Deep Jurassic Research Articles

Overpressure of Deep Jurassic System in the Central Junggar Basin and Its Influence on Petroleum Accumulation

Overpressure of Deep Jurassic System in the Central Junggar Basin and Its Influence on Petroleum Accumulation

Seismic Reprocessing Overcomes Uncertainty in Offshore Egyptian Field

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 214066, “Seismic Reprocessing Leads to New Breakthroughs—A Successful Case in the ASH Field, AG Basin in Egypt,” by Mohamed Elokr and Ahmed Lotfy, Kuwait Energy Egypt, and Wei Xing, United Energy Group, et al. The paper has not been peer reviewed. _ The ASH oil field is in the eastern portion of the AG Basin in Egypt. The Lower Cretaceous Alam El Bueib is the main oil-producing formation. Because of the high heterogeneity of the Abu Roash succession, in addition to the influence of thick limestone of the Upper Cretaceous and the influence of multiple complex faults, the quality of seismic data is very poor, requiring seismic reprocessing. Two key techniques were used to achieve the goal: New vertical seismic profile (VSP) well data were acquired to adjust the velocity model, and common reflection angle migration (CRAM) prestack depth migration (PSDM) was used for reprocessing. Introduction The ASH field produces oil from the Aptian Lower Cretaceous reservoir. The ASH structure is interpreted as an elongated east-northeast/west-southwest anticline with intensive breaching on the downthrown side of northwest/southeast to west-northwest/east-southeast-oriented normal faults with obvious signature of synsedimentary activities. The entire area was subject to a series of tectonic events affecting the Western Desert, which led to a complex fault regime clearly recognized in the ASH structure by a series of horizontal displacement across the northwest/southeast faults. Strike/slip to oblique-slip faults also are observed from thicknesses of the cretaceous formations, creating significant difficulty in fault definition. The 3D seismic data acquired in 2007 suffered from losing amplitude of the Lower Cretaceous and deep Jurassic reflectors because of the complex structure. The strong carbonate markers disappear north of the field, where high, dense fault intersection is present closing from the main bounding fault. Loss of seismic amplitude could have severe effects on of the future economics of the field. The seismic survey was reprocessed two times without significant enhancement of the fault imaging. PSDM was run without velocity control, resulting in an uncertain velocity model with inaccurate fault imaging and a mismatching between actual and prognosed depths of the drilled wells of greater than 50 m. This contradiction in reservoir mapping meant that the northern portion of the field remained unexplored. Acquiring VSP in the recently drilled well encouraged reprocessing of the 3D seismic survey using well velocity as a control point in adjusting the seismic velocity model, and application of CRAM technology to improve fault imaging and appraise the uncertain area of the ASH field. CRAM Methodology CRAM was developed as a type of beam migration. Asymptotic ray tracing is performed, assuming that one-way diffraction rays from the subsurface image point to the surface sources and receivers. The takeoff angles from the image point are measured around a given local normal to a background reflection surface for each source ray and receiver ray. A system of source and receiver ray pairs is formed, and the recorded seismic data can be mapped into a local angle domain based on the reflection angles at the subsurface image points.​​​​

Crustal waveguides of sedimentary deposits and conditions for their operation

The relevance of the article is determined by the need to develop the theory of crustal waveguides as a new look at the processes of formation of various hydrocarbon deposits. The purpose – identification of new structures – crustal waveguides – active fluid-saturated zones, within which certain types of traps for hydrocarbons can be formed. Research methods. The article analyzes literature sources that consider the phenomenon of oil formation as a result of the evolution of fluid-saturated zones of crustal waveguides and data from X-ray diffraction and electron microscopic analyzes of layered silicates. Results. The conditions for the operation of crustal waveguides are considered from the standpoint of the presence of free matter, methods of its delivery and energy sources, and features of the formation of their structure. The data of studies of the mineral composition of the rocks of the North-Tazovskaya depression of Western Siberia and the features of their hydrothermal-metasomatic alterations are presented. Minerals and their parageneses have been established, indicating the presence of hydrothermal processes accompanying crustal waveguides. The article argues that the processes of alteration of host rocks in various zones of crustal waveguides lead to the formation of special hydrocarbon traps of an unconventional type, established for deep Jurassic and pre-Jurassic deposits in the north of Western Siberia. The theory of crustal waveguides makes it possible to find the relationship between the formation of oil and gas fields and the geoenergetic aspects of the problem of the removal of intraterrestrial heat in various forms, the main of which is hydrothermal activity. The conclusions summarize the main provisions of the theory of crustal waveguides of sedimentary deposits, the conditions for their operation. The necessary conditions are: the presence of free matter in deep zones, energy sources of self-oscillations of the sedimentary strata and deep faults containing them as a result of unloading the stress field of a special kind, taking into account the depths of their formation. Conclusions are also drawn about the mineral composition of hydrothermal solutions associated with crustal waveguides, which affect host rocks and lead to the formation of a new type of metasomatic traps for hydrocarbons.

Hydrogeochemical characteristics and solute sources of groundwater in the Yuhengbei mining area, Shaanxi Province, China

In mining areas, groundwater resources are crucial for providing drinking water, as well as water needed for plant growth and mining. The Yuhengbei mining area is situated in arid and semi-arid areas in Western China. As there are a number of large-scale mining fields in the area, their exploitation may affect both the quality and quantity of groundwater. The hydraulic connection between aquifers directly determines whether deep coal mining activities affect water quality in shallow aquifers. This study investigates the hydrogeochemical characteristics and the solute sources of groundwater (including surface water, Quaternary water, water from Cretaceous and Jurassic coal strata aquifer) in the Yuhengbei mining area. The study uses conventional, multivariate statistical analysis, and dissolved organic matter (DOM) analysis. The results suggest that all water samples were slightly alkaline, while the water from the shallow aquifers (surface water, Quaternary and Cretaceous aquifers) was predominantly of the HCO3-Ca type. Furthermore, the groundwater in the deep Jurassic aquifer was mostly of the SO4-Na type. Major ion and DOM concentrations appeared to decrease with increasing aquifer depth. Moreover, the groundwater circulation is regulated by natural processes. Namely, rock weathering was the main mechanism controlling the chemical constituents of groundwater in the shallow aquifers, while groundwater quality in deep aquifers was mainly governed by the dissolution of evaporation. Water–rock interaction and cation exchange were the main mechanisms controlling the chemical constituents of groundwater. The R-mode HCA revealed that natural processes controlled the chemical composition of groundwater, as well as that the evolution of groundwater is primarily controlled by the nature of the geological structures and the local hydrogeological conditions. The TOC and UV254 results suggest that the coal strata aquifer had a weak hydraulic connection with the overlying aquifers. The DOM was mainly derived from autogenous biological sources. Based on the indicator values, the hydraulic connection between the deep aquifers and the upper shallow aquifers was weak overall. These results show that in this area, although drinking water and irrigation water come predominantly from shallow groundwater, mining activities temporarily have little impact on the quality of shallow groundwater. Better understanding hydrogeochemical characteristics and formation mechanisms will provide a technical basis for local groundwater management and support the sustainable use of water resources.

Roof-Breaking Characteristics and Ground Pressure Behavior in Deep Jurassic Coal Seams: A Thick-Plate Model and Field Measurements

Roof-breaking characteristics and ground pressure behavior of the coalface are instrumental in guiding deep Jurassic coal seam mining, in particular in the Shaanxi and Inner Mongolia regions of China. A thick-plate mechanical model (TPMM) of the main roof was developed and applied to the case study of 21102 first-mined coalface (FMC) of the Hulusu Coal Mine (HCM) in the Hujirt Mining Area (HMA), China. A theoretical analysis performed via the developed model revealed that the first and periodic breaking intervals of the main roof were 40.6 and 25.0 m, respectively. The roof failure occurred in the tensile mode, was controlled by the internal stress σ x in the rock strata, and started from the center of the long side with the fixed support in the goaf. The field measurement of roof weighting was also performed for the coalface advance from zero to 400 m. The measurement results showed that the first weighting average interval was 41.4 m, and the average interval of periodic weighting was 22.0 m, which agreed with the theoretical calculation and proved the proposed model’s feasibility. Finally, the frequency distribution features of the hydraulic support working resistance in the FMC were analyzed statistically. The results showed that the ZY10000-16/32D supports could adapt to the mining geological conditions of the FMC. However, the margin of the rated working resistance of supports was still small. Thus, roof management enhancement during the mining process was strongly recommended. These research findings could offer theoretical guidance for safe and high-efficiency production in the coal mines under similar geological conditions.

Catagenetic Transformation of the Composition and Structure of Insoluble Organic Matter in Deep Jurassic Sediments in the North of the West Siberian Basin

Catagenetic Transformation of the Composition and Structure of Insoluble Organic Matter in Deep Jurassic Sediments in the North of the West Siberian Basin

Lithofacies classification using Bayes theorem method : Case study Western Desert, Egypt

This paper shows the availability of using the Bayesian classification method to predict class membership probabilities in one of the deep tight reservoirs in Western Desert, Egypt. The workflow of our project that using the Bayesian method used the deterministic petrophysical results of three training wells to train the data and extract the classifiers. The classified data were modeled using Gaussian distribution for each lithofacies. The used wells were acquired from a deep Jurassic gas reservoir in the Western Desert of Egypt. The fitting between actual and modeled data has been reached by minimizing the L2 norm. Besides, a cross-validation process was used for validating the resulted classifiers. Finally, the Bayesian classification method can predict the GWC with an accuracy of 4 m. To avoid probability interference caused by the compacted shale more data should be added to the initial model.

Discovery of the high-yield well GT1 in the deep strata of the southern margin of the Junggar Basin, China: Implications for liquid petroleum potential in deep assemblage

The accumulation assemblages in the deep Jurassic and Cretaceous strata of the southern margin of the Junggar Basin (SMJB) have favorable petroleum accumulation conditions. Due to the huge burial depth, gas/condensate gas reservoirs are expected as the main targets, however, barely any exploration breakthrough has been achieved. The recently drilled well GT1 has obtained a high oil production of >1200 tons/day in the deep Cretaceous strata, which was classified as low gas-to-oil ratio (GOR) light oil. This is the first significant breakthrough in the deep assemblages in the SMJB which brings to the question of its genesis and preservation and the true petroleum potential in the area. In this study, we have presented integrated geochemical analyses including GC × GC-TOFMS, GC-MS and stable carbon isotopes and thus to conclude that the GT1 oil was sourced from Jurassic coal-measure source rocks with maturity of 0.7–0.9%Ro. As only a small amount of diamondoids and ethanodiamondoids were detected, the GT1 oil was classified as normal mature oil well preserved from thermal cracking despite its huge burial depth. The associated gas is coal-type gas with relatively high wetness and the similar maturity to that of the oil. According to the hydrocarbon accumulation analyses, the main source rocks were developed in the Lower-Middle Jurassic with present-day maturity of 1.0%–2.0%Ro. The sandstones developed in the transitional facies in the overlying Lower Cretaceous Qingshuihe Formation were impacted by the tectonic compression and preserved as belt-like distribution in anticline traps. Regional Cretaceous Tugulu Group also overlie these sandstone facies. This forms a favorable source-reservoir-seal assemblage for oil and gas accumulations in the area. Therefore, the deep assemblage in the SMJB is favorable for the accumulation and preservation of large-scale oil reservoirs and shows great potential for liquid petroleum exploration.

Discharge from an elevated ridge to a deep regional aquifer: a case study from Mt. Hermon, Middle East

As is the case for many high-elevation karstic ranges in the world, the water balance of the active Jurassic aquifer in Mt. Hermon is highly speculative. For the latter, this is due mostly to large uncertainty in the infiltration coefficient. Consequently, until recently, when the new deep Shamir wells (up to 1,580 m depth) were drilled, water flow beyond the natural discharge remained unknown. The new wells provide evidence for leakage from Mt. Hermon southward, towards a huge reservoir in the deep Jurassic aquifer. This newly identified thick and confined aquifer extends under the entire Golan Heights and eastward. It is characterized by long residence time, depleted isotopic composition (δ18O and δD of −7.1 and −38.1‰, respectively), low Cl (~18 mg/L) and high SO4 concentrations (~650 mg/L), with elevated δ34SSO4 (~22‰). This composition suggests dissolution of marine gypsum of the Upper Triassic at the summit of Mt. Hermon. Another water source in Mt. Hermon is represented by the SO4-poor (~10 mg/L) Dan Spring, which discharges at the southwestern end of the mountain. Mixtures of these two end members at variable ratios determine the composition of all other Jurassic water sources around Mt. Hermon. The current study exemplifies how geological, hydrological and geochemical insights obtained from well-designed deep wells can dramatically improve the understanding of the flow regime from a high-elevation karstic ridge downstream. The study also has a practical aspect, whereby production from this deep system is mostly at the expense of the huge storage under the Golan Heights.

A Prospective Deep Basin in Southern Papua New Guinea?

ExxonMobil's development of the Hides area to the northwest, and Inter Oil's giant gas discoveries at Elk and Antelope to the east, have revitalized exploration in the intervening area of PPL 319-PRL 13, southern Papuan Basin. With only limited seismic and well data available, the most time- and cost-efficient exploration option for the permit holder was to fly and interpret an airborne gravity and magnetic survey covering the permits and the adjacent surround. After completion of acquisition and processing, the gravity/magnetic data were analysed both qualitatively and quantitatively. Existing seismic data were reprocessed and reinterpreted. We then integrated the results by means of 2D structural models incorporating surface geology, seismic, and subsurface data in order to reach solutions compatible with all data sets. The final interpretation revealed what appeared to be a large, deep Jurassic basin which we have named Kikori Basin. If confirmed, it could be a hydrocarbon kitchen feeding both internal and surrounding prospective fold and fault structures. Several target leads in and around the deep basin were selected for detailing by a new seismic program which is not yet completed.

Geologic evolution of the Iraqi Zagros, and its influence on the distribution of hydrocarbons in the Kurdistan region

Whereas the vast majority of discovered hydrocarbon reserves in Iraq reside in Cretaceous and Cenozoic reservoirs, numerous oil and gas fields have been discovered recently in deeper Jurassic and Triassic reservoirs in the Kurdistan region of Iraq. This study presents a Middle–Upper Jurassic thermal maturity map for the Kurdistan region of Iraq and demonstrates that regional first-order trends in Jurassic source rock maturity show a close correlation to the spatial distribution of oil gravities within the overlying Jurassic (and Cretaceous) reservoirs. This distribution is consistent with compartmentalization of the active source rock kitchens due to Zagros folding, resulting in relatively short-distance migration and charge of the anticlinal structures from the adjacent synclinal lows. The thermal maturity map confirms relatively low maturity over the Mosul high, where the Cretaceous and Cenozoic section overlying the source rock interval is relatively thin, and increasing maturity to the southeast as the thickness of the Cenozoic foredeep sediments increases toward the depocenter located in the southeastern Iraqi Zagros and the adjacent Iranian Zagros. The correlative trend in oil gravities is exemplified by the recent Jurassic discoveries: low to medium gravity oils (14–27° API) in Shaikan and Atrush to the northwest, light oil (39–47° API) in Mirawa and Bina Bawi, and gas condensate (55° API) in Miran West to the southeast. Understanding thermal maturity patterns and hydrocarbon fluid-type distributions will help to guide risk assessment for remaining prospectivity and future exploration drilling within the Kurdistan region of Iraq.

Calibration of a Fractal Model Relating Porosity to Permeability and its Use for Modeling Hydrothermal Transport Processes in the Perth Basin, Australia

Numerical modeling of geothermal systems requires the best possible knowledge of porosity and permeability within the geothermal reservoir. The deep Jurassic Yarragadee Aquifer in the Perth Basin consists of various different lithofacies but is characterized by a high average permeability and the likely occurrence of free convection, making this Aquifer to a promising low-medium enthalpy geothermal reservoir unit.We define a log(permeability) - porosity model based on a combination of the Kozeny-Carman equation and fractal theory and calibrate it by nonlinear regression using a least-squares approach to the Yarragadee Aquifer in the Perth Basin. For the calibration, we use over 100 measurement pairs of porosity and permeability from three boreholes in the vicinity of our model domain. The calibrated model reasonably covers the control of lithofacies on permeability.

Analysis of Reservoir Forming Conditions and Prediction of Continuous Tight Gas Reservoirs for the Deep Jurassic in the Eastern Kuqa Depression, Tarim Basin

Abstract:The exploration targets in the Kuqa Depression at present are mainly structure traps in Cretaceous‐Tertiary. Due to the complexity of mountain distribution and reservoir forming conditions, the exploration of Jurassic in the eastern Kuqa Depression has been in a state of semi‐stagnation since the discovery of the Yinan‐2 gas reservoir. According to the concept and theory of “continuous petroleum reservoirs” and the re‐analysis of the forming conditions of the Yinan‐2 gas reservoir and regional natural gas in the eastern Kuqa Depression, it is believed that the deep Jurassic has good natural gas accumulation conditions as well as geological conditions for forming continuous tight gas reservoirs. The boundary of the Yinan‐2 gas reservoir is not controlled by a structural spillpoint. The downdip part of the structure is dominated by gas, while the hanging wall of the fault is filled by water and forming obvious inverted gas and water. The gas reservoir has the normal temperature and ultrahigh pressure which formed in the near source or inner‐source. All of these characteristics indicate that the Yinan‐2 gas reservoir is different from conventional gas reservoirs. The deep Jurassic in the eastern Kuqa Depression has multisets of source‐reservoir‐cap assemblages, which comprise interbedded sandstones and mudstones. These assemblages are characterized by a self‐generation, self‐preserving and self‐coverage model. Reservoir sandstones and coal measure mudstones are interbedded with each other at a large scale. As the source rocks, Triassic‐Jurassic coal measure mudstones distribute continuously at a large scale and can generate and expel hydrocarbon. Source rocks contact intimately with the overlying sandstone reservoirs. During the late stage of hydrocarbon expulsion, natural gas charged continuously and directly into the neighboring reservoirs. Petroleum migrated mainly in a vertical direction over short distances. With ultra‐high pressure and strong charging intensity, natural gas accumulated continuously. Reservoirs are dominated by sandstones of braided delta facies. The sand bodies distribute continuously horizontal. With low porosity and low permeability, the reservoirs are featured by strong heterogeneity. It is hypothesized that the sandstones of the interior depression tend to be relatively tight with increasing depth and structure stress weakness. Thus, it is predicted that continuous tight gas reservoirs of ultra‐high pressure may exist in the deep formations of the eastern and even the whole Kuqa Depression. So, it is worth evaluating the exploration potential.

Analysis of Reservoir Forming Conditions and Prediction of Continuous Tight Gas Reservoirs for the Deep Jurassic in the Eastern Kuqa Depression, Tarim Basin

Analysis of Reservoir Forming Conditions and Prediction of Continuous Tight Gas Reservoirs for the Deep Jurassic in the Eastern Kuqa Depression, Tarim Basin

Petrophysical assessment of a carbonate-rich caprock for CO2 geological storage purposes

We present an experimental study of the retention/flow properties of the caprock of the Rousse depleted gas field, where up to 120000 tons of CO 2 are to be injected over a 2-year period. One of the caprock samples examined comes from the carbonated breccias overlying the reservoir, a deep Jurassic horst, whereas the other samples come from the 2000 m-thick Upper Cretaceous marl (carbonated turbidites) located above these breccias and the reservoir. The single-phase (brine) intrinsic permeabilities of the marly samples turn out to be extremely low and stress-sensitive. They vary from 20-25 nanoDarcy for the most permeable sample at low stress, to below 1 nanoDarcy at effective stresses in the range of 10 MPa and above. These latter values are lower by at least one order of magnitude than the published values for the caprocks of the Weyburn field and Utsira aquifer. However, the permeability of the brecciated sample is considerably higher, and decreases from ca. 0.1 to 0.003 milliDarcy when the effective stress increases to 20 MPa, which possibly reflects some of the reservoir petrophysical behavior. Gas breakthrough (BT) experiments are undertaken on two of the marly samples fully saturated with brine, using either CO2 or nitrogen as the gas phase. One sample is able to resist CO2 excess pressures as large as 7.6 MPa, whereas in the other more permeable sample a relative pressure of 4.5-6 MPa is enough to induce the BT of nitrogen, a gas with interfacial properties more favourable than CO2 to capillary sealing. Nitrogen flow following BT is extremely limited, with an effective permeability below 1 nanoDarcy. Upon subsequent sample resaturation with brine and then BT with CO2, some alteration of the sealing behaviour is observed, manifested by a decreased BT pressure and an increased gas effective permeability.

ABSTRACT: Diagenetic Controls on Reservoir Quality and Seal Integrity: Deep Jurassic Targets, Norwegian Sea

ABSTRACT: Diagenetic Controls on Reservoir Quality and Seal Integrity: Deep Jurassic Targets, Norwegian Sea

Giant irregular pockmark craters in the Palaeogene of the Outer Moray Firth Basin, UK North Sea

A cluster of irregular pockmark craters of Palaeogene age in block 15/18 in the Outer Moray Firth, UK North Sea have been mapped on 3D seismic data with well control. The craters range from 0.5 to 4 km in diameter, between 50 and 200 m in depth, and have been buried by Tertiary subsidence and sedimentation to present-day depths of between 1000 and 1400 m sub-sea. They are circular to elliptical in plan view, and are located in a delta front and upper pro-deltaic slope position on the Dornoch delta. The structures are confined to the top of the Lower Eocene Balder Formation, and regional mapping across the Moray Firth Basin shows they are restricted to an area of ca. 15 × 15 km 2. Well data show that the craters are sand-filled, and have excavated an argillaceous and tuffaceous unit. The crater-filling sands are sealed by overlying mudstone units. Planar wing-like features emanate from the margins of the craters. Well control indicates that these seismically identified ‘wings’ consist of sandstone, suggesting that the craters have been modified by an episode of sand remobilisation and injection. The proposed mechanism for the formation of these structures is large-scale fluid eruption (probably shallow gas) at the Early Eocene seafloor. These craters are significantly larger in size than those typically observed on contemporary sea floors. We argue that the gas is more likely to have been sourced from deeper Jurassic or Carboniferous sediments within the Witch Ground Graben, than from shallow biogenic origins. These structures rank in scale alongside the largest known pockmarks and demonstrate that natural fluid and hydrocarbon seeps have a long ancestry in this basin.

Wiggins Arch, Southern Mississippi: New Exploratory Data from 3-D Seismic

Newly released data from a 65 mi2 (168 km2) 3-D (three-dimensional) seismic survey on the south flank of the Wiggins arch reopens the question of exploratory opportunity in this sparsely drilled region. Although surrounded by significant production from deep Jurassic and more shallow Cretaceous reservoirs, the Wiggins has not yet been the site of successful exploration. Only nine deep tests, however, exist within an area over 2000 mi2 (5180 km2) in size; thus, previous interpretations of low potential on the Wiggins suffer from limited data. The recently completed 3-D survey, in conjunction with systematic evaluations of existing sample, well log, and 2-D (two-dimensional) seismic information, shifts the focus of attention from the Smackover Formation to overlying Haynesville and Cotton Valley intervals. Data suggest that large-scale oolite shoals exist in the upper Haynesville, immediately below the Bossier Shale. Potential shoals coincide in occurrence with a major northeast-southwest-trending anticlinal structure. Changes in the character of the Haynesville reflector imply significant porosity development, while time structure mapping and isochron thinning in the Haynesville-Cotton Valley interval indicate an important degree of potential entrapment in Cotton Valley sandstones. A well planned for an updip portion of the anticline should yield extremely valuable information for future analyses of Wiggins arch geology.

The Sr‐isotope record of late quaternary hydrologic changes around Lake Frome, South Australia

The Sr-isotope ratios (87Sr/86Sr) of modern brines and salts from the Lake Frome closed-basin are remarkably consistent (0.71126±0.00005 to 0.71206±0.00006). However, gypsum deposited from lake brines during the late Quaternary show a much greater variation (0.71032 ± 0.00007 to 0.71219±0.00010). This variation is due to the changing sources of water and Sr to the Lake Frome basin. The present isotope ratio of the Lake Frome brines and the modern halite crust is similar to the isotopic composition of groundwater from the shallow (Willawortina Formation) aquifer of the adjacent Lake Frome Plains (0.71171±0.00004 to 0.71223±0.00004), reflecting the contemporary importance of this source of Sr and water. Isotope ratios of preserved gypsum deposits greater than those presently observed reflect either an increased importance of the moderate depth (Namba Formation) aquifer (0.71468±0.00004 to 0.71554±0.00007) or a generally increased residence time of groundwater entering the basin at times in the past. Sr-isotope ratios lower than the present brine and modern salts are interpreted to result either from an increased flux of marine Sr (˜ 0.7092) via aerosols or an increased flux of less radiogenic Sr from the deep Jurassic aquifer of the Great Artesian Basin (0.7118). On the basis of the Sr-isotopic composition of the preserved gypsum, periods of high rainfall ('wet') were identified at 3-6 Ka, 12-15 Ka, and > 17 Ka and dry periods at ˜ 10 and ˜ 17 Ka. These results, which are fairly consistent with previous palaeoclimatic inferences, indicate that variations in Sr-isotope ratios of dated authigenic sediments from closed-basin lakes may be used for palaeoenvironmental reconstruction.

Are numerical models useful in reconstructing the migration of hydrocarbons? A discussion based on the Northern Viking Graben

Abstract The 2D-finite-difference code TEMISPACK is used to discuss the history of hydrocarbon (HC) generation, expulsion and migration along a 160 km long transect across the Northern Viking Graben. In the first section, HC generation is discussed based on hypotheses on the crustal heat flow; a kinetic model is used to calculate the maturity. Observations are shown to be consistent with a 67 mW m −2 heat flow, possibly 60 mW m −2 in the graben axis. The precision of the calibrated heat flow is not more than 10%, which results in an uncertainty of the timing of HC generation of 5–10 Ma. The development of overpressures, observed today in the deep Jurassic reservoirs is shown to be essentially controlled by compaction, with a minor contribution due to gas generation; the model also shows that faults related to the Jurassic rifting are most certainly impermeable. In the second section, the previous overpressures and maturity reconstructions form the framework in which expulsion and migration of HC are modelled, based on the two-phase (water, hydrocarbon) Darcy law. The computation explains the filling of the tilted blocks located in the Tampen Spur zone as well as the presence of gas at Troll. A sensitivity analysis shows the importance of poorly constrained parameters such as the relative permeability curves, capillary pressures, or the properties of the petroleum fluids.