bicadinanes and related compounds as maturity indicators for oils and sediments abstract biomarker maturity indices have proven useful in petroleum exploration to assess the thermal maturity of oils and sediments however their application to deltaic petroleum systems is often hindered by source and diagenetic interferences recently we have described several new maturity indices based on the catagenetic products of the plant biopolymer polycadinene these indices should be largely immune to biochemical effects during diagenesis since little microbial activity will remain at temperatures sufficient to dissociate the polymer here we report changes in five polycadinene related maturity indices through a marine fluvio deltaic depositional sequence in the south sumatra basin indonesia the results are compared with the maturity required for oil generation as estimated by kinetic modelling and with conventional maturity indicators such as sterane epimerisation and the methyl phenanthrene index the data confirm that the polycadinene indices are less subject to source and diagenetic interferences than those based on the sterane and aromatic hydrocarbon distributions values for several of the indices have also been measured for 17 tertiary oils from southeast asia papua new guinea new zealand and australia contrary to an earlier report the bicadinane maturity indicator bmi 1 continues to change into the oil window and hence may be useful in ranking the relative maturity of oils as well as sediments the bmi 1 values for two oils having unusually low sterane maturities are normal leading us to question whether the low sterane maturity of many tertiary age oils is due to their source rocks having been heated more rapidly than those of older oils

the effect of maturity on the relative abundances of cadalene and isocadalene in sediments from the gippsland basin australia abstract two sedimentary sequences from the gippsland basin australia have been analysed for cadelene and 3 isopropyl 1 6 dimethylnaphthalene isocadalene the relative abundance of these compounds changes with increasing depth maturity of the sediments and is associated with an increase in the abundance of 1 6 dimethylnaphthalene relative to other dimethylnaphthalene isomers these changes have been attributed to dealkylation and isomerization reactions of cadalene in the sediments because analogous changes have been demonstrated in laboratory heating experiments in which cadalene was converted into a mixture containing mainly 1 6 dimethylnaphthalene and isocadalene when heated in the presence of aluminum smectite the ratio of isocadalene and cadalene may be a useful maturity parameter for some sediments and crude oils which contain these compounds

the effects of thermal maturity on distributions of dimethylnaphthalenes and trimethylnaphthalenes in some ancient sediments and petroleums abstract the distributions of di and trimethylnaphthalenes in two sedimentary sequences from western australia have been examined by capillary gas chromatography and combined gas chromatography mass spectrometry a general decrease was observed in the relative proportions of dimethylnaphthalenes and trimethylnaphthalenes with increasing thermal maturity similar trends were also observed for six crude oils which have very different ratios of ethylcholestane epimers indicating very different thermal histories these results suggest that changes in the relative abundances of certain methyl substituted naphthalenes may be useful indicators of thermal maturity of sedimentary organic matter and the use of a number of isomer ratios is illustrated

short term biomarker variability in the monterey formation santa maria basin abstract forty eight samples of a 370 ft miocene monterey formation core from the union leroy 51 18 well santa maria basin california are examined for source rock potential and molecular distributions total organic carbon content rock eval pyrolytic yield vitrinite reflectance and distribution of regular steranes tricyclic and pentacyclic terpanes and aromatic steroids are measured the present day temperature range through the sampled interval is estimated to be less than 5 c maturity differences among samples are negligible source rock potential and molecular distribution differences are considered to arise entirely from the effects of source input diagenesis including lithologic effects and migration the sampled section is comprised of siliceous and phosphatic carbonate lithofacies and covariances between geochemical parameters source potential and biomarker distribution and lithofacies are recognized several molecular parameters change systematically with depth lithology including ethylcholestane ratios and c 20 and c 22 sterane and hopane epimer ratios the siliceous lithofacies possesses higher moretane hopane tricyclic pentacyclic terpane and sterane terpane ratios and lower mono mono tri aromatic steroid ratios bisnorhopane hopane ratios are up to 40 times higher in the phosphatic carbonate lithofacies than in the siliceous lithofacies increased relative concentrations of aromatic steroids are attributed to sulfur sterol reactions during very early diagenesis whereas increased relative bisnorhopane amounts are considered to result from massive input of specific anaerobic biota although kerogen elemental ratios change abruptly at the lithofacies boundary molecular distributions within the extractable organic matter change gradually beginning about 30 ft 9m above this interface this gradual change is tentatively attributed to upward migration of hydrocarbons originating in the phosphatic carbonate lithofacies probably through fractures within the lower part of the siliceous lithofacies interval

diterpanes triterpanes steranes and aromatic hydrocarbons in natural bitumens and pyrolysates from different mimic coals abstract there is a significant difference in the distribution of terpanes in natural bitumen extracted from four coals identified as rocky mountain coal rmc australian gippsland latrobe eocene coal gec australian gippsland latrobe cretaceous coal gcc and texas wilcox lignite wl whereas pentacyclic triterpanes are dominant in gec gcc and wl diterpanes strongly predominate in the bitumen of rmc this indicates that resin is a more important constituent of rmc than in the other coals and releases the diterpenoids at an early stage of diagenesis furthermore the composition of diterpanes is also different among these coals for example tricyclic diterpanes are the only diterpanes present in rmc whereas tetracyclic and tricyclic diterpanes are both present in gec and gcc and tetracyclic diterpanes are most abundant in gec however diterpanes are nearly absent in wl surprisingly the diterpenoid content is negligible in the pyrolysates of all coals the sterane and triterpane distributions in the natural bitumen of coals are very different from those in pyrolysates c 31 22r hopane predominates in the m z 191 mass fragmentograms of the natural bitumens whereas a homologous series of hopanes c 27 c 35 except c 28 is present in the coal pyrolysates c 29 steranes dominate in all of the coal bitumens as well as pyrolysates c 27 steranes are absent in the natural bitumen but are generated in pyrolysates during lengthy heating of coal kerogen this indicates that formation of secondary steranes occurs by cracking the side chain c 29 monoaromatic steroid hydrocarbons are abundant in the natural bitumen of the four coals the m z 253 distribution is similar for gec rmc and wl but it differs from the more mature ggc which has a lower content of 20r of both 5 and 5 c 29 ma steroids triaromatic steroid hydrocarbons are only present in gcc whereas significant amounts of tetra and triaromatic triterpenoid hydrocarbons are present in the other three coals gec rmc and wl the ratio of benzo e pyrene perylene may be potentially useful as a maturity parameter because the amount of benzo e pyrene increases with thermal stress whereas perylene decreases with heating time benzohopanes are widely present in four humic coals but become unstable after 10 h heating at 300 c

synchronous fluctuations of glaciers in the alps and altai in the second half of the holocene the altai mountains occupy the northwestern part of the central asian collision belt the late holocene behavior of glaciers in the altai mountains was reconstructed for the khaidun river valley in the western part of the russian altai we report here 17 new radiocarbon datings from wood remains buried by moraines from peat layers and lacustrine sediments the glaciers advance took place in 2500 2200 bc 1100 bc 500 bc ad 400 ad 1200 1300 and ad 1500 1850 it is the first time that material which separates the so called historical stage of cooling into three stages has been collected in the altai mountains the comparison of time of moraine formation in the altai with glacier activity in the alps is evidence of the simultaneity of these processes

the effects of varying tissue preservation on the aliphatic hydrocarbons within a high volatile bituminous coal detailed petrographic analyses of the barrett coal an upper permian high volatile bituminous coal in the hunter coalfield australia reveal cycles of peat formation defined by the tissue preservation index these cycles correspond to coal plies that are separated by dirtbands the petrographic cyclicity is also manifested by a corresponding stratigraphic cyclicity in the biomarker composition of the extracted aliphatic hydrocarbons higher amounts of hopanes and lower carbon preference index values n c22 to n c30 show greater bacterial contribution in samples characterised by low values of tissue preservation index the inherited biodegradation signal from the diagenetic stage has now been partially diluted by the generation of new hydrocarbons from the macromolecule the c31 22s22s 22r hopane and c29 20s20s 20r sterane ratios show least variation through the seam the homohopane epimer ratios have attained equilibrium whilst the hopane moretane ratios are typical of coals with unsupressed vitrinite reflectance of 0 65 however other hopane and sterane maturity ratios appear slightly affected by the degree of preservation of organic matter the c29 and c29 20s20s 20r sterane ratios were not found to be useful for maturity comparisons with other coals because of the low values average 0 18 and 0 43 respectively due to heating rate effects these and other biomarker distributions are syndepositional features of the coal precursor and therefore are controlled by the environmental conditions during the stage of peat accumulation and its subsequent coalification all these changes are evident in a coal member only 2 2 m in thickness the petrographic and geochemical evidence suggest that the environmental affinity of the barrett coal is that of a delta plain depositional setting

petroleum generation and migration in coal seams of the carboniferous ruhr basin northwest germany a series of 64 coal and clastic rock samples of westphalian age was taken from five narrow 3 m depth intervals between 1200 and 1530 m depth and studied by geochemical and petrological methods maturity increases regularly with increasing depth as indicated by several maturity parameters the influence of organic facies is evident in the uppermost coal seam 0 70 rr where a hand picked vitrinite contains less total extract and less saturated long chain hydrocarbons than a directly adjacent sporinite rich coal extracts from deeper and more mature coals are less affected by facies changes probably due to enhanced petroleum redistribution in the seams total yield and gross composition of extracts in the deepest seam 0 92 rr which is directly overlain by sandstone seem to be influenced by the preferential expulsion of saturated hydrocarbons

biomarker and n alkane isotope profiles for tertiary oils relationship to source rock depositional setting abstract biomarker and n alkane isotope profiles have been measured for 29 late cretaceous tertiary oils from se asia china papua new guinea new zealand and the u s a the results are interpreted with respect to six kinds of source rock depositional setting fluvio deltaic fd freshwater transitional tr lacustrine lac saline lacustrine sal marine deltaic md and marine carbonate mc a comparison of biomarker and n alkane isotope results suggests that parameters such as the oleanane hopane ratio may overestimate the higher plant contribution to marine oils the abundance of bicadinanes is much more variable than that of oleanane probably because of a specific association with the highly resinous dipterocarp hardwoods of se asia however traces do occur in an oil from outside the paleogeographic range of the dipterocarp family confirming that there is also a more general angiosperm source the bicadinane isomeric distribution is shown to depend on maturity leading to new maturity parameters which are resistant to even very heavy biodegradation retention indices based on the regular hopane series are provided for the main bicadinane and methyl bicadinane isomers and for a group of oleanane related triterpanes of unknown structure the presence of the latter compounds may indicate locally deposited as opposed to transported higher plant matter in the source source rock depositional setting is the primary control on the shape of the n alkane isotope profile with negatively sloping curves being characteristic of fd and tr oils and flat or positively sloping curves typical of marine oils the difference is probably related to the bacterial reworking of higher plant matter in the fd environment a slight isotopic anomaly at n c 17 correlates with the abundance of algal derived steranes and may indicate a minor marine contribution to the source of an fd oil

sesquiterpanes diterpanes and other higher terpanes in oils from the taranaki basin of new zealand abstract the bi tri and tetracyclic terpenoids in crude oils from the taranaki basin of new zealand have been studied the distribution of c14 c15 and c16 bicyclanes was essentially invariant amongst the various fields a suggestion is made that the proportion of rearranged to total drimanes may be related to maturity and the ratio of homodrimane to drimane may be related to depositional environment mass spectra are reported for a number of unidentified sesquiterpanes including possibly norbisabolane and bisabolane the distribution of bi tri and tetracyclic diterpanes was very similar to that of crude oils from the gippsland basin of south east australia the occurrence of rimuane was confirmed 8 h labdane was identified and 4 h 18 norisopimarane was tentatively identified isopimarane dominated the diterpane distribution of oils from the maui field in the south west of the basin and its abundance gradually diminished in a north westerly direction until it became a minor component of oil from the new plymouth field this trend coincided with an increase in the complexity of the triterpane distribution and with the oleanane content of the oils these northwards trends also followed the evolution of coal deposits in the region the diterpanes in these oils had the same structure as those which occur in extant trees of the araucariaceae and podocarpaceae which flourish in new zealand today and did so even more extensively in the past coal measures formed from forests of such trees are probably the source of the diterpanes a situation analogous to that in the gippsland basin of south eastern australia tricyclanes of the cheilanthane series were not detected in any of the taranaki oils lending support to the view that these tricyclanes occur most abundantly in oils derived from marine sources c24 tetracyclanes secotriterpanes occurred abundantly in all the oils as they do in terrigenous oils from other parts of the world

organic geochemical characterisation of sediments from the sakoa coalfield madagascar abstract eleven coals and five mudstones of the sakoa coalfield southwest madagascar have been characterised by microscopy rock eval pyrolysis and detailed analysis of c 12 aromatic and c 15 saturated hydrocarbon fractions of the organic extracts average vitrinite reflectance of 0 77 r r and t max of 439 c indicate moderate thermal evolution of the organic matter corresponding to the beginning of catagenesis which is confirmed by molecular indicators based on aromatic hydrocarbons and saturated biological markers the average methylphenanthrene index mpi 1 of 0 69 corresponding to a calculated vitrinite reflectance r c of 0 81 is in agreement with an isomerisation of selected steranes and triterpanes having not yet reached equilibrium maceral compositions and molecular distributions of saturated and aromatic hydrocarbons are typical of organic matter mainly derived from terrestrial higher plants with significant contributions of bacteria and a minor input of algae the hydrocarbon potential in terms of hydrogen index hi values of the coal samples varies between 83 and 284 mg hydrocarbons hc g c org i e in approximately the same range as that commonly observed for high volatile bituminous coals carbon normalised concentrations of c 12 aromatic and c 15 saturated hydrocarbons generally are higher in the coals than in the mudstones thus suggesting expulsion of heavier hydrocarbons was relatively ineffective in sakoa coals

botanical control on banding character in two new zealand coal beds abstract two new zealand coal beds were examined using a palaeobotanical approach to determine controls on banding characteristics the morley cretaceous coal is thickly 5 to 1 or 32 to 2 mm and well 25 banded whereas the brunner eocene coal is essentially non banded palynological analyses indicate that the morley was formed from a gymnosperm dominated vegetation with a secondary component of angiosperms in contrast the brunner was dominated by angiosperm vegetation with only a minor component of gymnosperms however it was found that any xylite vitrain bands 1 mm in either coal bed were comprised exclusively of secondary xylem of probable gymnosperm origin the presence of gymnosperm wood i e secondary xylem forming the larger bands in both coals can be related to the relative resistance of this material to chemical and physical decay as compared to all other plant tissues an absence of angiosperm wood in either the morley or brunner coal beds may be the result of the relative chemical degradability of angiosperm wood as compared to gymnosperm wood alternatively angiosperms in either of the two palaeo mires may have been shrubby and thus did not produce abundant woody tissue in either case the banded nature of the morley coal bed is the result of the presence of gymnosperm wood and the non banded nature of the brunner is the result of being dominated by a flora angiosperm which is easily degraded

the volcano sedimentary evolution of a post variscan intramontane basin in the swiss alps glarus verrucano as revealed by zircon u pb age dating and hf isotope geochemistry the late palaeozoic glarus verrucano basin gvb glarus alps eastern switzerland formed as an intramontane graben in the aftermath of the variscan orogeny its fill the glarus verrucano consists of immature alluvial fan and playa lake deposits with intercalated bimodal volcanics basalts and rhyolites despite its importance for local and regional geology no modern sedimentologic or stratigraphic studies on the gvb exist by means of sedimentologic and geochronologic studies we reconstruct the volcano sedimentary evolution of the gvb it developed at the carboniferous permian boundary and experienced a first bimodal volcanic phase around 285 ma for the same time indications for temporarily humid climate in the otherwise rather arid early permian are demonstrated e g pyrite bearing sandstones during the middle and early late permian increasing aridity is indicated by playa deposits fanglomerates and subaerial ignimbrites which mark a second silicic volcanic phase at 268 ma the detrital zircon age spectra are dominated by late variscan ages and thus demonstrate that older sedimentary and metamorphic rocks once forming the variscan nappe edifice were already mostly eroded at that time finally some larger scale speculations are given which could indicate a causal connection between the widespread tectono magmatic mid permian episode and the local development of the glarus verrucano basin

relationship of biomarker distribution to depositional environment phosphoria formation montana u s a from biomarker distributions it is possible to distinguish extracts from four different members facies meade peak rex retort and tosi of the permian phosphoria formation at little sheep creek montana these distributions relfect changes in floral and faunal distribution in response to a changing depositional environment and suggest that the retort and tosi members the second cycle of deposition were deposited under more anoxic conditions and at higher salinity than meade peak and rex members the first cycle correlations between biomarker indicators of anoxia and salinity suggest that anoxia was in part the result of a chemocline separating normal marine waters above from more saline bottom waters anoxia and salinity in the bottom waters increased with time making conditions in the basin progressively more hostile to benthic organisms development of extreme environmental conditions in continental shelf habitats such as those suggested by biomarker parameters at little sheep creek may have compounded spatial problems created by continental suturing suggested to be a cause of permian extinctions factor analysis indicates that four factors for 75 3 of the variance in biomarker ratios these factors are suggested to be source organisms types of eukaryotic plankton salinity eh and mineralogy the grandeur member of the phosphoria which is a typical reservoir rock for phosphoria derived oils yields an extract containing a novel c30 nitrogen containing steroid this unique steroid is not abundant in any other known extracts or oils which along with the extremely low porphyrin maturity parameter of all the extracts pmp 0 01 confirms the absence of migrated oils in the phosphoria at little sheep creek the biomarker ratios c29 ga a steranes ts ts tm triaromatic triaromatic monoaromatic steroids short long short chain monoaromatic steroids and short long short chain triaromatic steroids are commonly used to determine the thermal maturity of oils and source rock extracts however these ratios show considerable variation in core extracts of the same thermal maturity from different facies in contrast the thermal maturity parameters c29 20s 20s 20r steranes and porphyrin maturity parameter pmp show little variation indicating that these parameters are less affected by changes in depositional environment and may be most useful for thermal maturation determinations of phosphoria related oils and source rocks this study supports previous work suggesting environmental effects must be considered if correct determination of thermal maturity of an oil or extract is to be made this is especially true of oils and extracts from source rocks with high sulfur concentrations and or those deposited at elevated salinities

alternative calibration of the methylphenanthrene index against vitrinite reflectance application to maturity measurements on oils and sediments abstract the methylphenanthrene index mpi 1 radke and welte advances in organic geochemistry 1981 pp 504 512 wiley chichester 1983 has been calibrated against vitrinite reflectance using australian carboniferous permian jurassic and tertiary sediments and coals containing terrestrial organic matter the correlation obtained was slightly different from that found previously and has resulted in an extension of the relationship to lower maturation levels vitrinite reflectance 0 5 ro in this sample set the methylphenanthrene distribution factor kvaldheim et al geochim cosmochim acta51 1883 1888 1987 showed a poorer correlation to vitrinite reflectance than mpi 1 hydrogen rich organic matter from an immature marine sequence gave anomalously high methylphenanthrene indices and confirms a source control on the index at low maturation levels the index has been used to confirm the low maturity of some australian terrestrial oils and to determine the degree of maturation of organic matter and oils in proterozoic sediments

aliphatic and aromatic triterpenoid hydrocarbons in a tertiary angiospermous lignite abstract the hydrocarbon biomarker assemlage of a liptinite rich early tertiary angiospermous lignite has been characterized the biomarker fistribution for the lignite confirms the low maturity and previous paleobotanic interpretations the aliphatic hydrocarbon fraction consists predominantly of n alkanes showing a strong odd carbon predominance and numerous de a triterpenoids and triterpenoids derived from higher plants bacterially derived triterpenoids were represented primarily by homohopanes and hopenes the aromatic hydrocarbon fraction contains a series of mono di and triaromatic de a triterpenoids and mono tri and tetraaromatic triterpenoids derived from higher plants the concentration and abundance of monoaromatic triterpenoids containing one to three additonal degress of unsaturation have not seen previously observed in a single deposit as such a diagenetic scheme for angiosperm derived triterpenoids is presented which confirms and extends previous hypothesis

trimethylnaphthalenes in crude oils and sediments effects of source and maturity abstract three sedimentary sequences and a number of crude oils have been analysed for trimethylnaphthalenes tmns using gas chromatography the sediments were found to contain enhanced relative abundances of 1 2 5 tmn and 1 2 7 tmn in samples of cretaceous age and younger this was especially pronounced in lower maturity sediments containing type iii organic matter older samples containing type iii organic matter were found to contain enhanced relative abundances of 1 2 5 tmn and lower relative abundances of 1 2 7 tmn 1 2 7 tmn appears to be derived directly from the structural degradation of oleanane type triterpenoids present in angiosperms and as such is proposed as a marker for this class of plant and indirectly therefore also for samples deposited since the early cretaceous the crude oils have been classified according to their trimethylnaphthalene compositions this classification enables crudes of higher plant origin to be identified and to be further classified according to whether the organic matter pre dated or post dated the evolution of angiosperms

tertiary deltaic and lacustrine organic facies comparison of biomarker and kerogen distributions abstract biological marker and kerogen compositional data in tertiary deltaic and lacustrine settings were compared and contrasted the two settings are represented by a coals and organic carbon rich shales in a ick deltaic sequence from the far east constituting three distinct organic facies and b humic coals and spropelic oil shales in a restricted intermontane freshwater lacustrine sequence from western montana u s a constituting two distinct organic facies wedistinguish the organic facies in each depositional setting by systematic differences in gross compound class distributions kerogen and infrared spectral characteristics molecular character of pyrolyzates sterane carbon number distribution and sterane hopane ratios the kerogens of the three deltaic organic facies are characterized by varying quantities of vitrinite resinite sporinite cutinite and fluorescent amorphinite triterpane distributions reveal numerous terrigenous compounds including oleanane oleanenes and unidentified c 30 presumably resin derived components the two organic facies of the freshwater lacustrine setting are distinguishedased upon either a vitrinite or an alganite fluorescent amorphinite predominance in the kerogen numerous characteristics facies related markers are also present including dehydroabietane land plant resins and 4 methylsteranes perhaps from freshwater dinoflagellate input the available molecular and petrographic data are evaluated and characteristics are proposed for distinguishing five organic facies of these two depositional settings the successful simultaneous use of molecular geochemistry and organic petrography in this study provides a format for refining the organic facies concept

changes in hydrocarbon maturity indices with coal rank and type buller coalfield new zealand abstract the buller coalfield south island new zealand is an inverted late paleogene basin that contains middle eocene bituminous coals which exhibit considerable variation in both coal rank across basin and coal type in seam twenty two fractionated bitumen extracts of brunner coal measures coal samples from 12 drillholes were analyzed by gc and gc ms to characterize the effect of coal rank and type on conventional hydrocarbon maturity indices at the beginning and end of the oil window 0 56 1 26 ro max the carbon preference index pristane phytane and isoprenoid n alkane ratios evolve throughout the high volatile bituminous b rank stage while other biomarker ratios 18 h 22 29 30 trisnorneohopane 17 h 22 29 30 trisnorhopane ts tm 18 h 21 h 30 norneohopane c 29 ts 17 h 21 h 30 norhopane and c 30 diahopane hopane do not show appreciable change in value until medium volatile bituminous rank various aromatic based ratios appear to be more effective in delineating rank throughout the entire oil window in particular the methylphenanthrene index and vitrinite reflectance are positively correlated over the entire bituminous rank range however subtle changes in depositional conditions variable coal type complicate these rank estimates within a given coal seam variation in cpi isoprenoid n alkane and hopane sterane ratios appear to be related to the hydrogen content of the coal while the homohopane index and the oleanane hopane ratio covary with sulfur content as with depressed vitrinite reflectance values mpi is similarly lowered in the perhydrous samples the mechanisms that control these hydrocarbon parameters during deposition and diagenesis are complex and convoluted however changes in bacterial activity and community with marine incursion appear to play an important role due to these anomalies none of the hydrocarbon maturity indices calculated can be singularly used to constrain coal rank

effects of source thermal maturity and biodegradation on the distribution and isomerization of homohopanes in petroleum abstract the distributions of c31 c35 17 21 h homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment for example high c35 homohopane indices c35 c31 c35 are typical of petroleums generated from source rocks deposited under anoxic marine conditions where the c35 backbone of the precursor bacteriohopanetetrol is preferentially preserved homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock homohopane distributions are altered by secondary processes for example the c35 homohopane index decreases with increasing api gravity and thermal maturity of related oils from the monterey formation offshore california similar decreases in the homohopane index are observed for expelled oils from monterey source rock during hydrous pyrolysis biodegradation can result in selective loss of high or low molecular weight homologs apparently depending on the bacterial population in the reservoir bacterial demethylation of homohopanes appears necessary to explain 25 norhopanes in at least one biodegraded oil hydrous pyrolysis studies of the phosphatic and siliceous members in the monterey formation show that identical heating conditions result in different homohopane c 22 epimer ratios furthermore a reversal in the trend of the epimer ratio at high experimental temperatures is observed for the phosphatic but not for the siliceous lithology this could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures possibly mediated by rock mineralogy