Geochemical Characteristics of Different Kinds of Crude Oils in the Tarim Basin,Northwest China

Based on the compositions and distributions of biomarkers in thirty-five representative oil samples, oils from the Tarim Basin of northwestern China are mainly divided into two oil families. One oil family contains relatively low amounts of C₁₅-C₂₀ isoprenoid hydrocarbons and shows pristane predominance with Pr/Ph ratios ranging from 1.50 to 3.00. The GC/MS analytical data of these oils show the occurrence of abundant hopanes, and low concentrations of steranes and tricyclic terpanes with hopanes/steranes ratios from 6.25 to 12.24 and tricyclic terpanes/hopanes ratios from 0.03 to 0.24. These oils contain low drimane relative to homodrimane (C₁₅/C₁₆ < 1.0) and abundant rearranged bicyclanes in bicyclic sesquiterpanes. They are dominated by low carbon number (C₁₉-C₂₁) compounds in the tricyclic terpanes, and are rich in rearranged hopanes, C₂₉Ts and an unknown C₃₀ compound in pentacyclic triterpanes. These geochemical characteristics suggest that the oils were generated mainly from terrigenous organic matter. The other oil family shows remarkably different biomarker compositions and distributions. The oils revealed Pr/Ph ratios of about 1.0, high drimane/homodrimane ratios (>1.0), low hopanes/steranes ratios (0.65–2.50), high tricyclic terpanes/hopanes ratios (0.30–2.00) and a dominant peak at C₂₃ in tricyclic tepanes, suggesting a marine organic origin. Oil-source rock correlation indicates that these two oil families seem to have been derived from Mesozoic Jurassic-Triassic terrestrial source rocks (shales and coal seams) and Lower Paleozoic Ordovician-Cambrian marine source rocks, respectively.     

Molecular composition and organic petrographic characterization of Madbi source rocks from the Kharir Oilfield of the Masila Basin (Yemen): palaeoenvironmental and maturity interpretation

The upper part of Madbi Formation organic-rich shale is considered an important regional source rock in the Masila Basin, Yemen. Ten cutting samples from this Upper Jurassic organic-rich shale were collected from wells drilled in the Kharir Oilfield, Masila Basin in order to geochemically assess the type of organic matter, thermal maturity and depositional environment conditions. Results reveal that Upper Jurassic organic-rich shale samples contain high organic matter more than 2.0 wt.% TOC and have very good to excellent hydrocarbon potential. Marine algae organic matter is the main source input for the Upper Jurassic shale sequence studied. This has been identified from organic petrographic characteristics and from the n-alkane distributions, which dominated by n-C₁₄-n-C₂₀ alkanes. This is supported by the high value of the biomarker sterane/hopane ratio that approaches unity, as well as the relatively high C₂₇ sterane concentrations. A mainly suboxic depositional environment is inferred from pr/ph ratios (1.75–2.38). This is further supported by relatively high homohopane value, which is dominated by low carbon numbers and decrease towards the C₃₅ homohopane. The concentrations of C₃₅ homohopane are very low. The depositional environment conditions are confirmed by some petrographic characteristics (e.g. palynofacies). Detailed palynofacies analysis of Madbi shales shows that the Madbi shale formation is characterised by a mix of amorphous organic matter, dinoflagellates cysts and phytoclasts, representing a suboxic, open marine setting. The Upper Jurassic marine shale sequence in the Masila Basin is thermally mature for hydrocarbon generation as indicated by biomarker thermal maturity parameters. The 22 S/22 S + 22R C₃₂ homohopane has reached equilibrium, with values range from 0.58 to 0.62 which suggest that the Upper Jurassic shales are thermally mature and that the oil window has been reached. 20 S/(20 S + 20R) and ββ/(ββ + αα) C₂₉ sterane ratios suggest a similar interpretation, as do the moretane/hopane ratio. This is supported by vitrinite reflectance data ranging from 0.74% to 0.90%Ro and thermal alteration of pollen and spore. The thermal alteration index value is around 2.6–3.0, corresponding to a palaeotemperature range of 60–120°C. These are the optimum oil-generating strata. On the basis of this study, the Madbi source rock was deposited under suboxic conditions in an open marine environment and this source rock is still within the oil window maturity range.     

北部湾盆地乌石凹陷流沙港组烃源岩地球化学特征

以大量烃源岩样品地球化学分析为基础,综合应用钻井以及地震等地质资料,从有机质丰度、类型、成熟度、母质来源、沉积环境以及生物标志物等方面对乌石凹陷流沙港组烃源岩地球化学特征进行了研究。研究结果表明,流三段发育于还原湖泊环境,有机质丰度较高,类型主要为Ⅱ1-Ⅱ2型;流二段沉积于弱还原至还原环境,有机质丰度最高,尤其是流二段油页岩基本为优质烃源岩,有机质类型以Ⅱ1-Ⅱ2型为主;流一段沉积于偏氧化性湖泊环境,有机质丰度最低,类型以Ⅱ2-Ⅲ型为主。依据烃源岩综合演化剖面,将其热演化过程划分为未成熟、低成熟、成熟和高成熟4个阶段,确定了烃源岩生烃门限深度为2900m,生烃高峰位于3600m。低伽马蜡烷指数显示流沙港组沉积时期水体为淡水环境,丰富的4-甲基甾烷表明水生低等植物对烃源岩母质来源有较大贡献。     

Terpenoid biomarker distributions in low maturity crude oils

Twenty-seven heavy crude oils of diverse origin were geochemically assessed with respect to both bulk and mlecular composition for the purpose of identifying and quanttfying valid biomarker parameters for low maturity oils. The low thermal maturity level of many of these oils is evident from the bulk and alipathic chromatographic data, and oil sourced from both marine and terrigenous organic matter are represented. Selective metastable ion monitoring (SMIM) was employed to measure separately the distribution of C₂₇, C₂₈, and C₂₉ sterane isomers. The useful maturity indicators include the C₂₉ 5α(H) 20S/20R ratio, the relative quantity of the biological sterane configuration in each of the total normal C₂₇, C₂₈, and C₂₉ steranes, and the rearranged to normal sterane ratio. In addition, C₂₇ rearranged steran es appear to form at a faster rate than C₂₈ or C₂₉ rearranged steranes. However, the isomerization of the C₂₇ biological component appears to occur at a slower rate than the C₂₉ counterpart suggesting that the former may be used as a maturity parameter at higher levels of thermal maturation. In the triterpane distributions, the C₂₇ trisnorhopane isomers and the moretane to hopane ratios appear to be both source and maturity related and cannot be used as successful maturity parameters in oils unless they share a common source. The C₃₁₊ hopane 22S/22R equilibrium ratio appears to increase with increasing molecular weight (C₃₁–C₃₄).     

Organic matter geochemistry and petrography of Late Cretaceous (Cenomanian-Turonian) organic-rich shales from the Belle Fourche and Second White Specks formations,west-central Alberta,Canada

Organic-rich mudstones with up to 10 wt% TOC from the upper portion of the Belle Fourche Formation and the lower part of the Second White Specks Formation in the Western Canada Sedimentary Basin were evaluated as source rocks. Both geochemistry and organic petrography indicate an open marine paleoenvironment with deposition of Type II kerogen based on the predominance of marine alginite and amorphous organic matter (OM), limited amounts of terrigenous vitrinite and inertinite macerals, the presence of marine fossils, and the low ratio of TOC to total sulfur (∼1.26). The prevalence of short-chain n-alkanes (n-C₁₃ to n-C₁₉), a predominance of C₂₈ αββ(H)-20S steranes, and small concentrations of oleanane confirm the dominantly algal and planktonic origin of OM. Alternating oxic to anoxic paleoenvironmental sedimentary conditions are proposed based on common bioturbation, abundant inoceramid prisms, and good organic richness. Biomarker distributions are consistent with intermittent anoxia, without unequivocal evidence for water column stratification or hypersalinity. The thermal maturity measured in seven sediment cores by different methods consistently indicates a westward increase in maturity according to vitrinite reflectance, T_max, and hopane and sterane biomarkers. Two cores are thermally immature (∼0.42 %Ro), one is early mature (∼0.65 %Ro), and four cores are within the oil window (∼0.78 to 0.89 %Ro). All thermally mature cores retain good to very good hydrocarbon potential (248 mg HC/g rock) and are dominantly oil-prone and minor gas-prone based on their maceral compositions. The upper Belle Fourche and lower Second White Specks Formations represent potential targets for unconventional light shale oil production.     

Evolution of polycyclic alkanes in the Espirito Santo Basin

The distributions of polycyclic alkanes were monitored in a Neocomian sequence (well 1-ESS-34) from the Espirito Santo Basin, southeast Brazil. The profiles included, apart from regular hopanes, significant concentrations of 18α(H), 28,30-bisnorhopane and subordinate amounts of gammacerane. Sterane concentrations, normally with hopane/sterane <5, were compatible with other geochemical data indicating a predominantly planktonic/microbial source of the deposited organic matter. Sample maturities ranged from very immature to the onset of oil generation, allowing biomarker distributions to be followed along a broad maturation range. The ability of certain molecular ratios (e.g. C₂₇ 17α(H)/17β(H)hopanes) to reflect a maturity sequence with depth in the closely-spaced strata of the immature upper levels (Jiquiá Stage) showed the value of molecular techniques over classical geochemical methods (e.g. vitrinite reflectance) for the study of immature sequences. The presence in the oils of southern Espirito Santo of 28,30-bisnorhopane, gammacerane and methyl steranes in similar concentrations as in extracts of the deepest levels of the 1-ESS-34 well qualify the Jiquia Stage as the probable source rock of oils accumulated in the basin.     

羌塘盆地昂达尔错地区油苗生物标志化合物特征及其指示意义

本文以油苗抽提物的饱和烃色谱-质谱分析为基础,剖析了羌塘盆地昂达尔错地区油苗的生物标志化合物特征,探讨了其沉积环境和成熟度指示意义。昂达尔错油苗含有丰富的正构烷烃、类异戊二烯烃、萜类化合物和甾类化合物。正构烷烃轻烃组分含量较高,无明显的奇偶优势。Pr/Ph比小于1,显示一定的植烷优势。萜类化合物中五环三萜烷相对含量最高,四环萜烷含量最少,并有一定的伽马蜡烷和升藿烷分布。规则甾烷相对含量为C27C29C28,呈不对称"V"字形分布,异构化成熟度参数值较高,且4-甲基甾烷化合物普遍存在。昂达尔错油苗母源岩沉积于具有较高盐度的还原环境,以藻类、菌类等海相有机质输入为主,油苗有机质处于成熟阶段。     

Potential source rocks, organic geochemistry and thermal maturation in the southern depocenter (Kipourio–Grevena) of the Mesohellenic Basin, central Greece

The study area is the southern depocenter (depth > 4200 m) of the Mesohellenic Basin which extends between Kipourio and Grevena, central Greece. The Mesohellenic Basin is a Middle-Tertiary intramontane basin developed within the Hellenide orogen. Previous studies have focused on the depositional environments, configuration and hydrocarbon potential of the basin. In this paper we present additional geochemical and petrographic data from outcrop samples of the basin's southern depocenter, which is considered the most promising area, in terms of hydrocarbon prospectivity. A total number of thirty six samples were analysed: Rock-Eval pyrolysis, maceral analysis, vitrinite reflectance and thermal alteration index, bitumens extraction, liquid chromatography, and GC-MS. The samples were collected from deltaic deposits and submarine fan sediments of Late Eocene to Late Oligocene age. The TOC values of the analysed samples range between rich and very rich and the organic matter consists mainly of type III kerogen and the organic matter consider to be predominately gas prone. The thermal maturity assessed from T_max and vitrinite reflectance shows an immature stage of the organic matter along with the presence of layers having reached the very early mature stage. Vitrinite reflectance measurements and maturity calculations (applying the Lopatin modeling), reveal that the lower part of the depocenter sediments falls within the ‘oil window’. The extractable organic matter (EOM) (mg bitumens/g TOC) indicate the existence of samples (from deltaic deposits) with high ratio of transformation (EOM) (> 100 mg bitumen/g TOC). The GC and GC-MS analyses of the biomarkers indicate mainly the occurrence of terrestrial organic matter reflecting oxidizing conditions and both immature and very early mature stages. The results of the Rock-Eval pyrolysis and the distribution of the isoprenoids support the assumption of the input of an organic matter mixture.     

Sedimentary organic matter and source rock potential of the Paleocene Aaliji Formation in Qumar Oil Field,NE Iraq

The organic matter content of the Paleocene Aaliji Formation has been studied from the Qm-1 well in the Qumar Oil Field, NE Iraq. A palynofacies analysis revealed the obvious domination of amorphous organic matter (AOM) in the samples studied in addition to the different ratios of palynomorphs, phytoclasts and opaque organic material. The deposition of the various percentages of organic matter components and types of palynomorphs appear to have occurred in a proximal suboxic–anoxic basinal environment. The samples analysed showed relatively low percentages of total organic carbon, indicating a generally poor source rock. The thermal alteration index for the palynomorphs (dinoflagellates) observed and identified ranged between 2, 2+ and 3?, indicating an immature to early stage of maturity for the section studied. No clear differentiation between the stages of maturity within the section was identified. The reflectance measured for a few vitrinite particles at a depth of 2,900 m showed uncertainly of 0.46 % Ro, indicating a still thermally immature stage. The values obtained from pyrolysis analysis also supported the formation being in the early stages of maturity, i.e. an immature condition, with T _max values between 416 and 435 °C. The quality of the organic matter examined and analysed appeared to be mostly type III gas-prone kerogen, as discerned from the hydrogen index, oxygen index and other pyrolysis parameters. The parameters obtained and calculated from gas chromatography analysis performed on a sample at a depth of 2,900 m found marginally mature marine source organic matter.     

Geochemical characteristics of Tertiary oils derived from siliceous sources in Japan, Russia and U.S.A.

Siliceous sourced Tertiary oils from the Circum-Pacific area of Japan, Russia and the U.S.A. have a heavy carbon isotope composition, monomodal n-alkane distributions, and nearly identical regular sterane compositions with a predominance of C₂₇ homologues. These are consistent with open marine depositional environments dominated by diatomaceous organic matter. However, a number of alkane and biomarker parameters such as Pr/Ph, CPI, relative concentration of 28,30-bisnorhopane, and the C₃₅/C₃₄ homohopane ratio indicate more oxic depositional environments for the source rocks of Japan and Russia. In contrast to the California Monterey Formation sourced oils, petroleums with low maturity levels from the North Sakhalin basin, Russia and the Akita basin, Japan have lower concentrations of asphaltenes and sulphur and are characterized by higher API gravities. A correlation of extractable organic matter from source rocks vs the least matured petroleums demonstrates that oil expulsion in siliceous shales of the Akita basin occurs at a maturity level corresponding to R_o≥0.65%, which is in the range of the conventional oil window (R_o = 0.6−1.1%).     

Hydrocarbon source potential and depositional environment of the Surma Group shales of Bengal basin,Bangladesh

Surma Group is the most important geological unit of Bengal basin, Bangladesh, because petroleum resources occur within this group. It is mainly composed of alternation of shale and sandstone and the shale fraction has long been considered as source rocks and the sandstone fraction as reservoir. These source and reservoir rocks have been studied by different authors by different approach but none of them adopted organic geochemistry and organic petrology as a means of study of source rock and their possible depositional environment. A total of thirty shale core samples have been collected from eight different gas fields to fulfill the short coming. The collected samples have been subjected to Source Rock Analysis (SRA) and/or Rock-Eval (RE) followed by pyrolysis gas chromatography (PyGC), gas chromatography mass spectrometry (GCMS), elemental analysis (EA) and organic petrological study such as vitrinite reflectance measurement and maceral analysis. The analyzed organic matter extracted from the shales of Surma Group consists mainly of Type III along with some Type II kerogen. The studied shales are mostly organically lean (TOC ±1%) and the extracted organic matter is fair to moderate. Based on these results, the analyzed shales have been ranked as poor (mostly) to fair quality source rock. The organic matter of the analyzed shale samples is thermally immature to early mature for hydrocarbon generation considering their T_max and measured mean vitrinite reflectance values. The hopane 22S/(22S + 22R), moretane/hopane ratio and sterane parameters are also in good agreement with these thermal maturity assessments. The predominance of odd carbons over even carbons (most common) and/or even carbons over odd carbon numbered n-alkanes, moderate Pr/Ph ratio, low to high Tm/Ts ratio, comparative abundance of sterane C₂₉ (i.e., C₂₉ >C₂₇>C₂₈), Pr/nC₁₇ — Ph/nC₁₈ values, C/S ratio and dominance of vitrinite macerals group with the presence of liptinite macerals demonstrate that the organic matter has derived mainly from terrestrial inputs with an insignificant contribution from the marine sources. The condition of deposition alternates from oxic to anoxic.     

Properties of gases and petroleum liquids derived from terrestrial kerogen at various maturation levels

This study deals mainly with shale-sandstone series in which the disseminated kerogen is mostly composed of land-derived debris. Organic matter was characterized by microscopic and chemical techniques. The kerogen maturity was assessed by microscopic studies, mainly by means of vitrinite reflectance measurements. The chemical properties of shale kerogens and of oil and gas shows, were examined in several sedimentary basins in different parts of the world.Oil and gas properties were tentatively interpreted in terms of maturity, using a comparison of oil properties with the kerogen features of shales interbedded in the impregnated sandstone reservoirs. The synthesis is documented in several case histories including those from New Zealand, Colombia, Australia, Indonesia.In low maturity stages (immature zone), dry gas with minor condensate is observed, whereas in higher maturity levels (oil window), wet gas with high paraffinic crudes is generally recorded. Pristane to n-C₁₇ ratios allow a distinction to be made between immature (often > 1.0) and mature (< 1.0) condensates, i.e. intensively cracked crude oils. I-C₄ to n-C₄ ratios enable an adequate discrimination to be made between gases produced with immature condensates ($\text{i-C}{}_4\text{n-C}{}_4\text{> 0.80}$) and those produced with high wax crudes or mature condensates ($\text{i-C}{}_4\text{n-C}{}_4\text{< 0.80}$). Shallow depth condensates and their related gases have been identified as immature fluids. Properties of some mature condensates are given as references.This study offers a maturity framework as a guide for oil and gas show prediction in shale-sandstone sequences containing land-derived kerogen.     

Carbon isotopic characteristics of hydrocarbon gases from coal-measure source rocks——A thermal simulation experiment

1 IntroductionThe compositional characteristics of natural gasesand their carbon isotopic composition are controlledmainly by the type of organic matter and the degree ofits thermal evolution in the source rocks (Song Yan,1995; Xu Yongchang et al., 1994; …     

Molecular evidence for recent land use change from a swampy environment to a pond (Lorraine,France)

Evidence of a changing environment in the catchment area of a small pond (Lansquenet, Lorraine, France) during the last millennium was provided by coupling palynology data and organic ratios [terrestrial to aquatic ratio: TAR_(HC); the C₂₉ vs. C₂₇ sterols, C₂₉/C_27(ST); average chain length, ACL and aquatic/macrophytes ratio, P_aq]. The depositional and alteration conditions of organic matter (OM) were provided by the 5α(H)-stanols/Δ⁵-sterols ratio. This study shows that quite simple organic ratios clearly allow identification of the different evolution steps of a watershed, from a swamp to the progressive settlement of a pond. During the swampy period, the C/N_(at) (ca. 11.1), TAR_(HC) (ca. 36.8)_, C₂₉/C_27(ST) (ca. 8.7) and P_aq (<0.3) show a high contribution of allochthonous OM associated with the presence of particular aquatic and mesohygrophilous species. A woody level dated to the XIIIth century reflects the period of the settlement of the pond in a more oxic environment [5α(H)-stanol/Δ⁵sterol ratio <0.2]. Then, the progressive increase in aquatic input and the development of submerged and floating macrophytes are evidenced by low C/N_(at) (<7.6), TAR_(HC) (ca. 2.7), C₂₉/C_27(ST) (ca. 2) and high P_aq (ca. 0.5), suggesting an increase in the water level and eutrophication of the pond. A second oxic event [5α(H)-stanols/Δ⁵sterols ratio averaging 0.16], associated with extensive OM degradation, corresponds to the most recent dredging in 2002 and reflects an important disturbance of sediments due to pond management.     

Study on the restitution coefficient of original total organic carbon for high mature marine source rocks

On the basis of the results of hydropyrolysis simulations for about 90 different types of immature to mature source rocks selected from about 5 000 marine source rocks in China, along with the natural thermal evolution profiles, the following conclusions were obtained. (1) Total organic carbon (TOC) content of excellent marine source rocks does not change obviously when Ro < 0.8% or Ro > 1.3%, and the residual TOC content is decreasing gradually with the maturity increase at 0.8% < Ro < 1.3%. (2) At the high-post mature stage (Ro > 1.3%), the decreased maximums of residual TOC content for the kerogen of sapropel (I), II₁, and II are usually 40%, 32% and 24%, respectively, and their TOC restitution coefficient is, respectively, 1.68, 1.48 and 1.32. (3) Both the TOC decreasing amplitude and the restitution coefficient decrease gradually with the decrease of TOC content for the source rocks with low organic matter abundance (usually 0.3% < TOC < 1.0%). The TOC restitution coefficients are, respectively, 1.20 and 1.0, when 0.3% < TOC < 0.5% and TOC < 0.3%. (4) TOC_res of solid bitumen and shale with high organic matter abundance (TOC > 30%) also require no restitution at the high mature stage. Such kind of TOC restitution is further supported by the coincidence of the decrease of residual TOC with the decreasing of S₁ + S₂ and the increasing of hydrocarbon quantity during the experimental simulation of hydrocarbon generation and expulsion for marine source rocks (0.3% < TOC < 30%) in natural thermal evolution profiles. __________ Translated from Earth Science—Journal of China University of Geosciences, 2007, 32(6): 853–860 [译自: 地球科学—中国地质大学学报]     

Organic inclusions as an indicator of oil/gas potential assessment of carbonate reservoir beds

Organic inclusions could be formed at the stages of either primary or secondary migration of hydrocarbons so long as mineral crystallization or recrystallization takes place in the sediments, presenting a direct indicator of oil/gas evolution, migration and abundance. Based on the study of organic inclusions in carbonate-type reservoir beds of commercial importance from North China, Xingjing, North Jiangsu, Jianghan, Sichuan and Guizhou in China, many inclusion parameters for oil/gas potential assessment of carbonate reservoir beds are summarized in this paper, including: 1) Types of organic inclusion: Commercially important oil beds are characterized by inclusions consisting of either pure liquid hydrocarbons or liquid plus minor gaseous hydrocarbons, while commercially important gas reservoirs are characterized by inclusions consisting of either pure gaseous hydrocarbons or gas plus minor liquid hydrocarbons. 2) Quantity of organic inclusions: The number of organic inclusions in commercially important oil/gas reservoirs is over 60% of the total inclusion percentage. 3) Temperature of saline inclusions: The homogenization temperatures of contemporaneous saline inclusions in oil reservoirs range from 91–161 °C, while in gas reservoirs from 150–250 °C). 4) Inclusion composition: In commercially important oil reservoirs, C₁/C₂=2−10, C₁/C₃=2−4, C₁/C₄=2−21, (C₂−C₄)/(C₁−C₄)(%)>20, (CH₄+CO+H₂)/CO₂ (molecules/g)=0.5−1.0, and in C₂−C₃−nC₄ triangle diagram there should be an upside-down triangle with the apex within the ellipse, while in commercial gas reservoirs, C₁/C₂=10−35, C₁/C₃=14−82, C₁/C₄=21−200, (C₂−C₄)/(C₁−C₄)(%)<20, (CH₄+CO+H₂)/CO₂>1, and there would be an upright triangle with the apex within the ellipse. The above-mentioned parameters have been used to evaluate a number of other unknown wells or regions and the results are very satisfactory. It is valid to use organic inclusions as an indicator to assess the oil/gas potential during oil/gas exploration and prospecting. This approach is effective, economic, rapid, and easy to popularize.     

Geochemistry of source rocks in the Es<Subscript>3</Subscript> and Es<Subscript>4</Subscript> members in the Shahejie Formation of the Western Depression,Liaohe Oilfield,China

The Western Depression of the Liaohe Basin is the major exploration area of the Liaohe Oilfield, and its main source rocks consist of the third and fourth members of the Shahejie Formation (Es3 and Es4). These source rocks are widely distributed in the depression, with semi-deep lake and fan delta as the main sedimentary facies, brown oil shale and black gray-dark gray mudstone as the main rocks, and a total thickness of 270-1450 m. The kerogens are mainly of the types I and IIA, and partly of the type IIB and least of the type III. The Ro values range from 0.4%-0.8%, indicating an evolution stage from immature to mature. The maturity of Es4 source rocks is rela-tively high, reaching the early mature stage, but their distribution and thickness are lower than those of Es3. Besides, according to biomarker analysis, it is thought that the source rocks of Es3 and Es4 are characterized by mixed input, and most of the source rocks were formed in the brackish water-saline and strongly oxygen-free environment. Fur-thermore, the Qingshui, Niuxintuo and Chenjia sags are believed to possess greater potential for hydrocarbon gen-eration and expulsion, for they are source rocks with a larger thickness, have higher organic carbon contents, belong to better organic matter types and possess higher maturities.     

Study on the geochemical correlation of crude oils of Palaeocene and Jurassic ages from the Potowar Indus Basin in northern Pakistan

This study deals with a detailed geochemical characterization of three crude oils from the Upper Indus Basin, Punjab, Pakistan. The samples were obtained from three productive oil fields of the Datta Formation (Jurassic), Lochhart (Palaeocene) and the Dhak Pass zone (Palaeocene). The GC parameters for and the bulk properties of Datta Formation oils are essentially coincident with those of the oils from the Dhak Pass Formation in the Upper Indus Basin, Pakistan and the oils likely originate from a marine source rock. In contrast, the Lockhart Formation oils show different behaviors and seem to be originated from dirty carbonate rocks although all three crude oils are mature, being of non-biodegraded and somewhat mixed organic matter origin. Low Pr/Ph values and high C₃₅ homohopane index for the Lockhart Formation oils suggest a source of anoxic environment with low Eh while oils from the Datta Formation and Dhak Pass Formation showed different trends, i.e., lower values of C₃₅ homohopane index indicating different depositional environment than oil from the Lockhart Formation. All three crude oils from the Upper Indus Basin are mature for the hopane ratios, i.e., Ts/Ts+Tm, C₃₂22S/(S+R) and C₃₀ αβ/(αβ+βα) and sterane ratios, i.e., C₂₉22S/(S+R) and C₂₉ββ/(ββ+αα) but oils from the Lockhart Formation seem to be less mature than those from the Palaeocene and Datta Formation according to plots like API° vs. homohopane Index, Pr/Ph vs. sterane. The relative composition of 5α(H), 14β(H), 17β(H)-24-ethylecholestanes and the C₂₉20S/20S+20R index, indicate that all three crude oils are equally mature, which makes it unlikely with respect to the above said plots. This difference is may be due to the migratory chromatography which alters the concentrations of sterane and hoapnes and hence gives different results. These oils do not exhibit UCM and have complete n-alkane profiles indicating non-biodegradation.     

New aromatic biomarkers and possible maturity indicators found in New Albany Shale extracts

Aromatic hydrocarbons from benzene extracts of New Albany Shale were characterized. A biomarker that has a molecular weight of 546 and a structural configuration consistent with that of an alkyl-aromatic hydrocarbon (C₄₀H₆₆) was tentatively identified. It was found that the relative concentrations of the biomarker are indicative of differing levels of thermal maturity of the shale organic matter. A 40-carbon bicyclic carotenoid (C₄₀H₄₈) is proposed as the geochemical precursor of this biomarker. Thermal maturity of the shale organic matter can also be differentiated by observing differences in “fingerprints” as obtained by field-ionization mass spectrometry on the aromatic hydrocarbon fraction. Using this technique, we found that the more mature shale samples from southeastern Illinois contain more low molecular weight extractable aromatic hydrocarbons and the less mature shale samples from northwestern Illinois contain more high molecular weight extractable aromatic hydrocarbons. It was demonstrated that field-ionization and tandem mass spectrometric techniques through fingerprint and individual compound identification, are useful for shale aromatic hydrocarbon fraction characterization and for thermal maturation interpretation.