Geology of the Devonian black shales of the Appalachian Basin

Black shales of Devonian age in the Appalachian Basin are a unique rock sequence. The high content of organic matter, which imparts the characteristic lithology, has for years attracted considerable interest in the shales as a possible source of energy. The recent energy shortage prompted the U.S. Department of Energy through the Eastern Gas Shales Project of the Morgantown Energy Technology Center to underwrite a research program to determine the geologic, geochemical, and structural characteristics of the Devonian black shales in order to enhance the recovery of gas from the shales. Geologic studies by Federal and State agencies and academic institutions produced a regional stratigraphic network that correlates the 15 ft black shale sequence in Tennessee with 3000 ft of interbedded black and gray shales in central New York. These studies correlate the classic Devonian black shale sequence in New York with the Ohio Shale of Ohio and Kentucky and the Chattanooga Shale of Tennessee and southwestern Virginia. Biostratigraphic and lithostratigraphic markers in conjunction with gamma-ray logs facilitated long-range correlations within the Appalachian Basin. Basinwide correlations, including the subsurface rocks, provided a basis for determining the areal distribution and thickness of the important black shale units. The organic carbon content of the dark shales generally increases from east to west across the basin and is sufficient to qualify as a hydrocarbon source rock. Significant structural features that involve the black shale and their hydrocarbon potential are the Rome trough, Kentucky River and Irvine-Paint Creek fault zone, and regional decollements and ramp zones.     

中三叠世鄂尔多斯盆地富有机质页岩发育的天文调控

地球轨道周期引起的气候波动是富有机质页岩发育的重要驱动因素。陆相页岩沉积相变快, 非均质性强, 天文信号识别和解译面临一定挑战。在中三叠世拉丁期, 鄂尔多斯盆地广泛发育了长7段富有机质页岩, 是研究陆相有机碳埋藏和古气候变化的优良素材。本研究选取鄂尔多斯盆地伊陕斜坡南部Y1011钻孔中三叠统长7段地层为分析对象。长7段取芯长度为65.93 m, 主要岩性为黑色页岩、灰色粉砂岩和灰黑色泥质粉砂岩组合。使用便携式X射线荧光光谱仪在长7段岩芯表面测得1265条数据, 使用碳/氮分析仪获取了长7段207块岩芯样品的总有机碳(TOC)与总氮(TN)含量。结果显示, TOC变化范围为0.38%~14.87%, C/N比值范围为3.98~15.63, 表明浮游藻类是沉积物有机质的主要来源, 同时部分混合了陆生维管束植物的贡献; 通过长7段Rb/Sr与TOC时间序列的频谱分析, 识别出稳定的地球轨道周期信号; 对最显著的21.8 m沉积旋回进行天文调谐, 获得长7段沉积持续时间约为1.29 Ma; 结合高精度年龄锚点, 揭示出长7段底界面和顶界面的年龄分别为241.56±0.22 Ma和240.27±0.22 Ma。长7段页岩沉积非均质性特征、古生产力与有机质保存旋回变化的响应周期均为405 ka; 偏心率对岁差具有调制作用, 通过驱动季风降雨与大陆风化的波动, 控制湖盆初级生产力输入旋回。对硫酸盐还原指数(SRI)时间序列提取到405 ka旋回, 揭示了有机质的消耗与保存在地球轨道力影响下的周期性演化。在405 ka长偏心率高值时期, 高生产力与保存条件的耦合有利于长7段页岩有机质富集。拉丁期湿润气候事件与秦岭造山带火山活动等地质事件对富有机质页岩沉积具有协同控制作用。本研究为探索地球多圈层相互作用控制的陆相有机碳埋藏提供了案例参考。

     

Bulge Migration and Pinnacle Reef Development, Devonian Appalachian Foreland Basin

Detailed stratigraphic analyses of Late Emsian and Early Eifelian (Lower to Middle Devonian) carbonate-dominated strata in the northern Appalachian Basin indicate anomalous, locally varying relative sea level changes and inversions of topography. The distribution of a major basal-bounding unconformity, basinal pinnacle reefs, local absence of parasequences, and eastward migration of shallow marine carbonate lithofacies and related biofacies in the Onondaga Limestone and underlying strata mark the retrograde migration of an elongate, northeast-southwest-trending area of positive relief, bordered on its cratonward side by a similarly migrating basin of intermediate depth. These features are thought to represent the forebulge and back-bulge basin of the Appalachian foreland basin system as it developed during a time of relative quiescence within the Acadian Orogeny. However, the relatively small size of the bulgelike feature (ca. 80-100-km-wide, 20-50-m positive relief), its great distance from the probable deformation front (>400 km), and the lack of a well-developed foredeep immediately adjacent to the bulgelike feature may indicate that it represents a smaller-scale flexural high ("flexural welt") superposed over the cratonward edge of the larger-scale classical forebulge of the basin. Development of shallow-water reefs on the crest of the bulge during sea level lowstand, followed by migration of the bulge and widespread transgression, permitted growth of economically significant pinnacle reefs in the deep basin center. Further subsurface reef exploration should concentrate along the projected position of the bulge during the basal Onondaga lowstand.     

Pyrolysis gas chromatography-mass spectrometry to characterize organic matter and its relationship to uranium content of Appalachian Devonian black shales

Gas Chromatographic analysis of volatile products formed by stepwise pyrolysis of black shales can be used to characterize the kerogen by relating it to separated, identified precursors such as land-derived vitrinite and marine-source Tasmanites. Analysis of a Tasmanites sample shows exclusively $\text{n-}\text{alkane}$ and -alkene pyrolysis products, whereas a vitrinite sample shows a predominance of one- and two-ring substituted aromatics. For core samples from northern Tennessee and for a suite of outcrop samples from eastern Kentucky, the organic matter type and the U content (<10?120ppm) show variations that are related to precursor organic materials. The samples that show a high vitrinite component in their pyrolysis products are also those samples with high contents of U.     

Geochemical characteristics and sedimentary environment of the Middle Devonian organic-rich shales in the Northwest of Guizhong Depression,Southwest China

In order to figure out the redox conditions and paleo-sedimentary environment of the Middle Devonian shales in the northwest of Guizhong Depression, the trace element analysis was conducted on the Middle Devonian cores (320.35–938.50 m) of the typical shale gas investigation well (GY-1) at a 1.50 m sampling interval through X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). According to the test result, the average values of V/(V+Ni), V/Cr and Ni/Co in Nabiao formation (Fm.) are larger than 0.67, 4.65 and 7.71 respectively, and Nabiao Fm. is rich in biological assemblages such as tabasheer, ammonite, etc. These evidences indicate the rising sea level rose relatively in the sedimentation period of Nabiao Fm. and a deepwater shelf environment, which was favorable for the preservation of organic matters. The V/(V+Ni), V/Cr and Ni/Co in Luofu Fm. and Tangting Fm. are 0.38–0.65, 0.73–4.10 and 3.70–6.72 respectively, indicating that the sea level dropped relatively in their sedimentation period, during which the water bodies became shallow, and the sedimentary environment was a weak oxidizing shallow water shelf environment. In addition, the variation of TOC has a high correlation with the enrichment degree of Ba element, indicating the favorable conditions for the enrichment and preservation of organic matters under an oxygen-deficient environment. Moreover, according to the identification of trace element indexes, the northwest of Guizhong Depression experienced the sedimentary cycle of relative rise to relative fall of sea level from bottom to top in the Middle Devonian sedimentation period. The relative sea level rose to the highest in the sedimentation period of Nabiao Fm., in which the organic-rich shales with stable thickness and high organic content were deposited. Hence, the Nabiao Fm. could be regarded as the favorable exploration target interval in this area.     

Petrophysical evaluation of subsurface reservoir sandstones of Bengal Basin,Bangladesh

The present study has been carried out for core and log based petrophysical evaluation of subsurface reservoir sandstones considering at least one well from each of the Kailas Tila, Titas, Bakhrabad and Shahbazpur gas fields of Bangladesh. Core analysis results show that average core porosity, permeability and pore throat values are 20%, 209 mD and 44020 Å, respectively. These measured values support that the sandstone reservoirs are categorized as good quality reservoirs. Core porosity values usually exceed thin section porosity. Different cross-plots indicate that permeability is largely dependent on porosity. Permeability and porosity are also dependent on textural parameters that include size, shape, sorting and matrix of the reservoir sandstones. It reveals from both thin section study and different cross plots that diagenetic cements are the main controlling factors of the reservoirs. Quantitative determination of the volume of cements shows that cements have inverse correlation with porosity and permeability. Log based petrophysical parameters include shale volume, porosity, water and hydrocarbon saturation, permeability, moveability index and bulk volume of water. The average values of the mentioned parameters are 20%, 22%, 26%, 74%, 110 mD, 0.28 and 0.05, respectively. The most important parameters are porosity and permeability, which indicate that log porosity exceeds core porosity and log permeability significantly lags behind core permeability. The study also reveals that 23 gas zones covering total thickness of 385 m sandstones mostly posses good quality reservoirs except few moderate quality reservoirs.     

Deformation partitioning during transpression in response to Early Devonian oblique convergence,northern Appalachian orogen,USA

Transpressive deformation was distributed heterogeneously within the Central Maine belt shear zone system, which formed in response to Early Devonian oblique convergence during the Acadian orogeny in the northern Appalachians. ‘Straight’ belts are characterized by tight folds, S>L fabrics and sub-parallel form lines, and asymmetric structures that together indicate dextral–SE-side-up kinematics. In contrast, intervening zones between ‘straight’ belts are characterized by open folds and L≫S fabrics. Within both types of zone, metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade, including a penetrative, moderately to steeply NE-plunging mineral lineation. Thus, we interpret accumulation of plastic deformation and regional metamorphic (re-) crystallization to have been synchronous across the Central Maine belt shear zone system. Discordance between inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in ‘straight’ belt rocks records shortening by tightening of folds and greater reorientation of matrix fabrics with respect to porphyroblasts. Kinematic partitioning of flow was responsible for the contrasting states of finite deformation recorded in the Central Maine belt shear zone system. Perturbations in the flow were caused by serially developed thrust-ramp anticlines in the stratigraphic succession immediately above the Avalon-like basement, at which décollement of the shear zone system was initially rooted. General shear deformation at the ramps involved strain softening with an enhanced component of noncoaxial flow. In contrast, deformation during extrusion in the intervening zones involved strain hardening with a greater component of coaxial flow. Part of the thickening stratigraphic succession exceeded T_solidus, reflected by the occurrence of migmatites and granites. The latter were partly sourced from the underlying Avalon-like basement that was involved in the deformation and melting.     

Pore water evolution in sandstones of the Groundhog Coalfield,northern Bowser Basin,British Columbia

The succession of sandstone cements in chert and volcanic lithic arenites and wackes from the northern Bowser Basin of British Columbia comprises a record of diagenesis in shallow marine, deltaic, and coastal plain siliciclastic sediments that pass through the oil window and reach temperatures near the onset of metamorphism. The succession of cements is consistent with seawater in the sandstones mixing with acid waters derived from dewatering of interbedded organic rich muds. Sandstone cement paragenesis includes seven discrete cement stages. From earliest to latest the cement stages are: (1) pore-lining chlorite; (2) pore-lining to pore-filling illite; (3) pore-filling kaolinite; (4) oil migration through some of the remaining connected pores; (5) chlorite dissolution; (6) quartz cement; and (7) calcite cement. These seven cement stages are interpreted as a record of the evolution of pore waters circulating through the sandstones after burial. The earliest cement stages, as well as the depositional environments, are compatible with seawater as the initial pore fluid. Seawater composition changed during transport through the sandstones, first by loss of Mg²⁺ and Fe²⁺ during chlorite precipitation (stage 1). Dewatering of interbedded organic-rich mudstones probably added Mg²⁺ and Fe²⁺ to partially buffer the loss of these cations to chlorite. Acids produced during breakdown of organic matter are presumed to have mixed into sandstone pore fluids due to further compaction of the muds, leading to reduction of initial alkalinity. Reduction in alkalinity, in turn, favours change from chlorite to illite precipitation (stage 2), and finally to kaolinite (stage 3). Pore waters likely reached their peak acidity at the time of oil migration (stage 4). Chlorite dissolution (stage 5) and quartz precipitation (stage 6) occurred when pores were filled by these hydrocarbon-bearing and presumably acidic fluids. Fluid inclusions in fracture-filling quartz cements contain petroleum, high-pressure methane, and methane-rich aqueous solutions. Homogenization temperatures from primary two-phase inclusions are consistent with quartz cementation during progressive heating between approximately 100 and 200°C. Following quartz precipitation, alkaline pore waters were re-established, as evidenced by late-stage calcite cement (stage 7).     

尼日尔Termit盆地上白垩统储层岩石学特征及控制因素分析

本文对尼日尔Termit盆地17口井218个井壁取芯样品开展普通薄片、阴极发光、X衍射、扫描电镜及重矿物分析,研究上白垩统储层的岩石学特征,并探讨其控制因素。实验分析结果表明,Termit盆地上白垩统储层的岩石类型主要为石英砂岩,成分成熟度高,以石英为主,含量达86%以上,钾长石、方解石及斜长石等含少量,占比约5%,这种岩石类型对原生孔隙的保存比较有利;填隙物主要为粘土杂基,含量为7%左右,矿物成分以高岭石为主,利于对晶间孔的保留;岩石中发育硅质、方解石和铁质等胶结类型,胶结物的含量占比较小,对储层的破坏作用不大;重矿物类型为磁铁矿、赤铁矿、锆石、磷灰石,推测物源区的母岩类型主要为花岗岩;以细粒结构和少量的不等粒结构为主,分选中等-差,磨圆度一般为次棱角-次圆状,磨圆差。与国内石英砂岩沉积于高能的滨浅海环境不同,研究区石英砂岩成分成熟度高,但分选程度和磨圆度较低,一方面反映了物源区母岩富含石英,同时也反映了这种石英砂岩沉积于近源的河流-三角洲环境。影响研究区上白垩统储层物性的主要因素是岩石类型及成岩作用,由于储层的岩石类型为石英砂岩,压实作用对储层的破坏作用较一般岩石类型偏小。砂岩埋深小于2500m时,主要发育原生孔隙,面孔率大于15%。溶蚀作用对本区储层产生一定的影响,最常见的为长石的溶蚀,溶蚀作用形成粒内溶孔和粒间溶孔,还会扩大原生孔隙形成混合孔,改善了砂岩的储集性能。     

川西洛带气田蓬莱镇组中上部的储层特征

洛带气田蓬莱镇组中上部天然气藏的储层主要为三角洲前缘的水下分流河道、河口砂坝、水下决口河道和远砂坝微相砂体。储集空间以粒间孔和粒内溶孔为主。储集岩的孔隙度一般为8%～ 16%,渗透率均值为1.970×10^-3μ m²,属低中孔隙度、低渗透率储层。储渗性主要受方解石、硬石膏、长石含量和粘土矿物的影响。沉积微相控制了储层砂体的类型和展布特征。根据区内储集岩的岩石学特征、物性参数和孔喉结构特征,将其划分为四种类型。     

东河储层敏感性参数与测井的响应关系

在岩心分析的基础上,分析和提取塔中"东河砂岩"储层的11个主要敏感性参数,同时利用常规测井方法可获得的岩石骨架、泥质含量、粘土矿物和物性参数等敏感性参数,然后在油田储层敏感性流动实验资料的基础上,运用逐步回归方法建立储层敏感性与测井资料的解释模型,对实际井进行处理分析。结果表明,由测井解释得到的储层敏感性结论与研究区的敏感性流动实验基本一致。     

Resistivity of reservoir sandstones and organic rich shales on the Barents Shelf:Implications for interpreting CSEM data

Marine controlled source electromagnetic(CSEM)data have been utilized in the past decade during petroleum exploration of the Barents Shelf,particularly for de-risking the highly porous sandstone reservoirs of the Upper Triassic to Middle Jurassic Realgrunnen Subgroup.In this contribution we compare the resistivity response from CSEM data to resistivity from wireline logs in both water-and hydrocarbon-bearing wells.We show that there is a very good match between these types of data,particularly when reservoirs are shallow.CSEM data,however,only provide information on the subsurface resistivity.Careful,geology-driven interpretation of CSEM data is required to maximize the impact on exploration success.This is particularly important when quantifying the relative re-sistivity contribution of high-saturation hydrocarbon-bearing sandstone and that of the overlying cap rock.In the presented case the cap rock comprises predominantly organic rich Upper Jurassic-Early Cretaceous shales of the Hekkingen Formation(i.e.a regional source rock).The resistivity response of the reservoir and its cap rock become merged in CSEM data due to the transverse resistance equivalence principle.As a result of this,it is imperative to understand both the relative contributions from reservoir and cap rock,and the geological sig-nificance of any lateral resistivity variation in each of the units.In this contribution,we quantify the resistivity of organic rich mudstone,i.e.source rock,and reservoir sandstones,using 131 exploration boreholes from the Barents Shelf.The highest resistivity(＞10,000 Ωm)is evident in the hydrocarbon-bearing Realgrunnen Subgroup which is reported from 48 boreholes,43 of which are used for this study.Pay zone resistivity is primarily controlled by reservoir quality(i.e.porosity and shale fraction)and fluid phase(i.e.gas,oil and water saturation).In the investigated wells,the shale dominated Hekkingen Formation exhibits enhanced resistivity compared to the background(i.e.the underlying and overlying stratigraphy),though rarely exceeds 20Ωm.Marine mudstones typically show good correlation between measured organic richness and resistivity/sonic velocity log signatures.We conclude that the resistivity contribution to the CSEM response from hydrocarbon-bearing sandstones out-weighs that of the organic rich cap rocks.     

塔中北斜坡碳酸盐岩岩溶储层油气差异富集特征

塔中北斜坡下奥陶统岩溶储层基本为低孔低渗储层,主要的储集空间为溶蚀孔、洞和断裂活动产生的裂缝。储层呈现“横向连片,纵向分层”特点,优质储层主要呈层状叠合分布在不整合面下0~200m范围内的垂直渗流带和水平潜流带。岩溶储层具有大面积、多储集段含油气的特点,平面上整体表现为“西油东气,内油外气”的分布特征。鹰山组直接盖层良3—5段致密灰岩平面上具有“东厚西薄,北厚南薄”的分布特点,剖面上呈现“块状分布,横向相连,纵向叠置”的展布特征。鹰山组内部多套高阻层相互叠置,与下伏含油气层构成良好的配置关系,形成一套或多套储盖组合,控制油气的分层聚集。塔中北斜坡发育着一系列NE向左行走滑断裂,以之为边界,可以分为若干个构造区块。区块内油气水正常分异,相对高的部位聚集油气、低部位出水。块体内部油气多富集在距主干走滑断裂0.5~4.0km范围内。      

Diagenesis in the Middle Jurassic Khatatba Formation sandstones in the Shoushan Basin,northern Western Desert,Egypt

The Middle Jurassic Khatatba Formation acts as a hydrocarbon reservoir in the subsurface in the Western Desert, Egypt. This study, which is based on core samples from two exploration boreholes, describes the lithological and diagenetic characteristics of the Khatatba Formation sandstones. The sandstones are fine‐ to coarse‐grained, moderately to well‐sorted quartz arenites, deposited in fluvial channels and in a shallow‐marine setting. Diagenetic components include mechanical and chemical compaction, cementation (calcite, clay minerals, quartz overgrowths, and a minor amount of pyrite), and dissolution of calcite cements and feldspar grains. The widespread occurrence of an early calcite cement suggests that the Khatatba sandstones lost a significant amount of primary porosity at an early stage of its diagenetic history. In addition to calcite, several different cements including kaolinite and syntaxial quartz overgrowth occur as pore‐filling and pore‐lining cements. Kaolinite (largely vermicular) fills pore spaces and causes reduction in the permeability of the reservoir. Based on framework grain–cement relationships, precipitation of the early calcite cement was either accompanied by or followed the development of part of the pore‐lining and pore‐filling cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar. Late kaolinite clay cement occurs due to dissolved feldspar and has an impact on the reservoir quality of the Khatatba sandstones. Open hydraulic fractures also generated significant secondary porosity in sandstone reservoirs, where both fractures and dissolution took place in multiple phases during late diagenetic stages. The diagenesis and sedimentary facies help control the reservoir quality of the Khatatba sandstones. Fluvial channel sandstones have the highest porosities and permeabilities, in part because of calcite cementation, which inhibited authigenic clays or was later dissolved, creating intergranular secondary porosity. Fluvial crevasse‐splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolinite matrix and pervasive, shallow‐burial calcite and quartz overgrowth cements, respectively. Copyright © 2013 John Wiley & Sons, Ltd.     

鄂尔多斯盆地海陆过渡相富有机质泥页岩特征及页岩气意义

以鄂尔多斯盆地石炭系-二叠系海陆过渡相富有机质泥页岩为研究对象,通过岩石热解分析、岩石抽提和族组分分析、有机元素分析以及煤岩光学分析等方法,对泥页岩中有机质丰度、类型、热成熟度、生烃潜力等地球化学特征参数进行测试;利用扫描电镜、X射线衍射、物性测试等手段,对泥页岩中矿物组成、孔渗特征等储层评价参数进行分析。研究结果表明：鄂尔多斯盆地海陆过渡相地层中泥页岩分布广泛,埋藏偏深,单层厚度较薄,但累计厚度大;泥页岩中有机碳(TOC)含量平均为2.5%,丰度较大;有机质母质中干酪根类型主要为Ⅲ型;镜质体反射率(Ro)平均为1.2%,热演化程度达到中高成熟阶段;具有较大的生烃潜力(S1+S2);同时,石英、长石等脆性矿物含量较高;发育多种孔、缝类型;保存条件良好,为页岩气提供有利的成藏环境。     

Possible indicators of microbial mat deposits in shales and sandstones: examples from the Mid-Proterozoic Belt Supergroup, Montana, U.S.A.

It has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the Proterozoic. However, domination of mat morphology by depositional processes, post-depositional compaction, and poor potential for cellular preservation of mat-building organisms make their positive identification a formidable challenge. Within terrigenous clastics of the Mid-Proterozoic Belt Supergroup, a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces. Among these are: (a) domal buildups resembling stromatolites in carbonates; (b) cohesive behaviour of laminae during soft-sediment deformation, erosion, and transport; (c) wavy–crinkly character of laminae; (d) bed surfaces with pustular–wrinkled appearance; (e) rippled patches on otherwise smooth surfaces; (f) laminae with mica enrichment and/or randomly oriented micas; (g) irregular, curved–wrinkled impressions on bedding planes; (h) uparched laminae near mud-cracks resembling growth ridges of polygonal stromatolites; and (i) lamina-specific distribution of certain early diagenetic minerals (dolomite, ferroan carbonates, pyrite). Although in none of the described examples can it irrefutably be proven that they are microbial mat deposits, the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other Proterozoic sequences.     

鄂尔多斯盆地冯地坑-洪德地区延长组长4+5段储层砂岩成岩作用研究

鄂尔多斯盆地冯地坑-洪德地区长4+5段储层以长石砂岩和岩屑长石砂岩为主。本文通过岩心观察、薄片鉴定、扫描电镜、粘土矿物分析等,研究延长组长4+5段的成岩作用特征,分析其对储层物性的影响,确定储层砂岩的成岩作用阶段。早成岩期的压实作用和多种矿物的胶结作用是造成本区岩石孔隙体积减少的重要因素,而溶解作用尤其是长石的溶蚀作用是储层变好的重要控制因素。     

Carbon isotopic composition and genetic types of natural gas in the Sichuan Basin,China

The origin and genetic types of natural gas in the Sichuan Basin are still disputed. To classify the origin and genetic types in different areas, the paper analyzes the carbon isotopic composition of gases and geologic features in the Sichuan Basin. The results showed that the gas sourced from terrestrial layers is typically characterized by terrestrial origin and was mainly accumulated nearby to form reservoir. The carbon isotopic composition of gas showed a normal combination sequence distribution, suggesting that natural gas in continental strata is not affected by secondary alteration or that this deformation is very weak. The gas source is singular, and only gas from the southern and northern Sichuan Basin shows the characteristic of mixed sources. However, marine gas presents the characteristics of an oil-formed gas. The carbon isotopic composition of natural gas in the western and central part of the basin mostly distributes in a normal combination sequence, and few of them showed an inversion, indicating that the gas perhaps had not experienced secondary alteration. The carbon isotopic composition of marine-origin gas in the southern, northern and eastern Sichuan Basin displays a completely different distribution pattern, which is probably caused by different mixing ratio of gas with multi-source and multi-period.     

Organic geochemistry and petrography of Lower Cretaceous Wealden black shales of the Lower Saxony Basin: The transition from lacustrine oil shales to gas shales

Ninety-seven Wealden black shale samples from three wells in the Lower Saxony Basin have been studied by organic geochemical and organic petrographical methods to determine their maturity, organic facies and depositional environment. The maturities of the three wells range from early mature (Ex-A), late to postmature (Ex-C) to overmature (Ex-B) as determined by vitrinite reflectance measurements, diamondoid ratios and other geochemical maturity parameters. Ex-C and Ex-B show distinct petrographic features related to oil generation and migration. In particular, the occurrence of dispersed solid bitumen replacing initial type I kerogen suggests a formerly active petroleum system. Structural and textural differences between early mature alginites and solid bitumen in postmature to overmature samples show an alteration of the pore system with increasing maturity. A freshwater depositional environment is indicated by widespread occurrence of botryococcus algae and other small alginite particles predominating in the immature well. These alginites are absent in the more mature gas shales of wells Ex-C and Ex-B. Geochemical evidence of algae and phytoplankton in general is provided by numerous biomarker parameters, while the occurrence of β-carotane in some samples indicates events of increased salinity, although no hypersaline conditions are inferred due to very low gammacerane indices. Increased amounts of vitrinite and inertinite in samples of Ex-B suggest locally significant terrigenous input of organic matter for some periods during Wealden Shale deposition. High sulfur/organic carbon ratios provide evidence for sulfate rich waters and (partly) anoxic bottom water conditions. While the lower mature lacustrine source rocks generate paraffinic/waxy oils, gas and condensates are produced at post-mature stages. Furthermore, maturity distribution maps from 3D numerical petroleum systems modeling reveal substantial differences in respect to petroleum generation.     

羌塘盆地侏罗系碎屑岩储集层综合评价

运用储层综合评判分析方法对羌塘盆地侏罗系碎屑岩储集层进行分析和评价。首先选择影响储层性质的8种参数,包括定性参数,如岩性、成岩作用、储集空间类型和定量参数,如储层厚度、孔隙结构、物性等,对其进行单因素评价。在此基础上,根据各单因素对储层贡献的大小,确定各单因素的加权系数。最后,利用评判分析方法,计算各剖面的综合判别得分。综合判别分析评价表明,北羌塘坳陷中部、东部和中央隆起带北缘发育好储层;北羌塘坳陷西部发育中等储层;而南羌塘坳陷储层较差。