Basal Heat Flow Research Articles

Crustal heat production and style of metamorphism: a comparison between two Archean high grade provinces in the Limpopo Belt, southern Africa

Different characteristics of late Archean metamorphism in the Northern and Southern Marginal Zones of the Limpopo Belt in southern Africa are examined in the light of differences between these provinces in contents of heat generating elements. In the Northern Marginal Zone intrusion of charnoenderbitic magmas lasted from 2.75 to 2.58 Ga, accompanied by repeated low to medium pressure granulite facies metamorphism with some anticlockwise P–T paths. In contrast, the medium pressure granulites of the Southern Marginal Zone experienced peak metamorphism at 2.69 Ga without much evidence of plutonism at that time, followed by near isobaric cooling at approximately 6.5 kbar. As a basis for our approach we used steady state geotherms, calculated for the deeply eroded Marginal Zones for late Archean times on the basis of their average K, Th and U concentrations, and assuming low basal heat flow values of 20 mW m −2. The Southern Marginal Zone and the adjacent northern Kaapvaal Craton are both characterised by low average Th and U concentrations (Th: 1.1 and 1.5 ppm, U: 0.3 and 0.65 ppm), which are not a consequence of high grade metamorphism. The average Th and U concentrations in the Zimbabwe Craton (8.8 and 2.2 ppm) are much higher than those in the Kaapvaal Craton, and those found in the Northern Marginal Zone are even higher (13.8 and 2.6 ppm). In the Southern Marginal zone, peak metamorphic temperatures are much above reasonable steady state geotherms and peak conditions could only have been attained through transient very high basal heat flux, which, in the context of a subduction zone, might have resulted from slab breakoff or slab windows. The Northern Marginal Zone in contrast has such a large internal heat generation that high grade metamorphic conditions plot close to steady state geotherms even with basal heat flux values of 20 mW m −2, and minor crustal thickening or basal heat flux increase would have sufficed to exceed fluid absent tonalite solidus temperatures in the lower crust. The repeated high grade metamorphism and charnoenderbite intrusions over a period of 1.7×10 8 years in the province can be explained by this observation. Both marginal zones of the Limpopo Belt can be understood as Archean active continental margins (of the Kaapvaal and Zimbabwe Craton, respectively). While the problems of explaining the metamorphism in the Southern Marginal Zone (i.e. the source of the required heat) are similar to those faced in many present day margins or orogens, the Northern Marginal Zone is non-uniformitarian in the sense that its extreme heat generation rate could not be a property of any geological province today.

Heat flow and thermal history of the South China Sea

A total of 589 heat flow values are available from the South China Sea. The values are widely scattered, ranging from 9 to 181 mW/m 2 with a mean of 77 mW/m 2. In the northern margin, the values are less scattered and the average is about 75 mW/m 2. Heat flow increases gradually from the northern margin to the central basin, in which two high heat flow centers appear, one in the east subbasin and the other in the southwest subbasin. The southern margin has an average heat flow of 80 mW/m 2, similar to the northern margin. The western margin where the Manila trench is located has an average heat flow as low as 49 mW/m 2. In order to understand the high heat flow in the basin in the context of its tectonic history, we carried out thermal modeling using a 2D finite-element method along a profile across the northern margin. A theoretical thermal history has been inferred on the basis of a multistage pure-shear extension model. The result shows that, before Miocene, the basement heat flow showed relatively low values, ranging from 48 to 57 mW/m 2. Since Late Miocene, the basement heat flow has increased as a result of greater extension. The present-day high heat flow is primarily the result of Pliocene extension.

Tectono-thermal modeling of the Yinggehai Basin, South China Sea

Based on the observed data, the average value of surface heat flow in the Yinggehai Basin is calculated and it turns out to be 84.1 mW/m 2 . The thermal evolution of the basin since the Cenozoic era has been attempted by tectono-thermal modeling. Three-phase extension made the basin become hotter and hotter, reaching its climax in paleo-temperature history since 5.2 Ma. And nowadays, the basin is in the heat flow decreasing period. During the Cenozoic era, the basement heat flow remained at 50—70 mW/m 2 all the time. This is related to the degree of each extension phase, stretching rate mode and also the limited basin scale. Modeling results also show that, the surface heat flow is controlled mainly by the basement heat flow, and less than 20% comes from radiogenic heat production in the sediments of the basin.

Recognition of tectonic events in undeformed regions: contrasting results from the Midland Platform and East Midlands Shelf, Central England

Apatite fission-track and vitrinite reflectance data from Central England demonstrate how these techniques can reveal otherwise unrecognized tectonic and/or palaeothermal episodes in apparently tectonically stable areas. The results document the transition from an inverted basinal region in the north (East Midlands Shelf), to a tectonically stable platform in the south (Midland Platform). AFTA data from the region reveal two discrete cooling episodes, in the Early and Late Tertiary. Maximum palaeotemperatures from AFTA and VR data in outcrop samples define a consistent increase from ≤50°C in Lower Cretaceous and Upper Jurassic units in the SE to around 80–90°C in Triassic and older units in the NW. These Early Tertiary palaeotemperatures reflect a combination of deeper burial and elevated basal heat flow. Results from the Rufford-1 well define an Early Tertiary palaeogeothermal gradient of 40.5°C km −1 (32–50°C km −1 at ±95% confidence limits), corresponding to deeper burial by 1450 m of additional section (1.1–2.2 km at ±95% confidence limits), subsequently removed by Tertiary erosion. In contrast, geological considerations suggest a maximum overburden of 800–900 m above the base of the Lower Jurassic in the vicinity of Rugby where palaeotemperatures at outcrop are similar to those near the Rufford-1 location. The discrepancy between stratigraphic and palaeo-thermal reconstruction of former burial depths, often noted in earlier studies, remains unresolved. The Late Tertiary episode is much less well-constrained, but results from Rufford- 1 may require between 910 and 1650 m of eroded section. Thus much of the total amount of removed overburden may have been removed during the Late Tertiary. Results from the Apley Barn Borehole (Oxfordshire) reveal a Late Tertiary palaeothermal episode characterized by a highly non-linear palaeotemperature profile which probably reflects local heating due to passage of hot fluids. Despite stratigraphic evidence for some Early Tertiary erosion results from this borehole show no evidence of Early Tertiary effects. Major Early and Late Tertiary exhumation was limited to regions underlain by older Palaeozoic basins while regions overlying Palaeozoic basement were more stable, experiencing significantly less exhumation. We suggest this reflects the preferential reactivation of the weaker basinal regions as a result of compressional events at plate margins. Our results emphasize the importance of incorporating results from both ‘inverted’ and ‘non-inverted’ areas in understanding the causal mechanisms of uplift and inversion, and highlight the importance of testing apparent stability using palaeo-thermal methods.

Heat flow evolution, subsidence and erosion in the Rheno-Hercynian orogenic wedge of central Europe

Abstract Numerical, thermal and rheological modelling techniques are applied to unravel the basin-forming processes and the heat flow and burial history of the Rheno-Hercynian fold belt (Rhenish Massif), the adjacent Subvariscan foreland (Ruhr Basin), and the intramontane Saar-Nahe Basin. Thermal history and crustal architecture in the study areas were affected mainly by the Variscan Orogeny during late Palaeozoic times. Calibration of the simulated thermal histories is primarily based on vitrinite reflectance and fission-track data. Mechanical modelling reveals average β values of 1.7, reaching a maximum of 2.4 in the central basin (Mosel Graben) and at the transition to the Giessen Ocean to the south during Early Devonian rifting. This stage was associated with tholeitic magmatism and an elevated heat flow of up to 110 mW m −2 , preserved in weakly overprinted syn-rift sediments. Average basal heat flow during maximum burial at the end of the Carboniferous period (i.e. the end of crustal shortening) was between 50 and 70 mW m −2 with a slight decrease from the Subvarian foreland basins towards the Rheno-Hercynian in the south. The values suggest average crustal thicknesses of between 32 and 36 km during late Carboniferous time. For the Saar-Nahe Basin, values between 50 and 75 mW/m 2 represent the thermal regime in the upper crust during the late Stephanian and early Permian time. Estimated eroded thicknesses of Palaeozoic sediments vary between 2500 m in the northern and central Ruhr Basin and more than 6000 m in the Osteifel and the Siegen Anticline within the Rheno-Hercynian, and between 1800 and 3600 m in the Saar-Nahe Basin. Fission-track data provide evidence for significant reheating during the Mesozoic era within the entire study area. This phase of heating, probably linked to North Atlantic rifting, coincides with post-Variscan ore formation and with major tectono-magmatic events in central Europe.

Random modelling of the lithospheric thermal regime: forward simulations applied in uncertainty analysis

A random modelling technique was applied in uncertainty analysis of forward geothermal modelling of the lithospheric thermal regime. We present results for estimating the effects of uncertainties in thermal conductivity, heat production rate, model basal temperature and basal heat flow density on calculated lithospheric temperature and heat flow density (HFD). We analysed two models: first a 4-layer synthetic model representative of typical shield conditions with thick crust and lithosphere, and second a two-dimensional case history from the Fennoscandian (Baltic) Shield. Thermal conductivity (normally distributed) and heat production (log-normally distributed) as well as temperature or heat flow density (normally distributed) used as the lower boundary condition in the mantle were randomly varied in the simulations. Calculations based on 1500 independent cases of the layered model indicate, for instance, that a standard deviation (STD) of 50 K in calculated Moho temperature results with uncertainties either in thermal conductivity of about ±0.5 W m −1K −1, in heat production rate of ±0.2 log A ( A in μW m −3) or ±115 K in basal temperature, but only ±2 mW m −2 in basal heat flow density. Respectively, the same values result in an uncertainty of ±2–10 mW m −2 in calculated surface heat flow density. If conductivity and heat production rate are varied simultaneously, the resulting uncertainty in calculated Moho temperature increases to about ±70 K. Adding also basal temperature variation increases the Moho temperature variation to about ±85 K. Results calculated with the two-dimensional transect in the Baltic Shield indicate analogously that uncertainty in temperature at 50 km depth (approximately at Moho) is ±35–60 K (using temperature as the lower boundary condition) and ±50–85 K (using heat flow as lower boundary condition). The corresponding variations in surface heat flow density are ±6–15 mW m −2. The choice of the lower boundary condition has an essential effect on the variation of mantle temperatures, and models using heat flow density as a lower boundary condition yield higher uncertainties in calculated mantle temperatures than those based on temperature as the lower boundary condition.

Early Tertiary heat flow along the UK Atlantic margin and adjacent areas

The sedimentary basins of the UK North Atlantic margin are characterized by extensive igneous activity, mostly of Early Tertiary age, related to continental rifting which led to the separation of Europe from Greenland. Given this setting, it might be expected that elevated basal heat flow during the Early Tertiary would have exerted a critical control on the thermal histories of potential hydrocarbon source rocks. Apatite fission track analysis and vitrinite reflectance data have been used to identify, characterize and quantify significant palaeothermal events which have affected the region. This approach allows reconstruction of thermal histories based on directly measured data, rather than relying on theoretical models. Results from the West of Shetland region show significant Tertiary heating, but this is often characterized by low palaeogradients or non-linear palaeotemperature profiles, suggesting heating caused by fluid movements. No evidence has been observed of enhanced Early Tertiary heat flow, and little evidence has been found to support significantly deeper burial during the Early Tertiary. Results from Hebridean basins also argue against a significantly increased Early Tertiary heat flow, while showing strong evidence for Late Tertiary uplift and erosion. Consistent evidence for increased Early Tertiary heat flow is seen to the south, in the Lake District and Solway basins. This may reflect the ability of extensive igneous activity to release heat from the base of the crust to shallower levels throughout the Atlantic margin and the Tertiary igneous province, whereas in the absence of igneous activity, the lack of a pathway for heat to escape results in elevated basal heat flow. Models based on high Early Tertiary heat flow are likely to over-predict maturity levels in Jurassic source rocks, and predict an earlier timing for hydrocarbon generation. Both factors could lead to false assessment of regional hydrocarbon prospectivity.

Influence of topographically driven convection on heat flow in the Swiss Alps: a model study

A 2-D thermohydraulic model for two sites in Switzerland is presented. Geologically both sites belong to Alpine nappe units and consist of generally low permeable rocks with hydraulic conductivities in the range of 10 −10–10 −8 m s −1. Elevation of topography attains 2.4–3.1 km, and the mountain water table exhibits a correspondingly high relief. The geothermal regime of the area is characterized by relatively high values of measured heat flow on the order of 80–100 mW m −2. The results reveal well developed subsurface fluid circulation systems in both cases. The vertical fluid velocities reach some cm/yr, which is higher by more than one order of magnitude than the flow rates in flat terrain. In mountainous terrain, flow systems may penetrate to depths of 2–3 km below sea level. The discharge zones in areas with high relief and steep slopes are rather narrow and concentrate at and near the valley bottoms. Such zones are the regions of heating where positive disturbances may reach up to 1.5–1.8 times the basal heat flow value. In the recharge areas at ridges, heat flow may in turn become quite low. The calculated thermohydraulic models have been validated by measured temperature distributions in drillholes. Reasonably good agreement between the measured and the calculated temperatures exists for both sites.

Beyond surface heat flow: An example from a tectonically active sedimentary basin

Thermal anomalies that have important geodynamic implications may not always be recognizable in present-day surface heat-flow patterns. The masking occurs because surface heat flow responds to mantle heat, crustal radioactivity, magmatism, crustal deformation, burial and/or exhumation, and fluid movement, any of which may offset the thermal effects of the others. Sedimentary basins are particularly suited to partitioning heat flow into its various components. We use Taranaki basin, New Zealand, as an example. It has a relatively undeformed (since the Miocene) western region that is used as a control against which the tectonically active eastern region can be compared. Although surface heat flow is roughly constant across Taranaki basin, basal heat flow modeled at lower crustal–upper mantle depths varies by a factor of two or more. A combination of low heat-producing crust and the heat sink effects of crustal thickening in the eastern region can account for the basal heat-flow anomalies. The tectonic thermal anomaly would have gone unnoticed without the aid of detailed basin analysis and thermal modeling.

The permeability of young oceanic crust east of Juan de Fuca Ridge Determined using borehole thermal measurements

Temperature measurements made in Ocean Drilling Program (ODP) Hole 1026B, drilled into a sediment‐buried basement ridge in 3.5 m.y. old crust on the eastern flank of Juan de Fuca Ridge, revealed fluid flow from the formation and into the overlying ocean. This flow indicates that the upper basaltic crust at this site is naturally overpressured relative to hydrostatic. This finding is consistent with output from previously published numerical models of hydrothermal circulation in the upper crust that include appropriate crustal geometry, sediment thickness, and basal heat flow. The fluid flowing into Hole 1026B enters the hole through the upper 10 m of basalt below the sediment‐basement contact. The flow rate estimated from the thermal data (80–120 m/hr), in combination with the inferred basement overpressure relative to hydrostatic (about 20–30 kPa), was used to estimate the bulk permeability of the shallowest oceanic crust: 5 to 9 × 10−12 m². Such high bulk permeability would allow regionally significant hydrothermal circulation, consistent with thermal homogenization of upper basement in this area.

Coal rank variations with depth related to major thrust detachments in the South Wales coalfield: implications for fluid flow and mineralization

Abstract Coal maturity data in the form of volatile matter (daf and dmmf) and random vitrinite reflectance have been analysed for the South Wales coalfield. They show that in general coals increase in rank with depth, obeying Hilt’s law, and increase in rank laterally from high volatile bituminous coals in the south and east of the coalfield to anthracite in the northwest of the coalfield. The lateral increase in rank does not coincide with the basin depocentre which was located to the southwest of the coal basin during Westphalian times. The rank pattern with depth in the Westphalian A-Lower Westphalian C Coal Measures of the eastern half of the coalfield suggests a palaeogeothermal gradient of approx. 310°C km −1 , equivalent to a basal heatflow of 295 m Wm −2 . Investigation of vitrinite reflectance in a coal sequence repeated by intense Variscan thrusting indicates that coal rank was acquired both pre- and syn-thrusting. Detailed analysis of the volatile matter data reveals the presence of excursions from Hilt’s law present in one or more coal seams close to the boundary between Westphalian A & B. Of the 154 data sets analysed from the coalfield, 94 (61%) show one or more excursion. It is shown that the excursions correlate with thrust detachments within the coal seams, and it is argued that the excursions represent an increase in maturity temperature caused by fluids carrying heat into the coal seam along the seismically active thrusts. The fluids may also have been responsible for the carbonate, oxide and sulphide mineralization of the coalfield. Preliminary comparisons with the Ruhr coal basin in Germany suggest that future studies involving computer generated thermal models are required to understand the thermal evolution of both basins.

Palaeoclimate and structure: the most important factors controlling subsurface temperatures in crystalline rocks. A case history from Outokumpu, eastern Finland

SUMMARY The present work contributes to the study of heat-transfer mechanisms in crystalline bedrock. We present evidence from a thoroughly investigated case history in Outokumpu, eastern Finland, in the Fennoscandian (or Baltic) Shield, which shows that the subsurface temperature field is controlled by the thermal conductivity structure and downward diffusion of palaeoclimatic ground-temperature variations. The subsurface temperature profiles from three continuously cored boreholes (790-1 100 m deep) were used for a detailed 2-D modelling of structural, hydrogeological and palaeoclimatic effects on the subsurface temperature field. The boreholes are situated in a subdued topography and they intersect Early Precambrian folded lithologies, such as mica gneisses, black schists, skarn rocks, quartzites and serpentinite-talc rocks. Finitedifference techniques were used in the numerical solution of the heat- and masstransport equations. The 2-D models of thermal conductivity and hydraulic permeability that were compiled were based on an extensive set of data relating to geological structure, in situ hydraulic permeability, groundwater composition and thermal conductivity of drill-core samples down to about 1 km depth. It was found that the thermal effect of topographically driven groundwater convection is very small, with Peclet numbers typically of the order of 10-4-10-5. The effect of anisotropy of thermal conductivity was found to be one order of magnitude smaller than the effects of heatflow refraction in the inclined rock layers which produce horizontal components of heat-flow density ranging from - 15 to 5 mW m-' (basal heat flow 35 mW m-'). Since the advective heat transfer could be neglected, we tried to find a palaeoclimatic groundtemperature history that would explain the measured data. First, an inversion algorithm based on heat conduction in laterally homogeneous media was used for the reconstruction of palaeoclimatic ground-temperature history, but it yielded spurious results because of the strong 2-D conductivity effects. The only means to reconstruct the past ground-temperature changes was forward modelling using a conductive transient 2-D model with time-dependent surface-temperature variations. The surface temperatures at different times (between 100000 yr BP and the present) and the basal heat-flow density were varied in order to reach a reasonable fit between the modelled and measured borehole temperatures. To reach this goal, the ground-temperature history must include the main climatic events during the last 100000 yr, such as the last glacial epoch and the Little Ice Age (from about 1300 AD to 1700 AD), followed by warmer temperatures in more recent times. Our results suggests that subsurface temperatures in conditions similar to those of the Outokumpu case can yield a wealth of palaeoclimatic information when an appropriate approach for modelling heat transfer in the bedrock is chosen.

Impact of seafloor sediment permability and thickness on off‐axis hydrothermal circulation: Juan de Fuca Ridge eastern flank

Sediment permeability plays a key role in controlling both the flux of solute and heat through marine sediments and patterns of hydrothermal circulation within the underlying oceanic crust. Interlayered silt‐rich and clay‐rich sediments almost completely bury basement on the eastern flank of the Juan de Fuca Ridge. Laboratory measurements indicate that silt‐rich sediment permeability is 1 to 2 orders of magnitude greater than clay‐rich sediment permeability at similar depths. Numerical simulations of coupled fluid flow and heat transfer using a simplified model of a sedimented ridge flank with smooth basement topography illustrate how differences in permeability between the two sediment types can influence patterns of hydrothermal circulation and the flux of heat and solute across the seafloor. The layered structure of the model domain is inferred from reflection seismic and seafloor heat flow data. Two distinct patterns of hydrothermal circulation are obtained, depending on whether silt‐rich or clay‐rich sediments compose the sediment layer above a permeable upper basement aquifer. A clay‐rich sediment column nearly isolates circulation within the basement aquifer, resulting in closed convection. Computed seafloor heat flow profiles resemble heat flow measurements made on the eastern flank of the Juan de Fuca Ridge. Computed vertical fluid fluxes across the sediment column are small, in agreement with fluid fluxes estimated from sediment geochemical profiles. When the sediments are silt‐rich, 17% of the flow within a convection cell crosses the sediment column. Open convection where basement topography is smooth is probably rare because sediment columns overlying oceanic crust are unlikely to be entirely silt. Each pattern of convection persists over a wide range of sediment thicknesses for a given sediment permeability and basal heat flow. A transition from open to closed convection occurs when the effective permeability of the sediment column is only slightly less than that of an entirely silt‐rich column. Thus the addition of only a small amount of clay to an otherwise silt‐rich sediment column may effectively isolate the crustal aquifer from the ocean.

Effects of strongly variable viscosity on three‐dimensional compressible convection in planetary mantles

A systematic investigation into the effects of temperature dependent viscosity on three‐dimensional compressible mantle convection has been performed by means of numerical simulations in Cartesian geometry using a finite volume multigrid code, with a factor of 1000–2500 viscosity variation, Rayleigh numbers ranging from 105–107, and stress‐free upper and lower boundaries. Considerable differences in model behavior are found depending on the details of rheology, heating mode, compressibility, and boundary conditions. Parameter choices were guided by realistic Earth models. In Boussinesq, basally heated cases with viscosity solely dependent on temperature and stress‐free, isothermal boundaries, very long wavelength flows (∼25,000 km, assuming the depth corresponds to mantle thickness) with cold plumes and hot upwelling sheets result, in contrast to the upwelling plumes and downwelling sheets found in small domains, illustrating the importance of simulating wide domains. The addition of depth dependence results in small cells and reverses the planform, causing hot plumes and cold sheets. The planform of temperature‐dependent viscosity convection is due predominantly to vertical variations in viscosity resulting from the temperature dependence. Compressibility, with associated depth‐dependent properties, results in a tendency for broad upwelling plumes and narrow downwelling sheets, with large aspect ratio cells. Perhaps the greatest modulation effect occurs in internally heated compressible cases, in which the short‐wavelength pattern of time‐dependent cold plumes commonly observed in constant‐viscosity calculations completely changes into a very long wavelength pattern of downwelling sheets (spaced up to 24,000 km apart) with time‐dependent plumelike instabilities. These results are particularly interesting, since the basal heat flow in the Earth's mantle is usually thought to be very low, e.g., 5–20% of total. The effects of viscous dissipation and adiabatic heating play only a minor role in the overall heat budget for constant‐viscosity cases, an observation which is not much affected by the Rayleigh number. However, viscous dissipation becomes important in the stiff upper boundary layer when viscosity is temperature dependent. This effect is caused by the very high stresses occurring in this stiff lid, typically 2 orders of magnitude higher than the stresses in the interior of the domain for the viscosity contrast modeled here. The temperature in the interior of convective cells is highly sensitive to the material properties, with temperature‐dependent viscosity and depth‐dependent thermal conductivity strongly increasing the internal temperature, and depth‐dependent viscosity strongly decreasing it. The sensitivity of the observed flow pattern to these various complexities clearly illustrates the importance of performing compressible, variable‐viscosity mantle convection calculations with rheological and thermodynamic properties matching as closely as possible those of the Earth.

Thermal History of the 1.1-Ga Nonesuch Formation, North American Mid-Continent Rift, White Pine, Michigan

The Nonesuch Formation, a black siltstone, was deposited in the Lake Superior portion of the mid-continent rift system during middle Proterozoic rifting. Despite its age of nearly 1.1 Ga, the Nonesuch Formation contains liquid petroleum and solid pyrobitumen. Numerical modeling techniques were used in this study to constrain the thermal history of the Nonesuch Formation at White Pine, Michigan. Thermal modeling addresses two issues: (1) the utility of illite-smectite expandability as a limiting parameter for describing the thermal history of ancient (Proterozoic) sedimentary rocks, and (2) the potential for hydrocarbon maturation within the Nonesuch Formation at White Pine. A range of potential burial histories for the Nonesuch Formation was constructed based on geologica evidence. Time-temperature histories were calculated for each burial history model assuming one-dimensional, transient, conductive heat flow. Results of numerical time-temperature models were then evaluated on the basis of organic and inorganic thermal maturity indicators including Rock-Eval® pyrolysis data and biomarker indices (correlated to equivalent vitrinite reflectance values), in addition to illite-smectite expandability. Illite-smectite expandability values appear generally consistent with measured organic thermal maturity values. The preferred set of cases also produces calculated illite-smectite expandability and vitrinite reflectance values consistent with thermal maturity indicators (that do not include vitrinite) collected in the field. Modeling results demonstrate tha a combination of elevated basal heat flow and rapid burial during the Proterozoic is required to produce the observed thermal maturities at White Pine, Michigan. More specifically, modeling indicates that maximum temperatures for the Nonesuch Formation, 110 to 125°C, were reached at about 1075 Ma, coincident with a maximum burial depth of about 6 km, although 4 km represents a more plausible value. The results support a thermal history consistent with in-situ oil generation at White Pine; however, thermal modeling alone cannot rule out the possibility that oil was generated and expelled elsewhere and migrated into the area with fluids expelled during an episode of postrift compression.

Development of Sediment Overpressure and Its Effect on Thermal Maturation: Application to the Gulf of Mexico Basin

High sedimentation rates can potentially lead to overpressuring and sediment undercompaction within basins. Sediments with anomalously high porosity, in turn, induce low thermal conductivities and so tend to act as a thermal insulator to the flow of heat. In the Gulf of Mexico basin (Gulf basin), the generation of overpressure is caused mainly by the inability of pore pressure fluids to escape at a rate commensurate with sedimentation. We modeled the generation and dissipation of abnormal sediment pore pressure due to variations in sedimentation rate, facies, formation porosity, and permeability within the Gulf basin using finite-element techniques to solve the differential equations of both heat and fluid transport within compacting sediments. We assume that the porosity effective stress relationship within the sediment follows a negative exponential steady-state form when the pore pressure is hydrostatic. An important feature of our modeling approach is the assumption that sediments are incapable of significant expansion in response to increasing pore pressure. Sediments are assumed to hydrofracture when the pore pressure approaches the lithostatic pressure, rather than a common assumption of porosity expansion even in lithified sediments. From our modeling, we conclude that significant overpressures have been created (and dissipated) at various times within the Gulf basin and track, in general, the west to east migration of sediment loads deposited since the Cretaceous. Although predicted overpressures of more than 0.75 kpsi (i.e., an equivalent excess hydraulic head of 500 m) of Campanian-Maastrichtian age remain to the present day, the main phase of overpressure development in the Gulf basin is predicted to have occured during the Miocene-Holocene. Maximum overpressures (~13.6 kpsi; excess hydraulic head of 9.4 km) are predicted for the present day. Overpressure development during the Miocene-Quaternary, a consequence of rapid sediment deposition associated with the Mississippi delta system, is also predicted to be associated with undercompaction. This undercompaction led to increased temperature gradients during the Miocene and Quaternary despite the fact that the anomalous basal heat flow engendered by extension had practically dissipated. We further predict that by the end of the Neogene, temperatures would have been approaching s eady state over broad regions of the Gulf basin implying that the highest temperatures occur in the deepest parts of the basin. In contrast, during the Quaternary, the rapid progradation of overpressured and undercompacted sediments resulted in a thick section that has yet to reach thermal equilibrium and thus is anomalously cold with respect to its present depth. The predicted vitrinite reflectance indicates that for most of the Gulf basin history, the depth to the top of the oil window remained at approximately 2.5±0.5 km below sea floor (bsf). Similarly, the depth to the base of the oil window ranged from 3.5 to 6.5 km bsf. This relatively constant position of the top of the oil window defines a maturation front that propagated from the offshore into the End_Page 1367------------------------------ onshore regions of the northern Gulf basin as a function of time. As such, hydrocarbon generation is predicted to have occurred continuously within the Jurassic and Cretaceous sections of the onshore region during the entire Cenozoic. Prior to this, maturation fronts within each of the onshore basins resulted in maturation of Upper Jurassic source rocks during the Early Cretaceous. In the offshore Gulf Coast area, pre-Tertiary source rocks are predicted to be overmature for liquid hydrocarbons at present. In the offshore regions affected by Quaternary sedimentation, the depth to the top of the oil window has been significantly depressed in response to sediment loading and subsidence.

History of hydrocarbon generation in the Tembungo field, offshore northwest Sabah

The Tembungo field in the Sabah Basin produces oil from upper Miocene turbidite reservoirs. The oils, possessing low sulphur and wax contents and API gravities of 38-40°, were derived from marginal marine source rocks subject to significant land plant input. The history of hydrocarbon generation in the Tembungo area including burial, fluid pressure and fluid-flow, thermal history, hydrocarbon generation and migration has been studied using a two-dimensional finite difference basin modelling approach. Backstripping exercises suggest that high sedimentation rates occurred during middle to late Miocene. The Tembungo structure itself began to grow in late Miocene (7.2 Ma), with an accelerated growth rate in early Pliocene. A constant basement heat flow of 55 m Wm-2 was determined for the area. The faults are known to be sealing, with very low associated permeabilities. The presence of barrier faults contributed to the development of overpressure in the area. The contribution of hydrocarbon generation to the overpressure is considered less significant. Maturity reconstruction based on kinetic models indicates that hydrocarbon generation began approximately 9.0 Ma and oil began to be trapped in the Tembungo structure in late Miocene to early Pliocene. These oils were most likely sourced from middle Miocene sediments. Sensitivity analysis of transient state versus steady state pressure calibration reveals different histories of hydrocarbon generation and migration.

A simple statistical method for correcting and standardizing heat flows and subsurface temperatures derived from log and test data

Data from subsurface temperature measurements provide widely used and vital input data for maturity modeling_ Because maturity calculations are very sensitive to thermal history, and because reconstruction of the thermal history begins with the modern temperature profile, accurate knowledge of true formation temperatures is vital. Temperature data used in maturity modeling come from a variety of sources, including BHTs derived from single logging runs, BHTs obtained from multiple logging runs at the same depth and corrected using the Horner plot method, RFTs, DSTs, and production tests (PTs)_ However, temperatures obtained using most of these techniques require some correction before they represent true formation temperatures. Unfortunately, the need for these corrections is not generally recognised, leading most modelers to consistently underestimate modern subsurface temperatures_ Such errors can lead to major errors in subsequent calculations of hydrocarbon generation and cracking, and can thus have profound effects on exploration decisions_ In an effort to evaluate the accuracy of data from single logging runs, Horner plots, RFTs, and DSTs or PTs, an extensive temperature data base was developed for the Malay Basin. Basal heat flows calculated for many wells using each type of temperature data were compared. It was found that all other temperature data considerably underestimated subsurface temperatures compared to DSTIPT data, which were assumed to represent true formation temperatures. Results were analyzed using two different statistical approaches, which gave quite consistent conclusions, and average correction factors for each type of temperature data were developed. To be equivalent to heat flows calculated from DSTI PT temperatures, heat flows calculated using single BHT data already subjected to a standard 10% correction had to be corrected upward by an additional 16%, those calculated from Horner plot extrapolations by an additional 14%, and those obtained from uncorrected RFT data by 9%. Measured subsurface temperatures were corrected using a more complex set of equations that take surface temperature (T.) into account. The corrected subsurface temperature To is given by one of the following formulas, where Tb is the uncorrected temperature from a single logging run, Th is the extrapolated temperature obtained from a Horner plot, and Tr is the uncorrected RFT temperature. To = (1.1 eTb - T.)e1.16 + T. To = (Th - T.)e1.14 + T. To = (Tr - T.)e1.09 + T. Although these correction factors were developed for the Malay Basin, evidence presented by other workers suggests that corrections are needed in other basins as well, and that the magnitude of the corrections suggested here is reasonable for other areas. Future work should test these hypotheses and extend this calibration to ·other types of basins in other parts of the world. The correction factors established in this study only represent statistical averages, and cannot be expected to work well in all cases. Whenever DST or PT data are available, they should be weighted considerably more heavily than any other type of data, even those corrected by the best methods available. In the absence ofDST or PT data, however, these correction methods will greatly increase our confidence in subsurface temperatures from RFTs or from wireline logs.

Effect of thermal conductivity anisotropy of rocks on the subsurface temperature field

SUMMARY The effect of thermal conductivity anisotropy on the heat flow is evaluated by solving numerically the heat conduction equation in 2-D models of anisotropic structures. The most common case of sedimentary rock anisotropy with two principal conductivities, parallel and normal to the bedding, is considered. For the effect due purely to anisotropy to be studied, both diagonal terms of the conductivity matrix of the anisotropic prism are put equal to the conductivity of the surrounding isotropic half-space. The results are presented in the form of figures showing isolines of horizontal and vertical heat flow densities (HFD). The greatest vertical HFD variations occur along the vertical sides of the prism and attain *7 per cent of the basal heat flow for the realistic conductivity model adopted. In addition, the vertical HFD within the prism was compared with the value calculated as if the data were measured in a vertical borehole. In this case, only the vertical temperature gradient is known. The difference between the two quantities represents the contribution of the horizontal temperature gradient to the vertical HFD. It attains 1-3 per cent of the HFD in models with an aspect (length-height) ratio of the prism of less than 2. An anisotropic body acting also as a heat flow refractor was investigated with a model of horizontally deposited sedimentary rock, with a factor of anisotropy of 1.5, surrounding a prism of the same rock with inclined layers. Vertical HFD changes concentrate along the sides of the prism and vary from -10 to +15 per cent of the HFD .

Far- and near- field deformation and granite emplacement

The thermal structure of the continental crust controls both the rheological behaviour of the crust and partial melting at depth. As far as material which can be melted, is there, granites are produced and intrude the upper crust. A layered crostai model is assumed with distinct rheologies, quartz or feldspar dependent for the upper and lower crust respectively. The model also assumes heat production in each layer and basal heat flow values. The thermal state of the crust, is examined by varying either the thickness of the uppermost crust, or the basal heat flow. This is equivalent to overthrusting upper crustal slivers as observed during contractional tectonics, or to increase the non-crustal component of heat flow as expected in continental rifts. In both cases (thinning and thickening), crustal deformation, usually brittle at low temperatures, turns to plastic flow with increasing temperature. The mode of emplacement of granitic plutons observed under many tectonic conditions is examined with reference to the proposed models. Granites are emplaced in all tectonic environments and the regional deformation (far-) field controls the bulk morphology of the plutons. The near-field conditions are of major importance in determining the location where granites develop. All plutons are emplaced within locally extensional zones (near-field) because a lower stress level is required to penetrate the brittle crust. The far-field conditions control the geometry, the number of roots and the thickness of the plutons, whereas the near-field conditions control the geometry of their roots.