episodic petroleum fluid migration in fault zones of the northwestern junggar basin northwest china evidence from hydrocarbon bearing zoned calcite cement hydrocarbon bearing zoned calcite cements occur widely in jurassic cretaceous fault zone cores and sandstone outcrops of the northwestern junggar basin northwest china hydrocarbon bearing bands alternate with nearly hydrocarbon free bands at a micron scale analytical results from biomarker organic geochemistry fourier transform infrared microspectroscopy and trace element geochemistry on these zoned cements suggest that at least three different types of fluids have participated in their formation the first fluid type is probably primary unmodified lacustrine formation water from which the hydrocarbon poor bands are formed and are characterized by mg rich calcite the other two types of fluids include basinal fluids e g hot hydrocarbon bearing fluids and meteoric water the hydrocarbon rich bands in which the hydrocarbons have been biodegraded and the mn content is relatively high suggest a mixture of hydrocarbon bearing basinal fluid and meteoric water the alternating growth of hydrocarbon bearing and hydrocarbon free bands of calcite cements implies that the cement formation is episodic it is related to alternating episodes of mixed petroleum bearing fluid and unmodified primary formation waters respectively the fault appears to have been a mixing zone where seismic pumping during the movement of associated regional faults occurred thus in the northwestern junggar basin the micron scale hydrocarbon bearing zoned structure of the calcite cements is likely a reflection of episodic petroleum fluid migration in fault zones

sedimentary record of mesozoic deformation and inception of the turpan hami basin northwest china the turpan hami basin is a major physiographic and geologic feature of northwest china yet considerable uncertainty exists as to the timing of its inception its late paleozoic and mesozoic tectonic history and the relationship of its petroleum systems to those of the nearby junggar basin to address these issues we examined the late paleozoic and mesozoic sedimentary record in the turpan hami basin through a series of outcrop and subsurface studies mesozoic sedimentary facies regional unconformities sediment dispersal patterns and sediment compositions within the turpan hami and southern junggar basins suggest that these basins were initially separated between early triassic and early jurassic time prior to separation upper permian profundal lacustrine and fan delta facies and triassic coarse grained braided fluvial alluvial facies were deposited across a contiguous junggar turpan hami basin permian through triassic facies were derived mainly from the tian shan to the south as indicated by northward directed paleocurrent directions this is consistent with the sedimentary provenance of triassic sandstone mean qm29f29lt42 qp23lvm49lsm28 and qm51p25k24 and conglomerate 32 granitic clasts in the northern turpan hami basin we interpret a relative increase in quartz and feldspar concentration and a relative decrease in volcanic lithic grains in the northern turpan hami basin to reflect unroofing in the tian shan and exposure of late paleozoic granitoid rocks in addition two basinwide unconformities of late permian early triassic and early triassic middle late triassic age attest to deformation within the turpan hami basin and associated continued uplift and erosion of the tian shan

generation of gas and oil from coal and other terrestrial organic matter abstract late carboniferous permian sedimentary rocks contain the world s largest coal reserves which have generated and released gas but little if any oil in comparison organic matter deposited in lacustrine fluvial deltaic or brackish water sediments has sourced large quantities of waxy oils in countries such as china australia the u s a argentina venezuela and indonesia plus gas and condensate in deltas worldwide why is more oil coming from land derived organic matter disseminated in sediments than from the carboniferous permian coal beds which also are land deposits the answer appears to be in the hydrogen content and the ease of migration when organic matter and coals have high hydrogen contents relative to carbon they can form oil when the hydrogen is low mainly gas is formed most of the world s coal is low in hydrogen whereas much of the terrestrial organic matter disseminated in sediments is higher in hydrogen so it forms more oil also both gas and oil are adsorbed on coal more strongly than on the disseminated kerogen of sedimentary rocks this makes migration of oil out of the coal more difficult compared to migration from conventional shale source rocks

upper permian lacustrine oil shales southern junggar basin northwest china upper permian organic rich lacustrine mudstones oil shales that crop out in the southern junggar basin rank among the richest and thickest petroleum source rock intervals in the world with maximum toc values reaching 34 and rock eval pyrolytic yields s 2 up to 200 kg hc t rock lacustrine sedimentary facies define an overall transgressive regressive cycle of approximately 2000 m gross thickness which includes approximately 800 m of source rocks averaging 4 1 toc and 26 2 kg hc t rock basinal facies comprise silicic organic rich laminated lacustrine mudstones and interbedded siltstones organic matter contained in the mudstones ranges in composition from type i to type iii basinal facies were deposited in a deep oxygen deficient stratified lake lake margi facies consist of nonlaminated siliciclastic mudstones rippled dolomitic siltstones and sandstones and minor limestones maximum toc values are approximately 6 desiccation cracks are common in the marginal facies but evaporite minerals are rare or absent biomarker correlation parameters measured from rock extracts exhibit significant stratigraphic variability but strongly support the hypothesis that upper permian lacustrine oil shales charge the giant karamay field in the northwestern junggar basin karamay oils are characterized by high relative abundances of beta carotane this characteristic is restricted to desiccated facies in the outcrop sections however we therefore propose that an abundance of beta carotane indicates elevated environmental salinities during deposition of the oil shales orographic precipitation induced by a paleo tian shan range may have provided fresh water to the southern areas of the basin while more evaporative conditions prevailed to the north outcrop oil seeps confirm that the outcrop sections have attained sufficient thermal maturity to generate oil t max values range from 431 to 451 degrees c vitrinite reflectances range from 0 73 to 0 88 and sterane isomerization ratios range from 0 30 to 0 48 20s 20s 20r matoil registered trademark computer modeling of burial history and thermal maturity of the outcrop sections indicates that approximately 4 5 km of triassic jurassic cover was needed to bring the oil shales to their current maturity the major phase of hydrocarbon generation in this area appears to have preceded the creation of traps by neogene thrusting which may account for the lack of discoveries in the southern junggar basin most of the oil discovered so far is trapped in structures which originated duri g the late paleozoic or mesozoic

the diagenetic fate of taraxer 14 ene and oleanene isomers abstract the extractable organic matter in holocene to early miocene deep sea sediments from site 645 odp leg 105 in baffin bay contains almost exclusively biological markers of terrigenous origin pentacyclic triterpenoids of the ursene and amyrin oleanene type often occur as the most abundant compounds in the aliphatic hydrocarbon ketone and alcohol fractions of the sediment extract a specific diagenetic reaction involves the conversion of taraxer 14 ene into olean 12 ene in the upper 700 metres of the sedimentary sequence the taraxerene to oleanene conversion is complete before the onset of isomerisation of diasterenes at c 20 in the sediment as well as in laboratory simulation experiments olean 12 ene further isomerises to olean 13 18 ene and olean 18 ene the latter of which may be the direct precursor of 18 h and 18 h oleanane found in sediments containing more mature organic matter and in crude oils the subsurface interconversion of these triterpenoid skeletons indicate that oleanane does not necessarily start life as an oleanoid

controls of oil family distribution and composition in nonmarine petroleum systems a case study from the turpan hami basin northwestern china controls on oil family distribution in tectonically complicated nonmarine petroliferous basins are commonly difficult to isolate because of the varying ages of potential source rocks the complex assemblage of organic rich sedimentary facies and the geographic variability of burial histories the turpan hami basin of northwestern china is an oil bearing intermontane basin where stratigraphic sedimentologic and geochemical controls are sufficient to address each of these issues independently and to determine how they influence the current distribution and composition of liquid hydrocarbons source rock age is one of three major statistically significant discriminators affecting oil family composition both lower middle lower or middle jurassic and upper permian rocks are important source rocks for the basin a newly developed diterpane biomarker parameter can distinguish permian rocks and their correlative oils from jurassic coals and mudrocks and their derivative oils source facies is a second key control on petroleum occurrence and character a variety of biomarker parameters that reflect source rock depositional conditions are indexed to rock samples from interpreted depositional environments by erecting rock to oil correlation models the biomarker parameters separate oil families into end member groups group 1 oils lower middle jurassic peatland swamp facies high land plant input less reducing conditions group 2 oils lower middle jurassic profundal lacustrine facies high algal input more reducing conditions and group 3 oils upper permian lacustrine facies high algal stratified anoxic conditions burial history exercises a third major control on petroleum distribution source rock maturation modeling can demonstrate that relatively uninterrupted burial in the asymmetrically subsiding northern turpan hami area taibei depression exhausted upper permian sourced rocks by the late cretaceous which led to southward migration of upper permiansourced oils group 3 into triassic reservoirs of southern and southwestern turpan hami tainan and tokesun depressions subsequent to uplift of the central basin thrust that currently partitions taibei from tainan lower middle jurassicsourced oils were expelled in the taibei depression by paleoceneeocene time which locally charged jurassic and cretaceous reservoirs groups 1 and 2 forming turpan hami s largest oil accumulations in the basin

thermal history of the periphery of the junggar basin northwestern china geochemical analyses of rock core samples show that the basin periphery has experienced low thermal stress present day heat flows are in the range of 25 35 mw m 2 and have not been significantly higher than the worldwide mean of approx 63 mw m 2 since the mid permian present day heat flows were determined from corrected borehole temperatures and rock thermal conductivities paleo heat flows were determined by first order reaction kinetic modeling of several geochemical paleothermometers vitrinite reflectance clay mineral diagenesis and relative proportions of sterane and hopane biological marker diastereomers

relationship between petroleum composition and depositional environment of petroleum source rocks crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters as demonstrated with a sample suite of nearly 40 oils a novel parameter based on the presence of c30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine derived organic matter a second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions concentrations of high molecular weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes certain algal derived nonmarine oils may show little high molecular weight paraffin response oils from carbonate sources with a few exceptions can be distinguished by having low pristane phytane ratios low carbon preference indexes and high sulfur contents gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis

origin and mixing of crude oils in triassic reservoirs of mahu slope area in junggar basin nw china implication for control on oil distribution in basin having multiple source rocks abstract the origin of crude oils and their varied geochemical features along the western and northern slope of the mahu depression in the junggar basin has been controversial based on the molecular and isotopic geochemistry of hydrocarbons from 46 drill stem test crude oil samples and 36 core samples three oil groups i ii iii and five subgroups i 1 i 2 ii iii 1 and iii 2 have been recognized and oil source correlations have been established the subgroup i 1 oils originated mainly from lower permian fengcheng p 1 f carbonate source rocks subgroup i 2 from p 1 f mudstone source rock and group ii oils from middle permian wuerhe p 2 w mudstone source rock whose potential was underestimated in the past group iii consists of mixed oils i e subgroup iii 1 received contributions from p 1 f carbonates and p 2 w mudstones and subgroup iii 2 is a mixture of oils from p 1 f mudstones and p 2 w generated oils in order to determine the main source rocks the laboratory oil mixing experiments were carried out tricyclic terpane biomarker parameters tts were used to quantify the source contributions whole oil carbon isotope ratios were also used to verify the proportions of different oils in each mixture the results show that subgroup iii 1 oils have diverse mixing ratios in different reservoir subgroup iii 2 oils are mainly from p 1 f mudstones and p 2 w source rock contributes little finally the accumulation process of oil fields along the northern slope of the mahu depression was analyzed and the contributions from different sources were revealed this study provides an effective quantitative method to identify contributions from different lacustrine source rocks to mixed oils in the mahu depression which could be helpful in predicting location and composition of undiscovered oils

multiple stage migration and accumulation of permian lacustrine mixed oils in the central junggar basin nw china abstract the permian lacustrine petroleum system in the central junggar basin northwest china has recently obtained exploration success these accumulations are characterized by mixed oils and multiple stage migration and accumulation this process however has not been well constrained limiting the understanding of the general laws of hydrocarbon migration and accumulation and making it difficult to define exploration strategies here we reconstruct this process mainly based on oil bearing fluid inclusion analyses including petrography homogenization temperature and sequential extraction inclusion petrography shows that petroleum inclusions dominantly display yellow fluorescence while intergranular free oils mostly display strong blue green fluorescence this implies that the reservoirs seem to have experienced at least two stages of hydrocarbon charge from different sources and or maturities the sources maturities are determined through reservoir sequential extraction analyses the free oils are mixed and sourced from the lower permian fengcheng formation p1f and the middle permian lower wuerhe formation p2w while the inclusion oils are also mixed and derived mainly from the p1f source both oils are characterized by moderate maturity homogenization temperature measurements of oil bearing fluid inclusions combined with reconstruction of reservoir burial and thermal history not only further verify multiple hydrocarbon charging events but also characterize the timing which indicates a three stage petroleum charging model during the first stage of the late triassic carboniferous reservoirs were charged by p1f sourced oils which were dysmigrated or remigrated to jurassic reservoirs during the second stage i e the early cretaceous in contrast the third stage from the late cretaceous represents another hydrocarbon charge event that is the primary oil accumulation sourced from the p2w rocks according to this model both primary and secondary oil accumulations contribute to the current region s exploration success and provide future exploration targets

geochemical and biological marker assessment of depositional environments using brazilian offshore oils abstract a combined geochemical and molecular characterization of a wide selection of oils from the major brazilian offshore basins has been undertaken the elemental sulphur nickel and vanadium and bulk oapi and 13c properties of each sample been considered together with its molecular composition determined using liquid and gas chromatography and quantitative biological marker investigations using gas chromatography mass spectrometry for alkanes the results reveal significant differences in the chemical features of the various oils which enable them to be divided into five groups the distinction of the groups appears to reflect differences in the depositional environment of the source rocks of the oils each group is correlated tentatively with source rocks laid down in a specific depositional regime namely lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate or marine deltaic the diagnostic features that allow this classification are the relative abundance and carbon number distributions of n alkanes pristane phytane ratios sulphur nickel and vanadium contents carbon isotope data the absolute concentrations of hopanes and steranes and their abundance relative to 4 methylsteranes and also the occurrence and abundance of several specific biological markers including 18 h oleanane gammacerane carotane tricyclic terpanes higher acyclic isoprenoids 28 30 bisnorhopane and 25 28 30 trisnorhopane this investigation shows the value of a combined geochemical and molecular approach in the assessment of the palaeoenvironment of deposition of the source rocks which gave rise to the oils

origin and accumulation mechanisms of petroleum in the carboniferous volcanic rocks of the kebai fault zone western junggar basin china abstract the kebai fault zone of the west junggar basin in northwestern china is a unique region to gain insights on the formation of large scale petroleum reservoirs in volcanic rocks of the western central asian orogenic belt carboniferous volcanic rocks are widespread in the kebai fault zone and consist of basalt basaltic andesite andesite tuff volcanic breccia sandy conglomerate and metamorphic rocks the volcanic oil reservoirs are characterized by multiple sources and multi stage charge and filling history characteristic of a complex petroleum system geochemical analysis of the reservoir oil hydrocarbon inclusions and source rocks associated with these volcanic rocks was conducted to better constrain the oil source the petroleum filling history and the dominant mechanisms controlling the petroleum accumulation reservoir oil geochemistry indicates that the oil contained in the carboniferous volcanic rocks of the kebai fault zone is a mixture the oil is primarily derived from the source rock of the permian fengcheng formation p 1 f and secondarily from the permian lower wuerhe formation p 2 w compared with the p 2 w source rock p 1 f exhibits lower values of c 19 tt c 23 tt c 19 20 tt tt ts ts tm and 20r sterane c 27 c 28 ratios but higher values of tt c 23 c 21 hhi gammacerane c 30 hopane hopane 20s c 34 c 33 c 29 and c 29 20s 20s 20r ratios three major stages of oil charge occurred in the carboniferous in the middle triassic late triassic to early jurassic and in the middle jurassic to late jurassic periods respectively most of the oil charged during the first stage was lost while moderately and highly mature oils were generated and accumulated during the second and third stages oil migration and accumulation in the large scale stratigraphic reservoir was primarily controlled by the top carboniferous unconformity with better porosity and high oil enrichment developed near the unconformity secondary dissolution pores and fractures are the two major reservoir storage space types in the reservoirs structural highs and reservoirs near the unconformity are two favorable oil accumulation places the recognition of the large scale carboniferous volcanic reservoirs in the kebai fault zone and understanding of the associated petroleum accumulation mechanisms provide new insights for exploring various types of volcanic reservoir plays in old volcanic provinces and will undoubtedly encourage future oil and gas exploration of deeper strata in the region and basins elsewhere with similar settings

temperature effects on kerogen and on molecular and isotopic composition of organic matter in pierre shale near an igneous dike abstract a suite of siltstone samples from the upper cretaceous pierre shale from the contact zone of a 130 cm thick igneous dike near wolcott colorado u s a was taken from the contact to 170 cm from the dike to study the effects of temperature on the organic matter the sampled bedding interval was about 10 cm thick so variation in lithology and type of organic matter is minimal vitrinite reflectance values r 0 increase from 0 4 far from the dike to 3 3 near the dike contact geochemical measurements show systematic thermal effects analogous to those often observed for catagenesis and metagenesis in the depth range of 1 4 km within a sedimentary basin the h c ratio of kerogen and the hydrogen index rock eval decrease most rapidly in the 0 6 1 7 r 0 range in which the transformation ratio rock eval increases from 0 1 to 0 3 based on extraction of c 15 compounds the main increase of hydrocarbons and total extractable organic matter occurs between 0 6 and 1 0 reflectance the saturated aromatic hydrocarbon ratio increases almost twofold in this range of maturity however the pristane phytane ratio is essentially constant through the hydrocarbon generation zone but decreases slightly at high levels of thermal alteration r 0 1 2 the 13 c values for aromatic and saturated hydrocarbons are about 27 and 29 respectively and are constant to about 1 0 r 0 then both become heavier by about 2 at higher r 0 values

petroleum secondary migration and accumulation in the central junggar basin northwest china insights from basin modeling the timing of petroleum generation migration and accumulation in the central junggar basin has been evaluated using basin modeling technology to decipher the processes of petroleum secondary migration and accumulation in the jurassic carrier beds during the yanshan and himalayan orogenies during the early yanshan orogeny j2 j3 crude oils sourced mainly from the permian source rocks migrated laterally from depocenters to adjacent structural high points during the late yanshan orogeny cretaceous eocene the hydrocarbon secondary migration pattern changed significantly specifically in the cretaceous a large scale regional second phase of generation and migration occurred in the western part of the central junggar basin in the eastern part of the central junggar basin crude oils emplaced in the depressions migrated mainly to the eastern uplifts in the early tertiary oils in the source kitchens generally began migrating to the north and mixed with the jurassic generated hydrocarbons during the himalayan orogeny neogene quaternary this migration was strengthened continually the modeled results agree with migration associated geochemical information of the jurassic accumulations integration of the regional geological setting and some petroleum geology data suggest that the jurassic tectonic high points and facies change and unconformity associated subtle traps are of crucial importance for current oil exploration both overpressured bodies in the triassic and permian systems and volcanic rock reservoirs of the carboniferous basement in the uplifts have important implications for future natural oil and gas exploration this work should help reduce exploration risk in this region

molecular and carbon isotopic geochemistry of crude oils and extracts from permian source rocks in the northwestern and central junggar basin china abstract the junggar basin is a major oil producing province in china most oil reservoirs found so far in this basin are in the mahu sag and neighboring uplifts northwestern junggar basin a total of 78 oils and 10 permian source rocks from the northwestern and central junggar basin and two oils and two permian source rocks from the eastern junggar basin were analyzed by gc gc ms and gc irms the 78 oils can be clearly classified into two groups based on these analytical results for group i oils 13c values of individual n alkanes are relatively higher and remain stable with increasing carbon number for group ii oils these values are relatively lower and decrease at first and then increase with carbon number differences in molecular parameters can be also observed between these two groups of oils group i oils generally have 1 higher pr n c17 and ph n c18 ratios and lower pr ph ratio and 2 higher gammacerane c30 hopane gammacerane and carotane carotane c30 hopane ratios compared with group ii oils in addition group i oils mainly have tricyclic terpane distribution patterns with either c20 c21 c23 while group ii oils mainly have the pattern of c20 c23 however molecular parameters overlap to some extent between these two groups of oils group i oils correlate well with the nine source rocks of the lower permian fengcheng formation p1f while group ii oils correlate well with the three source rocks of middle permian age based on carbon isotopic and molecular compositions the occurrence of these two groups of oils in the northwestern and central junggar basin are consistent with facies and thickness variations in the source rocks within the lower permian fengcheng formation p1f and middle permian lower wuerhe formation p2w

sedimentary facies and holocene progradation rates of the changjiang yangtze delta china abstract the changjiang yangtze river one of the largest rivers in the world has formed a broad tide dominated delta at its mouth during the holocene sea level highstand three boreholes cm97 js98 and hq98 were obtained from the changjiang delta plain in 1997 1998 to clarify the characteristics of tide dominated delta sediments and architecture based on sediment composition and texture and faunal content core sediments were divided into six depositional units in ascending order they were interpreted as tidal sand ridge prodelta delta front subtidal to lower intertidal flat upper intertidal flat and surface soil deposits the deltaic sequence from the prodelta deposits to the delta front deposits showed an upward coarsening succession overlain by an upward fining succession from the uppermost part of the delta front deposits to the surface soil thinly interlaminated to thinly interbedded sand and mud sand mud couplets and bidirectional cross laminations in these deposits show that tide is the key factor affecting the formation of changjiang deltaic facies sediment facies and their succession combined with ams 14c dating revealed that isochron lines cross unit boundaries clearly and delta progradation has occurred since about 6000 to 7000 years bp when the rising sea level neared or reached its present position the average progradation rate of the delta front was approximately 50 km kyear over the last 5000 years the progradation rate however increased abruptly ca 2000 years bp going from 38 to 80 km kyear the possible causes for this active progradation could have been an increase in sediment production in the drainage basin due to widespread human interference and or decrease in deposition in the middle reaches related to the channel stability caused by human activity and climatic cooling after the mid holocene

junggar basin northwest china trapped late paleozoic ocean abstract the junggar basin originated during the late paleozoic as either a remnant lower to mid paleozoic ocean basin or a mid carboniferous back arc intra or inter arc basin bounded by emergent volcanic arcs south of the siberian craton the retreating junggar sea left behind a regressive sedimentary section comprising at least 3 4 km of marine volcaniclastics in the southern junggar area sandstones deposited in the junggar basin since the devonian are exclusively volcaniclastic demonstrating that precambrian basement rocks have never been exposed in the basin and supporting the hypothesis that the junggar is underlain by oceanic crustal materials the mid carboniferous junggar ocean may have been a remnant ocean basin of early to mid paleozoic age or alternatively may have been an extending intra or inter arc basin formed behind an emergent volcanic arc in the northern turpan region subsequent strike slip deformation and the general sparsity of geologic and geophysical data from the junggar area make it difficult to distinguish between these models late early permian through triassic sediments of the junggar basin are exclusively non marine deposited in a flexurally subsiding foreland basin during initial uplift of the ancestral tian shan mountains the sedimentary section deposited during the late permian early triassic appears to be inconsistent with a proposed rifting episode during this period

petroleum isotopic and biomarker composition related to source rock organic matter and depositional environment abstract a multivariate approach has been developed that separates oils derived from organic matter in nonmarine shales marine shales and marine carbonates some or all of the following analyses were completed on 43 oils from known source rocks 1 c30 sterane and monoaromatic steroid biomarker ratios and 2 stable carbon and hydrogen isotope compositions of whole oil saturates and aromatics three group discriminant analysis of the 28 oils whereall biomarker and isotopic data are available results in no misclassifications at a significance level of 99 using only carbon and hydroogen isotopic data for whole oils saturates and aromatics results in correct classifications of 82 for oils derived from marine shale organic matter and 100 for oils from the other two sources triangular plots probabilities can be used to distinguish oils derived from the three source rock organic matter end members several publications describe classifications of marine vs nonmarine oils using stable carbon isotope ratios of the whole oil or the saturate and aromatic fractions our results confirm that these methods do not completely differentiate oils by source

an organic geochemical investigation of crude oils from shanganning jianghan chaidamu and zhungeer basins people s republic of china abstract eighteen oils from four different oil producing basins in china have been characterized using a variety of organic geochemical techniques the source materials for these oils were deposited under a variety of environmental conditions and this study investigates the use of geochemical parameters in differentiating the resultant oils the parameters discussed are based on relative amounts and distributions of both individual and groups of biomarkers including organosulphur compounds and the carbon isotopic composition of the aliphatic and aromatic fractions examination of the oils showed that those from the zhungeer basin thought to be derived from source rocks formed in a brackish lacustrine environment possessed high concentrations of tricyclic terpanes relative to pentacyclic hopanes and a predominance of 24 methyl and 24 ethylcholestanes c28 and c29 respectively over cholestanes c27 which were virtually absent oils believed to be derived from source rocks deposited in the saline depositional environment of the jianghan basin were characterized by a large amount of gammacerane relative to 17 21 30 homohopanes c31 22r and 22s in general those oils considered to be derived from source rocks deposited in fresh water lacustrine environments shanganning and chaidamu basins had small amounts of tricyclic terpanes relative to pentacyclic hopanes the presence of an unknown c30 pentacyclic terpane small amounts of extended hopanes c31 c35 relative to 17 21 hopane and relatively large amounts of 24 ethylcholestanes 20r and 20s relative to total stearanes it appears therefore that the distribution of biomarkers may be used to differentiate oils derived from different lacustrine environments

hydrocarbon generation potential of triassic mudstones in the junggar basin northwest china triassic mudstones in the junggar basin of northwest china especially those of the upper triassic baijiantan formation which formed during a lake flooding event are the most important set of cap rocks in the basin these mudstones presumably have hydrocarbon generation potential although this potential has not been documented here we provide new organic geochemical and geological background data on these mudstones and discuss their hydrocarbon generation potential geochemical analyses were performed on available samples collected from more marginal shallow lake facies in the central basin area these analyses indicated that these rocks contain about 1 0 total organic carbon toc the organic matter type is predominantly type iii kerogen derived mainly from terrestrial higher plants the organic matter underwent maturation in uplifted areas and reached higher maturity levels in depressions within the basin thus the triassic mudstones may have a potential for hydrocarbon generation during the lake flooding period especially for gas and hence deserve attention in future exploration more research needs to be conducted on the generation potential of these units including oil besides gas as few deep lake facies samples from depressions in the basin are currently available for investigation

geochemistry of oils from the junggar basin northwest china the junggar basin of northwestern china is a structural basin containing a thick sequence of paleozoic pleistocene rocks with estimated oil reserves of as much as 5 billion bbl analyses of 19 oil samples from nine producing fields and two oil stained cores in the junggar basin revealed the presence of at least five genetic oil types the geochemistry of the oils indicates source organic matter deposited in fresh to brackish lake and marine environments including coaly organic matter sources the volumetrically most important oil type discovered to date is produced from late carboniferous middle triassic reservoirs in the giant karamay field and nearby fields located along the northwestern margin of the junggar basin oil produced from the mahu field located downdip in a depression east of the karamay field is from a different source than karamay oils unique oil types are also produced from an upper permian reservoir at jimusar field in the southeastern part of the basin and from tertiary oligocene rocks at dushanzi field and lower jurassic rocks at qigu field both located along the southern margin of the basin previous studies have demonstrated the presence of upper permian source rocks and the possibility of mesozoic or tertiary sources has been proposed but not tested by geochemical analysis although analyses of some possible jurassic coal source rocks have been reported our findings indicate that several effective source rocks are present in the basin including local sources of mesozoic or younger age for oil accumulations along the southern and southeastern margins of the basin future exploration or assessment of petroleum potential of the basin can be improved by considering the geological relationships among oil types possible oil source rocks and reservoirs

polycyclic alkanes in a biodegraded oil from the kelamayi oilfield northwestern china abstract a comparison is made of the distributions of certain classes of saturated hydrocarbon biomarkers in an unaltered oil and a severely biodegraded oil from the kelamayi oilfield in the zhungeer basin of northwestern china both oils are thought to have the same source rock the n alkanes acyclic isoprenoids carotenoid derived alkanes and regular c27 c29 steranes which are present in the nondegraded oil were not detected in the biodegraded sample additionally the biodegraded oil shows an increase in the abundance oil and the presence of 17 h 25 norhopanes 17s h 25 norhopanes c 10 demethylated tricyclic terpanes and a c 10 demethylated c23 tetracyclic terpane which were not detected in the nondegraded oil c26 and c25 regular steranes appear to show a much greater resistance to microbial attack than the c27 c29 regular steranes as indicated by an isomer distribution for these low molecular weight steranes that is expected for nondegraded samples in the biodegraded oil unknown tricyclic compounds showing a base fragment ion of m z 219 and a similar carbon number abundance to the steranes in the nonbiodegraded sample are present in relatively high concentrations in the biodegraded oil these compounds are also present in very low abundance in some nondegraded oils from the kelamayi basin indicating that their much higher concentration in the microbialy altered oil is possibly due to their resistance to biodegradation however at present their derivation from the microbial alteration of steranes cannot be ruled out several series of bicyclic compounds are also present in higher concentrations in the biodegraded oil

hydrocarbon genesis of jurassic coal measures in the turpan basin china abstract a typical case of coal derived oils in china i e the crude oils from the middle lower jurassic coal measure strata in the turpan basin is presented by means of oil source correlation it is confirmed that low maturity crude oil in the shengjinkou oil field is derived from the coal bearing qiketai formation of middle jurassic age a brackish lacustrine sediment mature crude oils in the qiketai oil field and in well taican 1 are sourced from the badaowan formation of lower jurassic age which contains coal seams as thick as 100 m results show that commercial accumulations of liquid crude oils can be generated from coals and coal measure strata containing high volatile coal of bituminous rank despite unfavourable types of source material the total hydrocarbon generating potential can be great due to the unusual abundance of organic matter

carbon isotope composition of marine crude oils 1 a histogram of isotope 13 c values of 621 post ordovician marine oils shows a trimodal distribution within this distribution four groups of oils can be recognized on the basis of isotope 13 c values in conjunction with pristane phytane ratios and sulfur contents oils with isotope 13 c values of 32 0 to 28 0 o oo are mainly marine shale oils older than oligocene oils with isotope 13 c values of 28 0 to 23 5 o oo are mainly deltaic oils of varying geologic age or mesozoic carbonate oils and oils with isotope 13 c values heavier than 23 5 o oo are mainly marine shale oils of miocene age the isotope 13 c variation among oil groups is explained primarily by different factors that control the fractionation of carbon isotopes during primary production of rganic carbon miocene and younger marine shale oils are isotopically heavier than older marine shale oils because of decreased atmospheric co 2 concentration since 25 ma which has resulted in decreased isotope fractionation by marine plankton during photosynthesis deltaic oils are mostly derived from terrestrial organic matter and their isotope 13 c values reflect the time invariant average isotope 13 c value of land plant carbon at about 25 o oo mesozoic carbonate oils are isotopically heavy due mainly to salinity and temperature effects during primary production in restricted environments from which terrestrial organic matter is excluded processes that occur after oil generation generally have smaller effects on isotope 13 c values of oils as a result isotope 13 c val es of oils are useful for determining oil oil and oil source rock relationships and in conjunction with other geochemical properties can indicate the possible age and depositional environment of source rocks

organic geochemistry of carboniferous source rocks and their generated oils from the eastern junggar basin nw china abstract carboniferous source rocks have been gaining increasing attention after the discovery of the kelameili gasfield in the eastern junggar basin in 2005 two sets of source rocks within the lower carboniferous dishuiquan formation c 1d and upper carboniferous batamayineishan formation c 2b respectively have been identified in this area in this paper clear differences between these two source rocks are demonstrated in molecular and carbon isotopic compositions the c 1d source rocks in comparison with the c 2b source rocks have remarkably lower pr n c 17 ratio higher ts ts tm and c 30 diahopane c 30 diahopane c 30 hopane ratios lower concentration of c 24 tetracyclic terpane relative to c 23 and c 26 tricyclic terpanes and higher gammacerane c 31 hopane ratio 13 c values of individual n alkanes decrease with carbon number for c 1d source rocks while the opposite is true for c 2b source rocks among the 10 oils collected from the studied area four were generated exclusively from c 1d source rocks while the others were mainly derived from c 1d source rocks but contaminated by a small or trace amount of oil components which were derived from the c 2b middle permian pingdiquan formation p 2p and lower jurassic source rocks the free adsorbed and inclusion oils from three oil containing volcanic reservoir rocks within the c 2b formation generally correlate with c 1d source rocks based on molecular and carbon isotopic data however the three types of oil from the same reservoir rock vary significantly in molecular and carbon isotopic compositions reflecting facies and maturity changes of charging oil during the reservoir filling

generation and evolution of overpressure caused by hydrocarbon generation in the jurassic source rocks of the central junggar basin northwestern china the jurassic black mudstone and coal beds in the central junggar basin northwestern china are the major source rocks for the basin with type ii2 and type iii gas prone kerogens widespread overpressures are developed in the jurassic stratigraphic interval sonic and resistivity logs display strong characteristic responses of overpressure in the mudstones with anomalously high acoustic traveltimes and low resistivity compared with the normally pressured mudstones the overpressured jurassic sediment sequences appear to have undergone normal compaction because the mudstones exhibit no anomalously low bulk density the overpressured mudstones deviate from the normally pressured mudstones in density effective vertical stress space the overpressure in the jurassic source rocks is therefore not caused by disequilibrium compaction the overpressured jurassic sandstone reservoirs are predominantly oil and gas saturated or oil bearing the well log responses of the overpressured mudstones and seismic velocity characteristics indicate that the top depth of the overpressure zone ranges from 3800 to 4600 m 12 500 to 15 100 ft corresponding to formation temperatures of approximately 94 c to 111 c 201 f to 232 f with estimated vitrinite reflectance values of 0 6 to 0 75 the jurassic source rocks with overpressure are capable of generating hydrocarbons at present and are currently overpressured all the evidence suggests that the overpressure in the jurassic source rocks in the central junggar basin is caused by hydrocarbon hc generation the overpressure evolution was modeled quantitatively in response to pressure changes caused by hc generation during basin evolution the results indicate that multiple episodes of overpressure development and release occurred within the jurassic source rocks suggesting multiple episodes of hc expulsion the timing and numbers of these episodes of hc expulsion were thus determined from the modeled overpressure evolution

source and thermal maturity of crude oils in the junggar basin in northwest china determined from the concentration and distribution of diamondoids abstract here we discuss three types of diamondoid parameters for oils from the junggar basin these are absolute concentrations concentration ratios and isomerization ratios according to the absolute diamondoid concentrations the oils collected from different areas of the basin were broadly divided into three maturity stages 1 low mature 1000 ppm adamantanes 50 ppm diamantanes the oils in the northwestern region of the junggar basin are in the low mature to mature stages based on a combination of diamondoid concentration ratios and biomarker indices these oils can be divided into three groups i e group i oils in the wuxia zone derived from the lower permian fengcheng formation p1f group ii oils in the kebai zone sourced from middle permian lower wuerhe formation p2w and group iii oils in the mahu depression generated from more mature source rocks of jiamuhe formation p1j or the p1f diamondoid concentrations and isomerization ratios were used to precisely evaluate the thermal maturity of mature oils and highly mature condensates respectively our results indicate that the oils in the central part of the basin have decreasing thermal maturity from south to north whereas the oils in the kelameili area display increasing thermal maturity from east to west in this study we found that different diamondoid indices are useful only in certain thermal maturity ranges therefore at least for the junggar basin it is crucial to know which thermal region one is in before using diamondoid ratios for maturity assessment

stable carbon isotopes of amorphous kerogens from phanerozoic sedimentary rocks amorphous kerogens from phanerozoic sedimentary rocks may be divided into two groups on the basis of their 13c values h amorphous kerogens have isotopic values in the range of 24 to 20 per mil and l amorphous kerogens have isotopic values in the range of 35 to 26 per mil the occurrence of amorphous kerogens in the intermediate isotopic range from one rock unit in the data set suggests a weak continuum may exist between these two groups phytoplankton and to a lesser extent zooplankton and bacteria are considered to be the precursors for both kerogen types a plausible explanation for this dichotomy may be found in the source of co2 utilized by phytoplankton which is considered to be controlled by environmental factors h amorphous kerogens are expected to occur in silled basins that are overlain by expansive well circulated deep 500 m water where the prevailing source of carbon in the photic zone is atmospheric derived co2 conversely l amorphous kerogens are expected to occur in restricted basins that are overlain by stratified shallow 200 m water where the prevailing source of carbon in the photic zone may be organic derived co2

application of biological markers in the assessment of paleoenvironments of chinese non marine sediments abstract in this study we employ molecular organic geochemical data for 22 representative sediment samples from chinese non marine sedimentary basins in an attempt to characterize and assess the non marine depositonal paleoenvironments the samples investigated include oilshale shale mudstone and sodium sulphate rich sediments ranging in age from cretaceous to tertiary it is evidenced by geological and geochemical data that those sediments studied originate from lacustrine environments ranging from freshwater brackish fresh water to hypersaline the aliphatic fractions of the sediment extracts were analyzed by gc and gc ms and relative abundance of compounds and compound classes of normal acyclic and cyclic alkanes were measured from mass chromatograms the results reveal significant differences in the chemical features of the various sample groups which enable them to be characterized and identified in terms of source and or environment related parameters the diagnostic parameters that allow this assessment are the relative abundances of n alkanes acyclic isoprenoid alkanes and 4 methyl steranes hopane sterane ratio gammacerane and homohopane indexes etc in this study data interpretation was also assisted by pattern recognition through principal component analysis the results show that the sample groups of different environmental origin could be unambiguously distinguished on scores plot

sensitivity of biomarker properties to depositional environment and or source input in the lower toarcian of sw germany abstract a sequence of lower toarcian sediments which are highly contrasting in their depositional environment over a 5 m depth interval has been investigated in detail for the variability of geochemical properties which are used for source characterization and oil to source correlation lower liassic marls are representative of a well aerated shallow water environment in contrast to the immediately overlying bituminous shales which are deposited under extremely reducing conditions various properties have been found to vary considerably within these two units amongst the most important are carbon isotopic composition of the kerogen the pristane phytane ratios nickel as opposed to vanadyl porphyrins and the c27 dia regular steranes although maturation within the profile does not change some of the maturation dependent biomarker properties such as the monoaromatic steroid side chain cracking and the tm ts ratio exhibit large changes which can be assigned to diagenetic processes another maturation dependent property the 20s 20r epimerization of c29 steranes exhibits smaller changes which could also be due to early diagenetic processes the study suggests that reducing and oxidizing conditions i e eh and ph in the sediment exert an influence on several biomarker precursor product pathways in maturation studies initial variations due to depositional conditions have therefore to be taken into account

chlorobiaceae in palaeozoic seas revealed by biological markers isotopes and geology biomarker hydrocarbons in oils and sediments have considerable potential for assessing the nature of biological processes in the past but only a few can be uniquely related to particular organisms reef hosted oils in the silurian of the michigan basin canada and the devonian of western canada have high abundances of 1 alkyl 2 3 6 trimethylbenzenes their precise structures suggest that they are diagenetic products of aromatic carotenoids of the green sulphur bacteria individual components are enriched in 13c by 7 8 relative to the saturated hydrocarbons of the same oil an isotopic anomaly of this magnitude and direction correlates with a component of photosynthetic carbon assimilation by the reductive tricarboxylic acid cycle these results together with data on palaeoenvironments provide compelling evidence for the existence in ancient restricted seas of microbial communities containing chlorobiaceae organisms with distinctive biochemistry which leave morphologically identifiable remains

origin and mechanism of the formation of the low oil saturation moxizhuang field junggar basin china implication for petroleum exploration in basins having complex histories the moxizhuang field is a small oil field in the central junggar basin containing several low saturation low resistivity oil reservoirs which display a complex correlation between oil saturation and porosity and permeability that is atypical of both the filled and drained reservoirs biomarker associations of crude oil and grains containing oil inclusions gois of both the present day water bearing zones water zones and the oil and water bearing zones low oil saturation pay zones were analyzed to investigate the mechanisms for the formation of the low saturation low resistivity oil accumulations the biomarker assemblage and hierarchical cluster analysis indicate that oil in the moxizhuang field was mostly generated from permian source rock deposited in brackish to hypersaline anoxic environments the pay zones and several water zones display goi values as much as 38 greater than the generally accepted threshold goi value for an oil column 5 these goi values are similar to those for high saturation oil reservoirs in the bohai bay basin and oil zone samples from australian basins suggesting that both pay zones and water zones were high saturation oil reservoirs in the geologic past geologic history analysis shows that the moxizhuang field was located on the north wing of a paleoanticline during oil charge in the late cretaceous to paleogene this anticline has gradually evolved into a south dipping monocline since the neogene causing northward remigration of accumulated oil differential lateral leakage of accumulated oil in different sandstone layers during the remigration phase led to the formation of the water zones with high goi values completely drained reservoirs and the low saturation low resistivity pay zones partially preserved reservoirs and caused the complex correlation between oil saturation and porosity and permeability compared with other postaccumulation physicochemical alteration processes lateral leakage has rarely been recognized recognizing differential lateral leakage of accumulated petroleum with the existence of high quality unfaulted cap rocks has important implication for petroleum exploration in sedimentary basins having complex evolution histories

natural and artificial maturation of biological markers in a toarcian shale from northern germany steroid and terpenoid biological marker evolution was studied in a maturity sequence of lias toarcian shales and adjacent strata from three shallow boreholes in the hils syncline northern germany excellent facies control within the study area ensures the comparability of the organic matter type among the stratigraphically equivalent samples of the natural maturation series in the early hydrocarbon generation stage 0 68 r0 a strong facies dependence of some sterane and hopane isomerisation values was found within the same borehole at only a few metres depth difference the extent of sterane isomerisation at c 14 and c 17 was close to 80 in the toarcian shale and between about 55 and 65 in the adjacent dogger and lias sediments similar differences were observed for the 17 h hopane moretane and 22 29 30 trisnor 18 h neohopane 22 29 30 trisnor 17 h hopane ratios whereas the 20s 20s 20r sterane isomerisation ratio did not show this difference the observed differences mostly disappear by the end of the main oil generation phase 0 88 ro where several reactions appear to have reached completion hydrous pyrolysis experiments by variations of time and temperature were used to simulate the natural evolution of the biological markers results for sterane isomerisation in the high temperature range were compared to those published by mackenzie and mckenzie geol mag 120 417 470 1983 for natural sediments kinetic data derived for this reaction from the hydrous pyrolysis results alone or from the combination of hydrous pyrolysis and natural data sets differ markedly from those determined by mackenzie and mckenzie 1983 the hydrous pyrolysis data alone yield an activation energy for sterane isomerisation at c 20 of 131 kj mol the combined data of 170 kj mol the frequency factors are in the 105 108 s 1 range the significance of the data is discussed in detail

lake type controls on petroleum source rock potential in nonmarine basins based on numerous empirical observations of lacustrine basin strata we propose a three fold classification of lacustrine facies associations that accounts for the most important features of lacustrine petroleum source rocks and provides a predictive framework for exploration in nonmarine basins where lacustrine facies are incompletely delineated 1 the fluvial lacustrine facies association is characterized by freshwater lacustrine mudstones interbedded with fluvial deltaic deposits commonly including coal shoreline progradation dominates basin fill resulting in the stacking of indistinctly expressed cycles up to 10 m thick in map view the deposits may be regionally widespread but laterally discontinuous and contain strong facies contrasts transported terrestrial organic matter contributes to mixed type i iii kerogens that generate waxy oil type i kerogen is hydrogen rich and oil prone type iii kerogen is hydrogen poor and mainly gas prone the luman tongue of the green river formation wyoming and the honyanchi formation junggar basin china provide examples of this facies association which is also present in the songliao basin of northeastern china the central sumatra basin and the cretaceous doba doseo basins in west central africa 2 the fluctuating profundal facies association represents a combination of progradational and aggradational basin fill and includes some of the world s richest source rocks deposits are regionally extensive in map view having relatively homogenous source facies containing oil prone type i kerogen examples include the laney member of the green river formation wyoming the lucaogou formation junggar basin china the bucomazi formation offshore west africa and the lagoa feia formation campos basin brazil 3 the evaporative facies association represents dominantly aggradational fill related to desiccation cycles in saline to hypersaline lakes and may include evaporite and eolianite deposits sublittoral organic rich mudstone facies are relatively thin but may be begin page 1034 quite rich and widespread the highest organic enrichment coincides with the deepest lake stages low input of land plant organic matter results in minimal lateral contrasts in organic content in some cases a distinctive type i s sulfur rich kerogen may generate oil at thermal maturities as low as 0 45 vitrinite reflectance equivalent examples include the wilkins peak member of the green river formation wyoming the jingjingzigou formation junggar basin china the jianghan and qaidam basins china and the blanca lila formation argentina

molecular correlation of free oil and inclusion oil of reservoir rocks in the junggar basin china free oils and inclusion oils oil bearing fluid inclusions of 12 samples collected from the sandstone reservoir formations in the central eastern and northern areas of the junggar basin northwest china were analyzed by gc and gc ms analytical results indicate very similar biomarker distributions within each of the free oils and their associated inclusion oils in the two samples collected respectively from the northwestern and eastern border of the junggar basin the free oil and inclusion oil of sample md1 1 collected from the northwestern border correlate well with the permian source rock of the fencheng formation while those of sample dn1 1 from the eastern border correlate with the permian source rock of the pingdiquan formation in contrast in the central area the free and inclusion oils vary significantly in most cases which suggests variations of sources for oil charges during the filling process these data and the correlation between the free and inclusion oils are consistent with the field and seismic data which show that in areas where samples md1 1 and dn1 1 are located only one available source rock exists while in the central area multiple source rocks are present

characteristics of selected petroleum source rocks xianjiang uygur autonomous region northwest china 1 the sedimentary basins of xinjiang uygur autonomous region china are moderately to poorly explored for petroleum volumetric adequacy of petroleum source rocks is a critical exploration risk in these basins particularly because source rock data are limited this study provides new source rock data and speculatively assesses the source rock potential of xinjiang basins the junggar zhungaer basin the best explored of the xinjiang basins and containing a giant oil field is underlain in many areas by an upper permian lacustrine oil shale sequence remarkable for its organic richness and oil source quality depending on its position in the basin the permian section ranges from immature to overmature and is inferred to be the principal source of oil in the basin upper triassic middle jurassic coal measures including lacustrine rocks constitute a secondary source rock sequence in the basin the smaller intermontane turpan tulufan basin contains a very similar upper triassic middle jurassic sequence which where sufficiently buried probably comprises the only significant oil source sequence in the basin the vast tarim talimu basin offers the greatest variety of potential source rocks of all xinjiang basins but remains the least well documented from limited but geologically planned and focused sampling cambrian carboniferous and permian strata are not considered major oil contributors in the dominantly shallow marine paleozoic section of the northern tarim basin only ordovician black shales appear to have significant potential the upper triassic middle jurassic sequence of the northern tarim basin is similar to that of the junggar and turpan basins a section rich in coal and lacustrine shale that constitutes another potentially significant oil source due to the size stratigraphic packaging and structural relief of the northern tarim basin paleozoic and mesozoic potential il source beds range from immature to overmature

evaluating rare earth elements as a proxy for oil source correlation a case study from aer sag erlian basin northern china abstract the use of rare earth elements ree for oil source correlation has not yet been established in order to evaluate the potential of ree as proxies for oil source correlation we used traditional correlation approaches including biomarkers and trace elemental ratios v ni and ni co to identify the source rock of oil sands using isolated organic extracts of mudstones and oil sands and by analyzing the organic extracts and their corresponding whole rock material for ree compositions by inductively coupled plasma mass spectrometry icp ms oil source correlations using ree were not consistent with conclusions based on biomarker data and trace element ratios only one pair of oil source rock relationship was successfully established using ree ree concentrations of crude oils could possibly be controlled by the quantities of metal complexes and functional groups that provide complexing sites for v and ree as well as secondary alteration processes e g thermal alteration and biodegradation ree patterns might be linked to the organic matter types of source rocks although an accurate oil source relationship failed to be established using ree in this study ree could potentially be a novel complementary proxy for oil oil and or oil source correlations when supplemented by traditional correlation approaches including biomarkers

migration and accumulation of crude oils from permian lacustrine source rocks to triassic reservoirs in the mahu depression of junggar basin nw china constraints from pyrrolic nitrogen compounds and fluid inclusion analysis abstract the geochemical compositions of crude oils and permian source rocks from the mahu depression of the junggar basin were analyzed in this study based on tricyclic terpanes and isoprenoid biomarkers three oil groups i ii and iii can be identified that correspond to different source rock formations and organic facies after examination of the influence of source facies and thermal maturity and selected biomarker ratios based on carbazole isomers such as 1 mc 4 mc 1 8 dmc 2 7 dmc and a a c bc ratios in different oil groups allowed us to determine the direction of hydrocarbon migration in different reservoirs in addition abundant oil inclusions are present with fluorescence colors ranging from yellow to blue white these were analyzed by ftir to distinguish between different sources for the oils fluid inclusion petrography and microthermometry were also used to prove two stages of oil migration and their different sources taking into consideration possible migration pathways and likely driving forces we conclude that abnormally high pressure in the permian sequence acted as the main driving force for oil migration the major migration pathways were vertical faults in the fault belts and slope areas and sand bodies in the deeper areas group i1 oils from p1f carbonate source rocks in the fault belts mainly migrated vertically through the faults and accumulated in t1b reservoirs in the late triassic and were driven by excess formation pressure and buoyancy group i2 oils in the slope areas first migrated vertically within thrust faults from p1f mudstone source rock in the late triassic after which they migrated laterally over short distances to accumulate in t1b sand bodies group ii oils from adjacent p2w source rocks in the center of the depression migrated mainly in the late jurassic due to abnormally high pressure and accumulated in nearby t1b traps some reservoir rocks in the slope area contain two generations of fluid inclusions implying oil mixing from early stage p1f oils and late stage p2w oils resulting in the group iii oils combining these data from biomarkers pyrrolic nitrogen compounds and fluid inclusions allowed us to construct models of migration and accumulation for oils in the mahu depression