伦坡拉盆地始新统牛堡组烃源岩成熟史

伦坡拉盆地自下而上发育始新统牛堡组牛二中亚段、牛二上亚段及牛三下亚段3套烃源岩。基于钻井测试资料、地震资料及含烃流体包裹体测试分析,利用盆地模拟技术对伦坡拉盆地热史及牛堡组3套烃源岩的成熟史进行了系统分析。研究表明,伦坡拉盆地自始新统牛三段沉积以来直至渐新统丁青湖组三段沉积末期,古地温梯度总体表现为持续降低,变化介于6.67~4℃/hm,为典型热盆特征,古地温梯度升高与区域构造隆升作用具有明显的相关性。牛二中及牛二上亚段的2套烃源岩成熟较早且热演化程度高,分别于中-晚始新世(距今约46.4~37.5 Ma)及早渐新世(距今约36.6~33.5 Ma)进入生油门限,现今主体皆位于成熟生油阶段(R_o=0.7%~1.3%),是该区主力烃源岩。西部蒋日阿错洼陷牛二中及牛二上亚段主力烃源岩现今热演化程度最高,生供烃条件较中-东部江加错、爬错洼陷更为优越。      

西藏洞错盆地古近系丁青湖组烃源岩评价

洞错盆地位于西藏中部,为古近-新近纪陆相盆地,主要发育新近系康托组、古近系丁青湖组和牛堡组。对盆地地层、沉积相和烃源岩等评价分析后认为,丁青湖组泥岩、泥灰岩和灰岩为主要烃源岩,泥质烃源岩厚40 m,碳酸盐类烃源岩厚72 m. 其中泥灰岩及灰岩有机碳含量较高（平均值为0.51%）,属中等-好烃源岩。丁青湖组下段半深湖相的深黑色泥岩有机碳含量平均值高达5.92%,属优质烃源岩;有机质类型以Ⅰ、Ⅱ型为主;烃源岩热演化达到早期成熟阶段（热解峰温为431~456℃,镜质体反射率为0.36%~0.66%）。综合分析表明,洞错盆地具有较好的生烃潜力和勘探前景。     

西藏伦坡拉盆地古近系油藏成藏机理及有利区带预测

西藏伦坡拉盆地古近系始新统牛堡组-渐新统丁青湖组的早期勘探发现了良好的油气,展示了较好的勘探潜力,但单井测试多为低产,分析认为成藏机理及主控因素认识不够制约了早期勘探。为弄清伦坡拉盆地古近系油藏成藏机理,明确有利区带,通过地质与物探联合分析,认为伦坡拉盆地具有多套扇三角洲前缘砂岩体目的层系、有利生储盖组合、继承性古隆起（古斜坡）、立体输导体系、构造与成藏匹配好等有利成藏条件。油藏具“近源聚集、多期成藏、中浅层晚期破坏、中深层原地富集”成藏富集规律,评价落实了6个有利勘探区带,采用有机碳法计算资源量达数亿吨,资源丰度较高,具有形成中型油田的资源潜力,其中长山岩性区带成藏条件优越,估算资源量上亿吨,同时该区正好是丁青湖组中下部油页岩发育区,采用体积丰度法计算出其资源量3倍于常规资源量。总体上长山岩性区带为常规与非常规有利油气勘探区的叠合部位,展现出广阔的勘探前景。     

西藏伦坡拉盆地地温场特征及烃源岩热演化史

西藏伦坡拉盆地是在燕山期基底上发育的拉张型老第三纪陆相盆地。形成时间晚于中国东部新生代沉积盆地.它在始新世未发生短暂抬升,渐新世求突告结束,之后长期抬升剥蚀,沉积时间短。伦坡拉盆地具有典型热盆的特点,受区域异常热流影响,地温梯度高于50℃/km;其地表温度随青藏高原隆升而大幅降低。独特的构造、沉积演化史及地温场特征,导致了盆内牛堡组二、三段烃源岩成熟快、门限浅,利于油气生成和排运。但盆地后期改造复杂,裂缝发育,勘探证实原生油气藏破坏明显,对保存大规模的油气储量不利。      

IMPACT OF DIAGENESIS ON RESERVOIR-QUALITY EVOLUTION IN FLUVIAL AND LACUSTRINE-DELTAIC SANDSTONES: EVIDENCE FROM JURASSIC AND TRIASSIC SANDSTONES FROM THE ORDOS BASIN, CHINA

The reservoir quality of Jurassic and Triassic fluvial and lacustrine‐deltaic sandstones in the intracratonic Ordos Basin is strongly influenced by depositional facies and various types of diagenetic modifications. The fluvial sandstones have higher average He‐porosity and permeability (14.8% and 12.7 mD, respectively) than the deltaic sandstones (9.8% and 5.8 mD, respectively). In addition to extensive mechanical compaction, eodiagenesis (220–97 Ma; depth < 2000 m; T < 70°C) has resulted in dissolution and kaolinitization of detrital silicates in the Jurassic fluvial sandstones, and in smectite infiltration and minor cementation by calcite and siderite in the Triassic fluvial and deltaic sandstones. Pervasive eogenetic carbonate cementation (> 20 vol.%) occurred in Triassic deltaic siltstones and very fine‐grained sandstones which are closely associated with organic‐rich mudstones. Mesodiagenesis (97–65 Ma), which occurred during rapid subsidence to depths of 3700–4400 m, resulted in the albitization of plagioclase, checmical compaction, the conversion of kaolinite into dickite, and cementation by quartz overgrowths, chlorite, illite, ankerite (δ¹³C_VPDB=?2.4‰ to +2.6‰; δ¹⁸O_VPDB=?21.5‰ to ?10‰) and calcite (δ¹³C_VPDB=?4.7‰ to +3.7‰; δ¹⁸O_VPDB=?21.8‰ to ?13.4‰). Oil emplacement (95 Ma) retarded cementation by mesogenetic quartz and carbonate but had little influence on dickite, illite and chlorite formation. Retardation of quartz cementation was also due to the presence of chlorite fringes around detrital quartz grains. Dickitization of eogenetic kaolinite together with the short residence time at maximum burial temperatures (105–124°C) has retarded the albitization of K‐feldspars and illite formation and hence prevented severe permeability destruction. Telodiagenesis, which occurred after uplift (Eocene to end‐Neogene), caused slight dissolution and kaolinitization of feldspars. This study demonstrates that despite complex patterns of diagenetic modifications in the Triassic and Jurassic successions, depositional porosity and permeability are better preserved in fluvial meandering channel sandstones than in deltaic sandstones. These results should be important for modelling of reservoir‐quality distribution and exploration risk evaluation in the basin.     

伦坡拉盆地热演化史及有利区带预测

伦坡拉盆地是西藏地区已发现的最具油气勘探潜力的盆地,勘探前景广阔。为研究该区烃源岩热演化史和油气运聚规律,开展了钻井岩心分析、二维地震资料解释、单井热演化史模拟、油气运聚模拟及分析等研究工作。结果表明：①在W1和W2井牛堡组三段下亚段底部识别出厚层的浊积砂体;②始新世末期,受高热流和高地温梯度双重作用的影响,牛堡组二段中亚段烃源岩开始生烃,并一直持续至今,烃源岩的成熟呈现出西早东晚的趋势,期间油气主要在蒋日阿错凹陷和江加错凹陷发生少量运移,并在凹陷内部的隆起带及周缘聚集;③渐新世早期,爬错凹陷发生大规模运移,油气沿优势运移通道向北和向南,并通过断层向上覆地层运移。由此推测,在盆地北部的逆冲推覆带中段和南部冲断隆起带中西段,以及江加错凹陷的东部和爬错凹陷的西部存在10处有利油气聚集带。该研究成果为伦坡拉盆地的油气勘探提供了依据。     

ORGANIC GEOCHEMISTRY OF POTENTIAL SOURCE ROCKS IN THE TERTIARY DINGQINGHU FORMATION,NIMA BASIN,CENTRAL TIBET

The Tertiary Nima Basin in central Tibet covers an area of some 3000 km² and is closely similar to the nearby Lunpola Basin from which commercial volumes of oil have been produced. In this paper, we report on the source rock potential of the Oligocene Dingqinghu Formation from measured outcrop sections on the southern and northern margins of the Nima Basin. In the south of the Nima Basin, potential source rocks in the Dingqinghu Formation comprise dark‐coloured marls with total organic carbon (TOC) contents of up to 4.3 wt % and Hydrogen Index values (HI) up to 849 mg HC/g TOC. The organic matter is mainly composed of amorphous sapropelinite corresponding to Type I kerogen. Rock‐Eval T_max (430–451°C) and vitrinite reflectance (R_r) (average R_r= 0.50%) show that the organic matter is marginally mature. The potential yield (up to 36.95 mg HC/g rock) and a plot of S2 versus TOC suggest that the marls have moderate to good source rock potential. They are interpreted to have been deposited in a stratified palaeolake with occasionally anoxic and hypersaline conditions, and the source of the organic matter was dominated by algae as indicated by biomarker analyses. Potential source rocks from the north of the basin comprise dark shales and marls with a TOC content averaging 9.7 wt % and HI values up to 389 mg HC/g TOC. Organic matter consists mainly of amorphous sapropelinite and vitrinite with minor sporinite, corresponding to Type II‐III kerogen. This is consistent with the kerogen type suggested by cross‐plots of HI versus T_max and H/C versus O/C. The T_max and R_r results indicate that the samples are immature to marginally mature. These source rocks, interpreted to have been deposited under oxic conditions with a dominant input of terrigenous organic matter, have moderate petroleum potential. The Dingqinghu Formation in the Nima Basin therefore has some promise in terms of future exploration potential.     

The migration of hydrocarbons: a case from the Shahejie formation of the Wenliu area of Bohai Bay Basin,China

The formation waters derived from different members of the Shahejie Formation have the δD and δ¹⁸O values of ?67?～??44.6‰ and ?7.1?～?1.38‰, respectively. Results show that the formation water of the Shahejie Formation mainly origin from connate water and slightly influenced by meteoric water. Here, we present a new method to determine the different hydrocarbon migration force in different strata of the Shahejie Formation in the Wenliu area. The method described in this article uses the relationship between the formation water composition that composed of four different ion concentrations (Cl^?, K⁺+Na⁺, HCO₃^?, and SO₄^2?) and the represents or absence of hydrocarbon accumulations. Combining with the formation pressure coefficient data and formation water relative density in different members, results show that the two sets of figures in the deep members look like the same, indicating that the migration force of hydrocarbon is the hydrodynamic. While in the shallow members, the two sets of figures are different, indicating that the migration force of hydrocarbon is buoyancy. This methodology provides a tool to understand the migration force of hydrocarbon in different depth. It will be useful in understanding the migration pattern in the subsurface.     

西藏伦坡拉盆地第三系及其含油性

伦坡拉盆地位于藏南褶皱系昂拉仁错—奇林湖复向斜带北部边界断裂的南侧。它是一个东西长约200公里,南北宽约20公里,面积约4000平方公里的陆相含油盆地。盆地的石油普查工作开始于1956年。从1967年起,地质部第四普查勘探大队在此开展了大规模的多工种的石油普查工作,现将已经取得的一些资料和本人的初步看法提出来，与同行们讨论。     

Organic geochemical characteristics of Middle to Late Eocene Shahbazan Formation in Dezful Embayment,SW Iran: A case study from Qaleh-Nar oilfield

Abstract

In a first attempt, Middle to Late Eocene Shahbazan Formation as a possible source rock in Dezful Embayment was geochemically investigated. Maturity indicators derived from Rock-Eval pyrolysis (T_max and PI) and gas chromatography (CPI) show that the organic matter, which dominated by a mixed type II/III kerogen, is thermally mature and has already entered the oil window. A fair to good petroleum-generative potential is suggested by moderate to relatively high values of total organic carbon (TOC) and potential yield (S₁+S₂). Deposition of Shahbazan Formation under low-oxygen condition, which is represented by low pristane/phytane ratio (<1), conduced to preservation of organic matter. This is in accordance with considerable TOC contents, ranging from 1.01 to 1.72?wt%. The relation between pristane/nC₁₇ and phytane/nC₁₈ as well as terrigenous/aquatic ratio (～1) represent the mixed marine and terrestrially sourced organic matter. Based on the results obtained from this study, Shahbazan Formation could have been acted as a prolific oil and gas source rock.     

Petroleum source rocks characterization and hydrocarbon generation of the Upper Jurassic succession in Jabal Ayban field,Sabatayn Basin,Yemen

The Upper Jurassic samples of Jabal Ayban field, Sabatayn Basin, western Yemen were used to determine the source rock characteristics and petroleum generative potentials. Based on organic geochemical analysis, the Upper Jurassic source rocks have variable TOC content in the range of 0.82–13?wt%, indicating a fair to very good source rock generative potential. The bulk geochemical results showed that Safer Member contains Type III kerogen grading to mixed Type II–III, while the Lam and Meem members displaying generally Type II, mixed Type II–III and Type III kerogens, which have the ability to generate mixed oil and gas accumulations under thermal maturation level. Vitrinite reflectance in the range (0.35–0.78% Ro) and Tmax in the range (411–445?°C), generally proving that the Meem source rocks have sufficient thermal maturity for hydrocarbon-generation, whereas the Lam and Safer are immature source rocks. 1D basin modelling was performed to analyse the hydrocarbon generation and expulsion history of the study area. Calibration of the model with measured VR (% Ro) and (BHT) data shows that the paleo-heat-flow was high at Late Jurassic. The burial/thermal history models indicate that the source rocks in the Lam and Safer members are immature source rocks and the peak oil generation has not been reached yet. The models also indicate that the early hydrocarbon generation (0.64% Ro) in the Meem source rock occurred during late Jurassic at 150?Ma and the main hydrocarbon generation (0.79% Ro) has been reached approximately at 146?Ma. Therefore, the Meem Member can be consider as generative potentials of prospective source rock horizons in Jabal Ayban field, Sabatayn Basin.     

SOURCE ROCK POTENTIAL OF THE BLUE NILE (ABAY) BASIN, ETHIOPIA

The Blue Nile Basin, a Late Palaeozoic ‐ Mesozoic NW‐SE trending rift basin in central Ethiopia, is filled by up to 3000 m of marine deposits (carbonates, evaporites, black shales and mudstones) and continental siliciclastics. Within this fill, perhaps the most significant source rock potential is associated with the Oxfordian‐Kimmeridgian Upper Hamanlei (Antalo) Limestone Formation which has a TOC of up to 7%. Pyrolysis data indicate that black shales and mudstones in this formation have HI and S₂ values up to 613 mgHC/gCorg and 37.4 gHC/kg, respectively. In the Dejen‐Gohatsion area in the centre of the basin, these black shales and mudstones are immature for the generation of oil due to insufficient burial. However, in the Were Ilu area in the NE of the basin, the formation is locally buried to depths of more than 1,500 m beneath Cretaceous sedimentary rocks and Tertiary volcanics. Production index, T_max, hydrogen index and vitrinite reflectance measurements for shale and mudstone samples from this areas indicate that they are mature for oil generation. Burial history reconstruction and Lopatin modelling indicate that hydrocarbons have been generated in this area from 10Ma to the present day. The presence of an oil seepage at Were Ilu points to the presence of an active petroleum system. Seepage oil samples were analysed using gas chromatography and results indicate that source rock OM was dominated by marine material with some land‐derived organic matter. The Pr/Ph ratio of the seepage oil is less than 1, suggesting a marine depositional environment. n‐alkanes are absent but steranes and triterpanes are present; pentacyclic triterpanes are more abundant than steranes. The black shales and mudstones of the Upper Hamanlei Limestone Formation are inferred to be the source of the seepage oil. Of other formations whose source rock potential was investigated, a sample of the Permian Karroo Group shale was found to be overmature for oil generation; whereas algal‐laminated gypsum samples from the Middle Hamanlei Limestone Formation were organic lean and had little source potential     

西藏伦坡拉盆地烃源岩特征及资源条件

伦坡拉盆地主要发育下第三系牛堡组烃源层系,主力烃源岩为牛二段及牛三下亚段,牛三上亚段也有一定的生烃能力。烃源岩受沉积相带的控制,主要发育于深-半深湖相区,蒋日阿错凹陷烃源岩厚度最大。烃源岩具有机质丰度中等,母质类型优良,成熟度主体偏高的特点。自始新世末期以来,伦坡拉盆地长期具有成藏的油源条件,早期的源岩为牛二段,晚期则以牛三下亚段为主。总体上看,伦坡拉盆地具良好的油气资源前景。      

冀中坳陷霸县凹陷古近系中深层古地层压力演化及对储层成岩作用的影响

综合运用测录井、试油和实验分析测试资料及盆地模拟方法,在霸县凹陷古近系中深层今地层压力分析的基础上,恢复了古地层压力的演化过程,探讨了古地层压力对储层成岩作用的影响。分析结果表明,古近系中深层地层压力存在超压原始积累-超压降低2个阶段,其演化表现出“低-高-低”变化特征。沉积初期地层压力以常压为主,在距今约40 Ma开始发育超压,约25~20 Ma期间超压达到最高值,表现为强超压,随后地层压力持续降低,至今演化为常压、弱超压,局部地区发育中超压。早期快速埋藏导致的欠压实作用是地层超压形成的主要原因,而后期生烃增压作用不明显。古地层超压抑制了储层的成岩作用,具体表现为：①抑制压实作用的进行,从而保护储层的原生孔隙;②抑制黏土矿物的转化,减少Ca²⁺等的来源,从而减弱胶结作用发生,保护储层的孔隙;③抑制有机酸的生成,阻碍流体渗流与物质交换,从而不利于长石溶蚀作用的发生。     

Identification of the lower limit of high-quality source rocks and its relation to hydrocarbon accumulation— Taking the Beier Sag in the Hailaer Basin as an example

The theory of “source rock control” has evolved from source-rock-control hydrocarbon accumulation, to effective source-rock-control hydrocarbon accumulation, and to high-quality sourcerock- control hydrocarbon accumulation. However, there are problems, such as whether high-quality source rocks exist or not? What high-quality source rocks are, and how to identify them, are yet to be agreed upon. Aimed at this issue of concern to explorationists, and taking the Beier Sag in the Hailaer Basin as an example, this paper defines the high-quality source rocks and the lower limit for evaluation of high-quality source rocks, by using the inflection point on the relationship curve of hydrocarbon (oil) expulsion, which is calculated by the material balance principle, versus total organic carbon (TOC). The results show that when TOC is low, all source rocks have limited hydrocarbon expulsion and slow growth rate, thus they cannot be high-quality source rocks. However, when TOC rises to some threshold, hydrocarbon expulsion increases significantly with TOC. This inflection point should be the lower limit of high-quality source rocks: those with TOC greater than the inflection-point value are high-quality source rocks. In addition, the lower limit of high-quality source rocks is also related to the type and maturity of organic matters in the source rocks, as well as the mineral components of the source rocks affecting the residual hydrocarbons. Theoretically, the lower limit of high-quality source rocks depends on geological conditions rather than being a constant value. However, for the sake of simplicity and practicability, in this paper TOC=2.0% is regarded as the lower limit of high-quality source rocks. The examination of such standard in the work area indicates that the high-quality source rocks in members K1n2 and K1n1 of the Nantun formation contribute 76% and 82% to oil generation, and 96% and 91% to oil expulsion , respectively. The distribution of high-quality source rocks is also closely related to the distribution of hydrocarbon reservoirs in the region, demonstrating that high-quality source rocks control hydrocarbon accumulation.     

西藏伦坡拉盆地烃源岩特征及资源条件

伦坡拉盆地主要发育下第三系牛堡组烃源层系,主力烃源岩为牛二段及牛三下亚段,牛三上亚段也有一定的生烃能力。烃源岩受沉积相带的控制,主要发育于深－ 半深湖相区,蒋日阿错凹陷烃源岩厚度最大。烃源岩具有机质丰度中等,母质类型优良,成熟度主体偏高的特点。自始新世末期以来,伦坡拉盆地长期具有成藏的油源条件,早期的源岩为牛二段,晚期则以牛三下亚段为主。总体上看,伦坡拉盆地具良好的油气资源前景     

Characteristics and accumulation model of the late Quaternary shallow biogenic gas in the modern Changjiang delta area, eastern China

The Changjiang(Yangtze)is one of the largest rivers in the world.It formed a huge incised valley at its mouth during the Last Glacial Maximum;the incised-valley fill,approximately 80–110 m thick,supplies an important foundation for the generation of shallow biogenic-gas reservoirs.Two cores and 13 cone penetration tests were used to elaborate the characteristics,formation mechanism,and distribution of the shallow biogenic-gas reservoirs in the study area.The natural gas is mainly composed of CH_4(generally[95%)with a δ~(13)C_(CH4) and δ~(13)C_(CO2) of-75.8 to-67.7% and -34.5 to-6.6%,respectively,and a δD_(CH4) of-215 to-185%,indicating a biogenic origin by the carbon dioxide reduction pathway.Commercial biogenic gas occurs primarily in the sand bodies of fluvial-channel,floodplain,and paleo-estuary facies with a burial depth of 50–80 m.Gas sources as well as cap beds are gray to yellowish-gray mud of floodplain,paleoestuary,and offshore shallow marine facies.The organic matter in gas sources is dominated by immature type III kerogen(gas prone).The difference in permeability(about4–6 orders of magnitude)between cap beds and reservoirs makes the cap beds effectively prevent the upward escape of gas in the reservoirs.This formation mechanism is consistent with that for the shallow biogenic gas in the late Quaternary Qiantang River incised valley to the south.Therefore,this study should provide further insight into understanding the formation and distribution of shallow biogenic gas in other similar postglacial incised-valley systems.     

伦坡拉盆地下第三系储集层成岩作用和孔隙演化

藏北伦坡拉盆地下第三系储集层主要发育于始新统牛堡组二段至渐新统丁青湖一段，储集层所经历的成岩作用十分复杂，现象十分丰富。详尽描述了压实、４期溶蚀、３期胶结、交代、破裂等成岩作用的过程及特征，确认区内储集层所经历的主要成岩环境为：湖底／河流→大陆地表→埋藏→大陆地表。孔隙演化研究表明，区内储集层的原始孔隙度均较高，经过一系列成岩作用后，原生孔隙所剩无几；溶蚀作用特别是深埋溶蚀作用形成的各类溶孔，是进油期的主要孔隙，孔隙度一般为５％～１５％，部分充填烃类。照片１０图５参４（邹冬平摘）     

珠江口盆地AB凹陷古近系油气成藏控制因素分析

古近系文昌组-恩平组是珠江口盆地油气勘探的重要层位。通过对烃源岩展布、构造活动强弱、输导体类型、储盖条件、圈闭种类的研究表明,烃源岩分布控制油气富集程度,构造活动制约油气聚集能力,输导体系制约油气聚集能力,储集条件影响油气储量大小,圈闭发育决定油气保存结果。40~15 Ma时期,文昌组-恩平组大量生烃,具备形成早期油气藏的条件;中中新世开始的东沙运动使得文昌组-恩平组早期形成的油气藏发生调整,于晚期再次成藏;中中新世晚期,恩平组SQ7层序开始生烃,利于晚期成藏。文昌组-恩平组的成藏模式分为早期成藏、早期成藏-晚期补充以及晚期成藏3种。     

RELATIONSHIPS BETWEEN HYDROCARBON GENERATION, COAL TYPE AND RANK FOR MIDDLE EOCENE COALS, BULLER COALFIELD, NEW ZEALAND

The Buller Coalfield in the northern portion of the Paparoa Trough (NW South Island, New Zealand) contains a middle Eocene bituminous coal-bearing succession that exhibits marked variations in both coal rank and type. The across-basin rank changes result from differential late Palaeogene subsidence of the Paparoa Trough and subsequent inversion. Superimposed upon coalification trends are down-seam variations in coal type, evidenced by isorank variation in conventional chemical parameters. These type variations are not a consequence of changes in maceral group proportions, which are dominated by vitrinite. Forty-eight coal samples from fifteen drillholes through the Brunner Coal Measures have been examined for vitrinite reflectance, proximate analysis, specific energy, and sulphur content. Total Organic Carbon (TOC), Rock-Eval properties and the bulk composition of the bitumen were also determined. All the coals analysed lie within the high volatile bituminous B to medium volatile bituminous coalification stages. Variation in analytical properties within this suite, and more specifically within serial samples, provides insights into the nature of the organic matter comprising the coals. While TOC varies systematically with ash content, reflecting proximity of the depositional mires to fluvial systems, values increase with rank (68–86% ash free). The Hydrogen Index (HI) decreases from 334 to 190 mg hydrocarbons/g TOC over a range of 429 to 470 d?C Tmax, analogous to the volatile matter content and vitrinite reflectance, respectively. The more perhydrous coal samples at a given rank are characterised by depressed Tmax/vitrinite reflectance and elevated Hi/volatile matter contents. Bitumen and sulphur contents are not the primary control onperhydrous characteristics. The bitumen content crudely corresponds to the S1 peak plus the initial portion of the S2 peak liberated under low temperatures during Rock-Eval pyrolysis; however, the bulk of the generative potential is associated with the residual kerogen fraction. Suggate (S) Rank and maximum palaeotemperature appear to be the best indicators of coal rank. A period of significant hydrocarbon generation and release begins at Tmax～440d?C for the Buller coals. The main oil “window” is defined by the interval 440–455d?C Tmax or 0.8–1.1% vitrinite reflectance (Ro_max), and by correlation to maximum palaeo-temperatures, 125–155d?C. This is further substantiated by mass balance considerations, which indicate that hydrocarbon generation (Petroleum Generation Index (PG1) > 0.1) was occurring in the coals at a maturity level of O. 7–0.9 % Ro_max (Tmax 440–444 d?C) with an increase in PGI between ～0.9 and 1.1% Ro_max (～Tmax 445–455 d?C). Between PGI 0.1 and 0.4, the expulsion efficiency rapidly increased, presumably due to generation and expulsion of the bulk of the oil.