structural geology and regional tectonic significance of the ramgarh thrust himalayan fold thrust belt of nepal 1 the ramgarh thrust is one of the major fault systems of the himalayan thrust belt in nepal and northern india the ramgarh thrust sheet is 0 2 2 0 km thick and can be traced along strike the entire length of the himalaya in nepal the fault generally places the oldest paleoproterozoic rocks in the lesser himalayan series upon younger lesser himalayan rocks or lower miocene foreland basin deposits regional balanced cross sections suggest that the ramgarh thrust had at least 120 km of initial south vergent displacement subsequently the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying lesser himalayan rocks ramgarh hanging wall strata are greenschist grade phyllite quartzite and augen gneiss all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top to the south sense of shear structural fabrics in the ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet regional and local mapping of the ramgarh thrust in nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat except where local lateral ramps complicate its geometry similarly the structurally overlying main central thrust always places a hanging wall flat in greater himalayan series rocks upon the regionally flat ramgarh thrust sheet these geometric relationships preclude kinematic and thermal models that elevate greater himalayan and lower lesser himalayan rocks along high angle thrust ramps in the vicinity of the present traces of the ramgarh and main central thrust faults instead the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the main central thrust the present steep dips of the ramgarh and main central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the lesser himalayan zone the ramgarh thrust sheet overlaps a bed length of at least 100 km in lower miocene foreland basin deposits indicating that a significant amount of displacement on the thrust must have occurred after 15 ma growth of the lesser himalayan duplex and additional slip on the frontal part of the ramgarh thrust occurred from 12 to 5 ma the presence of a major greenschist grade metasedimentary thrust sheet composed of lesser himalayan rocks directly below the main central thrust suggests that the famous inverted metamorphism in this region is a result of structural inversion similarly the concept of a broad zone of intense shear strain related exclusively to emplacement of the main central thrust sheet is probably invalid in nepal

the composition and its impact on the methane sorption of lacustrine shales from the upper triassic yanchang formation ordos basin china abstract the organic geochemistry mineralogy and methane sorption of lacustrine shales of the upper triassic yanchang formation collected from the south eastern ordos basin were investigated to characterize them and clarify the effects of shale composition on their sorbed gas capacity these yanchang shales have recently been selected as a target area for shale gas exploration in typical terrestrial strata in china the two main sections of these shales containing type ii organic matters chang 7 and chang 9 have relatively high total organic carbon content toc of 2 10 the two shales also have similar mineralogies mainly comprising quartz clay minerals and feldspars both the chang 7 and chang 9 shales are generally in the oil window chang 9 is slightly more mature than chang 7 higher methane sorption capacity was observed for chang 9 than for chang 7 shales determined on a dried basis at 50 c methane sorption measurements were further performed on three samples from which the residual bitumen had been extracted and their corresponding kerogen fractions to gain insight into the effects of shale composition on methane sorption this was significantly higher in solvent extracted samples than in raw samples indicating that residual bitumen largely restricts methane sorption on such shales a positive correlation between the amount of clay minerals and methane sorption capacity of bulk rocks was evident suggesting that clay mineral content is relevant to methane sorption this result was also supported by the much higher methane sorption capacity of solvent extracted shales compared to the extracted kerogen from those shales when measured sorption data was normalized to toc values the effects of both residual bitumen and clay mineral on the methane sorption of bulk rocks have complicated the evaluation of methane sorption on organic matter in these mature shales

modeling mountain building and the seismic cycle in the himalaya of nepal a host of information is now available regarding the geological and thermal structure as well as deformation rate across the himalaya of central nepal these data are reconciled in a two dimensional mechanical model that incorporates the rheological layering of the crust which depends on the local temperature and surface processes over geological timescale 5 ma the 20 mm yr estimated shortening rate across the range is accommodated by localized thrust faulting along the main himalayan thrust fault mht the mht reaches the surface along the foothills where it is called the main frontal thrust fault mft the mht flattens beneath the lesser himalaya and forms a midcrustal ramp at the front of the higher himalaya consistent with the river incision and the anticlinal structure of the lesser himalaya farther northward the mht roots into a subhorizontal shear zone that coincides with a midcrustal seismic reflector aseismic slip along this shear zone is accommodated in the interseismic period by elastic straining of the upper crust increasing the coulomb stress beneath the front of the higher himalaya where most of the microseismic activity clusters negligible deformation of the hanging wall requires a low apparent friction coefficient less than 0 3 on the flat portion of the mht on the ramp might be as high as 0 6 sensitivity tests show that a rather compliant quartz rich rheology and a high radioactive heat production in the upper crust of 2 5 w m 3 is required erosion affects the thermal structure and interplays with crustal deformation a dynamic equilibrium is obtained in which erosion balances tectonic uplift maintaining steady state thermal structure topography and deformation field using a linear diffusion model of erosion we constrain the value of the mass diffusivity coefficient to 0 5 1 6 l0 4 m 2 yr this study demonstrates that the data are internally consistent and compatible with current understanding of the mechanics of crustal deformation and highlight the role of viscous flow in the lower crust and of surface erosion in orogeny processes on the long term as well as during interseismic period

spectrum of pore types and networks in mudrocks and a descriptive classification for matrix related mudrock pores matrix related pore networks in mudrocks are composed of nanometer to micrometer size pores in shale gas systems these pores along with natural fractures form the flow path permeability network that allows flow of gas from the mudrock to induced fractures during production a pore classification consisting of three major matrix related pore types is presented that can be used to quantify matrix related pores and relate them to pore networks two pore types are associated with the mineral matrix the third pore type is associated with organic matter om fracture pores are not controlled by individual matrix particles and are not part of this classification pores associated with mineral particles can be subdivided into interparticle interp pores that are found between particles and crystals and intraparticle intrap pores that are located within particles organic matter pores are intrap pores located within om interparticle mineral pores have a higher probability of being part of an effective pore network than intrap mineral pores because they are more likely to be interconnected although they are intrap om pores are also likely to be part of an interconnected network because of the interconnectivity of om particles in unlithifed near surface muds pores consist of interp and intrap pores and as the muds are buried they compact and lithify during the compaction process a large number of interp and intrap pores are destroyed especially in ductile grain rich muds compaction can decrease the pore volume up to 88 by several kilometers of burial at the onset of hydrocarbon thermal maturation om pores are created in kerogen at depth dissolution of chemically unstable particles can create additional moldic intrap pores

coal maceral ratios as indicators of environment of deposition do they work for ombrogenous mires an example from the miocene of new zealand abstract maceral indices were applied to sub bituminous coals of the maryville coal measures as a means of interpreting environment of deposition very low amounts of inertinite and ubiquitous bituminite lead to difficulties in interpretation of the maceral ratios comparison with modern analogues suggests the theoretical basis for discrimination of mire type by maceral ratios is not well founded on the following grounds absence of well structured wood tissues does not imply the absence of arborescent vegetation inertinite formation may be unrelated to vegetation gelification is used in a geochemical sense and does not indicate groundwater table level as supposed for biochemical gelification detrital macerals often result from in situ fragmentation and not transportation

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

the importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs abstract the effect of shale composition and fabric upon pore structure and ch 4 sorption is investigated for potential shale gas reservoirs in the western canadian sedimentary basin wcsb devonian mississippian d m and jurassic shales have complex heterogeneous pore volume distributions as identified by low pressure co 2 and n 2 sorption and high pressure hg porosimetry thermally mature d m shales 1 6 2 5 vro have dubinin radushkevich d r co 2 micropore volumes ranging between 0 3 and 1 2 cc 100 g and n 2 bet surface areas of 5 31 m 2 g jurassic shales which are invariably of lower thermal maturity ranging from 0 9 to 1 3 vro than d m shales have smaller d r co 2 micropore volumes and n 2 bet surface areas typically in the range of 0 23 0 63 cc 100 g co 2 and 1 9 m 2 g n 2 high pressure ch 4 isotherms on dried and moisture equilibrated shales show a general increase of gas sorption with total organic carbon toc content methane sorption in d m shales increases with increasing toc and micropore volume indicating that microporosity associated with the organic fraction is a primary control upon ch 4 sorption sorption capacities for jurassic shales however can be in part unrelated to micropore volume the large sorbed gas capacities of organic rich jurassic shales independent of surface area imply a portion of ch 4 is stored by solution in matrix bituminite solute ch 4 is not an important contributor to gas storage in d m shales structural transformation of d m organic matter has occurred during thermal diagenesis creating and or opening up microporosity onto which gas can sorb as such d m shales sorb more ch 4 per weight percent wt toc than jurassic shales inorganic material influences modal pore size total porosity and sorption characteristics of shales clay minerals are capable of sorbing gas to their internal structure the amount of which is dependent on clay type illite and montmorillonite have co 2 micropore volumes of 0 78 and 0 79 cc 100 g n 2 bet surface areas of 25 and 30 m 2 g and sorb 2 9 and 2 1 cc g of ch 4 respectively dry basis a reflection of microporosity between irregular surfaces of clay platelets and possibly related to the size of the clay crystals themselves mercury porosimetry analyses show that total porosities are larger in clay rich shales compared to silica rich shales due to open porosity associated with the aluminosilicate fraction clay rich sediments low si al ratios have unimodal pore size distributions

barnett shale gas production fort worth basin issues and discussion newark east barnett shale field fort worth basin texas is currently the most productive gas field in texas in terms of daily production and is growing at an annual rate of more than 10 however despite the fact that the barnett play has been studied intensely by very capable geologists and engineers from several companies over a period of many years there continues to be several misunderstandings concerning fundamental factors controlling the success of the barnett play of north texas barnett gas production is poorer in areas near faults and structural flexures anticlines and synclines fractures which are most abundant in these structural settings are detrimental to barnett production open natural fractures are rare in the barnett and have little or nothing to do with barnett productivity in areas where barnett shale is thermally mature with respect to gas generation it is slightly overpressured about 0 52 psi ft 11 76 kpa m limestone beds within the barnett formation are the product of debris flows that originated on a carbonate shelf to the north of the present basin center it appears that the barnett can be used as an exploration model for other basins especially analogous basins of the ouachita trend the history of the development of the barnett reservoir in north texas provides an excellent example of how persistence can lead to success in nonconventional gas plays

geochronologic and thermobarometric constraints on the evolution of the main central thrust central nepal himalaya the main central thrust mct juxtaposes the high grade greater himalayan crystallines over the lower grade lesser himalaya formation an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust garnet bearing assemblages sampled along the marysandi river and darondi khola in the annapurna region of central nepal show striking differences in garnet zoning of mn ca mg and fe above and below the mct thermobarometry of mct footwall rocks yields apparent inverted temperature and pressure gradients of 18 c km 1 and 0 06 km mpa 1 respectively pressure temperature p t paths calculated for upper lesser himalaya samples that preserve prograde compositions show evidence of decompression during heating whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature in situ i e analyzed in thin section ion microprobe ages of monazites from rocks immediately beneath the greater himalayan crystallines yield ages from 18 to 22 ma whereas late miocene and pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence a lesser himalayan sample collected near the garnet isograd along the marysandi river transect contains 3 3 0 1 ma monazite ages p 0 72 gpa t 535 c this remarkably young age suggests that this portion of the mct shear zone accommodated a minimum of 30 km of slip over the last 3 ma i e a slip rate of 10 mm yr 1 and thus could account for nearly half of the convergence across the himalaya in this period the distribution of ages and p t histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the mct formed by the transposition of right way up metamorphic sequences during late miocene pliocene shearing

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

formation of monazite via prograde metamorphic reactions among common silicates implications for age determinations three lines of evidence from schists of the great smoky mountains nc indicate that isogradic monazite growth occurred at the staurolite in isograd at 600 c 1 monazite is virtually absent below the staurolite in isograd but is ubiquitous several hundred grains per thin section in staurolite and kyanite grade rocks 2 many monazite grains are spatially associated with biotite coronas around garnets formed via the reaction garnet chlorite muscovite biotite plagioclase staurolite h2o 3 garnets contain high y annuli that result from prograde dissolution of garnet via the staurolite in reaction followed by regrowth and rare monazite inclusions occur immediately outside the annulus and in the matrix but not in the garnet core larger monazite grains also exhibit quasi continuous th zoning with high th cores and low th rims consistent with monazite growth via a single reaction and fractional crystallization during prograde growth common silicates may host sufficient p and lrees that reactions among them can produce observable lree phosphate specifically phosphorus contents of garnet and plagioclase are hundreds of parts per million and dissolution of garnet and recrystallization of plagioclase could form thousands of phosphate grains several micrometers in diameter per thin section lrees may be more limiting but sheet silicates and plagioclase can contain tens to 100 ppm lree so recrystallization of these silicates to lower lree contents could produce hundreds of grains of monazite per thin section monazite ages determined via electron and ion microprobes are 400 ma directly linking prograde barrovian metamorphism of the western blue ridge with the acadian orogeny in contrast to previous interpretations that metamorphism was taconian 450 ma interpretation of ages from metamorphic monazite grains will require prior chemical characterization and identification of relevant monazite forming reactions including reactions previously viewed as involving solely common silicates

metamorphic reactions related to decompression and synkinematic intrusion of leucogranite high himalayan crystallines bhutan the high himalayan crystallines hhc of bhutan were penetratively deformed intruded by leucogranite and metamorphosed during the collision of the indian and asian plates metamorphic reaction textures in the hhc show that it experienced decompression while maintaining a laterally heterogeneous and locally inverted internal temperature range of c 600 750c this thermal structure was produced by thrusting hot migmatitic rocks over lower grade rocks within the hhc and by the advection of heat from the intrusion of leucogranite dykes and sills during decompression a variable velocity field within the hhc during exhumation and extrusion between india and tibet caused the inversion of top to the south sense of shear present throughout most of the hhc to top down to the north shear near its top

young rift kinematics in the tadjoura rift western gulf of aden republic of djibouti the tadjoura rift forms the westernmost edge of the westerly propagating sheba ridge between arabia a nd somalia as it enters into the afar depression fro m structural and remote sensing dataset the tadjou ra rift is interpreted as an asymmetrical south facing half gr aben about 40 km wide dominated by a large boundary fault zone to the north it is partially filled up by the 1 3 ma old gulf basalts which onlapped the older s omali basalts along its shallower southern flexural margin the m ajor and trace element analysis of 78 young onshore lavas allows us to distinguish and map four distinct basa ltic types namely the gulf somali goumarre and h ayyabley basalts these results together with radiometric a ge data lead us to propose a revised volcano strat igraphic sketch of the two exposed tadjoura rift margins an d to discriminate and date several distinct fault n etworks of this oblique rift morphological and statistical an alyses of onshore extensional fault populations sho w marked changes in structural styles along strike in a dir ection parallel to the rift axis these major fault disturbances are assigned to the arrest of axial fault tip propagati on against pre existing discontinuities in the ns o riented arta transverse zone according to our model the sinist ral jump of rifting into the asal ghoubbet rift seg ment results from structural inheritance in contrast with the en echelon or transform mechanism of propagation that prevail ed along the entire length of the gulf of aden extensi onal system

the age and rate of displacement along the main central thrust in the western bhutan himalaya abstract in western bhutan the main central thrust mct is broadly folded creating multiple exposures of the fault surface over a 70 km across strike distance this unusual map pattern presents a unique opportunity to map the mct and document both the magnitude and age of displacement in situ th pb sims and la icp ms geochronology of metamorphic monazite from the immediate hanging wall of the mct indicates that prograde monazite growth in greater himalayan gh rocks continued until 20 8 1 1 ma whereas crystallization of in situ melts characterized by high y monazite overgrowths occurred during cooling from ca 15 10 ma prograde monazite growth at 15 ma in lesser himalaya lh rocks in the immediate footwall requires that lh footwall strata began to be buried at this time and the mct had reached its southernmost exposed extent by combining prograde monazite ages in the immediate hanging wall and footwall the duration of mct displacement is bracketed between 20 8 1 1 and 15 0 2 4 ma immediately north of our study area a published estimate of shearing along the outer south tibetan detachment std argues for displacement between 20 and 15 ma coeval with the age range for mct displacement that we document in this study however retrograde monazite grains as young as 10 ma suggest that gh rocks were cooling until 10 ma 5 myr later than motion on the outer std immediately to the north this cooling was either the result of continued displacement on the mct or growth of a duplex that passively folded the mct using a sequential reconstruction we estimate a total displacement of 230 km which is the sum of displacements on the mct and the structurally lower paro thrust over a duration of 5 8 2 6 myr this indicates a horizontal shortening rate of 4 0 3 2 1 3 cm yr which exceeds present rates estimated from geodetic measurements across the himalaya and mct displacement rates c 2 cm yr inferred from petrologic and thermal models in central nepal but is indistinguishable from plate convergence rates calculated for eastern bhutan between 23 and 20 ma 3 3 0 7 cm yr our study highlights that displacement on the mct alone achieved plate velocity rates in western bhutan and that the age and rate of mct displacement varied significantly across the himalaya

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

geological and geochemical characteristics of marine continental transitional shale from the lower permian taiyuan formation taikang uplift southern north china basin abstract in recent years extensive studies have been conducted on conventional shale gas resources but little work has been performed on the resource potential of marine continental shales this paper takes the lower permian taiyuan shale in the taikang uplift of the southern north china basin as an example to characterize a transitional shale reservoir the results indicate that the taiyuan shale has high toc values averaging 2 67 and contains type iii gas prone organic matter the shale has high ro values averaging 3 44 and tmax ranges from 424 c to 601 c with respect to fracability or brittleness the content of brittle minerals in the taiyuan shale ranges from 21 to 71 averaging 45 55 and the average content of clay minerals is 44 1 based on sem and sem with ar ion milling observation interparticle pores intraparticle pores and microfractures are developed in the taiyuan shale which can provide reservoir space for the accumulation of shale gas organic matter pores rarely appear due to the organic type and high maturity the results of this study can guide the shale gas exploration of taiyuan formation in the taikang uplift of the southern north china basin

a new kinematic evolutionary model for the growth of a duplex an example from the rangit duplex sikkim himalaya india abstract the lesser himalayan duplex lhd is a prominent structure through much of the lesser himalayan fold thrust belt in the darjeeling sikkim himalaya a component of the lhd is exposed in the rangit window as the rangit duplex rd the rd consists of ten horses of the upper lesser himalayan sequence gondwana buxa upper daling the duplex varies from hinterland dipping in the north through an antiformal stack in the middle to foreland dipping in the south the ramgarh thrust rt is the roof thrust and based on a balanced cross section the main himalayan sole thrust is the floor thrust at a depth of 10 km and with a dip of 3 5 n retrodeformation suggests that the rd initiated as a foreland dipping duplex with the early ramgarh thrust as the roof thrust and the rt as the floor thrust the rt became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent rt reactivation this kinematic history best explains the large translation of the overlying mct sheets the restoration suggests that rd shortening is 125 km and the original gondwana basin extended 142 km northward of its present northernmost exposures within the window

geologic correlation of the himalayan orogen and indian craton part 2 structural geology geochronology and tectonic evolution of the eastern himalaya despite being the largest active collisional orogen on earth the growth mechanism of the himalaya remains uncertain current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the himalayan and tibetan crust during cenozoic india asia collision a major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the himalayas from 91 e to 97 e which makes up about one quarter of the mountain belt to address this issue we conducted detailed field mapping u pb zircon age dating and 40ar 39ar thermochronology along two geologic traverses at longitudes of 92 e and 94 e across the eastern himalaya our dating indicates the region experienced magmatic events at 1745 1760 ma 825 878 ma 480 520 ma and 28 20 ma the first three events also occurred in the northeastern indian craton while the last is unique to the himalaya correlation of magmatic events and age equivalent lithologic units suggests that the eastern segment of the himalaya was constructed in situ by basement involved thrusting which is inconsistent with the hypothesis of high grade himalaya rocks derived from tibet via channel flow the main central thrust in the eastern himalaya forms the roof of a major thrust duplex its northern part was initiated at ca 13 ma while the southern part was initiated at ca 10 ma as indicated by 40ar 39ar thermochronometry crustal thickening of the main central thrust hanging wall was expressed by discrete ductile thrusting between 12 ma and 7 ma overlapping in time with motion on the main central thrust below restoration of two possible geologic cross sections from one of our geologic traverses where one assumes the existence of pre cenozoic deformation below the himalaya and the other assumes flat lying strata prior to the india asia collision leads to estimated shortening of 775 km 76 strain and 515 km 70 strain respectively we favor the presence of significant basement topography below the eastern himalaya based on projections of early paleozoic structures from the shillong plateau i e the central shillong thrust located 50 km south of our study area since northeastern india and possibly the eastern himalaya both experienced early paleozoic contraction the estimated shortening from this study may have resulted from a combined effect of early paleozoic and cenozoic deformation

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

geological and geochemical characteristics of marine continental transitional shale from the upper permian longtan formation northwestern guizhou china abstract organic rich shale deposited in a marine continental transitional facies was well developed in the upper yangtze area during the late permian it is considered to contain a large amount of shale hydrocarbon resources this paper takes the upper permian longtan shale in the central guizhou uplift as an example to characterize the transitional shale reservoir based on organic geochemistry data x ray diffraction xrd analysis and scanning electron microscopy sem observation the geochemical characteristics of this transitional shale were systematically studied and the shale gas potential was investigated the results indicate that the longtan shale has fair to good hydrocarbon potential with high total organic carbon toc and contains type iii gas prone organic matter its high thermal maturity favors the generation of gas ro 1 10 2 74 xrd analyses show that the mineral components of the transitional shale are dominated by high clay content 58 7 wt the clay minerals are dominated by illite 44 and mixed illite smectite 43 sem observation shows that intra particle pores inter particle pores and microfracture were well developed in the longtan shale which can provide reservoir space for the accumulation of shale gas organic matter pores appear rare due to its organic type in summary the longtan shale has high organic content high maturation and high gas content all of which demonstrate its great potential as a shale gas resource but the high clay content of the longtan shale may be a challenge to shale gas production

shale characteristics in the southeastern ordos basin china implications for hydrocarbon accumulation conditions and the potential of continental shales abstract continental shales in china are generally characterised by low thermal maturity and high clay content characteristics that are significantly different from those of marine shales documented in the usa and elsewhere whether such continental shales have commercial hydrocarbon potential as marine shale in the usa is a question for the china petroleum industry recently vertical drills in the lacustrine shales in the yanchang formation produced an average of two to five tons of oil and 1000 3000 m3 of gas per day by reservoir hydraulic fracturing which demonstrates that the continental shales can also achieve viable hydrocarbon production shale and gas samples collected from the chang 7 c7 and chang 9 c9 members yanchang formation in the southeastern ordos basin were examined for geochemical petrologic and gas content analysis the results show that the c7 and c9 members developed a huge volume of organic rich shales with 2 4 wt toc in the deep or semideep lacustrine characterised primarily by type ii kerogen with a relatively low thermal maturity and a vitrinite reflectance ranging from 0 5 to 1 5 ro which decreases from west to east the hydrocarbon product varies with the thermal maturity in the southwestern corner the gas content measured by canister desorption equals 1 15 3 49 m3 t rock and the gas absorption capacity ranges from 3 to 5 m3 t rock whereas in the eastern part the gas content is low and oil production is 1 4 m3 day on average nanometre scale pores and micro fractures are well developed all of this indicates that the continental shales in the southeastern ordos basin might have huge shale hydrocarbon resources however the clay content of the continental shales of c7 and c9 ranging from 40 to 60 of the bulk mineral content are much higher than for gas produced in marine shales which might lead to significant challenges for successful development

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

the kinematic evolution of the nepalese himalaya interpreted from nd isotopes neodymium nd isotopes from the himalayan fold thrust belt and its associated foreland basin deposits are useful for distinguishing between himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen neodymium isotopic data from the himalayan fold thrust belt in nepal n 35 reveal that the lesser himalayan zone consistently has a more negative nd 0 value than the greater and tibetan himalayan zones our data show the average nd 0 value in the lesser himalayan zone is 21 5 whereas the greater and tibetan himalayan zones have an average nd 0 value of 16 these consistently distinct values throughout nepal enable the use of nd isotopes as a technique for distinguishing between lesser himalayan zone and greater himalayan zone rock the less negative nd 0 values of the greater himalayan rocks support the idea that the greater himalayan zone is not indian basement but rather a terrane that accreted onto india during early paleozoic time neodymium isotopic data from eocene through pliocene foreland basin deposits n 34 show that sediment provenance has been dominated by greater and tibetan himalayan detritus across nepal the nd t values in the synorogenic rocks in western and central nepal generally show an up section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones at 10 ma in khutia khola and 11 ma in surai khola a shift in nd t values from 16 to 18 marks the erosional breaching of a large duplex in the northern part of the lesser himalayan zone this shift is not seen in eastern nepal where the nd t values remain close to 16 throughout miocene time because there has been less erosional unroofing in this region

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

insights into the effects of matrix retention and inert carbon on the petroleum generation potential of indian gondwana shales abstract the rock eval pyrolysis and total organic carbon toc analysis technique is widely used for organic geochemistry screening of source rocks and potential unconventional petroleum reservoirs the rock eval derived parameters the hydrogen index hi which is the ratio between hydrocarbons released under the s2 curve hydrocarbon formed by thermal pyrolysis s2 is an indicator of petroleum generation potential and total organic carbon toc can be used to infer the type of organic matter present in a rock however hi is often under estimated due to retention of some hydrocarbons included under the s2 curve by the rock matrix and the presence of inert carbon within the rock here we describe and correct the matrix retention and inert carbon effects on hydrocarbon generation from a suite of shale samples from indian gondwana shale reservoirs removal of the petrographically identified inert carbon component from the samples tested leads to less scatter in the toc s2 relationship obtained the ratio of volume percentage of organic matter identified through optical microscopy to toc is calculated and that ratio was least in the one heat affected sample but higher in low toc shales 12 5 with decreasing toc content in the sample set analyzed the corrected hi values calculated using the s2 toc intercept increase significantly this correction can therefore lead to false indications about the type of organic matter present a key novel finding of this work is the need while correcting hi for matrix retention effects to consider samples with a specific range of toc contents and to match them to the kerogen types present e g types iii and iv in the samples analyzed filters should also be applied to adjust for the degree of thermal maturity and organic facies

depositional environment and thermal maturity of the coal bearing longtan shale in southwest guizhou china implications for shale gas resource potential abstract significant knowledge has been assembled on the marine longmaxi shale in the sichuan basin over the past ten years however the distribution of shale gas resources on the periphery of the sichuan basin and in other source rock formations are still not sufficiently understood to improve our understanding on the depositional environment and thermal maturity and their relevance to the source rock quality and shale gas potential of the thick coal bearing longtan shale 36 organic rich shale samples were selected from a single borehole located in the southwestern guizhou province for geochemical and petrographic analysis total organic carbon toc contents are high with 7 7 wt on average reaching partly up to 28 21 wt while carbonate contents are low vitrinite reflectance vrr values and rock eval tmax temperatures range from 1 48 to 1 84 and 482 to 583 c respectively indicating a high thermal maturity which is in accordance with aromatic molecular parameters the remaining present day hydrocarbon generation potential rock eval s2 is rather low due to the high maturity microscopic investigations further show that the organic matter in the rocks is mainly composed of higher land plant particles present as large and poorly rounded vitrinite and inertinite particles indicating a rather short transport distance toc vs total sulfur ts relationships as well as specific hydrocarbon ratios e g pristane phytane and framboidal pyrite size distributions indicate a variable either marine or freshwater environment for the shales in the coal bearing sequence and suggest that original kerogen was either type iii or type ii iii bottom water during deposition was probably oxygen depleted but not anoxic high sedimentation rates can be assumed for the now 700 m thick compacted sequence favorable for the preservation of organic matter overall the present results are in accordance with previous studies of the longtan shale which initially had a high hydrocarbon mainly gas generation potential furthermore the thickness of the formation is much larger in the here studied southwestern part of the guizhou province than in the previously studied northern parts

a review of shale wettability characterization using spontaneous imbibition experiments abstract wettability is an important reservoir property that can affect the distribution and migration of geofluids within shale reservoirs which significantly influences the occurrence state of methane in shale pore systems and the selection of fracturing fluids during shale gas exploitation the wettability characteristics of organic matter om rich shale are much more complicated than those of conventional reservoir rocks with a relatively homogeneous composition in addition the low porosity and extremely low permeability as well as the strong heterogeneity of shale complicate the evaluation of shale wettability as a result traditional wettability tests including contact angle measurements and oil water displacement experiments are inapplicable for shale samples consequently spontaneous imbibition experiments become a practical and effective way to establish the wettability characteristics of different shales this paper reviews shale wettability characterization using spontaneous imbibition experiments and discusses the close relationship between shale wettability and pore structure as reported in the literature most shale samples can imbibe both water and oil which indicates the co existence of water wet and oil wet connected pore spaces and the mixed wet property of shales however different shales show a range of oil and water imbibition behaviors indicating the wettability differences amongst them this review reveals that the development degrees of water and oil wet pores within shale determine its wettability as well as its imbibition behaviors and four simplified pore network models are proposed for more water wet more oil wet mixed wet and intermediate wet shale samples respectively this work also shows the pronounced effects of thermal maturity on shale pore structure development and shale wettability characteristics consequently a simplified and empirical wettability evolution model with four stages only considering the effect of thermal maturity is established in this study

a short duration pulse of ductile normal shear on the outer south tibetan detachment in bhutan alternating channel flow and critical taper mechanics of the eastern himalaya in easternmost bhutan the south tibetan detachment std is a ductile shear zone that juxtaposes the radi or sakteng klippe of the tethyan sedimentary series from underlying high grade greater himalayan rocks in situ la icpms u th pb analysis of metamorphic monazite from the immediate footwall and hanging wall of the std within the shear zone at the base of the klippe constrains north vergent normal shear to between 25 and 20 ma coeval thrusting on the main central thrust during this time supports a phase of channel flow viscous wedge model activity lasting only ca 3 ma geochronologic data from the eastern himalaya indicate alternating mechanisms for extrusion of the metamorphic core of the orogen from the late oligocene through to the late miocene switching from channel flow viscous wedge behavior to critical taper frictional wedge behavior each phase lasting approximately only 2 to 5 ma the tectonic evolution of the eastern himalaya is comparable to central and western himalayan tectonics during the early miocene but during the middle miocene metamorphism and magmatism in the eastern himalaya migrated towards the orogenic hinterland a process not widely documented elsewhere in the himalaya thus highlighting the need for an orogenic model in three spatial dimensions

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

preliminary stratigraphic and structural architecture of bhutan implications for the along strike architecture of the himalayan system article i nfo preliminary mapping and stratigraphic correlation of lesser himalayan rock in eastern bhutan using field characteristics u pb detrital zircon dating and nd geochemistry define the first order stratigraphic architecture of the indian passive margin sequence in the eastern himalaya we use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern himalayan fold thrust belt in bhutan we propose that lesser himalayan rock in bhutan can be divided into lower lesser himalayan rocks with a paleoproterozoic detrital zircon signal and upper lesser himalayan rocks with detrital zircon signals of 1000 500 ma the 500 ma detrital grains are from rocks in the frontal portions of the fold thrust belt north of the main boundary thrust as well as directly in the footwall of the main central thrust in the hinterland our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross section through the kuru chu valley in eastern bhutan which provides the first image of the geometry and amount of shortening through bhutan the main frontal thrust tilts a 6 km of thick section of neogene foreland basin deposits these deposits are separated from folded and faulted upper lesser himalayan rocks by the main boundary thrust north of the main boundary thrust we propose two duplex systems the southernmost duplex contains 9 repeated sections of upper lesser himalayan units including the permian gondwana sequence and the cambrian or younger baxa group the northernmost duplex is located in the footwall of the main central thrust and is comprised of two repeated sections of the proterozoic shumar and daling formations the southern boundary of this northern duplex is the shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the ramgarh thrust in nepal and india we propose that the development of the duplex systems passively folded overlying tethyan and greater himalayan rocks minimum shortening for this part of the himalayan fold thrust belt is 359 km all of which has occurred from 22 ma to present defining a long term shortening rate of 16 mm yr

paleogeography and shale development characteristics of the late permian longtan formation in southeastern sichuan basin china abstract the late permian longtan formation is extensively developed within the southeastern sichuan basin which lies on the western margin of the yangtze plate in southwestern china the stratigraphy and sequence boundaries of the longtan formation were interpreted from a combined study of field data well log data and various data analysis in order to determine the extent and thickness of shale sequences this interpretation defined the broad sequence stratigraphic framework and separated the longtan formation into three 3rd order sequences in addition the formation was divided into five sedimentary facies using sedimentary paleontological geophysical and other data the facies identified were fluvial low energy shore tidal flat melanged accumulated shelf and platform basin three paleogeographic maps were created based on the 3rd order sequences sq1 sq2 and sq3 the study shows that sq1 sq3 sedimentary environment of the study area which extends from the southwest to northeast was developed followed by fluvial shore tide shallow water melanged accumulation shelf deep water melanged accumulation shelf and platform basin the mud shale rock types of the longtan formation that were identified are carbonaceous mud shale silty mudstone and lime mudstone according to the divided standard of mud shale effective thickness there are three mud shale assemblages present in the sequences sq1 sq2 sq3 namely mud shale with sandstone mud shale with limestone and mud shale sq1 has the thickest mud shale identified on well logs mud shale with sandstone was well developed in the southwest of the study area and was interpreted to have been deposited in a shore or tidal flat sedimentary environment this rock assemblage has a large effective thickness and extensive lateral distribution in contrast the assemblage of mud shale with limestone was mainly developed in the northeast of the study area and was interpreted to have formed in a melanged accumulation shelf sedimentary environment the effective thickness of the mud shale with limestone is small and the distribution is limited there is developed mud shale in above two type s sedimentary environment but the distribution is also limited the longtan formation s mud shale reached stage b diagenesis and is principally composed of quartz and clay minerals the average quartz content is 43 40 and the average clay mineral content which is mainly composed of illite mixed illite smectite chlorite and kaolinite is 39 58 the sedimentary environment has controlled the distribution of minerals within the sequences the clay mineral content gradually decreases as the sedimentary environment changes from shore tidal flat in the southwest to deep water melanged accumulation shelf in the northeast therefore this comprehensive analysis shows that the southwest of the study area is the key area for shale gas exploration in the longtan formation

geochemical and geological characterization of marine continental transitional shales from longtan formation in yangtze area south china abstract although previous research has provided useful insights into unconventional shale gas resources in recent years the geochemical and geological characteristics of marine continental transitional shale have not been studied systematically during the late permian longtan formation shales were widely deposited in the yangtze area of south china these shales clearly indicate the need to deepen our understanding of marine continental transitional shales this study describes the various characteristics of black shales from the longtan formation in this area on the basis of a field investigation and relevant laboratory analyses longtan shale reservoirs with a thickness and burial depth range of 20 650 and 800 2400 m respectively are usually interbedded with coal and compact sandstone reservoirs due to their special sedimentary environment geochemical analysis of the samples of longtan shales from seven wells in the yangtze area indicates the presence of high organic matters oms with a total organic carbon content in the range of 0 85 35 7 average of 7 33 but low hydrocarbon genetic potential s2 with a value below 3 mg hc g rock the analysis of carbon isotopes and organic macerals demonstrates that the investigated samples mainly contain type iii kerogen data from gas chromatography and gas chromatography mass spectrometry analyses illustrate a weakly oxidizing to a weakly reducing environment with most organic materials originating from algal bacterial organisms and terrestrial plants the terrestrial plants contribute significantly to the om the longtan shales are thermally over matured and they have entered the dry gas generation window according to ro production index data and biomarker maturity ratios x ray diffraction analyses show that the content of brittle minerals of the longtan shales is generally lower than that of marine shales in north america the longtan shales show a high clay content averaging 60 32 wt and low calcite mineral content averaging 3 73 wt field emission scanning electron microscopy shows that various types of nanometer to micrometer scale pores including interparticle and intraparticle pores om pores and microfractures are well developed in the shales generally high om abundance thermal evolution degree clay mineral content and pore space porosity of 0 56 10 6 positively influence shale gas content 1 0 3 8 however high clay content and variable reservoir thickness hinder the successful production of shale gas

monazite allanite phase relations in metapelites abstract calculations of the transition from allanite to monazite bearing assemblages in typical pelitic bulk compositions have been made using thermodynamic data estimated from oxide sums and inferred from natural parageneses calculations in the cfashpce and mnnckfmashpce systems place the allanite to monazite transition in the middle amphibolite facies 525 600 c for a bulk composition similar to shaw s average pelite the temperature of the transition is pressure dependent and strongly dependent on the bulk rock cao content consistent with inferences from natural parageneses the transition is also a function of the bulk al 2 o 3 content although the calculated result is opposite to that inferred from natural samples comparison with published results in the la mg system suggest that the nature of the ree phosphate does not greatly influence the conditions of the transition

a model for the origin of himalayan anatexis and inverted metamorphism the origin of the paired granite belts and inverted metamorphic sequences of the himalaya has generally been ascribed to development of the main central thrust mct although a variety of models have been proposed that link early miocene anatexis with inverted metamorphism recent dating studies indicate that recrystallization of elements of the mct footwall occurred in the central himalaya as recently as 6 ma the recognition that hanging wall magmatism and footwall metamorphism are not spatially and temporally related renders unnecessary the need for exceptional physical conditions to explain generation of the high himalayan leucogranites and north himalayan granites which differ in age petrogenesis and emplacement style we suggest that their origin is linked to shear heating on a continuously active thrust that cuts through indian supracrustal rocks that had previously experienced low degrees of partial melting numerical simulations assuming a shear stress of 30 mpa indicate that continuous slip on the himalayan decollement beginning at 25 ma could trigger partial melting reactions leading to formation of the high himalayan granite chain between 25 and 20 ma and the north himalayan belt between 17 and 8 ma the ramp flat geometry we apply to model the himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crustal levels above the decollement throughout the early and middle miocene late miocene out of sequence thrusting within the broad shear zone beneath the mct provides a mechanism to bring these rocks to the surface in their present location i e well to the north of the present tectonic front and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the mct we envision that formation of the mct zone involved successive accretion of tectonic slivers of the lesser himalayan formations to the hanging wall and incorporate these effects into the model the model predicts continued anatexis up to 400 km north of the himalayan range consistent with the timing and geochemistry of leucogranites exhumed on the flank of a south tibetan rift

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

sedimentological and geochemical characterization of the upper permian transitional facies of the longtan formation northern guizhou province southwest china insights into paleo environmental conditions and organic matter accumulation mechanisms abstract characterized by high toc content and large cumulative thickness the upper permian marine continental transitional facies of shales are widely distributed in southwest china and are currently becoming a prospective area for shale gas exploration and production compared to marine shales the organic matter om accumulation model and paleo environmental conditions remain unclear in transitional settings in this study core descriptions thin section and scanning electron microscopy sem observations x ray diffraction xrd mineralogical analysis and a major trace and rare earth element analysis were integrated to document the varying lithofacies paleo environmental conditions and om accumulation mechanisms for the upper permian transitional facies of the longtan formation in northern guizhou province southwest china six major lithofacies including carbonaceous mudstones siliceous clay mixed mudstones argillaceous mudstones silty shales muddy siltstones and calcareous clay mixed shales were identified the xrd analysis and sem images suggest that clay average 51 2 wt and siderite average 17 3 wt are the dominant minerals in the longtan formation which is different from the mineralogical composition of typical marine shales which are generally dominated by quartz e g horn river shale in the western canada sedimentary basin and the barnett shale in the fort worth basin furthermore the sedimentological and geochemical analyses suggest that the paleo environmental conditions and om accumulation mechanisms of the transitional facies of the longtan formation are different from those of marine shales and can be summarized as follows 1 terrestrial plant fragments mainly contribute to the toc contents of the longtan formation 2 paleoproductivity is generally the major influencing factor of om accumulation in marine shales but is relatively low during the deposition of the longtan formation 3 terrestrial om input redox conditions and sedimentation rate are the major factors controlling om accumulation and 4 most of the silica content has detrital origin instead of biogenic origin in the longtan formation

using isotopic and chronologic data to fingerprint strata challenges and benefits of variable sources to tectonic interpretations the paro formation bhutan himalaya 1 we combine detrital zircons dz and epsilon neodymium nd signatures with field mapping in the paro formation in western bhutan dz age spectra are strongly variable and display signatures that have been used to uniquely identify both greater himalayan gh and lesser himalayan lh strata dz age peaks from six quartzite samples require sources for 0 5 0 8 1 2 1 4 1 7 1 8 and 2 5 ga zircons in the paro formation the youngest 0 5 ga zircons argue for a cambrian maximum deposition age two samples have a youngest 1 8 ga peak typically attributed to paleoproterozoic lh rocks a 450 ma crystallization age from two granite samples constrains the minimum deposition age as ordovician new nd signatures from six detrital samples from the paro formation show significant variation with lithology schists have nd 0 values between 12 0 and 16 9 while quartzite values vary between 18 8 and 24 5 these data imply that the paro formation was derived from both young and old sources with dz and nd values obtained from the same quartzite samples requiring old detritus while the nd values obtained from interbedded schist require younger detritus using published isotopic and chronologic definitions of himalayan strata schist rich layers wouldbeconsideredgh whiletheinterbeddedquartzite

quantifying internal strain and deformation temperature in the eastern himalaya bhutan implications for the evolution of strain in thrust sheets abstract quartz microstructural analysis together with quantification of 3 d crystal plastic strain of quartz clasts normalized fry and rf methods constrain deformation temperature ranges and internal strain magnitude orientation within himalayan thrust sheets in bhutan lesser himalayan lh thrust sheets display an inverted deformation temperature gradient from 250 to 310 c at the main boundary thrust to 500 670 c at the main central thrust mct attributed primarily to stacking of lh thrust sheets deformed at progressively higher temperatures toward the hinterland the hot iron effect of the mct hanging wall only affects the upper few hundred meters of hinterland lh thrust sheets frontal thrust sheets exhibit 7 layer parallel shortening lps strain all other thrust sheets exhibit layer normal flattening lnf strain with 1 9 1 8 1 0 lh rocks and 2 1 1 8 1 0 greater and tethyan himalayan rocks median ellipsoids x parallel to lineation we propose that lps strain developed foreland ward of the thrust deformation front and that lnf strain resulted from later tectonic loading the lps to lnf transition occurs at minimum temperatures for quartz plasticity ca 250 270 c lnf ellipsoids are folded along with strata within thrust sheets indicating that internal strain preceded thrust imbrication and translation at the scale of 10 s of meters structural distance strain magnitude of hanging wall rocks does not increase near thrusts which supports the existence of discrete faults with large translations as generally depicted in balanced cross sections strain magnitudes in low grade greater and tethyan himalayan rocks in central bhutan indicate that the top to the north component of channel flow extrusion is most likely less than 19 km

porosity of devonian and mississippian new albany shale across a maturation gradient insights from organic petrology gas adsorption and mercury intrusion the evolution of porosity in shales with increasing maturity was examined in a suite of five new albany shale samples spanning a maturity range from immature vitrinite reflectance ro 0 35 to postmature ro 1 41 devonian to lower mississippian new albany shale samples from the illinois basin used in this study contain marine type ii kerogen having total organic carbon contents from 1 2 to 13 0 wt organic petrology co2 and n2 low pressure adsorption and mercury intrusion capillary pressure techniques were used to quantify pore volumes pore sizes and pore size distributions increasing maturity of the new albany shale is paralleled by many changes in the characteristics of porosity the total porosity of 9 1 vol in immature new albany shale decreases to 1 5 vol in the late mature sample whereas total pore volumes decrease from 0 0365 to 0 0059 cm3 g in the same sequence reversing the trend at even higher maturity the postmature new albany shale exhibits higher porosity and larger total pore volumes compared to the late mature sample with increasing maturity changes in total porosity and total pore volumes are accompanied by changes in pore size distributions and relative proportions of micropores mesopores and macropores porosity related variances are directly related to differences in the amount and character of the organic matter and mineralogical composition but maturity exerts the dominant control upon these characteristics we conclude that organic matter transformation due to hydrocarbon generation and migration is a pivotal cause of the observed porosity differences

kerogen origin evolution and structure kerogen commonly defined as the insoluble macromolecular organic matter om dispersed in sedimentary rocks is by far the most abundant form of om on earth this fossil material is of prime importance as the source of oil and natural gas moreover kerogen can provide essential information on major topics such as past environments climates and biota this review reports the main advances in kerogen studies since the comprehensive synthesis edited by durand durand b ed kerogen insoluble organic matter from sedimentary rocks editions technip paris 1980 it is organized into eight sections the first two are concerned with the successive definitions of kerogen and the definition used here the different techniques used for kerogen isolation without loss or degradation and basic kerogen analysis the third and fourth focus on sedimentary om sources and preservation processes in relation to depositional environment including sedimentation conditions favourable for kerogen accumulation and extrapolation to past geological time great strides have been made in the latter topics over the last 25 years owing to a combination of classical studies in organic geochemistry and studies in other domains such as biogeochemistry oceanography hydrology and soil science along with the development of powerful analytical tools the next two sections deal with the different kerogen classifications by type and kerogen evolution and maturation upon burial in sediments structural modelling of coal and kerogen based on physical and or chemical structural analysis is described in the following section although only statistical the models thus derived provide a synthetic view of the main structural resemblances and differences among various samples in relation to source maturity or physicochemical properties finally the last section explores some of the advances in kerogen understanding expected for the near future the review includes a list containing about 500 references

stratigraphy and sedimentary history of the nepal tethys himalaya passive margin abstract the sedimentary history of the nepal tethys himalaya began with deposition of thick carbonates in the cambro ordovician followed by a mixed siliciclastic carbonate epicontinental succession recording two major deepening events in the early silurian and late devonian fossiliferous carbonate ramp deposits in the tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins rift stage break up in the mid early permian locally associated with effusion of tholeiitic lava flows was followed by a transgressive sandy to shaly locally coal bearing or bioclastic unit capped by condensed pelagic carbonates in the middle to late permian juvenile ocean stage subsidence of the cooling stretched crust led close to bathyal water depths in the olenekian but then slowed down in the middle triassic to increase again sharply in the late triassic owing to renewed extensional tectonic activity and sediment loading during up and out building of the indian continental terrace deposition of tropical platform carbonates finally became widespread in the middle liassic mature passive margin stage the initial fragmentation of gondwana in the middle jurassic led to rejuvenation of the indian craton and deposition of quartzo feldspathic hybrid arenites capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late bathonian middle callovian next a discontinuous pelagic grey marly limestone unit was followed by the ammonoid rich offshore spiti shale in the late jurassic the final disintegration of gondwana began in the berriasian when quartzose siliciclastics derived again from the rejuvenated indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the valanginian followed in the hauterivian to albian by more felsic differentiates such as the trachyandesites exposed in the lesser himalaya 120 km to the south a widespread drowning episode fostered by waning volcaniclastic supply during a global eustatic rise is documented by a major glauconitic horizon deposited at middle southern latitudes in the late albian overlain by scaglia like pelagic limestones in the latest albian the final part of sedimentary history during the rapid northward flight of india and its collision with eurasia is not documented anywhere in nepal due to later erosion of upper cretaceous to lower tertiary strata

dynamic palaeogeographic reconstructions of the wuchiapingian stage lopingian late permian for the south china block abstract as an important tool in natural resource exploration palaeogeography provides a portrayal of deep time earth the south china block located in the eastern palaeotethys includes multiple natural resources e g coal shale gas bauxite etc in the permian which are well known for their important economic significance in this paper data gathered from literature amount to 431 stratigraphic sections of wuchiapingian age from south china these data were quantitatively analyzed and dynamically reconstructed in the global context under the revised plate motion model by gplates software for the first time a schematic 3 d model was constructed based on the high resolution reconstructions in this paper showing two relatively large highlands developed in the south china block during the wuchiapingian including the kangdian highland in the west and the cathaysia highland in the east and a small yunkai highland in the south a broad platform dominated depositional system with small uplifts and depressions was developed between these two highlands these reconstructions show great potential to explore enriched coal seams and shale gas in the peripheral areas of the two ancient highlands in the west and east of the south china block based on the dynamic evolution of this depositional system south china experienced an overall sea level rise with frequent secondary transgressions and regressions during the wuchiapingian this is in contrast to the widespread continental and evaporite deposits on much of pangea and its surrounding margins after the end guadalupian global regression compilation of conodont biostratigraphic data suggest that the temporal and spatial constraints on the unconformity across the guadalupian lopingian boundary glb resulted from dongwu uplift movement the regression began during the jinogondolella shannoni zone and ended in the clarkina leveni zone but the response time varies in different localities of south china these results provide detailed information about sea level eustacy tectonic uplift sedimentary hiatus resource accumulation and palaeoclimate change from the south china block during the wuchiapingian

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

gas geochemistry from conventional to unconventional domains abstract gas geochemistry is developing into a powerful tool to understand geological processes and affirm source origins of geo fluids major and trace gases including abundances and isotopes have shown considerable application in natural gas systems for example progress in unconventional gases such as shale gas tight gas and extreme conditions in the deep oceans represent more emerging areas and application of these novel gas related techniques examples where gas geochemistry continues to place key constraints on the origin and migration characteristics of natural gas the p t characteristics of fluids in both subaerial and deep geothermal reservoirs and the dynamics of the accumulation cycles to name but a few this volume will reflect this diversity in scope and application of gas geochemistry focusing on deeper and broader applications in unconventional domains of novel gas geochemical techniques and applications

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

geometry and crustal shortening of the himalayan fold thrust belt eastern and central bhutan we present a new geologic map of eastern and central bhutan and four balanced cross sections through the himalayan fold thrust belt major structural features from south to north include 1 a single thrust sheet of subhimalayan rocks above the main frontal thrust 2 the upper lesser himalayan duplex system which repeats horses of the neoproterozoic cambrian baxa group below a roof thrust shumar thrust carrying the paleoproterozoic daling shumar group 3 the lower lesser himalayan duplex system which repeats horses of the daling shumar group and neoproterozoic ordovician jaishidanda formation with the main central thrust mct acting as the roof thrust 4 the structurally lower greater himalayan section above the mct with overlying tethyan himalayan rock in stratigraphic contact in central bhutan and structural contact above the south tibetan detachment in eastern bhutan and 5 the structurally higher greater himalayan section above the kakhtang thrust cross sections show 164 267 km shortening in subhimalayan and lesser himalayan rocks 97 156 km structural overlap across the mct and 31 53 km structural overlap across the kakhtang thrust indicating a total of 344 405 km of minimum crustal shortening 70 75 our data show an eastward continuation of lesser himalayan duplexing identified in northwest india nepal and sikkim which passively folded the overlying greater himalayan and tethyan himalayan sections shortening and percent shortening estimates across the orogen although minima do not show an overall eastward increase which may suggest that shortening variations are controlled more by the original width and geometry of the margin than by external parameters such as erosion and convergence rates

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

the influence of depositional environments on coal petrographic composition of the lower kittaning seam western pennsylvania abstract lateral variations in maceral composition of the lower kittanning seam can be explained in terms of differential preservation of plant materials which is influenced by the ph eh of water within the peat swamp and by the rate of subsidence within the central part of the basin high telocollinite contents are observed this type of vitrinite represents large well preserved pieces of woody tissue whose preservation was favored by rapid burial towards the margins of the basin high desmocollinite contents are seen slower burial combined with the influx of neutral oxygenated fresh water permitted decomposition of the precursor materials producing this highly degraded type of vitrinite vertical variations in telocollinite and desmocollinite content can also be related to burial rates and ph eh conditions within the swamp throughout the basin high telocollinate contents observed in the lower part of the seam resulted from rapid burial under relatively low ph conditions continued high rates of subsidence led to an influx of neutral ph oxidizing waters associated with the encroaching marine environment under these new conditions more degradation of plant tissues occurred producing the high desmocollinite contents observed in the upper part of the seam this effect was most pronounced in the center of the basin an alternative explanation would be a change in vegetation type in response to the changing conditions the high desmocollinite content would then reflect a shift to a community dominated by smaller possibly herbaceous plants in addition to a change in vitrinite type towards the top of the seam a decrease in total vitrinite content and increases in total liptinite and inertinite contents are observed the increase in liptinite content is due to either 1 the higher resistance to decomposition of liptinite precursor materials when compared to vitrinite precursors in the more neutral conditions or 2 a change from a dominantly arborescent vegetation to a more herbaceous one the increase in inertinite results from an increase in transported material inertodetrinite and or an increase in the formation of inertinite macerals such as degrado fusinite possibly in response to the encroaching marine conditions an additional control on maceral composition is the level of metamorphism of the coal total liptinite content decreases towards the southeast in this area the coal has attained a sufficiently high rank that the reflectance of liptinite macerals approaches that of vitrinite thus making their identification difficult

organic geochemistry of the woodford shale southeastern oklahoma how variable can shales be woodford shale samples obtained from a cored outcrop in southeastern oklahoma were geochemically analyzed to determine vertical variations of organic facies thermal maturity and an evaluation of their depositional environments total organic carbon values ranged from 5 01 to 14 81 indicating a good source rock potential in this area the woodford shale is marginally mature as indicated by vitrinite reflectance values rock eval data revealed that the samples are dominated by type ii kerogen biomarker ratios based on pristane phytane steranes and hopanes show a mix of marine and terrigenous organic matter high salinity conditions and water density stratification also prevailed during deposition of this formation as indicated by the presence of gammacerane the woodford shale was subdivided into lower middle and upper members based on the integration of geochemical and geologic data moreover the presence and extent of photic zone anoxia pza were determined by the presence of aryl isoprenoids the lower and upper woodford shale members were deposited under dysoxic to suboxic conditions and episodic periods of pza the middle member was deposited under anoxic conditions and persistent pza in addition aryl isoprenoids helped infer the position of the chemocline during deposition of the different members the relative hydrocarbon potential parameter was used in determining transgressive and regressive cycles within the woodford shale this study undoubtedly demonstrates the significant lithologic and chemical variability that occurs within shales the application of this workflow to regional studies can have a direct influence on exploration and production activities in shale gas systems

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

miocene faulting at plate tectonic velocity in the himalaya of central nepal abstract combination of geochemical zoning in metamorphic garnet and monazite plus in situ th pb isotopic dating of monazite yields p t conditions ages and convergence rates for the main central thrust mct and affiliated faults in central nepal inferred rates were 1 5 0 9 cm yr langtang thrust 19 ma 2 2 0 7 cm yr main central thrust 15 ma and 7 3 cm yr ramgarh thrust 10 ma the lower values are similar to modern convergence rates across the himalaya but the ramgarh thrust may have briefly absorbed all indo asian convergence at 10 ma when foreland and marine sedimentation rates markedly increased and at least one major strike slip fault in tibet experienced a hiatus in movement variable rates of convergence across the himalaya on myr timescales imply myr variations in strain rates throughout all components of the indo asian orogen

fractured shale gas systems the first commercial united states natural gas production 1821 came from an organic rich devonian shale in the appalachian basin understanding the geological and geochemical nature of organic shale formations and improving their gas producibility have subsequently been the challenge of millions of dollars worth of research since the 1970s shale gas systems essentially are continuous type biogenic predominant thermogenic or combined biogenic thermogenic gas accumulations characterized by widespread gas saturation subtle trapping mechanisms seals of variable lithology and relatively short hydrocarbon migration distances shale gas may be stored as free gas in natural fractures and intergranular porosity as gas sorbed onto kerogen and clay particle surfaces or as gas dissolved in kerogen and bitumen five united states shale formations that presently produce gas commercially exhibit an unexpectedly wide variation in the values of five key parameters thermal maturity expressed as vitrinite reflectance sorbed gas fraction reservoir thickness total organic carbon content and volume of gas in place the degree of natural fracture development in an otherwise low matrix permeability shale reservoir is a controlling factor in gas producibility to date unstimulated commercial production has been achievable in only a small proportion of shale wells those that intercept natural fracture networks in most other cases a successful shale gas well requires hydraulic stimulation together the devonian antrim shale of the michigan basin and devonian ohio shale of the appalachian basin accounted for about 84 of the total 380 bcf of shale gas produced in 1999 however annual gas production is steadily increasing from three other major organic shale formations that subsequently have been explored and developed the devonian new albany shale in the illinois basin the mississippian barnett shale in the fort worth basin and the cretaceous lewis shale in the san juan basin in the basins for which estimates have been made shale gas resources are substantial with in place volumes of 497 783 tcf the estimated technically recoverable resource exclusive of the lewis shale ranges from 31 to 76 tcf in both cases the ohio shale accounts for the largest share

ediacaran cambrian ordovician silurian and permian shales of the upper yangtze platform south china deposition thermal maturity and shale gas potential abstract for many years the yangtze platform has been successfully explored and exploited for petroleum the lower silurian longmaxi shale proved to contain commercial quantities of gas but several other formations might also have a high potential for shale hosted gas exploration this paper comprises of geochemical petrographical petrophysical and mineralogical information on the major proterozoic and paleozoic gas shales in the upper yangtze area which provides insights on the depositional environment the thermal maturity of organic matter shale gas storage capacity and the fracability of these rocks the total organic carbon toc content varies from 0 1 to 22 5 with an average value of 2 7 and most of these shales were deposited in an oxygen depleted marine environment equivalent vitrinite reflectance vrr values range from 2 20 to 4 25 indicating that all samples are in the gas generation window and lost a large amount of primary organic carbon porosity varies between 1 5 and 13 6 with an average value of 5 8 while the excess sorption capacity ranges from 0 10 to 0 22 mmol g rock minerology data show that quartz is the predominant mineral except for the upper permian longtan formation in which clay minerals account for 29 to 84 new data were combined with those from other publications and compared to information on other well studied shale hosted gas systems

plio pleistocene vegetation changes in the north china plain magnetostratigraphy oxygen and carbon isotopic composition of pedogenic carbonates abstract the plio pleistocene history of c4 plant biomass in northwestern china has been documented from the loess soil sequences of the loess plateau region however how c4 plants evolved in the warmer and low elevation eastern china monsoon zone is still poorly known mainly because of the unavailability of well dated geological records in this study a 203 6 m core of floodplain deposits was recovered from the north china plain near tianjin and dated magnetostratigraphically the results define a chronosequence for the last 3 3 ma the late quaternary portion of the core consists of fluvio marine sediments while the rest of the section 3 3 0 6 ma contains abundant paleosols formed on the floodplain as confirmed by soil micromorphological evidence the stable carbon and oxygen isotopic composition of pedogenic carbonates was measured to document vegetation and climate changes the results reveal mixed c3 and c4 vegetation with an estimated c4 abundance of 40 60 from 3 1 to 2 2 ma and a subsequent gradual decline to 25 until 0 6 ma this trend is consistent with the data from the loess soil sequences further west on the loess plateau suggesting they are regionally significant changes the lowering of growing season temperature and or drier conditions induced by global cooling would explain this overall decline

morphology genesis and distribution of nanometer scale pores in siliceous mudstones of the mississippian barnett shale research on mudrock attributes has increased dramatically since shale gas systems have become commercial hydrocarbon production targets one of the most significant research questions now being asked focuses on the nature of the pore system in these mudrocks our work on siliceous mudstones from the mississippian barnett shale of the fort worth basin texas shows that the pores in these rocks are dominantly nanometer in scale nanopores we used scanning electron microscopy to characterize barnett pores from a number of cores and have imaged pores as small as 5 nm key to our success in imaging these nanopores is the use of ar ion beam milling this methodology provides flat surfaces that lack topography related to differential hardness and are fundamental for high magnification imaging nanopores are observed in three main modes of occurrence most pores are found in grains of organic matter as intraparticle pores many of these grains contain hundreds of pores intraparticle organic nanopores most commonly have irregular bubblelike elliptical cross sections and range between 5 and 750 nm with the median nanopore size for all grains being approximately 100 nm internal porosities of up to 20 2 have been measured for whole grains of organic matter based on point count data from scanning electron microscopy analysis these nanopores in the organic matter are the predominant pore type in the barnett mudstones and they are related to thermal maturation nanopores are also found in bedding parallel wispy organic rich laminae as intraparticle pores in organic grains and as interparticle pores between organic matter but this mode is not common although less abundant nanopores are also locally present in fine grained matrix areas unassociated with organic matter and as nano to microintercrystalline pores in pyrite framboids intraparticle organic nanopores and pyrite framboid intercrystalline pores contribute to gas storage in barnett mudstones we postulate that permeability pathways within the barnett mudstones are along bedding parallel layers of organic matter or a mesh network of organic matter flakes because this material contains the most pores

apatite monazite and xenotime in metamorphic rocks this chapter focuses on phosphates that are significant in metamorphic rocks a quick survey of phosphate mineral descriptions at a commercial mineral web site revealed over 500 phosphate mineral names remarkably only three are common in metamorphic rocks apatite monazite and xenotime and this chapter is restricted to discussion of these minerals apatite monazite and to a lesser extent xenotime have enjoyed intensive study during the previous half century following world war ii considerable study was made of minerals that contain fissionable materials and the sometimes large concentrations of u and th in monazite culminated in a number of seminal papers on the occurrence of that mineral e g overstreet 1967 during the 1970s and early 1980s the study of apatite and monazite turned towards their role as a sink for rees and other trace elements in rocks e g watson 1980 watson and capobianco 1981 throughout this period monazite and to some extent xenotime enjoyed attention as a geochronometer e g parrish 1990 and through today this attention has continued to grow accessory minerals such as monazite xenotime and apatite have come to the forefront of research in metamorphic petrology in recent years for two reasons first it has recently been recognized that trace elements in metamorphic rocks and especially trace element zoning in garnet and other major phases contain considerable detailed information about the reaction history a rock has experienced coupled with the fact that diffusion of many trace elements in metamorphic minerals is in general considerably slower than diffusion of major elements in those same minerals the possibility has recently emerged of recovering details of metamorphic petrogenesis that were previously unattainable inasmuch as accessory minerals play a dominant role in the mass budget of many trace elements attention has naturally turned to their paragenesis it has also recently been discovered that

trace element distributions in silicates during prograde metamorphic reactions implications for monazite formation to assess the petrogenetic relationship between monazite and major silicates during prograde metamorphism ree were measured across coexisting zoned silicates in garnet through kyanite grade pelitic schists from the great smoky mountains western blue ridge terrane southern appalachians to establish ree concentrations and distributions before and after the monazite in isograd and to identify the role major silicates play in the formation of monazite results indicate significant scavenging of light rare earth elements lree from silicates during the monazite in isograd reaction however the absolute concentration of lree hosted in the silicates was insufficient to produce monazite in the quantity observed in these schists monazite must have formed mainly from either the dissolution of allanite or some other source of concentrated lree possibly adsorbed onto grain boundaries even though direct evidence for allanite is lacking in a majority of the samples laser ablation icp ms analyses and theoretical thermodynamic calculations show that monazite may have formed as a result of contributions from both allanite and major silicates allanite breakdown initially formed monazite and monazite production drew lree liberated from allanite major silicates and possibly from crystal boundaries in many rocks the reaction was further promoted by the staurolite in reaction allowing for rapid isogradic monazite growth

textural chemical and isotopic insights into the nature and behaviour of metamorphic monazite monazite is a mineral of choice for dating metamorphism in amphibolite and granulite grade metapelites however there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems the two main obstacles addressed in this contribution are firstly the minor but significant e g 1 30 ma dispersal in duplicate isotope dilution thermal ionisation mass spectrometry id tims u pb age data commonly recorded from a single rock and secondly the difficulty of attaching monazite age data to pressure and temperature information through a multidisciplinary approach utilising tims and laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms isotope data quantitative and qualitative emp chemical analyses of monazite and textural studies we assess the significance of pb loss older components and continuous and episodic monazite growth in the generation of dispersed age data three samples from the canadian cordillera and one sample from the himalaya of pakistan are examined each sample exhibits an age dispersion of between 1 and 12 ma for single crystal and multi grain tims u pb monazite age determinations consideration of the closure temperature for pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive pb loss did not play a significant part in generating this age dispersal the la mc icpms study indicates that an older component 100 ma older than the tims ages contributed to the age dispersal in three of the four samples in all the samples however chemical analyses identified that the majority of monazites examined exhibited significant intra crystalline zoning in y content the la mc icpms analysis of one sample that was constrained to zones of distinct y content indicates that these zones are of distinct age we suggest that monazite grown before the appearance of garnet and during garnet breakdown is relatively rich in y whereas monazite grown after garnet is relatively poor in y a combination of these chemical data with textural observations suggests that once monazite had entered the mineral assemblage it grew or recrystallised episodically throughout the prograde and retrograde paths of the metamorphic event this behaviour contributes to and in one of the samples controls the observed age dispersal this recognition allows the generation of pressure temperature time points by combining textural and chemical information of monazite with in situ age determinations and pressure temperature information from garnet thus the episodic growth of compositionally distinct monazite throughout a metamorphic event provides the geochronologist with a very valuable chronological tool