gas condensate migration and oil fractionation in deltaic systems abstract the term evaporative fractionation is proposed to describe several phenomena involved in the secondary alteration of reservoired oil firstly it is suggested that oil is frequently partially vaporized in the reservoir secondly that gas bearing substantial portions of the oil in solution is conducted along faults to form independent gas condensate accumulations thirdly that residual oils formed in this fashion bear internal evidence of fractionation in addition to the conspicuous loss of light ends there is an increase in the content of light aromatic and naphthenic hydrocarbons relative to paraffins in the residual oil evaporative fractionation effects are recognized for the first time in the distillation and physical property data of us bureau of mines routine method oil analyses fractionation effects are postulated to be the cause of sigmoid as opposed to normal curvilinear aromatic profiles the effects of evaporative fractionation are widespread in oils throughout the united states they are evident in approximately 75 of oils in the gulf coast region and in a large proportion of the gas condensate accumulations the process is a major and apparently unrecognized means of petroleum alteration and of generation of gas condensates experimental evidence shows that partial vaporization results in enhanced levels of light aromatic hydrocarbons in residual oils oil samples were partially vaporized in excess methane at elevated pressure and temperature followed by removal of the vapour the gas chromatographic features of many gulf coast oils were reproduced various levels of light end depletion and enhancement of aromaticity were brought about

petroleum accumulation in the deeply buried reservoirs in the northern dongying depression bohai bay basin china new insights from fluid inclusions natural gas geochemistry and 1 d basin modeling abstract the deeply buried reservoirs dbrs from the lijin shengtuo and minfeng areas in the northern dongying depression of the bohai bay basin china exhibit various petroleum types black oil gas condensates and pressure systems normal pressure overpressure with high reservoir temperatures 154 185 c the pressure volume temperature composition pvtx evolution of petroleum and the processes of petroleum accumulation were reconstructed using integrated data from fluid inclusions stable carbon isotope data of natural gas and one dimensional basin modeling to trace the petroleum accumulation histories the results suggest that 1 the gas condensates in the lijin area originated from the thermal cracking of highly mature kerogen in deeper formations two episodes of gas condensate charging which were evidenced by the trapping of non fluorescent gas condensate inclusions occurred between 29 25 5 ma and 8 6 5 0 ma with strong overpressure pressure coefficient pc 1 68 1 70 resulting in the greatest contribution to the present day gas condensate accumulation 2 the early yellow fluorescent oil charge was responsible for the present day black oil accumulation in well t764 while the late blue white oil charge together with the latest kerogen cracked gas injection resulted in the present day volatile oil accumulation in well t765 and 3 the various fluorescent colors yellow blue white and blue and the degree of bubble filling fv 2 3 72 5 of the oil inclusions in the minfeng area show a wide range of thermal maturity api gravity ranges from 30 to 50 representing the charging of black oil to gas condensates the presence of abundant blue white fluorescent oil inclusions with high grain obtaining oil inclusion goi values 35 8 usually 5 in oil reservoirs indicate that a paleo oil accumulation with an approximate api gravity of 39 40 could have occurred before 25 ma and gas from oil cracking in deeper formations was injected into the paleo oil reservoir from 2 8 ma to 0 ma resulting in the present day gas condensate oil accumulation this oil and gas accumulation model results in three oil and gas distribution zones 1 normal oil reservoirs at relatively shallow depth 2 gas condensate reservoirs that originated from the mixture of oil cracking gas with a paleo oil reservoir at intermediate depth and 3 oil cracked gas reservoirs at deeper depth the retardation of organic matter maturation and oil cracking by high overpressure could have played an important role in the distribution of different origins of gas condensate accumulations in the lijin and minfeng areas the application of oil and gas accumulation models in this study is not limited to the dongying depression and can be applied to other overpressured rift basins

migration induced compositional changes in oils and condensates of a single field thirty one oils and condensates from the vermilion 39 field offshore louisiana u s gulf coast are examined using techniques of elemental isotopic and molecular geochemistry in an effort to determine the causes of compositional differences among them compositional variability in elemental concentration carbon isotope ratios and molecular distributions suggests that at least two sub groups occur in the sample set the sub groups are characterized by differences in n c 15 content relative to n c 15 pristane phytane ratio 13 c sulfur and nitrogen concentration and s n ratio although the separation into these two sub groups is superficially indicative of distinct sources and or variable levels of thermal maturity interpretation of the data set as a whole indicates that source influences maturity differences and biodegradation do not control observed variations in composition reliable source e g r c 27 29 sterane distribution and maturity e g 20s 20s 20r parameters provide clear evidence of a common source and constant maturity level for all 31 oils condensates the remaining geochemical differences including various molecular pristane phytane isotopic 13 c oil and elemental s n parameters correlate with the extent of migration fractionation that each sample has undergone as measured by the relative amount of n c 15 in each sample the migration fractionation process in the vermilion 39 field is facilitated by fault initiated pressure releases within reservoirs that originally contained full range crudes vertical migration of the released saturated hydrocarbon gases has led to a concentration of lighter components in the most fractionated samples this geochemical interpretation is supported by geologic data inasmuch as the extent of fractionation is least in the deepest samples and greatest in the shallowest samples the results presented here indicate the potential for misinterpretation of geochemical parameters previously considered to be influenced solely by source influence or thermal maturity

fractured tight sandstone oil and gas reservoirs a new play type in the dongpu depression bohai bay basin china although conventional reservoirs dominate the bohai basin china a new type of sandstone reservoir also exists in the dongpu depression that has a low matrix porosity tight in which natural fractures govern both permeability and porosity these fractured sandstones are located on a structurally modified buried hill underlying paleogene mudstones and are truncated along an angular unconformity the fractured sandstone oils of the triassic liujiagou heshanggou and ermaying formations are derived from the paleogene shahejie formation which reached peak oil generation and expulsion during the oligocene to early miocene 32 8 15 6 ma gas was generated primarily during the paleogene from carboniferous and permian coals petrographic evidence suggests that oil and gas emplacement followed the compaction and cementation of the triassic sandstone reservoirs fluid inclusion evidence and burial history analysis suggest that fractures developed before oil emplacement but may have coincided with peak gas generation which suggests that oil and gas mainly migrated and accumulated in fractures

precambrian basement reservoirs case study from the northern bongor basin the republic of chad bongor basin in the south of chad is a cretaceous paleogene rift basin developed on precambrian crystalline basement and is part of the central african rift system cars high oil flows have been tested from five basement buried hills indicating that the precambrian basement is a good reservoir for hydrocarbon accumulation core analyses from 10 wells indicate that the basement rocks are composed of plutonic rocks including granite syenite monzonite quartz monzonite and diorite and orthometamorphites including migmatitic granite migmatitic gneiss and granulite structural mapping of the top basement shows that faults in basement have a principal strike orientation of west northwest east southeast and a secondary orientation of northeast southwest a combination of mud losses well testing results cores wire line logs formation microimager sonic scanner logs and three dimensional seismic data were used to identify and detect the basement reservoirs two main sets of high angle fractures striking west northwest east southeast and west southwest east northeast were identified three facies are divided based on the degree of weathering intensity of fracturing and sonic anisotropy the basement can be divided vertically into four zones weathered and leached fractured semifilled fractured and tight zones the weathered and leached and fractured zones are the main basement reservoirs in the study area the effective porosities range from 3 to 10 in places up to 25 the porosity and permeability of basement reservoirs depend on the structural location and hydrothermal and authigenic precipitates developed during and after the basin formation the discovery of basement reservoirs in the bongor basin will aid the hydrocarbon exploration of a new play in the cars

origin of diamondoid and sulphur compounds in the tazhong ordovician condensate tarim basin china implications for hydrocarbon exploration in deep buried strata abstract in this paper we analyse the ordovician condensate in the tazhong area of the tarim basin nw china using a new high resolution comprehensive two dimensional gas chromatography with time of flight mass spectrometry gc gc tofms results show that 4778 compounds at a signal to noise ratio of more than 100 were detected reflecting the advantage of this method in the analysis of condensate abundant adamantane series compounds and some diamantane series compounds as well as a series of sulphur containing compounds including the benzothiophene series and the dibenzothiophene series were detected for the first time in the study area combined with the results of a study on oil and gas sources and migration accumulation we address the origin of these compounds it is implied that the diamondoids were not formed in situ due to oil cracking in reservoir approximately 5500 m deep at temperature around 139 c whereas the cracking of oil to gas may have occurred in the deeper buried cambrian dolomite reservoir at a reservoir temperature of 180 220 c 8500 11 000 m depth the oil cracked gas secondary cracking product might have migrated to and accumulated in the shallower buried ordovician reservoir accompanied by abundant diamondoids the formation of sulphur containing compounds may be related to thermochemical sulphate reduction tsr which similar to the oil cracking took place in the deeper buried cambrian reservoir under high temperature tsr so 4 2 in the formation water is reduced to h 2 s and organic sulphur compounds leading to the enrichment of sulphur in crude oils e g benzothiophene and dibenzothiophene series as such it is implied that the tazhong area could potentially be explored for deep buried oil cracked gas

light hydrocarbons and diamondoids of light oils in deep reservoirs of shuntuoguole low uplift tarim basin implication for the evaluation on thermal maturity secondary alteration and source characteristics abstract the shuntuoguole low uplift in tarim basin is particular because it provides insights into geochemistry of light oils at ultra deep ordovician oil reservoirs 7200 8200 m the light hydrocarbons and diamondoids of 16 oil samples from shunbei no i fault zone f1 and v fault zone f5 were analyzed to investigate their thermal maturity source characteristics and secondary alteration the oils show high similarity in the characteristics of light hydrocarbons and diamondoids the light hydrocarbons are characterized by high contents of alkanes and low contents of cycloalkanes and aromatics and the diamondoids show a domination of adamantanes and their alkylated isomers compared diamantanes thermal maturity indicators derived from diamondoids and c5 c7 light hydrocarbons indicate that the oils are highly mature and the corresponding equivalent vitrinite reflectance rc determined by light hydrocarbons and methyldiadamantane index are in the ranges of 1 2 1 5 signatures of biodegradation and thermochemical sulfate reduction were not observed in the oils the geochemical information from the light hydrocarbons and distribution patterns of molar concentration of n alkanes indicate that the oils from f1 experienced noticeable evaporative fractionation and the oils from f5 did not this was supposed to be the primarily reason for the physical difference between oils from the both fault zones the parameters derived from the light hydrocarbons and diamondoids indicate the oils are from same source rocks which are associated with marine materials with type ii kerogen such as algae and bacteria and both of cambrian lower ordovician o1 and middle upper ordovician o2 3 source rocks in the deep lower palaeozoic strata contributed to the oils

early cenozoic two phase extension and late cenozoic thermal subsidence and inversion of the bohai basin northern china abstract the bohai basin is one of a family of early cenozoic extensional basins that lie along the eastern margin of asia from russia to vietnam initial extension was probably triggered by subduction roll back of the oceanic pacific plate from the asian continent there were two phases in the bohai basin s rift history the earlier paleocene early eocene phase resulted in the deposition of the kongdian formation and the fourth lowest member of the shahejie formation in a series of elongate half grabens these half grabens have master faults with a nne ssw orientation secondary normal faults are typically clockwise oblique to the master faults indicating a component of dextral transtension deposition was focused in the west and south of the present basin these rocks are mainly alluvial fan and fluvial red beds the architecture of the basin underwent an important change at ca 43 45 ma middle eocene beginning with the deposition of the third member of the shahejie formation in part these sediments were deposited in the same half grabens as the kongdian formation but the bozhong depression in the central part of the basin originated at this time and became the major depocentre the bozhong depression superficially resembles a pull apart basin it formed when continued transtension of the earlier tertiary fault systems to the east and west created an extensional overlap between them during this phase the basin as a whole had a geometry with elements typical of both pull apart and transtensional basins regional extension in many east asian basins ended at the end of the oligocene probably because of the onset of transpression within eastern asia caused by the collision of australia with the philippine sea plate dextral transpression caused minor inversion of some of the earlier normal faults in bohai but as a whole the basin began to subside in a post rift phase of thermal subsidence that has lasted until the present day

the complexity secondary geochemical process genetic mechanism and distribution prediction of deep marine oil and gas in the tarim basin china abstract most paleozoic marine craton basins in china are rich in natural gas except the tarim basin where both oil and gas are prolific various hydrocarbon types have been found in this basin including deep ultra heavy oil heavy oil normal oil volatile oil condensate and natural gas petroleums with diverse types even coexist within a single reservoir generally the ordovician and cambrian reservoirs are deeply buried 5500 8000 m and intensively altered after initial oil and gas accumulations bringing challenges to physical property prediction before drilling and evaluation of exploration targets based on recent exploration discoveries and experimental data we systematically analyzed the origin of oil and gas demonstrated the mechanisms and processes of secondary geochemical alteration e g biodegradation gas washing fractionation tsr and high temperature cracking moreover the preservation mechanism of ultra deep normal oil was also clarified and the distribution patterns of oil and gas were predicted tarim basin has been uplifted and shallowly buried in the early accumulation stage then rapidly and deeply buried in the late stage resulting in various geochemical alteration processes at different burial stages through the reconstruction of complex deep strata oil and gas accumulation histories the spatial distribution of deep petroleum types were predicted this review has provided detailed assessment with case studies in the tarim basin and shed light on the preservation mechanisms and the alteration processes of deep liquid petroleums in addition to massive gas accumulations liquid oils still have great exploration potential in strata deeper than 8000 m in the tarim basin

the origin migration and accumulation of the ordovician gas in the tazhong iii region tarim basin nw china abstract the ordovician large scale gas reservoir in the tazhong iii region burial depth 6000 m is a typical deep strata gas reservoir in the tarim basin of nw china the origin migration and accumulation of the ordovician natural gas in the tazhong iii region were investigated by integrating geological geophysical and geochemical parameters the parameters that were successfully analyzed included the gas compositions and carbon isotope ratios production data burial and geothermal histories oil compositions and properties together with the diamantane index the results indicated that the ordovician natural gas in the tazhong iii region originated from cambrian ordovician paleo oil cracking accompanied by a large amount of h2s generated from thermochemical sulfate reduction tsr when the oil cracking gas migrated vertically into the ordovician reservoir through the tazhong i fault and strike slip faults it was preferentially charged in the pinnate fault zones of strike slip faults with high porosity and permeability leading to the enrichment of natural gas and an abnormal distribution of the compositions and properties of hydrocarbon then oil cracking gas intruded into and altered the previous oil reservoir from northeast to southwest through the strike slip faults unconformities and pore fracture reservoir belt leading to the differential accumulation of hydrocarbon in districts of the tazhong iii region the ordovician natural gas migration accumulation model in the tazhong iii region indicates a great exploration potential in the deep or super deep strata of the tarim basin further exploration activities in the ordovician reservoir of the tarim basin should be conducted in the regions where cambrian ordovician source rocks are well developed and the stress fracture zones related deep fault should be emphasized

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

hydrocarbon accumulation processes and mechanisms in lower jurassic tight sandstone reservoirs in the kuqa subbasin tarim basin northwest china a case study of the dibei tight gas field the dibei gas field is a large tight gas field located in the kuqa subbasin tarim basin northwestern china the reservoir is within the lower jurassic ahe formation j1a and has porosity and permeability ranges of 2 8 and 0 01 1 md respectively two episodes of hydrocarbon charge are identified based on a detailed study of fluid inclusion petrography and microthermometry fluorescence spectroscopy characteristics and the thermal maturity of both gas and light oil low maturity oil as represented by hydrocarbon inclusions with yellow green fluorescence entered the reservoir circa 23 12 ma whereas high maturity hydrocarbons as indicated by hydrocarbon inclusions with blue white fluorescence have charged the reservoir since 5 ma the hydrocarbon charge process combined with porosity evolution determined the present gas water distribution characteristics in the dibei gas field porosity in the j1a sandstone reservoir was relatively high during the first episode of hydrocarbon charge which allowed oil to migrate upward and accumulate in structural highs under buoyancy from 5 ma to the present the dibei gas field experienced strong tectonic compression associated with intense thrust fault reactivation causing deformation and oil leakage from the reservoir continuous tight sand deposits along the slope areas located far away from the active faults became favorable accumulation sites for gas derived from the underlying triassic source rocks hydrocarbon accumulation along the slope area in the ahe formation is dominantly controlled by equilibrium between hydrocarbon generation pressure and capillary pressure

the potential benefits to the basin modeling enterprise of modularization and incorporating organic and inorganic chemical change mineral and hydrocarbon resources in basins are produced by fluid movements whose nature is not fully understood flow models in realistically defined basins that predict chemical change are needed to make progress but these models will be difficult to develop and keep vital two simple examples are given to illustrate what is required and the challenges to basin modeling are then outlined it is argued that the best strategy is to modularize existing physical and chemical codes and place as many of these modules as possible in the public domain this will keep the modeling enterprise open to the new entry and continuing invigoration

relationship between the fluorescence colour of oil inclusions and thermal maturity in the dongying depression bohai bay basin china part 2 fluorescence evolution of oil in the context of petroleum generation expulsion and cracking under geological conditions abstract abundant measured data from source rocks crude oils and oil inclusions in the dongying depression were used to identify the oil generation expulsion and cracking stages under geological conditions so as to build a fluorescence evolutionary model that considers all petroleum generation expulsion and cracking processes in the model oil generation consists of three successive processes bitumen generation bitumen decomposition into oil and oil cracking these three stages are strongly controlled by thermal stress in the dongying depression kerogen decomposition into bitumen occurs at around 85 c 0 42 ro while nso compounds in the bitumen begin to crack into oil at around 120 c 0 67 ro competition between bitumen generation and nso compound cracking leads to peak bitumen generation occurring at around 130 c 0 75 ro after that the main phase of oil generation from intense bitumen decomposition occurs a transition at approximately 160 c between bitumen decomposition into oil and the inferred start of oil cracking corresponds to 4000 m 0 98 1 11 ro during the bitumen generation and main oil generation stages the fluorescence colours of the expelled oils are mainly yellow and orange and a fluorescence blueshift of the oils only occurs during the main oil generation stage in the late oil generation stage 160 c the fluorescence colours of the oils may be close to blue with increasing burial once the source rock and reservoir temperatures reach the threshold conditions for oil cracking 160 c an intense blueshift of the oil occurs in both source rocks and reservoirs and even in oil inclusions this blueshift is shown by a decreased dispersion in the fluorescence colour distribution of oil inclusions with increasing burial from multiple colours at temperatures 160 c therefore in oil prone source rocks containing type i and ii kerogen a blue fluorescence colour of oil appears to be strongly related to the secondary cracking process blue fluorescent oil inclusions generally have higher thermal maturities than oil inclusions with green or yellow fluorescence for oil prone source rocks this relationship is based on the assumption that the oil was not from a gas prone source rock and that secondary alteration processes can be excluded including migration fractionation gas deasphalting trapping fractionation and phase separation furthermore blue fluorescent oil inclusions may be a good indicator for oil prone source rocks which is significant for evaluation of source rock quality in deepwater and ultra deepwater areas in offshore basins

classification of offshore gulf of mexico oils and gas condensates five groups of petroleums can be recognized in the offshore gulf of mexico on the basis of carbon isotope and sulfur isotope ratios sulfur content and relative abundances of nickel and vanadium of 355 oils analyzed 341 96 1 are evidently closely related and cannot be subdivided on the basis of carbon or sulfur isotopes we suggest that these oils represent a single broad superfamily however two subgroups within this set are distinguishable on the basis of sulfur content and relative abundances of nickel and vanadium group 1a sigma 13 c 26 5 0 4 parts per mil sigma 34 s 5 6 2 4 parts per mil s 0 25 0 15 v ni v 0 27 0 10 and group 1b sigma 13 c 26 5 0 3 parts per mil sigma 34 s 5 9 1 2 parts per mil s 0 73 0 43 v ni v 0 67 0 07 group 1a oils are broadly distributed across the louisiana and adjacent texas continental shelf whereas group 1b oils are found along the louisiana shelf edge and continental slope group 1b oils have substantial genetic affinities to those from the campeche shelf offshore mexico of mesozoic origin the remaining 14 analyzed petroleums include supermature condensates from offshore texas and oils from offshore southern florida believed to be cogenetic with those from the jurassic smackover formation a three dimensional appraisal of geohistory suggests that petroleum generation began in the eocene in the region of the rio grande delta progressed coastwise to the mississippi delta area by the miocene and then occurred in regions of progres ively deeper water during the pliocene pleistocene

classification and thermal history of petroleum based on light hydrocarbons abstract classifications of oils and kerogens are described two indices are employed termed the heptane and isoheptane values based on analyses of gasoline range hydrocarbons the indices assess degree of paraffinicity and allow the definition of four types of oil normal mature supermature and biodegraded the values of these indices measured in sediment extracts are a function of maximum attained temperature and of kerogen type aliphatic and aromatic kerogens are definable only the extracts of sediments bearing aliphatic kerogens having a specific thermal history are identical to the normal oils which form the largest group 41 in the sample set this group was evidently generated at subsurface temperatures of the order of 138 149 c 280 300 f defined under specific conditions of burial history it is suggested that all other petroleums are transformation products of normal oils

seepage in vietnam onshore and offshore examples abstract hydrocarbon seepages in vietnam are generally linked to migration from tertiary source rocks and seeps are present both onshore and offshore residual oil can be observed at outcrop in breached traps within tertiary sections and in exhumed buried hill traps in pre tertiary rocks in the onshore hanoi basin there is active oil seepage on the western margin of the offshore phu khanh basin through fractured granites that outcrop at the basin margin there are examples of onshore gas seepages in southern vietnam these are predominantly non hydrocarbon mantle derived gases sometimes mixed with biogenic methane and are seeping via hot springs from fractured mezosoic igneous rocks in active fault zones offshore in the nam con son basin sonar seismic data and sea bed cores show a variety of seepage features including shallow gas anomalies mounds and pockmarks on the sea bed and gas effects in the water column sometimes visible on sonar records however sea bed seepage features are not particularly common and have a low preservation potential in the area due to powerful sea bed currents aircraft surveys of the sea surface have revealed several oil slicks that could be related to active oil seepages in the nam con son basin seepage appears to be occurring in vietnam at two fundamentally different rates background seepages flow at low rates typically tens to thousands of cubic feet per day of gas and up to tens of barrels per day of oil these seeps flow episodically for extended periods of months or years and leading to the formation of sea bed mounds higher flow rate eruptive seepages may have formed some of the pockmarks and have a far shorter duration probably only a few hours or days with flow rates of millions of cubic feet per day of gas and any associated liquids

application of aromatic compounds as maturity indicators in source rocks and crude oils concentration ratios of aromatic compounds such as naphthalenes phenanthrenes and dibenzothiophenes are at present attracting increasing attention as maturity indicators in sedimentary rocks and crude oils generally these indicators rely either on an increase with maturity in the degree of alkylation of a given parent compound or a shift in the isomer distribution of alkyl aromatic homologues towards thermally more stable isomers a combination of both concepts in the methylphenanthrene index mpi resulted in an excellent maturity parameter as demonstrated for rock extracts by an improved correlation with mean vitrinite reflectance rm between 0 65 and 1 4 i e in the zone of oil formation in cases that rm measurements were ambiguous this empirical relationship has proved useful in the calculation of theoretical vitrinite reflectance rc values from the mpi on the other hand for potential petroleum source rocks reaching maturity at relatively low temperatures the influence of the organic matter type on the aromatic distributions cannot be neglected consequently organic facies effects should be taken into account in the interpretation of aromatic distributions of such source rocks and related crude oils since the impact of the organic facies is strongly reduced with increasing maturity aromatic maturity parameters are particularly useful in the maturity evaluation of post mature crude oils and condensates in contrast to biological markers substantially higher concentrations of the key alkyl aromatics persist at elevated maturation levels thus aromatic maturity parameters should be considered as versatile tools in organic maturation studies and can complement established maturity indicators based on steranes and terpanes

archean oil evidence for extensive hydrocarbon generation and migration 2 5 3 5 ga archean sedimentary rocks from the pilbara craton australia contain evidence for petroleum generation and migration in the form of bitumen nodules produced by radiogenic immobilization of fluid hydrocarbons around detrital uraninite thorite and monazite grains the nodules are preserved in sandstones at several stratigraphic levels in the fortescue group 2 75 ga and lalla rookh formation 3 0 ga both nonmarine successions and in deltaic sediments of the mosquito creek formation 3 25 ga the most ancient evidence comes from the warrawoona group 3 46 ga where hydrocarbon droplets were apparently formed in situ from kerogenous sediments by flash maturation during early hydrothermal silicification bituminous relics of petroleum are also commonly preserved in shallow marine sandstones of the black reef formation 2 59 ga and the witwatersrand supergroup 2 85 ga from the kaapvaal craton south africa along with subeconomic methane accumulations in all cases the petroleum was apparently sourced from archean shales generated during the archean and migrated before the late archean or early early proterozoic metamorphism occluded fluid pathways given this widespread and abundant evidence for hydrocarbon generation and migration in archean depositional basins it seems that primordial bacterial biomass producing labile type i kerogen was often buried in sufficient quantities to successfully generate and expel petroleum depositional basins on ancient cratons clearly contained permeable rocks amenable to the migration and probably to the accumulation of petroleum thus the main factors precluding the discovery of economically exploitable hydrocarbon accumulations in archean basins are the subsequent destructive effects of deformation and metamorphism which causes trap breaching imperfect sealing or thermal obliteration however there are ancient stable cratons where such disruption may not have occurred and so petroleum explorers may wish to reassess the possibility of finding valuable hydrocarbon resources in archean rocks

bed material size and the riffle pool sequence relationships between the bed material size distribution and riffle pool geometry of a small coarse bedload upland stream channel are described downstream sequences of bed material size are highly variable although series analysis reveals some persistence in the data over short distances cross correlation of bed material series with bed elevation series indicates riffles tend to coincide with coarser bed material sequences comparison of the bed material size distributions of individual riffles pools and bars is however more revealing mean particle size was found to be the most useful criterion for distinguishing between bed forms and in these terms bars are the most distinctive features moreover riffles and pools can be viewed as simply downstream oscillations in elevation of a single coarser grained sedimentary unit coarse sediment supply from erosion scars created where the channel meets confining bluffs in valley fill deposits does mean that the average particle size of adjacent and downstream bed forms is increased however conclusions remained unchanged after reanalysis following the identification and subtraction of immobile particle diameters plan geometry and coarse sediment supply in the upland environment are also shown to influence bed material size distributions tightly curved pools tend to have the finest pool sediments but also the least difference between bar and pool sediments size of material in pools is also negatively related to an index of cross section asymmetry

the framework of hydrocarbon generation and migration gulf of mexico continental slope abstract the occurrence of large volumes of crude oil and thermogenic gas in plio pleistocene reservoirs and holocene seeps of the gulf of mexico slope argues that the process of hydrocarbon generation and migration continues at the present time calculated thermal maturity models based on deep seismic stratigraphy indicate that cretaceous and possibly early tertiary sediments are presently within the thermal maturity range for generation and expulsion of crude oil the cretaceous and early tertiary sediments are suggested as possible source rocks because younger sediments are thermally immature for crude oil generation and older sediments are thermally overmature for crude oil preservation the calculated thermal maturity profile for the slope indicates that some generation of crude oil could occur as shallow as 6 km and that liquid hydrocarbons could be preserved as deep as 9 km thermogenic gas is stable at even greater depths migration with a strong vertical component must be invoked to explain thermogenic hydrocarbons in shallow reservoirs and seeps of the slope the great depth at which liquid hydrocarbons and dry gas can be preserved in reservoirs of the gulf of mexico salt basin indicates that huge volumes of prospective section remain essentially unexplored

geochemistry of palaeozoic marine petroleum from the tarim basin nw china part 3 thermal cracking of liquid hydrocarbons and gas washing as the major mechanisms for deep gas condensate accumulations abstract the lunnan lower uplift in the tarim basin nw china contains a composite petroleum system with mainly biodegraded heavy oil in the west normal oil in the center and gas condensate in the east twenty three gas samples and 37 oil samples taken from three major hydrocarbon bearing intervals in the lunnan lower bulge were analyzed for their stable carbon isotopes and molecular biomarkers on the basis of their isotopic compositions biomarkers especially diamondoids and integrating the physical properties of the hydrocarbons including densities gor and pvt relationships it has been concluded that the recently discovered deep 6500 m eastern lungu giant ordovician gas condensate pool with an estimated reserve of 723 million bbl oil equivalent is a secondary hydrocarbon accumulation derived from the mixing of an early formed oil and a late formed gas the extremely dry gases with a gas dryness of 0 98 were derived from thermal cracking of crude oils and charged to an existing oil reservoir causing extensive gas washing and secondary alteration compared with most of the hydrocarbons in the lunnan area the gas from the eastern lungu gas condensate pool has a heavier carbon isotopic composition higher dryness and a higher maturity level the unique physical chemical and isotopic compositions of the gas condensate are believed to be a direct result of gas flushing of an early formed eastern lungu oil pool by a late formed dry gas from oil cracking the eastern lungu gas condensate pool is presently characterized by containing dry gas heavy oil abundant aromatics and high wax

distribution of hydrocarbon source potential in the jurassic smackover formation abstract there are regionally consistent differences in source potential between the upper and lower members of the smackover formation measurements of total organic carbon and visual kerogen assessments indicate that the lower smackover has source potential for crude oil from texas to florida source potential is largely restricted to the laminated lime mudstone facies of the lower smackover where deposition occurred in a subtidal environment that was anoxic and probably hypersaline algal derived amorphous kerogen is concentrated in laminations of depositional origin carbonate diagenesis during burial enhanced source potential by further concentrating kerogen along stylolites we suggest that the efficiency of hydrocarbon expulsion has been enhanced by depositional and diagenetic factors that concentrate source rock kerogen along conduits for fluid movement the upper smackover serves mainly as a reservoir rock and other jurassic formations lack source potential

dating and characterizing primary gas accumulation in precambrian dolomite reservoirs central sichuan basin china insights from pyrobitumen re os and dolomite u pb geochronology abstract primary gas accumulation with trillions of cubic meters of gas volumes was discovered recently in the precambrian dengying formation of the gaoshiti moxi block central sichuan basin china in this study we investigated the pervasive saddle dolomite cements and solid bitumens within karst vugs in the dengying dolomite reservoirs using laser ablation inductively coupled plasma mass spectrometry la icp ms in situ u pb dating we obtained a high precision absolute age of 259 4 3 0 ma for the saddle dolomite cementation consistent in timing with the eruption of emeishan flood basalts 262 251 ma combined with high homogenization temperatures th and salinities of primary fluid inclusion as well as rare earth element ree compositions we consider that the saddle dolomite precipitation was likely related to hydrothermal activity driven by the emeishan mantle plume the identification of rare surviving oil inclusions and extensive solid bitumens within the dolomite vugs is indicative of the presence of paleo oil reservoirs these bitumens are characterized by high equivalent vitrinite reflectance requ no fluorescence and deformed fibrous textures typical of highly mature pyrobitumens gas geochemistry data indicates that only minor thermochemical sulfate reduction tsr has occurred thus the pyrobitumen was interpreted to result mainly from the thermal cracking of paleo oils and its associated re os isochron age of 154 21 ma representing the timing of dry gas formation petrographical evidence suggests that the paleo oil charge must have taken place after sparry dolomite cementation but prior to pyrobitumen generation this combined with coeval aqueous inclusion th data and published thermal burial models constrains the time of paleo oil emplacement to ca 222 205 ma suggesting an oil filling event related to the indosinian orogeny this study documented the first chronological data constraining the complex evolution history of a petroleum system in one of the oldest primary gas reservoirs in the world

reservoir alteration of crude oils in the junggar basin northwest china insights from diamondoid indices abstract reservoir alteration of crude oils is ubiquitous in petroleum reservoirs alteration can change the composition and physicochemical properties of crude oils and complicate oil source correlations and origin studies of crude oils thus the identification and assessment of reservoir alteration could provide a better understanding of the formation and evolution of crude oils in this study the effects of reservoir alteration e g biodegradation gas washing water washing and mixing on oils in the junggar basin northwest china were identified and assessed by combining diamondoid indices with other commonly studied crude oil properties the results indicate that 1 diamondoid indices can be used to evaluate the intensity of reservoir alteration on oils e g slight biodegradation and water washing has no effect on diamondoids moderate biodegradation and water washing can result in a marked increase in diamondoid concentrations severe biodegradation and water washing significantly changes diamondoid concentration and isomerization ratios 2 diamondoid indices can assist in determining the type of reservoir alteration of oils e g condensate formed by gas washing has a lower diamondoid concentration than that generated by thermal cracking differing from biodegradation and water washing gas washing can affect diamondoid ratios mixing can be identified by a combination of diamondoid concentrations and other parameters such as api gravity and biomarker indices in oils therefore diamondoids provide specific information as to the reservoir alteration of oils which is useful in further understanding of the origin and evolution of crude oils in the junggar basin

petroleum generation in the southeast texas basin implications for hydrocarbon occurrence at the south liberty salt dome geochemical characteristics of hydrocarbons from the south liberty field liberty county texas were integrated with local stratigraphy pressure temperature seismic data and formation water chemistry to determine the source maturity and migration pathways of hydrocarbons associated with the salt dome in the southeast texas gulf coast fourteen crude oil samples from the soft geopressured cook mountain and yegua both eocene and frio oligocene reservoirs 989 2886 m 3245 9469 ft were analyzed by whole oil high resolution gas chromatography gc and gc mass spectrometry pressure and temperature data from 36 wells were used to model the thermal maturation history in the vicinity of the dome south liberty oils were found to belong to a single genetic family sourced from rocks with a similar level of maturity biomarkers indicate they were formed in a marginal marine environment with notable terrestrial input from a likely lower tertiary source rock probably the downdip lower claiborne and or wilcox group oils were generated within the peak oil window vitrinite reflectance 0 6 0 9 at expulsion temperatures of 125 to 130c thermal modeling indicates the lower claiborne and or upper wilcox beds attained optimum maturity in and around the dome between 8 to 20 ma and 18 to 31 ma respectively oil migration toward the structure probably occurred through faults breaching the deep seated lower claiborne upper wilcox source two severely biodegraded samples were found and these were associated with cooler formation water 50 to 55c with a greater meteoric water component 48 60 comparison with brazoria county oils 100 km 62 mi to the southwest indicates that the south liberty oils belong to the same genetic family with minor differences resulting from a greater input from terrestrial kerogen

alteration and multi stage accumulation of oil and gas in the ordovician of the tabei uplift tarim basin nw china implications for genetic origin of the diverse hydrocarbons abstract the ordovician is the most important exploration target in the tabei uplift of the tarim basin which contains a range of petroleum types including solid bitumen heavy oil light oil condensate wet gas and dry gas the density of the black oils ranges from 0 81 g cm 3 to 1 01 g cm 3 20 c and gas oil ratio gor ranges from 4 m 3 m 3 to 9300 m 3 m 3 oil source correlations established that most of the oils were derived from the mid upper ordovician marine shale and carbonate and that the difference in oil properties is mainly attributed to hydrocarbon alteration and multi stage accumulation in the tabei uplift there were three main periods of hydrocarbon accumulation in the late caledonian stage ca 450 430 ma late hercynian stage ca 293 255 ma and the late himalayan stage ca 12 2 ma the oil charging events mainly occurred in the late caledonian and late hercynian stage while gas charging occurred in the late hercynian stage during the late caledonian stage petroleum charged the reservoirs lying east of the uplift however due to a crustal uplifting episode in the early hercynian ca 386 372 ma most of the hydrocarbons were transformed by processes such as biodegradation resulting in residual solid bitumen in the fractures of the reservoirs during the late hercynian stage a major episode of oil charging into ordovician reservoirs took place subsequent crustal uplift and severe alteration by biodegradation in the west central basin resulted in heavy oil formation since the late himalayan stage when rapid subsidence of the crust occurred the oil residing in reservoirs was exposed to high temperature cracking conditions resulting in the production of gas and charged from the southeast further altering the pre existing oils in the eastern reservoirs a suite of representative samples of various crude oils including condensates lights oils and heavy oils have been collected for detailed analysis to investigate the mechanism of formation based on the research it was concluded that the diversity of hydrocarbon physical and chemical properties in the tabei uplift was mainly attributable to the processes of biodegradation and gas washing the understanding of the processes is very helpful to predict the spatial distribution of hydrocarbon in the tabei uplift and provides a reference case study for other areas

bed forms and bend arc spacings of some coarse bedload channels in upland britain seventy eight riffle to riffle and 80 bend spacings along eleven coarse bedload low sinuosity stream channels in upland britain have been surveyed frequency distributions of these spacings are notably right skewed the most common repeating distances between riffles and bend inflections are between 4 and 6 channel widths although spacings up to 20 widths are also present riffle and pool locations around bends at different stages of planform development indicate that change is largely through increased sinuosity between two consecutive riffles of an original straight reach observed straight segments exhibit alternating riffles and pools evenly spaced at 4 6 widths and most bends have similarly spaced riffles at their inflections in plan with the intermediate pool at their apex however angular deflections between axial lines joining inflections indicate existing sequences of bends did not develop from a single straight reach bends which are significantly longer than 4 6 widths are of low sinuosity and represent variability in naturally irregular planforms rather than arcs in advanced stages of meander growth adjusted bed topography around such long bends takes two forms either a single riffle pool cycle is present with one or both bed forms being longer than average or a 4 6 widths spacing is maintained by more than one riffle pool sequence locally the cross section characteristics of riffles and pools are also influenced by planform location

phase fractionation and oil condensate mass balance in the south marsh island block 208 239 area offshore louisiana abstract phase fractionation can strongly deplete oil of its volatile compounds in a regular and characteristic fashion this process has affected oils to a remarkably uniform extent throughout the 30 15 km south marsh island 208 239 and vermilion 30 31 area including the tiger shoal starfak mound point lighthouse point amber trinity shoal and aquamarine fields just offshore louisiana fractionation of the original parent oil likely occurred in the deep relatively flat lying rob l sand that underlies the area and produced gas washed oils mean api 33 and gas condensates mean api 50 in a volume ratio of 1 3 5 both fractionated oil and vapor migrated from the fractionation site to shallower reservoirs however the estimated ultimate production ratio of gas washed oil to gas condensate in this group of fields is 1 0 32 about 11 times higher than would be expected on mass balance considerations alone thus there is an apparent deficiency of producible gas condensate relative to the amount of producible oil for the entire study area and for every field in that area in the case of the tiger shoal field the ratio of industry estimated ultimately producible oil to gas condensate is 1 1 1 based on the production data we conclude that either there is an additional 6 4 10 6 m 3 43 mmbbl of undiscovered and or unproduced condensate in the area or that condensate has escaped preferentially in vapor form to the seafloor the well studied and nearly depleted tiger shoal field provides a good example of how chemical data can be analyzed in a way that contributes insight into the phase fractionation process and the remaining exploration potential of an area

petroleum migration faults and overpressure part ii case history the haltenbanken petroleum province offshore norway abstract petroleum inclusion and geochemical data from core extracts were applied to deduce a model for oil migration overpressure development and palaeo leakage of oil from currently dry structures in the haltenbanken vest area the existence of fluorescent oil type inclusions in quartz in the smorbukk asgard 2 field suggest that oil migrated into this structure 70 50 million years before present ma bp this is also the case for the dry structures 6506 12 4 6506 11 3 and 6506 11 1 west of the main smorbukk fault zone black oil inclusions with medium gas oil ratio gor occur in these fields together with condensate type petroleum inclusions this suggests that the dry structures transformed from containing oil to condensate before leakage petroleum extracted from inclusions in these structures and in nearby fields have identical marine type ii kerogen signatures source rocks at the spekk formation level in the current drainage area of smorbukk and these dry structures were immature 70 50 ma bp and the smorbukk sor asgard 3 field did not fill at this early time thus oil must initially have entered into smorbukk from areas to the w sw through the currently pressure sealing smorbukk fault zone which today marks the westward limit of the smorbukk field diagenesis in this fault zone caused the much later overpressure development and petroleum was lost from the 6506 12 4 6506 11 3 and 6506 11 1 structures as overpressure built up regionally petroleum loss from these structures with their often thick seals must have occurred via self propagating open fracture induced mechanisms lack of petroleum in the cretaceous strata above these structures suggests that leakage occurred to even shallower strata this could imply that the cretaceous strata in halten vest were overpressured at the time of leakage in contrast the oil in the cretaceous lysing and lange formation above the jurassic reservoirs in smorbukk and smorbukk sor most likely originated based on geochemistry and gors from the jurassic reservoirs below and not from cretaceous strata this migration event would have been facilitated if it occurred before these sands became overpressured as they are today modelling suggests that the spekk formation became mature in the smorbukk sor region

hydrocarbon migration and accumulation of the suqiao buried hill zone in wen an slope jizhong subbasin bohai bay basin china abstract the hydrocarbon migration and accumulation of the suqiao deep buried hill zone in the jizhong subbasin the bohai bay basin eastern china was investigated from the perspective of paleo fluid evidence by using fluid inclusions quantitative fluorescence techniques qgf total scanning fluorescence method tsf and organic geochemical analysis results show that the current condensate oil gas reservoirs in the study area once were paleo oil reservoirs in addition the reservoirs have experienced at least two stages of hydrocarbon charge from different sources and or maturities during the deposition of the oligocene dongying formation ed the deep ordovician reservoirs were first charged by mature oils sourced from the lacustrine shale source rocks in the fourth member of shahejie and kongdian formations es4 ek and then adjusted at the end of ed period subsequently by virtue of the tectonic movement since the deposition of the neogene minghuazhen formation nm the reservoirs were mainly charged by the gas that consisted of moderate to high maturity condensate and wet gas sourced from the es4 ek lacustrine shale source rocks and mature coal derived gas sourced from the carboniferous permian c p coal bearing source rocks meanwhile the early charged oil was subjected to gas flushing and deasphalting by the late intrusion of gas the widely distributed hydrocarbon inclusions the higher qgf index and foi the frequency of oil inclusions values in both gas oil and water zone are indicative of early oil charge in addition combined with the homogenization temperatures of the fluid inclusions

variation in cross sectional asymmetry of coarse bedload river channels alternative indices of cross section asymmetry are compared observed values of two preferred indices for 567 natural sections across eleven confined upland channels are predictably low along such low sinuosity reaches variation in asymmetry of sections located around confined unconfined but stable and freely migrating bends is evaluated consideration of the influence of bed topography emphasizes local plan curvature of bed forms as an important control on degree of asymmetry variation in indices for bed forms in similar planform locations indicates the levels of asymmetry at different stages of the transformation of channel geometry over time

carbon and hydrogen isotopic compositions of n alkanes as a tool in petroleum exploration compound specific isotope analysis csia of individual organic compounds is a powerful but underutilized tool in petroleum exploration when integrated with other organic geochemical methodologies it can provide evidence of fluid histories including source maturity charge history and reservoir processes that can support field development planning and exploration efforts the purpose of this chapter is to provide a review of the methodologies used for generating carbon and hydrogen isotope data for mid and high molecular weight n alkanes we discuss the factors that control stable carbon and hydrogen isotope compositions of n alkanes and related compounds in sedimentary and petroleum systems and review current and future applications of this methodology for petroleum exploration we discuss basin specific case studies that demonstrate the usefulness of csia either when addressing particular aspects of petroleum exploration e g charge evaluation source rock oil correlation and investigation of maturity and in reservoir processes or when this technique is used to corroborate interpretations from integrated petroleum systems analysis providing unique insights which may not be revealed when using other methods csia of n alkanes and related n alkyl structures can provide independent data to strengthen petroleum systems concepts from generation and expulsion of fluids from source rock to charge history connectivity and in reservoir processes

buried hill play jizhong subbasin bohai bay basin a review and future prospectivity as one of the six subbasins in the lacustrine bohai bay basin the jizhong subbasin is characterized by a dominance of the petroleum reserves located in buried hill traps of paleozoic and proterozoic marine carbonates particularly the latter this paper documents the revitalization of exploration of the buried hill play and discusses other play fairways not previously considered through use of case studies discovery of the largest field renqiu field in 1975 led to the establishment of the buried hill play in this play oil derived from the paleogene lacustrine source rocks charged into and accumulated in the underlying proterozoic marine carbonate reservoirs a number of buried hill fields were discovered within a time span of ca 10 yr in the subbasin with more and more large buried hill structures at depths of less than 5000 m 16 400 ft being drilled the annual reserve addition showed a rapid decline until 2006 the application of new technologies including reprocessing of merged three dimensional seismic data improved logging and testing techniques together with innovative exploration ideas made it possible to revitalize the buried hill play the exploration success lies with the focus change from targeting the conventional shallow to moderately buried hills to the unconventional buried hill pools which include deeply buried hilltop hillslope and intrahill pools case studies of one conventional buried hill field renqiu field and three unconventional buried hill fields chang 3 wengu 3 and niudong 1 fields together with modern geological investigations indicate that there still exists significant exploration potential for the buried hill play in the jizhong subbasin the potential largely lies with the unconventional buried hills the baxian wuqing and baoding depressions are favorable fairways for the deeply buried hill pools represented by the niudong 1 field the wenan slope is the favorable fairway for the intrahill pools represented by the chang 3 and wengu 3 fields

effect of water washing on crude oil compositions crude oils from venezuela oklahoma and new mexico were water washed in the laboratory at temperatures from room temperature to 80 c and with water salinities from 0 to 300 000 mg l in experiments lasting from 7 to 338 hours the effects of water washing were determined by gas chromatographic and gas chromatography mass spectrometric analysis of the residual oil these experiments show that water washing is particularly effective for the c15 fraction and hydrocarbons are removed in the sequence aromatics then n alkanes then naphthenes in the c15 fraction no loss of pristane phytane steranes or terpanes occurs but some aromatics and sulfur compounds especially dibenzothiophene are depleted although water washing reduces api gravity water washing is unlikely to produce tar layers the effects of water washing were simulated with a numerical model based on the equation of diffusion predicted oil water ratios were in good agreement with the experimental values for the light ends and the model suggests that water washing is very fast in the subsurface comparison of oils that appeared to have been water washed in nature with the equivalent unwashed oils showed the expected compositional trends water washing is probably the dominant process affecting crude oil composition in the subsurface when water flows past oils under conditions where bacterial degradation is precluded by temperature 80 c or by lack of dissolved oxygen and the temperatures are too low for thermal cracking

quantitative evaluation of the transporting capacity of unconformities a case study from the zhongguai area junggar basin nw china abstract unconformities are special geologic bodies with a three layer structure including the rock above the unconformity a weathered clay layer and a leached rock the upper wuerhe upper permian bottom unconformity in northwest junggar basin is the main pathway system linking the zhongguai zg lift with hydrocarbon source kitchens based on the log response characteristics of the unconformity s vertical structure and core calibration a principal component analysis pca was applied to extract and identify the evaluation parameters of the unconformity s vertical structure on the basis of darcy s law the quantitative characterization formula of the unconformity s transport capacity with an actual geological significance was established after taking several factors into consideration the migration probability of the unconformity was used to evaluate the possibility that the unconformity acted as a migration pathway consequently a relationship between the migration probability and the evaluation parameters was developed to analyze the effectiveness of these parameters in characterizing the transporting capacity of the unconformity this study shows that the quotient of the two principal components extracted from the conventional logging curves pc1 pc2 have different reference values in the leached and parent rocks the distribution of thicknesses of the weathered clay layer and the leached rock is well matched with the distribution of paleogeomorphology the parameters representing only a single geological factor are not enough to determine whether the unconformity acted as a migration pathway the flow coefficient cf containing multiple factors can effectively and clearly identify the transporting capacity of the unconformity considering the positive correlation between the parameter cf and the migration probability of the unconformity the oil and gas shows of the area with a parameter cf greater than 10 10 4 cm2 s are abundant which is the superior transporting area of the unconformity and there is a good matching relationship between the distribution of the superior transporting area of the unconformity and that of the hydrocarbon distribution

kerogen asphaltene sulfur relationships in sulfur rich monterey oils abstract monterey oils in the santa maria basin onshore and offshore are generally low maturity oils because of low thermal exposure relatively shallow burial for short times they are generated as heavy high sulfur oils because of the composition of the unusual type ii source kerogens which are exceptionally rich in organic sulfur 8 14 these high sulfur kerogens which we designate type ii s appear to generate oil at significantly lower thermal exposures than ordinary type ii kerogens with less than 6 s in contrast to common type ii kerogens which generate oil largely by cleavage at oxygen bridge groups and carbon carbon bonds eliminating co2 h2o and hydrocarbon fragments type ii s kerogens have an additional complement of weak bonds associated with bound sulfur the preferential cleavage at weak sulfur linkages tends to produce larger fragments leading to high initial amounts of asphaltenes resins and sulfur rich aromatics together with smaller amounts of saturated hydrocarbons elemental analyses of monterey kerogens and asphaltenes clearly support the concept that the asphaltenes are essentially fragments of kerogen cleaved sufficiently to be soluble or peptized in this heavy oil system solubilization of asphaltenes is attributed to their interactions with the abundant resins and aromatics this sysstem is sufficiently stable under source and reservoir conditions to allow expulsion and migration for reasonable distance without significant fractionation or changes in gross properties initial asphaltenes have essentially the same atomic h c ratios as the source kerogen but have significantlyy lower o c ratios and slightly lower s c ratios the n c ratios are nearly the same for kerogens and their derived asphaltenes with increasing maturity the h c ratios in asphaltenes decrease much like maturation changes in kerogen the s c ratio also decreases significantly however the o c ratios initially low in asphaltenes and n c ratios increase slightly due to material balance caused by other losses these element composition and atomic ratio relationships allow some important inferences concerning source kerogen composition from analysis of asphaltenes alone this can be useful when potential source rock samples are not available for analysis and or when a choice between alternate source rock possibilities must be made

buried hill discoveries of the damintun depression in north china several buried hill oil fields recently have been found in the damintun depression all the buried hills except one consist of archean metamorphic rocks of these the dongshenpu buried hill is the largest the metamorphic rocks have no original porosity but have secondary porosity such as weathering fractures structural fractures brecciated pores secondary corrosion pores and secondary replacement pores fractures especially structural fractures form the only migration passages and main reservoir spaces for hydrocarbons in the buried hill crude oil comes from paleogene continental source rocks and is very waxy based on the interpretation of log data the porosity averages 3 24 according to calculations using pressure build up curves the effective permeability averages 0 140 greek mu sq meters

oil migration in a major growth fault structural analysis of the pathfinder core south eugene island block 330 offshore louisiana the pathfinder core collected in the south eugene island block 330 field offshore louisiana provides an outstanding sample of structures associated with a major growth fault that abuts a giant oil field and that is thought to have acted as a conduit for hydrocarbon migration into the producing reservoirs where cored the growth fault zone cuts semiconsolidated pliocene pleistocene mudstone and is over 100 m wide the fault zone in the core consists of three structural domains each characterized by a distinct rock type distribution of fault dips and dip azimuths and distribution of spacing between adjacent faults and fractures although all of the domains contain oil bearing sands only faults and fractures in the deepest domain contain oil even though the oil barren fault domains contain numerous faults and fractures that are parallel to those containing oil in the deepest domain the deepest domain is also distinguished from the other two domains by a greater degree of structural complexity and by a well defined power law distribution of fault and fracture spacings sediments in this domain behaved as competent rock with respect to fault and fracture spacing whereas the departure from power law distribution of fault and fracture spacing in the other two domains may reflect deformation of unconsolidated sediment this departure from a power law spacing distribution in the upper two domains combined with stable isotope data that indicate low temperature water rock interaction within a gouge zone that separates these two fault domains indicates that the faults in those domains may have been active only early in the history of the growth fault zone when the sampled sediments were at shallow burial depths thus these faults may predate oil migration in contrast the faults in the oil bearing domain appear to have been active later in the fault zone s history when the sediments faulted as competent rock and when geologic and organic geochemical investigations indicate oil migrated into the block 330 reservoirs even though oil is present in sands throughout the core its restriction to faults and fractures in the youngest sampled portion of the fault zone implies that oil migrated only through that part of the fault that was active during the time when oil had access to it the absence of oil in fractures or faults in the other probably older fault domains indicates that the oil was never sufficiently pressured to flow up the fault zone on its own either by hydraulic fracture or by increased permeability as a result of decreased effective stress instead fluid migration along faults and fractures in the pathfinder core was enhanced by permeability created in response to relatively far field stresses related to minibasin subsidence

hydrocarbon migration in the multiple sourced petroleum system in the northwestern junggar basin northwestern china constraint from geochemical and phase fractionation analysis oil mixing and absence of source rock samples make it difficult to identify the hydrocarbon migration routes in a petroleum system we provide a case study in the northwestern junggar basin to show that combined geochemical and phase fractionation analyses are robust tools to unravel the complex hydrocarbon migration processes the study rebuilds a migration history that multiple sourced hydrocarbons migrated mixed accumulated and fractionated along the evolution of regional tectonics in detail the shawan and mahu sags expelled the early stage hydrocarbons during the late triassic and late jurassic respectively because of their variable subsidence these hydrocarbons charged the entire area based on the evidence from bitumen and oil inclusions during the early cretaceous both sags subsided rapidly and expelled their late stage hydrocarbons these hydrocarbons first mixed along unconformities in the sags which generated mixed source oils and induced gas washing subsequently they further mixed with or displaced the encountered early stage oils during migration along the basal unconformity of the upper permian into the area causing a horizontal distribution of oil maturity zones in addition gases flowing through the early stage oils induced gas washing again creating heavy oils condensate oils and mixed gases after the late stage oils finally accumulated in fractured volcanics migration fractionation caused the remigration of light end compositions this study also shows the strong control of structures on hydrocarbon migration the unconformity network provided opportunities for long distance migration and widespread mixing of multiple sourced hydrocarbons whereas the paleoridge line of the zhongguai high defined the boundary of regional migration

reservoir fluids and their migration into the south eugene island block 330 reservoirs offshore louisiana this study in the well documented pliocene pleistocene south eugene island block 330 sei330 field offshore louisiana unravels a complex petroleum system by evaluating both the inorganic and organic geochemical characteristics of reservoir fluids the brines at sei330 dissolved halite prior to entering the reservoirs and equilibrated with reservoir sediments exchanging sodium for calcium magnesium and other cations the systematically varying extent of brine sodium depletion in two reservoirs defines south to north flow in those sands these sands were filled from a fault that bounds the reservoirs on the south oil compositional parameters also show north south variation across these reservoirs the sei330 oils and gases each had different sources in contrast to published jurassic sources for oil carbon isotope data indicate that sei330 hydrocarbon gases probably sourced from early tertiary or cretaceous sediments after oil had migrated through them distribution of biodegraded vs unbiodegraded oils indicates the reservoirs filled much more recently than formation of the salt weld beneath the field oil compositions indicate that some sei330 oils were partially stripped of low molecular weight compounds by their dissolution in a mobile vapor phase gas washed by large volumes of gas several hundred meters below the deepest reservoir modeling of this gas oil interaction aids in identifying deep potential targets in which gas washing occurred hydrocarbon distribution combined with oil chemistry and reservoir pressures indicate reservoirs filled from fault systems on both the north and south sides of the field the fault feeders are wide 100 m structurally complex zones that can direct different types of fluids into different reservoirs

thresholds in small rivers hypotheses developed from fluvial morphological research in western germany the objective of this study was the formulation of fluvial morphological regularities for small rivers with a wide range of morphological and geological characteristics in north rhine westphalia germany based on a statistical research methodology such empirical quantitative information on reference conditions is required for the restoration of small rivers in the former highly industrialised ruhr area following the approach of some classic empirical works in fluvial morphology of the last century several natural reference rivers in the entire research area have been observed in order to provide a statistical correlation between independent and dependent morphological variables regressions between valley floor slope bankfull discharge and stream power on the one hand and several variables describing the longitudinal profile and river planform on the other hand have shown some significant results the regularities found are a quantitative contribution to the establishment of reference conditions as well as a useful tool for the restoration of small rivers if the specific properties and values of the underlying random sampling are taken into account in addition the relation between stream power and sinuosity shows the likely existence of a threshold exceeding a stream power of 100 w m the sinuosity decreases after an increase for lower stream power values comparable thresholds were found for the relation between stream power and pool depth as well as stream power and step steepness the thresholds could be explained by a change in the type of energy dissipation due to different physio geographical settings in highland rivers within forested v shaped valleys here large woody debris seems to increase the channel roughness and possibly replaces the significance of coarse grained bed material pool depth and step steepness as contributors to energy dissipation

gas exsolution during fluid migration and its relation to overpressure and petroleum accumulation abstract petroleum systems in the tertiary sections of the gulf of mexico and niger delta among other basins are characterized by the close association between petroleum accumulation and over pressure although overpressure is generated mainly by compaction disequilibrium of the low permeability sediments exsolution of gases from water and petroleum during migration over great vertical distance may create overpressure or modify the existing overpressure in this paper the author combined relative permeability data and petroleum phase behavior modelling to explain and predict the presence of an overpressure and petroleum baffle due to gas exsolution generalized isoperm diagrams for oil gas and water phases in sandy and silty shaly sediments were created based on published data aqueous solubility of methane and carbon dioxide as a function of temperature pressure and salinity is summarized based on literature data phase diagrams of petroleum with a wide range of composition api gravity and gor were produced using phase behavior software with calibration using field data these tools were integrated to model overpressure and petroleum phase variations in two oil gas fields in the gulf of mexico the pressure gradients in these fields are likely modified by gas exsolution from the petroleum which migrated and accumulated within and below the pressure transition zone ptz petroleum trapped within ptz tends to be fractionated whereas that accumulated below ptz tends to be single phase with high api gravity these results provide a model which explains both the effect of gas exsolution on ptz and the occurrence of different types of petroleum in these fields these tools may be integrated with geological and seismic data to assess more accurately the economic value of petroleum accumulations in petroleum systems governed by overpressure and petroleum phase separation and fractionation

synthetic river valleys creating prescribed topography for form process inquiry and river rehabilitation design abstract the synthesis of artificial landforms is complementary to geomorphic analysis because it affords a reflection on both the characteristics and intrinsic formative processes of real world conditions moreover the applied terminus of geomorphic theory is commonly manifested in the engineering and rehabilitation of riverine landforms where the goal is to create specific processes associated with specific morphology to date the synthesis of river topography has been explored outside of geomorphology through artistic renderings computer science applications and river rehabilitation design while within geomorphology it has been explored using morphodynamic modeling such as one dimensional simulation of river reach profiles two dimensional simulation of river networks and three dimensional simulation of subreach scale river morphology to date no approach allows geomorphologists engineers or river rehabilitation practitioners to create landforms of prescribed conditions in this paper a method for creating topography of synthetic river valleys is introduced that utilizes a theoretical framework that draws from fluvial geomorphology computer science and geometric modeling such a method would be valuable to geomorphologists in understanding form process linkages as well as to engineers and river rehabilitation practitioners in developing design surfaces that can be rapidly iterated the method introduced herein relies on the discretization of river valley topography into geometric elements associated with overlapping and orthogonal two dimensional planes such as the planform profile and cross section that are represented by mathematical functions termed geometric element equations topographic surfaces can be parameterized independently or dependently using a geomorphic covariance structure between the spatial series of geometric element equations to illustrate the approach and overall model flexibility examples are provided that are associated with mountain lowland and hybrid synthetic river valleys to conclude recommended advances such as multithread channels are discussed along with potential applications

aspects of petroleum basin evolution due to gas advection and evaporative fractionation abstract criteria are proposed for the recognition of depletion by evaporative fractionation in reservoir fluid analyses pvt data the characteristics of original as generated molar concentration profiles of marine oils are postulated oils derived from clastic or carbonate sediments bearing respectively type ii or iis kerogens in unaltered marine oils two decreasing exponential concentration series occur separated by a discontinuity between p5 the 5 carbon pseudo component and p6 such that p6 exceeds p5 rendering the profile abruptly bimodal a suite of 195 detailed pvt analyses c1 to p30 of reservoir fluid oils shows that concentrations in the p6 fraction are frequently perturbed the maximum migrating to p7 p8 or a higher carbon number alteration attributable to additive gas advection followed by evaporative fractionation in the subject oils four variables are adduced which track these changes c1 c2 p4 p8 e3 a light gas liquid heavy end ratio and e7 a gasoline range heavy end ratio increase in c1 c2 is accompanied by decreasing values of p4 p8 for which there is no a priori rationale other than the precepts of evaporative fractionation involving the advection of highly mature dry gas dividing a suite of 535 pvt analyses detailed analyses plus 340 analyzed to p7 of world wide provenance into seven subsets on the basis of c1 c2 ratios a well defined separate regression of gas oil ratio on saturation pressure is revealed in each subset such correlations depend primarily upon the processes involved in evaporative fractionation the curves embrace the full commonly encountered range of combinations of gas oil ratio and saturation pressure both of which correlate poorly with level of maturity expressed in a liquid fraction slope factor these observations lead to the hypothesis that progressive evaporative fractionation is a widespread if not universal evolutionary process in petroleum basins

jolliet field thrust fault structure and stratigraphy green canyon block 184 offshore louisiana abstract jolliet field is believed to be the first development in the offshore u s gulf of mexico to exploit a major thrust fault structure and as such represents a new and potentially important type of deep water structural play the field s multiple stacked reservoirs consist of pleistocene lowstand gravity flows deposited in upper slope fans the thrust fault is documented by 3 d seismic interpretation repeated paleontologic markers and correlation of up to 3000 feet of repeated pay section this fault is believed to be the toe thrust of a major gravity slide which was triggered in the lower pleistocene when sediments were tilted to a critical angle by a rising salt diapir seismic data suggest that the resulting slide surface is approximately cylindrical connecting a major growth fault on the west side of green canyon block 184 to the jolliet field thrust fault salt withdrawal in the area produced normal faults some of which cut the thrust fault adding structural complexity to the field in addition to the geologic and geophysical evidence the structural model is supported by slope stability computer modelling using realistic sediment shear strengths densities and pore pressures

gas washing of oil along a regional transect offshore louisiana abstract gas chromatogram data for 219 oils in a 190 km n s transect offshore louisiana reveal a spatially coherent pattern of compositional change which is caused by gas washing near the louisiana shoreline as much as 91 of the original n alkanes have been removed from the oils the maximum intensity of depletion decreases southward in a nearly regular fashion to nil at the jolliet field 190 km offshore the oils show a parallel change in the maximum carbon number of the removed n alkanes implying that the pressure at which gas washing took place also decreased in the offshore direction the systematic change in maximum extent of depletion crosscuts tectonostratigraphic boundaries as well as oil source provinces models of gas washing suggest that the maximum depth of washing reflects the distribution of deeply buried continuous sands suggesting deep sands may have provided sites for efficient gas oil interaction

organic geochemical indicators of dynamic fluid flow processes in petroleum basins a variety of preliminary geophysical geochemical and geological observations anderson et al geophysics 12 17 april 1991a b anderson oil gas j 85 91 26 april 1993 along with well production data schumacher aapg ann conv abstr april 1993 have led to the hypothesis that fluid injection into eugene island block 330 ei 330 in the u s louisiana gulf coast may be a continuing episodic process with the most recent injections having occurred on short geologic time scales 10 000 yr or less organic geochemical evidence presented here suggests these processes may occur on very short time scales years a process called dynamic fluid injection was proposed to explain these observations and is envisioned as episodic pressure build up followed by rapid escape of gas and oil through geopressure and injection into overlying reservoirs

effect of water washing on c15 hydrocarbon fraction of crude oils from northwest palawan philippines recognition and subsequent correlation of biodegraded and or water washed oils with their unaltered counterparts has been a long recognized problem in petroleum geochemistry because water washing and biodegradation can occur concurrently it is difficult to delineate the individual effects of these two oil altering processes especially on the c15 hydrocarbon fraction both processes can cause an oil to become enriched in sulfur and to have a lower api gravity owing to loss of the more volatile components thus the altered oil may appear to be a high sulfur crude of low maturity altered and unaltered oil fractions extracted from cores of a water washed carbonate reservoir offshore northwest palawan were studied to observe the effect of alteration on correla ion parameters water washing resulted in the following changes 1 depletion of 13c in the c15 branched cyclic alkane fraction whereas virtually no change occurred in isotope composition of the aromatic fraction 2 selective loss of dibenzothiophenes followed by loss of aromatic hydrocarbons and 3 virtually no change in steranes or tricyclic and pentacyclic terpanes with the exception of enhancement of 17 agr h norhopane and decrease of 18 agr h trisnorhopane in situations of severe water washing mild biodegradation also occurred as indicated by the loss of n alkanes and possible selective loss of n alkanes having less than 25 carbon atoms relative to branched alkanes cycloalkanes and aromatic components because cycloalkanes and aromatic hydrocarbons are not generally preferentially utilized by bacteria as long as n alkanes are present the changes observed in these two fractions most likely result from water washing because of the potential for extreme compositional changes in the n alkane and aromatic fractions the most useful correlation parameters between the altered and unaltered oils are sterane and terpane distributions stable carbon isotope ratios are also useful however more studies of oils in which water washing is known to have occurred need to be made to explain the general response or various responses of the isotopic composition of oils to water washing

natural gas washing induces condensate formation from coal measures in the pinghu slope belt of the xihu depression east china sea basin insights from fluid inclusion geochemistry and rock gold tube pyrolysis abstract a systematic study into the origin and potential formation processes of condensates in the pinghu slope belt of the xihu depression east china sea basin was conducted using fluid inclusion analysis petroleum geochemistry and rock gold tube pyrolysis grains containing oil inclusions goi technique well logging and formation testing together indicate that the present day gas condensate layer has evolved from a paleo oil layer biomarkers from oil inclusions indicate that these paleo oils were generated from coal measures at a moderate maturity level based on organic macerals elemental ratios of kerogens rock eval pyrolysis and results from gold tube pyrolysis experiments the coal measures are evaluated as source rocks prone to gas generation and capable of oil formation at moderate levels of maturity this maturity level coincides with present day condensate maturities determined from a series of molecular maturity ratios vitrinite reflectance 0 7 1 0 ro biomarkers related to specific sources suggest that the condensates are also coal derived these observations suggest that the condensates are derived from secondary alteration following paleo oil accumulation rather than being related to the thermal degradation of resin rich organic matter at a low maturity level or the thermal cracking of kerogen at a high maturity level some condensates are characterized by a significant loss of n alkanes with low carbon numbers and relatively high toluene n heptane tol nc7 ratios this indicates that the paleo oil reservoirs have been subjected to varying degrees of gas washing which resulted in condensate formation as opposed to the process of oil cracking under high temperatures subsequent interactions between gas and paleo oils resulted in oil inclusions showing fluorescence varying from near yellow to blue variable vapor volume fractions measured in the same oil assemblage are the result of two phase trapping of oil and gas the widely observed presence of bitumen asphaltene in the reservoir pores and bitumen bearing oil inclusions is considered to be due to gas deasphalting homogenization temperatures th of associated aqueous inclusions and burial thermal curves suggest that the initial paleo oil reservoirs formed in the late miocene excess gas composed of local mature gas and heterochthonous high to over mature gas migrated into the paleo oil reservoirs later in the quaternary this process not only resulted in the formation of condensates but also led to vertical differentiation of density and wax content in the reservoirs

quantitatively predicting the thermal maturity of oil trapped in fluid inclusions based on fluorescence and molecular geochemical data of oil inclusions in the dongying depression bohai bay basin china the fluorescence characteristics of individual oil inclusions and molecular composition analysis of oil inclusions in 20 eocene sandstone samples from the dongying depression were used to assess the relationship between the fluorescence colors of the oil inclusions and their thermal maturity the oil inclusions from the shahejie formation member iii middle submember member iii lower submember and member iv upper submember samples primarily have yellow fluorescence colors 500 nm wavelength of the maximum intensity max 590 nm whereas the oil inclusions in the shahejie formation member iv lower submember reservoir sample only have blue fluorescence 440 nm max 500 nm the molecular maturity parameters from steranes hopanes and aromatic hydrocarbons showed that the oil inclusions with yellow fluorescence colors have relatively low thermal maturities within a narrow range 0 75 0 82 vitrinite reflectance equivalent requ which is significantly lower than that of the sample containing oil inclusions with blue fluorescence colors 1 26 requ the yellow fluorescent oil was interpreted to be related to the main oil generation and expulsion process whereas the blue fluorescent oil was mainly controlled by the thermal cracking process of c14 aromatic hydrocarbons in source rocks reservoirs and oil inclusions this study demonstrates that the fluorescence colors of oil inclusions may be good indicators of thermal maturity in oil systems charged from oil prone source rocks and for the first time makes it possible to quantitatively predict the thermal maturity of oil inclusions based on their fluorescence characteristics however biodegradation water washing phase separation and trapping fractionation should be considered before using this correlation because these processes can also influence fluorescence colors

petroleum charge history in the baiyun depression and panyu lower uplift in the pearl river mouth basin northern south china sea constraints from integration of organic geochemical and fluid inclusion data the source thermal maturity and charge history of oil and gas condensate in the baiyun depression and the panyu lower uplift were determined using organic geochemical and fluid inclusion data nearly all the gas condensates and oil in the study area originated from eocene enping formation source rocks whereas the condensate fluids in the eastern area originated from the enping and zhuhai formations the condensate fluids have a relatively low thermal maturity with a calculated vitrinite reflectance equivalent that typically ranges from 0 8 to 1 0 based on the c29 sterane isomerization ratios and aromatic hydrocarbon maturity parameters four types of oil inclusions with various fluorescence colors yellow blue and api gravities 25 44 were recognized in the oligocene and lower miocene reservoir units which suggest multiple oil charging events the homogenization temperatures of the fluid inclusions recorded the oil charge events accompanying large scale hot fluid movement which was most likely caused by the charging of high temperature natural gas from the deeper wenchang formation the trapping pressures of the oil inclusions and the reconstructed burial histories were used to determine the timing of oil charge the main oil charge occurred from 16 7 ma throughout the baiyun depression and panyu lower uplift whereas the panyu lower uplift experienced an additional oil charge from 5 0 ma the natural gas charge occurred during the main oil charging period and may continue to the present day multiple physical processes such as gas injection without gas exsolution gas washing with gas exsolution and migration fractionation have played important roles in gas condensate formation and controlling the petroleum distribution in the baiyun depression and panyu lower uplift

the origin and accumulation of multi phase reservoirs in the east tabei uplift tarim basin china abstract a giant deep strata oil gas field with complex fluid characteristics was recently discovered in the hade yuke area of the east tabei uplift tarim basin nw china the deep fluids show a lateral co existence of several gas condensate and oil phase reservoirs characterized through the integration of complimentary geochemical e g bulk composition biomarker and stable carbon isotope and geological data typical features of the gas and condensate reservoirs which were mainly distributed in the eastern parts of the study site included high gor gas to oil ratio hydrocarbon gases with relatively heavy 13c values and crude oils with high wax content in contrast oil reservoirs which mostly occurred in more westerly locations had lower gor values lighter 13c values and lower wax content indicating gradual change of fluid characters the petroleum accumulations derive from two hydrocarbon charge events an early oil charge in the late hercynian and a later gas charge in the late himalayan which migrated from east to west leading to a gradual variation in fluid phase and characteristics alteration of paleo oil reservoirs by this secondary gas charge was therefore primarily responsible for the complex fluid character and multi phase reservoirs presently in place secondary condensates were formed from the retrograde condensation of paleo reservoirs our re construction of the hydrocarbon accumulation process encourages future exploration endeavours to target commercially viable gas accumulations in the deep east strata and oil reservoirs in the west

tectonic and stratigraphic evolution of the mongolian people s republic and its influence on hydrocarbon geology and potential abstract the stratigraphy of mongolia can be divided into five megasequences each characterized by a common tectonic and stratigraphic history the earliest megasequence 1 precambrian silurian records the amalgamation of precambrian early palaeozoic passive and active margin terranes and the resulting growth of a caledonian fold belt in western mongolia megasequence 2 devonian permian appears to be the most complex with a variety of active margin collisions in southern and eastern mongolia and the continued deformation of the caledonian fold belt in the west a substantial fold belt underpinned by a variety of granitic plutons existed across mongolia by the triassic there was widespread erosion and limited deposition in small foreland and intermontane basins megasequence 3 triassic early jurassic megasequence 4 mid jurassic cretaceous records extension across the partially eroded fold belt with deposition confined to a series of alluvial and lacustrine rift basins these basins later experienced transpression inversion and erosion followed by onlap and overstep giving more widespread sedimentation from large post rift alluvial systems during the late cretaceous and tertiary megasequence 5 megasequences 1 3 are usually extensively folded megasequences 4 and 5 have been involved in transpressive inversion and mongolia is an area of present day compressive and transpressive tectonics erosion and general uplift the late jurassic early cretaceous sequences megasequence 4 are the most prospective for finding extractable hydrocarbons oil has been successfully produced from megasequence 4 alluvial and lacustrine rift sequences with rich oil prone lacustrine mudrocks which can form both source and seal facies interbedded sandstones siltstones and mudstones form various combinations of reservoirs and seals a large number of hydrocarbon trap types are yet to be tested past drilling has been concentrated around surface seeps and on simple structures that can be mapped at the surface with usually only shallow tests