Mechanisms of coal metamorphism: case studies from Paleozoic coalfields

Controls on coal metamorphism can be complex. In this paper, we examine four Paleozoic coalfields: the western Kentucky portion of the Illinois Basin, the Pennsylvania anthracite fields, the South Wales Coalfield, and the Bowen Basin. An increase in temperature with depth of burial is certainly a factor in coal metamorphism. In many coalfields, however, including the coalfields reviewed here, it has become apparent that such a simple mechanism does not explain the coal rank patterns observed. The flow of hydrothermal fluids through the coals has been proposed as a cause of coal metamorphism. Evidence includes inverted rank gradients, elevated CFL as an indicator of brine fluids, isotopic evidence for hydrothermal fluids, and vein and cleat mineral assemblages. In any case, multiple hypotheses must often be evaluated in the examination of any coalfield since the simple paradigm of coal rank increases with a simple increase in temperature with increasing depth does not fit the evidence observed in many cases.     

Coal rank variation in the Intrasudetic Basin,SW Poland

The Upper Carboniferous, coal-bearing sequence of the Intrasudetic Basin (SW Poland) includes coals ranging from high-volatile bituminous to anthracitic rank. The lowest values of reflectance are recorded around the basin margins (0.6% R₀ max), the highest ones appear in the center of the basin (exceeding 4% R₀ max). Reflectance gradients are very high, reaching 0.6%/100 m in the centre of the basin.A comparison of the isoreflectance maps for three lithostratigraphical units—the Walbrzych, Bialy Kamien and Zacler Formations, with the present-day burial depth and the depth of burial during the Westphalian B/C—indicates that there is a strong relationship between reflectance and the sediment cover during the Westphalian B/C, particularly in the vicinity of Walbrzych and Lubawka. This suggests that the increase in coal rank is related to the increase in cover which permitted the temperature to build-up to high values.In the eastern and central parts of the basin and the Nowa Ruda area, higher reflectance than that derived from burial depth is observed which is believed to result from higher heat flow from the basement. The volcanic rocks of the Intrasudetic Basin appear to have little effect on coal rank and are not considered to be a significant contributor to the heat flow of the region.During coalification, the oldest Westphalian coal seams were buried to about 700 m and the youngest seams of the Walbrzych Formation to 900 m. Around the basin margins the coals had reached their present-day rank by the Westphalian B/C and in the central part probably by the end of the Stephanian. Most effective coalification took place during the Westphalian A,B occupying a period of less than 20 million years. The coalification temperature is calculated to be 160–170°C with a geothermal gradient of 8–10°C/100 m. These geothermal conditions support the suggestion of a volcanic origin for the Intrasudetic Basin.     

Determination of rank and petrographic composition of Jurassic coals from eastern Surat Basin,Australia

Petrographic analysis and rank determination were carried out on coals from a Jurassic sequence in eastern Surat Basin, Australia. The coals consist mainly of exinite-rich clarite, with desmocollinite as dominant maceral of the vitrinite group. Petrographically there is no significant variation in the composition of the coals. A herbaceous swamp type, free from severe oxidation/ dehydration, appears to have been a dominant depositional environment during the peat accumulation.The coal rank ranges from sub-bituminous B to high-volatile bituminous C/B. Vitrinite reflectance/ depth profile shows a uniform increase in coalification with depth of burial.     

划分成岩作用与埋藏变质作用的指标及其界线

本文是对“陕北鄂尔多斯盆地埋藏变质作用研究”一文的讨论,它综述并讨论了划分成岩作用和埋藏变质作用的指标,界线及其在鄂尔多斯盆地中的应用。伊利石结晶度、粘土矿物、浊沸石胶结物,石油形成,镜质体反射率,煤阶和地温等指标一致表明,鄂尔多斯盆地中生代地层处于中期成岩作用阶段,从而否定了本区为埋藏变质地区的结论。     

In situ stress field of eastern Australia

New in situ data based on hydraulic fracturing and overcoring have been compiled for eastern Australia, increasing from 23 to 110 the number of in situ stress analyses available for the area between and including the Bowen and Sydney Basins. The Bowen Basin displays a consistent north‐northeast maximum horizontal stress (σ_H) orientation over some 500 km. Stress orientations in the Sydney Basin are more variable than in the Bowen Basin, with areas of the Sydney Basin exhibiting north‐northeast, northeast, east‐west and bimodal σ_H orientations. Most new data indicate that the overburden stress (σ_V) is the minimum principal stress in both the Bowen and Sydney Basins. The Sydney Basin is relatively seismically active, whereas the Bowen Basin is relatively aseismic. Despite the fact that in situ stress measurements sample the stress field at shallower depth than the seismogenic zone, there is a correlation between the stress measurements and seismicity in the two areas. Mohr‐Coulomb analysis of the propensity for failure in the Sydney Basin suggests 41% of the new in situ stress data are indicative of failure, as opposed to 13% in the Bowen Basin. The multiple pre‐existing structural grains in the Sydney Basin further emphasise the difference between propensity for failure in the two areas. Previous modelling of intraplate stresses due to plate boundary forces has been less successful at predicting stress orientations in eastern than in western and central Australia. Nonetheless, stress orientation in the Bowen Basin is consistent with that predicted by modelling of stresses due to plate boundary forces. Variable stress orientations in the Sydney Basin suggest that more local sources of stress, such as those associated with the continental margin and with local structure, significantly influence stress orientation. The effect of local sources of stress may be relatively pronounced because stresses due to plate boundary forces result in low horizontal stress anisotropy in the Sydney Basin.     

宁武-静乐含煤盆地构造-热演化史恢复

宁武-静乐盆地是华北地区重要的含煤构造盆地,是山西省煤层气勘探主要区域。本文基于镜质组反射率及磷灰石裂变径迹古温标,恢复了盆地的构造-热演化史。研究结果表明,石炭系太原组煤层的Ro值主要反映中生代晚期地层达到最大埋深时的古地温状况,根据最大古埋藏深度,获得盆地的古地温梯度约3.88 ℃/100 m～4.34 ℃/100 m。裂变径迹资料抬升冷却分析,得出盆地经历了70～63 Ma和38～20 Ma以来的两次快速抬升。构造-热演化史研究表明,盆地经历的最大古地温时期是在晚侏罗世-早白垩世达到的,该时期是煤层气主要生成期。晚白垩世期盆地抬升冷却,地温明显降低,生气作用减弱及停止。镜质组反射率和裂变径迹所揭示的热演化史对于深化认识宁武-静乐盆地的煤层气勘探开发具重要意义。     

Inertinite-rich Tertiary coals from the Zeya–Bureya Basin, Far Eastern Russia

Selected Tertiary coals from the Zeya–Buryea Basin, Far Eastern Russia, were investigated for aspects of their coal type, rank, depositional environment and post-depositional history. The coals have been examined in outcrop (lithotype logging), microscopically (maceral, reflectance and fluorescence), and geochemically (proximate analysis).Two laterally extensive coal-bearing horizons occur: one of Palaeocene age and the other of early Miocene age. The Palaeocene coals were investigated in active open-cut mines at Raichikhinsk and Yerkovtsi and the early Miocene deposit in an abandoned open-cut mine at Cergeyevka.Palaeocene coals at Raichikhinsk and Yerkovtsi were indistinguishable from each other macroscopically, microscopically, and geochemically. The deposits were sufficiently coalified that brightness logging could be undertaken. Dull coals, with numerous fusainous wisps, were dominant. Four dulling-up sequences, which represent stacked peat deposits, were observed at Raichikhinsk. At Yerkovtsi, only a small section of the middle of the seam, which was mostly dull and muddy coal, was investigated. Petrographically, these coals were dominated by inertinite group macerals, which is unusual in non-Gondwanan coals and rare in the Tertiary. Rank classification was problematic with volatile matter (VM) content of vitrain (daf), macroscopic appearance, and microscopic textures suggesting subbituminous B rank, but carbon content, moisture content and specific energy indicating a lignite rank.Notwithstanding complications of rank, estimates of the maximum-range burial depths were calculated. Taking the VM (daf) content of vitrain as 48%, burial depth estimates range from 900 m for a high geothermal gradient and long heating time to a maximum of 3300 m for a low geothermal gradient and short heating time. These estimates are maxima as the coal rank may be lower than implied by the VM.The Cergeyevka deposit is a soft brown coal. Limited sampling of the upper-most portion indicated a high moisture content (75% daf) and an unusual, hydrogen-rich geochemistry. Lack of identifiable liptinites using either reflected light or fluorescence microscopy suggested a significant bituminite component. Otherwise, the coals appear to be typical for the Tertiary. An estimate of 125 m maximum burial depth was obtained using the bed-moisture content of the coal, which is around the present burial depth.Comparison of present-day thicknesses with inferred burial depths suggests that at least 500 m of section is missing between the Palaeocene coals and the early Miocene coals.Palaeoenvironmental considerations suggest that fire played a significant role in the accumulation of the peats at Raichikhinsk and Yerkovtsi. At Cergeyevka, peat accumulation ended by drowning of the mire.Two tuff beds were recognised within the seam at Raichikhinsk and one in the seam at Yerkovtsi. Correlation of the tuff beds is uncertain but they should prove useful in regional coal seam correlation and interpreting coal depositional environments. Geochemical analysis by XRF was complicated by high loss-on-ignition (LOI) values. Despite extensive alteration, an acid igneous source is implied from the presence of free quartz and TiO₂/Al₂O₃ ratios of 0.02 to 0.05.     

Estimation of relative depth of burial using vitrinite reflectance: Implications for a sedimentary basin formation by basement tilting

The kinetics of coalification are still debatable; the role of time has not been conclusively assigned. If accepting the importance of time, we have not reached consensus in formulation of the kinetics.Here we show that estimation of burial depth is possible under some conditions using vitrinite reflectance neither with computation of a specific kinetic equation of organic metamorphism nor with postulating plausible geothermal gradients of the past. The present technique is based only on empirical linear relations between vitrinite reflectance and burial depth, and is demonstrated to be effective for investigation of syn-depositional tectonics. The authors applied the technique to Miocene coals of the Chichibu Basin, Central Japan, and this study indicates that the sedimentary basin was formed by tilting of the basement rocks.     

Modeling of gas generation from the Alam El-Bueib formation in the Shoushan Basin, northern Western Desert of Egypt

The Shoushan Basin is an important hydrocarbon province in the northern Western Desert, Egypt, but the burial/thermal histories for most of the source rocks in the basin have not been assigned yet. In this study, subsurface samples from selected wells were collected to characterize the source rocks of Alam El-Bueib Formation and to study thermal history in the Shoushan Basin. The Lower Cretaceous Alam El-Bueib Formation is widespread in the Shoushan Basin, which is composed mainly of shales and sandstones with minor carbonate rocks deposited in a marine environment. The gas generative potential of the Lower Cretaceous Alam El-Bueib Formation in the Shoushan Basin was evaluated by Rock–Eval pyrolysis. Most samples contain sufficient type III organic matter to be considered gas prone. Vitrinite reflectance was measured at eight stratigraphic levels (Jurassic–Cretaceous). Vitrinite reflectance profiles show a general increase of vitrinite reflectance with depth. Vitrinite reflectance values of Alam El-Bueib Formation range between 0.70 and 0.87 VRr %, indicating a thermal maturity level sufficient for hydrocarbon generation. Thermal maturity and burial histories models predict that the Alam El-Bueib source rock entered the mid-mature stage for hydrocarbon generation in the Tertiary. These models indicate that the onset of gas generation from the Alam El-Bueib source rock began in the Paleocene (60 Ma), and the maximum volume of gas generation occurred during the Pliocene (3–2 Ma).     

贵州织纳煤田水公河向斜上二叠统8煤层三史模拟

埋藏史、受热史和有机质成熟史(简称"三史")模拟技术已广泛应用于油气地质研究,但其在煤和煤层气地质领域则应用较少。根据实测煤的镜质组最大反射率、古地温梯度和地层残余厚度等资料,运用Petromod 1D模拟软件对贵州织纳煤田水公河向斜上二叠统8煤层"三史"演化过程进行研究。结果显示,水公河向斜地质演化史可分为3个阶段:第1阶段从晚二叠世至侏罗纪末,煤层埋深增加,最大至5 925~5 959 m,地温梯度为2.91/hm℃,受深成变质作用影响,煤级从褐煤演化至焦煤阶段;第2阶段从侏罗纪末至白垩纪末,受燕山运动影响,煤层埋深降低,地温梯度为3.29~3.46/hm℃,区域岩浆热变质作用使煤级从焦煤演化至无烟煤阶段;第3阶段从白垩纪末至今,地层少量沉积后发生抬升作用,地温梯度为2.78℃/hm,煤化作用基本停止。总之,深成变质作用是向斜煤层气生成的主要影响原因,距今230~170 Ma是煤有机质生烃的关键时期。      

Rare earth element fractionation in authigenic illite–smectite from Late Permian clastic rocks,Bowen Basin,Australia: implications for physico-chemical environments of fluids during illitization

REE analyses were performed on authigenic illitic clay minerals from Late Permian mudrocks, sandstones and bentonites from the Bowen Basin (Australia). The mixed-layer illite–smectite exhibit REE patterns with an obvious fractionation of the HREE from the LREE and MREE, which is an apparent function of degree of illitization reaction. The highly illitic (R≥3) illite–smectite from the northern Bowen Basin show a depletion of LREE relative to the less illitic (R=0 and 1) clays. In contrast, an enrichment of HREE for the illite-rich clays relative to less illitic clays is evident for the southern Bowen Basin samples. The North American Shale Composite-normalized (La/Lu)_sn ratios show negative correlations with the illite content in illite–smectite and positive correlations with the δ¹⁸O values of the clays for both the northern and southern Bowen Basin samples. These correlations indicate that the increasing depletion of LREE in hydrothermal fluids is a function of increasing water/rock ratios in the northern Bowen Basin. Good negative correlations between (La/Lu)_sn ratios and illite content in illite–smectite from the southern Bowen Basin suggest the involvement of fluids with higher alkalinity and higher pH in low water/rock ratio conditions. Increasing HREE enrichment with δ¹⁸O decrease indicates the effect of increasing temperature at low water/rock ratios in the southern Bowen Basin.Results of the present study confirm the conclusions of some earlier studies suggesting that REE in illitic clay minerals are mobile and fractionated during illitization and that this fact should be considered in studies of sedimentary processes and in identifying provenance. Moreover, our results show that REE systematic of illitic clay minerals can be applied as an useful technique to gain information about physico-chemical conditions during thermal and fluid flow events in certain sedimentary basins.     

柴达木盆地石炭系油气调查最新进展

柴达木盆地石炭系是海相油气勘探的新领域,通过对柴达木盆地石炭系油气地质条件进行调查,评价其资源潜力,为该区寻找油气战略接替区提供科学依据。以柴达木盆地石炭系为主要目标,通过调查和研究,首次获得柴达木盆地石炭系油气流,在柴达木盆地古生界海相新层系油气方面取得重要发现;综合区域调查、平衡剖面反演及镜质体反射率等分析,获取柴达木盆地主要构造运动期次;结合盆地模拟技术,重建了石炭系埋藏史,分析了柴达木盆地德令哈坳陷石炭系烃源岩油气地化指标,研究了烃源岩生烃演化史。研究表明:柴达木盆地德令哈坳陷石炭系烃源岩广泛发育,有机质丰度较高,埋深较大,但未发生变质,处于成熟-高成熟阶段;德令哈坳陷石炭系埋藏史主要表现为快速埋藏期、稳定期与强烈抬升剥蚀期,新生代以来沉降史与抬升剥蚀史存在着差异;石炭系烃源岩热演化史主要表现为“存在二次生烃,晚期生烃为主”的特点,主要受该区构造运动控制;柴达木盆地热演化总体表现为缓慢降低的趋势,主要受控于柴达木盆地岩浆热事件与构造活动。调查分析表明,新层系石炭系油气条件良好,资源前景广阔,是柴达木盆地下一步勘探的接替层系。     

柴达木盆地石炭系油气调查最新进展

柴达木盆地石炭系是海相油气勘探的新领域,通过对柴达木盆地石炭系油气地质条件进行调查,评价其资源潜力,为该区寻找油气战略接替区提供科学依据。以柴达木盆地石炭系为主要目标,通过调查和研究,首次获得柴达木盆地石炭系油气流,在柴达木盆地古生界海相新层系油气方面取得重要发现; 综合区域调查、平衡剖面反演及镜质体反射率等分析,获取柴达木盆地主要构造运动期次; 结合盆地模拟技术,重建了石炭系埋藏史,分析了柴达木盆地德令哈坳陷石炭系烃源岩油气地化指标,研究了烃源岩生烃演化史。研究表明: 柴达木盆地德令哈坳陷石炭系烃源岩广泛发育,有机质丰度较高,埋深较大,但未发生变质,处于成熟—高成熟阶段; 德令哈坳陷石炭系埋藏史主要表现为快速埋藏期、稳定期与强烈抬升剥蚀期,新生代以来沉降史与抬升剥蚀史存在着差异; 石炭系烃源岩热演化史主要表现为“存在二次生烃,晚期生烃为主”的特点,主要受该区构造运动控制; 柴达木盆地热演化总体表现为缓慢降低的趋势,主要受控于柴达木盆地岩浆热事件与构造活动。调查分析表明,新层系石炭系油气条件良好,资源前景广阔,是柴达木盆地下一步勘探的接替层系。     

鄂尔多斯盆地东部深层煤层气成藏地质条件分析

       鄂尔多斯盆地东部目前的煤层气开发深度浅于1200 m,丰富的深部煤层气资源有待勘探开发。鉴于此,本文分析了 区内深部煤层赋存特征、热演化程度、煤层含气量及其分布规律。认为上古生界埋深的总体分布格局与两个向西凸出浅埋 带的叠加,可能导致煤层含气性和渗透性区域分布格局的进一步复杂化。发现上古生界煤阶区域分布格局与煤层埋深类似, 指示深成变质作用对区域煤阶分布起着主要控制作用。预测了深部煤层含气量的分布格局,认为煤层埋藏一旦超过临界深 度时煤层含气量随深度的加大反而有所降低。建立了自生自储、内生外储两类煤成气藏模式,认为吸附型煤层气藏与游离 型煤成气藏的共生为合采提供了丰厚的资源条件。     

Thermal history of the Hodgkinson Province and Laura Basin,Queensland: Multiple cooling episodes identified from apatite fission track analysis and vitrinite reflectance data

Apatite fission track analysis and vitrinite reflectance data from outcrop and well samples in the Hodgkinson Province and Laura Basin reveal regional Cretaceous cooling. Apatite fission track analysis appears to define two discrete cooling episodes, in the mid‐Cretaceous (110–100 Ma) and Late Cretaceous (80–70 Ma), although in most samples data allow only definition of a single episode. Rocks now at outcrop cooled from Cretaceous palaeotemperatures generally between 50 and 130°C in the south of the region, and from >100°C in the north. Some samples from the Hodgkinson Province also show evidence for an Early Jurassic cooling episode, characterised by maximum palaeotemperatures varying from at least 95°C (from apatite fission track analysis) to ～200–220°C (from vitrinite reflectance), with cooling beginning at around 200 Ma. Apatite fission track analysis data do not reveal the earlier event in the Laura Basin, but on the basis of vitrinite reflectance data from Permian? units this event is also inferred to have affected the pre‐Jurassic basin units in this region. The regional extent of the Cretaceous cooling episode in the Hodgkinson Province suggests that the elevated palaeotemperatures in this region were most likely due to greater depth of burial, with subsequent cooling due to kilometre‐scale denudation. For a palaeogeothermal gradient of 30°C/km and a palaeosurface temperature of 25°C the total degree of Cretaceous cooling experienced by the samples corresponds to removal of between ～0.8 and >3.0 km of Triassic and younger section removed by denudation, beginning some time between ca 110 and 80 Ma. Higher palaeogradients would require correspondingly lower amounts of removed section. The geology of the Laura Basin suggests that an explanation of the observed Cretaceous palaeotemperatures in this region in terms of deeper burial may be untenable. Heating due to hot fluid flow may be a more realistic mechanism for producing the observed Cretaceous palaeothermal effects in the Laura Basin.     

Cooling in rifting sequences during increasing burial depth due to heat flow decrease

Two case histories are presented to give evidences for sediment cooling during increasing burial depth due to heat flow decrease at the end of crustal stretching in extensional settings. The first refers to the Lower Cretaceous succession accumulated in a strongly subsiding trough within the Sirt Basin (Libya); the second relates to the Mesozoic succession of the Lombardian Basin (NW Italy) formed during Late Triassic–Early Jurassic rifting of the northern margin of the Adriatic microplate. In both cases, heat flow decreasing at the end of crustal stretching overbalanced the thermal effect of increasing burial depth causing a net cooling of rocks. These examples provide an alternative to exhumation for explaining cooling events recorded by rifting sedimentary sequences.     

Thermal maturity in the Central European Basin system (Schleswig-Holstein area): results of 1D basin modelling and new maturity maps

Thermal maturity information has been compiled for one of the deepest parts of the Central European Basin system, the Schleswig-Holstein area in northern Germany. New vitrinite reflectance data were obtained and old data were evaluated from a total of 31 wells. Furthermore, numerical 1D basin modelling was performed in order to interpolate/extrapolate vitrinite reflectance data to base Zechstein and base Keuper. For these two horizons, maturity maps were finally compiled revealing large differences in present-day thermal maturity within the basin. For example, vitrinite reflectance at base Zechstein ranges from greater than 4.5% in the Glueckstadt Graben to less than 1% in the northern part of the study area. Highest thermal maturity of base Keuper occurs in the West Holstein Trough and in the northern part of the Glueckstadt Graben. The timing of maturation is greatly affected by the complex tectonic evolution of the area. For six key wells distributed over the entire study area, burial and temperature histories as well as evolution of maturity were evaluated using 1D basin modelling. Deepest burial and maximum temperatures occurred either during Jurassic, Upper Cretaceous or Neogene times in these wells. Only some parts of the sedimentary package in Schleswig-Holstein show a significant increase in maturity during the Tertiary leading to additional hydrocarbon generation and entrapment.     

Maturity studies and source-rock potential in the southern Sverdrup Basin, Arctic Canada

The purpose of the study is to better understand the relationship between organic matter optical properties and the presence of potentially large oil and gas accumulations in Arctic Canada. The type and thermal maturity of the dispersed organic matter of the Mesozoic formations in the southern Sverdrup Basin, Melville Island, have been studied using organic petrology and Rock-Eval pyrolysis.All types of organic matter are present in the strata of Mesozoic age. Hydrogen-rich liptinite is dominated by alginite (Botryococcus and Tasmanites), dinoflagellate cysts and amorphous fluorescing matrix. Sporinite, cutinite, resinite and liptodetrinite made up the lesser hydrogen-rich exinite. Vitrinite reflectance in Cretaceous sediments ranges from 0.36 to 0.65% R_o; in Jurassic sediments it ranges from 0.40 to 1.0% R_o and in the Triassic from 0.45 to 1.30% R_o, showing an overall increase with depth of burial.Cretaceous sediments of the Deer Bay Formation are thermally immature and contain organic matter of terrestrial origin. The Upper Jurassic shales of the Ringnes Formation contain predominantly organic matter of liptinitic and exinitic origin with a considerable vitrinitic input. At optimum maturation levels, potential source beds of this formation would have a good hydrocarbon-generating potential. The hydrocarbon potential, however, would be limited to the generation of gases due to the leanness of the source rocks. Parts of the Lower Jurassic Jameson Bay Formation are organic-rich and contain a mixed exinitic/vitrinitic organic matter, Botryococcus colonial algae but visible organic matter is dominated by high plant remains (mainly spores). The Schei Point Group shales and siltstones contain organic matter of almost purely marine origin, whereas the predominantly higher plant-derived organic matter found in the Deer Bay, Jameson Bay and partly in the Ringnes formations have higher TOC. Among the Schei Point Group samples, the Cape Richards and Eden Bay members of the Hoyle Bay Formation are richer in TOC (>2.0%) than the Murray Harbour Formation (Cape Caledonia Member). This may reflect differences in the level of maturity or in the depositional environment (more anoxic conditions for the former).Regional variations in the level of thermal maturity of Mesozoic sediments in Sverdrup Basin appear to be a function of burial depth. The Mesozoic formations thicken towards the basin centre (NNE direction), reflecting the general pattern of increasing thermal maturity north of Sabine Peninsula. However, the regional thermal-maturation pattern of the Mesozoic is not solely a reflection of the present-day geothermal gradient, which indicates that anomalous zones of high geothermal gradient may have existed in the past, at least since when the Mesozoic sediments attained maximum burial depth. The contour pattern of the regional variation of maturity at the base of numerous Triassic formations is similar to that of the structural contours of the Sverdrup Basin, indicating that present-day maturation levels are largely controlled by basin subsidence.     

A study of hydrocarbon generation and expulsion of the Nkporo Shales in Anambra Basin,Nigeria

This study is a contribution to our understanding of the hydrocarbon potentials of the Nkporo Shale in the Anambra Basin, Nigeria. The main objective was to simulate the hydrocarbon generation and expulsion in the Nkporo source horizons encountered in Igbariam-1, Iji-1, and Ajire-1 wells drilled in the basin, using the one-dimensional Genesis Zetaware basin modeling software. This software enabled the integration of burial and thermal influences on Nkporo source rocks with kinetic parameters, in order to define the timing of hydrocarbon generation and expulsion in relation to the main structural episodes in the basin. The Nkporo source horizons encountered in Igbariam-1 well (2,650 m) has not expelled hydrocarbons but has a remaining potential of 380 mg/gTOC. Conversely, in Iji-1 well (3,450 m) and Ajire-1 well (3,250 m), the Nkporo source rocks expelled oil of about 110–127 mg/gTOC and gas of about 85–125 mg/gTOC. The time of expulsion was in Eocene–Oligocene (51–30 My) at the rate of 10.5–11.5 mg/gTOC/My. The remaining potentials for Nkporo source rocks in Iji-1 and Ajire-1 wells are 160 and 125 mg/gTOC, respectively. The result shows an increasing trend in maturation, generation, and expulsion of hydrocarbons with increasing depth towards the southwestern part of the basin. This resulted in an increase in the generated hydrocarbon volumes to the south due to (a) increased burial depth and early maturation and (b) changing source lithofacies—more marine. The fault system linking potential reservoirs such as Owelli Sandstone to the source rock suggests a possible migration of the generated hydrocarbons to those reservoirs at that time.     

柴达木盆地德令哈坳陷石炭系烃源岩成熟度演化史

德令哈坳陷是柴达木盆地石炭系具有勘探前景的地区。综合区域调查、平衡剖面反演、镜质体反射率等分析,获取了柴达木盆地主要构造运动期次,并结合盆地模拟技术,重建了石炭系埋藏史;结合石炭系烃源岩油气地化指标,研究了德令哈坳陷石炭系烃源岩热演化史。研究表明,德令哈坳陷内石炭系烃源岩广泛发育,有机质丰度较高,埋深较大,但未发生变质,处于成熟-高成熟阶段。德令哈坳陷石炭系埋藏史主要表现为晚石炭世、中侏罗世早期-晚白垩世、渐新世晚期-中新世为快速埋藏期,早二叠世-晚三叠世、早白垩世-始新世为稳定期,晚三叠世末-晚侏罗世、始新世末-中新世、中新世末以来为强烈抬升剥蚀期,沉降史与抬升剥蚀史新生代以来存在着差异;石炭系烃源岩热演化史主要表现为"二次生烃、晚期生烃为主"的特点,主要受控于区域构造运动。晚古生代以来柴达木盆地热演化总体表现为缓慢降低的特征,主要受控于柴达木盆地岩浆热事件与构造活动特征。