Molecular correlation of crude oils and oil components from reservoir rocks in the Tazhong and Tabei uplifts of the Tarim Basin, China

Molecular data from a large set of source rock, crude oil and oil-containing reservoir rock samples from the Tarim Basin demonstrate multiple sources for the marine oils in the studied areas of this basin. Based on gammacerane/C₃₁ hopane and C₂₈/(C₂₇ + C₂₈ + C₂₉) sterane ratios, three of the fifteen crude oils from the Tazhong Uplift correlate with Cambrian-Lower Ordovician source rocks, while the other crude oils from the Tazhong Uplift and all 39 crude oils from the Tahe oilfield in the Tabei Uplift correlate with Middle-Upper Ordovician source rocks. These two ratios further demonstrate that most of the free oils and nearly all of the adsorbed and inclusion oils in oil-containing reservoir rocks from the Tazhong Uplift correlate with Cambrian-Lower Ordovician source rocks, while the free and inclusion oils in oil-containing carbonates from the Tahe oilfield correlate mainly with Middle-Upper Ordovician source rocks. This result suggests that crude oils in the Tazhong Uplift are partly derived from the Cambrian-Lower Ordovician source rocks while those in the Ordovician carbonate reservoirs of Tahe oilfield are overwhelmingly derived from the Middle-Upper Ordovician source rocks.The scatter of C₂₃ tricyclic terpane/(C₂₃ tricyclic terpane + C₃₀ 17α,21β(H)-hopane) and C₂₁/(C₂₁ + ΣC₂₉) sterane ratios for the free and inclusion oils from oil-containing carbonates in the Tahe oilfield possibly reflects the subtle organofacies variations in the source rocks, implying that the Ordovician reservoirs in this oilfield are near the major source kitchen. In contrast, the close and positive relationship between these two ratios for oil components in the oil-containing reservoir rocks from the Tazhong Uplift implies that they are far from the major source kitchen.     

Sequential Extraction on Oil Sandstones from TZ401 Well——A Case Study on Filling History of Hydrocarbon Reservoir

Sequential extraction was performed on two oil sandstones from the Upper Carboniferous oil columns of TZ401 well.The free oils of these two oil sandstones and a crude oil from the Lower Carboniferous oil column of this well have low ratios of C28/C27+C28+ C29) steranes and gammacerane/C31 hopanes,ranging of 0.11-0.16 and 0.09-0.15,respectively,similar to those from the Middle-Upper Ordovician source rock.However,these two ratios for the adsorbed and inclusion oils of these two oil sandstones are relatively high,ranging of 0.29-0.31 and 0.26-0.40,respectively,similar to those of the Cambrian-Lower Ordovician source rock.This result demonstrates that the initial oil charging the reservoirs was derived from the Cambrian-Lower Ordovician source rock,whereas the later charging oil was derived from the Middle--Upper Ordovician source rock.     

Correlation of crude oils and oil components from reservoirs and source rocks using carbon isotopic compositions of individual n-alkanes in the Tazhong and Tabei Uplift of the Tarim Basin,China

Carbon isotopic compositions were determined by GC–IRMS for individual n-alkanes in crude oils and the free, adsorbed and inclusion oils recovered by sequential extraction from reservoir rocks in the Tazhong Uplift and Tahe oilfield in the Tabei Uplift of Tarim Basin as well as extracts of the Cambrian–Ordovician source rocks in the basin. The variations of the δ¹³C values of individual n-alkanes among the 15 oils from the Tazhong Uplift and among the 15 oils from the Triassic and Carboniferous sandstone reservoirs and the 21 oils from the Ordovician carbonate reservoirs in the Tahe oilfield demonstrate that these marine oils are derived from two end member source rocks. The major proportion of these marine oils is derived from the type A source rocks with low δ¹³C values while a minor proportion is derived from the type B source rocks with high δ¹³C values. Type A source rocks are within either the Cambrian–Lower Ordovician or the Middle–Upper Ordovician strata (not drilled so far) while type B source rocks are within the Cambrian–Lower Ordovician strata, as found in boreholes TD2 and Fang 1. In addition, the three oils from the Cretaceous sandstone reservoirs in the Tahe oilfield with exceptionally high Pr/Ph ratio and δ¹³C values of individual n-alkanes are derived, or mainly derived, from the Triassic–Jurassic terrigenous source rocks located in Quka Depression.The difference of the δ¹³C values of individual n-alkanes among the free, adsorbed and inclusion oils in the reservoir rocks and corresponding crude oils reflects source variation during the reservoir filling process. In general, the initial oil charge is derived from the type B source rocks with high δ¹³C values while the later oil charge is derived from the type A source rocks with low δ¹³C values.The δ¹³C values of individual n-alkanes do not simply correlate with the biomarker parameters for the marine oils in the Tazhong Uplift and Tahe oilfield, suggesting that molecular parameters alone are not adequate for reliable oil-source correlation for high maturity oils with complex mixing.     

Molecular fossils and sources of Cambrian heavy oil of Well Tadong-2 in the Tarim Basin,Xinjiang, China

Research on the molecular fossil characteristics of heavy oil from Well Tadong-2 is of great importance to constrain the source of marine crude oils in the Tarim Basin, Xinjiang, China. The authors synthetically applied the isotope mass spectrograph, chromatography and chromatography-mass spectrography to the studies of molecular fossil characteristics of heavy oil from Well Tadong-2 in the Tarim Basin, and the results obtained revealed that heavy oil from Well Tadong-2 is characterized by high gammacerane, high C₂₈ sterane, low rearranged sterane and high C₂₇-triaromatic steroid, these characteristics are similar to those of Cambrian-Lower Ordovician source rocks, demonstrating that Cambrian crude oils came from Cambrian-Lower Ordovician source rocks; condensed compounds (fluoranthene, pyrene, benzo[a]anthracene, bow, benzo fluoranthene, benzopyrene) of high abundance were detected in heavy oil from Well Tadong-2, and the carbon isotopic values of whole oil are evidently heavy, all the above characteristics revealed that hydrocarbons in the crude oils became densified in response to thermal alteration.     

Reservoir geochemistry of the Tazhong oilfield in the Tarim Basin, China, Part I. Geochemical characteristics and genetic classification of crude oils

1 Introduction Since the 1990’s of the 20th century, oil & gas geochemists have shifted their research focus on hydrocarbon source rocks in the past to that on reservoir rocks and oil reservoirs at present; their research field has been expanded from oil & gas exploration to the assessment of oil reservoirs and production & management. Therefore, reservoir geochemistry as a branch disciplinary of organic geochemistry is now attracting great concern of many oil & gas explorers and oilfield en…     

Geochemistry and possible origin of petroleum in Palaeozoic reservoirs from Halahatang Depression

The Halahatang Depression in the Tabei Uplift of the Tarim basin is an active exploration area because it has substantial reservoir potential and contains or is near to many commercial oil fields. Geochemical analysis indicates that Halahatang oils were derived from marine carbonate source rocks deposited under anoxic reducing conditions. The maturities for Halahatang oils are corresponding to the peak of the oil window and slightly higher than the neighboring Tahe oils. The Halahatang oils feature low Pr/Ph, C₂₁/C₂₃ tricyclic terpane and, C₂₈/C₂₉ sterane ratios, high C₂₉/C₃₀ hopane and C₃₅/C₃₄ hopane ratios, a “V” shape in the distribution of C₂₇–C₂₈–C₂₉ steranes and light carbon isotope ratios, similar to the Tahe oils and correlate well with the Middle-Upper Ordovician source rock. However, some source-related biomarker parameters imply a more reducing source organofacies with more zooplanktonic contribution than that for the Tahe oils.     

Geochemical characteristics of saturate hydrocarbons in crude oils and source rocks of the Qaidam, Tarim, Tupran basins, NW China

Based on the systematic analyses of fifteen typical crude oils and ten typical potential source rocks col-lected from the Qaidam,Tarim and Turpan basins,Northwest China,the geochemical characteristics of the oils and source rocks were investigated and oil-source rock correlations undertaken.The oils and source rocks deposited in saline lacustrine environment from the western Qaidam Basin were characterized by n-alkanes with even car-bon-number preference in the C20-C28 range,low pristane/phytane(Pr/Ph) ratios(less than 0.5),and high abundances of C27 steranes,gammacerane and C35 hopanes.The oils and source rocks deposited in marine environment from the Tarim Basin were characterized by n-alkanes with even carbon-number preference in the C14-C18 range,relatively low Pr/Ph ratios(near to 1),high abundance of C28 steranes,and relatively high gammacerane.In contrast,the oils and source rocks deposited in terrigenous bog environment from the Turpan Basin were characterized by relatively high Pr/Ph ratios(oil samples greater than 6) high abundance of C29 steranes,and relatively low gammacerane and C31-35 hopanes.The higher amounts of C37 and C38 n-alkanes of source rocks from the western Qaidam Basin and the Tarim Basin suggest an origin of these alkanes from functionalized C37 and C38 n-alkadienes and alkenones in prymnesiophytes living in lacustrine and marine environments.Oil-source rock correlations suggest oils in the west-ern Qaidam Basin were derived from the Oligocene Lower Ganchaigou Formation(E3),oils in the Tabei and Tazhong uplifts from the Tarim Basin have a genetic relationship with the Middle-Upper Ordovician source beds.Oils in the Turpan Basin generally fall into two genetic types.Most oils in the Taibei depression from the Turpan Basin were derived from the Lower-Middle Jurassic coal measures,but the fewer oils in this region are a mixed source derived from the Lower-Middle Jurassic coal measure and the Upper Permian source rocks.     

The occurrence of ultra-deep heavy oils in the Tabei Uplift of the Tarim Basin,NW China

Deeply buried heavy oils from the Tabei Uplift of the Tarim Basin have been investigated for their source origin, charge and accumulation time, biodegradation, mixing and thermal cracking using biomarkers, carbon isotopic compositions of individual alkanes, fluid inclusion homogenization temperatures and authigenic illite K–Ar radiometric ages. Oil-source correlation suggests that these oils mainly originated from Middle–Upper Ordovician source rocks. Burial history, coupled with fluid inclusion temperatures and K–Ar radiometric ages, suggests that these oils were generated and accumulated in the Late Permian. Biodegradation is the main control on the formation of these heavy oils when they were elevated to shallow depths during the late Hercynian orogeny. A pronounced unresolved complex mixture (UCM) in the gas chromatograms together with the presence of both 25-norhopanes and demethylated tricyclic terpanes in the oils are obvious evidence of biodegradation. The mixing of biodegraded oil with non-biodegraded oil components was indicated by the coexistence of n-alkanes with demethylated terpanes. Such mixing is most likely from the same phase of generation, but with accumulation at slightly different burial depths, as evidenced by overall similar oil maturities regardless of biodegradation level and/or amount of n-alkanes. Although these Ordovician carbonate reservoirs are currently buried to over 6000 m with reservoir temperatures above 160 °C, no significant secondary hydrocarbon generation from source rocks or thermal cracking of reservoired heavy oil occur in the study area. This is because the deep burial occurred only within the last 5 Ma of the Neogene, and there has not been enough heating time for additional reactions within the Middle–Upper Ordovician source rocks and reservoired heavy oils.     

Improved understanding of the petroleum systems of the Niger Delta Basin,Nigeria

Biomarker and n-alkane compound specific stable carbon isotope analyses (CSIA) were carried out on 58 crude oil samples from shallow water and deepwater fields of the Niger Delta in order to predict the depositional environment and organic matter characteristics of their potential source rocks. Using a source organofacies prediction approach from oil geochemistry, the presence in the western deepwater oils relatively abundant C₂₇ steranes, C₃₀ 24-n-propyl cholestane, low oleanane index, relatively low pr/ph ratios, gammacerane, and positive to nearly flat C₁₂–C₃₀ n-alkane compound specific stable carbon isotope profiles, suggests that the source facies that expelled these oils contain significant marine derived organic matter deposited under sub-oxic and stratified water column conditions. This contrasts with the terrigenous organic matter dominated source rocks accepted for shallow water Niger Delta oils. Oils in the shallow water accumulations can be separated into terrigenous and mixed marine-terrigenous families. The terrigenous family indicates expulsion from source rock(s) containing overwhelmingly higher plant source organic matter (average oleanane index = 0.48, high C₂₉ steranes) as well as having negative sloping n-alkane isotope profiles. Oxic source depositional conditions (pr/ph > 2.5) and non-stratified conditions (absence to low gammacerane content) are inferred for the terrigenous family. The mixed marine-terrigenous family has biomarker properties that are a combination of the deepwater and terrigenous shallow water oils. Bitumen extracts of the sub-delta Late Cretaceous Araromi Formation shale in the Dahomey Basin are comparable both molecularly and isotopically to the studied western deepwater oil set, but with an over all poor geochemical correlation. This poor geochemical match between Araromi shale and the western deepwater oils does not downgrade the potential of sub-delta Cretaceous source rock contribution to the regional oil charge in the deepwater Niger Delta.     

Quantitative assessment and significance of gas washing of oil in Block 9 of the Tahe Oilfield, Tarim Basin, NW China

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 chroma- tograms,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.     

普光气田及邻区碳酸盐储层沥青的分子地球化学研究

从普光气田及邻近地区二叠系和下三叠统15个含沥青碳酸盐储集岩样品分步提取了自由态油气组分和油气包裹体组分,并且应用限定体系(金管)热解实验获取了固体沥青热解组分.各组分进一步做色谱和色谱-质谱分析获取了生物标志物组成.分析结果表明,从油气包裹体组分、自由态油气组分至固体沥青热解组分,伽马蜡烷/C31升藿烷比值依次增高.同时油气包裹体组分 C23三环萜烷/(C23三环萜烷+C30藿烷)、C21/(C21+ΣC29)甾烷、C27重排甾烷/(C27重排甾烷+C27规则甾烷)、20S/(20R+20S) C29甾烷和αββ/(ααα+αββ) C29甾烷等成熟度指标与自由态组分相近,而明显高于固体沥青热解组分.根据分子指标,可以推断古油藏的原油来源于不同的烃源岩,早期充注的原油来源于下志留统烃源岩,具有较低的伽马蜡烷含量和较高的成熟度;晚期充注的原油来源于上二叠统烃源岩,具有较高的伽马蜡烷含量和较低的成熟度.固体沥青主要为晚期充注、来源于上二叠统烃源岩的原油裂解生气的产物     

塔中4油田石炭系储层不同赋存态烃类分子和碳同位素对比研究

塔中421井和塔中402井石炭系油层2个原油样和8个油砂样连续抽提组分甾烷、萜烷分布特征和正构烷烃单体碳同位素组成具有明显的差异,具有不同的来源。塔中421井上石炭统3个油砂样自由态组分、束缚态组分和油气包裹体具有伽马蜡烷和C28甾烷相对含量高、正构烷烃单体碳同位素组成重的特征,划分为Ⅰ类原油,对比认为主要来源于寒武系-下奥陶统烃源岩。塔中421井和塔中402井上石炭统的2个油样具有伽马蜡烷和C28甾烷相对含量低、并且正构烷烃单体碳同位素组成轻的特征,划分为Ⅱ类原油,其来源尚不明确。塔中402井石炭系上、中和下统的5个油砂样各类组分具有介于Ⅰ、Ⅱ类原油之间的特征,为Ⅰ和Ⅱ类原油的混合物。5个油砂样从自由态组分、束缚态组分至油气包裹体Ⅰ类原油含量依次增高,Ⅱ类原油含量依次降低。2口井8个油砂样从自由态组分、束缚态组分至油气包裹体C23三环萜烷／（C23三环萜烷＋C30藿烷）和C21／（C21＋∑C29）甾烷比值都依次降低,反映了油气充注过程中,原油成熟度不断升高。塔中4井区储层油砂不同吸附态烃类分子与碳同位素的研究结果反映塔中4油田具有多种油气来源,经历长期油气充注过程,寒武系-下奥陶统烃源岩在地史上对该区具有成烃贡献。     

A geochemical investigation of crude oils and source rocks from Biyang Basin, China

China has a number of petroliferous lacustrine sedimentary basins of varying salinity and age (mainly Eocene). A geochemical investigation has been undertaken on several oils and source rocks from the Eocene lacustrine Biyang Basin. The distributions of n-alkanes, isoprenoids, steranes, and terpanes have been studied and used to characterize the sedimentary environment of deposition, maturity, biodegradation and undertake possible correlations. The ratios of C₃₀-hopane/gammacerane, 4-methyl-steranes/regular steranes, steranes/hopanes, C₂₁ tricyclic/C₃₀ hopane are proposed to be indicative of the depositional environment whereas ß-carotane appears to be a source related indicator. The geochemical data obtained in this study suggest that the major source rocks in the Biyang Basin were deposited in a saline/hypersaline depositional environment.     

准噶尔盆地滴南凸起含油储集岩分子与碳同位素地球化学研究

从滴南凸起10个含油储集岩样品分步提取了自由态油气组分、束缚态油气组分和油气包裹体组分,各组分进一步做色谱、色谱-质谱和正构烷烃单体碳同位素分析。根据生物标志物组成,可将10个含油储集岩样分为两类：第一类包括D2-1和D18-12个侏罗系油砂样,第二类包括其他8个采自侏罗系、二叠系和石炭系含油储集岩样。两类样品生物标志物组成差异主要有：（1）第一类样品各类油气组分三环萜烷含量明显低于第二类样品;（2）第一类样品 C20、C21和 C23三环萜烷含量比较接近,其分布模式为 C20＆lt;C21＆gt;C23,第二类样品这3个化合物含量差异较大,且分布模式为C20＆gt;C21＆gt;C23;（3）第一类样品伽马蜡烷和β-胡萝卜烷相对含量高于第二类样品;（4）第一类样品C27甾烷含量较低而C28甾烷含量较高,第二类样品则相反。可以推断第一类样品自由态组分、束缚态组分和油气包裹体均来源于二叠系烃源岩而第二类样品各类油气组分则来源于石炭系烃源岩。第一类样品油气包裹体成熟度明显高于自由态组分和束缚态组分,表明早期充注原油的成熟度高于晚期充注的原油,总体上各类油气组分成熟度位于生油高峰阶段（Ro 0.8%~1.1%）。第二类样品从自由态组分、束缚态组分至油气包裹体成熟度依次降低,表明早期充注原油的成熟度低于晚期充注的原油,总体上各类油气组分成熟度位于高-过成熟阶段（Ro〉1.25%）。第一类样品各类油气组分正构烷烃单体碳同位素组成相对较轻,第二类样品各类油气组分正构烷烃单体碳同位素组成有一定的差异,组成较轻者与第一类样品各类油气组分接近。     

The organic geochemistry of oil seeps from the Sierra de Perijá eastern foothills, Lake Maracaibo Basin, Venezuela

The organic geochemistry of samples from 11 oil seeps was studied. The samples were collected from the Cachirí area, Carboniferous Region of Tulé (Lake Maracaibo Basin, Venezuela), associated with the Tigre Fault. Biomarkers (hopanes, steranes, n-alkanes, acyclic isoprenoids, and aromatic steroids) were analyzed using gas chromatography-mass spectrometry (GC-MS). These hydrocarbon rich fluids have undergone biodegradation (2-6 on the Peters and Moldowan scale), showing both the partial loss of n-alkanes and the microbial degradation of isoprenoids and steranes. These oil seeps were generated from a mature calcareous source rock that was deposited in a marine paleoenvironment under reducing conditions. Moreover, these seeps are likely derived from the Cretaceous La Luna Formation that reached a level of maturity near the peak of oil generation in the study area. The nature of the studied oil seeps, together with the oil generation models reported for this rock unit in the study area, suggests that these oils are a mixture of an initially heavy, altered oil and a second migrated light crude oil resulting from two generation pulses from the La Luna Formation. Evidence for the presence of light oil trapped in the study area should prompt re-exploration in the northwestern coast of Lake Maracaibo in shallow reservoirs, previously discarded because they usually demonstrated a lack of light oils.     

Origin and quantitative source assessment of deep oils in the Tazhong Uplift,Tarim Basin

A large amount of deep oil has been discovered in the Tazhong Uplift, Tarim Basin whereas the oil source is still controversial. An integrated geochemical approach was utilized to unravel the characteristics, origin and alteration of the deep oils. This study showed that the Lower Cambrian oil from well ZS1C (

_1x) was featured by small or trace amounts of biomarkers, unusually high concentration of dibenzothiophenes (DBTs), high δ³⁴S of DBTs and high δ¹³C value of n-alkanes. These suggest a close genetic relationship with the Cambrian source rocks and TSR alteration. On the contrary, the Middle Cambrian oils from well ZS1 (

_2a) were characterized by low δ¹³C of n-alkanes and relatively high δ³⁴S of individual sulfur compounds and a general “V” shape of steranes, indicating a good genetic affinity with the Middle–Upper Ordovician source rocks. The middle Cambrian salt rock separating the oils was suggested to be one of the factors responsible for the differentiation. It was suggested that most of the deep oils in the Tazhong Uplift were mixed source based on biomarkers and carbon isotope, which contain TSR altered oil in varied degree. The percentage of the oils contributed by the Cambrian–Lower Ordovician was in the range of 19–100% (average 57%) controlled by several geological and geochemical events. Significant variations in the δ³⁴S values for individual compounds in the oils were observed suggesting a combination of different extent of TSR and thermal maturation alterations. The unusually high DBTs concentrations in the Tazhong-4 oilfield suggested as a result of mixing with the ZS1C oil (

_1x) and Lower Ordovician oils based on δ³⁴S values of DBT. This study will enhance our understanding of both deep and shallow oil sources in the Tazhong Uplift and clarify the formation mechanisms of the unusually high DBTs oils in the region.     

Application of sulfur and carbon isotopes to oil–source rock correlation: A case study from the Tazhong area,Tarim Basin,China

Up until now, it has been assumed that oil in the Palaeozoic reservoirs of the Tazhong Uplift was derived from Upper Ordovician source rocks. Oils recently produced from the Middle and Lower Cambrian in wells ZS1 and ZS5 provide clues concerning the source rocks of the oils in the Tazhong Uplift, Tarim Basin, China. For this study, molecular composition, bulk and individual n-alkane δ¹³C and individual alkyl-dibenzothiophene δ³⁴S values were determined for the potential source rocks and for oils from Cambrian and Ordovician reservoirs to determine the sources of the oils and to address whether δ¹³C and δ³⁴S values can be used effectively for oil–source rock correlation purposes. The ZS1 and ZS5 Cambrian oils, and six other oils from Ordovician reservoirs, were not significantly altered by TSR. The ZS1 oils and most of the other oils, have a “V” shape in the distribution of C₂₇–C₂₉ steranes, bulk and individual n-alkane δ¹³C values predominantly between −31‰ to −35‰ VPDB, and bulk and individual alkyldibenzothiophene δ³⁴S values between 15‰ to 23‰ VCDT. These characteristics are similar to those for some Cambrian source rocks with kerogen δ¹³C values between −34.1‰ and −35.3‰ and δ³⁴S values between 10.4‰ and 21.6‰. The oil produced from the Lower Ordovician in well YM2 has similar features to the ZS1 Cambrian oils. These new lines of evidence indicate that most of the oils in the Tazhong Uplift, contrary to previous interpretations, were probably derived from the Cambrian source rocks, and not from the Upper Ordovician. Conversely, the δ¹³C and δ³⁴S values of ZS1C Cambrian oils have been shown to shift to more positive values due to thermochemical sulfate reduction (TSR). Thus, δ¹³C and δ³⁴S values can be used as effective tools to demonstrate oil–source rock correlation, but only because there has been little or no TSR in this part of the section.     

塔里木盆地一类新海相原油的地球化学特征

对塔中52等井奥陶系储层产出的原油进行的分析结果表明,它们的三环萜烷系列较为特殊,主要表现为其相对丰度呈C19>C20>C21>C23>C24>C25>C26阶梯状的模式,C24四环萜烷异常丰富,且其丰度远高于C26三环萜烷,这一分布模式一般出现在淡水沼泽相和浅湖相沉积地层与原油中。在三萜烷分布特征上,其伽马蜡烷含量很低,甾烷系列和藿烷系列的分布与组成特征与该地区来源于中上奥陶统烃源岩的海相原油十分接近,同时它们的全油均具有轻的碳同位素组成,其δ13Ｃ值都小于-30‰,具有海相成因原油的特征。而塔中12井上奥陶统良里塔格组4-5段烃源岩中生物标志物的分析结果进一步证实了该类原油与那些富含宏观藻残片,且有机质类型偏腐殖型的上奥陶统海相烃源岩关系密切,是该地区油气勘探中值得关注的对象。      

Scarcity of the C30 sterane biomarker, 24-n-propylcholestane,in Lower Paleozoic marine paleoenvironments

24-n-Propylcholestane (24-npc), a C₃₀ sterane compound derived from sterol precursors which are the major sterol constituents of modern pelagophyte microalgae, occurs in certain Neoproterozoic rocks and oils and throughout the Phanerozoic rock record. This broad distribution leads 24-npc to be widely considered a reliable indicator of open to partially restricted marine depositional conditions for source rocks and oils. Here we report two significant hiatuses in the occurrences of 24-npc in the Lower Paleozoic marine rock record: the first in the Middle–Late Cambrian and the second in the Late Ordovician–early Silurian transition for a range of lithofacies (carbonates and siliciclastic rocks), organic carbon contents (both organic-lean and organic-rich), and paleoceanographic environments (shelf and deeper water marine settings) and observed offshore of two paleocontinents, Laurentia and Baltica. The Ordovician–Silurian gap is at least 9 million years, and possibly up to 20 million years, in duration. Robust older occurrences of 24-npc steranes in some Neoproterozoic rocks and oils suggest that oceanographic conditions in our intervals of Lower Paleozoic time were unfavorable for the proliferation of pelagophyte algae as phytoplankton. Caution should therefore be applied when interpreting a lacustrine versus marine depositional environmental setting for source rocks and oils in these intervals of Early Paleozoic time using lipid biomarker assemblages.     

Geochemical analysis of mixed oil in the Ordovician reservoir of the Halahatang Depression, Tarim Basin, China

In this study,12 crude oil samples were collected and analyzed from the Ordovician reservoir in the Halahatang Depression,Tarim Basin,China.Although the density of oil samples varies considerably,based on saturated hydrocarbon gas chromatographic(GC),saturated and aromatic hydrocarbon gas chromatographic-mass spectrometric(GC/MS) and stable carbon isotopic composition analyses,all the samples are interpreted to represent a single oil population with similar characteristics in a source bed or a source kitchen,organic facies and even in oil charge history.The co-existence of a full suite of n-alkanes and acyclic isoprenoids with UCM and 25-norhopanes in the crude oil samples indicates mixing of biodegraded oil with fresher non-biodegraded oil in the Ordovician reservoir.Moreover,according to the conversion diagram of double filling ratios for subsurface mixed crude oils,biodegraded/non-biodegraded oil ratios were determined as in the range from 58/42 to 4/96.Based on oil density and oil mix ratio,the oils can be divided into two groups:Group 1,with specific density>0.88(g/cm3) and oil mix ratio>1,occurring in the north of the Upper Ordovician Lianglitage and Sangtamu Formation pinchout lines,and Group 2,with specific density<0.88(g/cm3) and oil mix ratio<1,occurring in the south of the pinchout lines.Obviously,Group 2 oils with low densities and being dominated by non-biodegraded oils are better than Group 1 oils with respect to quality.It is suggested that more attention should be paid to the area in the south of the Upper Ordovician Lianglitage and Sangtamu Formation pinchout lines for further exploration.