plate tectonic history basin development and petroleum source rock deposition onshore china abstract china comprises a mosaic of distinct continental fragments separated by fold belts these fold belts are suture zones resulting from the accretion of various fragments formerly separated by intervening areas of oceanic crust the major sedimentary basins onshore china can be classified into four groups those in western china are flexural developing as a result of north south compression in contrast those in the east are extensional and related to development of the pacific oceanic margin in central china basins have a more problematic origin those of north central china ordos sichuan are flexural basins controlled by eastward directed thrusting along their western margin in contrast basins further south chuxiong shiwandashan are predominantly extensional and related to major strike slip movements by synthesizing basin stratigraphies across china in tectonostratigraphic terms and in particular comparing the nature and timing of unconformities it is possible to formulate a coherent model for the palaeoreconstruction of china we identify five major tectonostratigraphic breaks which equate with the collision of the following continental fragments tarim north china carboniferous permian south china block permian triassic qiantang late triassic early jurassic lhasa block late jurassic early cretaceous and india early tertiary prior to permian times the southern margin of eurasia ran approximately along the northern border of modern china the late carboniferous collision of tarim north china with eurasia resulted in the development of a flexural basin junggar and deposition of non marine clastics to the south of the suture shallow marine deposition continued in the late permian early triassic the progressive collision from east to west of the south china block with the north china block resulted in a change to fluvial lacutrine sedimentation across the entire north china tarim block open marine carbonate deposition in the north of the south china block passed southward into a deeper marine clastic sequence deposited in a backarc basin further south a subduction zone existed along the southeastern margin of the south china block in western china northward subduction throughout the triassic resulted in the development of the songban ganzi accretionary prism with retroarc thrusting resulting in flexure and the first development of the tarim basin oblique collision of the qiantang block in the late triassic along the east of the south china block resulted in east west directed thrusting which initiated the suchuan and ordos basins continued strike slip deformation along the south western margin of the south china block resulted in the development of basins with a significant extensional component such as chuxiong the collision of the qiantang block with the southern edge of the tarim basin early jurassic resulted in a renewed clastic influx in both the tarim and junggar basins along the eastern pacific margin a compressional arc and retroarc basin in the south passed northwards into an extensional arc system subduction rollback of the extensional arc initiated rifting in the late jurassic in the eren and songliao basins the late jurassic early cretaceous collision of the lhasa block in the west rejuvenated the thrust systems bordering the western basin and resulted in a renewed clastic influx in the southeast the compressional arc phase culminated in widespread thrusting and folding of early cretaceous age in the northeast extension continued with the progressive migration of the rift system southward with time the arrival of the indian block in the early tertiary rejuvenated the bounding thrust belts of all the western basins in the east the change in convergence of the pacific plate to a more westerly direction is marked by extension and widespread rifting along the entire length of eastern china throughout most of china mesozoic and cenozoic deposition occurred in predominantly non marine environments source rocks in such settings comprise principally mudstones deposited in lakes organic rich mudstones these can accumulate in both deep and shallow lakes in order to accumulate substantial volumes the lake must be significant in space and time in china lacustrine orms occur in both rift and flexural basins lacustrine orms deposited under humid climatic conditions are restricted to the period of maximum tectonic subsidence in the flexural basins of western china source rock deposition follows basin initiation by 20 30 ma in the extensional basins of eastern china source rock deposition takes place 5 15 ma after basin initiation by contrast semi arid and arid climate lacustrine orms whilst being best developed during the period of maximum tectonic subsidence occur at all stages in the basin history

deep hydrocarbons in the northwestern junggar basin nw china geochemistry origin and implications for the oil vs gas generation potential of post mature saline lacustrine source rocks abstract deep 4500 m sedimentary units in petroliferous basins are a frontier field for petroleum exploration and exploitation worldwide this study focuses on the northwestern junggar basin of nw china and presents the results of a comprehensive geochemical analysis of the source rocks and hydrocarbons hosted by deep reservoirs in this region including data obtained from rock eval pyrolysis carbon isotope biomarker and light hydrocarbon analyses these data provide insights into the exploration potential of deep hydrocarbons in the study area and in other areas with similar geologic settings i e saline lacustrine petroleum systems results indicate that the deep hydrocarbons in the northwestern junggar basin are dominated by oil with both the oil and natural gas being derived mainly from the lower permian fengcheng formation this formation was deposited in a heterogeneous saline lacustrine environment which led to the generation of geochemically diverse oil compositions consequently fengcheng derived oils can be divided into subcategories based on source rock lithology i e hypersaline alkaline muddy dolomites saline dolomitic mudstones and brackish mudstones a small number of deep oils in the study area were derived from the carboniferous lower permian jiamuhe formation the deep hydrocarbons in the study area generally accumulated in near source environments indicating that prospective areas are present in the form of fengcheng derived oils in the depocenter of the mahu sag especially oils derived from hypersaline alkaline muddy dolomites and saline dolomitic mudstones these source rocks are likely to have generated oil even under the highly mature conditions associated with deep burial this might explain why discoveries within saline lacustrine basins are generally dominated by oil implying that these conditions might also exist within the deeper parts of saline lacustrine basins worldwide

controls on organic matter accumulation in the triassic chang 7 lacustrine shale of the ordos basin central china abstract the triassic chang 7 formation is a rich lacustrine source rock in the ordos basin central china petrographic and geochemical analyses were performed in order to research the mechanism of organic matter om enrichment in the chang 7 shales bulk geochemical data indicate that the om rich shales were formed in a fresh and brackish water which is not usually the case for high toc total organic carbon content sediments therefore there must be some special conditions favorable for om enrichment in chang 7 shales petrography and element geochemistry show that the chang 7 om rich shales are abundant in om enriched laminae framboidal pyrites collophanes and some elements including iron fe diphosphorus pentoxide p2o5 copper cu vanadium v molybdenum mo and uranium u high bio productivity algae blooming and anoxic depositional environments are critical for om enrichment as the organic carbon contents increase the fe p2o5 cu v mo and u element contents increase as well and the ratios of u th and v v ni are growing many geochemical data demonstrate that the water was oxic while the environment under the sediment water interface was anoxic as a result of a high organic supply that sufficiently consumed oxygen taking into account the geological settings and our previous studies we propose that the frequent volcanic events and hydrothermal activity caused by the collision of the north china landmass with the yangtze landmass and consequential formation of the qingling mountains had a significant role in the om enrichment in the fresh and brackish lacustrine shales deposition of volcanic ash into aqueous environments might lead to the increase of some key nutrients such as fe and p2o5 which might enhance primary bio productivity new evidence for hydrothermal activity the reddingite was demonstrated in this study not only can the hydrothermal activity provide elements necessary for life but is also favorable for formation of anoxic settings the deposition of the chang 7 lacustrine om rich shales can be explained using the high bio productivity driven model which is related to volcanic events and hydrothermal activity as a consequence of regional tectonic movements an anoxic depositional setting is beneficial for om preservation and slow sedimentation rates in the deep lacustrine environments are favorable for om enrichment

reappraisal of anoxia and organic richness with emphasis on cretaceous of north atlantic accumulation of anoxic sediments in the north atlantic during barremian through turonian time early to middle cretaceous was controlled mainly by local conditions although global climatic and oceanographic factors played a supporting role evidence does not support the popular hypothesis developed over the past few years of global or oceanwide anoxia data which contradict this hypothesis are based on 1 a reevaluation of criteria for identifying sediments that accumulated in anoxic water 2 a critical reappraisal of reported worldwide occurrences of anoxic sediments of these ages and 3 an examination of the spatial and temporal distribution of anoxic sediments in the north atlantic and elsewhere a model is proposed in which the earliest anoxic sediments of cretaceous age in the north atlantic were deposited in deep restricted basins by middle cenomanian time about 97 m y ago sluggish circulation had led to a gradual expansion of the oxygen minimum layer permitting deposition of anoxic sediment in nonbasinal settings as well expansion of the oxygen minimum layer caused the calcite compensation depth to rise promoting oxidation of organic carbon and causing contraction of the oxygen minimum layer development of anoxia was thus self damping anoxia effectively disappeared by early senonian time 88 m y ago when improved circulation created oceans more like those of the present some evidence indicates that enhanced biologic productivity and upwelling may have been local factors in fostering anoxia but the early and middle cretaceous were generally not highly productive times

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 stratigraphic assessment of pre mt simon clastics precambrian of michigan basin previous workers have synthesized the results of recent deep drilling in the michigan basin with the concept of keweenawan rifting their model correlates the pre mt simon clastics in the sparks et al 1 8 well with those in the state beaver island 1 well identifies them both as turbidites and places the pre mt simon sequence stratigraphically higher in the section than the jacobsville formation and freda sandstone reexamination of the cores and samples from the sparks et al 1 8 suggests a lacustrine origin for the pre mt simon clastics whereas paleomagnetic age data suggest a time equivalence with the freda sandstone the pre mt simon clastics at beaver island are possibly of marine origin and are considered to be time equivalents of the jacobsville formation

late triassic liassic paleoclimatology of the photo central north atlantic rift system abstract the photo central north atlantic rift system was a complex of grabens and half grabens trending southwest northeast sedimentological and paleontological evidence suggests increasing aridity toward the northern end of the rift system and increasing aridity from the late triassic into the liassic the entire rift system drifted slowly northward during the triassic and liassic its northern end had reached about 20 n at the end of the triassic salt deposits in the eastern and central part of the rift synchronous with coal swamps fresh water lakes and zeolitic playas in basins on the western margin of the rift system suggest that the local climatology was controlled by adiabatic processes associated with the lower eastern and higher western border mountains

organic petrological composition of triassic source rocks and their clastic depositional environments in some australian sedimentary basins abstract kerogen from terrestrial plant debris type iii has commonly been considered to be a good source for hydrocarbon gas but not for oil compared with types i and ii kerogen from marine and lacustrine sediments the gippsland basin australia contains giant oil fields producing from organic matter of land plant origin clearly some terrestrial paleodepositional environments have produced organic matter of land plant origin with the potential to generate large volumes of oil an attempt has been made here to identify some environments that contain organic matter of terrestrial origin with the potential to generate oil the dispersed organic matter dom in sediments from various paleodepositional environments in the northern carnarvon clarence moreton simpson desert bowen and gunnedah basins of australia has been analysed petrographically to reduce variations in organic matter type due to differences in geological age examples of triassic age only have been compared dom with relatively high contents of liptinite which is widely accepted as having a better potential to generate oil than vitrinite and inertinite was found in the following environments fluvio deltaic bowen basin proximal lacustrine gunnedah basin and fluvio deltaic northern carnarvon relationships between triassic dom types and paleodepositional environments found in one basin did not necessarily hold true for other basins it is not valid to infer a unique paleodepositional environment from dom type but within a given basin dom type may be predicted from environment

source rock deposition controlled by tectonic subsidence and climate in the western pearl river mouth basin china evidence from organic and inorganic geochemistry abstract interest in factors controlling lacustrine source rock deposition has increased over the last few decades because this type of deposits contain significant petroleum resources generally tectonic subsidence and climate are the two root causes as they control the accommodation potential water column properties and sources of organic matter in this study coupling organic geochemical and elemental geochemical data two potential source rocks i e the eocene wenchang formation e 2 w and oligocene enping formation e 3 e were investigated two models were finally raised to explain deposition of the two set of source rocks according to their paleoclimatic and tectonic properties the source rock potential shows a strong heterogeneity the second member of the eocene wenchang formation e 2 w 2 is characterized by high organic matter content and oil prone kerogen type in contrast the first member of the eocene wenchang formation e 2 w 1 and the oligocene enping formation e 3 e are characterized by low organic matter content and gas prone kerogen type the primary productivity and depositional environment exhibit notable differences between the two potential source rocks horizons and show an obvious variation from the depocenter to the slope and can be best explained by the coevolution of tectonic subsidence and climate during the e 2 w depositional stage low sediment supply led to mudstone deposited in deep lacustrine environment and resulted in underfilled lake basin the low water inflow provided little terrigenous organic matter low bicadinane perylene and floranthene contents and oxygen besides the low area depth ratio impeded the water circulation thus resulted in shallow thermocline and anoxic suboxic bottom environment abundant dibenzothiophene and high c 35 c 31 22s hopane ratios therefore abundant algae which contributed to the high amorphous organic matter aom content can be preserved the warm and wet climate high mn mg ratios gave birth to autochthonous organism such as dinoflagellates and pavlova gyrans abundant 4 methyl sterane during the e 3 e depositional stage the sufficient sedimentary supply resulted in expanding shallow lacustrine and swamp environment the higher area depth ratio and high sediment supply made environment unstable and can be strongly influenced by external environment broader range of mn mg ratios enough terrigenous organic matter tom was transported to the slope but little to the depocenter the slightly hot and dry climate low mn mg ratios led to decreasing autochthonous organism and evaporation environment the shallow water depth and relative dry climate resulted in saline suboxic dysoxic acid bottom environment the co variation of organic and inorganic indexes indicates the combination is a valid method in reconstructing source rock depositional models

sem electron channelling analysis of dynamic recrystallization in a quartz grain abstract the sem electron channelling ec technique has been used to investigate the processes involved in the dynamic recrystallization of an individual quartz porphyroclast by using a combination of crystal axis polefigure diagrams and theoretical crystal axes dispersion paths for individual crystal slip systems it is possible to establish whether a relationship exists between recrystallized grains and parental grain it is then possible to distinguish between rotational and migrational dynamic recrystallization processes and to establish a dynamic recrystallization history for the porphyroclast dynamic recrystallization initiates as a subgrain rotation process initial rotation is accommodated by slip on individual systems leading to grain bending elongate polygonal subgrains and a systematic orientation relationship between subgrains and parent larger rotations and neoblast formation require more than one slip system but a systematic relationship between grains and parent can still be recognized rotational recrystallization produces an unstable microstructure which is stabilized by grainboundary migration

petrological and geochemical investigations of potential source rocks of the central congo basin democratic republic of congo paleozoic and mesozoic outcrop and core samples remina dekese and remina samba wells covering various stratigraphic intervals from the central congo basin were analyzed for total organic carbon corg total inorganic carbon cinorg and total sulfur content rock eval analysis and vitrinite reflectance ro measurements were performed on the basis of the corg content fifteen samples were chosen for molecular organic geochemistry nonaromatic hydrocarbons hcs were analyzed by gas chromatography gc flame ionization detection and gc mass spectrometry samples of the alolo shales from the aruwimi group lindi supergroup late neoproterozoic to early paleozoic are in general very poor in corg most samples 0 5 and contain a high amount of degraded organic matter om all samples of this group revealed a type iii to iv kerogen and cannot be considered as a potential source rock permian carboniferous sediments from the lukuga group dekese well and outcrop samples contain moderate contents of organic carbon 2 the tmax values heating temperature at which the top peak of s2 occurs indicate early mature om partly also a higher level of maturity because of ro 0 6 0 7 and production index values s1 s1 s2 0 2 all samples contain hydrogen poor type iii to iv kerogen with low hc generation potential only having a very minor gas generation potential the kinds of om as well as the biological markers indicate a terrestrial dominated depositional environment organic geochemical investigations on upper jurassic stanleyville group to lower cretaceous loia group samples from the samba well and outcrops in the northeastern part of the congo basin reveal moderate to high contents of organic carbon as much as 25 the kerogen has very high hydrogen index hi values reflecting type i kerogen of excellent quality in the stanleyville group as much as 900 mg hc g corg and type i to ii kerogen in the overlying loia group as much as 900 mg hc g corg outcrop samples from the stanleyville group have variable partly high corg contents and are also characterized by very high hi values as much as 900 mg hc g corg the samples studied are too immature for petroleum generation based on biomarker analysis an aquatic anoxic depositional environment can be assumed for the stanleyville group whereas a lacustrine deposition is likely for the samples of the loia group based on the geologic knowledge of the area deposition under lacustrine conditions is most likely also for the stanleyville group both the stanleyville and loia groups can be regarded as excellent petroleum source rocks and could be part of a petroleum system if sufficient burial and maturation have occurred the presence of resedimented vitrinite particles in the lukuga group of the dekese well with a slightly higher maturation rank than the autochthonous vitrinites suggests that 3000 4000 m 9840 13 120 ft of carboniferous to devonian sediment has been eroded from the eastern margin of the congo basin finally ro data were used to create one dimensional models for the dekese and samba wells giving an overview of the burial thermal and maturity histories of the area

patterns of organic carbon enrichment in a lacustrine system across the k t boundary insight from a multi proxy analysis of the yacoraite formation salta rift basin argentina abstract in this study an integrated approach was applied using available sedimentary geochronology geochemical and isotopic datasets to better understand the complex interactions between production destruction and dilution processes that characterize the organic rich sediments dynamic across the k t boundary in a lacustrine system this approach was tested here on the late cretaceous to early cenozoic yacoraite formation a typical lacustrine source rock from the salta rift basin nw argentina the yacoraite formation corresponds to a mixed carbonate siliciclastic lacustrine sedimentary system deposited during the sag phase post rift we demonstrated here that new ashes u pb dating tie the k t boundary during the deposition of the yacoraite formation the yacoraite formation recorded major climate changes that can be documented in terms of catchment dynamic erosion processes carbonate accumulation trends lacustrine dynamic and source rock quality the pattern of organic carbon enrichment in the yacoraite formation illustrates how a biological pump came across a major climatic change the background organic matter correspond to type i dominated by algal growth mean hi 600 800 mghc gtoc toc0 1 2 wt the k t boundary was the climax of a climate change initiated ca 0 3 myr before and induced a major change in the catchment weathering processes which temporally corresponds to the accumulation of poor quality source rock intervals toc0 0 2 wt and hi

source rock facies distribution predicted from oil geochemistry in the central sumatra basin indonesia the molecular and compound specific isotopic composition of the n alkanes in 30 oil samples from the central sumatra basin were analyzed with the purpose of determining facies variations of the source rock in this basin the oil samples were generated by the eocene oligocene lacustrine brown shale formation biomarkers in the oil samples indicate varying contributions from algal and terrigenous organic matter primarily under oxic or suboxic depositional conditions five source rock facies were identified based on the relative concentrations of n alkanes isoprenoids tricyclic terpanes and steranes the individual facies were also distinguished by the compound specific isotopic composition of the n alkanes source rock depositional environments vary from deep lacustrine shallow lacustrine and lacustrine saline to coal and mixed coal and lacustrine shale deep lacustrine facies predominate in the aman and bengkalis troughs whereas saline and shallow lacustrine facies predominate in the south aman and kiri troughs respectively oils derived from coal and mixed coal and lacustrine shale facies predominate in the northern part of the central sumatra basin in the tanjung medan and north of the balam grabens

rift basins of interior sudan petroleum exploration and discovery in early 1975 chevron overseas petroleum inc commenced what became a major petroleum exploration effort in previously unexplored interior sudan with the cooperation of the sudanese government chevron has acquired a vast amount of geological and geophysical data during the past 12 years these data include extensive aeromagnetic and gravity surveys 36 040 mi 58 000 km of seismic data and 86 wells this information has defined several large rift basins now recognized as a major part of the central african rift system the sedimentary basins of interior sudan are characterized by thick nonmarine clastic sequences of jurassic cretaceous and tertiary age over 45 000 ft 13 716 m of sediment was deposited in the deepest trough and extensive basinal areas are underlain by more than 20 000 ft 6 096 m of sedimentary rocks the depositional sequences include thick lacustrine shales and claystones floodplain claystones and lacustrine fluvial and alluvial sandstones and conglomerates those lacustrine claystones deposited in a suboxic environment provide good oil prone source rocks reservoir sandstones have been found in a wide variety of nonmarine sandstone facies the extensional tectonism that formed these basins began in the jurassic early cretaceous movement along major fault trends continued intermittently into the miocene this deformation resulted in a complex structural history that led to the formation of several deep fault bounded troughs major interbasinal highs and complex basin flanks this tectonism has created a wide variety of structures many of which have become effective hydrocarbon traps during the past eight years several important oil discoveries have been made significant accumulations have been delineated in the heglig and unity areas where estimated recoverable reserves are 250 300 million bbl of oil

paleoreconstruction by biological markers abstract during diagenesis and conversion of the original lipid fraction of biological systems to petroleum hydrocarbons the following four basic events needed for paleoreconstruction can be monitored by biological markets 1 sourcing 2 maturation 3 migration and 4 biodegradation actual cases of applying biological markers to petroleum exploration problems in different parts of the world are demonstrated cretaceous and phosphoria sourced oils in the wyoming thrust belt can clearly be distinguished from one another by high quality source fingerprinting of biomarker terpanes using gas chromatography mass spectrometry identification of recently discovered biological markers head to head isoprenoids allows subtle source differentiation between oils from sumatra the degree of crude oil maturation in basins from california alaska russia wyoming and louisiana can be assessed by specific novel biomarker ratios 20s 20r sterane epimers field evidence from such interpretation is confirmed by laboratory pyrolysis of the rock an example of immature sumatra rock is given extensive migration is documented by biomarkers in several oils the effect of biodegradation on biomarker steranes and terpanes is exemlified in a california basin all biological marker results are consistent with the geological setting and add a new dimension in assisting the petroleum explorationist toward paleo reconstruction

new aspects on the formation of hydrocarbon source rocks episodic volcanism favors as shown by examples from europe the usa and brazil the formation of organic rich sediments hydrocarbon source rocks the examples examined from europe comprise the lower carboniferous oil shale group scotland upper carboniferous coal measures western germany upper jurassic kimmeridge shale north sea area lower cretaceous blattertonstein northwest germany and eocene messel oil shale rheingraben examples from the usa are devonian mississippian gas shales permian phosphoria shale middle cretaceous mowry shale eocene green river shale and miocene monterey shale examples from brazil are permian irati oil shale and oligocene paraiba valley oil shale volcanism is reflected during sedimentation 1 by exhalation of so2 and or co2 2 volcanic ashfalls 3 by generation of anoxia and preservation of organic matter and 4 by halmyrolytic alteration of volcanic ashes into highly surface active clay minerals montmorillonite

petrology of selected carbonaceous limestones and shales in late carboniferous coal basins of atlantic canada carbonaceous limestones and shales are associated with coal in late carboniferous alluvial to coastal successions of nova scotia the units are thin 45 km they consist of micrite and or illitic and kaolinitic clay and some contain abundant bivalves ostracodes pyrite up to 11 vol and phosphatic and sideritic nodules trace fossils are scarce to absent the organic material 2 43 toc is characterised as type ii iii and is mainly of terrestrial derivation predominantly vitrinite and sporinite with lamalginite and telalginite as minor constituents vitrinite reflectance measurements indicate immaturity ro 0 5 but are lower than those of associated coals source rock and hydrocarbon recovery potentials are good to very good and the potentials of some units compare favourably with those of alginite rich carboniferous oil shales in adjacent basins the carbonaceous units were deposited in shallow lakes and or interdistributary bays with aerobic to anaerobic bottom waters at times of minimal detrial input

formation and evolution of nonmarine petroleum in songliao basin china in large lake basins source rocks containing sapropelic kerogen have a high transformation ratio and a high potential for petroleum and they offer the material basis for the formation of a large nonmarine oil field on the basis of geologic and geochemical data and the results of thermal simulation of kerogen it is confirmed that the maturation sequence of kerogen is type i type ii and type iii

evolution of lacustrine systems along the miocene mur murz fault system eastern alps austria and implications on source rocks in pull apart basins tectonically controlled lakes developed during miocene lateral extrusion of the eastern alps mineralogy and the inorganic and organic geochemistry of rocks from three boreholes were investigated to reconstruct the evolution of lake ingering and lake groisenbach and to study the distribution of source rocks in pull apart basins gas prone coal and oil prone sapropelic shale accumulated during the initial shallow stages of lake ingering thereafter the lake deepened rapidly 125 m thick prodelta shale containing a type ii kerogen was deposited in the brackish several hundred meter deep hydrologically closed lake afterwards decreasing subsidence allowed the filling of the lake by prograding deltas during the advance of the deltaic systems the lake became shallower hydrologically open and the brackish influence terminated source rock quality decreased significantly during the filling stage of the lake a consequence of dilution of autochthonous organic matter and of enhanced input of land plants despite its considerable dimensions formation and filling of lake ingering lasted only two million years lake groisenbach was considerably smaller and more susceptible to high frequency changes in lake chemistry although the water body was temporarily oligosaline brackish conditions did not occur high sulphur contents were due to anoxic events and the inflow of ca rich waters abundant dissolved silica favoured diatoms blooms

hydrogeochemistry of lake turkana kenya mass balance and mineral reactions in an alkaline lake abstract lake turkana in northwestern kenya is a closed basin alkaline ph 9 2 lake of moderate salinity tds 2500 ppm principal ions are na hco 3 and ci the lake is essentially polymictic in the northern basin and little compositional variation occurs in surface waters the omo river is the principal influent providing some 80 90 of water input to the lake chloride has an apparent accumulation time of about 2500 years after accounting for burial of interstitial water the bottom sediments are predominantly detrital and fine grained yet mineral water reactions are very important for the geochcmical budget ca 2 is precipitated as calcite na is removed as an exchangeable cation on smectite mg 2 is probably incorporated into a mg silicate phase most likely poorlycrystalline smectite as it enters the lake water k may be used in illite regradation cation exchange is a very important process in the mass balance of this lake over 40 of incoming na is removed as an exchangeable cation after cation exchange and interstitial water burial na has a response time of 2650 years which compares favorably with that of chloride these processes seem to occur rapidly within the water mass of the lake other reactions may be important in regulating interstitial water compositions several changes occur in the upper 3m of sediment interstitial water ph drops to 8 3 and alkalinity increases slightly with depth so 2 4 decreases slightly and amorphous silica saturation is approached these changes are a response to organic matter oxidation and the dissolution of unstable silicates rather than a reversal of reactions occurring in the lake water high rates of sedimentation up to 1 cm per year may minimize the effects of diffusion between the interstitial waters and the lake water although burial of interstitial water assumes considerable importance

a surprising asymmetric paleothermal anomaly around el gordo diapir la popa basin mexico thirty seven mudstone samples were collected from the uppermost lower mudstone member of the potrerillos formation in el gordo minibasin within la popa basin mexico the unit is exposed in a circular pattern at the earth s surface and is intersected by el gordo diapir in the northeast part of the minibasin vitrinite reflectance ro results show that samples along the eastern side of the minibasin i e south of the diapir are mostly thermally immature to low maturity ro ranges from 0 53 to 0 64 vitrinite values along the southern western and northwestern part of the minibasin range between 0 67 and 0 85 values of ro immediately northwest of the diapir are the highest reaching a maximum of 1 44 the results are consistent with two different possibilities 1 that the diapir plunges to the northwest or 2 that a focused high temperature heat flow existed along just the northwest margin of the diapir if the plunging diapir interpretation is correct then the thermally immature area south of the diapir was in a subsalt position and the high maturity area northwest of the diapir was in a suprasalt position prior to tertiary uplift and erosion if a presumed salt source at depth to the northwest of el gordo also fed el papalote diapir which is located just to the north of el gordo diapir then the tabular halokinetic sequences that are found only along the east side of el papalote may be subsalt features however if the diapir is subvertical and the high maturity values northwest of the diapir are caused by prolonged high temperature fluid flow along just the northwestern margin of the diapir then both of these scenarios are in disagreement with previously published numerical models this disagreement arises because the models predict that thermal anomalies will extend outward from a diapir a distance roughly 1 5 times the radius of the diapir but the results reported here show that the anomalous values on one side of the diapir are about two times the radius whereas they are as much as five times the radius on the other side of the diapir the results indicate that strata adjacent to salt margins may experience significantly different heat histories adjacent to different margins of diapirs that result in strikingly different diagenetic histories even at the same depth

lacustrine source rock deposition in response to co evolution of environments and organisms controlled by tectonic subsidence and climate bohai bay basin china three paleogene syn rift intervals from the bohai bay basin the most petroliferous basin in china were analyzed with sedimentological and geochemical techniques to characterize the lateral source rock heterogeneities to reveal the environmental and ecological changes through geologic time and to construct depositional models for lacustrine source rocks under different tectonic and climatic conditions the third es3 and first es1 members of the eocene shahejie formation and the oligocene dongying formation ed display widely variable total organic carbon contents hydrogen indices and visual kerogen compositions suggesting changes in organic facies from deep to marginal sediments carefully selected deep lake facies samples from any interval however display fairly uniform biomarker composition these three intervals have distinctly different biomarker assemblages which indicate weakly alkaline freshwater lakes with a moderately deep thermocline during es3 deposition alkaline saline lakes with shallow chemocline during es1 deposition and acidic freshwater lakes with deep unstable thermocline during the deposition of the dongying formation such environmental changes corresponded to changes in subsidence rate and paleoclimate from rapid subsidence and wet climate during es3 deposition through slow subsidence and arid climate during es1 deposition to rapid subsidence and wet climate during ed deposition and resulted in synchronous changes in terrigenous organic matter input phytoplankton community and primary productivity the co evolution of environments and organisms controlled by tectonic subsidence and climate accounted for the deposition and distribution of high quality lacustrine source rocks with distinctly different geochemical characteristics most rift basins experienced changes in subsidence rates and possibly changes in climates during their syn rift evolutions the models constructed in this paper may have important implications for source rock prediction in other lacustrine rift basins

depositional facies paleogeography and palaeoclimatology of the lower jurassic mccoy brook formation fundy rift basin nova scotia the mccoy brook formation is a 230 m sequence of redbeds of early jurassic age that accumulated in the subtropical fundy rift basin the basin was asymmetrical bordered on the northern side by the cobequid fault system and adjacent highlands the formation is a mosaic of terrestrial facies a multi and single storey fluvial sandstones were deposited by rivers that collected drainage from the norhern highlands and flowed southwest across the valley floor b graded bed sequences of sandstone mudstone were deposited by flood events on the fluvial floodplains and the margins of playas c interbedded sandstones and finely laminated to disrupted red mudstones represent deposits of interfingering sandflats at the toes of alluvial fans and playas respectively the fabrics of both lithologies are commonly disrupted by desiccation and intrastratal precipitation of calcite and minor gypsum d plane bedded conglomerate and pebbly sandstone record deposition in shallow channels that traversed alluvial fans e non bedded breccias of basalt boulders up to 8 m in size in a sand to mud matrix formed as talus against palaeocliffs of the lower jurassic north mountain basalt the palaeocliffs resulted from synsedimentary faulting within the basin f basalt conglomerate beds with matrix to clast supported fabrics comprise sequences deposited by debris flows probably generated by remobilization of talus g a thin sequence of limestone siltstone and muddy sandstone containing fish scales and bones formed in a shallow lake on a basalt flow h sandstones containing m scale crossbeds and a three fold hierarchy of erosional surfaces accumulated as barchans and barchanoid transverse ridges that moved southwest in the valley all of these facies occur within an area of 18 km2 located south of the fault bounded highlands synsedimentary faulting within the basin during early deposition of the mccoy brook formation created palaeocliffs of north mountain basalt talus and debris flow deposits accumulated adjacent to these cliffs interfingering with eolian fluvial and lacustrine sediments during later deposition of the mccoy brook formation the early fault controlled topography was buried as alluvial fans prograded southward from the highlands broad sandflats extended basinward from the toes of the fans interfingering with playa mudflats on the valley floor the palaeoclimate was semi arid as evidenced by gypsum bearing playa redbeds eolian sandstones and caliche palaeosols

organic geochemical and paleoenvironmental characterization of the brown shale formation kiliran sub basin central sumatra basin indonesia abstract the central sumatra basin csb formed during the late eocene to early oligocene is the most prolific hydrocarbon basin in indonesia the brown shale formation of the pematang group is the sole known source rock for the tens of billions of barrels of oil located in the csb samples analyzed in this study were from the karbindo coal mine in the kiliran graben of the kiliran sub basin located on the western side of the csb this study incorporates advanced organic geochemical and petrographical data of samples from the brown shale formation as well as interpretations from previous studies at this location to construct a detailed paleoenvironment model for this tertiary lake the lacustrine source rock was deposited under fresh water conditions with subtle changes in the environment that resulted in significant changes in ecologies based on new data from organic petrography biomarkers microscale sealed vessel pyrolysis and characterization of the polyaromatic hydrocarbons the organic matter in the brown shale formation was determined to be sourced primarily from lacustrine organisms with a small amount of terrestrial plant input 4 methylsterane concentrations and n alkane distributions indicate that non marine dinoflagellates and botryococcus braunii a race were likely a significant portion of the local biosphere

geological controls on the formation and distribution of gas in british coal measure strata abstract the methane ethane higher alkanes and nitrogen found in british coal seams are residual products of coalification occasionally complemented by petroleum derived hydrocarbons carbon dioxide a minor constituent of coal seam gas may have been introduced during the phase of cleat mineralisation following uplift helium and argon are considered to be radiogenic in origin variations in seam methane content with depth geographic location and rank can be interpreted in terms of the coalification and erosion history of a coalfield however the limitations inherent in an analysis in which coal rank is represented by a single parameter must be recognised there are indications that the methane content of a seam is not likely to exceed the methane sorption capacity of the coal determined at a temperature commensurate with that obtaining at the maximum burial depth research in the east midlands coalfield suggests that structurally controlled migration of coalification gas was contemporaneous with folding during the permo carboniferous erosion period gas was lost from outcropping seams subsequently the bulk permeability of the seams must have been reduced perhaps as a result of cleat mineralisation because no re distribution of methane appears to have occurred in response to later tertiary deformation and gas contents of seams at incrop below the base of the permo triassic have remained zero data on the effects of faulting on seam gas contents are few but the expectation is that faults of appropriate age and orientation whilst inhibiting in seam migration may in some instances have aided cross measures movement of gas elevated seam ethane and propane contents are frequently encountered in seams disturbed by oil bearing faults

dynamics of anoxia abstract a simple steady state one dimensional advective diffusive model of the ocean s coupled carbon oxygen and phosphorous system is developed the model is an extension of wyrtki s 1962 model of the oxygen distribution the occurrence of anoxia requires two conditions 1 sufficiently high productivity and 2 sufficiently low oxygen concentration in the bottom water it is shown that anoxic layers are most likely to occur fairly high in the water column anoxic events may be associated with timesof vigorous thermohaline circulation in contrast to stagnation arguments often invoked the model results are used to interpret both modern and ancient oceanic anoxic events

laboratory studies of oxic and anoxic degradation of chlorophyll a in long island sound sediments abstract laboratory incubations of long island sound lis sediments show that chlorophyll a chla degradation generally follows first order kinetics but that the rates and mechanisms of degradation differ under oxic and anoxic conditions chl a in fresh sediment can be operationally defined as being present in three pools bound chl a nonextractable by acetone free acetone extractable anoxically degradable chl a and free anoxically stable chl a a major pathway of degradation is initial release of bound chl a into the free pool the release rate constants of bound chl a into free chl a are remarkably faster 10 under oxic 4 d 1 compared to anoxic 0 1 d 1 conditions and a much greater fraction of total chl a degrades under oxic conditions however the apparent degradation rate constants of free chl a are two to three times higher in the absence of oxygen in sediments with high bound chla 0 2 0 3 d 1 for anoxic 0 1 d 1 for oxic and about seven times higher in sediments without bound chl a 0 3 d 1 for anoxic 0 04 d 1 for oxic the differences between anoxic and oxic decomposition in incubation experiments together with naturally observed concentration profiles which reach low levels of chl a imply that chl a in natural sediments degrades during the oscillation between oxic and anoxic conditions caused by physical and biological mixing processes oscillation experiments oxic vs anoxic and anoxic vs oxic suggest that the activities of aerobic organisms may be an important factor affecting chl a degradation examination of effects of meiofauna on chl a degradation under oxic conditions implies that microorganisms may play a stronger role in chl a degradation than meiofauna 0 125 mm the relative temperature independence of anoxic degradation and temperature dependence of oxic degradation suggest that anoxic degradation may be controlled largely by chemical factors while oxic degradation may be more strongly controlled by biophysical and biochemical processes

the late pleistocene and holocene stratigraphy and paleolimnology of lakes kivu and tanganyika abstract evidence from diatoms chemistry and sedimentology shows that lakes kivu and tanganyika experienced late pleistocene low stands 86 to 300 m for kivu and 250 to 350 m for tanganyika both lakes had higher 2 10 fold sedimentation rates prior to 11 500 yr b p about 9900 yr b p upwelling became important in southern lake tanganyika which had been deepening for at least several millenia by 9400 yr b p kivu had become deep and dilute with overflow via the ruzizi river to lake tanganyika southern lake tanganyika became more stably stratified about 5200 yr b p volcanism and hydrothermal activity in kivu began about 5000 yr b p and the salinity reached 3 ppt and the lake became strongly stratified the ruzizi river outflow was interrupted from 3500 yr b p until recently the sedimentary record implies that sudden hydrothermal activity and sublacustrine volcanism in kivu has probably been the cause of its impoverished fauna these results show that high lake levels in the eastern rift of africa from 10 000 to 7000 yr b p were paralleled in the western rift given the great volumes of these lakes climate has not caused great variations in salinity the diatom floras however have been remarkably sensitive to changes in stratification and nutrient supply volcanism has been a major force in the limnological and biological evolution of lake kivu while both kivu and tanganyika have responded to changes in precipitation and wind stress

deep gas potential of aquitaine basin france the presence of paleozoic source rocks and various possible reservoirs indicate the possibility of a deep gas play in the aquitaine basin france subsidence history provided the key input to a modified form of the crustal stretching model developed by d p mckenzie from which a record of paleotemperatures was calculated paleotemperatures in turn were used in a theoretical maturation study to determine areas of effective gas generation

quantitative filling model for continental extensional basins with applications to early mesozoic rifts of eastern north america in many half graben strata progressively onlap the hanging wall block of the basins indicating that both the basins and their depositional surface areas were growing in size through time based on these constraints we have constructed a quantitative model for the stratigraphic evolution of extensional basins with the simplifying assumptions of constant volume input of sediments and water per unit time as well as a uniform subsidence rate and a fixed outlet level the model predicts 1 a transition from fluvial to lacustrine deposition 2 systematically decreasing accumulation rates in lacustrine strata and 3 a rapid increase in lake depth after the onset of lacustrine deposition followed by a systematic decrease when parameterized for the early mesozoic basins of eastern north america the model s predictions match trends observed in late triassic age rocks significant deviations from the model s predictions occur in early jurassic age strata in which markedly higher accumulation rates and greater lake depths point to an increased extension rate that led to increased asymmetry in these half graben the model makes it possible to extract from the sedimentary record those events in the history of an extensional basin that are due solely to the filling of a basin growing in size through time and those that are due to changes in tectonics climate or sediment and water budgets

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

controls of oil family distribution and composition in nonmarine petroleum systems a case study from inner mongolia erlian basin northern china abstract the erlian basin is a continental rift basin located in inner mongolia northern china it is a typical representative of cretaceous northeast asian rift system which includes many small petroliferous basins in mongolia republic and northern china although lower cretaceous source rocks are understood to be most important in the erlian petroleum systems the precise identification of these source rock intervals and their determination on oil families distribution and composition are poorly understood in this tectonically complicated nonmarine basin new bulk data have been gathered from source rock intervals oil sands and crude oil samples in eight main oil producing subbasins geochemical analyses indicate that lower cretaceous aershan formation k1ba and tengger 1 formation k1bt1 are two main source intervals in the erlian basin and their source rock facies vary from profundal lacustrine to marginal lacustrine according to biomarker and trace elements calibration the profundal lacustrine facies is characterised by brackish water and anoxic environment which is similar to their correlative oils family 1 oils the marginal lacustrine facies is characterised by freshwater and suboxic environment which sourced the most common family 2 oils meanwhile different maturation processes exercise the second control on oil groups and their compositions the profundal lacustrine source rocks characterised by their sulphur rich kerogens lead to two oil groups group 1 and group 2 oils whose maturity range from low to normal while the marginal lacustrine source rock only lead to normal maturity oils at last biodegradation affected the composition of a certain oils and formed group 4 heavy oils in addition short migration distance in small subbasins made the contamination or fractionation less notable in the erlian basin

characteristics and origin of lacustrine source rocks in the lower cretaceous erlian basin northern china abstract four lacustrine rift sub basins in the erlian basin one of the most petroliferous basins in northern china have been analyzed to characterize source rock potential to reveal environmental and ecological changes and to construct depositional models for lacustrine source rocks in different tectonic settings the first member of the tengger formation in these sub basins is characterized by a wide range of total organic carbon contents hydrogen indices and kerogen types these sub basins have dissimilar boundary fault activities and sedimentation rates the differences in border fault activity and sedimentation rate are consistent with changes in productivity redox conditions and clastic influx input the carbon isotopic composition of carbonates in mudstone samples suggests that the productivity of aer and wuli sub basins is higher than that of naoer and saihan sub basins differences in tectonic conditions also result in different redox conditions of depositional environments total reduced sulfur and pr ph ratios show an oxidizing environment in aer and wuli sub basins while a reducing environment was present in naoer and saihan sub basins with more active boundary faults and higher sedimentation rates the terrigenous organic matter inputs to aer and wuli sub basins are higher than naoer and saihan sub basins this can be confirmed by relative steranes abundances different tectonic conditions existed in different sub basins which lead to variations in ecological communities in the lake systems these environmental and ecological changes enable us to construct models for the deposition of lacustrine source rocks under different tectonic backgrounds during source rock deposition in aer and wuli sub basins the high influx of clastic sediments carrying high terrigenous organic matter and dissolved inorganic carbon and nitrate into the lake might have sustained high productivity and oxidizing conditions due to high influx of clastic sediments and unstable water column stratification the depositional environment of aer and wuli sub basins are oxidizing in a high productivity lake there is still significant organic matter accumulation in oxidized environment after oxidative degradation the high productivity is the dominant factor controlling the formation of organic rich sediments and the deposition of source rocks in aer and wuli sub basins in naoer and saihan sub basins the depositional environment during source rock formation is reducing due to low influx of clastic sediments with low productivity in low productivity lakes such as naoer and saihan sub basins which are low productivity lakes redox condition substitutes for productivity and becomes the predominant factor controlling the formation of lacustrine source rocks it is evident that the synergetic evolution of depositional environments and organisms in a lake system accounts for the formation of source rocks with different characteristics the proposed models may have important implications for source rock prediction in other lacustrine rift basins

paleocurrent reconstruction of the deep pacific inflow during the middle miocene reflections of east antarctic ice sheet growth today the deep western boundary current dwbc east of new zealand is the most important route for deep water entering the pacific ocean large scale changes in deep water circulation patterns are thought to have been associated with the development of the east antarctic ice sheet eais close to the main source of bottom water for the dwbc here we reconstruct the changing speed of the southwest pacific dwbc during the middle miocene from 15 5 12 5 ma a period of significant global ice accumulation associated with eais growth sortable silt mean grain sizes from ocean drilling program site 1123 reveal variability in the speed of the pacific inflow on the timescale of the 41 kyr orbital obliquity cycle similar orbital period flow changes have recently been demonstrated for the pleistocene epoch collectively these observations suggest that a strong coupling between changes in the speed of the deep pacific inflow and high latitude climate forcing may have been a persistent feature of the global thermohaline circulation system for at least the past 15 myr furthermore long term changes in flow speed suggest an intensification of the dwbc under an inferred increase in southern component water production this occurred at the same time as decreasing tethyan outflow and major eais growth between 15 5 and 13 5 ma these results provide evidence that a major component of the deep thermohaline circulation was associated with the middle miocene growth of the eais and support the view that this time interval represents an important step in the development of the neogene icehouse climate

productivity and paleoclimatic controls on source rock character in the aman trough north central sumatra indonesia abstract the central sumatra basin csb is a prolific oil producing basin in indonesia where a single source rock has been identified the brown shale formation of the paleogene pematang group despite the presence of a single source rock the oils in the central sumatra basin and particularly in the aman trough exhibit variable molecular and isotopic compositions previously described as reflective of lateral facies variations within the source rock analyses by gas chromatography gc gas chromatography mass spectrometry gc ms and gas chromatography isotope ratio mass spectrometry gc irms of 15 oil samples from the aman trough allowed identification of vertical and lateral facies changes within the brown shale formation as well as paleoproductivity and paleoclimatic changes that took place during deposition of this prolific source rock the molecular and isotopic data indicate the source rock for these oils was deposited in a fresh to brackish water stratified lake in which co 2 limiting conditions prevailed compound specific isotopic data also indicate that changes in paleoclimatic conditions possibly associated with the eocene oligocene paleoclimatic transition had an effect on the geochemical composition of the source rock in the aman trough

shallow water evaporitic environments and their source rock potential in the major evaporitic environments on the world s surface today most organic matter accumulates in shallow subaqueous to seasonally subaerially exposed algal mat sediments given the present depositional setting and the most likely flushing by oxidizing water it is unlikely this organic matter could be preserved to form source rocks however if we place such evaporite deposition into a geologic context then there are times in the past when source rocks could have formed in shallow water settings such settings were characterized by hydrological conditions which allowed the retention of hypersaline anoxic pore waters to depths where the organic material was buried deeply enough to generate hydrocarbons when deep basin shallow water evaporite successions were laid down in basins such as the mediterranean in the late miocene the michigan basin in the silurian and other large saline giants then conditions were right for the formation of source rocks within shallow water and salt flat evaporitic environments the evaporites in these saline giants were deposited under conditions of relatively shallow water 50 m the basin never appears to have dried out but water levels changed quickly nearly equal 10 000 years from shallow to deep continual water saturation coupled with saline pore fluids prevented the inflow of fresh oxidizing groundwater into the basin center of shallow water organic rich evaporites immature hydrocarbons derived from such rocks today drip from the 5 5 m y evaporites of sicily in active salt and sulfur mines organic rich sediments could also be preserved to generate hydrocarbons in rapidly subsiding rift basins in such basins rapid burial has prevented the entrance of fresher oxygenated waters and the associated degradation and destruction of the organic matter the early continental riff stage generates the source rocks the ephemeral streams wadis and dune fields generate the reservoirs and the subsequent evaporite stage seals the reservoir

the gulf of suez northern red sea neogene rift a quantitive basin analysis abstract subsidence analysis backstripping was carried out on a series of wells from the gulf of suez and northern red sea region of egypt in order to examine the interplay between tectonic events basin subsidence sedimentation and sea level changes in a young developing ocean basin and continental margin using constraints on chronostratigraphy and paleodepth from various sources combined with stratigraphic and structural information from industry wells and other geophysical sources it has been possible to compile the data necessary to perform geohistory analyses throughout the region major subsidence due to crustal thinning began 25 ma with sedimentation initially occurring in isolated sub basins these earliest sediments record the transition from continental to marine depositional environments subsequently during early and middle miocene times subsidence was rapid and uniform along and across the entire rift basin open marine sedimentation occurred across all structural regimes the mid clysmic tectonic event 16 5 ma resulted in structural rearrangement of the rift basin and uplift of the rift shoulders rapid subsidence continued as global sea level fell producing a series of prograding siliciclastic fan deltas at the rift margins at 15 5 ma opening of the suez rift was terminated tectonic subsidence decreased dramatically in the southern rift and ceased entirely in the northern rift tensional plate motion probably was transferred from the gulf of suez to sinistral strike slip movement on the dead sea transform at this time the quiescence in subsidence combined with a lowered global sea level resulted in the deposition of a thick up to 4 km series of evaporites within the central trough of the rift from the middle to latest miocene the accumulation of such a thick sequence of sediments during a phase of decreased tectonic subsidence is interpreted as a filling in of the rift topography which developed during the earlier period of rapid subsidence and rift shoulder uplift and continued compaction a rapid global sea level rise concomitant with a subsequent pulse of increased tectonic activity in the latest miocene earliest pliocene returned the rift to dominantly marine conditions

hydrocarbon potential of the meso cenozoic turkana depression northern kenya ii source rocks quality maturation depositional environments and structural control abstract source rocks identified by drilling loperot 1 well in the lokichar basin of the northern kenya rift were evaluated for source potential using rockeval pyrolysis and palynofacies analysis results indicate moderate to good oil source potential from algal prone source rocks the original sediments accumulated in a large freshwater lake similar to some modern rift lakes in the loperot 1 well the section with the highest source potential lies within the lokhone shale member up to 17 total organic carbon toc average 2 4 toc from well log calculation the other potential source interval is the deeper loperot shale member which is highly mature to post mature r 0 1 1 average 1 2 toc from well log calculation the poorer quality of the lower shale interval while partly due to maturity is thought to be largely a result of the basin evolution the loperot shale member was restricted to the lokichar basin while the lokhone shale member was deposited during a time when displacement on the main boundary fault system was greater and adjacent basins had been established consequently lacustrine conditions appear to have extended into adjacent basins thus allowing the accumulation of relatively higher concentrations of algal material in sediment starved parts of the basin