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The Ordovician reservoir in the Halahatang Depression is one of the most commercially prolific oil reservoirs in the Tabei Uplift of the Tarim Basin, NW China. For this study, a total of 94 oil samples were collected from 88 exploration/production wells in this oilfield. All of the oils possess similar organic molecular compositions and belong to the same oil population. Oil-to-source correlation results indicate that oil accumulations discovered in Halahatang Oilfield were derived from the Upper Ordovician source rocks. The homogenization temperatures of fluid inclusion in the Ordovician reservoir in the Halahatang Oilfield range from 90 °C to 100 °C and from 140 °C to 160 °C. The bimodal distribution pattern shown in a histogram of the homogenization temperatures indicates that the Ordovician oil reservoir in the Halahatang Oilfield has been charged twice. Coupling this information with the reconstructed burial and geothermal histories by 1-D basin modeling, the homogenization temperatures can be related to the relative geological ages of 419 Ma to 410 Ma and 16 Ma to 8 Ma, namely, the Middle to Late Silurian and the Middle Miocene respectively, representing the two phases of oil charging and entrapment. The oil migration orientation and charging pathways can be traced using molecular indicators, such as Ts/(Ts + Tm) and TMNr parameters. The overall oil migration orientation in the oilfield is generally from south to north. Therefore, the source kitchen for the Halahatang Oilfield is predicted to be to the south side of the depression. The southern part of the Halahatang Depression, along the oil charging pathways, is identified as the most promising region for oil exploration. [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]

Abstract: Two oil families in Ordovician reservoirs from the cratonic region of the Tarim Basin are distinguished by the distribution of regular steranes, triaromatic steroids, norcholestanes and dinosteroids. Oils with relatively lower contents of C28 regular steranes, C26 20S, C26 20R+C27 20S and C27 20R regular triaromatic steroids, dinosteranes, 24-norcholestanes and triaromatic dinosteroids originated from Middle–Upper Ordovician source rocks. In contrast, oils with abnormally high abundances of the above compounds are derived from Cambrian and Lower Ordovician source rocks. Only a few oils have previously been reported to be of Cambrian and Lower Ordovician origin, especially in the east region of the Tarim Basin. This study further reports the discovery of oil accumulations of Cambrian and Lower Ordovician origin in the Tabei and Tazhong Uplifts, which indicates a potential for further discoveries involving Cambrian and Lower Ordovician sourced oils in the Tarim Basin. Dinosteroids in petroleum and ancient sediments are generally thought to be biomarkers for dinoflagellates and 24-norcholestanes for dinoflagellates and diatoms. Therefore, the abnormally high abundance of these compounds in extracts from the organic-rich sediments in the Cambrian and Lower Ordovician and related oils in the cratonic region of the Tarim Basin suggests that phytoplankton algae related to dinoflagellates have appeared and might have flourished in the Tarim Basin during the Cambrian Period. Steroids with less common structural configurations are underutilized and can expand understanding of the early development history of organisms, as well as define petroleum systems. [Copyright &y& Elsevier]

Some Ordovician and Triassic oils in Block 9 are characterized by light oils, which have distinctly differentiated from heavy oils in other blocks in the Tahe Oilfield, Tarim Basin. Based on the whole oil gas chromatograms, this paper estimates the effect of oil migration and fractionation and the amount of depletion (Q) in terms of the n-alkanes depletion model. The results showed that the amount of depletion in the Ordovician reservoir is highest in the east of this block, e.g. the depletion is 97% in Well T904. The amount of Q gets lower to the west, e.g. the depletion is 53.4% in Well T115 and there is no sign of depletion in Well S69. It is suggested that the direction of gas washing is from the east to the west. The compositions and isotopic characteristics of associated gas in Ordovician oils indicated that the gas might be derived from Cambrian source rocks of the Caohu Depression which lies to the east of Block 9. In contrast, no obvious depletion of n-alkanes in Triassic oils was found, suggesting that the migration pathway of natural gas has been limited to the Ordovician karst fracture system formed in the Early Hercynian Orogeny. Different depletions of the Ordovician and Triassic oils can reveal fault activities in this region. [ABSTRACT FROM AUTHOR]

Based on the chromatograms of oils and saturated hydrocarbons, biomarkers and stable carbon isotope analyses, the geochemical characteristics and oil family Classification of crude oils from the Markit Slope in the southwest of the Tarim Basin were investigated. The results showed that crude oils from the Markit Slope are divided into two oil family Classification. Oils collected from the Bashituo oilfield in the western part of the Markit Slope are characterized by high contents of tricyclic terpanes, pregnane, and homopregnane, low contents of garmmacerane (G/H<0.20), dibenzofuran, and methyl cyclohexane, and light stable carbon isotopic values (the δC values of saturated hydrocarbons and aromatic components are less than −34‰ and −32‰, respectively), with the distribution type of steranes being C≫C −32‰ and −30.6‰, respectively), with the distribution type of steranes being C>C≪C. In addition, oils from Well Qu 1 have some characteristics similar to those of the above oils, with some special characteristics including high contents of β-carotenes and special distribution of steranes (C

Abstract: The paleo-heat flow evolution of the Tabei Uplift in the Tarim Basin is investigated based on burial and thermal history reconstruction of 14 wells and using basin modeling. Numerous geological parameters, such as, temperature data and missing sediment thickness by erosion were used in the modeling. The basin model was calibrated using 460 measured vitrinite reflectance (%VR o) and vitrinite-like maceral reflectance (%VLMR o) values to constrain the validity of the maturity model. The heat flow history of the Tabei Uplift, Tarim Basin shows the following characteristics: (1) the highest paleo-heat flow was predicted to have occurred in the Early Ordovician as 65±5mW/m2, and gradually decreased to 55±5mW/m2 during the Late Carboniferous; (2) a thermal kick was modeled to have occurred in the Permian as suggested by an abrupt rise in the heat flow; (3) the heat flow gradually decreased since the Triassic; (4) the present day heat flow was predicted to be as low as 38mW/m2. This heat flow history honors the geologic and tectonic evolution history of the Tabei Uplift and is suggested as the best case heat flow model. [Copyright &y& Elsevier]

The geochemistry, origin and charge history of oils from the Yuqi area of Tarim Basin have been investigated, through GC, GC-MS and fluid inclusion microthermometry analysis. The Yuqi oils accumulated mainly in three intervals: (1) in the Lower-Middle Ordovician Yingshan Formation (O 1-2 y ) carbonate reservoirs; (2) in the overlying Upper Triassic Halahatang Formation (T 3 h ); and (3) in the Lower Cretaceous Yageliemu Formation (K 1 y ) sandstones. Oils from different reservoirs have distinct physical properties, varying from extra-heavy (O 1-2 y), heavy (T 3 h ), to light oils (T 3 h and K 1 y ). However, their geochemical compositions show a high degree of similarity, which indicates that they derive from the same source rock. Abundant tricyclic terpanes, gammacerane, dibenzothiophene and C 21 C 22 steranes, together with a low level of diasteranes, indicate an anoxic marine source rock for oils in the Yuqi area. Oil-oil correlation shows that Yuqi oils derive from the same source bed as Tahe oils. The co-occurrence of intact n -alkanes and 25-norhopanes in all the samples supports the proposition that there is a mixture of an early filled severely biodegraded oil and a late filled fresh oil. In this study, charge history is examined on the basis of integration of fluid inclusion homogenization temperature data with 1D burial-thermal history models. Two episodes of oil charging are identified in the O 1-2 y reservoir (well YQX1-1) at around 436-420 Ma (Middle-Late Silurian) and 10-3 Ma (Miocene to Pliocene), respectively. For the samples from the T 3 h and K 1 y intervals, only one episode of oil charge is indicated by the homogenization temperatures of coexisting aqueous inclusions with an inferred timing around 10-3 Ma. The T 3 h heavy oil reservoir is assumed to be a secondary hydrocarbon pool, which accumulated by re-migration and re-distribution of hydrocarbons from O 1-2 y hydrocarbon pools. The few early biodegraded oils in the K 1 y light oils were probably picked up along the migration pathway during the late fresh oil charging. [ABSTRACT FROM AUTHOR]

The Tabei Uplift is one of the largest petroleum accumulation region in China. After decades of exploration and development a number of commercial oil reservoirs have been discovered in the Mesozoic formations suggesting a great potential for hydrocarbons. The Mesozoic oils were sampled from 18 wells in the Tabei Uplift. A total of 6 Ordovician oil samples were also collected for contrast analysis. On the basis of oil–oil correlation, all of the 24 crude oils belong to the same oil population and probably derived from the same source kitchen. Despite the coexistence of “UCM”,25-norhopane and intact n -alkanes which is generally interpreted as twice oil filling history, the homogenization temperature ( T h ) of brine inclusion indicates a single oil filling event which happened at 4–3 Ma, with the major frequency ranging from 110 °C to 120 °C. By means of T s /( T s + T m ), MPI−1, 4-/1-MDBT and total concentration of carbazole compounds, the oil filling orientation in Yakela and Yuqi areas were from NEE to SWW and NW to SE, respectively. The Luntai fault is believed to be an oil-migrating fault connecting the Ordovician reservoirs with Mesozoic reservoirs. Based on the physical properties, the geochemical characteristics and the oil filling histories, it can be concluded that the Mesozoic reservoirs in the Tabei Uplift were of secondary oil accumulations derived from the deeper Ordovician reservoirs. [ABSTRACT FROM AUTHOR]

Dibenzothiophenes (DBTs) and benzo[ b ]naphthothiophenes (BNTs) are the main sulfur-containing heterocyclic aromatic compounds in crude oils and sedimentary rock extracts. Because of the relatively higher electronegativity and an unshared pair of electrons in the outer shell orbital of the sulfur atom, hydrogen bonds can be formed between the sulfur and hydrogen atoms of the surrounding medium in carrier beds. Therefore, DBTs and BNTs are potential geochemical markers for tracing oil filling pathways. A correlation of oils in the Halahatang Oilfield of the Tabei Uplift in the Tarim Basin (NW China) shows that the molecular marker compositions of all oils are quite similar, implying that these oils have been sourced from the same source kitchen and belong to a same oil family. Using molecular indicators relative to DBTs and BNTs, namely, 4-/1-MDBT (4-/1-methyldibenzothiophene), 4,6-/(1,4 + 1,6)-DMDBT (4,6-/(1,4 + 1,6)-dimethyldibenzothiophene), (2,6 + 3,6)-/(1,4 + 1,6)-DMDBT ((2,6 + 3,6)-/(1,4 + 1,6)-dimethyldibenzothiophene), and [2,1]BNT/([2,1]BNT + [1,2]BNT) (benzo[ b ]naphtho[2,1- d ]thiophene/(benzo[ b ]naphtho[2,1- d ]thiophene + benzo[ b ]naphtho[1,2- d ]thiophene)), this paper traces the oil migration orientation and filling pathways in this oilfield. The isopleth maps of these molecular indicators unequivocally reveal an overall oil migration, filling points, and preferred filling pathways orientation from south to north. Based on this, a potential source kitchen is predicted in the southern part of the Halahatang Depression, most probably at the Manxi (Shuntuoguole) Low Uplift, just between the Awati and Manjiaer depressions. The southern part of the Halahatang Depression, along the oil charging pathways, is predicted to be the most promising region for oil exploration. [ABSTRACT FROM AUTHOR]

In order to select biomarker parameters that have potential to quantitatively distinguish between the Cambrian–Lower Ordovician and Upper Ordovician source rocks in the Tarim Basin with a total organic carbon content >0.5%, 42 Lower Paleozoic shale samples, were collected from a number of wells and outcrops in the Tarim Basin, northwestern China. The biomarkers in the extracts of the source rocks were analyzed by gas chromatography–mass spectrometry. The results show that C 23 and C 30 were dominant in the tricyclic terpanes and pentacyclic triterpanes, respectively, and C 27 and C 29 were both dominant in C 27–29 steranes of the source rock samples. The distribution of n -alkanes, isoprenoids and terpanes indicate that the source rock samples have been slightly to moderately biodegraded, and they originated from a reducing depositional environment with considerable marine algal input in. Comparatively, Upper Ordovician source rock samples presented lower values of C 28 /C 29 regular steranes and higher values of C 29 ααα20S/(20S + 20R) and C 29 αββ/(αββ+ααα). Abnormally lower values of C 29 ααα20S/(20S + 20R) and C 29 αββ/(αββ+ααα) were observed in the Cambrian–Lower Ordovician strata. The set of data in this study show that homohopane index including C 31 homohopane/C 30 hopane, C 31-35 homohopanes/C 30 hopane, and the ratios related to the triaromatic steroids including C 26 −20S/C 28 −20S TAS, C 27 −20R/C 28 −20R, (C 26 −20R + C 27 −20S)/C 28 −20S and C 28 /(C 26 +C 27 ) TAS, could serve as potential biomarker parameters to distinguish between the two groups of source rocks in the Tarim Basin. The values of those parameters (in above order) in Cambrian–Lower Ordovician source rocks were <0.6, <1.5, >0.3, >0.5, >0.85, and <1.2, respectively, whereas those in Upper Ordovician source rocks were >0.8, >1.9, <0.3, <0.5, <0.85, and >1.2, respectively. The correlation between the source rock samples and 19 oils obtained from the Tadong, Tabei and Tazhong uplifts in the Tarim Basin show that most of the oils in Tabei uplift are probably from Upper Ordovician source rocks, the oils in Tadong uplift are more likely from Cambrian- Lower Ordovician source rocks, and the Tazhong uplift is more like a mixed oil region with oils from both of the two groups of source rocks. [ABSTRACT FROM AUTHOR]