modelling of hydrocarbon generation in the cenozoic song hong basin vietnam a highly prospective basin abstract the cenozoic song hong basin situated on the northern part of the vietnamese shelf has been only sporadically explored for hydrocarbons a review of the results of the exploration efforts so far shows that the distribution of potential source rocks and their time of hydrocarbon generation are the critical risks for finding commercial amounts of hydrocarbons in the song hong basin including the hanoi trough the rocks most likely to have source potential are 1 oil prone eocene lower oligocene lacustrine mudstones and coals 2 oil and gas prone middle miocene coal beds 3 gas prone upper oligocene lower miocene coals and 4 gas and oil prone miocene marine mudstones to assess the time of hydrocarbon generation from these units relative to the formation of traps the generation history was modelled at 32 well and pseudo well locations the modelling demonstrates that the two first mentioned source rock units are especially important in the northern and northeastern part of the basin and along its western margin traps may have been charged by eocene lower oligocene source rocks in the hanoi trough the excellent middle miocene coal beds have probably generated hydrocarbons within the last few million years thus the huge and still underexplored song hong basin provides attractive areas for further exploration

evolution of the early proterozoic colorado province constraints from u pb geochronology the colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the archean wyoming craton during the early proterozoic between about 1790 and 1660 ma correspondence in ages between metamorphism deformation and plutonism association of volcanic rocks with comagmatic calc alkalic plutons and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton

geodynamic mechanism and classification of basins in the earth system abstract the sedimentary basin is a basic geological unit in the earth system its formation and evolution are controlled mainly by plate tectonics and geodynamics which have produced a variety of basin types the development and evolution of various basins depend not only on the tectonic effect of lithospheric plate but also on sublithospheric and earth surface actions basin classification schemes based on the theories of platform geosyncline or plate tectonics could neither include all basin types nor reflect the complex processes of basin formation and evolution from the viewpoint of earth system we proposed a new basin classification scheme based on geodynamic mechanism that is lithospheric plate sublithospheric and earth surface actions in our geodynamic classification we employed a series of geodynamic settings of basin formation and evolution including the lithospheric plate sublithospheric and earth surface environments the dynamic regimes of basin formation and evolution such as tectonic setting extension compression and shearing the thermal processes and gravity effects the various natures of basin basement and crustal types continental oceanic or transitional crust and the history of basin sedimentary filling and tectonic paleogeography successively as the controlling factors for basin classification this new geodynamic classification scheme highlights both the role of the lithosphere and the effects of the sublithospheric and the earth surface actions on controlling the basin formation and evolution the basic dynamic types of basin classification include the prototype basin forming under a single tectonic thermal or gravity regime and the superimposed basin as well as residual basin forming under multistage thermal and or tectonic regimes thus this classification scheme could provide an evolving understanding of lithosphere crust mantle interaction and geodynamic processes as well as of hydrocarbon accumulation and metallogenic mechanisms in sedimentary basins and would help in the synthetic exploration of various resources

life in extreme environments hydrothermal vents eruption of volcanic rocks at the midocean ridges is the major mechanism by which heat is lost from the interior of the earth approximately one third of the heat is removed from the spreading centers by convective circulation of seawater 1 the magnitude of this heat loss requires that the entire volume of the oceans circulates through the midocean ridges in approximately 10 million years seawater interaction with volcanic rocks at near 400 c results in substantial chemical flux and makes an important contribution to buffering the composition of some elements in seawater cations from seawater mg 2 ca 2 and na form hydroxyl bearing alteration minerals in the volcanic rocks releasing hydrogen ion to solution the hot acidic altered seawater releases metals fe mn zn and cu and reduced sulfur h2s from the volcanic rock these are transported by hydrothermal solutions to the seafloor and form metallic mineral deposits

deep shallow coupling response of the cenozoic bohai bay basin to plate interactions around the eurasian plate abstract the bohai bay basin bbb is a meso cenozoic intracontinental petroliferous basin where four pre existing continental scale fault zones intersect in east china the bbb has experienced complex deformational and evolutionary history that records significant information about how the lithosphere there has responded to the coeval plate interactions around the eurasian plate and or their resulting deep crust mantle processes in the cenozoic now both deep and shallow deformational and evolutionary processes of the bbb have been well studied but many geodynamic mechanisms previously proposed for those processes are too general to explain them well based on various data and principles we first summarized both deep and shallow responses and processes of the bbb in the cenozoic and then matched reasonable geodynamic mechanisms for each of them the results show as follows the cenozoic differential reactivation of pre existing continental scale weak zones controlled the coeval deep shallow differential coupling evolution of the bbb the paleogene extension in the bbb was mainly caused by crust mantle processes triggered by the stagnant izanagi slab the cenozoic strike slip faulting was influenced by various plate interactions in which the oblique subduction and anticlockwise rotation of young pacific plate resulted in the complex pre oligocene strike slip history of the tan lu fault zone to the south of the zhang peng fault zone while the india eurasia collision has caused the intense and synchronous strike slip faulting of various faults there since 35 ma which was then weakened by the oblique subduction and clockwise rotation of young philippine sea plate in the neogene the compression resulting from both the india eurasia collision and the philippine sea plate s subduction contributed to the latest paleogene tectonic inversion in the bbb the arrival of mantle flow triggered by both the india eurasia collision and the stagnant slab of the pacific plate resulted in the tectonic reactivation there in the late neogene

geologic characteristics and oil exploration of small depressions in eastern china the geology and exploration histories of the biyang damintun changwei and baise depressions each covering an area less than 1000 km 2 reveal four major characteristics of hydrocarbon exploration in small depressions 1 thick sediments and source rocks 2 graben or wedge shaped structures 3 predominantly nonanticline reservoirs and 4 high wax crude oil in view of the discovery of shuanghe and other significant fields in these small depressions the results of such studies may have important implications for oil explorations in other small depressions

geochemistry and correlation of crude oils from reservoirs and source rocks in southern biyang sag nanxiang basin china abstract the paleogene hetaoyuan formation a sequence of gray to dark calcareous mudstone and dolomitic mudstones is the main source rock and reservoir in the biyang sag nanxiang basin eastern china organic geochemical analysis study performed on 57 source rock samples from the hetaoyuan formation demonstrates poor to abundant organic matter content ranging from 0 24 5 4 wt toc content characterized by oil prone type ii kerogen eleven crude oil samples from the hetaoyuan formation in southern biyang sag were analyzed using bulk properties and molecular methods to determine their maturity and origin the results suggest that the crude oils exhibit different maturity and source organic matter three groups of oils were identified by integrated analysis of gas chromatography gas chromatography mass spectrometry stable carbon isotope physical properties and gross composition of crude oil group a oils defined as low mature oil are characterized by high density viscosity sulfur content and low saturated hydrocarbon content this group of oils has relatively high values of ph n c 18 c 24 c 26 and gammacerane index indicating that the original organic matter had more contribution from terrigenous plants and was deposited in saline and stratified water columns under anoxic conditions group b oils defined as normal maturity oil are characterized by moderate density viscosity sulfur content and moderate saturated hydrocarbon content this group of oils has relatively low values of ph n c 18 c 24 c 26 and gammacerane index and displays a slight predominance of c 29 steranes indicating that the original organic matter had more contribution from marine algae and was deposited under sub oxic to anoxic conditions with low salinity group c oils defined as light oil are characterized by low density viscosity and abundant saturated hydrocarbons representative source rock samples were selected for further correlation the results of oil source rock correlation suggest that crude oils from the eh 2 member of hetaoyuan formation are mixture derived from source rocks in the eh 2 member and upper layer of the eh 3 member while oils from the upper and lower layer of the eh 3 member correlate with source rocks in the upper and lower layers of the eh 3 member respectively

lake bogoria kenya hot and warm springs geysers and holocene stromatolites abstract i carried out the first regional geological survey of the central gregory rift valley in kenya in 1958 60 and review here the numerous subsequent specialised studies focused on the unique endoreic lake bogoria formerly hannington studies which embraced the sedimentology of the holocene sediments around the lake shores the hot spring and geyser activities and the coring of the sediments beneath the lake i focus on the occurrences of stromatolites in a hydrothermal environment both in two closely spaced late holocene 4500 yr bp generations at the lake margin associated with algae and cyanobacteria which represent a final more humid climatic phase after the several interglacial more humid phases also represented by stromatolite occurrences in other rift valley lakes and also at present being formed at the edge of the now highly saline lake in the very hot springs in association with thermophilic bacteria and with silica i briefly mention the older occurrences in lake magadi to the south which are quite different and form three generations and also present day occurrences of stromatolites in a flood plain environment unlike the present day environment at lake bogoria other stromatolite occurrences are mentioned around lake turkana and the former lake in the suguta river valley to the north i suggest that the hot waterfall at kapedo at the head of the suguta river and the central island of ol kokwe with hot springs amidst the fresh water lake baringo could well be investigated for stromatolite occurrences lake bogoria an empty wilderness occupied only by flamingos when i mapped it is now more accessible and provides a unique open air laboratory for such researches but like all the rift valley lakes is unique sui generis results of detailed investigations of the type reviewed here can only be applied to other occurrences of stromatolites elsewhere in the rift system or beyond the rift system with reservation

small scale petroliferous basins in china characteristics and hydrocarbon occurrence a large number of small scale lacustrine sedimentary basins are widely distributed across china studies of such basins have been limited these basins have complex characteristics and thus exhibit significant differences in terms of their hydrocarbon potential at present among the known 348 small scale basins with areas less than 20 000 km2 200 m m y 656 ft m y during deposition of the source rocks within underfilled and balanced filled lakes

chronologic and isotopic framework for early proterozoic crustal evolution in the eastern mojave desert region se california the early proterozoic geologic evolution of the eastern mojave desert region as defined by characteristics of its supracrustal rocks granitoids metamorphism structural history and pb and nd isotopic signature contrasts sharply with other proterozoic provinces of the southwestern united states the oldest supracrustal rocks of the mojave desert region contain zircons over 2 0 ga corroborating nd isotopic evidence for a much older crust here than elsewhere in the southwestern united states granitoids widely emplaced within these supracrustal rocks range from 1 76 to 1 64 ga the earlier plutons and surrounding supracrustal rocks were metamorphosed to granulite and high amphibolite facies throughout the province at about 1705 ma in a migmatite producing event that we term informally the ivanpah orogeny subsequent granitoids emplaced from 1 69 to 1 67 ga were voluminous along a north trending belt in the middle of the mojave province younger plutons were emplaced at about 1 66 ga in several places and at about 1 64 ga along the extreme southern part of the province commonalities between the proterozoic evolutions of the mojave and arizona crustal provinces do not conclusively establish the time that the provinces were juxtaposed the data only suggest that the juxtaposition occurred between about 1 76 and 1 64 ga

basin evolution thermal history and hydrocarbon potential of the st george basin bering sea alaska a comparative study using one and two dimensional models abstract the hydrocarbon potential of the st george basin alaska was evaluated using both one and two dimensional basin models whereas the st george basin was probably cooler in the past the sensitivity of the results to uncertainties in lithology depth to basement depth to formation tops and palaeo water depth suggests that a range of heat flow histories is the appropriate way to model the hydrocarbon potential on the margins of the basin the models predict that small amounts of hydrocarbons have been generated from eocene and early oligocene sediments starting at about 37 0 million years before present mybp high paleeo heat flux or at 5 0 mybp cool palaeo heat flux in the deepest part of the st george basin the models predict that the pre pliocene sediments have generated hydrocarbons from at least 24 0 mybp at both locations the different heat flow scenarios affect the predicted timing of hydrocarbon generation but not the age of the sediments which generate hydrocarbons nor the amount of hydrocarbons accumulated a regional seismic line which intersects the cost 1 well was depth converted and drilled by regularly spaced pseudo wells from which a cross section was constructed modelling the cross section using a two dimensional model predicted the same results as those obtained from the one dimensional model indicating that a one dimensional approach was sufficient to approximate this section both models predict a negligible accumulation of hydrocarbons at cost 1 i e cost 1 is a dry well a synthesis of modelled results and qualitative information indicates that the st george basin has a very low potential for hydrocarbon accumulation on the basin margin the limited source quality and lack of seals result in a low accumulation of any generated hydrocarbons hydrocarbons generated elsewhere in the basin may be lost vertically or trapped within the graben in difficult to identify traps overall the st george basin is an unattractive exploration target

destruction of the eastern north china craton in a backarc setting evidence from crustal deformation kinematics abstract the eastern north china craton ncc west of the pacific ocean has a long history of rifting from early cretaceous to the end of paleogene when the craton experienced destruction and significant thinning of the lithosphere the cause and geotectonic environment for this destruction remain controversial we use a set of field observation including basin patterns fault geometry and kinematics fault plane slip data dike distribution and fabric data from metamorphic core complexes to determine the kinematic evolution of the deformation in the eastern ncc during this period we show that the principal extension direction in the eastern ncc evolved from wnw ese in the earliest middle early cretaceous via nw se in the latest early cretaceous to nearly n s in the late cretaceous paleogene the movement history from late mesozoic to early cenozoic of the oceanic plates in the pacific ocean with respect to the eastern ncc is available from previous studies the izanagi plate first subducted nearly orthogonally wnw wards during earliest middle early cretaceous time and then moved obliquely nnw wards in the latest early cretaceous while the relaying pacific plate moved generally northwards from the late cretaceous to paleogene both the movement direction of oceanic plates and the principal extension direction of the continental deformation rotated clockwise we suggest that such a correlation can be explained by the eastern ncc being in a backarc setting in the cretaceous paleogene period the results support the backarc extensional model for the destruction and significant lithospheric thinning of the eastern ncc

tectonic evolution of the qinghai tibet plateau abstract the qinghai tibet plateau composed of several continental slivers within the eastern tethyan domain is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on earth this plateau is generally inferred to be a collage of several continental blocks that rifted from gondwanaland and subsequently accreted to the asian continent however recent recognition of over twenty ophiolite melange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision based on 177 recently finished 1 250 000 scale geological maps and related studies we summarize the main tectonic context of the qinghai tibet plateau and propose a new integrated model to account for the new findings the complex orogen of the immense qinghai tibet plateau consisting of multiple island arc basin systems that developed at different stages while surrounded by the north china yangtze tarim and indian plates is emphasized the entire orogen surrounded by suture zones that mark the locations of oceanic closure is investigated by examining i the first order tectonic units and ophiolitic melanges including arc arc continent collision zones and ii their internally enclosed blocks as the second order tectonic units therefore the qinghai tibet plateau is divided into three major orogenic systems namely from northeast to southwest the early paleozoic qinling qilianshan kunlunshan qin qi kun the late paleozoic triassic qiangtang sanjiang and the late paleozoic to cenozoic gangdese himalaya orogenic systems which are separated by the kangxiwa muzitagh maqin mianxian and the bangong shuanghu changning menglian sutures respectively we propose that the formation and evolution of the qinghai tibet plateau to have been intrinsically related to those of the eastern tethys recorded by the longmu co shuanghu ophiolite melange zone the southern qiangtang paleozoic accretionary arc basin system the bangong nujiang suture zone and their associated composite island arc basin systems the present day bangong shuanghu changning menglian suture system marks the final closure of the tethyan ocean the yarlung zangbo ocean opened as a back arc basin in response to the southward subduction of the tethyan ocean lithosphere in the middle triassic and closed as a result of the india asia collision at the end of cretaceous followed by the northward indention of the indian plate that resulted in significant intra continental deformation and plateau uplift in the cenozoic

cretaceous early tertiary paleolatitude migration in the sanshui basin of south china and the evolution of the south china sea abstract the evolutionary model of the south china sea scs is discussed based on data from the sanshui basin which indicates the paleolatitude migration of south china the results show that south china played a prevailing role in the evolution of the scs which has been basically formed by the southward migration and later reversal of the south china continent in the course of extension faulting and breaking up in the south of south china during the late cretaceous to miocene the microcontinent fragments which are found scattered in the scs today were separated there when south china migrated back the philippine arc migrated northwards with philippine sea plate until the pliocene to form the east boundary of the scs the indochina block only played a transitional boundary role

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

u th pb rb sr and ar ar mineral and whole rock isotopic systematics in a metamorphosed granitic terrane southeastern california mesozoic structural domes are developed in an older proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the halloran hills southeastern california isotopic analyses of whole rocks and mineral separates from these rocks by u th pb rb sr and ar ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history individual mineral isotopic systems have variably equilibrated with each other in response to mesozoic regional metamorphism and locally to later heating during mesozoic batholith emplacement discordant u th pb zircon data indicate that the granitic core rocks are 1 710 ma and that one dioritic phase may be slightly older rb sr whole rock model dates scatter about 1 700 ma rb sr amphibole whole rock and u th pb amphibole dates are also proterozoic potassium feldspars retain a 207 pb 206 pb signature of their proterozoic age ar ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1 450 ma to 1 100 ma and the dates become younger toward the structurally deeper core of the dome all remaining isotopic determinations yield mesozoic or younger dates for mineral whole rock systems rb sr whole rock apatite feldspar biotite analyses show nonequilibration of strontium isotopes with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 ma no single metamorphic age is indicated by the rb sr data rb sr whole rock biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water u pb sphene and apatite analyses from rocks that yield 1 710 ma zircon dates are nearly concordant at 140 ma an amphibole from the structurally deepest rocks of the main dome that yield 140 to 150 ma u pb sphene dates has an ar ar plateau date of 144 ma the u pb sphene and ar ar amphibole analyses are believed to be the best age estimate for the end of the highest temperature phase of regional metamorphism th pb sphene and apatite dates and ar ar biotite dates cluster at 90 5 ma as a consequence of regional cooling during late cretaceous time following extensive mesozoic plutonism in the region at 97 to 90 ma we interpret the discordant mineral date patterns to have resulted from metamorphism of 1 700 ma plutonic rocks during the jurassic 140 50 ma and subsequent uplift and cooling to 200 c at about 90 ma on the basis of this study the isotope dating systems ranked in decreasing order of resistance to resetting are u th pb zircon concordia intercept rb sr whole rock rb sr amphibole u th pb amphibole pb pb whole rock ar ar amphibole rb sr sphene u pb sphene and apatite rb sr plagioclase potassium feldspar apatite th pb sphene and apatite ar ar biotite u pb feldspars rb sr biotite

low frequency reflection from a nonmeridional eastern ocean boundary and the use of coastal sea level to monitor eastern pacific equatorial kelvin waves abstract analytical theory and wind sea level and atmospheric pressure data were used to examine low frequency dynamics near the equatorial eastern pacific ocean boundary the analytical technique involves linking low frequency solutions near a nonmeridional boundary with the deep ocean equatorial kelvin wave for both unforced and wind forced flows using an equatorial wave orthogonality condition the following results were obtained mathematical and physical arguments show that nonmeridional boundaries should be less reflective than meridional ones and that the poleward coastal kelvin wave energy flux should be greater the more the boundary tilts from north to south the eastern pacific ocean boundary is more nonmeridional in the northern hemisphere and as a consequence calculations for the intraseasonal semiannual annual and interannual frequencies an indicated that poleward coastally trapped energy flux is greater in the northern hemisphere than in the southern hemisphere for most frequencies

neogene quaternary postrift tectonic reactivation of the bohai bay basin eastern china bohai bay basin located in eastern china is considered a cenozoic rifted basin the basin is atypical in terms of its neogene quaternary postrift subsidence history in that it experienced intensive tectonic reactivation rather than the relative tectonic quiescence experienced during this stage by most rift basins this neogene quaternary tectonic reactivation arose principally in response to two tectonic events 1 activity on a dense array of shallow faults and 2 accelerated tectonic subsidence that occurred during the postrift stage these two events were neither strictly temporally nor spatially equivalent the dense array of shallow faults form a northwest southeast trending belt in the central part of the basin with displacement induced by the reactivation of older northeast and northwest trending basement faults and an associated substantial component of strike slip displacement occurring after 5 3 ma the intensive reactivation of these faults contributed to the atypically accelerated rate of postrift tectonic subsidence of the basin that commenced ca however this was not the sole cause of this accelerated tectonic subsidence a combination of geological activity deep within the crust led to the buildup of intraplate stresses and this combined with ongoing thermal subsidence acted as an additional contributory factor that drove unusually high rates of subsidence for this basin this episode of accelerated postrift tectonic reactivation resulted in conditions favorable for hydrocarbon accumulation

lead isotopic compositions of diverse igneous rocks and ore deposits from southwestern new mexico and their implications for early proterozoic crustal evolution in the western united states u pb zircon measurements from some of the basement rocks in southwestern new mexico have ages that range from 1 650 to 1 450 m y the largest ore deposits occur near silver city in the southwest part of the region studied they are associated with laramide alkali to calc alkaline plutons and their lead isotopic compositions are the least radiogenic in the region this lead exhibits lower crustal upper mantle characteristics and for the major producing porphyry copper deposits the 206 pb 204 pb ratios are less than 18 0 lead from more silicic mid tertiary volcanic rocks and associated ore deposits has somewhat higher 208 pb 204 pb ratios that reflect a greater crustal involvement in their origin in a regional trend to the northeast ore lead becomes more radiogenic and at hansonburg 200 km from silver city lead in the mississippi type deposits is clearly derived from upper crustal sources on the 207 pb 204 pb 206 pb 204 pb plot data from all of the rocks and ores that we have analyzed form an array that lies below the average orogene curve of doe and zartman 1979 a common source is implied for the lead the isotopic composition of which was similar to that found in 1 750 m y old stratiform deposits as far apart as pecos new mexico and jerome arizona such a composition indicates that over a large region of the southwestern united states continental crust developed between 1 750 and 1 450 m y ago possibly in an island arc environment basement rocks that are 1 750 m y old extend northward through colorado to utah galena data obtained in previous studies show that the fraction of older sialic lead in those rocks increases toward the archean craton in wyoming the crust apparently developed southward from wyoming in stages at 2 400 m y ago or before 2 100 m y ago and 1 750 m y ago with incorporation of older sialic material in each stage as far south as milford utah

relationship between sedimentation rate and total organic carbon content in ancient marine sediments an analysis of deep sea drilling project dsdp cores indicates quantitative relations between sedimentation rate and total organic carbon content in fine grained ancient marine sediments of jurassic cretaceous and cenozoic ages these relations are largely independent of geographic setting geologic age and differential compaction but are strongly affected by lithology for any given sedimentation rate the total organic carbon content toc by weight percent increases from calcareous sediments to calcareous siliceous sediments to siliceous sediments to black shale the total organic carbon content also varies with the sedimentation rate for each lithology starting at low rates of sedimentation toc first increases with sedimentation rate owing to a more rapid passage of the sediment through the near surface zone of intense organic degradation above a critical sedimentation rate toc typically decreases with increasing sedimentation rate owing to clastic dilution of the organic input near surface organic degradation however continues to be less efficient at higher burial rates therefore the oil generation potential of oil prone organic constit ents should continue to increase with increasing sedimentation rate similar relations were also established between total organic carbon accumulation rate and grain accumulation rate and support both reduced near surface organic degradation and the clastic dilution effect by using the toc cutoff for a petroleum source rock these relations can be used to identify the presence of petroleum source rock formations in marine sedimentary basins the results of this study may have important implications for petroleum exploration in frontier regions sedimentation rate can be determined from seismic reflection records or seismic isopachs in basins where only limited well data are generally available seismic records can thus be used to identify potential petroleum source rocks and to estimate the relative oil generation potential of source rocks in various frontier sedimentary basins prior to and during the early phases of exploratory drilling

cenozoic tectonic evolution of asia a preliminary synthesis abstract asia has been a major testing ground for various competing models of continental deformation due to its relatively well understood plate boundary conditions in the cenozoic exceptional exposure of active structures and strain distribution and widespread syn collisional igneous activity as a proxy for the thermal state of the mantle and crust two cenozoic orogens dominate the continent the himalayan tibetan orogen in the east induced by the india asia collision and the turkish iranian caucasus orogen in the west induced by the arabia asia collision the development of the two orogens was accomplished by shortening in the early stage followed by strike slip faulting and extension in the late stage in the himalayan tibetan orogen shortening across two discrete thrust belts at 55 30 ma in southern and northern tibet created a large intracontinental basin the paleo qaidam basin in between subsequent crustal thickening and a possible thermal event in the mantle e g convective removal of central tibetan mantle lithosphere may have raised the elevation of this early intra plateau basin up to 2 3 km to its current height collision between india and asia also caused lateral extrusion of southeast asia between 32 ma and 17 ma the latest stage of the india asia collision was expressed by north trending rifting and the development of trench facing v shaped conjugate strike slip faults in central mongolia central tibet eastern afghanistan and southeast asia in the turkish iranian caucasus orogen early crustal thickening in the orogenic interior began at or prior to 30 20 ma this style of deformation was replaced by strike slip faulting at 15 5 ma associated with further northward penetration of arabia into asia westward extrusion of the anatolia turkey block and rapid extension across the sea of crete and sea of aegean the late stage extension in both orogens was locally related to extensional core complex development the continental margin extension of east asia was developed in two stages initially in a widely distributed zone that has an east west width of 500 800 km during 65 35 ma which was followed by localized extension and opening of back arc basins associated with the development of spreading centers at 32 17 ma e g japan sea or east korea sea bohai bay and south china sea opening of the back arc basins could be induced by 1 rapid eastward migration of the western pacific trench system or 2 oblique subduction of pacific plate beneath asia that had produced a series of en echelon right slip primary shear zones linking with back arc spreading centers oriented obliquely to the strike of the nearby trench since 15 ma the eastern margin of asia became contractional in the east west direction as indicated by the collapse of back arc basins in the western pacific and the development of fold thrust belts along the eastern continental margin coeval with the contraction is widespread east west extension in siberia north china and the tibetan plateau the above observations can be explained by a change in boundary condition along the eastern margin of asia that allowed the thickened asian continent to spread eastward causing east west extension in its trailing edge and east west compression in its leading edge in west asia continental margin extension started at about 25 20 ma in the aegean and cretan regions which was associated with a rapid southward retreat of the hellenic arc the complex evolution of cenozoic deformation in asia may be explained by a combined effect of temporal changes in plate boundary conditions thermal evolution of the upper mantle perturbed by collisional tectonics and the built up of gravitational energy through crustal thickening and thermal heating although the past research in asia has treated the india asia and arabia asia convergence as separate collisional processes their interaction may have controlled the far field cenozoic deformation in asia the most pronounced result of this interaction is the creation of a northeast trending 300 400 km wide and 1500 km long zone of northwest striking right slip faults which extends from the zagros thrust belt in the south to western mongolia in the north and links with the active tian shan and altai shan intracontinental orogens cenozoic deformation and coeval igneous activity spatially overlap with one another in the himalayan tibetan and turkish iranian caucasus orogens a large cenozoic magmatic gap exists between tibet in the south and mongolia in the north where cenozoic deformation has not been associated with any coeval igneous activity finally cenozoic igneous activity is always associated with jurassic cretaceous magmatic arcs suggesting a causal relationship between the early arc magmatism and later syn collisional magmatism

on the use of global potential field models for regional interpretation of the west and central african rift system abstract the use in regional interpretations of the earth gravity model egm08 and earth magnetic model emag2 is evaluated by comparison to ground gravity and aeromagnetic data in the central sector of the west and central african rift system wcars the comparison includes upward continuation spectral analysis and pseudogravity calculation and statistical evaluation a correlation between emag2 which contains roughly 25 km resolution aeromagnetic data in the region and near surface aeromagnetic data over wcars is only true for the very low wavelength part but a strong similarity between egm08 and ground gravity data can be confirmed interpretation of the egm08 data allows identifying and confirming the position of major structural trends and provides new information on the crustal architecture the lineaments limiting different grabens forming the logone birni basin and in the southern chad basin are identified the results presented here show the use of egm08 data for regional interpretations over cameroon and adjacent countries which can overcome the absence and sparseness of data in developing countries and remote areas

detailed structure of the top of the melt body beneath the east pacific rise at 9 40 n from waveform inversion of seismic reflection data we have applied waveform inversion to multichannel seismic reflection data collected at the east pacific rise at 9 40 n in order to determine the precise velocity structure of the magma body causing the axial magma chamber reflection our analysis supports the idea of a molten sill as previously suggested from forward modeling of seismic data from this location our inverted solution has a 30 m thick sill with a p wave seismic velocity of 2 6 km s 1 although not well constrained by the data we believe that the s wave velocity in the sill is not significantly different from 0 0 km s 1 the low p and s wave velocities in the sill imply that it contains less than 30 crystals the molten sill is underlain by a velocity gradient in which the p wave velocity increases from 2 6 to 3 5 km s 1 over a vertical distance of 50 m the shape of our velocity depth profile implies that accretion of material to the roof of the sill is minor compared to accretion to the floor the underlying velocity gradient zone may represent crystal settling under gravity we suggest that only material from the 30 m thick layer can erupt

organic matter preservation on the pakistan continental margin as revealed by biomarker geochemistry in order to assess whether the oxygen minimum zone omz in the arabian sea has an eaect on the preservation and composition of organic matter in surface sediments we investigated samples from three diaerent transects on the pakistan continental margin across the omz in addition to determining the total amount of organic carbon toc we analyzed the extractable lipids by gas chromatography combined gas chromatography mass spectrometry and compound specific stable carbon isotope measurements the extractable lipids are dominated by marine organic matter as indicated by the abundance of lipids typical of marine biota and by the bulk and molecular isotopic composition sediments from within the omz are enriched in organic carbon and in several extractable lipids i e phytol n alcohols total sterols n c35 alkane relative to stations above and below this zone other lipid concentrations such as those of total n fatty acids and total n alkanes fail to show any relation to the omz only a weak correlation of toc with mineral surface area was found in sediments deposited within the omz in contrast sediments from outside the omz do not show any relationship between toc and surface area among the extractable lipids only the n alkane concentration is highly correlated with surface area in sediments from the hab and makran transects in sediments from outside the omz the phytol and sterol concentrations are also weakly correlated with mineral surface area the depositional environment of the indus fan oaers the best conditions for an enhanced preservation of organic matter the omz together with the undisturbed sedimentation at moderate rates seems to be mainly responsible for the high toc values in this area overall the type of organic matter and its lability toward oxic degradation the mineral surface area the mineral composition and possibly the secondary productivity by sedimentary bacteria also appear to have an influence on organic matter accumulation and composition 2000 elsevier science ltd all rights reserved

evolutionary characteristics of the sags to the east of tan lu fault zone bohai bay basin china implications for hydrocarbon exploration and regional tectonic evolution abstract some cenozoic half grabens sags have been found to the east of the tan lu fault zone in the bohai bay basin east china the evolutionary history of these sags is consistent with the entire bohai bay basin besides some remarkable characteristics which had not been mentioned in previous studies of the bohai bay basin are presented and they are the keys to understand the regional tectonic evolution among them two characteristics are emphasized 1 the structural and evolutionary features of the sags were markedly changed at about 38 ma 2 the paleogene basin underwent several times of uplifts and denudations during paleogene the first characteristic shows that the normal faulting was the dominant structural deformation before 38 ma and after then the strike slip movement might initiate to influence the basin the second characteristic indicates remarkable reducing of the present day residual basin relative to the original basin and implies the episodic uplifting of the jiaoliao massif besides our estimation on the hydrocarbon exploration potential of these sags shows that part of them can sustain the formation of medium large oilfield independently while part can contribute to the formation of large oilfield

fluvial facies of the miocene ridge route formation ridge basin california abstract the upper miocene nonmarine ridge route formation consists of more than 9000 m of fluvial rocks deposited along the northeast margin of ridge basin california five major fluvial tongues of the ridge route formation cross the basin to interfinger with the violin breccia on the southwest two styles of fluvial sedimentation are inferred 1 deposition in small braided channels on fan deltas resulting from streamflow and sheet flood processes on the northeast side of the basin and 2 deposition on large meandering channel complexes in the lower parts of fan deltas and in elongate deltas which formed by point bar accretion and overbank flooding near the center of the basin these fluvial strata are composed of crossbedded and laminated sandstone interbedded with mudstone the braided channel deposits have a high sandstone mudstone ratio 10 1 and contain lenticular beds averaging 1 m thick and 20 m long these channels are small and of low relief

opaline cherts associated with sublacustrine hydrothermal springs at lake bogoria kenya rift valley an unusual group of cherts found at saline alkaline lake bogoria in the kenya rift differs from the magadi type cherts commonly associated with saline alkaline lakes the cherts are opaline rich in diatoms and formed from a siliceous probably gelatinous precursor that precipitated around submerged alkaline hot springs during a holocene phase of high lake level silica precipitation resulted from rapid drop in the temperature of the spring waters and possibly ph lithification began before subaerial exposure ancient analogous cherts are likely to be localized deposits along fault lines

identifying updip pinch out sandstone in nearshore subaqueous fans using acoustic impedance and the instantaneous phase in the liangjia area yitong basin china abstract drilling has shown that there is a large petroleum reserve in the sheling formation in the liangjia area of the yitong basin from reservoirs of the nearshore subaqueous fan facies to reduce the risk associated with exploration the updip pinch out or isolated sandstone in the nearshore subaqueous fan should be identified and described analysis of the 3d seismic reflection instantaneous phase and acoustic impedance was conducted to identify and interpret the sandstone seismic reflection instantaneous phase and acoustic impedance alone can not clearly identify the updip pinch out of sandstone in the nearshore subaqueous fan however the combination data of acoustic impedance and the instantaneous phase has better detectability it indicates the ability for identifying sandstone distribution and is a more reliable indicator of the sandstone distribution than these seismic data alone the sandstone and the updip pinch out of the sandstone in the nearshore subaqueous fan can be identified more clearly and directly by the combination data of acoustic impedance and instantaneous phase than the 3d seismic reflection data alone combined analysis of the acoustic impedance and instantaneous phase in the sedimentary system is a valid approach for identification and description of the updip pinch out sandstone or the isolated sandstone that can reduce the risk associated with exploration of lithological traps

proterozoic geochronologic and isotopic boundary in nw arizona u pb ages determined from zircon sphene and apatite in conjunction with pb isotopic analyses of alkali feldspar establish a regional geochronological framework and constrain the location of a major north trending proterozoic crustal boundary in northwestern arizona two regions west of the boundary hualapai mountains and lost basin range garnet mountain are characterized by complex u pb zircon systematics evidence for inheritance of an older zircon component 1 8 2 3 ga and elevated sup 207 pb sup 204 pb from feldspars compared to the east although the discordia patterns are complex supracrustal rocks are interpreted to be ca 1 73 ga and are intruded by plutonic rocks ca 1 70 ga deformation is younger than ca 1 70 ga foliated granites and older than the 1 682 plus minus 4 ma garnet mt monzogranite the rocks in one area east of the boundary cottonwood cliffs are characterized by relatively simple u pb zircon systematics no evidence for inheritance of any older component and feldspar sup 207 sup 204 pb near model mantle values supracrustal rocks are older than 1 73 ga as they are intruded by a 1 730 plus minus 9 ma foliated granodiorite timing of deformation is constrained by the late syn kinematic valentine granite dated at 1 713 plus minus 12 ma mineral ages indicate thatmore the rocks on either side of the boundary had different cooling histories and inferentially different uplift histories west of the boundary the cooling history is inferred from minerals separated from an amphibolite metamorphic zircon is 1 687 13 minus 8 ma sphene is 1 660 plus minus 5 ma hornblende has a sup 40 ar sup 39 ar age of 1 552 plus minus 5 ma and apatite has a u pb age of 1 520 plus minus 45 ma east of the boundary an amphibolite has sphene with an age of 1 670 plus minus 11 ma and apatite with an age of 1 630 plus minus 8 ma less

cenozoic deformation in western yunnan china myanmar border in the present paper we describe ductile and brittle deformation styles in western yunnan and ne myanmar using field data and landsat 7 imagery we show that this complex area located at the northern termination of sunda plate three rivers area was wedged during the tertiary between the left lateral ailao shan chong shan metamorphic belts to the east and the right lateral shan scarp gaoligong metamorphic belt in west this triangular region therefore underwent the effects of these continental size ductile strike slip faults separating major blocks with a dominant ew to ene compression since the late miocene date of the reversal of motion along the rrf the incipient eastward motion of the sunda block and the persisting right lateral motion along its western boundary sagaing fault created n s compression and e w to wnw extension underlined by left lateral transtension along the wanding nanting fault zones at the same time the diangcan shan situated along strike the ailao shan metamorphic belt was slightly impinged by the blocks extruded from the syntaxis and exhumed again from the early pliocene in accordance with this late and still active state of stress q 2004 elsevier ltd all rights reserved

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

outcrop architecture of a fluvio lacustrine succession upper triassic yanchang formation ordos basin china abstract although several studies have considered the sedimentary facies of the upper triassic yanchang formation in terms of their hydrocarbon potential in the petroliferous ordos basin such studies have mostly interpreted subsurface data and few have systematically examined the detailed outcrop based anatomy of this succession this study characterizes a series of well exposed outcrop sections along an 80 km long e w oriented series of road cuttings in the southeast ordos basin to reconstruct the sedimentary architecture of a fluvio lacustrine succession and determine the principal controls that governed the style of accumulation and preservation the yanchang formation is divided into 10 depositional units chang 10 base to chang 1 top that record the sedimentary evolution of a series of large scale fluvial systems that constructed delta front and pro delta bodies as they entered a large interior draining lake detailed analysis of the internal architectures of the delta front bodies records the mechanism of growth of a large scale shallow water delta succession lacustrine subaqueous distributary channels are composed of 3 15 m thick sandstone fills such channelized bodies are highly elongate and many can be traced several kilometres such bodies can be shown to be commonly associated with thin bedded mouth bar and subaqueous interdistributary bay deposition the progradational architecture of the delta front elements is difficult to recognize on outcrop scale deposits of the chang 10 to 1 units record 4 major lacustrine transgressive regressive cycles during the late triassic transgressive events are signified by the widespread occurrence of 10 20 m thick oil shale prone intervals regressive events are signified by the progradational stacking towards the basin centre of sandstone prone facies associations mainly accumulated in fluvial channel delta plain and upper delta front environments the uppermost chang 1 interval records late stage filling of the lacustrine basin the transgressive regressive cycles equate to sequences that can be correlated across the region development of which was controlled by a combination of changes in the rate of tectonic subsidence the rate of sediment delivery from the basin margin hinterland and climate factors that were themselves influenced by intracontinental growth of the neighbouring qinling orogen

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

gravity study of the central african rift system a model of continental disruption 1 the ngaoundere and abu gabra rifts abstract a regional compilation of published and unpublished gravity data for central africa is presented and reveals the presence of a major rift system called here the central african rift system it is proposed that the junction area between the ngaoundere and abu gabra rift arms in western sudan forms an incipient intraplate triple junction with the as yet unfractured but domally uplifted and volcanically active darfur swell it is only the darfur swell that shows any similarities to the uplift and rift history of east africa the other two rifts arms are considered to be structurally similar to the early stages of passive margin development and thus reflect more closely the initial processes of continental fragmentation than the structures associated with rifting in east africa

structurally controlled hydrothermal dolomite reservoir facies an overview structurally controlled hydrothermal dolomite htd reservoir facies and associated productive leached limestones are major hydrocarbon producers in north america and are receiving increased exploration attention globally they include multiple trends in the ordovician locally silurian and devonian of the michigan appalachian and other basins of eastern canada and the united states and in the devonian and mississippian of the western canada sedimentary basin they also occur in jurassic hosts along rifted atlantic margins in the jurassic cretaceous of the arabian gulf region and elsewhere hydrothermal dolomitization is defined as dolomitization occurring under burial conditions commonly at shallow depths by fluids typically very saline with temperature and pressure t and p higher than the ambient t and p of the host formation the latter commonly is limestone proof of a hydrothermal origin for htd reservoir facies requires integration of burial thermal history plots fluid inclusion temperature data and constraints on timing of emplacement hydrothermal dolomite reservoir facies are part of a spectrum of hydrothermal mineral deposits that include sedimentary exhalative lead zinc ore bodies and htd hosted mississippi valley type sulfide deposits all three hydrothermal deposits show a strong structural control by extensional and or strike slip wrench faults with fluid flow typically focused at transtensional and dilational structural sites and in the hanging wall transtensional sags above negative flower structures on wrench faults are favored drilling sites for htd reservoir facies saddle dolomite in both replacive and void filling modes is characteristic of htd facies for many reservoirs matrix replacive dolomite and saddle dolomite appear to have formed near contemporaneously and from the same fluid and temperature conditions the original host facies exerts a major influence on the lateral extent of dolomitization resultant textures pore type and pore volume breccias zebra fabrics shear microfractures and other rock characteristics record short term shear stress and pore fluid pressure transients particularly proximal to active faults high temperature hydrothermal pulses may alter kerogen in host limestones a process designated forced maturation basement highs underlying sandstone and or carbonate aquifers probably overpressured and overlying and internal shale seals and aquitards also may constrain or influence htd emplacement although many questions and uncertainties remain particularly in terms of mg and brine source and mass balance recognition and active exploration of the htd play continues to expand increasing use of three dimensional seismic imagery and seismic anomaly mapping combined with horizontal drilling oblique to linear trends defined by structural sags helps to reduce risk