Present Heat Flow Research Articles

Age and thermal history of the Geysers plutonic complex (felsite unit), Geysers geothermal field, California: a 40Ar/ 39Ar and U–Pb study

Sixty-nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the Geysers geothermal field yield 207Pb/ 206Pb vs. 238U/ 206Pb concordia ages ranging from 1.13±0.04 Ma to 1.25±0.04 (1 σ) Ma. The weighted mean of the U/Pb model ages is 1.18±0.03 Ma. The U–Pb ages coincide closely with 40Ar/ 39Ar age spectrum plateau and `terminal' ages from coexisting K-feldspars and with the eruption ages of overlying volcanic rocks. The data indicate that the granite crystallized at 1.18 Ma and had cooled below 350°C by ∼0.9–1.0 Ma. Interpretation of the feldspar 40Ar/ 39Ar age data using multi-diffusion domain theory indicates that post-emplacement rapid cooling was succeeded either by slower cooling from 350° to 300°C between 1.0 and 0.4 Ma or transitory reheating to 300–350°C at about 0.4–0.6 Ma. Subsequent rapid cooling to below 260°C between 0.4 and 0.2 Ma is in agreement with previous proposals that vapor-dominated conditions were initiated within the hydrothermal system at this time. Heat flow calculations constrained with K-feldspar thermal histories and the present elevated regional heat flow anomaly demonstrate that appreciable heat input from sources external to the known Geysers plutonic complex is required to maintain the geothermal system. This requirement is satisfied by either a large, underlying, convecting magma chamber (now solidified) emplaced at 1.2 Ma or episodic intrusion of smaller bodies from 1.2 to 0.6 Ma.

Composition and thermal evolution of cratonic mantle beneath the central Archean Slave Province, NWT, Canada

The composition and thermal evolution of the upper mantle lithosphere beneath the central Archean Slave Province has been studied using mineral chemical and petrographic data from mantle xenoliths entrained in the Torrie kimberlite pipe. Coarse-, granuloblastic-, and porphyroclastic- textured harzburgite, lherzolite, and pyroxenite xenoliths yield equilibration temperatures ranging between 850 and 1350 °C. Thermobarometry of these samples requires a minimum lithospheric thickness of approximately 180 km at the time of kimberlite magmatism. The distribution of pressures and temperatures of equilibration for the xenoliths lie on a calculated 42 mWm−2 paleogeotherm, ∼10 mWm−2 lower than the present heat flow measured at Yellowknife, near the SW margin of the Slave Province. The Mg# [Mg/(Mg + Fe)] of olivine in peridotites varies between 0.906 and 0.938 with an average of 0.920. The Torrie xenolith suite shows variable degrees of serpentinization and/or carbonation with the rim compositions of many clinopyroxene grains showing Ca enrichment, but in general, the xenoliths are homogeneous at all scales. The Torrie xenoliths are rich in orthopyroxene similar to low temperature (<1100 °C) peridotites from southern Africa, and Siberia. Estimates of bulk rock composition based on mineral chemical and modal data reveal a negative correlation between Si and Fe, similar to peridotite xenoliths from Udachnaya. The similarity of olivine Mg#s with other cratons combined with the negative correlation of Fe and Si suggest that the lithosphere beneath the Slave craton has experienced a evolution similar to other cratons globally.

Modeling Petroleum Generation in the Southern Muglad Rift Basin, Sudan1

Maturity and petroleum generation have been modeled in a 120-km-long northeast-southwest cross section of the Southern Muglad basin, Sudan. The section passes close to the Unity and Kaikang areas, which provide well control on the structure interpreted from gravity and seismic reflection profiles. Modeling of geotherms from corrected bottom-hole temperatures indicates present heat flows around 60 mW/m2 in the well control areas. Using measured vitrinite data as calibrant, maturity can be modeled equally well with a constant paleoheat flow similar to the present day or with a complex heat flow pattern related to rifting. A geologically realistic complex heat flow model was employed to calculate the timing of generation and expulsion from the Lower Cretaceous lacustrine source rocks of the Abu Gabra and Sharaf formations. Kinetic (Easy% Ro) calculation of vitrinite maturity indicates a projected oil preservation depth limit around 4000 m shallowing to around 3500 m northeast of Unity near the basin margin.

Present heat flow and paleo-geothermal regime in the Canadian Arctic margin: analysis of industrial thermal data and coalification gradients

Calculations of the present geothermal gradient and terrestrial heat flow were made on 156 deep wells of the Canadian Arctic Archipelago. Corrected bottom hole temperature (BHT) data and drill stem test (DST) temperatures were used to determine the thermal gradients for sites for which the quality of data was sufficient. Thermal gradients evaluated for depths below the base of permafrost for the onshore wells and below sea bottom for the offshore wells were combined with the estimates of effective thermal conductivity to approximate heat flow for these sites. The present geothermal gradient is in the 15–50 mK/m range (mean = 31 ± 7 mK/m). Present heat flow is mainly in the 35–90 mW/m 2 range (mean = 53 ± 12 mW/m 2). Maps of the present geothermal gradient and present heat flow have been constructed for the basin. The analysis of vitrinite reflectance profiles and the calculation of logarithmic coalification gradients for 101 boreholes in the Sverdrup Basin showed large variations related in many cases to regional variations of present terrestrial heat flow. Paleo-geothermal gradients estimated from these data are mostly in the range of 15–50 mK/m (mean = 28 ± 9 mK/m) and paleo-heat flow is in the 40–90 mW/m 2 range (mean = 57 ± 18 mW/m 2) related to the time of maximum burial in the Early Tertiary. Mean values of the present heat flow and paleo-heat flow for the Sverdrup Basin are almost identical considering the uncertainties of the methods used (53 ± 12 versus 57 ± 18 mW/m 2, respectively). Present geothermal gradients and paleo-geothermal gradients are also close when means are compared (31 ± 7 versus 28 ± 9 mK/m respectively). A zone of high present heat flow and a paleo-heat flow zone coincide in places with the northeastern-southwestern incipient rift landward of the Arctic margin first described by Balkwill and Fox (1982). Correlation between present heat flow and paleo-heat flow for the time of maximum burial in the earliest Tertiary suggests that the high heat flow zone has prevailed since that time.

Whole-mantle versus layered mantle convection and the role of a high-viscosity lower mantle in terrestrial volatile evolution

Terrestrial noble gas isotope geochemistry provides one of the most powerful geochemical tools presently available for understanding the volatile evolution of the Earth. The data require the existence of isolated reserviors with very different time integrated U/He ratios within discrete portions of the mantle. One mechanism proposed to account for the apparent isolation is a high-viscosity lower mantle. We have conducted numerical experiments to study the influence of a higher viscosity lower mantle on the mixing efficiency of mantle convection and the rate of mantle degassing. We consider cylindrical mantle convection models in which the lower mantle viscosity is increased to up to two orders of magnitude higher than that of the upper mantle, while maintaining the overall vigor of convection as measured by the total surface heat flow. The models incorporate radiogenic ingrowth and degassing of He. We show with models that match the present day heat flow that the high viscosity of the lower mantle alone does not prevent large-scale mixing of the mantle. Both 3He and 3He 4He remain relatively homogeneous through the entire mantle, showing only small-scale heterogeneity. Degassing is more rapid in the case of the high-viscosity lower mantle. Present day plate velocities, heat flow and estimates for the amount of degassed radiogenic noble gases over the age of the Earth are best matched by the model with a 30-times increase of viscosity in the lower mantle.

Subsidence analysis, maturity modelling and hydrocarbon generation of the Alpine sedimentary sequence in the NW of the Iberian Ranges (Central Spain)

The burial, thermal and hydrocarbon histories of late Permian-Tertiary sedimentary succession in the NW of the Iberian Ranges were investigated using a single-well model. The reconstructed geobistories indicate four stages of evolution during early Alpine to late Alpine infill in the studied area: late Permian, Triassic, early and middle Jurassic, and latest Jurassic-middle Cretaceous. The differences between interpreted basement behaviour interpreted from tectonic subsidence curves differentiates four major «rift-uplift realms». The «riftluplift» realm is defined either as an individual fault-bounded structural domain of ifie continental phase of the rift system or as an uplift that displays similar extensional/uplift proeesses. Thcse «riftluplift realms» show a propagation of rifting/uplift stages through time. The initial Permian, Triassic and Jurassic episodes show a north-eastem propagation foliowed by a south-western propagation from the Central realm. Each of these realms shows aLo a gradual north-eastern progress of the extension strain. The Cretaceous rift/uplift stage seems to behave in the opposite direction. Present day bottom heat flow and thermal history were modelled, using both a steady sUite thermal aud a paleothermal heat flow-invariant model. Thc present average bottom heat flow oblained faills within the range of values reported in the literature from similar tectonic contexts. Different hcat flow values were tentatively adopted to determine maturation. Ml the quantitative estimations of generated hydrocarbons indicate that insignificant volumes of hydrocarbons were generated by source rocks considered in this study.

Implications of vitrinite-reflectance suppression for the tectonic and thermal history of the Malay Basin

Vitrinite-reflectance profiles for wells in the Malay Basin are generally consistent, and appear at first glance to accurately represent present-day thermal maturities. However, these measured Ro values are much lower than one would expect for wells with such high present-day geothermal gradients. Consequently, calculated Ro values can only be fitted to the measured Ro data by proposing a strong and recent heat pulse. In this scenario, the paleoheat flow was much lower than the present heat flow, and rose to the present levels within the last few million years or less. A plausible tectonic history for the Malay Basin can be constructed that justifies this scenario, because Quaternary volcanics and hot springs are known, and because the last 10 million years has seen renewed subsidence after a period of uplift during the Middle Miocene. However, FAMM (Fluorescence Alteration of Multiple Macerals) data obtained from seven wells indicate that the measured Ro values are much too low in most of the Malay Basin. Ro values have been suppressed by the presence of abundant liptinite and perhydrous vitrinite, probably as a result of marine influence, except along the western margin of the basin and in the far northwestern end. Calibration of the paleoheat flow with F AMM data permits use of a much more constant thermal history at each location. In this model, the main heat flow increased during Oligocene rifting in proportion to the amount of crustal extension, and then has subsequently decayed exponentially to modern levels. Using this paleoheat flow model, hydrocarbons are generated much earlier and maturities in the basin are much higher than if the paleoheat flow model is. calibrated using the measured Ro data. These conclusions in turn indicate that the recent tectonic history of the Malay Basin has probably been rather gentle, in keeping with evidence from sedimentation rates. Although we are not yet certain how common vitrinite suppression is globally or in the Malay Basin, these results indicate that (1) all data sets should be routinely checked for vitrinite suppression, especially in areas where the phenomenon has been recognized; (2) any thermal model requiring a significant recent heat pulse to match measured and calculated Ro values should be viewed with suspicion until validated independently; and (3) errors in reconstruction of thermal and tectonic history can often lead to significant errors in exploration decisions.

Surficial extent and conceptual model of hydrothermal system at Mount Rainier, Washington

A once massive hydrothermal system was disgorged from the summit of Mount Rainier in a highly destructive manner about 5000 years ago. Today, hydrothermal processes are depositing clayey alteration products that have the potential to reset the stage for similar events in the future. Areas of active hydrothermal alteration occur in three representative settings: 1. (1) An extensive area (greater than 12,000 m 2) of heated ground and slightly acidic boiling-point fumaroles at 76–82 °C at East and West Craters on the volcano's summit, where alteration products include smectite, halloysite and disordered kaolinite, cristobalite, tridymite, opal, alunite, gibbsite, and calcite. 2. (2) A small area (less than 500 m 2) of heated ground and sub-boiling-point fumaroles at 55–60 °C on the upper flank at Disappointment Cleaver with smectite alteration and chalcedony, tridymite, and opal-A encrustations. Similar areas probably occur at Willis Wall, Sunset Amphitheater, and the South Tahoma and Kautz headwalls. 3. (3) Sulfate- and carbon dioxide-enriched thermal springs at 9–24 °C on the lower flank of the volcano in valley walls beside the Winthrop and Paradise Glaciers, where calcite, opal-A, and gypsum are being deposited. In addition, chloride- and carbon dioxide-enriched thermal springs issue from thin sediments that overlie Tertiary rocks at, or somewhat beyond, the base of the volcanic edifice in valley bottoms of the Nisqually and Ohanapecosh Rivers. Maximum spring temperatures of 19–25 °C and 38–50 °C, respectively, and extensive travertine deposits have developed in these more distant localities. The heat flow, distribution of thermal activity, and nature of alteration minerals and fluids suggest a conceptual model of a narrow, central hydrothermal system within Mount Rainier, with steam-heated snowmelt at the summit craters and localized leakage of steam-heated fluids within 2 km of the summit. The lateral extent of the hydrothermal system is marked by discharge of neutral sulfate-enriched thermal water from the lower flank of the cone. Simulations of geochemical mass transfer suggest that the thermal springs may be derived from an acid sulfate-chloride parent fluid which has been neutralized by reaction with andesite and highly diluted with shallow groundwater. The model may accomodate some of the thermal springs beyond the base of the edifice. Present heat flow from Mount Rainier is substantial relative to other Cascade Range volcanoes and does not appear to have diminished since at least the late 19th century. Evidence of older hydrothermal processes found in Holocene lithic tephra and debris avalanches record activity more extensive but similar in chemical composition to that of today.

An upper limit to palaeo heat flow: theory and examples from the Danish Central Graben

The surface heat flow yields a condensed expression of the subsurface thermal regime. While the present heat flow can be measured as the product of thermal conductivity and temperature gradient, resolution of the past surface heat-flow variations calls for indirect methods such as the modelling of vitrinite reflectance and other organic maturity indicators, which represent integrals of the past temperature history. This paper argues that regarding the reflectance of vitrinite, which is the most common maturity indicator, the feature of the past thermal regime which can survive the effects of measuring and modelling uncertainties is the maximum temperature, present or past. How the maximum temperature was obtained is only rarely resolved. Given maximum temperature, burial history, thermal conductivity structure, and surface temperature history, the one-dimensional steady-state heat equation yields an approximate upper limit to the surface heat flow. Artificial examples show that the past heat flow may be similar to the maximum heat-flow constraint when the past was significantly warmer than the present. However, this requires a delicate balancing of burial history and heat-flow history to occur: the thermal relaxation rate of the hot past must compensate for heating due to burial. Generally, the past heat flow may be well below the maximum constraint. Examples from the Central Graben show that vitrinite reflectance samples of Upper Jurassic age constrain the maximum palaeo heat flow back to 150 Ma. This is due to the very rapid and significant sedimentation of Upper Jurassic sediments resulting in immediate deep burial of vitrinite. The data of the present study are consistent with a Early Cretaceous heat flow elevated by about 20–30% above the present, and with a heat flow throughout the Cenzoic only marginally higher than the present. However, the data are also fully consistent with a time independent background heat-flow history. This illustrates the limits of resolution in palaeo thermal reconstruction based on vitrinite reflectance and temperature data. The problem is one of finding the space of admissible solutions rather than one of finding a single solution.

Extension in south-central British Columbia: mechanical and thermal controls

Geological data indicate that the Omineca Belt, in the Canadian Cordillera, experienced a rapid episode of extension between 58 and 48 Ma following a compressional event and crustal thickening in the Early Tertiary. Restored crustal sections have been constructed using listric fault geometries to infer that the amount of Eocene extension affecting the southern Omineca Belt exceeded 50%. At the end of compression in the Early Tertiary the Omineca and Intermontane belts of southern British Columbia reached a crustal thickness of 60 km. Isostatic compensation implies that these belts formed elevated ranges; this is consistent with sediment deposition west of these ranges and the absence of in-situ sedimentary basins. The stress induced by topography and crustal thickening is calculated: the calculations show that the stress in homogeneously thickened lithosphere is small and that tensile stress in the crust is compensated by compression in the mantle. Removal of the mantle lithosphere induces a stress field that is tensile throughout the lithosphere. This local stress field combined with stress induced at the plate boundary by the oblique impingement of the Kula-Farallon ridge controlled the position and direction of crustal extension in south-central British Columbia. The present high heat flow on the west flank of the Omineca Belt, 80 mW m −2, is interpreted as the relict of Eocene extension following removal of the lithospheric mantle.

Crustal reflectors, metamorphic reactions and crustal temperatures in the southern Canadian Cordillera

In Phanerozoic areas, including the southern Canadian Cordillera, the lower crust commonly exhibits bands of subhorizontal seismic reflectors; such reflectors are rare in Precambrian areas. The origin of the reflectors has been extensively discussed (e.g., Warner, 1990); the main options are layered mafic intrusions, mylonitic horizontal shear zones, and layered porosity. Klemperer (1987) and Wever et al. (DEKORP Research Group, 1987) pointed out that the depth to the top of these characteristic reflectors appears to be correlated with present heat flow and thus present crustal temperature, the top of the reflectors being shallower where the heat flow is higher. Other compilations have substantiated this correlation and shown that the top of the reflective layer is everywhere at nearly the same temperature of 450°C (Lewis et al., 1988; DEKORP Research Group, 1990; Marquis and Hyndman, 1992). An associated correlation might be the decrease in the shallowest depth of these reflectors beneath some batholiths (e.g., Matthews, 1987) that have high heat production and thus higher crustal temperatures, The southern Canadian Cordillera is made up primarily of oceanic terranes accreted to the North American craton in the Paleozoic and

Paleo heat flow of eastern Canada's passive margins

Tectonic subsidence rates, derived from several wells in the Labrador Sea and offshore Nova Scotia and Newfoundland, were used to estimate the paleo heat flow along the margins of eastern Canada. If tectonic subsidence is caused by the cooling of the lithosphere only, the transient component of the heat flow (i.e., the heat flow in excess of background heat flow) can be estimated directly from subsidence data. For the Nova Scotia and the Labrador Sea margins, the analysis shows that the transient component of the heat flow decreased markedly from 28–56 mW · m −2, immediately after rifting, to a present value between 7 and 14 mW · m −2 depending on boundary conditions. The analysis shows a distinctive difference between the evolution of the Labrador Sea and that of the northeastern Newfoundland margin where it is estimated that the transient component of the heat flow exceeded 100 mW · m −2 after rifting and dropped rapidly to very low values. The study suggests that the subsidence of the Newfoundland margin was not caused by cooling only but required an additional mechanism such as lower-crustal phase transformations or continuing extension after continental breakup with ductile deformation in the lower crust and upper mantle. The estimated transient heat flow was used to calculate the average present surface heat flow in eastern Canada's margins, which is the sum of heat flow from the mantle, crustal heat production, and the transient cooling of the lithosphere. The calculated heat flow compares well with heat flow density data obtained from bottom hole temperature measurements.

Models of burial diagenesis for deep exploration plays in Jurassic fault traps of the Central and Northern North Sea

With increasing burial depth, intergranular porosity declines in a predictable manner according to theoretical and empirically established mathematical relationships. In basins with simple structural and thermal histories, average porosity can often be represented as an exponential function of depth. There are, however, common significant deviations from such a simple model of porosity evolution. Four mechanisms are recognized by which enhanced porosity may be generated or maintained during burial: development of overpressures due to restricted pore fluid escape; inhibition of mechanical compaction by selective cementation; inhibition of cementation as a result of hydrocarbon accumulation; or generation of secondary porosity due to either grain or cement dissolution together with simultaneous removal of reaction products. Secondary porosity may be generated either near the surface by the ingress of dilute meteoric-derived waters or at depth by the generation of ‘aggressive’ pore fluids. Deep burial diagnesis may encompass both closed and open system porosity generating reactions. In closed systems, increase in temperature concomitant with burial overcomes kinetic barriers enabling dissolution of metastable detrital minerals. By contrast, in more open systems, movement of fluids from either mudstones or evaporites to either overlying or adjacent sandstones introduces chemical disequilibrium that may cause mineral dissolution. Rapid, intermittent movement of fluids along active faults may additionally introduce temperature disequilibrium to further facilitate mineral dissolution in the proximity of faults. Over geological periods of time, however, faults are barriers to fluid flow and provide a mechanism for generating overpressure. In the Central and Northern North Sea basins, an upper diagenetic regime is separated from a lower diagenetic regime by a regional cover of Cretaceous mudstones and chalks. The Cretaceous sediments provide a regional seal which often coincides with the development of overpressure above deep graben centres reflecting restricted pore water escape. Fluid and solute movement between these two regimes is only possible where faults or gas chimneys penetrate the Cretaceous seal. In the upper regime, porosity loss in Tertiary sediments broadly conforms to the predicted decline of porosity with depth. In the lower regime, Jurassic sandstones on structural highs are characterized by extensive secondary porosity resulting from feldspar grain and carbonate cement dissolution, best preserved where oil accumulation rapidly follows porosity generation. Enhanced porosity can be expected either where sandstones interdigitate with mudstones or in graben centre plays adjacent to faults. This contrasts with graben margin plays where faults are the locus of pronounced cementation and complex diagenetic sequences are developed. In the absence of either oil accumulation or overpressure, secondary porosity either compacts or is occluded by a characteristic deep burial authigenic mineral assemblage. The present heat flow in the North Sea can be interpreted in terms of a residual thermobaric fluid flow from basin centres to basin margins which was probably initiated during the early Tertiary, coincident with the onset of rapid thermal relaxation subsidence.

Skagerrak evolution derived from tectonic subsidence

Stratigraphic data gathered from 32 points in the continental Skagerrak Basin have been analyzed by basin modelling methods. Non-tectonic effects of sediment loading were removed by backstripping, giving the “observed” tectonic subsidence. This was compared to synthetic subsidence curves constructed on the basis of a combined lithospheric extension and metamorphism model, where gabbroic magma, which was emplaced during the Permian, is transformed to granulits-eclogite facies. The overall picture is that prior to 230 Ma subsidence was roughly similar north and south of the Fennoscandian Border Zone (FBZ). After this time the northern area sagged due to the still-preserved lithospheric thermal anomaly and continued metamorphism, whereas the southern area was also affected by extension. A physical explanation for this contrast in evolution is given by considering the relative lithospheric strength history of the areas, the northern area becoming progressively stronger with time relative to the southern area as crustal thicknesses are reduced. Total post-570 Ma extension is approximately 1.4 (40%) north of the FBZ and 1.5 (50%) south of it. North of the FBZ the average thickness of the metamorphic layer is between 3 and 7 km, whereas south of the zone a thickness of 1–2 km is estimated. This difference may reflect variations in the degree of Permian magmatism. The temperature history of the sediments and the degree of organic maturation has been calculated by the TTI method. The youngest sediments to have experienced temperatures high enough for significant oil generation are estimated to be of Bathonian age. The present model heat flow of about 57 mW m −2 is in agreement with observations. Lateral displacements along the FBZ are of the order of 10–20 km, and have consequently only played a minor role in the development of the Skagerrak Basin. A thermal lithosphere thickness of about 125 km is inferred.

Challenge Facing Oil Explorers in Nevada

Can operators in Nevada maintain the current trend of rising oil production In October 1990, Nevada oil production averaged over 11,500 b/d, up about 30% from 1989 average daily production and double 1985 rates. This extraordinary surge reflects improvements in the understanding of Nevada's unique oil-field geology. Future increases in production will result from meeting geological, geophysical, and engineering challenges, such as the following. (1) Enhancing subsurface structural imaging. Integration of outcrop geology, borehole geology, and remote sensing with seismic, potential field, and electrical geophysics will improve structural interpretations. (2) Predicting pre-Miocene subcrop stratigraphy. Better understanding of complex Mesozoic and Tertiary structural overprinting, combined with additional well control, will permit more precise balancing of cross sections. Such sections will better predict the position of reservoir-quality rocks relative to sealing surfaces like the pre-Miocene unconformity. (3) Understanding local thermal histories and source rocks. Mapping variations in present and past heat flow, along with local source rock quality will allow better judgment as to whether oil and gas may have been generated and preserved in a given area. (4) Optimal handling of fluid-sensitive reservoirs. Use of air drilling has doubled production rates at Trap Spring field and may give similar results inmore » other sensitive reservoirs. Application of sound geological and geophysical concepts and advances in drilling and completion technology will allow Nevada operators to find and develop significant new oil reserves in the 1990s.« less

Gas Hydrates as Potential Resource of Energy and Pathfinders for Conventional Type Hydrocarbon Deposits

Solid compounds of water and gaseous hydrocarbons are known as gas hydrates, clathrates, or cryohydrates. They occur naturally in offshore and terrestrial environments, in the areas where temperature is at least seasonally low (i.e. close to or below freezing), bathymetric, geostatic, ice, or permafrost pressure is sufficiently high, and the source of hydrocarbons is available. These factors (regional and local geological conditions of 21 locations grouped into 13 study regions worldwide offshore and one in permafrost environments with proven, reported, and inferred presence of gas hydrates) have been recently researched by Geoexplorers International, Inc. Conservative estimations from Geoexplorers International suggest that the world's total gas hydrates may contain 7,000 to 50,000 tcf of natural gas. Although at this time exploitation of gas trapped in the hydrate zone and below is not economically viable, because estimated reserves are enormous, they should be seriously considered as potential energy resource. Smaller, but less dispersed massive gas hydrate deposits associated with fault zones may be the first offshore gas resource to become economic. This research, particularly of the Messoyakh gas field, has proved that the presence of gas hydrates provides very useful information in exploration for conventional oil and gas deposits. Gas hydrates indicatemore » ongoing hydrocarbon generation in the sediments. Hydrates are valuable to assess the present heat flow and thermal history of a region. Since gas hydrates exist only under a very limited range of pressure and temperature, deviation in patterns of their occurrence can be related to changes in pore water chemistry, hydrocarbon composition, or pressure and temperature gradient anomalies.« less

Geological information obtained from coal and coalified dispersed organic matter in the Tertiary system of Japanese islands

The geological structure of the Tertiary system of the Japanese islands is rather complicated because the islands were subjected to the Alpine orogeny at the northeastern margin of the Asian continent which faces the active Pacific and Philippine Sea plates that moved toward the continent. The major coal-bearing formations are Tertiary in age and were affected by tectonic movements throughout their geologic history that began with sedimentation, diagenetic alteration, and ended in final geotectonic deformation. Because geological conditions were unstable, coal and oil fields are smaller in size and tend to induce short-lived mining operations and difficult or fruitless exploration. Though the geological background is thus unfavorable for economic development of low-cost hydrocarbon resources, coal and coalified dispersed organic matter (MOD) in the sediments offer abundant valuable information to solve geological problems on the Cenozoic island-arc system within limited areas of the islands. The Japanese islands are composed of several arcs: Ryuku, Honshu, Kurile, and Hidaka-Saghalien from southwest to northeast. The Honshu arc is subdivided into two parts northeastern and southwestern by the Fossa Magna (great fissure). The major coal and oil fields are restricted in distribution; i.e., the former are distributed in the northeastern Pacific side and contrasting continent side of the western edge of southwestern Honshu. The major oil fields are restricted mostly to the continental side of northeastern Honshu. Regional variation of coal-seam thickness in the major coal fields suggests a change in the sedimentary conditions of the basins (Aihara, 1980a,b,c). Variation of maceral composition provides more detailed information about differences of sedimentary conditions in peat swamps of the coal basins. The characteristic maceral composition of less fusinite and micrinite in the Japanese Tertiary coal suggests that the peat swamp was kept wet and buried quickly, thus preventing desiccation (Takahashi and Aihara, 1989). Diagenetic and thermal alteration of the Tertiary System can be analyzed by the degree of coalification (rank) of coal and coalified MOD in the sediments. vitrinite reflectance ( R 0) was adopted as the rank parameter. The variation of R 0 regionally, with stratigraphic position, and at various depth in boreholes was studied. The result has revealed regional variation of the paleogeothermal gradient which can be related to the present terrestrial heat flow conditions of the islands (Aihara, 1980a,b,c; Takahashi and Aihara, 1989). Paleogeothermal conditions in characteristic geotectonic provinces of the coalfields is recognized; i.e., lower heat flow conditions in the northeastern Pacific side where there are thicker sediments, and higher heat flow conditions in the southwestern continental side where there are thinner sediments. Geological structure also contrasts between the two provinces; i.e., coalfields in the former are folder and faulted, whereas those in the latter are gently dipping and simple half-grabens. These characteristic geotectonic conditions provided contrasting temperature-pressure conditions during coalification (or organic metamorphism) of each province; i.e., lower temperature and/or higher pressure conditions predominated in the northeastern Pacific side, and higher temperature and/or lower pressure conditions predominated in the southwestern continental side behind the active volcanic front. The delicate difference in temperature-pressure conditions of coalification reactions affected the chemical composition in vitrinite of the two provinces. Minute examination of ultimate analytical data of vitrinite from major coalfields (Sugimura et al., 1966) reveals that the vitrinite in the northeastern Pacific has slightly higher H/C atomic ratios than that on southwestern continental side. The hydrogenous active fractions generated during coalfication tended to be more preserved in the chemical structure of the coal under lower temperature and/or higher pressure conditions than under higher temperature and/or lower pressure conditions that caused their release (Aihara, 1980a,b,c). Gas chromatographic analysis of released gas from fresh samples collected from coal-mine faces has detected aliphatic fractions only from coalfields in the northeastern Pacific side. Coaly laminae and coalified MOD in the non-coal-bearing sediments in the Pacific side of southwestern Honshu, which are contemporaneous with the coal-bearing sediments on the continental side, can be utilized to analyze the paleogeothermal history and tectonic deformation of the extremely complicated accreted body of the Shimanto Terrain. Regional and stratigraphic variation of R 0 of MOD from the Kyushu Shimanto Terrain has been studied, and thermal effects from the newly born, warm Philippine Sea plate was detected. Measurement of the direction of optical anisotropy ofo R 0 (bireflectance, R bi = R max − R min) of coaly laminae in regions of high R 0 value (> 1.0) reveals the direction of stress and strain during the low-temperature metamorphism. Geological information obtained from coal and coalified MOD of the Japanese Tertiary system in the field and the laboratory was successfully utilized to analyze the sedimentological, diagenetic, paleogeothermal, and tectonic problems. it is evident that the same methods can also be adapted to older clastic sediments if they contain MOD.

Constraints on the Central Indian Basin thermal structure from heat flow, seismicity and bathymetry

The lithosphere beneath the Central Indian Basin is characterized by high heat flow, widespread deformation of sediment and acoustic basement, and unusually high seismicity. The high heat flow suggests that temperatures in the lithosphere may be higher than expected for its age. We explore the constraints on the temperature distribution with depth by combining heat flow, bathymetry, and seismicity data. The high heat flow (~30 mW m −2 greater than expected) constraints the near-surface temperature distribution. The bathymetric depths (on average those expected for these lithospheric ages after corrections for sediment loads) constrain the integral of the temperature with depth. Since the depths of oceanic intraplate earthquakes appear to be limited by an isotherm (~ 750°C), the maximum earthquake depth (40 km) constrains the minimum depth of deformation and the maximum temperature there. The present thermal structure is investigated by examining different models matching these three observational constraints. First, the possibility is examined that the high heat flow results from anomalous lithosphere with a basal temperature or thickness different from typical oceanic lithosphere. In this case the temperature constraints from the earthquake depths restrict the additional heat flux to less than 20 mW m −2. However, since the heat flux anomaly requires plate thicknesses substantially thinner than usually assumed, it appears that at most 10 mW m −2 can be added by this mechanism. Second, the possibility is examined that the extra flux results from reheating the bottom portion of the lithosphere to asthenospheric temperatures. Substantial reheating 30–40 km below the surface is required to match the high heat flow. Such reheating should produce $ ̃ 1 km of uplift, whereas the average basement depth is no shallower than expected for its age. Thus, significant deep lithospheric reheating cannot be widespread over the deformed region. Third, the effects of a temperature perturbation within the lithospheric column and the resulting change in surface heat flux with time are examined. A temperature perturbation at shallow depths could produce the present heat flow anomaly, but no evidence for shallow intrusion or other mechanism for such an effect exists. It is concluded that, despite the heat flow anomaly, the lack of an average bathymetric anomaly and the observation of seismicity to a depth of 40 km indicate that lithospheric temperatures in the Central Indian Basin are not significantly different from those expected for its age.

Effects of asthenospheric heat flow in basin modelling exemplified with the Danish Basin

A mathematical model is developed to simulate the process of basin formation by rifting. Subsidence is driven by crustal thinning and influenced by the transient temperature variation within the lithosphere. The model offers possibilities for multiple phases of rifting superimposing non-steady state conditions. Important features in the model are: two-layered heat production, and a definition of the lower boundary in terms of heat flow instead of one of constant temperature as opposed to other models. Furthermore, phase transformations caused by rift-induced pressure and temperature changes are suggested as an additional syn-rift subsidence mechanism. The relevant phase transformations are assumed to be those occurring during an amphibolite to granulite facies transition. Perturbation of the steady state temperature profile is performed by one or several phases of rifting (with finite stretching rates), by sill intrusion, or by changes in asthenospheric heat flow. The greatest deviation from the steady state geotherm occurs at the lower boundary because of the flow definition of the lower boundary. The consequence of this is a much longer duration of anomalous heat flow. The model is applied to the Danish Basin, North Jylland-Denmark. This basin was formed in Permian times and since then it has been thermally subsiding apart from a minor event of stretching in the Late Cretaceous. The higher present-day heat flow in the basin compared to the adjacent Fennoscandian Shield area is successfully explained by the model presented in this paper. It is concluded that transient temperature conditions still today contribute significantly to present surface heat flow, although main rifting took place for more than 240 m.y. ago. This cannot be explained by models using a fixed temperature definition of the lower boundary. The modelled contribution by phase transformations to subsidence, explains why the syn-rift Early and Middle Triassic subsidence was accompanied by insignificant faulting activity, as evidenced by the seismic data available.

Splitting of normal modes and travel time anomalies due to the magnetic field of the Earth

We examine the effects of a toroidal magnetic field on two types of seismic data: first, on splitting of normal modes and, second, on body wave travel times. The main contribution of this paper is the derivation of formulas for these problems. Comparison between data and theory indicates that the magnitude of the toroidal magnetic field (T20) has to be as large as 250 T (tesla) to explain the data. There are, however, some inconsistencies among the fit to the data set. For example, some modes require a strong magnetic field near the core mantle boundary, while other modes require a weak field. Also the travel time difference between polar and equatorial paths is not explained by the T10 or T20 field. A field with odd angular degree 𝓁 (T𝓁0 or S𝓁0) is needed to produce the travel time difference between the polar and equatorial paths. We also point out that ohmic dissipation due to a strong magnetic field with 100 T is about 4–5 orders of magnitude larger than the present heat flow output of the Earth. The analysis suggests that the magnetic field in the outer core is not the cause of seismic anomalies.