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

According to the assemblage characteristics of saturated hydrocarbon biomarkers in crude oils and their geochemical implications, this study has proposed, for the first time, the criteria for the genetic classification of crude oils in the Tazhong area of the Tarim Basin, China. Crude oils from the area studied are classified as three genetic types: type-Ⅰ is characterized by the low contents of C29 norhopane, extremely abundant contents of gammacerane, low contents of rearranged sterane and relatively high contents of regular C28 sterane; the geochemical properties of type-Ⅱ crude oils are opposite to those of type-Ⅰ crude oils; the parameters for type-Ⅲ crude oils are intermediate between type-Ⅰ and type-Ⅱ. Results of oil correlation indicated that type-Ⅰ crude oils were derived from Cambrian-Lower Ordovician hydrocarbon source rocks, type-Ⅱ curde oils originated from Middle-Upper Ordovician hydrocarbon source rocks and type-Ⅲ crude oils are of mixed origin.     

Geochemical Characteristics of Crude Oils from Zao-V Oil Measures in Shenjiapu Oilfield

The geochemical characteristics of crude oils from Zao-V oil measures in the Shen-jiapu oilfield are systematically described in terms of the fractional composition of crude oils, GC characteristics of saturated hydrocarbon fraction of crude oils and the characteristics of their bio-markers. The deposifional environment, type and evolution of the biological source are also discussed. All pieces of evidence such as low saturated hydrocarbon fraction, high resin and asphalt, high isoprenoid alkane, weak odd-carbon number predominance ( CPI ranging from 1.23 to 1,29, OEP ranging from 1.14 to 1.16) and low sterane and terpane maturity parameters show these crude oils are immature oils. Low Pr/Ph ratios (0.66 -0.88) and high gammacer-ante/C31 hopane ratios ( 0.59 - 0.86 ) indicate the source rocks were formed in a slightly saline to brackish reducing lake depositional environment. Gas chromatographic characteristics of the saturated hydrocarbon fraction and the predominance of C30 hopane in terpane series and C29 sterane in sterane series indicate the biological source of the crude oils is composed mainly of bacterial and algal organic matter, and some algae are perhaps the main contributor of organic matter to the source rocks.     

The geochemical characteristics and origin of crude oils in the Kekeya Oilfield,Xinjiang, China

GC/MS and GC/MS/MS techniques were employed to describe the characteristics of biomarker assemblages in two sets of hydrocarbon source rocks, Jurassic and Permian, in southwestern Tarim, and the parameters for the classification of the two sets of hydrocarbon source rocks have been established. It is found that diahopane and C30-unknown terpane are abundant in Permian samples, the contents of diahopane in Jurassic samples are relatively low, and terpenoids have been detected in Jurassic samples but not in Permian source rock samples. Kekeya crude oils are abundant in diahopane and C30-unknown terpane. The results of fine oil-rock correlation indicated that Kekeya crude oils were derived mainly from the Permian hydrocarbon source rocks. However, a small amount of diterpenoid was detected in the crude oils, indicating that the Jurassic hydrocarbon source rocks also made a certain contribution to Kekeya crude oils.     

Geochemical characteristics of crude oils from Well Zheng-1 in the Junggar Basin, Xinjiang, China

Well Zheng-1 is located in the combined area of the central uplift and the north Tianshan piedmont depression in the Junggar Basin. Two oil-bearing beds are recognized at 4788–4797 m of the Lower Cretaceous Tugulu Formation (K1tg) and 4808.5–4812.5 m of the Lower Jurassic Sangonghe Formation (J1s). The geochemical characteristics of family composition, carbon isotopic composition, saturated hydrocarbons, sterane and terpane biomarkers and carotane of two crude oils are described in this paper. The results show that the geochemical characteristics of the two crude oils are basically similar to each other, indicating they were all derived mainly from the high mature, brine, algae-rich lake facies sediments. Oil-source correlation revealed that crude oils of the two beds were derived mainly from the source rocks of Permian and mixed by the oil derived from the source rocks of Jurassic and Triassic. This is consistent with the geological background with several sets of source rocks in the area studied.     

Reservoir geochemistry of the Tazhong Oilfield in the Tarim Basin, China, Part II. Migration, accumulation and mixing of crude oils

Measurements of the absolute and relative concentrations of nitrogen-containing compounds in crude oils from different reservoir strata (Ordovician, Silurian and Carboniferous) in the Tazhong region of the Tarim Basin, Xinjiang, China, showed that even though there are quite a number of factors affecting the distributional and compositional characteristics of neutral nitrogen-containing compounds in crude oils, the distributional and compositional characteristics of crude oils whose source conditions are approximate to one another are influenced mainly by the migration and fractionation effects in the process of formation of oil reservoirs. In addition, crude oils in the Tazhong region show obvious migration-fractionation effects in the vertical direction. Carboniferous crude oils are characterized by high migration parameters and low compound concentrations, just in contrast to Ordovician crude oils. This indicates that crude oils from shallow-level oil reservoirs were derived from those of deep-level oil reservoirs via faults, unconformable contact or carrier beds. Crude oils from the Tazhong region show some migration-fractionation effects in the lateral direction, but mixing of crude oils derived from different hydrocarbon source rocks in the process of formation of oil reservoirs made it more complicated the migration and accumulation of crude oils, as well as the formation of oil reservoirs.     

Geochemical Evidence for Coal and Carbonaceous Mudstone as the Possible Major Oil Source Rock in the Jurassic Turpan Basin,Northwest China

Petroleum geologists have debated whether the hydrocarbons from Jurassic coal measures are derived from the coals, carbonaceous mudstones or coal-measure mudstones in the Turpan Basin. Based on the geochemistry analysis of the 20 crude oils and 40 source rocks from the Turpan Basin, some data have been obtained as follows: carbon preference index and methylphenanthrene index of the Jurassic oils are 1.16–1.45 and 0.28–0.80, and the ααα C29 sterane 20S/(20S+20R) and C29 sterane ββ/(ββ+αα) are 0.44–0.51 and 0.4–0.54 respectively, which show the normal maturity of oils; the vitrinite reflectance of the source rocks from the Xishanyao to Badaowan Formations range from 0.47% to 0.97%, which indicate immature to mature thermal evolutionary stage and sufficient conditions for generating mass mature oil. The effect of hydrocarbon expulsion should be considered when studying the source of coal-derived oil by using Biomarkers. Biomarkers in the Jurassic oils from the basin are similar to those in the coals and carbonaceous mudstones, with a strong predominant content of pristane, relatively high ratio of C15/C16 sesquiterpenoids (>1), a relatively high content of low carbon number tricyclic terpanes and C24 tetracyclic terpane, little gammacerane and C29 Ts detected, an absolute predominant content of C29 sterane and a relatively high content of diasterane. However, the opposite characteristics are shown in mudstones, with an approximately equal content of pristane and phytane, relatively low ratio of C15/C16 sesquiterpenoids (<1), a relatively high content of high carbon number tricyclic terpanes and a low content of C24 tetracyclic terpane, peaks of gammacerane and C29 Ts detected obviously and an increasing C27 sterane content. All of these characteristics identify the coals and carbonaceous mudstones as the possible major oil source rocks in this area, and they were formed in the stronger oxidizing environment with shallower water than mudstones.     

Origin of the Silurian Crude Oils and Reservoir Formation Characteristics in the Tazhong Uplift

<正>The Silurian stratum in the Tazhong uplift is an important horizon for exploration because it preserves some features of the hydrocarbons produced from multi-stage tectonic evolution.For this reason,the study of the origin of the Silurian oils and their formation characteristics constitutes a major part in revealing the mechanisms for the composite hydrocarbon accumulation zone in the Tazhong area.Geochemical investigations indicate that the physical properties of the Silurian oils in Tazhong vary with belts and blocks,i.e.,heavy oils are distributed in the TZ47-15 well-block in the North Slope while normal and light oils in the No.Ⅰfault belt and the TZ16 well-block,which means that the oil properties are controlled by structural patterns.Most biomarkers in the Silurian oils are similar to that of the Mid-Upper Ordovician source rocks,suggesting a good genetic relationship. However,the compound specific isotope of n-alkanes in the oils and the chemical components of the hydrocarbons in fluid inclusions indicate that these oils are mixed oils derived from both the Mid-Upper Ordovician and the Cambrian-Lower Ordovician source rocks.Most Silurian oils have a record of secondary alterations like earlier biodegradation,including the occurrence of "UCM" humps in the total ion current(TIC) chromatogram of saturated and aromatic hydrocarbons and 25-norhopane in saturated hydrocarbons of the crude oils,and regular changes in the abundances of light and heavy components from the structural low to the structural high.The fact that the Silurian oils are enriched in chain alkanes,e.g.,n-alkanes and 25-norhopane,suggests that they were mixed oils of the earlier degraded oils with the later normal oils.It is suggested that the Silurian oils experienced at least three episodes of petroleum charging according to the composition and distribution as well as the maturity of reservoir crude oils and the oils in fluid inclusions.The migration and accumulation models of these oils in the TZ47-15 well-blocks,the No.Ⅰfault belt and the TZ16 well-block are different from but related to each other.The investigation of the origin of the mixed oils and the hydrocarbon migration and accumulation mechanisms in different charging periods is of great significance to petroleum exploration in this area.     

Charactristics of the Biomarkers from Nonmarine Crude Oils in China—A Review

The present paper deals with the biomarker characteristics of crude oils and source rocks from different environments(fresh,fresh-brackish and salt waters)of nonmarine depositional basins of different ages in China.Their characters are summarized as follows:1)Souce rocks and crude oils derived from fresh-water lacustrine facies have an odd/even predominance of n-alkanes and high pristine/phytane ratios.Oils from the fresh-water lacustrine facies differ from typical marine oils in the relative contents of total steranes and terpanes,the concentrations of hopanes and organic sul-phur compounds and the values of methylphenanthrene indices and C,H,S stable isotopes.2)The source rocks and crude oils derived from saline lacustrine facies possess an even/odd predominance of n-alkanes and high phytane/pristine ratios.There are also some differences between saline lacustrine oils and freshwater lacustrine oils in the concentrations of steranes,tricyclic terpanes and organic sulphur compounds,as well as in the values of methylphenanthrene indices and C,H,S stable isotopes.3)Oils derived from fresh-brackish water lake facies differ from oils from fresh-water lacustrine or samline lacustrine environments in respect of some biomarkers.According to the various distributions of these biomarkers,a number of geochemical parameters can be applied synthetically to differentiating and identifying the nature of original depositional environments of crude oils and source rocks and that of organisms-primary source materials present in those environments.     

Ruthenium-ion-catalyzed oxidation of asphaltenes and oil-source correlation of heavy oils from the Lunnan and Tahe oilfields in the Tarim Basin, NW China

The identification of marine source rocks in the Tarim Basin is debated vigorously. The intention of this paper is to investigate the asphahenes in heavy oils from the Lunnan and Tabe oilfields and Well TD2 with ruthenium-ions-catalyzed oxidation technique (RICO), in order to explore its role in oil-oil and oil-source correlations, The RICO products included n-alkanoic acids, α,ω-di-n-alkanoic acids, branched alkanoic acids, tricyclic terpanoic acids, hopanoic acids, gammacerane carboxylic acid , regular sterane carboxylic acids and 4-methylsterane carboxylic acids. The n-alkyl chains and biomarkers bounded on the asphaltenes were of unsusceptibility to biodegradation. The distribution and absolute concentrations of n-alkanoic acids in the RICO products of heavy oils from the Lunnan and Tabe oilfields are different from those of Well TD2. The biomarkers bounded on the asphahenes, especially steranes, have a distribution trend similar to that of the counterparts in saturates. The sterane carboxylic acids and 4-methylsterane carboxylic acids in the RICO products of heavy oils from the Lunnan and Tahe oilfields, dominated by C30 sterane and C31 4-methylsterane carboxylic acids, respectively, are significantly different from those of the heavy oils of Well TD2, whose dominating sterane and 4-methylsterane carboxylic acids are C28 sterane and C29 4-methylsterane acids, respectively. The RICO products of the asphaltenes further indicate that the Middle-Upper Ordovician may be the main source rocks for heavy oils from the Lunnan and Tabe oilfields.     

Geochemical characteristics of Ordovician crude oils in the northwest of the Tahe oil field,Tarim basin

Forty-six crude oil samples were selected from the Ordovician in the northwestern part of the Tahe oilfield for detailed molecular geochemical and isotopic analysis, including group compositions, carbonhydrogen isotopes and gas chroma-tograms of saturated hydrocarbons, as well as the characteristics of terpane, sterane and other biomarkers, indicating that crude oils are of the same origin from different districts in the Tahe oilfield and were derived from the same source kitchen (or oil source formation), i.e., mainly stemming from marine hydrocarbons. Detailed studies of oil physical properties of 25-honpane revealed that such oils have heavy or thick oil qualities due to biodegradation. Comprehensive assessment in terms of five maturity parameters shows that the oils from the Ordovician with Ro values varying from 0.80% to 1.59% are widely distributed in the northwest of the Tahe oilfield.     

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.     

Reservoir Geochemistry of the Kuche Petroleum System in the Tarim Basin, China

The geochemical characteristics of crude oils and reservoir core extracts from the Kuche petroleum system are described and studied systematically by means of various geochemical techniques and methods to acquire molecular information. The results suggest crude oils from the petroleum system can be divided into two groups: marine oils and non-marine oils. The former represents the dominant oils found in the area. Tar mats were firstly discovered and determined accurately in terrestrial oil and gas reservoirs, with Lower Tertiary sandstone reservoirs in the Yaha oilfield of the Tarim Basin. However, based on the ratio of 20S/(20S 20R)C29 sterane as a maturity parameter, lacustrine oils filled into the Tertiary reservoirs in the direction toward the western part of the petroleum system. In contrast, according to the fact that methylcyclohexane indices of eastern oils are greater than those of western oils, the location in which coalgenerated oils filled into the Tertiary reservoirs lies in the eastern part of the petroleum system.     

Origin of Oils in “Subtle pools” in the Dongying Depression, Bohai Bay Basin, China

Subtle traps or oil pools have become an important exploration play in the Dongying Depression, Bohai Bay Basin, east China. Despite recent successes in exploration, the formation mechanisms of subtle traps are still not well understood. The majority of subtle oil pools in the Dongying Depression are developed in the middle interval of the Es3 Member of the Paleogene Shahejie Formation with the subtle traps being primarily of lenticular basin-floor turbidite sands encompassed in mudstones. Oil in the subtle traps was previously thought to have migrated directly from the surrounding source rocks of the same formation （Es3）. Detailed geochemical investigation of 41 oils and 41 rock samples from the depression now indicates that the oils from the subtle traps cannot be correlated well with the surrounding Es3 source rocks, which are characterized by high Pr/ Ph （〉1）, low Gammacerane/C30hopane, representing a freshwater lacustrine setting. In contrast the oils features low Pr/Ph （〈1） and relatively high Gammacerane content, showing a genetic affinity with the underlying Es4 source rocks, which also have the same qualities, indicating a brackish lacustrine setting. Oils in the Es3 subtle traps are probably derived from mixed sources with the contribution from the upper Es4 source rocks predominating. Therefore unconventional oil migration and accumulation mechanisms need to be invoked to explain the pooling of oils from the ES4 source rocks, which probably came through a thick low interval of the Es3 source rocks with no apparent structural or stratigraphic pathways. We suggest that the subtle oil migration pathway probably plays an important role here. This finding may have significant implications for future exploration and the remaining resource evaluation in the Dongying Depression.     

Distribution and Significance of Carbazole Compounds in Palaeozoic Oils from the Tazhong Uplift, Tarim

Carbazole compounds in crude oils from the Tazhong uplift of the Tarim basin have been fractionated and detected and successfully used to study petroleum migration and trace source rocks in the study area. Alkylcarbazoles have been found in large amounts in the oil samples analyzed and alkylbenzocarbazoles detected in a small concentration only in part of the samples, but alkyldibenzocarbazoles have not been found in oils. Based on the distribution of G1, G2 and G3 of C2-alkylcarbazoles, the ratio of C3-carbazoles to C2-carbazoles and the relative amounts of alkylcarbazoles and alkylbenzocarbazoles, one can know that the vertical oil migration in the Tazhong uplift is generally from below upward, i.e. from the Ordovician through the Silurian to the Carboniferous. Evidently, source rocks in the uplift should be lower Palaeozoic strata (Ordovician and Cambrian). This study shows that carbazoles are of great importance in the study of petroleum migration and source rocks.     

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.     

The evolution characteristics and fractionation mechanism of carbon isotopes in the process of “multi-stage hydrocarbon generation”

The Jiyang Sag and the Liaohe Basin are the two important areas where immature oil resources are distributed in China. From these two areas immature-low mature to mature oil samples were collected for carbon isotopic analysis. The extracts of source rocks are dominant in the Jiyang Sag while crude oils are dominant in the Liaohe Basin. The maturity index, Ro, for source rocks varies from 0.25%(immature) to 0.65% (mature). Studies have shown that within this range of Ro values the extracts of source rocks and crude oils, as well as their fraction components, have experienced observable carbon isotope fractionation. The carbon isotopic values tend to increase with burial depth, the oils become from immature-low mature to mature, and the rules of evolution of oils show a three-stage evolution pattern, i. e. ,light→heavy→light→heavy oils. Such variation trend seems to be related to the occurrence of two hydrocarbon-generating processes and the main hydrocarbon-forming materials being correspondingly non-hydrocarbons and possessing MAB characteristics, lower thermodynamic effects and other factors. In the process towards the mature stage, with increasing thermodynamic effects, the thermal degradation of kerogens into oil has become the leading factor, and correspondingly the bond-breaking ratio of ^12C-^13C also increases,making the relatively ^12C-rich materials at the low mature stage evolve again towards ^13C enrichment.     

The origin and accumulation model of crude oils from oil reservoirs Chang 9 and Chang 10 in the Yanchang Formation of the Ordos Basin

In the lower parts of oil reservoirs Chang 9 and Chang 10 of the Yanchang Formation are oil-bearing layers newly found in oil exploration in the Ordos Basin.Based on GC,GC-MS analyses of saturated hydrocarbons from crude oils and source rocks,reservoir fluid inclusions and BasinMod,the origin of crude oils,accumulation period and accumulation models are discussed in combination with other petroleum geology data in this paper.The result shows that(1) there are two different types of crude oils in oil reservoir Chang 9 in the Longdong and Jiyuan regions:crude oils of typeⅠ(Well D86,Well A44,Well A75,Well B227,Well X62 and Well Z150) are mainly de-rived from the Chang 7 source rocks(including mudstones and shales) and distributed in the Jiyuan and Longdong regions;those of typeⅡ(Well Z14 and Well Y427),are distributed in the Longdong region,which are derived from the Chang 9 source rocks.Crude oils from oil reservoir Chang 10 in the Shanbei region are mainly derived from the Chang-9 source rocks;(2) there are two phases of hydrocarbon filling in oil reservoir Chang 9 in the Jiyuan and Longdong regions and oil reservoir Chang 10 in the Shanbei region:The first phase started at the early stage of J2z.The process of hydrocarbon filling was discontinuous in the Late Jurassic,because of the tectonic-thermal event in the Ordos Basin.The second phase was the main accumulation period,and hydrocarbons began to accumulate from the late stage of J2a to the middle-late of K1,mainly at the middle-late stage of K1;(3) there exist two types of accu-mulation models in oil reservoirs Chang 9 and Chang 10 of the Yanchang Formation:source rocks of the reservoirs in oil reservoir Chang 9 in the Jiyuan region and oil reservoir Chang 10 in the Shanbei region,the mixed type of reservoirs on the lateral side of source rocks and source rocks of the reservoirs in oil reservoir Chang 9 in the Long-dong region.     

Thermogravimetry-differential thermal analysis coupled with chromatography as a thermal simulation experimental method and its application to gaseous hydrocarbons from different source rocks

In this paper a thermogravimetry-differential thermal analysis method coupled with chromatography （TG-DTA-GC） has been adopted to simulate the generation of gaseous hydrocarbons from different hydrocarbon source rocks such as coals, mudstones, and carbonate rocks with different maturities. The temperature programming for thermal simulation experiment is 20℃/min from ambient temperature to 700℃. As viewed from the quantities and composition of generated gaseous hydrocarbons at different temperatures, it is shown that low-mature coal has experienced the strongest exothermic reaction and the highest loss of weight in which the first exothermic peak is relatively low. Low-mature coal samples have stronger capability of generating gaseous hydrocarbons than high-mature samples. The amounts and composition of gaseous hydrocarbons generated are closely related not only to the abundance of organic carbon in source rocks, but also to the type of kerogen in the source rocks, and their thermal maturity. In the present highly mature and over-mature rock samples organic carbon, probably, has already been exhausted, so the production of gaseous hydrocarbons in large amounts is impossible. The contents of heavy components in gaseous hydrocarbons from the source rocks containing type- Ⅰ and - Ⅱ kerogens are generally high ; those of light components such as methane and ethane in gaseous hydrocarbons from the source rocks with Ⅲ-type kerogens are high as well. In the course of thermal simulation of carbonate rock samples, large amounts of gaseous hydrocarbons were produced in a high temperature range.     

Evidence of Oil Sources and Migration in Triassic-Jurassic Reservoirs in the South Tianhuan Depression of the Ordos Basin,China Based on Analysis of Biomarkers and Nitrogen-Bearing Compounds

Abstract: The Ordos Basin is an important intracontinental sedimentary basin in western China for its abundant Mesozoic crude oil resources. The southern part of the Tianhuan Depression is located in the southwestern marginal area of this Basin, in which the Jurassic and Triassic Chang-3 are the main oil-bearing strata. Currently, no consensus has been reached regarding oil source and oil migration in the area, and an assessment of oil accumulation patterns is thus challenging. In this paper, the oil source, migration direction, charging site and migration pathways are investigated through analysis of pyrrolic nitrogen compounds and hydrocarbon biomarkers. Oil source correlations show that the oils trapped in the Jurassic and Chang-3 reservoirs were derived from the Triassic Chang-7 source rocks. The Jurassic and Chang-3 crude oils both underwent distinct vertical migration from deep to shallow strata, indicating that the oils generated by Chang-7 source rocks may have migrated upward to the shallower Chang-3 and Jurassic strata under abnormally high pressures, to accumulate along the sand bodies of the ancient rivers and the unconformity surface. The charging direction of the Jurassic and Chang-3 crude oils is primarily derived from Mubo, Chenhao, and Shangliyuan, which are located northeast of the southern Tianhuan Depression, with oils moving toward the west, southwest, and south. The results show that an integration of biomarker and nitrogen-bearing compound analyses can provide useful information about oil source, migration, and accumulation.     

Origin of Crude Oil in the Lunnan Region,Tarim Basin

<正>The oil source of the Tarim Basin has been controversial over a long time.This study characterizes the crude oil and investigates the oil sources in the Lunnan region,Tarim Basin by adopting compound specific isotopes of n-alkanes and biomarkers approaches.Although the crude oil has a good correlation with the Middle-Upper Ordovician(O_(2+3)) source rocks and a poor correlation with the Cambrian-Lower Ordovician((?)-O_1) based on biomarkers,theδ~(13)C data of n-alkanes of the Lunnan oils show an intermediate value between(?)-O_1 and O_(2+3) genetic affinity oils,which suggests that the Lunnan oils are actually of an extensively mixed source.A quantification of oil mixing was performed and the results show that the contribution of the Cambrian-Lower Ordovician source rocks ranges from 11%to 70%(averaging 36%),slightly less than that of the Tazhong uplift.It is suggested that the inconsistency between the biomarkers andδ~(13)C in determining the oil sources in the Lunnan Region results from multiple petroleum charge episodes with different chemical components in one or more episode(s) and different sources.The widespread marine mixed-source oil in the basin indicates that significant petroleum potential in deep horizons is possible.To unravel hydrocarbons accumulation mechanisms for the Lunnan oils is crucial to further petroleum exploration and exploitation in the region.