impact of inversion tectonics on hydrocarbon entrapment in the baggara basin western sudan abstract the baggara basin of sudan is an ene wsw trending basin located within the west and central africa rift system wcars and nw of the muglad basin the trend of the baggara basin is similar to other basins in eastern chad where oil has been discovered since there are no published studies for the baggara basin the present paper represents an addition to the geology of eastern extension of the wcars in sudan the 2d seismic data interpretation sheds light on the development and evolution of key regional and local structures of the basin the baggara basin is characterized by graben geometry with two boundary faults characterized by dextral strike slip movement and a depocenter at the central part the interpretation reveals the presence of two types of structures within the baggara basin inversion tectonics restricted to the larger basin bounding faults in the western part and en echelon faults in the central part the timing of inversion tectonics is during the eocene and it is related to a brief major compressional event due to the collision of the african and eurasian plates although the inversion created a four way dip closed anticline structure this anticline postdated the initial migration from the source rock accordingly the anticline structural is unsuccessful for hydrocarbon entrapment

oil and gas developments in central and southern africa in 1985 oil industry activity in central and southern africa appears to have stabilized in the last few years but some activities such as field development and exploration acreage acquisition are still declining several countries have advertised open concessions and invited bids only in a very few cases have the offers stimulated the interest of oil companies an exceptional increase in exploration concession acquisition took place in the east africa rift area land seismic acquisition increased mostly in nigeria but 1985 marine seismic activity was half the 1984 activity exploration drilling was higher than in 1984 but development drilling was considerably lower significant discoveries were made in gabon and angola production increased 8 with the largest and most significant increase in angola followed by gabon the production share of non opec versus opec countries remained stable at 33

sedimentological and palyno environmental appraisal of the late quaternary sediments north eastern bornu basin abstract an integrative study has been carried out on some quaternary sediments of the chad formation in tuma 1 and sa 1 wells they are located between 11 n and 13 43 38 n 8 21 49 and 14 40 22 e sedimentological study entailed textural analysis and thin section preparation on the disaggregated samples while the palynological study involves the recovering of the age diagnostic species which gave clues to the paleoenvironmental settings for these sediments the sediments consist predominantly of terrigenous and argillaceous mudstone with thin sandstone interbeds of point bar deposits of laterally migrating moderate to high sinuosity river channels the dominant sandstones are angular to subrounded moderately poor sorted fine modal mineralogically and texturally immature the presence of botryococcus braunii freshwater algae suggests a freshwater lacustrine environment

late cretaceous intraplate silicic volcanic rocks from the lake chad region an extension of the cameroon volcanic line silicic volcanic rocks at hadjer el khamis near lake chad are considered to be an extension of the cameroon volcanic line cvl but their petrogenetic association is uncertain the silicic rocks are divided into peraluminous and peralkaline groups with both rock types chemically similar to within plate granitoids in situ u pb zircon dating yielded a mean 206pb 238u age of 74 4 1 3 ma indicating the magmas erupted 10 million years before the next oldest cvl rocks i e 66 ma the sr isotopes i e isr 0 7021 0 7037 show a relatively wide range but the nd isotopes i e 143nd 144ndi 0 51268 0 51271 are uniform and indicate that the rocks were derived from a moderately depleted mantle source thermodynamic modeling shows that the silicic rocks likely formed by fractional crystallization of a mafic parental magma but that the peraluminous rocks were affected by low temperature alteration processes the silicic rocks are more isotopically similar to late cretaceous basalts identified within the late cretaceous basins i e 143nd 144ndi 0 51245 0 51285 of chad than the uncontaminated cvl rocks i e 143nd 144ndi 0 51270 0 51300 the age and isotopic compositions suggest the silicic volcanic rocks of the lake chad region are related to late cretaceous extensional volcanism in the termit basin it is unlikely that the silicic volcanic rocks are petrogenetically related to the cvl but it is possible that magmatism was structurally controlled by suture zones that formed during the opening of the central atlantic ocean and or the pan african orogeny

aeromagnetic anomalies and tectonic trends in and around the benue trough nigeria four major fracture zones cutting the atlantic sea floor off the west coast of africa st paul s romanche chain and charcot have been delineated offshore using bathymetrie seismic gravity and magnetic data respectively1 but the extrapolation of these fractures onto the african continent is uncertain here we analyse recently compiled aeromagnetic data2 for the benue trough and adjacent areas of nigeria and demonstrate the existence of narrow magnetic lineaments parallel to the margins of the benue trough which can be matched with the oceanic fractures alkaline ring complexes which predate the opening of the atlantic ocean are concentrated along these lineaments suggesting that they represent pre existing zones of weakness in the african crust which may have influenced the rifting of the continental margins and the early development of atlantic fracture zones

differential opening of the central and south atlantic oceans and the opening of the west african rift system abstract plate tectonic studies of the development of the central and south atlantic oceans using seasat and geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time such models enable an initial assessment of the differential opening between these two oceanic basins to be determined the equatorial atlantic is an integral part of this oceanic rifting process allowing stresses arising from the differential opening to be dissipated into both the caribbean and africa along its northern and southern boundaries respectively the tectonic model for the west african rift system based on geological and geophysical studies shows a series of strike slip fault zones diverging into africa from the gulf of guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones the development of the west african rift system was contemporaneous with the early opening of the south atlantic continued to develop well after the final breakup of south america from africa and did not cease until the late cretaceous when there was a major phase of basin inversion and deformation santonian 80 ma deformation across the benue trough nigeria is broadly contemporaneous with dextral shear reactivation of the central african fracture system which in turn resulted in renewed extension in the sudan basins during the late cretaceous and early tertiary this paper illustrates the close linkage in both time and space between the history of the african rift basins and the opening of the atlantic both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the central and south atlantic in order to dissipate stresses generated by this opening the mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics

the causes of apparent overconsolidation in the namak marine deposit korea an intensive experimental study was carried out to investigate the causes of the overconsolidation phenomena in the namak new city site korea geological events in conjunction with sea level changes groundwater fluctuation weathering leaching and cementation were examined as possible causes of overconsolidation in addition to oedometer and triaxial tests radiocarbon dating geochemical components and pore water chemistry were analysed it was shown that the overconsolidation in the upper marine clay deposit was not caused by stress reductions such as erosion and groundwater fluctuation furthermore weathering or leaching seemed unlikely the apparent overconsolidation can be explained by cementation which is indicated by profiles of the sulphate and potassium ion concentrations in the pore water at the early stage of the holocene marine deposition the rate of deposition was 4 4 mm yr thereafter it was reduced to 1 4 mm yr controlling chemical processes such as sulphate reduction and ion exchan

a regional gravity study of the west african rift system in nigeria and cameroon and its tectonic interpretation abstract this study brings together existing and new gravity data to investigate the nature and cause of the bouguer gravity field associated with the cretaceous west african rift system in nigeria and cameroon the new gravity measurements include data collected over the basement area between the benue trough and the cameroon border and fill an important gap in the gravity coverage after removal of the long wavelength negative anomaly from the observed gravity field the remaining positive gravity anomaly associated with the rift system is interpreted in terms of two and three dimensional crustal models these models are constrained by crustal thicknesses derived from a seismic refraction study carried out across and to the south of the yola rift the results of simple three dimensional gravity modelling indicate the crust beneath the lower and middle benue is approximately 20 km thick while beneath the gongola rift the crust is approximately 25 km thick relative to a normal crustal thickness of 34 km away from the rift assuming the thinned crust associated with the rift system is the result of simple lithospheric stretching then the maximum possible crustal extensions of 95 65 and 55 km have taken place perpendicular to the benue gongola and yola rifts respectively these crustal extensions do not necessarily reflect the total crustal movement affecting these rifts since the benue and yola rifts have been subjected to varying amounts of shear displacement during the cretaceous and early tertiary times crustal extension estimates across the west african rift system could be as much as four times greater than the published extension estimates across the east african rift system these rift systems have the same plate tectonic setting being the failed third arms of successful triple junctions and are the result of extensional and shear tectonics the differences in crustal extension and the resulting isostatic response of the lithosphere beneath these rifts can explain why the west african rift system has been associated with subsidence processes throughout its development whereas the east african rift system in northeast africa has been strongly affected by uplift

geometry of propagating continental rifts reconstruction of past plate configurations through palinspastic restoration of continental margins1 4 requires a detailed knowledge of the geometries of horizontal continental extension extension is often restricted to discrete linear zones termed rifts5 6 and rift models that invoke propagating vertical dykes and cracks7 10 are useful for assessing the relative significance and timing of uplift subsidence and extension in the tectonic evolution of rift basins however many models do not adequately account for the observed asymmetry of rifts i describe here the general three dimensional character of young and aborted continental rifts which can be used to derive a structural model for the propagation of rifts in continental lithosphere the rifts become asymmetric as a consequence of the role played by low angle normal faults in the overall rift geometry

displacement geometry in the volume containing a single normal fault fault displacements measured in coal mines and from seismic data are used to develop a model describing the near field displacements associated with an ideal single normal fault displacement on a fault surface ranges from a maximum at the center of the fault to zero at the edge or tip line the tip line is elliptical with the shorter axis of the ellipse parallel to the displacement direction contours of equal displacement form concentric ellipses centered on the point of maximum displacement displacement gradients vary with fault size and with mechanical properties of host rock fault radius to maximum displacement ratios range from 5 to 500 plotting of displacement contour diagrams and knowledge of displacement gradients are useful in interpreting seismic reflectio data both for quality control of interpretations and for quantitative extrapolation of limited data displacements associated with faulting decrease systematically with increasing distance along the normal to the fault surface this decrease is seen as reverse drag in both hanging wall and footwall hanging wall rollover and tilting of the reflectors cannot be used to distinguish listric from planar normal faults even where fault block rotation can be demonstrated neither listric fault geometry nor a flat detachment surface is geometrically necessary because faulting is accommodated by ductile deformation rigid fault bounded blocks cannot exist except in some special circumstances related to a free surface the displacements within the rock volume affected by a single fault are not simply related to regional extension apparent horizontal extension by faulting varies from one layer to another and a significant proportion of the extension in a basin may be due to ductile deformation

facies and porosity 3d models constrained by stochastic seismic inversion to delineate paleocene fluvial lacustrine reservoirs in melut rift basin sudan abstract the paleocene sandstones of the yabus and samma formations are the main oil reservoirs in the melut rift basin of interior sudan in our study area we implemented a multi modeling approach to reveal heterogeneity and reservoir characteristics of this sandstone it estimates acoustic impedance from seismic data using deterministic and stochastic inversion algorithms we integrated petrophysical and geostatistical analyses of the well logs to produce facies and porosity 3d models the study populated and evaluated three different porosity models one from well logs only a second from well logs constrained by inversion results and a third from well logs constrained by inversion results and facies stochastic inversion reveals a significant vertical facies heterogeneity as less as two meters in yabus sandstone this heterogeneity is a lithology dependent and facies distribution is structurally controlled by major faults that developed during the third rift phase the geostatistical integration of seismic inversion sedimentologic and stratigraphic analyses reveal lateral facies distribution in the study area the significant correlation between facies distribution and porosity allows reliable mapping of thin sandbodies the facies and porosity 3d models indicate vertically stacked channels with isolated sandbodies in yabus sandstone this strongly suggests a meandering origin for this sandstone however the thick sandbodies observed in samma formation might indicate a braided channel setting the dominant claystone and mudstone facies deposited in melut formation indicate a shallow lacustrine setting the reservoir quality decreases with depth from the paleocene yabus and samma formations at the top towards the cretaceous melut formation at the bottom the porosity estimates support this conclusion as detected low porosity as less as 5 in melut formation with respect to higher porosity as high as 25 in yabus and samma formations the facies and porosity 3d models enhanced our understanding in predicting the reservoir heterogeneity quality and architecture of this sandstone

extraction of deep crustal reflections from shallow vibroseis data using extended correlation lower crustal seismic reflections can be extracted from shallow crustal seismic profiles through the application of extended correlation to uncorrelated vibroseis seismic data fixed bandwidth extended correlation shortens the correlation operator before crosscorrelation producing reflections over an increased correlation time range all with lowered bandwidth self truncating extended correlation preserves the full bandwidth in the original seismic reflection times but loses bandwidth in a predictable manner at the additional later arrival times correlation wavelet shape and extra correlation time are directly related and can be calculated for specific acquisition parameters pre correlation tapering is necessary to avoid undue wavelet distortion at extended correlation times seismic data collected in the basin and range province illustrate the application of the method the results are verified with conventional correlations of long sweep records and with impulsive source data

the role that plate tectonics inferred stress changes and stratigraphic unconformities have on the evolution of the west and central african rift system and the atlantic continental margins the muglad rift basin of sudan is a good example of polyphase rifting with at least three major phases of basin development each phase has resulted in the generation of source rock reservoir and seal geology with structural traps often closely linked to basement highs in this paper we investigate on a regional scale the tectonic processes that have contributed to rift basin development on a regional scale the evolution of the africa wide mesozoic rift system is intimately linked to relative movements of african sub plates and to global plate tectonic processes and plate interactions changes in plate interactions are observed in the oceanic crust as azimuth changes of fracture zone geometries and by inference have caused significant modifications to both the orientation and magnitude of the motions of the african sub plates such plate motion processes have controlled the polyphase development of the west and central african rift system on the basinal scale changes of sub plate motions have resulted in changes in the stress field which have had a clear impact on the deformation and fault geometries of rift basins and on the resulting stratigraphy the construction of the first unified stratigraphic chart for the west and central african rift system shows a close correlation in the timing of the major unconformities with the timing of changes in relative plate motion as observed in the changes of the azimuthal geometry of the oceanic fracture zones in the central atlantic since similarly timed unconformities exist along the continental margins of africa and south america we propose that the causative mechanism is change in relative plate motion which leads to an increase or decrease in the tension on the plate and thus controls the strength or effective elastic thickness te of the crust plate beneath the margins this results in a focused change in isostatic response of the margin during short period changes in relative plate motion i e more tension will mean that loads are not compensated locally resulting in local uplift of the margin

seismic inversion as a predictive tool for porosity and facies delineation in paleocene fluvial lacustrine reservoirs melut basin sudan abstract the melut basin is a rift basin in the interior sudan linked to the mesozoic cenozoic central and western african rift system the paleocene yabus formation is the main reservoir deposited in heterogeneous fluvial lacustrine environment delineation of channel sandstone from shale is a challenge in reservoir exploration and development we demonstrate a detailed 3d quantitative seismic interpretation approach that integrates petrophysical properties derived from well logs analysis a porosity transform of acoustic impedance inversion provided a link between elastic and rock properties thus we used seismic porosity to discriminate between different facies with appropriate validation by well logs at the basin scale the results revealed lateral and vertical facies heterogeneity in the melut basin good reservoir quality is observed in the paleocene yabus formation the sand facies indicated high porosity 20 corresponding to low acoustic impedance 20000 24000 g ft cm3 s however lower quality reservoir is observed in the cretaceous melut formation the porosity of sand shale facies is low 5 corresponding to high acoustic impedance 29000 34000 g ft cm3 s this suggests that the yabus sandstone is potentially forming a better reservoir quality than melut formation at the reservoir scale we evaluated the facies quality of yabus formation subsequences using petrophysical analysis the subsequences yb1 to yb3 yb4 to yb7 and yb8 to yb10 showed relatively similar linear regressions respectively the subsequence of yb4 to yb7 is considered the best reservoir with higher porosity 25 however subsequence yb1 to yb3 showed lower reservoir quality with higher shale volume 30 this attributed to floodplain shale deposits in this subsequence similarly the high porosity 20 recognized in deeper subsequences yb6 to yb9 is due to clean sand facies we learnt a lesson that appropriate seismic preconditioning exhaustive petrophysical analysis and well log validation are important keys for improved reservoir quality prediction results in fluvial lacustrine basins

geochemical characterisation of early cretaceous lacustrine sediments of bima formation yola sub basin northern benue trough ne nigeria organic matter input preservation paleoenvironment and palaeoclimatic conditions the bima formation is the oldest lithogenetic unit occupying the base of the cretaceous successions in the yola sub basin of the northern benue trough it is differentiated into lower b1 middle b2 and upper b3 bima members integrated organic and inorganic geochemical studies were applied on the lacustrine sediments of the bima formation with the aim of reconstructing the palaeodepositional environment and organic matter input in response to climate and tectonism the analysed sediments were deposited in a freshwater lacustrine environment with a low salinity stratified water column and suboxic to relatively anoxic conditions as indicated by the bulk geochemical parameters the biomarkers provide evidence for a contribution of aquatic algae and microorganisms with a significant amount of terrigenous organic matter input the preservation of the organic matter is possibly attributed to the stratified water column with low salinity and suboxic to relatively anoxic conditions based on the geochemistry of major and trace elements the sediments were deposited during semi arid climatic conditions within passive continental margin setting this agrees with the tectonic events in the west and central african rift system wcars during the early cretaceous period

petroleum geology of benue trough and southeastern chad basin nigeria cretaceous cyclic sedimentation in the southern benue trough together with unconformities provide a tripartite subdivision of the sedimentary succession into 1 the albian asu river group 2 the late cenomanian to early santonian cross river group new name comprising the marine nkalagu formation new name and interfingering marginal marine sandstones and 3 the post santonian coal measures sequence most of the albian to eocene marine shales in the benue trough and the turonian shales in the southern chad basin contain well over 0 5 total organic carbon with values of up to 7 4 in turonian anaerobic shales based on the high content of soluble organic matter thermal maturity and the predominantly terrigenous character of the late cretaceous shales mostly natural gas was probably generated in both basins the late santonian folding and uplift would have disrupted petroleum reservoirs also crude oil accumulations which were not dissipated by tectonism would be relocated at relatively shallow depths and hence become accessible to invading meteoric waters

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

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

depositional environments of late cretaceous gongila and fika formations chad bornu basin northeast nigeria abstract organic geochemical and palynofacies studies of 172 ditch cuttings samples of possible source rock shales from the late cretaceous gongila and fika formations in the chad basin of ne nigeria were carried out to determine their paleoenvironments of deposition although dominated by amorphous organic matter c s ratios and molecular parameters suggest the mostly organic lean shales toc contents typically below 1 5 were deposited in a normal marine environment levels of oxygenation influenced by water depth in the depositional environment appear to control organic richness and quality of the dark grey shales the organic rich toc 2 0 upper part of the fika formation was deposited under anoxic conditions during the late cretaceous and could represent an oceanic anoxic event mature intervals where such conditions prevailed would have generated liquid hydrocarbon although none were sampled here a trend of increasing organic richness towards the central part of the larger chad basin observed in this and other studies supports the development of organic rich marine shales average toc contents of 2 3 of equivalent age in the termit basin where water depth would have been deeper and oxygen conditions at levels that permitted preservation of marine organic matter

the messinian salinity crisis past and future of a great challenge for marine sciences forty years after the image of the mediterranean transformed into a giant salty lake was first conceived the fascinating history of the messinian salinity crisis msc still arouses great interest across a large and diverse scientific community early outcrop studies which identified severe palaeoenvironmental changes affecting the circum mediterranean at the end of the miocene were followed by investigations of the marine geology during the 1950s to 1970s these were fundamental to understanding the true scale and importance of the messinian event now after a long period of debate over several entrenched but largely untested hypotheses a unifying stratigraphic framework of msc events has been constructed this scenario is derived mainly from onshore data and observations but incorporates different perspectives for the offshore and provides hypotheses that can be tested by drilling the deep mediterranean basins the msc was an ecological crisis induced by a powerful combination of geodynamic and climatic drivers which had a great impact on the subsequent geological history of the mediterranean area and on the salinity of the global oceans these changed the mediterranean s connections with both the atlantic ocean and the freshwater paratethyan basins causing high amplitude fluctuations in the hydrology of the mediterranean the msc developed in three main stages each of them characterized by different palaeoenvironmental conditions during the first stage evaporites precipitated in shallow sub basins the msc peaked in the second stage when evaporite precipitation shifted to the deepest depocentres and the third stage was characterized by large scale environmental fluctuations in a mediterranean transformed into a brackish water lake the very high resolution timescale available for some late miocene intervals in the mediterranean makes it possible to consider environmental variability on extremely short time scales including in some places annual changes despite this fundamental questions remain some of which could be answered through new cores from the deepest mediterranean basins improvements in seismic imaging and drilling techniques over the last few decades make it possible to plan to core the entire basinal messinian succession for the first time the resulting data would allow us to decipher the causes of this extreme environmental change and its global scale consequences

determination of widths of meander belt sandstone reservoirs from vertical downhole data mesaverde group piceance creek basin colorado paleohydrologic reconstructions have been applied to downhole measurements made in the u s department of energy s multi well experiment mwx wells in order to derive sand body widths for the nonmarine part of the mesaverde group cretaceous in the east central part of the piceance creek basin of northwestern colorado these sediments were deposited in a predominantly meandering fluvial system paleochannel depths recognized in the wells from the heights of fining upward trends in core can be converted to channel widths using a published formula the resulting channel widths are used to calculate meander belt amplitudes sand body widths from relationships derived from published empirical data the estimate of width derived from this technique can be compared with sa d body widths derived by three other methods these various techniques give compatible results indicating the validity of the downhole technique

onocerane and other triterpenoids in late cretaceous sediments from the upper benue trough nigeria tectonic and palaeoenvironmental implications abstract the aliphatic hydrocarbon bitumen fraction has been investigated in late cretaceous outcrop samples from the upper benue trough inundated by the sea only during the maximum mid cretaceous transgression thermal maturity in nearly all formations is pre oil window indicating only limited mainly tertiary burial the santonian deformation a significant thermal influence to the south west appears to have had little effect here onocerane i is the dominant triterpenoid in the pindiga formation but is absent elsewhere its origin is uncertain but it appears to be associated with deposition in a restricted basin possibly a brackish lagoon receiving dominantly terrestrial organic input a review of possible natural product precursor compounds suggests onoceradiene from a fern like plant was a likely source even carbon number predominance of n alkanes in the range c 16 to c 22 in some formations is tentatively interpreted to indicate hypersalinity

geochemical characteristics of crude oils their asphaltene and related organic matter source inputs from fula oilfields in the muglad basin sudan abstract bulk geochemical characteristics of crude oils coupled with geochemical analyses of reservoir oil asphaltenes from fula oilfields in the muglad basin have provided information on source organic matter input depositional environment and the correlation between these crude oils and their potential source rocks in the basin only one oil family is present within the fula oilfields as indicated by biomarker and non biomarker parameters the fula oils are characterized by moderate api gravity low sulphur and moderate trace metal ni v contents and high wax contents this suggested that the oils were generated from source rock that was deposited in a principally lacustrine environment this is supported by bulk geochemical characteristics and biomarker compositions a variety of biomarker specific parameters obtained from n alkanes regular isoprenoids terpanes and steranes indicate that the fula oils were generated predominantly from algae derived organic matter that was deposited in a lacustrine environment which then experienced a slight seawater influence under suboxic to relatively anoxic conditions these oils were generated from source rock with a wide range of thermal maturity ranging from early to peak oil window the biomarker characteristics of these oils are consistent with those of the early cretaceous abu gabra shales the oil source rock relationship is also demonstrated by similarities in the structural moieties of the fula oil asphaltene and the abu gabra shale kerogens the pyrolysis gc of the analyzed fula oil asphaltene indicates that the oils were derived from type i algal organic matter consistent with paraffinic high wax oils

senonian basin inversion and rejuvenation of rifting in africa and arabia synthesis and implications to plate scale tectonics abstract the late paleozoic to tertiary stratigraphic record of much of the african plate reflects the effects of continental rifting and passive margin development several short lived but widespread and tectonically important compressional or wrench dominated events occurred however during the permian to recent evolution of africa we focus here on the best documented of these events which occurred during the late santonian at that time older sedimentary basins mostly ene wsw trending were folded and inverted including some basins along the tethyan margin from morocco to the syrian arc and the intraplate benue southern chad basins and the lugh mandera basin in oman ophiolites were obducted following the santonian tectonism an extensive phase of rifting occurred in central and north africa and northern arabia spanning from campanian to maastrichtian or paleocene times rejuvenation or acceleration of subsidence occurred in several basins located along the tethyan margin along the atlantic and indian ocean margins as well as within the intraplate domain rifting was sometimes accompanied by magmatic activity especially in offshore northern libya and along the indian ocean margin compressional deformations rejuvenated or developed by latest maastrichtian times along the tethyan margin and locally within the intraplate domain one of the most remarkable attributes of the santonian tectonic event which we consider representative of the major trans african stress field changes is the rapidity with which the change was reflected stratigraphically across north and central africa and its short duration the cause of the santonian compressional event is directly linked to the change in poles of rotation for the opening of the atlantic at 83 85 ma the end of the cretaceous normal magnetic quiet zone this was also synchronous with the obduction of ophiolites along the northeast margin of arabia oman the onset of separation between india and madagascar with formation of the mascarene oceanic basin and the development of the european alpine chain we interpret these phenomena as causally related aspects of a global tectonic event the santonian compressional event as well as the campanian maastrichtian rifting event and the end cretaceous compressional event illustrate the critical connection between intraplate tectonic histories and processes occurring at sometimes very distant plate boundaries

the mediterranean hydrologic budget from a late miocene global climate simulation during the late miocene the mediterranean experienced a period of extreme salinity fluctuations known as the messinian salinity crisis msc the causes of these high amplitude changes in salinity are not fully understood but are thought to be the result of restriction of flow between the mediterranean and atlantic eustatic sea level change and climate results from a new atmospheric general circulation model agcm simulation of late miocene climate for the mediterranean and adjacent regions are presented here the model hadam3 was forced by a late miocene global palaeogeography higher co2 concentrations and prescribed sea surface temperatures the results show that fluvial freshwater fluxes to the mediterranean in the late miocene were around 3 times greater than for the present day most of this water was derived from north african rivers which fed the eastern mediterranean this increase in runoff arises from a northward shift in the intertropical convergence zone caused by a reduced latitudinal gradient in global sea surface temperatures the northwards drainage of the late miocene chad basin also contributes numerical models designed to explore late miocene salt precipitation regimes in the mediterranean which typically make use of river discharge fluxes within a few tens of percent of present day values may therefore be grossly underestimating these fluxes although the agcm simulated late miocene river discharge is high the model predicts a smaller net hydrologic budget river discharge plus precipitation minus evaporation than for present day we discuss a possible mechanism by which this change in the hydrologic budget coupled with a reduced connection between the mediterranean and the global ocean could cause the salinity fluctuations of the msc

deformation of oceanic crust in the eastern gulf of guinea role in the evolution of the cameroon volcanic line and influence on the petroleum endowment of the douala rio muni basin

the petroleum geochemistry of the termit basin eastern niger abstract sixty crude oils from the termit basin eastern niger were analysed using biomarker distributions and bulk stable carbon isotopic compositions comprehensive oil to oil correlation indicates that there are two distinct families in the termit basin the majority of the oils are geochemically similar and characterized by low pr ph pristane to phytane ratios and high gammacerane c 30 hopane ratios small amounts of c 24 tetracyclic terpanes but abundant c 23 tricyclic terpane and lower 13 c values for saturated and aromatic hydrocarbon fractions all of these geochemical characteristics indicate possible marine sources with saline and reducing depositional environments in contrast oils from well dd 1 have different geochemical features they are characterized by relatively higher pr ph and lower gammacerane c 30 hopane ratios higher amounts of c 24 tetracyclic terpane but a low content of c 23 tricyclic terpane and relatively higher 13 c values for saturated and aromatic hydrocarbon fractions these geochemical signatures indicate possible lacustrine sources deposited under freshwater suboxic oxic conditions this oil family also has a unique biomarker signature in that there are large amounts of c 30 4 methylsteranes indicating a freshwater lacustrine depositional environment the maturity of the termit oils is assessed using a number of maturity indicators based on biomarkers alkyl naphthalenes alkyl phenanthrenes and alkyl dibenzothiophenes all parameters indicate that all of the oils are generated by source rocks within the main phase of the oil generation stage with equivalent vitrinite reflectance of 0 58 0 87

petroleum geology of cretaceous tertiary rift basins in niger chad and central african republic this overview of the petroleum geology of rift basins in niger chad and central african republic car is based on exploration work by exxon and partners in the years 1969 1991 the work included 50 000 km of modern reflection seismic 53 exploration wells 1 000 000 km2 of aeromagnetic coverage and about 10 500 km of gravity profiles the results outline ten cretaceous and tertiary rift basins which constitute a major part of the west and central african rift system wcars the rift basins derive from a multiphased geologic history dating from the pan african approximately 750 550 ma to the holocene main rifting was probably caused by the stretching and subsidence of african crustal blocks during the breakup of gondwana in the early cretaceous and by r activated plate movements in the early tertiary wcars in the study area is divided into the west african rift subsystem was and the central african rift subsystem cas was basins in niger and chad are chiefly extensional and are filled by up to 13 000 m of lower cretaceous to holocene continental and marine clastics the basins contain five oil 19 43 api and two oil and gas accumulations in upper cretaceous and eocene sandstone reservoirs the hydrocarbons are sourced and sealed by upper cretaceous and eocene marine and lacustrine shales the most common structural styles and hydrocarbon traps usually are associated with normal fault blocks cas rift basins in chad and car are extensional and transtensional and are filled by up to 7500 m of chiefly lower cretaceous continental clastics the basins contain eight oil 15 39 api and one oil and gas discovery in lower and upper cretaceous sandstone reservoirs the hydrocarbons are sourced by lower cretaceous shales and sealed by interbedded lacustrine and flood plain shales structural styles range from simple fault blocks through complex flower structures the main hydrocarbon traps are in contractional anticlines geological conditions favor the discovery of potentially commercial volumes of oil in wcars basins of niger chad and car

aridity and humidity two aspects of the late miocene climate of north africa and the mediterranean abstract the physiography of north africa is not greatly changed from that of the messinian with the drawdown of the mediterranean in the late messinian the then existing river systems were incised into the landscape and emptied into lake cyrenaica which occupied the substantially drained eastern mediterranean basin the record of incision provides a record of the magnitude of the messinian river systems an analysis of these river systems demonstrates that messinian rainfall a consequence of the zeit wet phase was greatest in east and south central north africa in keeping with the derivation of the water mainly from the indian ocean in central north africa the eosahabi river flowed from messinian lake chad eroding the east tibesti valley and cutting a channel which is especially well preserved near the coast of the gulf of sirt the penetrated sediments of lake cyrenaica and the marginal basins of the messinian transgression are known as the upper evaporites and are generally fine clastics gypsarenites and gypsum anhydrite they were rapidly deposited underlying them is the halite of the lower evaporites this sequence of upper and lower evaporites is much like the lower halite of the tortonian south gharib formation and the overlying clastics and anhydrite of the messinian zeit formation in the gulf of suez red sea area this type of sequence is termed an evaporitic couplet and as has been demonstrated for the gulf of suez red sea is a consequence of a transition from a dry to a more humid climate in a restricted or semirestricted basin this transition occurred at about 7 5 ma in the gulf of suez red sea and at about 5 8 ma in the mediterranean the zeit wet phase manifest in the above events developed in association with the initiation and development of the asian monsoon and the drying of the mediterranean it can be understood in terms of the development of an evolving monsoon desert system the asian monsoon was initiated 8 7 ma in association with the uplift of tibet at this time the north african desert zone was displaced northwards to be over the mediterranean and central and eastern north africa became seasonally humid a concept is advanced in which the early stage of the development of the asian monsoon is seen as having two phases alternating at the precessional ca 21 ka cycle one phase transferred moisture from the indian ocean mainly to southern asia the other phase transferred moisture mainly to north africa with the drawdown of the mediterranean at about 5 8 ma the zeit wet phase intensified a further northward movement of the humid and desert zones occurred at the time of drawdown with the messinian and early pliocene transgressions the wet phase ameliorated but a substantial river system still crossed central north africa at about 4 6 ma north africa became drier probably in response to the developing dominance of the features of the monsoon that transferred moisture mainly to southern asia features of the monsoon that are well recognised today

an integrated remote sensing and gis analysis of the kufrah paleoriver eastern sahara abstract a combined remote sensing optical and radar imagery and gis hydrologic network delineation analysis allows mapping of the kufrah paleoriver of libya and sheds light on its geomorphic evolution during the neogene the kufrah system which is now largely buried beneath the windblown sands of the eastern sahara drained an area of about 236 000 km 2 in central and southern libya the river discharged across a large inland delta to the al jaghbub depression in northern libya and ultimately through the sirt basin to the mediterranean sea radar imagery reveals buried features of the landscape including drainage divides locations of possible stream capture deeply incised valleys and the distal margins of the inland delta previous studies have shown that the kufrah paleoriver is the successor of the sahabi river which drained most of central libya during the late tertiary satellite imagery supports the concept of large scale drainage rearrangement in the quaternary driven by tectonic subsidence that diverted streamflow and sediment discharge away from the sahabi basin toward the inland delta of the lower kufrah basin paleochannels crossing the delta suggest that at various times during the quaternary the kufrah paleoriver either drained externally through the deeply incised sahabi paleochannel to the mediterranean sea or drained internally to paleolakes in the al jaghbub depression thick alluvial deposits on the delta and lake margins likely provided a major sediment source to build the great sand sea which covers the region today the southwestern branch of the kufrah drainage is aligned with an elongated trough that connects to the amatinga river system in chad thus the kufrah watershed may have served as an outlet from megalake chad to the mediterranean sea during humid phases of the neogene if so the combined amatinga kufrah system may have served as one of the proposed natural corridors used by human and animal populations to cross the sahara during the pleistocene these findings hold promise for modeling past lake levels and paleoclimates locating groundwater sources in the region and exploring for reservoirs of oil and natural gas in the region

nonmarine cretaceous palynology of northern kordofan sudan with notes on fossil salviniales water ferns a palynological investigation of 164 samples from 18 water wells in northern kordofan sudan enabled the recognition of five informal zones based on pollen and spore assemblages ranging in age from albian to maastrichtian the youngest late campanian maastrichtian assemblages are restricted to the bagbag basin whereas albian cenomanian to turonian sediments are widespread to the east and west of the bagbag area impressions of salvinia floating leaves from outcrops of the upper hamrat el wuz formation western part of the study area are among the oldest occurrences of this water fern and indicate a campanian maastrichtian age for these sediments

a practical model for organic richness from porosity and resistivity logs a practical method the delta log r technique for identifying and calculating total organic carbon in organic rich rocks has been developed using well logs the method employs the overlaying of a properly scaled porosity log generally the sonic transit time curve on a resistivity curve preferably from a deep reading tool in water saturated organic lean rocks the two curves parallel each other and can be overlain since both curves respond to variations in formation porosity however in either hydrocarbon reservoir rocks or organic rich non reservoir rocks a separation between the curves occurs using the gamma ray curve reservoir intervals can be identified and eliminated from the analysis the separation in organic rich intervals results from two effects the orosity curve responds to the presence of low density low velocity kerogen and the resistivity curve responds to the formation fluid in an immature organic rich rock where no hydrocarbons have been generated the observed curve separation is due solely to the porosity curve response in mature source rocks in addition to the porosity curve response the resistivity increases because of the presence of generated hydrocarbons the magnitude of the curve separation in non reservoirs is calibrated to total organic carbon and maturity and allows for depth profiling of organic richness in the absence of sample data this method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs

palaeogeography of africa during the cretaceous period abstract a first attempt is made to synchronize the major palaeogeographical events of the african cretaceous where recognizable palaeoclimatological interpretations are given an overview of the nature of the continental margins and the plate tectonic setting for africa during the cretaceous provides a convenient backdrop for presenting the palaeogeographical development of the continent lacustrine deposits occur along the line of south atlantic rifting late jurassic oceanic crust in the south atlantic dates from the valanginian with continental volcanism adjacent to the walvis ridge continuing until the albian locally there was a barremian incursion in southeast africa the falkland plateau and walvis ridge areas created shallow water barriers in the aptian which facilitated the massive accumulation of salt tectono eustatic transgressions took place in the middle to late albian cenomano turonian late turonian to coniacian and campano maastrichtian anoxic conditions prevailed during the late jurassic agulhas bank falkland plateau aptian albian the cenomanian turonian and the coniacian these are relatable to the epicontinental transgressional maxima and the heavy influx of organic debris however there was no campano maastrichtian anoxic event

unusual high acidity oils from the great palogue field melut basin sudan abstract crude oils produced from the great palogue field in the melut basin sudan display unusually high total acid number values tan up to 10 4 mg koh g oil this field was discovered in 2002 and produces oils of highly variable 15 31 api gravity the main production comes from the paleogene samma and yabus formations with the upper cretaceous melut formation being a minor contributor the effective hydrocarbon source rocks are most likely within lower cretaceous lacustrine shales deposited during the syn rift stages of the basin s evolution this study addressed 20 crude oil samples from five exploration and appraisal wells in the great palogue field and four oil samples from other oil fields in the basin to represent the various production fluids from the upper cretaceous and paleogene pay zones these oil samples were characterized using routine molecular geochemical techniques to determine the primary geological and geochemical controls on oil acidity based on source and maturity constraints two genetic types of high acidity oils have been recognized from the melut basin the first group of high acidity oils is likely primary high tan oils with a tan value of 0 5 0 99 mg koh g oil they are buried deeper than 2000 m where the temperature is higher than 80 c these include the oils only in the upper cretaceous melut formation of the anbar 1 and palogue south 2 blocks the second group of high acidity oils show higher tan values 1 0 mg koh g oil and various levels of biodegradation these oils represent the dominant oil type in the melut basin occurring mainly in the paleogene yabus and samma formations a multiple phase oil charging model is proposed to explain the horizontal and vertical variation in the oil tan values across the great palogue field

organic geochemical characteristics and depositional models of upper cretaceous marine source rocks in the termit basin niger abstract upper cretaceous marine mudstones are widely known as the most important source rocks in the western african rift basins however geochemical studies on their occurrence and formation mechanism are scarce in this study a detailed geochemical investigation within a sequence stratigraphic framework was carried out to reconstruct the palaeoenvironmental conditions and to establish models on the depositional history of upper cretaceous marine mudstones in the termit basin niger western african a total of six third order sequences dsq1 dsq2 ysq1 ysq2 ysq3 and msq1 from bottom to top were identified in the upper cretaceous the distribution of biomarkers from 33 mudstone samples in different sequences indicate that mudstones from ysq3 were mainly deposited in a suboxic to oxic environment with significant contributions from mixed terrigenous higher plants and lower aquatic organisms in fresh brackish water column in contrast the other sequences ysq2 ysq1 dsq2 and dsq1 mainly represent a suboxic to anoxic marine environment with a stratified water column 0 59

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

identification of source rocks on wireline logs by density resistivity and sonic transit time resistivity crossplots source rock formations generally show a lower density a lower sonic transit time and a higher resistivity than other sediments of equal compaction and comparable mineralogy this phenomenon can be used to identify source rocks on wireline logs provided the source rocks have a minimum thickness within the resolution of the sondes used and are sufficiently rich in organic matter classification rules have been established to assist in the recognition of source rocks on a combination of logs because of the low density contrast between water and organic matter the method becomes inaccurate at high water saturations i e low compaction within limits the amount of organic matter contained in a sediment can be estimated from log anomalies when source rocks become mature free oil is present in addition to kerogen and the resistivity increases by a factor of 10 or more

depositional and structural styles in chad basin of northeastern nigeria seismic reflection data in the maiduguri area of the chad basin reveal a significant juxtaposition of faults and folds the primary structural orientation of which is northeast southwest and parallel to the surface structural pattern in the contiguous benue trough the faults in the basin consist of both basement involved faults and detached faults the folds are simple symmetrical structures restricted to the deeper parts of the basin concomitant compressive structures are notably scarce the structural pattern is complicated further by the presence of intrusions that are presumed to be the subsurface extension of volcanic outcrops that rim the southeastern part of the maiduguri subbasin seismic reflection data also reveal distinguishable seismic sequences that correl te with the established stratigraphic sequence of the basin the apparent structure and the seismically derived stratigraphy suggest a rift origin for the basin furthermore the seismic stratigraphy sedimentologic implications and seismically determined structural features suggest that the maiduguri area of the chad basin has good prospects for hydrocarbon plays in cretaceous rocks with high potentials for both structural and stratigraphic traps

geophysical evidence of cretaceous volcanics in logone birni basin northern cameroon central africa and consequences for the west and central african rift system abstract detailed analyses and interpretation realized by combining existing 2d reflection seismic and gravity magnetic data of the logone birni basin lbb in the west and central african rift system wcas have revealed the distribution of the main buried volcanic bodies as well as their relationships with the structural and tectonic evolution of this basin the volcanic activity in the lbb is restricted to the cretaceous period three main volcanic episodes are identified and are associated to the neocomian late albian and cenomanian turonian rifting phases respectively the volcanic bodies within the lower cretaceous are either lying directly on basement or are mainly interbedded with the contemporaneous sediments whereas the upper cretaceous bodies are morphologically expressed in the forms of dykes and sills the volcanic activity was more intense in the western region of the central lbb zina sub basin along the cameroon nigeria border whereas it was scanty and scattered in the other parts of the basin the main volcanic dykes are found on the flanks of the major faults bounding basement horsts or in crestal positions in association with syndepositional faults although wcas is associated with large amount of crustal extension and minor volcanism the intense volcanic activity observed in lbb during the cretaceous suggests that the intrusive zone during this period was confined to the basement beneath the study area flanked respectively to the north south and southwest by the lake chad poli and chum triple junctions tensional stresses generated by this localized domal uplift accounts for most of the observed tectonic structures where major faults transected the entire lithosphere thus providing conduits for magma migration

sedimentology diagenesis and reservoir quality of the upper abu gabra formation sandstones in the fula sub basin muglad basin sudan abstract successful hydrocarbon exploration of the lower cretaceous abu gabra formation has established the need to understand the diagenesis and reservoir quality of the sandstones reservoirs in the area therefore this study contains a subsurface facies analysis based on description of three conventional cores from the moga21 2 well methods include wireline log analysis a petrographic study with both thin sections and scanning electron microscope investigations grain size x ray diffraction xrd and heavy minerals analysis of the abu gabra sandstone sedimentary composition and facies sequences of the well logs indicate a complex fluvial channel paleo environment grain size analysis revealed that the sandstone is a mixture of mainly sand size petrographically the sandstones are classified based on standard rock classification schemes into subfeldspathic arenites and feldspathic arenite lithofacies quartz grains are the most abundant detrital components in the samples both polycrystalline and monocrystalline followed by feldspars mainly k feldspar micas and detrital and authigenic clays with carbonatic detritus although lithic fragments iron oxides and heavy minerals occur in minor amounts presence of sillimanite epidote and hornblende compared to zircon tourmaline and rutile as well as the presence of chlorite the dominance of polycrystalline quartz and high crystallinity of kaolinite could be responsible for the low grade metamorphism kaolinite is an authigenic component while quartz overgrowths occur in variable quantity and carbonates calcite and siderite in a relatively minor amount pore types are basically primary and secondary interparticle with few secondary intraparticle pores due to the dissolution of feldspars although porosity ranges from 9 20 to 31 20 the presence of kaolinite carbonate and quartz overgrowth cement reduce the porosity of the samples analyzed samples are characterized by poor to very good pore interconnectivity despite the fact that most of the sandstones display point concavo convex and long contacts with poor to good compaction few sutured grain contacts which reflect a higher degree of compaction still exist xrd and sem analyses show that kaolinite chlorite illite smectite and illite smectite mixed layer clay are the common clay minerals in the studied samples the presence of vermiform morphology besides the sharp peak pattern of the kaolinite in most of the xrd pattern indicates that part of the kaolinite was authigenically formed

the role of excess co2 in the formation of trona deposits abstract the prevailing theory for the formation of trona na3 co3 hco3 2 h2o relies on evaporative concentration of na hco 3 water produced by silicate hydrolysis of volcanic rock or volcaniclastic sediments given the abundance of closed drainage basins dominated by volcanics it is puzzling that there are so few trona deposits and present day lakes that would yield dominantly na co3 minerals upon evaporation groundwater in the san bernardino basin southeastern arizona usa and northeastern sonora mexico would yield mainly na co3 minerals upon evaporation but waters in the surrounding basins would not analysis of the chemical evolution of this groundwater shows that the critical difference from the surrounding basins is not lithology but the injection of magmatic co2 many major deposits of trona and na co3 type lakes appear to have had excess co2 input either from magmatic sources or from the decay of organic matter it is proposed that along with the presence of volcanics addition of excess co2 is an important pre condition for the formation of trona deposits

petroleum geology of the southern bida basin nigeria the southern bida basin is located in central nigeria and is a major sedimentary area with a 3 5 km thick sedimentary fill however it is the least understood of nigeria s sedimentary basins because serious oil and gas exploration has not been undertaken in the basin the surrounding precambrian basement rocks experienced severe deformation during the late panafrican phase 600 plus minus 150 m y and developed megashears that were reactivated during the late campanian maestrichtian the ensuing wrenchfault tectonics formed the basin the sedimentary fill which comprises the lokoja formation are chiefly if not wholly nonmarine clastics these have been characterized into facies that rapidly change from basin margin to basin axis and have undergone only relatively mild tectonic distortion subsurface relations of the lokoja formation are postulated from outcrop study the potential source rocks are most likely within the basinal axis fill and have not been deeply buried based on vitrinite reflectance of 0 65 these findings with the largely nonmarine depositional environment suggest gas and condensate are the most likely hydrocarbons alluvial fans and deltaic facies that interfinger with lacustrine facies provide excellent reservoir capabilities potential traps for hydrocarbon accumulation were formed by a northwest southeast trending campanian maestrichtian wrench system with associated northeast southwest oriented normal faults more the traps include strata in alluvial fans fractured uplifted basement blocks and arched strata over uplifted blocks however the size of hydrocarbon accumulations could be limited to some extent by a lack of effective hydrocarbon seal because the dominant seals in the formation are unconformities less

climate driven paleolimnological change controls lacustrine mudstone depositional process and organic matter accumulation constraints from lithofacies and geochemical studies in the zhanhua depression eastern china abstract the influence of paleoclimate on the depositional process of lacustrine mudstone and organic matter accumulation is important to both paleoclimate reconstruction and hydrocarbon exploration here we study the lower third member es3l of the eocene shahejie formation in the zhanhua depression bohai bay basin eastern china in order to understand how paleoclimate influenced depositional processes and organic matter accumulation of lacustrine organic rich mudstone by combining detailed core descriptions and microscopic observations and high resolution mineralogical and geochemical analyses we identified two major lithofacies including massive calcareous mudstone and laminated calcareous mudstone from the luo 69 drilling core the sedimentologic observations and changes of geochemical proxies including detritus index ln al2o3 na2o b ga and v v ni suggest that the massive calcareous mudstone was deposited in a small shallow salt lake that was dysoxic anoxic and the paleoclimate in the lake catchment was cool and arid and the laminated calcareous mudstone was deposited in a large and deep stratified lake which has anoxic highly saline bottom and oxic less saline surface water and the lake catchment was more humid and warm the dominant lithofacies changed from laminated calcareous mudstone to massive calcareous mudstone in the studied core suggesting that the lake became shallower and smaller when the paleoclimate became cooler and drier through time such a climate trend may be a response to global cooling during the middle eocene the average total organic content toc in both lacustrine highstand and lowstand are comparable even though the lake water chemistry and amount of terrigenous input are different we infer that the accumulation of organic matter within the lacustrine highstand was controlled by the combination of primary productivity carbonate production and preservation in anoxic bottom water while accumulation within lacustrine lowstand was only controlled by primary productivity

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

geochemical and petrographic characterisation of organic matter from the upper cretaceous fika shale succession in the chad bornu basin northeastern nigeria origin and hydrocarbon generation potential abstract the upper cretaceous fika shales in the chad bornu basin northeastern nigeria were analysed to evaluate the origin of the organic matter and the hydrocarbon generation potential this study is based on geochemical analyses total organic carbon toc content pyrolysis sra pyrolysis gas chromatography py gc bitumen extraction and biomarker distributions and kerogen analysis of organic matter by microscopy the present day toc contents range between 0 50 and 2 37 wt and hydrogen index hi values are largely below 200 mg hc g toc suggesting that the shales are organically lean and contain type iii and type iv kerogens this is evidenced by palynofacies investigation whereby the kerogen is characterised by large amounts of non fluorescent amorphous organic matter aom and phytoclasts pyrolysis gc analysis proved that all the investigated fika shale samples contain type iii kerogen that produces mainly gaseous hydrocarbons the fika shales have high gas generation potential due to a combination of kerogen type and thermal maturity the fika shales have vitrinite reflectance and tmax values in the range of 0 70 1 34 and 430 465 c respectively indicating early to late oil window maturity thus higher maturity levels have affected the hi of the samples and the hydrocarbon generation potential decreasing both the higher thermal maturity level is attributed to the presence of volcanic intrusives which occur as diorite sills at numerous horizons in the fika formation

petroleum geology of the melut basin and the great palogue field sudan abstract this paper overviews the framework structural and petroleum geology aspects of the melut rift basin in sudan the data base is from mainly proprietary exploration work consisting of more than 20 000 km of seismic profiles 2000 km2 of 3d seismic and more than 50 exploration wells there have been more than 15 oil and oil and gas discoveries the melut basin is a meso cenozoic rift basin accompanied by the formation and development of the central african shear zone casz on the pre cambrian crystalline and metomorphic basement of lower relief three stages of rift development and fracturing have been identified stronger in the early cretaceous and paleogene and weaker in the late cretaceous source rocks are the lower cretaceous lacustrine shales whereas reservoirs and seals are both paleogene and upper cretaceous dominant structural styles are large scale anticlines in the paleogene sequences and antithetic normal fault blocks in the upper cretaceous and paleogene heavy to light oils 8 87 61 5 api and gas have been discovered in the paleogene and only light oils and gas in the upper cretaceous pay zones the primary pay zone in the melut basin is the paleogene the great palogue field with ultimate recoverable oil reserves of more than 900 million barrels was discovered in the northern melut basin the gentle slopes of each subbasin are favorable areas for petroleum accumulation and enrichment the faulted anticlines within the paleogene syn rift sequences are the main trap types whereas the antithetic fault blocks of the upper cretaceous sequences are secondary

clay mineral diagenesis and source rock assessment in the bornu basin nigeria implications for thermal maturity and source rock potential abstract an understanding of the mineral and organic matter modifications during burial are critical in assessments of potential source rocks in petroleum exploration studies here we examined the relationships between clay mineral diagenesis and organic thermal indicators vitrinite reflectance and rock eval tmax to constrain the thermal maturity and hydrocarbon potential of source rocks in the mbeji 1 well in the northeastern part of the cretaceous tertiary bornu basin x ray diffraction mineralogical data show that the

petroleum potential of the reggane basin algeria the intracratonic reggane basin is located on the saharan platform southwest of algeria the basin covers an area of approximately 140 000 km sup 2 extending between the eglab shield in the south and the ougarta ranges in the north although exploration started in the early 1950s only a few wells were drilled in this basin gas was discovered with a number of oil shows the sedimentary fill mainly paleozoic shales and sandstones has a thickness exceeding 5 000 m in the central part of the basin the reservoirs are cambrian ordovician siegenian emsian tournaisian and visean sandstones with prospective petrophysical characteristics silurian upper devonian and to a lesser extent carboniferous shales are the main source rocks an integrated study was done to assess the hydrocarbon potential of this basin tectonic evolution source rocks and reservoirs distribution maturation analyses followed by kinetic modeling and hydrogeological conditions were studied results indicate that gas accumulations could be expected in the central and deeper part of the basin and oil reservoirs could be discovered on the basin edge

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