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Since the discovery of the Tahe oilfield, it has been controversial on whether the main source rock is in the Cambrian or Middle-Upper Ordovician strata. In this paper, it is assumed that the crude oil from the Wells YM 2 and TD 2 was derived from the Middle-Upper Ordovician and Cambrian source rocks, respectively. We analyzed the biomarkers of the crude oil, asphalt-adsorbed hydrocarbon and saturated hydrocarbon in bitumen inclusions from the Lunnan and Hade areas in the North Uplift of the Tarim Basin. Results show that the ratios of tricyclic terpane C/C in the crude oil, asphalt-adsorbed hydrocarbon and saturated hydrocarbon in bitumen inclusions are less than 1.0, indicating that they might be from Upper Ordovician source rocks; the ratios of C/(C+C+C) steranes in the saturated hydrocarbon from reservoir bitumen and bitumen inclusions are higher than 25, suggesting that they might come from the Cambrian source rocks, however, the ratios of C/(C+C+C) steranes in oil from the North Uplift are less than 25, suggesting that they might be sourced from the Upper Ordovician source rocks. These findings demonstrate that the sources of crude oil in the Tarim Basin are complicated. The chemical composition and carbon isotopes of Ordovician reservoired oil in the Tarim Basin indicated that the crude oil in the North Uplift (including the Tahe oilfield) and Tazhong Depression was within mixture areas of crude oil from the Wells YM 2 and TD 2 as the end members of the Cambrian and Middle-Upper Ordovician sourced oils, respectively. This observation suggests that the crude oil in the Ordovician strata is a mixture of oils from the Cambrian and Ordovician source rocks, with increasing contribution from the Cambrian source rocks from the southern slope of the North Uplift to northern slope of the Central Uplift of the Tarim Basin. Considering the lithology and sedimentary facies data, the spatial distribution of the Cambrian, Middle-Lower Ordovician and Upper Ordovician source rocks was reconstructed on the basis of seismic reflection characteristics, and high-quality source rocks were revealed to be mainly located in the slope belt of the basin and were longitudinally developed over the maximum flooding surface during the progressive-regressive cycle. Affected by the transformation of the tectonic framework in the basin, the overlays of source rocks in different regions are different and the distribution of oil and gas was determined by the initial basin sedimentary structure and later reformation process. The northern slope of the Central Uplift-Shuntuo-Gucheng areas would be a recent important target for oil and gas exploration, since they have been near the slope area for a long time. [ABSTRACT FROM AUTHOR]

Polychaete serpulids are globally distributed tubeworms mostly in marine environments from Late Triassic to modern time. These calcareous tubeworms could be rock-forming, reef-building, or a principal fouling organism in harbor and bays. Carbonates of the Paleogene Kalatar Formation in southwest Tarim Basin yield abundant serpulid fossils, which, together with oyster fossils, constitute the characteristic fossil assemblage of the Kalatar Formation. Other common fossils include bivalves, gastropods, ostracods, echinoderms, and bryozoans. Lithologies that yielded serpulid fossils are characterized by micritic bioclastic limestone, sandy limestone, and shelly limestone, indicating a semi-restricted to open shallow marine environment with medium to low water energy. The research data about serpulids and their fossil materials from China are relatively rare. Based on the studies of fossils taxonomy, community palaeoecology, and fossil taphonomy, this paper analyzed and studied the types, occurrence, distribution, and morphological characteristics of serpulids and their palaeoecological features in the Kalatar Formation. Two serpulid community compositions were recognized in the Kalatar Formation, including a rock-forming Ditrupa community and a cluster-growth Propomatoceros community. The Ditrupa community was distributed in coastal environment of the west Kunlun piedmont, lived on sandy hard substrates with little mud, and rarely occurred in lagoon and tidal settings. The Propomatoceros community occurred in offshore middle carbonate ramp in the piedmont of the south Tianshan Mountains and in offshore shelf in the piedmont of the west Kunlun Mountains. According to the analysis on the host-rock lithologies, preservation and symbionts, it is inferred that serpulids in the Kalatar Formation grew on the oyster shell or other hard substrate, and they did not form reefs or bioherms. [ABSTRACT FROM AUTHOR]

The salt beds of the Middle-Lower Cambrian are widespread in the middle-west parts of the Central Uplift and adjacent areas, the Tarim Basin. This paper presents the results of seismic interpretation and drilling data analysis, which discovered that the salt beds were formed in an old geologic age, deeply buried, with relatively small scaled flowing and gathering and uneven distribution. As the regional detachment layers, the salt sequences considerably control the structural deformation of the up-salt Paleozoic, forming a series of hydrocarbon traps. In due course, the salt beds of the Middle-Lower Cambrian provide excellent cap rocks and trap conditions; thus the value of exploring hydrocabon reservoir in the target strata of the sub-salt Sinian-Cambrian is greatly increased. Research has shown that the salt-related structures of the Middle-Lower Cambrian in the area mainly exist in the form of salt pillow, salt roller, up-salt anticline, salt diapir, assemblage of the salt arch and up-salt fault-block, assemblage of basement fault and salt anticline, assemblage of the basement fault-block and salt dome, assemblage of salt detachment and fault-related fold, and assemblage of basement fault-block, salt arch and up-salt imbricated thrusts. The evolution and deformation mechanisms of the salt-related structures are controlled largely by basement faulting, compressional shortening, plastic flowing and gathering, superstratum gravitation, and up-salt faulting and detaching. They are distributed in rows or belts along basement faults or fault block belts. [ABSTRACT FROM AUTHOR]

A detailed correlation of Hetianhe condensates versus typical source rocks from the Tarim Basin was established. Moreover, the genetic relationship between the condensates and their associated gases was also studied based on their geochemical compositions and fluid inclusion data. Hetianhe condensates are characterized by high pristine/phytane (Pr/Ph) ratios, high relative abundances of C regular steranes, C-C triaromatic steroids, and triaromatic dinosteranes, and relatively heavy stable carbon isotopic compositions. They geochemically correlate well with the Cambrian source rocks, indicating that these condensates are derived from the Cambrian rocks. Based on several molecular maturity parameters, the condensates are recognized as being moderately to highly mature, which are too low in maturity to extensively crack into gases. The gases and condensates of the Hetianhe Field are derived from the same source rocks and the gases are from oil-cracking; however, clear evidence indicates that the gases were not cracked from their associated condensates and that the gas generation from oil-cracking did not occur in the present reservoirs. The liquid condensates produced from wellheads were likely dissolved in their associated gases underground and carried into the reservoirs by the gases. [ABSTRACT FROM AUTHOR]

Based on the analysis of sedimentary facies and chemical index of alteration (CIA) in Nanhua and Ediacaran Systems in the southwest of Tarim Block, some features of glacial records in Neoproterozoic become more clear. Six sedimentary facies have been divided in the study area, including alluvial fan facies, lacustrine facies, glacial facies, littoral facies, neritic facies, and lagoonal facies, showing that this area underwent a process from continent to marine, with mainly littoral and neritic sedimentation. Two cold events have been recognized by analysis of CIA values in the study area, called Bolong and Yutang glaciation, respectively. They present as thick-layer tillite deposition in the Bolong Formation and thin-layer tillite deposition in the Yutang Formation, respectively. The Bolong glacial period in the study area can be correlated to the Yulmeinak glacial period in Aksu area, Tereeken glacial period in Qurugtagh area, and the Nantuo glacial period in South China, which is equivalent to the universally acknowledged Marinoan glacial period. The Yutang glacial period can be correlated to the Hankalchough glacial period in Qurugtagh, which is equivalent to Gaskers glaciation in Newfoundland. [ABSTRACT FROM AUTHOR]

The Upper Ordovician Lianglitag Formation carbonate rocks are important oil and gas reservoirs in the Tarim Basin, Northwest China, with obviously distinct reservoir properties among different oilfields. As a case area of the above carbonate reservoirs correlation, Tahe and Tazhong oilfields are chosen for the study through an integrated analysis of biostratigraphic isochronous framework, cores, thin sections, logs, and strontium isotopes. The research shows that the two areas were respectively attributed to different carbonate platforms during the Lianglitag Formation depositional phase. For the Lianglitag Formation, five fourth-order sequences may be divided in Tazhong, whereas only four fourth-order sequences are preserved in Tahe that correspond to the lower ones observed in Tazhong. The Lianglitag Formation carbonate platform in Tahe was characterized by ramp and/or weak rimmed types, with low depositional rates (37 μm/a), higher whole-rock clastic contents, and higher Sr/Sr isotope ratios of carbonate matrix, showing relatively deep-water environment and evident terrigenous input for carbonate deposition. However, a strongly rimmed carbonate platform was developed in Tazhong, with high depositional rates of 250 μm/a and less whole-rock clastic contents, indicating a high-energy depositional environment. Because of the sedimentary environment differences mentioned above, less high-energy facies but relatively intense syndepositional (early diagenetic) sea-water cementation of carbonates were developed in Tahe, and ubiquitous high-energy reef and shoal facies, with a frequent exposure to subaerial, and weak syndepositional sea-water cementation of carbonates in Tazhong. Therefore, fewer primary pores occurred in the Lianglitag Formation carbonate rocks of Tahe, whereas more primary pores were preserved in the same lithostratigraphic unit of the Tazhong (No. 1 fault zone). Available high-energy reef-shoal facies and early diagenesis, with more primary pores preserved, provided a key material basis for the Lianglitag Formation carbonate reservoirs that largely occurred in Tazhong. [ABSTRACT FROM AUTHOR]

U-Pb ages of Devonian detrital zircons from Tabei Uplift have been determined through LA-ICP-MS test technology. The results revealed that most zircon ages concentrate on 460-414 Ma, especially around 436-423 Ma, indicating possible occurrences of strong tectonic events at the northern margin of Tarim Basin during that period. Combined with previous researches on the basin marginal orogenic belts, intense tectonic activities developed at the northeast margin of Tarim Basin and its obvious sedimentary responses in basin during the end of Ordovician to Early-Middle Silurian are discussed. These include (1) several unconformities within the Late Ordovician-Silurian, showing truncation, erosion, and onlap characteristics, which reflected the local uplift formed during the surrounding extrusion process; (2) the Silurian fluvial delta system from northeast to southwest in Keping, Yingmaili, Hade, and Caohu areas, which reflected the partial uplift at the northeast margin and provided clastic supply to basin; and (3) as indicated by heavy mineral analysis, the Silurian sediments came mainly from the recycles of orogenic belts provenance, which indicated the compress tectonic setting. In addition, a wide range of red mudstone layer distribution in the upper part of the Silurian may be closely related to the surrounding tectonic uplift and the rapid decline of sea levels. [ABSTRACT FROM AUTHOR]

A typical ancient karst cave is found in the Yijianfang Formation of Ordovician at Liuhuanggou, North Tarim Basin. With an entrance of 4.8 m wide and 20 m high, the Liuhuanggou ancient karst cave extends to NW 315° and has been filled by sediments, which can be classified into three types according to their nature and origin: clastic sediments (gravels), chemical sediments (palaeotravertine), and mixed sediments. Among them, the Paleotravertine and gravels are most common. The palaeotravertine is laminated type and yellowish brown or black in color due to oil inside. When saturated with oil, the palaeotravertine is black. Under microscope, oil mainly accumulates in the cracks between calcite crystals. The filling period of the Liuhuanggou karst cave can be divided into three stages: mixed filling, chemical filling, and clastic (palaeotravertine breccia) filling. Numerous fractures are distributed on one side of the karst cave. The fractures can be as wide as 40 cm and over 40 m long. Fractures of different periods and sizes constitute a fracture network and are all filled with calcite veins. Oil can be found inside the fractures. In the chemical fillings, sulfur is also found, which can indicate the magma influence on karst caves. The existence of sulfur also confirms that the Liuhunggou karst cave was formed in ancient times rather than recently or nowadays. Different from normal sedimentary limestone, the ancient Liuhuanggou karst cave has a distinct isotope fractionation and the depletion of heavy carbon and oxygen isotope. Controlled by the northwest oriented vertical strike-slip shear fault, the karst cave is actually a dissolved vertical fault belt. [ABSTRACT FROM AUTHOR]

Ordovician black shales are widely distributed in the Tarim Basin and its peripheral regions, and some of them may serve as potential hydrocarbon source rocks. The present study of the Ordovician graptolite fauna from these shales, together with the yielded conodonts and chitinozoans etc., permits a refined correlation of the rocks. Based mainly on a new collecting of the graptolites and a study of the faunas in Kalpin and Kuruktag regions, and the successful identification of the new graptolite material from a few drill cores within the basin, we are able to update our knowledge of the Ordovician in the regions, and draw some conclusions: (1) The most widespread distribution of the black shales in the Tarim Basin and its peripheral regions, which correspond to the Nemagraptus gracilis Zone, may be related to a global sea-level rise during this time interval. (2) Black shales of Ordovician occur most frequently and extensively in the Kuruktag (also spelled as Quruq Tagh) region in eastern Tianshan Mountains, spanning Tremadocian to mid-Katian ( D. spiniferus Zone) temporally and extending southeastwards into Manjiaer Depression. The black shales of the D. spiniferus Zone may even extend into the central Tarim Basin. (3) Three different bio- and litho-facies belts (Bachu: carbonate platform and reef belt; Kalpin-Aksu: marginal platform and upper slope belt; Wushi: slope belt) are recognized in the northwestern Tarim Basin. (4) The internationally well-correlated Saergan black shale, which has been considered to possess high potential for hydrocarbon source rock, may possess a restricted distribution in the Kalpin and Aksu areas. [ABSTRACT FROM AUTHOR]