Palaeo-heat Flow Research Articles

An introduction to the Bowland Shale Formation, UK: processes and resources

Abstract This volume showcases recent geological, geophysical and geochemical research on the Carboniferous Bowland Shale Formation. The Bowland Shale is a relatively thick and extensive Palaeozoic black shale unit with a long history of debate and controversy in the UK. The Bowland Shale is proven in many of the key Carboniferous basins in the Midlands, northern England and North Wales, and represents a significant, near-continuous temporal record spanning 16 Myr including the important mid-Carboniferous boundary (Mississippian–Pennsylvanian). Since the first geological surveys in the late nineteenth century, the Bowland Shale has been of interest for a variety of reasons, including the search for Irish-type Pb–Zn base metal mineral deposits, and as a source rock in conventional hydrocarbon systems. In the mid-2000s, attention turned to the Bowland Shale as a target for unconventional hydrocarbon extraction, shale gas, following success in the USA. This placed the Bowland Shale at the centre of a series of interconnected controversies and debates from the local to national scale. The geological credibility of the purported shale gas resource in the UK was – and continues to be – highly contentious. This volume contributes to a more updated view of the Bowland Shale, covering topics such as sedimentary, geochemical and physical properties and processes, basin-forming events, hydrocarbon prospectivity, mineralization and heat and fluid flow in the subsurface. The volume also includes a field guide to some of the key localities in the UK. With the benefit of hindsight offered by the latest generation of research, the early regional shale gas assessments failed to attach sufficient weight to the compositional heterogeneity, structural complexity, compartmentalization and highly variable exhumation, erosion and palaeo-heat flow history of the Bowland Shale. The following topics are identified as promising avenues for future research beyond shale gas: (1) the role of the Bowland Shale in the context of mineral systems; (2) the role of the Bowland Shale as a cap rock in the Lower Carboniferous limestone geothermal play and potentially as an analogue of CO 2 or radioactive waste storage in the UK; and (3) pathways and mechanisms for weathering, alteration and trace element release from the Bowland Shale into the surface and/or subsurface environment.

Thermal evolution and resources of the Bowland Basin (NW England) from apatite fission-track analyses and multidimensional basin modelling

Abstract Once highlighted for having significant shale gas resource potential, the Bowland Basin has been at the centre of both scientific and political controversy over the last decade. Previous shale gas resource estimates range from 10 3 to 10 1 TCF. Repeated events of induced seismicity following hydraulic fracturing operations led to an indefinite government moratorium and abandonment of operations across the mainland UK. We use apatite fission-track analyses to investigate the magnitude and timing of post-Triassic uplift and exhumation. Results indicate that maximum palaeotemperatures of 90–100°C were reached in the stratigraphically younger Sherwood Sandstone. We combine palaeotemperature predictions to constrain palaeo heat flow and erosion in regional basin models for the first time. Our results indicate variable maximum Late Cretaceous palaeo heat flow values of 62.5–80 mW m −2 and the removal of 800–1500 m of post-Triassic strata at wells across the basin. Regional 2D basin modelling indicates a gas-in-place estimate of 131 ± 64 TCF for the Bowland Shale. This reduces to a resource potential of 13.1 ± 6.4 TCF, assuming a recovery factor of 10%. These values are significantly lower than previous resource estimates and reflect the highly complex nature of the Bowland Basin and relatively unknown history of post-Triassic uplift, exhumation and erosion.

Timing of rapid cooling and erosional decay of two volcanic islands of the Canary Archipelago: implications from low-temperature thermochronology

The Canary Archipelago comprises seven volcanic islands formed by the activity of the Canary mantle anomaly that might have been caused by an ascending plume at the NW-African passive margin. The “Basal Complex (BC)”, which contains the islands pre-shield rock formations, is exposed in the northwest and central Fuerteventura and NW-La Gomera and preserves the archive of giant landslides that caused the removal of most of the shield-stage volcanic rocks. Tools, like low-temperature thermochronology (LTT) are sensitive to rapid cooling activities that accompany landslides. In addition, integrating LTT data with time–temperature (t–T) numerical modelling are a powerful tool for reconstructing the thermo-tectonic evolution as well as defining and quantifying long-term landscape evolution in a variety of geological settings. To unravel part of the long-term landscape evolution of Fuerteventura and La Gomera, zircon and apatite fission-track, and (U–Th)/He data combined with t–T numerical modelling were applied to 39 samples representing the main rock units of the BCs and younger magmatic rocks on both islands. In Fuerteventura, the Northwest and Central Basal Complexes reveal rapid cooling/exhumation of more than 200 °C at ~ 20 Ma. The quantification of the thickness of the rock column using the t–T cooling path would need the knowledge of the palaeo-heat flow. The published thickness of the moved rock column in Fuerteventura and La Gomera does not point to an extreme high heat flow. Therefore, the formation of a giant landslide leads to the removal of ~ 2.0 (± 0.5) km of the volcano rock column. Offshore, such a landslide has led to part of the Puerto Rosario large debris avalanche. The “Central Basal Complex” revealed two more rapid cooling/exhumation events at ~ 16 Ma and ~ 14 Ma that might also be related to landslides. The three landslides might be responsible for the formation of the nowadays Puerto Rosario Debris Avalanche Unit offshore. What might have caused the landslides in Fuerteventura. Age data published provide evidence for magmatic and tectonic activity that occur at the time of the formation of the giant landslides. In addition, the Miocene climate significant changes lead to changes in precipitation, and such changes might also provide a destabilisation of pyroclastic units. Therefore, the causes of the giant landslides might be related to more than only one process. The La Gomera BC has experienced two rapid cooling/exhumation events: the first at ~ 9 Ma, which might have caused ~ 2.0 (± 0.2) km of erosion forming the offshore Tazo avalanche, also known as the Tazo landslide. The second rapid cooling at ~ 8.0 Ma is located at the northwest of the Island and might have been caused by the Garajonay caldera collapse and followed by landslides. The landslides are assumed to have formed the Segments I, II, III, and VIII of the submarine debris avalanches offshore. Like Fuerteventura, both landslides might have been triggered by tectonic and magmatic activities as well as due to variation in precipitation caused by climate variation.

A bulk kinetic, burial history and thermal modeling study of the Albian Kazhdumi and the Eocene-Oligocene Pabdeh formations in the Ahvaz anticline, Dezful Embayment, Iran

The Ahvaz anticline which is located in the middle region of the Dezful Embayment is certainly one of the most important oilfields in Iran. Burial history reconstruction and one-dimensional thermal maturity modeling of the Late Jurassic to Present-day successions have been conducted in the Ahvaz anticline. The petroleum generation bulk kinetic parameters of the Kazhdumi and Pabdeh formations were computed by a non-isothermal open system pyrolysis at different heating rates. Bulk kinetic experiments reveal that a relatively narrow distribution of activation energies, range from 40 to 52kcal/mol for the Kazhdumi samples and from 42 to 54kcal/mol for Pabdeh shales, are directly related to the homogeneity in the organic matter. The principal activation energies of 46kcal/mol and 48kcal/mol, frequency factors of 4.36×10+11s−1 and 1.91×10+12s−1 and hydrogen indexes (HI) of 123 and 351mg/g TOC have been computed for the Kazhdumi and Pabdeh formations, respectively. The lithology composition (percentages of shale, marl, gypsum, limestone and sandstone) of the Late Jurassic-Present formations in the selected wells was determined by using cutting samples and wireline well log data. The computed kinetic parameters and derived lithology composition of strata were applied to thermal maturity modeling. Thermal maturity modeling and burial history reconstruction which is calibrated by vitrinite reflectance data and formation temperature indicate that the palaeo-heat flow ranges from 37mW/m2 to 41mW/m2.The Kazhdumi Formation entered the main stage of oil generation during the Late Miocene (6.5Ma) and the Pabdeh Formation reached early stage of oil window from the Early Pliocene (3.5Ma). Our results provide the bulk petroleum generation kinetic parameters description of the Kazhdumi and Pabdeh formations and demonstrate the thermal maturation state of source rocks, especially the Albian shales, that has been recognized as most important source rock in the Dezful Embayment.

A comparative study between present and palaeo-heat flow in the Qiangtang Basin, northern Tibet, China

The Qiangtang Basin is a significant prospective area for hydrocarbon and gas hydrate resources in the Tibetan Plateau, China. However, relatively little work has been performed to characterise heat flow in this basin, which has restricted petroleum and gas hydrate exploration. In this study, we compare present and palaeo-heat flow in the Qiangtang Basin to provide information on geothermal regime, hydrocarbon generation and permafrost that is necessary for further petroleum and gas hydrate exploration. We base our study on temperature data from a thermometer well, thermal conductivity tests, vitrinite reflectance data, homogenisation temperature data from fluid inclusions, stratigraphic information and a time-independent modelling approach. Our results indicate that in the central Qiangtang Basin, the present thermal gradient is approximately 15.5 °C/km, and heat flow is approximately 46.69 mW/m2. Heat flow in the Qiangtang Basin is not relatively stable since the Early Jurassic, as previous research has suggested, and it is generally decreasing with time. Additionally, there is a clear difference between the hottest thermal regime of the southern and northern Qiangtang Depressions during Cretaceous to Pleistocene time. In the southern Qiangtang Depression, the palaeogeothermal gradient is approximately 32.0 °C/km, and palaeo-heat flow is approximately 70 mW/m2. However, in the northern Qiangtang Depression, the palaeogeothermal gradient exceeds 81.8 °C/km, and palaeo-heat flow is greater than 172.09 mW/m2. The high thermal regime in the northern Qiangtang Depression is driven mainly by hydrothermal convection. Gas reservoirs are possible targets for hydrocarbon exploration in this depression. Currently, the northwestern part of the northern Qiangtang Depression is the most favourable area for gas hydrate exploration in the Qiangtang Basin.

Thermal and maturation history of Jurassic source rocks in the Kuqa foreland depression of Tarim Basin, NW China

Kuqa foreland depression of the Tarim Basin is one of the largest gas production provinces in China. Thermal history reconstruction using vitrinite reflectance data indicates that the palaeo-heat flow in Kuqa depression was relatively high (50–55mW/m2) during the Mesozoic, but gradually decreased during the Cenozoic to reach the present value of 40–50mW/m2. The cooling of the Kuqa depression is probably attributed to the crust thickening and the rapid sedimentary rate. The Jurassic source rocks entered conventional oil window at 100Ma, and began to generate gas at approximately 75Ma in the Kelasu area. Thermal maturation of the Jurassic source rocks accelerated significantly since 23.3Ma, especially in the recent 5.2Ma. In this foreland depression, source rock maturation, which is likely controlled mainly by burial history, also influenced by the presence of fault thrusting and salt-bearing formations.

3D BASIN MODELLING IN THE CENTRAL PERSIAN GULF, OFFSHORE IRAN

In the Central Persian Gulf, super‐giant natural gas accumulations in Permo‐Triassic reservoirs are assumed to be derived from “hot shale” source rocks in the lower Llandoverian (base‐Silurian) Sarchahan Formation, whereas oil in Mesozoic reservoirs is derived from Mesozoic source rocks. A 3D basin model has been established for a study area located in the Iranian part of the Central Persian Gulf in order to help understand the petroleum systems there. Sensitivity analyses considered different heat flow scenarios, and differences in the timing of Cenozoic uplift and erosion. For the Palaeozoic petroleum system, different thicknesses and distributions of the Silurian source rocks were considered.From current temperature profiles measured in five wells, present‐day heat flow was found to be in the order of 65 mW/m2, while palaeo heat flow was probably between 60 and 68 mW/m2 during Cenozoic maximum burial. For Llandoverian source rocks, oil and gas generation commenced during Jurassic and Late Cretaceous time respectively, and gas generation continued until the Neogene. Sensitivity analyses show that different assumptions on the timing of Cenozoic erosion do not have significant effects on the calculated timing of hydrocarbon generation or on the volume of generated hydrocarbons. As expected however, different heat flow scenarios (e.g. time‐constant heat flow of 65 mW/m2 in the entire study area) had a significant influence.With an assumed 50 m thick Sarchahan “hot shale” succession developed uniformly in the study area (8 % TOC; 470 mg HC / g TOC HI), the model calculated gas accumulations which are of the same order of magnitude as those which have been discovered in this region (e.g. South Pars, Golshan and Balal fields). By contrast, scenarios with thinner “hot shales” and models without the Sarchahan Formation along the Qatar‐South Fars Arch do not predict the known accumulations. These scenarios suggest that prolific Silurian source rocks must be present on both sides of the South Pars / North Dome field, or that lateral gas migration from the south may have supplied the Permo‐Triassic reservoirs.This study shows that the Jurassic (mainly Hanifa / Tuwaiq Mountain Formation) and Cretaceous (Shilaif Formation) source units are not sufficiently mature in the study area to have generated significant volumes of oil. This result supports previous suggestions which envisaged lateral migration from the south of the oil present in Mesozoic reservoirs in the study area.

Paleo-Heat Flow Anomaly at the End of Middle Permian in NE Sichuan Basin, SW China: Did an Ocean Trough Exist?

It is an issue whether the Kaijiang-Liangping area (in the northeastern Sichuan Basin, SW China) was a controversial ocean trough during Changxing age in the late Permian-early Triassic. Some vitrinite reflectance (Ro) profiles from wells in the northeastern Sichuan Basin show obvious breaks, and the breaks are coincident with the boundary between the middle Permian and late Permian. Based on the Ro data, the heat flow history in the northeastern Sichuan Basin was reconstructed. The result shows that the heat flow reached its peak value at the end of middle Permian (~260Ma). The spatiotemporal feature of the heat flow evolution is not in agreement with the fact that there was a Kaijiang-Liangping Trough existing in the late-Permian to early Triassic. The palaeo-heat flow anomaly was, in fact, a response to the igneous activity in late middle Permian.

The hidden benefits of maturity modelling

Most 1-D modelling studies are carried out with the explicit objective of calculating maturity and hydrocarbon generation. This focus on bottom-line output has caused most modellers to overlook a major additional value of such studies: the knowledge acquired while building internally consistent geologic models. During model construction one should carefully consider the following factors: events during unconformities, subsidence and sedimentation rates, palaeobathymetry and palaeoelevation, changes in lithology, tectonic events, isostasy, eustasy, and palaeoheat flow. The process of building such models requires that we integrate these disparate types of data derived from disciplines that rarely communicate with each other. Maturity modelling thus becomes a general tool for geologic investigation and integration. These techniques are illustrated using examples from around the world. The education we receive in developing internally consistent, fully integrated geological models will often result in new exploration concepts, and thus can aid exploration programs in unanticipated ways.

Thermal maturity of the Upper Triassic–Middle Jurassic Shemshak Group (Alborz Range, Northern Iran) based on organic petrography, geochemistry and basin modelling: implications for source rock evaluation and petroleum exploration

AbstractOrganic petrography and geochemical analyses have been carried out on shales, carbonaceous shales and coals of the Shemshak Group (Upper Triassic–Middle Jurassic) from 15 localities along the Alborz Range of Northern Iran. Thermal maturity of organic matter (OM) has been investigated using vitrinite reflectance, Rock-Eval pyrolysis and elemental analysis of kerogen. Reflectance of autochthonous vitrinite varies from 0.6 to 2.2% indicating thermally early-mature to over-mature OM in the Shemshak Group, in agreement with other maturity parameters used. The shales of the Shemshak Group are characterized by poor to high residual organic carbon contents (0.13 to 5.84%) and the presence of hydrogen-depleted OM, predominantly as a consequence of oxidation of OM at the time of deposition and the hydrogen loss during petroleum generation. According to light-reflected microscopy results, vitrinite/vitrinite-like macerals are dominant in the kerogen concentrates from the shaly facies. The coals and carbonaceous shales of the Shemshak Group show a wide range in organic carbon concentration (3.5 to 88.6%) and composition (inertinite- and vitrinite-rich types), and thereby different petroleum potentials. Thermal modelling results suggest that low to moderate palaeo-heat flow, ranging from 47 to 79 mW m−2 (57 mW m−2 on average), affected the Central-Eastern Alborz basin during Tertiary time, the time of maximum burial of the Shemshak Group. The maximum temperature that induced OM maturation of the Shemshak Group seems to be related to its deep burial rather than to a very strong heat flow related to an uppermost Triassic–Liassic rifting. The interval of petroleum generation in the most deeply buried part of the Shemshak Group (i.e. Tazareh section) corresponds to Middle Jurassic–Early Cretaceous times. Exhumation of the Alborz Range during Late Neogene time, especially along the axis of the Central-Eastern Alborz, where maximum vitrinite reflectance values are recorded, probably destroyed possible petroleum accumulations. However, on the northern flank of the Central-Eastern Alborz, preservation of petroleum accumulations may be expected. The northern part of the basin therefore seems the best target for petroleum exploration.

Palaeogeothermal response and record of Late Mesozoic lithospheric thinning in the eastern North China Craton

Abstract The palaeotemperature recorded by vitrinite reflectance ( R o ) in the pre-Cenozoic uplifted stratigraphic strata, and in Palaeozoic–Mesozoic remnant basins outside the Cenozoic depocentres, has not been overprinted by later thermal events in the eastern North China Craton (NCC). Based on downhole R o data from the Palaeozoic and the Mesozoic subsections, we reconstruct the temperature gradients when the subsections reached their maximum palaeotemperatures in the Middle Triassic and the Cretaceous, and calculate the corresponding heat flow histories since the early Mesozoic. The temperature gradient and heat flow were much higher in the Cretaceous (35–43 °C km −1 and 73–83 mW m −2 , respectively) than in the Middle Triassic and at the present. The high palaeo-heat flow during the Late Mesozoic implies that the thickness of the ‘thermal’ lithosphere at that time was c . 65 km, about half the thickness of c . 135 km estimated for the Early Mesozoic. The change from a stable thermal regime to an active thermal regime took place during the Late Jurassic–Early Cretaceous ( c . 110 Ma). This tectonothermal event was accompanied by extensive surface erosion, and is also evidenced in the areas adjacent to the NCC, such as the South Yellow Sea and East China Sea basins. Our study provides not only geothermal evidence for the Late Mesozoic lithospheric thinning, but also additional constraints on the thinning mechanism, which is currently being debated.

Tectono-thermal evolution of the Junggar Basin, NW China: constraints from R o and apatite fission track modelling

The thermal evolution of the Junggar Basin, northwest China, was evaluated based on the thermal modelling results of 59 wells by using vitrinite reflectance (R o ) and apatite fission track (AFT) data. The thermal history indicates a cooling process of the basin since the Permian, but some differences in thermal evolution existed among the six structural units of the basin due to tectonic movements. The Junggar Basin was a ‘hot basin’ during the Permian, after which a cooling process with normal heat flow values occurred during the Mesozoic. Then the basin became a ‘cool basin’ from the beginning of the Tertiary. The average heat flow of the whole basin was 80 mW m −2 at the beginning of the Permian, then it decreased to 68 mW m −2 at the end of the Permian, to 63 mW m −2 at the end of the Triassic, 55 mW m −2 at the end of the Jurassic, 50 mW m −2 at the end of the Cretaceous and 42 mW m −2 at the present day. The heat flow distribution of the basin at different geological times also shows the thermal evolution characteristics of the Junggar Basin. At the beginning of the Permian, the highest heat flow, 85 mW m −2 , occurred in the central basin and the eastern part of the basin, but the lowest heat flow was distributed along the southern and western basin margins, down to 70 mW m −2 . The heat flow values were between 45 mW m −2 and 65 mW m −2 at the end of the Jurassic, with the lower value of 45 mW m −2 at the southern basin margin. The highest heat flow value again occurred at the southern end of the Luliang Uplift, at the northern part of the Central Depression and at the Eastern Uplift area during that period. At the end of the Cretaceous, it was down to 40–55 mW m −2 . The lowest heat flow occurred at the Southern Margin and in the Wulungu Depression, and the highest value in the Eastern Uplift area. The tectonic subsidence also supports this thermal evolution of the basin. The rapid decrease of heat flow during the Tertiary in the Southern Margin of the basin may be caused by the uplift of the Tianshan Mountain. These heat flow data can provide useful parameters for the study of the Junggar Basin. Palaeoheat flow data are the critical parameter for hydrocarbon generation calculations. The results of this study provide a foundation for hydrocarbon generation history modelling and petroleum resource assessment in the Junggar Basin, which are important factors in the exploration of the Wulungu Depression and the study of stratigraphic and subtle traps in the Central Depression.

Kaolin polytype evidence for a hot-fluid pulse along Caledonian thrusts during rifting of the European Margin

AbstractSedimentary basins developed along the European margin during the earliest, Permian, stage of proto-Atlantic rifting, during a phase of high heat flow. The proximity of some basins to Caledonian thrusts has implied that rifts locally utilized the basement fabric. New mineralogical and palaeomagnetic data show that thrust planes in the Moine Thrust Zone channelled a pulse of hot fluid in Permian time. The fluids precipitated kaolin in fractures in the thrust zone, and with decreasing intensity away from the zone. The high-temperature polytype dickite is largely confined to major thrust planes. Stable H and O isotope analyses indicate that the parent fluid included meteoric water involved in a hydrothermal system. Coeval hydrothermal hematite has a chemical remanence that dates the fluid pulse as Permian. This is direct evidence for post-orogenic activity in the thrust zone, in which the thrusts vented excess heat during regional crustal extension. The example from the European margin exemplifies the importance of deep-seated structures in the release of heat, and the value of kaolinite polytype mapping as a tool to record anomalous palaeo-heat flow.

Coalification history of the Stephanian Ciñera-Matallana pull-apart basin, NW Spain: Combining anisotropy of vitrinite reflectance and thermal modelling

The Stephanian Cinera-Matallana Basin of NW Spain comprises 1,500 m of alluvial to lacustrine coal-bearing sediments, which were deposited in a late Variscan transtensional/transpressional pull-apart setting. The relationship between coalification pattern and rock deformation was evaluated by measurements of the anisotropy of vitrinite reflectance (AVR). The AVR ellipsoids reveal both pre-tectonic elements related to the bedding fabric and syn-tectonic elements related to folding, producing biaxial ellipsoid shapes with the maximum reflectance parallel to fold axes. The mean coalification gradient for the Stephanian succession is about 0.62 %Rr/km. Calculations of the mean palaeo-geothermal gradient are presented on the basis of three different empirical equations. A palaeo-geothermal gradient of 85 °C/km is considered the most realistic, with an overburden of about 1,000 m. 1-D numerical modelling of the burial history results in two possible scenarios, the most preferable involving a palaeo-heat flow of 150 mW/m2 and an overburden of ca. 1,050 m. These results indicate that maximum coalification was related to a localised but high palaeo-heat flow/-geothermal gradient. The anisotropy of vitrinite reflectance highlights the interactive and transitional nature of sedimentary compaction and rock deformation on the maturation of organic material within strike-slip fault zones.

Characteristics and origin of underpressure system in the Shiwu Fault Depression, south-east Songliao Basin, China

The vertical characteristics of underpressure, pore fluids and sealing condition in the Shiwu Fault Depression, south-east Songliao Basin, are investigated. Based on the pore pressure data from drill stem tests collected from 40 wells, the vertical distribution of pore pressure in this depression consists of a shallow hydrostatic pressure system and a deep underpressure system. The observed low pressures range from about 4 to 10 MPa in the depth interval of about 1550–2800 m. The pore water chemistry data document that the ionic evaporite trends of the pore water in the underpressure zone are different from overlying sediments in the hydrostatically pressured section, indicating that the underpressure system is sealed. The study indicates that the depression has undergone rapid deposition in a rift period during the Cretaceous followed by a long-term slow uplift and erosion in the Tertiary. On the basis of the modelled results of the abnormal pressure evolution in a typical cross-section using a two-dimensional numerical flow model, we believe that the predecessor of the underpressure system was an overpressure system. Since the end of the Cretaceous, the observed underpressure system has developed as a result of a geotemperature decrease ranging from 30 to 70 °C, owing to palaeoheat flow reduction and long-term uplift and erosional cooling. The mudstones below a depth of 1550 m in the deep subnormal pressure system have small measured porosities ranging from 4% to 1.2% with calculated permeability of about 10−21–10−24 m2. The observed underpressure can be modelled if we assign permeabilities below 10−20 m2 with a linear reduction of geotemperature. The geotemperature decrease, in combination with very low permeability in the Lower Cretaceous mudstones, is therefore a possible origin for the formation of the underpressure system in this depression.

Coalification pattern and thermal modelling of the Permo-Carboniferous Saar Basin (SW-Germany)

Palaeo-heat flow values and thicknesses of eroded Permo-Carboniferous sediments in the Saar Basin were evaluated using one dimensional thermal modelling techniques. Thermal, burial and erosion histories for 16 wells were calibrated by comparing measured and calculated vitrinite reflectance using the kinetic EASY% R o algorithm and by comparing measured and calculated temperature data. On the basis of 37 wells, coalification maps were constructed revealing a syn-kinematic coalification pattern. Thermal maturity of the sediments can only be explained by deep burial and moderate heat flows during time of maximum burial, i.e., in the Permo-Carboniferous. Calculated heat flow data range between 50 and 75 mW/m 2, which implies a crustal thickness between 30 and 40 km during the time of maximum burial. These values are in accordance with the geodynamic setting of the basin. The influence of the Permo-Carboniferous volcanism on the palaeo-temperature distribution was overwhelmed by the subsequent deep burial. During Permian times, between 1800 and 3000 m of Permo-Carboniferous sediments were eroded. Different sedimentation and erosion histories are characteristic for anticlines and synclines, respectively.

Thermal diagenesis of clay minerals within volcanogenic material from the Tonga convergent margin

Clay minerals are examined in detail in the sediment from the Tonga Trench margin at Site 841 (Leg 135 ODP). The changes in amount and nature of secondary clays with depth provide an alternative explanation for the intensive alteration of volcanogenic material at convergent margins. A characteristic distribution of clay minerals with depth shows four distinct zones unexplainable by simple burial diagenesis processes. These are named the upper, reactive, lower and rhyolitic zones. The reactive zone is intercalated with numerous sills and is characterized by the dominant iron-rich clays such as saponite, corrensite and chlorite associated with analcime. The occurrence of such iron-rich clays, mostly associated with a large amount of analcime, yields chemical and mineralogical evidence for thermal diagenesis. The required heat for the diagenetic process was transferred from recently intruded basaltic andesite sills. In the vicinity of these intrusions, the iron-rich clay minerals may have formed at temperatures up to 200°C. A zoning with respect to clay and zeolite minerals indicates that the influence of the palaeoheat flow decreased with the distance from the intrusion. The formation of interlayered I/S, illite, kaolinite and aluminous chlorite, which are recognized as major secondary minerals within the rhyolitic complex, was mainly controlled by both early diagenesis at moderately elevated temperatures, and since the Eocene by burial diagenesis at low temperatures. The occurrence of a steam zone in an early stage of the intrusion is restricted to Miocene tuffs and has overprinted the early alteration of the volcanogenic material within the tuffs and has changed the originally pristine composition of the pore fluids.

Subsidence analysis and quantitative basin modelling in the Styrian Basin (Pannonian Basin System, Austria)

We present the results of subsidence analysis and of quantitative basin modelling using isostatical and flexural models for basin evolution along four cross-sections in the Styrian Basin, the westernmost subbasin of the Pannonian Basin System. Subsidence analysis reveals a first Ottnangian-Karpatian synrift phase. Our local isostatic models predict crustal stretching values up to 1.3 and subcrustal stretching values of 1.6 for this event. Stretching factors of a minor Sarmatian extension phase are below 1.04. The termination of subsidence during the Pannonian and a rapid Quaternary uplift phase can be explained by major changes in the regional stress field. A W-E cross-section through the northern Fürstenfeld Subbasin provides a key for the understanding of the dynamics of basin formation. It crosses a narrow Karpatian rift basin, the metamorphic core complex of the Penninic Eisenberg Window and shows an eastward tilting of the easternmost part of the basin during Pannonian times. Uplift of the Penninic window can only be modelled with an extremely weak lithosphere (equivalent elastic thickness (EET) ⪡ 2 km), whereas a best fit between observed and modelled tilting is obtained with an EET value of 5 km. These results suggest that the lithosphere was extremely weak during the onset of basin evolution in Ottnangian-Karpatian times, probably caused by high extension rates and high heat flows associated with Karpatian to early Badenian magmatic activity. Subsequent cooling led to a pronounced increase in flexural rigidity. An EET of 5 km fits well with estimations in other parts of the Pannonian realm. Depth-dependent rheology models based on palaeo-heat flow estimates indicate a similar increase in lithospheric strength with time. The impact of Plio-Pleistocene volcanism on rheology appears to be relatively modest, which can be explained by a deep position of the magma chamber for this event.

FLEXURAL ISOSTATIC MODELLING AS A CONSTRAINT ON BASIN EVOLUTION, THE DEVELOPMENT OF SEDIMENT SYSTEMS AND PALAEO-HEAT FLOW: APPLICATION TO THE VULCAN SUB-BASIN, TIMOR SEA

Although the petroleum industry is commonly interested in the upper few kilometres of the lithosphere, it is the deeper stretching events which may drive the development of regional thermal perturbations and which may overprint a significant thermal signature onto the shallower section. The Vulcan Sub-basin, which is located in the Timor Sea, northwestern Australia, has undergone a period of rifting during the Late Jurassic and shows a classic transition from intra-continental rifting to passive margin subsidence during the Late Jurassic to Early Cretaceous. A model has been developed of the Late Jurassic rifting history of the basin, which includes the flexural and stratigraphic response, and the development of the Cretaceous to Recent post- rift basin history. Quantification of the associated vertical motion of the lithosphere suggests that the transition is related to increased ductile extension in the lower crust and lithospheric mantle with little attendant upper crustal faulting to record the magnitude of this event in the structural history of the Vulcan Sub-basin. This lack of upper crustal deformation has resulted in an under- appreciation of the importance of this extensional event.By modelling the Jurassic to Recent basin history, a thermal model may be built allowing predictions of palaeo-heat flow during the critical time of hydrocarbon generation. The model predicts that during the Jurassic and Early Cretaceous, increased lower crust and lithospheric mantle extension produced a thermal anomaly of ~20mW/m2 across the Vulcan Sub-basin. The relaxation of this thermal anomaly in the Cretaceous and Tertiary produced a rapid post-rift subsidence which allowed flooding of the margin, with increased subsidence towards the northwest. However, the evolution of this thermal perturbation beneath the upper crust resulted in a time lag between Late Jurassic rifting and maximum basin heat flow in the Early Cretaceous of up to 30 million years after Callovian breakup Therefore, the simple relationship between upper crustal faulting and total lithosphere stretching common in intra-continental rifts is predicted to break dow n immediately preceding conti nental breakup and necessitates modelling of the transition from syn-rift to post-rift stratigraphy in order to predict the thermal history of the Vulcan Sub-basin.

Temperature and uplift history of Bjornoya (Bear Island), Barents Sea

Apatite fission track age data from the majority of the 14 samples studied indicate cooling below c. 110 degrees C beginning 60 to 90 Ma ago. Vitrinite reflectance modelling suggests temperatures prior to cooling reached > 160 degrees C in the north and 150 to 160 degrees C elsewhere. Temperatures from fluid inclusions in quartz cement from the Carboniferous Nordkapp Formation may have reached 139 to 151 degrees C. Zircon fission track data from the basement indicate that the maximum temperature during Hecla Hoek metamorphism was less than 290 degrees C. Circumstantial evidence suggests that temperatures prior to deposition of the Permian Miseryfjellet Formation were probably much lower than those during the Mesozoic. Maximum eroded overburden above the Miseryfjellet Formation can be estimated at 2900 to 4200 m using various different methods. The resulting geothermal gradients suggest palaeoheat flow conditions close to those of today.