N2and CO2abundances. The helium abundance of natural gases in gas fields from the Kuche Depression general has 1 order of magnitude higher than the air value. Comparatively, it has more than 2 orders of magnitude higher than the atmospheric value in the Akemomu gas field from the Southwestern Depression. The neon, argon, krypton, and xenon abundances in both Kuche and Southwestern Depressions are lower than the corresponding air values. (2) Natural gases from gas fields in the Kuche Depression and the Southwestern Depressions are generally typical coal-formed gases. The rare gases in the Kuche Depression have typical crustal genesis, mainly deriving from the radioactive decay of elements in the crust, while in the Akemomu gas field from the Southwestern Depression, the rare gases have main crustal genesis with a proportion of 92.5%, probably accompanied with a little mantled genetic contribution. (3) Natural gases in the Kuche Depression are generally derived from coal measure source rocks of Jurassic and Triassic, which principally originated from Jurassic in strata period and coals in source rock types. The Jurassic source rocks account for 55%-75% and the Triassic source rocks account for 25%-45% approximately, while coals occupy 68% and mudstones occupy 32% separately. Natural gases from the Akemomu gas field in the Southwestern Depression mainly originated from humic mudstones of marine and continental transitional source rocks of Carboniferous to Permian.

Fluid–rock interactions during continuous diagenesis of sandstone reservoirs and their effects on reservoir porosity

AbstractThe Cretaceous sandstone reservoir in the Kuche Depression, Tarim Basin, western China, was investigated with reference to its reservoir property evolution during diagenesis. Six general diagenetic stages were recognized through petrographic, mineralogical and geochemical analyses. Fluid–rock interaction experiments were conducted under these six continuous diagenetic conditions, that is with a simulation sequence of compaction → early diagenesis → organic acid incursion I → elevated temperature/pressure → organic acid incursion II → late diagenesis. Corresponding to these six experimental stages, a total of six models were constructed. Finally, an extended model of fluid–rock interaction during diagenesis at a geological timescale (from 30 Ma to present) was constructed after various parameters had been validated. Results demonstrate that the diagenetic stages from both the experimental and numerical simulations generally matched findings obtained from the petrographic and geochemical analyses: (i) With compaction becoming weakened, cementation by various minerals was gradually increased. (ii) Quartz overgrowth occurred because the contemporaneous sedimentary water was alkaline. (iii) Most minerals (for example, calcite and feldspar minerals) displayed dissolution owing to the first organic acid incursion, resulting in the visual porosity increasing to 29·26%. (iv) Increases in temperature and pressure caused a minor fluctuation of the porosity change. (v) The cement that formed during earlier stages largely dissolved with the second organic acid incursion. (vi) During the last stage, the reservoir fluid was diluted by sedimentary alkaline water and most minerals precipitated under an alkalic environment. The present porosity simulated is about 11·4%, comparable with the actually measured data. This study demonstrates that the combination of petrographic observations, laboratory experiments and numerical simulations can not only reconstruct the diagenetic process, but also provide a quantitative evaluation and prediction of reservoir petrophysical properties.

Braided river delta outcrop architecture: A case study of Triassic Huangshanjie Formation in Kuche depression, Tarim Basin, NW China

Abstract Guided by the theory of reservoir architecture, the architecture of braided river delta of Triassic Huangshanjie Formation in Kuche Depression of Tarim Basin was studied using cycle constraints and hierarchical dissection analysis method. According to sedimentary cycle and lithofacies assemblage characteristics, two types (distributary channel and distributary bar) of fourth level architectural units and their boundaries were identified on outcrops. Vertically, the reservoir sand bodies overlay in the three patterns, distributary channel-distributary bar, distributary bar-distributary bar, and distributary channel-distributary channel. Laterally, the architecture units stitching in four patterns, distributary channel-distributary channel intersecting and stacking, distributary channel-distributary bar intersecting and stacking, distributary bar-distributary bar and distributary bar-distributary channel intersecting and stacking. The fourth level units overlay and joint together, forming complex fifth level architectural units of channel filling divided by architectural boundaries of different levels. The fourth and fifth level boundaries reduce the vertical connectivity and enhance the heterogeneity of the reservoir. Through field measurement, profile architecture element database, was established and the quantitative relationship among the parameters was characterized. The research results provide a geological basis for the characterization of fine reservoir architecture of the similar sedimentary conditions in the Tarim Basin.

Comment on “Cenozoic tectonic deformation and uplift of the South Tian Shan: Implications from magnetostratigraphy and balanced cross-section restoration of the Kuqa depression” by Tao Zhang, Xiaomin Fang, Chunhui Song, Erwin Appel, and Yadong Wang [Tectonophysics, 2014, doi:10.1016/j.tecto.2014.04.044

The recent paper entitled: “Cenozoic tectonic deformation and uplift of the South Tian Shan: Implications from magnetostratigraphy and balanced cross-section restoration of the Kuqa depression” by Zhang et al. [Tectonophysics, 2014, doi:10.1016/j.tecto.2014.04.044] discusses the Cenozoic tectonic deformation and uplift of the South Tianshan Mountains by integrating tectonic investigations, seismostratigraphic analysis and paleomagnetic dating of the thrust–fold belt in the Kuqa Depression at the southern border of the Tianshan. To support their conclusions the authors have reinterpreted a high resolution magnetostratigraphic study of the Kezilenuer Section in a paper entitled “Magnetostratigraphic study of the Kuche Depression, Tarim Basin, and Cenozoic uplift of the Tian Shan Range, Western China” by Huang et al. [Earth and Planetary Science Letters Volume 251, pages 346–364 (2006)]. We note here (i) that apparent conflicts in definition of the Xiyu Formation in the Kuche Depression as proposed by Zhang et al. (2014) require amplification and (ii) argue that their age assignment for the Kezilenuer Section is incorrect and yields an anomalously low sedimentation rate for the infilling of this foreland basin.

A high-resolution geochemical record from the Kuche depression: Constraints on early Miocene uplift of South Tian Shan

The building of Tian Shan is often simplified to a Cenozoic reactivation of a Paleozoic fold belt. During the past decades extensive case studies have arrived at the conclusion that Tian Shan experienced diachronous uplift during the Cenozoic, however, understanding of the specific location and duration of the mountain building is still hindered by complex tectonic history of the orogenic belt. Considering the geochemical differences of various tectonic units within the mountains, we studied the chemical composition variability for the carbonate-free fine-grained samples collected from the longest terrestrial outcrop in the Kuche depression, southern Tian Shan, at a high resolution in order to better constrain the orogenic process. Our data show significant increases in mobile elements during the interval of 23–17Ma, departing substantially from the stable long-term weathering trends and limited sedimentary sorting effects. We interpret the trend as provenance changes, which provide crucial evidence to support that the South Tian Shan experienced significant surface uplift during the early Miocene. This argument is supported by other studies conducted on both local and regional scales. Mineralogical analyses for the study section have shown that, since the early Neogene, a marked increase of detrital limeclasts derived from the South Tian Shan is accompanied by the decreases of the abundance of garnets derived from an ultrahigh-pressure metamorphic fold belt in the central Tian Shan. Moreover, low-temperature thermochronological results revealed that the central Tian Shan merely experienced pre-Neogene cooling, contrasting to the southern piedmont of the mountain where Neogene cooling dominated. Taken all together, we conclude that the uplift of the South Tian Shan initiated at ~23Ma and the paleoelevation may exceed the height of the central Tian Shan at ~17Ma.

Rare gases geochemical characteristics and gas source correlation for Dabei gas field in Kuche depression, Tarim Basin

The total components and isotopes of rare gases as well as hydrocarbon gases for Dabei gas field are analyzed firstly, then their geochemical characteristics and genesis are discussed respectively, finally gas source correlation and their contributions are evaluated. The research results show that: (1) In Dabei gas field, methane is the main component, accompanying with a few N2 and CO2, as well as extremely low rare gases. In total component contents of rare gases, the He content is from 56.0 to 60.4 ppm with an average of 57.8 ppm and generally 1 order of magnitude higher than the air content value, while the Ne, Ar, Kr, and Xe contents are generally 1–3 orders of magnitude lower than the corresponding air content values. (2) The δ13C1 value in Dabei gas field usually ranges from −31.9‰ to −29.4‰. The δ13C2 value generally ranges from −24.2‰ to −19.4‰, which is obviously heavier than −28.5‰, The natural gases in Dabei gas field can be identified as typical coal-formed gas. (3) The 3He/4He value in natural gases is generally from (6.31–11.24) × 10−8 with an average value of 8.42 × 10−8. The 20Ne/22Ne value is from 9.563 to 9.734, and the 21Ne/22Ne value is from 0.0298 to 0.0307. The 40Ar/36Ar value is usually from 390 to 858 with an average value of 552, and the 38Ar/36Ar value is from 0.1955 to 0.2035. The 129Xe is relative loss, while the 132Xe is relative surplus. The comprehensive genesis identifications of He, Ne, Ar, and Xe indicate that rare gases are crustal derived genesis, mainly originating from the decay of related radioactive elements in crust. (4) The gas source correlation indicates that the natural gases in Dabei gas field are originated from humic coal series source rocks of Triassic-Jurassic, mainly contributed by Jurassic in vertical strata and coal rocks in type, coal rocks accounting for 63% and mudstones accounting for 37%.

Research of Drilling Complexities in Dabei Belt of Tarim Basin

Kuche depression of Tarim Basin went through tectogenesis many times, due to the southward thrust compressional stress from Tianshan fold uplift, the geology condition of this area is extraordinarily complex, there are problems such as depth difference in geological horizons, abnormal pressure formations and irregular distribution of special lithologies, etc. All of these problems have brought even more difficulties in engineering design and construction. Accidents such as well leakage, drilling sticking and complex situations happened frequently. All of those have influenced the engineering progress seriously and lowered the actual effect of drilling. In order to resolve the problems and complex situations above, we did detailed researches on accidents in the Kuche piedmont belt of Tarim Basin and studied reasons for complex situations, and then we proposed engineering resolutions and evaluated them.

Magnetostratigraphic Construct of Awate Section in the North Tarim Basin: the Impulse Uplift of Tianshan Range

AbstractMagnetostratigraphic results of Awate Section from 1606 horizons in Neogene strata between the upper Jidike Formation and the base of the Xiyu Formation in the Kuche Depression of the Tarim Basin, are used to evaluate the tectonic evolution of Tianshan Range. Progressive thermal demagnetization successfully isolated a linear characteristic remanent magnetization of dual polarity between 450 and 680°C, from which a magnetic polarity sequence is constructed. Twenty‐six reversed and twenty‐six normal polarity chrons can be identified from the composite magnetostratigraphy, which correlate between ~1.7 and ~15.1 Ma with the geomagnetic polarity time scale of Cande and Kent. Sedimentation rates from magnetostratigraphic age versus height show a decrease from ~22 to ~7 cm/ka at ~11 Ma, and around 6.5 Ma there was another abrupt increase from ~7 to ~68 cm/ka. These episodic changes can be connected with Late Cenozoic impulse uplift of Tianshan Range.

Magnetostratigraphic and rock magnetic study of the Neogene upper Yaha section, Kuche Depression (Tarim Basin): Implications to formation of the Xiyu conglomerate formation, NW China

Magnetostratigraphic study of 251 horizons through the younger Yaha succession in the Kuche Depression of the Tarim Basin, NW China, lying beneath a massive regionally extensive (Xiyu) conglomerate formation identifies nine reversed and eight normal polarity chrons correlating with the Geomagnetic Polarity Time Scale to show that deposition spanned the interval ∼5.3 to ∼1.7 Ma. Sedimentation rates fell episodically from ∼49 to ∼24 cm/kyr as neotectonic deformation in the southern Tian Shan thrust belt became focused on two major anticlines with the northern (Qiulitage) anticline being initiated at ∼5.5 Ma and developing a northern hinged limb that embraces the basal part of the studied section. Rock magnetic parameters show multiple signatures of lithologic change, deformation, burial diagenesis, and climate with the latter identifying an early Pliocene warm/humid interval followed by cooling and desertification after ∼2.6 Ma. Diachronous commencement of Xiyu conglomerate deposition ranged from mid‐Miocene in the north of the southern flank of the Tian Shan to Pleistocene in the south advancing as a clastic wedge derived from the uplifted range front to the north. This southward progradation was not climatically controlled although climatic effects may have modulated deposition during the Pleistocene. Uplift in the Tian Shan at ∼16–15 Ma correlates with rapid increase in sedimentation rate and episodic increases occurred subsequently until the initiation of the Qiulitage anticline. Reduced rates since ∼5.0 Ma contrast with increases more commonly recorded in foreland basins and have been controlled by accelerated growth of this regional structure counterbalancing uplift of the mountain front to the north.

Contractional Structure Model of the Transition Belt between Kuche Depression and South Tianshan Uplift

The Cenozoic contractional deformation in the transitional belt between Kuche Depression and South Tianshan Uplift can be hardly deciphered by the typical “A-type” subduction model with the structural features like geological outcrop, seismic data, well-log, and CEMP (Continuous Electromagnetic Profile) data. This article advanced a “nonsubduction” contractional deformation model with “delaminate contraction and vertical stack”. The former, “delaminate contraction”, means that the basement and the cover, even multilevel cover, are of their own distinct contractional structural styles. The latter, “vertical stack”, means the strong deformation zones within various levels are stacked up vertically. No regional detachment fault developed. Detachment folds were developed in shallower cover, whereas fault system assembled with reverse fault, thrust, and inverse fault or normal fault developed in the lower cover and basement. The cover folds express structure pattern of a closed anticline combined with an opened syncline. The asymmetric anticline detached within salt rock of Eocene or coal strata of Jurassic and was cut off by a break thrust in the steep limb. A series of basement-involved thrust with high-angle dipping to South Tianshan Uplift dominate the contractional deformation in the basement. A miniscale imbricate wedge thrust system developed in the footwall of basement-involved thrust with high-angle dip and a faulted-anticline with basement-cored developed in the hang-wall. Most of the fault system in the lower level disappeared toward the salt rock of Eocene or coal strata of Jurassic. The high-angle basement-involved thrust tip in the core of detachment anticline is in the shallower level so that the salt rock of Eocene or coal strata of Jurassic might be an important transfer level. The high-angle basement-involved thrust could be an inverse fault utilizing the preexisting normal fault and leads to the local structure deformation, thus attributing to the dynamics of vertical shearing resulted from the uplift of Southern Tianshan mountains.

Structural pattern and its control on hydrocarbon accumulations in Wushi Sag, Kuche Depression, Tarim Basin

Affected by the South Tianshan orogenesis, the faulted structure patterns in the Wushi Sag are very complex. Aimed to find out the basic characteristics of the structures, this article studies many seismic profile sections, the regional geology, the structural patterns, and the formative stage of the structures. The Wushi Sag has three main fault patterns (face to face thrust, back to back thrust, sphenoid thrust), and many local fault structural patterns (fault-bend fold, fault-propagation fold, duplex structure, outburst structure, growth structure, etc.). There mainly develop four groups of reverse faults (NE, NEE, NW, EW) and one group of NNW trending strike-slip faults. According to the structural background, seismic profiles, and balanced cross sections, the main fault in the studied area experiences at least five evolving stages, which are, pre-Mesozoic, late Permian – early Tertiary, Jurassic, late Jurassic – early Cretaceous, and late Neogene. The structural styles have a close relationship with source rocks, hydrocarbon reservoirs, and the character and distribution of petroleum systems.

Comment on “Magnetostratigraphic study of the Kuche Depression, Tarim Basin, and Cenozoic uplift of the Tian Shan Range, Western China” Baochun Huang, John D.A. Piper, Shoutao Peng, Tao Liu, Zhong Li, Qingchen Wang, Rixiang Zhu [Earth Planet. Sci. Lett., 2006, doi:10.1016/j.epsl.2006.09.020

The recent publication of “Magnetostratigraphic study of the Kuche Depression, Tarim Basin, and Cenozoic uplift of the Tian Shan Range,Western China” by B.C. Huang, J.D.A. Piper, S.T. Peng, T. Liu, Z. Li, Q.C. Wang, R.X. Zhu [Earth Planet. Sci. Lett., 2006, doi:10.1016/j.epsl.2006.09.020] discusses the Cenozoic uplift history of the Tianshan Mountains by studying the magnetostratigraphy of Paleogene to Neogene continental sediments from two sections located in the Kuche basin at the northern edge of the Tarim basin. To support their conclusion they reinterpreted a magnetostratigraphic study of the Yaha section, which lies ~ 10 km south of their sections, we previously published [J. Charreau, S. Gilder, Y. Chen, S. Dominguez, J.-P. Avouac, S. Sen, M. Jolivet, Y. Li and W. Wang, Magnetostratigraphy of the Yaha section, Tarim Basin (China): 11 Ma acceleration in erosion and uplift of the Tianshan Mountains, Geology 34(3), 2006, 181­184.]. Here, (1) we argue that the interpretations of the sedimentation rate changes they proposed for the Kuche sections are partially invalid, (2) we disagree with their reinterpretation of the age of the Yaha section, and (3) we think that the way they interpret their AMS data is incorrect.

Magnetostratigraphic study of the Kuche Depression, Tarim Basin, and Cenozoic uplift of the Tian Shan Range, Western China

Magnetostratigraphic and rock magnetic results from 1006 horizons in Paleogene and Neogene sediments between the upper Kumugeliemu Formation and the base of the Kuche Formation within the Kuche Depression of the Tarim Basin, NW China, are used to evaluate postcollisional uplift and deformation in the Tian Shan Range. Characteristic remanences with both normal and reversed polarity were isolated by thermal demagnetization, generally between 300 and 690 °C, and a composite magnetostratigraphy is constructed from data at 969 accepted levels correlating with polarity chrons C12n to C3r dated between ∼ 31.0 and ∼ 5.5 Ma on the CK95 geomagnetic polarity time scale. Observed height-dependent changes of rock magnetic parameters ( k m and AMS ellipsoid parameters) indicate that these sediments were influenced by weak deformation with the succession accumulated before ∼ 15 Ma recording the effects of compressive deformation. This finding, together with two substantial increases in accumulation rate at ∼ 16–17 and ∼ 7 Ma, define a framework for Cenozoic uplift and deformation of the Tian Shan Range. We argue that thrusting in the south of the range was probably initiated at ∼ 20 Ma and followed by rapid uplift at ∼ 16–17 Ma. On the assumption that sediment accumulation in western China was still predominately tectonically controlled at ∼ 7 Ma, another later pulse of rapid uplift with a much larger regional influence is also recognized in the Tian Shan. The Kuche Depression was subjected to two postcollisional phases of rotational deformation with clockwise rotation prior to ∼ 12 Ma (∼ 1.36°/Ma) succeeded by counterclockwise rotation (at ∼ 1.64°/Ma) after ∼ 12 Ma and possibly continuing to the present day. This rotational deformation is not temporally related to the history of basin formation resolved from the magnetostratigraphy and may be linked to block interactions with adjoining terranes.

06/01552 Gas systems in the Kuche Depression of the Tarim Basin: source rock distributions, generation kinetics and gas accumulation history: Zhao, W. et al. Organic Geochemistry, 2005, 36, (12), 1583–1601

06/01552 Gas systems in the Kuche Depression of the Tarim Basin: source rock distributions, generation kinetics and gas accumulation history: Zhao, W. et al. Organic Geochemistry, 2005, 36, (12), 1583–1601

Gas systems in the Kuche Depression of the Tarim Basin: Source rock distributions, generation kinetics and gas accumulation history

Six petroleum source beds have been developed in the Kuche Depression (also known as “Kuqa Depression”) of the Tarim Basin, including three lacustrine source rocks (Middle and Upper Triassic Kelamayi and Huangshanjie formations, and Middle Jurassic Qiakemake Formation) and three coal measures (Upper Triassic Taliqike Formation, Lower Jurassic Yangxia Formation, and Middle Jurassic Kezilenuer Formation). While type I–II organic matter occurs in the Middle Jurassic Qiakemake Formation (J 2q), other source beds contain dominantly type III organic matter. Gas generation rates and stable carbon isotopic kinetics of methane generation from representative source rocks collected in the Kuche Depression were measured and calculated using an on-line dry and open pyrolysis system. Combined with hydrocarbon generation history modelling, the formation and evolution processes of the Jurassic–Triassic highly efficient gas kitchens were established. High sedimentation rate in the Neogene and the fast deposition of the Kuche Formation within the Pliocene (5 Ma) in particular have led to the rapid increase in Mesozoic source rock maturity, resulting in significant dry gas generation. The extremely high gas generation rates from source kitchens have apparently expedited the formation of highly efficient gas accumulations in the Kuche Depression. Because different Mesozoic source rocks occur in different structural belts, the presence of both lacustrine and coaly gas kitchens during the Cenozoic time can be identified in the Kuche Depression. As shown by the chemical and stable carbon isotope compositions of the discovered gases, the formation of the giant gas pools in the Kela 2, Dina 2, Yaha and Wucan 1 have involved very different geological processes due to the difference in their gas source kitchens.

Factors controlling the enrichment of natural gas in Kuche depression, Tarim Basin, NW China: Molecular geochemical evidence from sedimentary organic matter

Using molecular geochemical data from infrared spectrometer and pyrolysis gas chromatography-mass spectrometry, this paper investigates the petroleum generation characteristics of Jurassic coal measures from Kuche depression, Tarim Basin, NW China. The results showed that the Jurassic coaly rocks with medium maturity (R0%: 0.8–1.1) were enriched in gas-prone functionalities (-CH3) and low molecular weight pyrolysates (< nC21), indicating that the coaly rocks from Kuche depression were gas/condensate prone at the stages of middle to high maturation, and it was further supported by the oil/source correlation from well Yinan 2 in this region.

Aromatic compounds in crude oils and source rocks and their application to oil–source rock correlations in the Tarim basin, NW China

Ten series of aromatic hydrocarbons (biphenyls; naphthalenes; phenanthrenes; anthracenes; retenes; chrysenes; benzoanthracenes; dibenzofurans; fluorenes; and dibenzothiophenes) isolated from Cambrian and Ordovician marine carbonate source rocks in Tabei and Tazhong areas of the Tarim Basin, and Triassic and Jurassic lacustrine mudstones and swamp coals in the Kuche depression have been characterized by GC-MS. The same series of aromatic compounds have been isolated and characterized from crude oils of the Tabei and Tazhong uplift. Results presented in this paper describe the distribution of these aromatic compounds in crude oils and source rocks from the three different kinds of sedimentary environments. Similarities in distributions of the fluorenes and the dibenzothiophenes in the aromatic fractions suggest the Tabei uplift oils are derived predominantly from the Lower Ordovician marine source rocks of Tabei Lunnan area (on the south of Luntai) and the oils from Tazhong uplift are mainly derived from Upper Cambrian, Lower Ordovician, and Middle–Upper Ordovician marine source rocks of Tazhong area.

Dibenzofuran series in terrestrial source rocks and curde oils and applications to oil-source rock correlations in the Kuche depression of Tarim Basin, NW China

Ten series of aromatic hydrocarbon compounds (biphenyl, naphthalene, phenanthrene, anthracene, retene, chrysene, benzoanthracene, dibenzofuran, fluorene, dibenzothiophene) isolated from seven Triassic and Jurassic lacustrine mudstone samples and three swamp coal samples, as well as five crude oil samples collected in the Kuche depression of the Tarim Basin, NW China, have been analysed by GC-MS techniques. It is found that the relative abundances of dibenzofuran series are higher in the three swamp coal samples than those in the lacustrine mudstone samples. Based on the similar relative abundances of dibenzofuran series, especially dibenzofuran compound, in the TICs of aromatic hydrocarbons, crude oils from wells SA3 (K), YTK5 (E, K) and QL1 (E) are thought to have been derived predominantly from the coals of the Lower Jurassic Yangxia Formation or Middle Jurassic Kezilenuer Formation, whereas those from wells YM7 (O) and YH1 (E) were derived mainly from Triassic and Jurassic lacustrine mudstones in the Kuche depression. This is the first report about how to distinguish coal-generated oils from lacustrine mudstone-generated oils in the Kuche depression in terms of the dibenzofuran series. The present paper has enlightening and directive significance for further oil-source rock correlations and oil and/or gas exploration in the Kuche depression of the Tarim Basin.

Distribution of Upper Cenozoic Carbonate-Cemented Sandstones, Kuche Depression, Northwest China: Insights from High-Resolution Borehole Micro-Resistivity Imaging

ABSTRACT High-resolution micro-resistivity images are used to study the distribution of neoformed carbonate concretions in Upper Cenozoic shallowly buried fluvial sandstones in the Kuche Depression, northwest China. The carbonate concretions are high-resistivity white facies on borehole images. Most of them occur as patches 1-40 cm thick and in shapes ranging from equant, oblate, to elongate. These concretions contrast markedly with the poorly cemented or uncemented host sandstones, which appear as dark to immediate grayscale facies on the images. Three patterns of occurrence of carbonate concretions are identified: isolated concretions, discontinuously cemented layers, and continuously cemented layers. Most concretions occur in fine-grained, low-permeability laminae, suggesting that their growth was controlled more probably by diffusion transport rather than advective transport. Grain size, lamination, and distribution of intergranular clay all potentially influenced the selective distribution of carbonate cement in the sandstones. Shale interbeds and terrigenous carbonate rock fragments in the sandstones are potential sources of carbonate cement.

Early Cretaceous Paleomagnetic Results from the Kuche Depression, Tarim Block and the Cause of their Inclination Shallowing

AbstractThe paleomagnetic results of the Baxigai Formation of early Cretaceous in Kuche depression, Tarim block are reported. Two magnetic components were clearly isolated by the principal component analysis and the Linefind methods. The overprint component is a remagnetization of Himalayan. The characteristic magnetic remanence (ChRM) deviates obviously from the present magnetic direction. After tilt correction, ChRM shows good consistence and passed reversal tests. The Early Cretaceous magnetic mean direction was obtained by summarizing most of published paleomagnetic data in this area. Inclination shallowing is noticed in the early Cretaceous stratum. We excluded some factors such as the quality of paleomagnetic data, sedimentary process as well as long period no dipole field which could result in inclination shallowing. We suggest that compaction is the most important factor leading to inclination shallowing.