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The exploration activities in the eastern Tarim Basin have been thwarted over the last decade after the discovery of several Jurassic gas condensate fields. In this study, the two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC-TOFMS) and compound-specific carbon isotope analysis were performed on a Jurassic condensate (Yingnan2) and the associated gas to determine its genesis and the accumulation process in this area. The geochemical and isotopic features suggested that the condensate analyzed was a mixture of the Ordovician cracked paleo-oil with the Jurassic intact oil as evidenced by the concentrated diamondoids and ethanoadamantanes, the high gas generation temperature (∼195 °C), the heavy whole oil δ13C (−28.6‰), and the significant variation in the isotopic profile of n-alkanes. As the gas amount was constantly elevated due to both oil cracking and the mixing of the kerogen-cracking gas from the Cambrian source rock, phase transition occurred and thus formed the Yingnan2 secondary condensate. The constant mixing of nitrogen-rich kerogen-cracking gas complementarily caused an increase in the nitrogen gas content. The accumulation model of the condensate in the eastern Tarim Basin was proposed with consideration of the post-accumulation alterations including thermal cracking and mixing, which complicated the quality and distribution of subsurface petroleums. It was further speculated that abundant gas and condensate resources may be preserved in favorable reservoir-seal assemblages in this field.

Petroleum fluids in the deep Ordovician reservoirs of the Tarim Basin vary in phase and molecular composition. An improved understanding of the secondary geochemical alteration processes is critical for successful exploration and fluid property prediction. Three oil samples from the Ordovician condensate and oil reservoirs were analyzed using comprehensive 2D gas chromatography/time-of-flight mass spectrometry (GC×GC-TOFMS). Molecular signatures revealed varying levels of diamondoids and organosulfur compounds (OSCs) that were preferentially enriched in the condensate and a gas saturated oil; however, these molecular signatures were not generated through in-reservoir oil cracking or thermochemical sulfate reduction (TSR) as favorable thermal and medium conditions were not available. Severe cracking and TSR occurred in deeper Cambrian source-reservoirs and generated secondary geochemical products including diamondoids, OSCs, and H2S. Such secondary products were carried by dry gases derived for oil cracking that migrated upward through the fault system and filled shallower Ordovician oil reservoirs. The differential secondary gas charge can account for the variable fluid composition and varying phase behavior, i.e. the transition from unsaturated oil to gas saturated oil and then to condensate. Condensates were formed from the dissolution of primary oils due to extra gas mixing.

Despite the absence of regional cap rocks in the Lower Paleozoic for the entire Tazhong Low Rise, several sets of effective local cap rocks are well preserved on the Northern Slope. Of these the best is the Ordovician mudstone of the Sangtamu Formation; the second is the Silurian Red Mudstone Member of the Tatairtag Formation and the marl of the Ordovician Lianglitag Formation; and the third is the gray mudstone of the Silurian Kepingtag Formation. The dense limestone of the Ordovician Yingshan Formation and the gypsum of the Middle Cambrian have shown initial sealing capacity. These effective cap rocks are closely related to the distribution of Lower Palaeozoic hydrocarbons in the Tazhong Low Rise. With well-preserved Sangtamu Formation mudstone and its location close to migration pathways, rich Lower Paleozoic hydrocarbon accumulation can be found on the Northern Slope. Vertically, most of the reserves are distributed below the Sangtamu Formation mudstone; areally, hydrocarbons are mainly found in the areas with well-developed Sangtamu Formation mudstone and Lianglitag Formation marl. Burial history and hydrocarbon charging history show that the evolution of Lower Palaeozoic cap rocks controlled the accumulation of hydrocarbon in the Tazhong Low Rise. Take the Red Mudstone Member of the Tatairtag Formation and Sangtamu Formation mudstone for examples: 1) In the hydrocarbon charging time of the Late Caledonian — Early Hercynian, with top surfaces at burial depths of over 1,100 m, the cap rocks were able to seal oil and gas; 2) During the intense uplifting of the Devonian, the cap rocks with top surfaces at burial depths of 200–800 m and 500–1,100 m respectively were denuded in local areas, thus hydrocarbons trapped in earlier time were degraded to widespread bitumen; 3) In the hydrocarbon charging time of the Late Hercynian and Himalayan, the top surfaces of the cap rocks were at burial depths of over 2,000 m without intense uplifting and denudation thereafter, so trapped hydrocarbons were preserved. Based on cap rocks, the Ordovician Penglaiba Formation and Lower Cambrian dolomite could be potential targets for exploration on the Tazhong Northern Slope, and combined with hydrocarbon migration, less risk would be involved.

The Tahe-Lunnan hydrocarbon province is China’s largest region with oil-and-gas-producing marine carbonate rocks. However, in terms of multi-source hydrocarbon generation, multi-episode reservoir adjustment and reconstruction, it remains unsettled how to determine the geological period of primary hydrocarbon filling of the Ordovician reservoir in this region. Based on the analysis of distribution and properties of reservoir, hydrocarbon filling of the Cambrian source rocks in the Late Caledon stage has largely been destroyed. However, hydrocarbon filling of the Middle Ordovician source rocks in the Early Carboniferous resulted in the major body of crude oil. During the charging process, the hydrocarbons were oxidatively degraded to heavy oil due to the poor closure conditions, which is corroborated by homogenization temperature of inclusions. Moreover, the capturing of hydrocarbon inclusions with high-temperature does not represent the filling of mature petroleum in the later period, but represents the result of the natural gas containing light fraction. Therefore, the Tahe-Lunnan area underwent two hydrocarbon filling processes, and the invasion of excessive dry gas led to a gas-washing fractionation upon the original Ordovician reservoirs.

Conventional seismic exploration method based on post-stack data usually fails to identify the distribution of fractured and caved carbonate reservoirs in the Tarim Basin, so the rich pre-stack information should be applied to the prediction of carbonate reservoirs. Amplitude-preserved seismic data processing is the foundation. In this paper, according to the feature of desert seismic data (including weak reflection, fast attenuation of high frequency components, strong coherent noises, low S/N and resolution), a set of amplitude-preserved processing techniques is applied and a reasonable processing flow is formed to obtain the high quality data. After implementing a set of pre-stack amplitude-preserved processing, we test and define the kernel parameters of amplitude-preserved Kirchhoff PSTM (pre-stack time migration) and subsequent gathers processing, in order to obtain the amplitude-preserved gathers used to the isotropic pre-stack inversion for the identification of caved reservoirs. The AVO characteristics of obtained gathers fit well with the synthetic gathers from logging data, and it proves that the processing above is amplitude-preserved. The azimuthal processing techniques, including azimuth division and binning enlargement, are implemented for amplitude-preserved azimuthal gathers with the uniform fold. They can be used in the anisotropic inversion to detect effective fractures. The processing techniques and flows are applied to the field seismic data, and are proved available for providing the amplitude-preserved gathers for carbonate reservoir prediction in the Tarim Basin.

The major storage space types in the carbonate reservoir in the Ordovician in the TZ45 area are secondary dissolution caves. For the prediction of caved carbonate reservoir, post-stack methods are commonly used in the oilfield at present since pre-stack inversion is always limited by poor seismic data quality and insufficient logging data. In this paper, based on amplitude preserved seismic data processing and rock-physics analysis, pre-stack inversion is employed to predict the caved carbonate reservoir in TZ45 area by seriously controlling the quality of inversion procedures. These procedures mainly include angle-gather conversion, partial stack, wavelet estimation, low-frequency model building and inversion residual analysis. The amplitude-preserved data processing method can achieve high quality data based on the principle that they are very consistent with the synthetics. Besides, the foundation of pre-stack inversion and reservoir prediction criterion can be established by the connection between reservoir property and seismic reflection through rock-physics analysis. Finally, the inversion result is consistent with drilling wells in most cases. It is concluded that integrated with amplitude-preserved processing and rock-physics, pre-stack inversion can be effectively applied in the caved carbonate reservoir prediction.

The carbonate reservoirs in the Tarim Basin are characterized by anisotropy and strong heterogeneity. Combined with an integrated analysis of data from seismic, geology, and drilling results, a series of attributes which are suitable for fractured and caved carbonate reservoir prediction is discussed, including amplitude, coherence analysis, spectra decomposition, seismic absorption attenuation analysis and impedance inversion. Moreover, 3-D optimization of these attributes is achieved by integration of multivariate discriminant analysis and principle component analysis, where the logging data are taken as training samples. Using the optimized results, the spatial distribution and configuration features of the caved reservoirs can be chara cterized in detail. This technique not only improves the understanding of the spatial distribution of current reservoirs but also provides a significant basis for the discovery and production of carbonate reservoirs in the Tarim Basin.

The Tazhong Uplift of the late Ordovician is a drowned rimmed carbonate platform. the carbonate rock of the late Ordovician Lianglitage Formation in the northern slope of the Tazhong region is one of the significant petroliferous intervals. Based on petrofacies, depositional cycles, natural gammaray spectrometry and carbon/oxygen isotope data from the Lianglitage Formation, one 2nd-order, three 3rd-order and several 4th-order sequences have been recognized, and the late Ordovician relative sealevel fluctuation curve has been established. The sequences O3l-1 and O3l-2 on the platform are composed of highstand and transgressive systems tracts, but lack the lowstand systems tract. The sequence O3l-3 is a drowning sequence. The sequence O3l-1 overlapped the eroded slope and pinched out to the northwest and landward. The highstand systems tract in the sequence O3l-2 consists of low-angle sigmoid and high-angle shingled progradation configuration. Major sedimentary facies of the Lianglitage Formation include reef and shoal in the platform margin and lagoon, which can be subdivided into coral-sponge-stromatoporoid reef complex, sand shoal, lime mud mound, and intershoal sea. Reefs, sand shoals and their complex are potential reservoir facies. The reefs and sand shoals in the sequence O3l-1 developed in the upper of its highstand systems tract. In the sequence O3l-2, the highstand systems tract with an internal prograding configuration is a response to the lateral shifting of the complex of reef and sand shoal. The transgressive systems tract, in particular the sand shoals, developed widely on the slope of the platform margin and interior. The reefs in the sequence O3l-3 migrated towards high positions and formed retrograding reefs in the western platform and low relief in the platform interior. Basinward lateral migration of the reefs and pure carbonate rock both characterize highstand systems tract and show that the rise of the relative sea-level was very slow. Shingled prograding stacking pattern of the 4th-order sequences and reefs grow horizontally, which represents the late stage of highstand systems tract and implies relative sealevel stillstand. Reefs migrating towards high land and impure carbonate rock both indicate transgressive systems tract and suggest that the relative sea-level rose fast. Erosional truncation and epidiagenetic karstification represent a falling relative sea-level. The relative sea-level fluctuation and antecedent palaeotopography control the development and distribution of reef complexes and unconformity karst zones. Currently, the composite zone of epidiagenetic karstic intervals and high-energy complexes of reefs and sand shoals with prograding configuration is an important oil and gas reservoir in the northern slope of the Tazhong carbonate platform.

Abstract: Lunnan (轮南) low uplift, where developed amounts of condensate pools in both Ordovician and Carboniferous reservoirs, locates in the north uplift of Tarim basin, Northwest China. Lunnan area has experienced intensive gas invasion since the Himalayan movement, causing multiple types of accumulation coexisting in the Ordovician reservoir. Geochemical evidence shows that these condensate oils are of low maturity (vitrinite reflectance levels near 1.0%) and the dominant component of the condensate gas is dry gas (the content of CH4 over 94%), which reflects that the condensate pool in Lunnan area is not the product of thermal cracking, but the subsequent accumulation derived from the phase fractionation. Furthermore, our observations on the C6-C7 hydrocarbons in Lunnan area are qualitatively similar to the effect of phase fractionation with condensates having higher paraffin and lower aromaticity than residual oils. Thus, two types of the condensates are defined by their phase behavior, which are saturated condensate pool and the nonsaturated one. The saturated condensate pool with an oil ring mainly developed in Ordovician reservoir, with large difference between formation pressure (FMP) and the dew point pressure (DPP). The origin of these condensates is considered as the in-situ segregation triggered by the invasion of excessive dry gas. On the other hand, the nonsaturated condensate pool that almost existed in the Carboniferous reservoir is a hydrocarbon reservoir containing slight difference between FMP and DPP, without oil ring, and evaporative fractionation due to the fault movement controls its generation.

In order to predict favorable exploration areas of the Paleozoic, Carboniferous and Silurian clastic reservoirs in the Tazhong area of the Tarim Basin, west China, we studied the basic characteristics of Paleozoic clastic reservoirs in the Tazhong area based on a lot of data. Several issues about the hydrocarbon accumulation related to the reservoirs were also discussed. The results were concluded that: the high-value areas of the porosity and permeability of clastic reservoirs were located in the southeast of the Tazhong area; the content of cement (carbonate cement in particular) was the main factor controlling the porosity and permeability of clastic reservoirs; the hydrocarbon distributions of Carboniferous and Silurian clastic reservoirs were closely related to the porosity and permeability; the favorable hydrocarbon accumulation areas of the two sets of strata were located in the southeast of this area, especially in the updip pinch-out area.

The Tazhong reef‐flat oil‐gas field is the first large‐scale Ordovician organic reef type oil‐gas field found in China. Its organic reefs were developed in the early Late Ordovician Lianglitag Formation, and are the first large reefs of the coral‐stromatoporoid hermatypic community found in China. The organic reefs and platform‐margin grain banks constitute a reef‐flat complex, mainly consisting of biolithites and grainstones. The biolithites can be classified into the framestone, bafflestone, bindstone etc. The main body of the complex lies around the wells from Tazhong‐24 to Tazhong‐82, trending northwest, with the thickness from 100 to 300 m, length about 220 km and width 5–10 km. It is a reef‐flat lithologic hydrocarbon reservoir, with a very complex hydrocarbon distribution: being a gas condensate reservoir as a whole with local oil reservoirs. The hydrocarbon distribution is controlled by the reef complex, generally located in the upper 100–200 m part of the complex, and largely in a banded shape along the complex. On the profile, the reservoir shows a stratified feature, with an altitude difference of almost 2200 m from southeast to northwest. The petroleum accumulation is controlled by karst reservoir beds and the northeast strike‐slip fault belt. The total geologic reserves had reached 297.667 Mt by 2007.

Abstract: Carbonate karst reservoir is the emphases of Tarim’s carbonate exploration. However, it is buried at a large depth, which results in weak seismic reflection signal and low S/N ratio. In addition, the karst reservoir contains great heterogeneity, so reservoir prediction is very difficult. Through many years of research and exploration, we have established a suite of comprehensive evaluation technology for carbonate karst reservoir using geophysical characteristics and a geological concept model, including a technique for reconstructing the paleogeomorphology of buried hills based on a sequence framework, seismic description of the karst reservoir, and strain variant analysis for fracture estimation. The evaluation technology has been successfully applied in the Tabei and Tazhong areas, and commercial production of oil and gas has been achieved. We show the application of this technology in the Lunguxi area in North Tarim in this paper.