GEOCHEMISTRY OF CRUDE OILS,SEEPAGE OILS AND SOURCE ROCKS FROM BELIZE AND GUATEMALA: INDICATIONS OF CARBONATE‐SOURCED PETROLEUM SYSTEMS

This study reviews the stratigraphy and the poorly documented petroleum geology of the Belize‐Guatemala area in northern Central America. Guatemala is divided by the east‐west trending La Libertad arch into the North and South Petén Basins. The arch is the westward continuation of the Maya Mountains fault block in central Belize which separates the Corozal Basin in northern Belize from the Belize Basin to the south. Numerous petroleum seeps have been reported in both of these basins. Small‐scale oil production takes place in the Corozal Basin and the North and South Petén Basins. For this study, samples of crude oil, seepage oil and potential source rocks were collected from both countries and were investigated by organic geochemical analyses and microscopy. The oil samples consisted of non‐biodegraded crude oils and slightly to severely biodegraded seepage oils, both of which were generated from source rocks with similar thermal maturities. The crude oils were generated from marine carbonate source rocks and could be divided into three groups: Group 1 oils come from the North Petén Basin (Guatemala) and the western part of the Corozal Basin (Belize), and have a typical carbonate‐sourced geochemical composition. The oils correlate with extracts of organic‐rich limestones assigned to the Upper Cretaceous “Xan horizon” in the Xan oilfield in the North Petén Basin. The oils were generated from a single source facies in the North Petén Basin, but were charged from two different sub‐basins. Group 2 oils comprise crudes from the South Petén Basin. They have characteristics typical of carbonate‐sourced oils, but these characteristics are less pronounced than those of Group 1 oils. A mixed marine/lacustrine source facies deposited under strongly reducing conditions in a local kitchen area is inferred. Group 3 oils come from the Corozal Basin, Belize. A carbonate but also a more “shaly” source rock composition for these oils is inferred. A severely biodegraded seepage oil from Belmopan, the capital of Belize, resembles a nearby crude oil. The eastern sub‐basin in the North Petén Basin may potentially be the kitchen area for these oils, and for the seepage oils found in the western part of the Corozal Basin. The seepage oils from the Corozal and Belize Basins are moderately to severely biodegraded and were generated from carbonate source rocks. Some of the seepage oils have identical C_27–29 sterane distributions to the Group 2 oils, but “biodegradation insensitive” biomarker ratios show that the seepage oils can be divided into separate sub‐groups. Severely and slightly biodegraded seepage oils in the Belize Basin were probably almost identical prior to biodegradation. Lower Cretaceous limestones from the Belize Basin have petroleum generation potential, but the samples are immature. The kitchen for the seepage oils in the Belize Basin remains unknown.     

OILS FROM CENOZOIC RIFT-BASINS IN CENTRAL AND NORTHERN THAILAND: SOURCE AND THERMAL MATURITY

Oil is produced from the Suphan Buri, Phitsanulok and Fang Basins onshore central and northern Thailand. Most of the Cenozoic rift‐basins onshore Thailand are 2–4 km deep, but the Phitsanulok Basin is the deepest with a basin‐fill up to 8 km thick. In this basin, the Sirikit field produces ～18,000–24,000 bbl/day of crude oil. In the Suphan Buri Basin, about 400 bbl/day of crude oil is produced from the U Thong and Sang Kajai fields. Approximately 800 bbl/day of crude oil is produced from the Fang field (Fang Basin), which in reality consists of a number of minor structures including Ban Thi, Pong Nok, San Sai, Nong Yao and Mae Soon. A total of eight oil samples were collected from these structures and from the Sirikit, U Thong and Sang Kajai fields. The oils were subjected to MPLC and HPLC separation and were analysed by gas chromatography (GC) and gas chromatography‐mass spectrometry (GC‐MS and GC‐MS‐MS). The U Thong oil was investigated in more detail by separating the oil into a number of fractions suited for the analysis of various specific compounds. The Sirikit oil appears to be the most mature, whereas the Suphan Buri oils and the oil from the San Sai structure (Fang Basin) are the least mature. Apart from the San Sai oil, the other oils in the Fang Basin are of similar maturity. The oils contain small amounts of asphaltenes and the asphaltene‐free fractions are completely dominated by saturated hydrocarbons (generally >60%). Long‐chain n‐alkanes extend to at least C₄₀ and the oils are thus highly waxy. In general the oils were generated from freshwater lacustrine source rocks containing a large proportion of algal material, as indicated by the presence of long‐chain n‐alkanes, low C₃₁22R/C₃₀ hopane ratios, the presence of 28‐Nor‐spergulane, T26/T25 (tricyclic triterpanes) ratios of 1.07–1.57 and tetracyclic polyprenoid (TTP) ratios close to 1. Occasional saline conditions may have occurred during deposition of the Sirikit, Ban Thi and Pong Nok source rocks. The Fang Basin oils were sourced from two different kitchens, one feeding the Ban Thi and Pong Nok structures and one feeding the Mae Soon, Nong Yao and San Sai structures. The presence ofcadalene, tetracyclic C₂₄ compounds, oleanane, lupane, bicadinane and trace amounts ofnorpimarane or norisopimarane indicate a contribution from higher land plant organic matter to the oils. The terrestrial organic matter may occur disseminated in the lacustrine facies or concentrated in coal seams associated with the lacustrine mudstones. Thermally immature oil shales (lacustrine mudstones) and coals exposed in numerous basins in central and northern Thailand could upon maturation generate oils with a composition comparable to the investigated oils.     

BIOMARKER CHARACTERISTICS OF CRUDE OILS FROM THE MURZUQ BASIN,SW LIBYA

Twenty crude oil samples from the Murzuq Basin, SW Libya (A‐, R‐ and I‐Fields in Blocks NC115 and NC186) have been investigated by a variety of organic geochemical methods. Based on biomarker distributions (e.g. n‐alkanes, isoprenoids, terpanes and steranes), the source of the oils is interpreted to be composed of mixed marine/terrigenous organic matter. The values of the Pr/Ph ratio (1.36–2.1), C₃₀‐diahopane / C₂₉ Ts ratio and diasterane / sterane ratio, together with the low values of the C₂₉/ C₃₀‐hopane ratio and the cross‐plot of the dibenzothiophene/phenanthrene ratio (DBT/P) versus Pr/Ph ratio in most of oil samples, suggest that the oils were sourced from marine clay‐rich sediments deposited in mild anoxic depositional environments. Assessment of thermal maturity based on phenanthrenes, aromatic steroids (e.g. monoaromatic (MA) and triaromatic (TA) steroid hydrocarbons), together with terpanes, and diasterane/sterane ratios, indicates that crude oils from A‐Field are at high levels of thermal maturity, while oils from Rand I‐Fields are at intermediate levels of thermal maturity. Based on the distributions of n‐alkanes and the absence of 25‐norhopanes in all of the crude oils analysed, none of the oils appear to have been biodegraded. Correlation of the crude oils points to a single genetic family and this is supported by the stable carbon isotope values. The oils can be divided into two sub‐families based on the differences in maturities, as shown in a Pr/nC₁₇ versus Ph/nC₁₈ cross‐plot. Sub‐family‐A is represented by the highly mature oils from A‐Field. Sub‐family‐B comprises the less mature oils from R‐ and I‐Fields. The two sub‐families may represent different source kitchens of different thermal maturity or different migration pathways. In summary, the geochemical characteristics of oil samples from A‐, R‐, and I‐Fields suggest that all the crude oils were generated from similar source rocks. Depositional environment conditions and advanced thermal maturities of these oils are consistent with previously published geochemical interpretations of the Rhuddanian “hot shale” in the Tanezzuft Formation, which is thought to be the main source rock in the Murzuq Basin.     

COMPOSITION OF DIAMONDOIDS IN OIL SAMPLES FROM THE ALPINE FORELAND BASIN,AUSTRIA: POTENTIAL AS INDICES OF SOURCE ROCK FACIES,MATURITY AND BIODEGRADATION

Twenty‐seven oil samples from Cretaceous, Eocene and Rupelian reservoir rocks in the Alpine Foreland Basin (Austria) were analysed to evaluate the composition of diamondoid hydrocarbons using gas chromatography – triple quadrupole mass spectrometry. The oils were generated from marly and shaly Oligocene source rocks buried beneath the nappes of the Alpine foldbelt to the south of the study area. Diamondoid hydrocarbons were detected in the saturated fraction of all the analysed oils. A biodegraded oil sample from a shallow reservoir in the NE part of the study area showed an enrichment in diamondoids due to the molecule's high resistance to microbial degradation. Variations in the organic matter type of the source rock facies and differences in maturity are known to influence the composition of diamondoids. However in this study, biomarker‐derived maturity parameters do not show a convincing correlation with diamondoid maturity parameters. Moreover, no cracking trend based on biomarkers and diamondoid concentrations was observed. The results indicate that the composition of diamondoids in oils from the Austrian part of the Alpine Foreland Basin is mainly controlled by heterogeneities in the Lower Oligocene source rocks, including the occurrence of a redeposited source rock succession in the western part of the study area. By contrast, EAI‐1 (the ethyladamantane index) shows a good correlation with various maturity parameters and seems to be independent of source rock facies.     

C4-C8轻烃在原油地球化学研究中的应用——以塔里木盆地大宛齐油田凝析油为例

全油样品的GC分析表明,塔里木盆地大宛齐油田不同层位的81个原油样品富含轻烃组分,且部分样品遭受了不同程度的微生物降解。以C₄-C₈轻烃地球化学参数为工具,对原油成熟度和次生蚀变等方面进行地球化学研究。轻烃成熟度参数表明,原油的折算R_c在0.9%左右,处于成熟阶段。对遭受生物降解作用的原油分析发现,随着生物降解作用的增强,其Mango轻烃参数K₁值减小,K₂值明显增大,甲基环己烷指数增大,甲基环戊烷较乙基环戊烷更容易遭受生物降解,表明单烷基环戊烷中烷基取代基越大的烃类越难遭受生物降解作用。结合轻烃参数指示的原油成熟度和母质来源,推测研究区原油为次生凝析油。通过轻烃参数对比研究发现,大宛齐油田凝析油轻烃参数特征与大北和其南部地区正常原油相似。依据塔里木盆地大宛齐油田的地质背景和原油成藏模式可知,研究区原油是大北和南部地区两类原油通过深大断裂运移到大宛齐浅部,通过蒸发分馏作用聚集成藏。      

东营凹陷南斜坡特高蜡油的成因

东营凹陷南斜坡王古1、新角1奥陶系古潜山原油具有特高含蜡特征(40%～60%),同区下第三系孔店组原油含蜡量也明显偏高(28.85%～31.7%)。不同层系的原油烃类化学成分分析、油源对比结果表明,研究区高蜡油成因不同于上覆第三系沙河街组,前者为深部油源,第三系孔店组为其重要烃源岩之一;后者主要源自正常油窗内沙河街组四段烃源岩。生物标志物定量分析结果表明,区内烃源岩沥青与原油中的蜡质烃含量随成熟度增加而增加,反映热成熟度是控制该区高蜡油形成的重要因素之一。东营凹陷湖沼相成因的孔店组烃源岩含有相对较高的陆源高等植物,是导致该区高蜡油形成的另一重要因素。综合分析认为,东营凹陷南斜坡高蜡油的形成主要与烃源岩相对较高的热成熟度及烃源岩含有丰富的可提供蜡质烃的成烃母质(主要指孔店组二段)有关,气侵与油气运移分馏效应的影响甚微。     

蒸发分馏作用对原油中金刚烷类化合物分布与组成的影响 ——以塔里木盆地库车坳陷羊塔克构造上的原油为例

借助于色谱和色谱—质谱分析技术,对塔里木盆地库车坳陷羊塔克构造上两组原油中烃类组成进行了系统分析,以研究蒸发分馏作用对金刚烷类化合物分布与组成特征的影响.轻烃分析结果表明,YT5和YT101井两组原油都经历了蒸发分馏作用的改造,其中上部储层产次生凝析油,而下部储层产经蒸发分馏作用改造的残留油.相似的甾、萜烷分布与组成表...     

CRUDE OIL GEOCHEMISTRY AND SOURCE ROCK POTENTIAL OF THE UPPER CRETACEOUS – EOCENE SUCCESSION IN THE BELAYIM OILFIELDS,CENTRAL GULF OF SUEZ,EGYPT

This study evaluates the petroleum potential of source rocks in the pre‐rift Upper Cretaceous – Eocene succession at the Belayim oilfields in the central Gulf of Suez Basin. Organic geochemical and palynofacies investigations were carried out on 65 cuttings samples collected from the Thebes, Brown Limestone and Matulla Formations. Analytical methods included Rock‐Eval pyrolysis, Liquid Chromatography, Gas Chromatography and Gas Chromatography – Mass Spectrometry. Four crude oil samples from producing wells were characterised using C₇ light hydrocarbons, stable carbon isotopes and biomarker characteristics. The results showed that the studied source rocks are composed of marine carbonates with organic matter dominated by algae and bacteria with minimal terrigenous input, deposited under reducing conditions. This conclusion was supported by n‐alkane distributions, pristane/ phytane ratios, homohopane and gammacerane indices, high concentrations of cholestane, the presence of C₃₀ n‐propylcholestanes, and low diasterane ratios. The source rocks ranged from immature to marginally mature based on the Rock‐Eval T_max together with biomarker maturity parameters. The analysed crude oil samples are interpreted to have been derived from source rock intervals within the Eocene Thebes Formation and the Upper Cretaceous Brown Limestone. The similarity in the geochemical characteristics of the crude oils suggests that there was little variation in the organofacies of the source rocks from which they were derived.     

Organic geochemistry investigations of crude oils from Bayoot oilfield in the Masila Basin,east Yemen and their implication for origin of organic matter and source-related type

Thirteen crude oil samples from fractured basement reservoir rocks in the Bayoot oilfield, Masila Basin were studied to describe oil characteristics and to provide information on the source of organic matter input and the genetic link between oils and their potential source rock in the basin. The bulk geochemical results of whole oil and gasoline hydrocarbons indicate that the Bayoot oils are normal crude oil, with high hydrocarbons of more than 60%. The hydrocarbons are dominated by normal, branched and cyclic alkanes a substantial of the light aromatic compounds, suggesting aliphatic oil-prone kerogen. The high abundant of normal, branched and cyclic alkanes also indicate that the Bayoot oils are not biodegradation oils.The biomarker distributions of isoprenoid, hopane, aromatic and sterane and their cross and triangular plots suggest that the Bayoot oils are grouped into one genetic family and were generated from marine clay-rich source rock that received mixed organic matter and deposited under suboxic conditions. The biomarker distributions of the Bayoot oils are consistent with those of the Late Jurassic Madbi source rock in the basin. Biomarker maturity and oil compositions data also indicate that the Bayoot oils were generated from mature source rock with peak oil-window maturity.     

辽河盆地东部凹陷欧利坨子地区原油性质与成因

油—油精细对比揭示辽河盆地欧利坨子地区低熟油和成熟油共存,与东部凹陷其它地区原油相比总体上成熟度较低.表现在原油族组成中饱和烃和饱芳比值低(饱/芳值小于3.0)、生物标志物中反映成熟度的参数明显偏低、饱和烃和芳烃碳同位素值(分别为-26.00‰~-27.50‰和-25.00‰~-26.50‰)也明显低于东部凹陷其它地区,单体烃碳同位素类型曲线特征也揭示其原油成熟度较低.油—源对比揭示欧利坨子地区原油来自沙三中下段的暗色泥岩,生物标志物资料揭示源岩热演化程度明显低于东部凹陷其它地区,因而造成该区原油成熟度较低.      

GEOCHEMISTRY AND ORIGIN OF CRUDE OILS AND CONDENSATES FROM THE CENTRAL PERSIAN GULF,OFFSHORE IRAN

Biomarker‐ and compound‐specific carbon isotope analyses were used to compare oil samples recovered from Late Jurassic and Early to Middle Cretaceous reservoirs at South Pars and nearby fields in the Iranian portion of the Persian Gulf, and condensate samples associated with the super‐giant gas accumulation in Permo‐Triassic reservoirs at South Pars. The results indicate that all of the oil samples, including heavy oil from South Pars and oil from the Salman, Reshadat, Resalat and Balal fields, are genetically related. The most probable source rocks for these oils are Jurassic marine limestones or marls deposited under anoxic conditions. Based on the methyl phenanthrene index, source rock maturity was inferred to be equivalent to vitrinite reflectance values of about 0.8% Rc. The distribution and maturity pattern of the source rocks suggest migration from a depocentre located to the south, with inferred migration distances of up to 250 km. There is no genetic relationship between the heavy oil which has accumulated in Mesozoic reservoirs at South Pars and condensates which are associated with the super‐giant gas accumulation in Permo‐Triassic reservoirs there. Based on biomarker compositions, the condensates at South Pars appear to be derived from shaly marine or lacustrine source rocks deposited under dysoxic conditions. The δ¹³C values of short‐chain n‐alkanes and isoprenoids in condensate samples suggest a common source and an equal maturity for the source rocks. Pristane/n‐C₁₇ versus phytane/n‐C₁₈ characteristics are in agreement with published data for Silurian‐sourced condensates. High thermal maturities equivalent to 1.7% Rc are also consistent with a Palaeozoic (Silurian) source rock.     

东海陆架盆地西湖凹陷原油“气洗”地球化学特征及成因

“气洗”是油气藏后期改造的重要营力之一,在东海陆架盆地西湖凹陷曾发生过大规模气洗作用,但现有研究仅限于描述阶段,并未对原油的“气洗”地球化学特征展开系统研究。通过对西湖凹陷平湖斜坡带和天台反转带原油进行地球化学分析,结果表明：原油的甾烷、萜烷指纹特征及碳同位素特征相似,显示出具有相同母源的特征,均为成熟原油,但不同原油的物性特征、轻烃、正构烷烃及金刚烷分布特征存在较大差异,通过计算原油的石蜡度、芳香度和正构烷烃损失量及对金刚烷系列化合物进行定量分析,认为气洗作用应是造成该差异的主要原因,并非生物降解或热裂解。为进一步厘清原油的气洗作用过程及成因,探讨了原油气洗作用的主控因素,并结合油气藏地质和地球化学资料进行了综合分析。结果表明：①平湖斜坡带北部原油主要为强烈蒸发分馏后的残余油,晚期可能被东部横向运移而来的高熟天然气混入,使原油中部分轻烃和金刚烷化合物含量升高;②平湖斜坡带南部主要受运移分馏作用影响,浅层聚集次生凝析油,分馏后的残余油在深层形成轻质油和蜡质油藏;③天台反转带原油主要为“气洗”晚期阶段形成的次生凝析油,其残余油目前尚未发现,暗指其深部储层或许具有一定规模的轻质、蜡质油藏。     

GEOCHEMICAL COMPOSITION OF ORDOVICIAN OILS FROM THE TUOPUTAI REGION,NORTHERN TARIM BASIN,NW CHINA: SOURCE ROCK CORRELATION

Molecular and stable carbon isotope compositions of 46 Ordovician crude oil samples from wells in the Tuoputai region of the northern Tarim Basin were investigated using GC–MS, MRM GC–MS and IRMS to determine their genetic relationships and to identify possible source rocks. Thirty-three source rock samples from outcrops and cores were also investigated. The oil samples varied from light to heavy crudes and showed very narrow δ¹³C value ranges for the whole oil, saturated and aromatic fractions. The majority of the oils displayed very similar molecular compositions with relatively high concentrations of n-alkanes and isoprenoids and low concentrations of terpenoids and steroids. Comparison of the compositions of these crude oils strongly suggested their genetic affinity, while maturity parameters indicated maturity variations from the peak to the late oil generation stages. The samples also showed the characteristics of mixtures of biodegraded and fresh oil charges. Bitumen extracts from Cambrian and Ordovician source rocks were studied in detail. The oil compositions suggested a marine marl source deposited in anoxic, hypersaline conditions with significant bacterial and algal organic matter inputs. The distributions of C₂₆–C₂₈ triaromatic steroids, tricyclic terpanes and regular steranes appear to have been greatly influenced by thermal maturation, making them unreliable for correlating the oils and the source rocks. In contrast, dinosteranes and triaromatic dinosteroids seem not to have been affected by maturation and were more useful for correlation studies. They indicated that there was no or little genetic relationship between the Cambrian – Lower Ordovician source rocks and the oils, but in general suggested a possible Middle – Upper Ordovician source for the oil accumulations in the Tuoputai field. However, the occurrence of triaromatic dinosteroids in oil from well TP28XCX may also suggest a minor contribution from Cambrian – Lower Ordovician source rocks.     

GENERATION AND ACCUMULATION OF HYDROCARBONS IN A DEEP “BURIED HILL” STRUCTURE IN THE BAXIAN DEPRESSION,BOHAI BAY BASIN,EASTERN CHINA

Significant volumes of hydrocarbons have been produced from karstified Infracambrian dolomites in a “buried hill” structure at depths of 5860m to 6027m and reservoir temperatures of 190–201°C in well Niudong‐1 in the Baxian depression, Bohai Bay Basin. This is the deepest oil and gas discovery made in eastern China so far and structures at similar depths are targets for exploration elsewhere in the Bohai Bay Basin. However the origin and accumulation of the hydrocarbons at Niudong‐1 is not clear: they may have been generated from highly mature lacustrine source rocks in the Sha‐4 Member of the Eocene Shahejie Formation; or they may have been derived from thermal cracking of previously‐accumulated oil. This paper investigates the organic geochemistry of the Sha‐4 Member source rocks and the crude oils produced from well Niudong‐1. Analyses of molecular parameters show that the hydrocarbons originated from the pyrolysis of organic matter in Sha‐4 Member source rocks, rather than from cracking of previously accumulated oil. Infracambrian dolomites at the Niudong‐1 location may have been charged with low‐maturity oil around 34 Ma ago, when the Sha‐4 Member source rocks were buried to depths of about 3500m and first entered the oil window. During further rapid burial to more than 5500m starting at about 15Ma, these source rocks became highly mature and generated significant volumes of light oil and gas. Overpressures in the source rock interval forced these hydrocarbons to migrate into unconformably‐underlying Infracambrian dolomite reservoir rocks at the Niudong‐1 structure. Significant risks are associated with future exploration of deep “buried hill” structures in the Bohai Bay Basin. Not all the structures were charged with oil, and accumulations were not necessarily preserved during Neogene burial as the reservoirs may have been breached by faulting.     

塔里木盆地西南部皮山北新1井白垩系油气成因

皮山北新1井位于塔里木盆地麦盖提斜坡皮山北1号背斜构造高部位,该井在白垩系获得油气,但油气源存在较大争议。原油轻烃特征表明,该井原油具有Ⅰ型母质类型来源的特征;原油饱和烃特征表明,生烃母质以泥质岩为主,但与塔河、玉北奥陶系原油有一定差异。原油二维色谱分析表明,该井原油与玉北、塔河、巴楚泥盆系原油均位于海相泥页岩的范畴;原油碳同位素特征表明,该井原油母质类型差于玉北奥陶系及巴楚巴什托石炭系原油。该井原油成熟度高于玉北奥陶系原油。通过该井白垩系油气与其周缘塔西南4套主力烃源岩生标特征的对比,认为皮山北新1井白垩系原油主要来源于塔西南海相石炭系烃源岩;天然气生源母质类型与塔西南柯克亚气田类似,有侏罗系腐殖型有机质来源天然气的贡献。皮山北新1井白垩系新层系油气成因研究将有助于拓展塔西南中新生界碎屑岩油气勘探领域。      

塔河油田成藏期次的地球化学示踪研究

应用油藏地球化学的方法对塔河油田的成藏期次进行了探讨。塔河油田稠油正构烷烃分布完整,但色谱基线不同程度抬升;原油非烃和沥青质碳同位素偏轻,族组成碳同位素发生倒转;原油中普遍含有25-降藿烷。这些特征表明该区油藏经历了至少两期成藏过程,早期充注原油遭受生物降解作用后又受到高成熟原油充注。塔河油田天然气为典型的油型气,成熟度较高,为成熟—过成熟阶段产物,亦显示了两期充注的特征:早期充注的为典型的原油伴生气,充注时间与后期原油的充注时间相同;晚期充注的为高温裂解气,充注方向为自东向西。因此,塔河油田至少存在着3次油气充注过程。      

盐湖相原油和生油岩中芳烃及硫化芳烃的分布特征

作者采用GC/MS技术分析了江汉、东濮和晋县第三系盐湖相原油和生油岩中的芳香族化合物和非芳香族杂环化合物,鉴定出80多种多环芳烃和硫化芳烃.本文讨论了它们的分布特征和含硫化合物的成因.     

塔里木盆地西部油源初步探讨

塔里木盆地是一个中间地块型沉积盆地,经历了两个发展阶段,形成古生界和中、新生界两套沉积盖层。塔里木盆地西部,指库尔勒与和田一线以西地区,多处见油,柯克亚构造获得高产油、气流。本文对它们的油源问题作一初步探讨。     

原油中烷基萘的形成机理及其成熟度参数应用

原油和沉积有机质中烷基萘系列主要通过异构化反应和歧化反应生成,并可用芳香烃的亲电取代反应历程进行解释,所生成各种异构体的相对含量受“电子效应”和“空间效应”的控制。多甲基萘系列地球化学参数,对于原油及沉积有机质的成熟度、生物降解作用和生源组成具有表征意义。渤海湾盆地大民屯凹陷原油样品中的多甲基取代萘系列分析结果表明,在常规的甾萜类异构化作用达到平衡终点的情况下,多甲基萘取代系列地球化学参数仍能反映出大民屯凹陷高蜡油成熟度比正常油偏低,并与该区的石油地质背景和盆地模拟结果相吻合,表明多甲基萘系列相关的地球化学参数是一类非常有用的地球化学指标。      

孟加拉盆地油气分布特征及主控因素

孟加拉盆地是发育在印度次大陆东北角比较典型的残留洋盆地,其经历了裂陷期、裂后期和前渊期。在残留洋盆地形成阶段发育了最重要的生储盖组合,主要烃源岩是渐新统Jenam组和中新统Bhuban组页岩;主要储集层是中新统Surma群Bhuban组和Boka Bil组陆架到潮控三角洲砂岩;盖层主要为中新统—上新统的海相页岩,分布广泛;圈闭类型以背斜为主。孟加拉盆地以天然气为主,绝大部分气田平面上分布在Surma坳陷、Tangail凸起、Hatia坳陷和东部褶皱带。天然气富集层位主要位于中新统Bhuban组和Boka Bil组。气田的分布与残留洋盆地的形成密切相关,盆地演化控制了含气层系的发育,盆地结构控制了天然气的平面分布。今后勘探的重点为盆地东部Surma坳陷及周缘、Hatia坳陷和东部褶皱带,处于压力过渡带的Surma群砂岩储层应是油气勘探的重点层系。通过对孟加拉盆地演化、石油地质条件和油气分布特征以及天然气成藏规律和主控因素的研究,可为残留洋盆地油气勘探提供借鉴和参考。