Kerry A. Hegarty

Thermal annealing of fission tracks in apatite 4. Quantitative modelling techniques and extension to geological timescales

Abstract A methodology is presented for the prediction of fission-track parameters in geological situations from a laboratory-based description of annealing kinetics. To test the validity of extrapolation from laboratory to geological timescales, the approach is applied to a number of geological situations for which apatite fission-track analysis (AFTA) data are available and where thermal history is known with some confidence. Predicted fission-track parameters agree well with observation in all cases, giving confidence in the validity of the extrapolation, and suggesting that fission-track annealing takes place by a single pathway in both laboratory and geological conditions. The precision of predicted track lengths is considered in some detail. Typical levels of precision are ∼ ±0.5 μm for mean lengths ⪷ 10 μm, and ∼ ±0.3 μm for length ⪆ 10 μm. Precision is largely independent of thermal history for any reasonable geological thermal history. Accuracy of prediction is limited principally by the effect of apatite composition on annealing kinetics. The development of fission-track parameters is illustrated through a series of notional thermal histories to emphasise various key aspects of the response of the system. Temperature dominates over time in determining final fission-track parameters, with an order of magnitude increase in time being equivalent to a ∼ 10°C increase in temperature. The final length of a track is determined predominantly by the maximum temperature to which it is subjected. Aspects of AFTA response are further highlighted by prediction of patterns of AFTA parameters as a function of depth and temperature from a series of notional burial histories embodying a variety of thermal history styles. The quantitative understanding of AFTA response not only affords the basis of rigorous paleotemperature estimation, but also allows a better understanding of the situations in which AFTA can be applied to yield useful information.

Fission track lengths in the apatite annealing zone and the interpretation of mixed ages

Abstract Measurements of confined fission tracks in apatites from deep boreholes show that their mean length is reduced, and the length distribution becomes progressively broader through the fission track annealing zone. At subsurface temperatures of around 100°C and above, the length distribution characteristically becomes very broad without a pronounced peak. The lengths of the longest tracks remain essentially constant at all stages of natural annealing observed in the boreholes. This pattern is similar to that found in laboratory annealing of spontaneous fission tracks in apatite from an outcrop sample of the same formation. These observations show that confined track lengths can be used as important indicators of the type of thermal history that a sample has experienced in the temperature zone of increasing track stability. The length distributions reported here provide a basis for interpretation of fission track ages that might otherwise be ambiguous. An apatite age which results from a uniform slow-cooling history will have a broad, negatively-skewed length distribution, while a bimodal distribution provides clear evidence of a two-stage history involving partial annealing by a discrete thermal event. The apparent fission track age associated with a bimodal length distribution will be a “mixed” age intermediate between the original age and that of the later event.

Recognition of the thermal effects of fluid flow in sedimentary basins

Abstract Moving fluids are capable of transporting a large amount of heat over long distances in sedimentary basins, but the effects are often ignored when modelling the thermal evolution of sedimentary basins. If significant, the passage of these heated fluids through the sediment pile will leave a thermal signature which can be measured at the present-day using palaeotemperature determinants such as vitrinite reflectance and apatite fission track analysis (AFTA™). The interpretation of palaeotemperature-depth profiles, particularly the slope of a palaeotemperature-depth profile, i.e. the palaeogeothermal gradient, allows fluid-induced temperature profiles to be distinguished from those due to simple conduction of basal heat flow. Steady-state systems, typified by large-scale lateral fluid flow in foreland basins or where low-temperature hydrothermal circulation systems occur associated with intrusions and thick volcanic piles, are characterized by dog-length geothermal gradients, with high-interval gradients near the surface, above the shallowest aquifer, and lower interval gradients beneath an aquifer. It is argued that the classic occurrence of near vertical vitrinite reflectance-depth profiles observed in many ancient foreland basins can result from this mechanism. Thermal effects of transient fluid flow are most easily observed at the present-day, where they are characterized by low or negative geothermal gradients beneath aquifers in active geothermal systems, and the same criterion enables their recognition in ancient situations. Similarly, shallow-level igneous intrusion into porous and permeable sediments can produce observable thermal signatures further from the intrusion than will result from simple conduction, but which can be readily explained by movement of fluids heated by the intrusion. Complex steady-state profiles that may be difficult to distinguish from transient profiles without additional geological data may arise where multiple aquifers are separated by aquitards. The thermal consequences of fluid flow in two of these situations, a foreland basin and igneous intrusion into porous sediments, are illustrated by examples of thermal history reconstruction from the Pliocene Papua New Guinea Fold Belt and the Canning Basin of Australia, respectively.

Quantifying exhumation from apatite fission-track analysis and vitrinite reflectance data: precision, accuracy and latest results from the Atlantic margin of NW Europe

Abstract In areas where significant unconformities are present, palaeotemperatures derived from apatite fission-track analysis (AFTA) and vitrinite reflectance (VR) data through a vertical rock section can be used to estimate palaeogeothermal gradients and (by extrapolation to an assumed palaeo-surface temperature) amounts of exhumation (palaeoburial). AFTA also provides a direct estimate of the timing of exhumation. These parameters can be used to reconstruct more complete histories than those based purely on the preserved rock record. Precision and accuracy of these estimates are controlled by a range of theoretical and practical factors, perhaps the most important being the use of appropriate kinetic models. In extracting thermal history information from fission tracks in apatite, it is essential to use models that can describe variation in response between apatite grains within a sample. It is also important to recognize the limitations of the methods. AFTA and VR are dominated by maximum temperatures, preserving no information on events prior to a palaeo-thermal maximum. Recognition of this allows definition of key aspects of the history with greater precision. Results from NW Europe define a series of regionally synchronous palaeo-thermal episodes, with cooling beginning in Early Cretaceous, Early Tertiary and Late Tertiary times. Latest results show that Early Tertiary palaeo-thermal effects in NW England can be understood as being due to a combination of higher basal heat flow and deeper burial, and emphasize the importance of obtaining data from a vertical sequence of samples. Comparison with similar results from other parts of the world suggests that events at plate margins exert a key influence on the processes responsible for regional exhumation, as recognized through Mesozoic and Cenozoic times across NW Europe.

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.

Complexities in the Development of the Caroline Plate Region, Western Equatorial Pacific

Johnson and Moinar (1972) and Bracey and Andrews (1974) speculated that the region north of New Guinea might consist of a lithospheric plate separate and distinct from the adjacent Pacific, Indo-Australian, and Philippine plates. The boundaries of the Caroline plate (Fig. 1) were first identified by Weissel and Anderson (1978) using marine geophysical and seismological evidence. They concluded that an additional plate (the Caroline plate) currently exists, using arguments based on (1) plate motion models and (2) tectonic characteristics of the Sorol Trough, the Mussau Trench, and the curiously disrupted seafloor between these features (Fig. 2).

The post-Carboniferous evolution of Ireland: evidence from Thermal History Reconstruction

Integration of Apatite Fission Track Analysis (AFTA ® ) and vitrinite reflectance (VR) data from onshore Ireland reveals a complex thermal history, characterized by multiple cooling episodes of late Carboniferous, Jurassic, early Cretaceous, early Tertiary and late Tertiary age. Peak palaeotemperatures in each episode decrease through time to produce an overall long-term cooling trend since the late Carboniferous. Thermal history styles across the region are very similar, though the magnitude of peak palaeotemperatures in individual episodes shows some variation. Similar thermal histories are also identified in the surrounding offshore regions. The regional nature of all these palaeo-thermal episodes, and their correlation with regionally significant unconformities, suggests that heating was due primarily to greater depth of burial, with subsequent cooling representing the progressive unroofing of the present onshore region since late Carboniferous times. In Northern Ireland, explanations of early Cretaceous and early Tertiary palaeotemperatures in terms of greater depth of burial are more difficult to reconcile with geological evidence, and heating due to hot fluid movement appears more likely. This applies particularly to early Tertiary effects, for which the Tertiary Igneous Province provides a ready explanation. Over the entire onshore region, maximum maturity levels in Carboniferous and older units were reached at the end of the Carboniferous, and preservation of hydrocarbons to the present day, through several tectono-thermal episodes, appears unlikely.

Dating and duration of hot fluid flow events determined using AFTA® and vitrinite reflectance-based thermal history reconstruction

Abstract Heating due to lateral introduction of hot fluids is becoming an increasingly recognized feature of the thermal histories of sedimentary basins. In some basins, heating by fluids may have an important effect on hydrocarbon source rock maturation history, so that quantification of the magnitude and timing of heating become essential elements in hydrocarbon prospectivity. In other cases, determining the time of fluid heating in a reservoir may provide a key constraint on hydrocarbon migration history. Examples are presented using AFTA apatite fission track analysis and vitrinite reflectance (VR) data to identify and quantify fluid heating in well sequences from several regions. In the West of Shetland region, in the vicinity of the Rona Ridge, non-linear palaeotemperature profiles defined by AFTA and VR results provide evidence of local heating shallow in the section. AFTA timing constraints suggest introduction of heated fluids produced by nearby Tertiary intrusive activity, although the time constraints are broad because of the low maximum palaeotemperatures involved (R0max < 0.6%). In a well from Asia, transient maximum palaeotemperatures > 120°C resulted in R0max > 0.6% in an Eocene section, with AFTA constraining the fluid flow event responsible for the early to mid Miocene (25 to 10 Ma). On the North West Shelf of Australia transient fluid flow associated with hydrocarbon leakage, and possibly charge, has been previously identified by a combination of AFTA, VR and fluid inclusion homogenization temperature (Th) results. In the East Swan-2 well, a fracture inclusion in quartz from shallow Eocene sandstones gives a minimum Th value of 88°C, c. 40°C higher than the present temperature. AFTA and VR data show no direct evidence of sustained heating at such a temperature, and can only be reconciled if the duration of heating was c. 20 000 years. The results are consistent with this event being associated with passage of a hot brine and hydrocarbon fluid (O’Brien and Woods, 1995). These case studies demonstrate that a combination of thermal history tools can be used to identify and quantify the thermal effect of fluid flow, potentially allowing much tighter constraints on hydrocarbon generation and migration histories.

Thermal and tectonic evolution of the Falkland Islands: implications for hydrocarbon exploration in the adjacent offshore region

Apatite fission track and vitrinite reflectance data combine to suggest that the Falkland Islands experienced three discrete episodes of heating and cooling during the Phanerozoic, each of which can be related to major changes in the plate tectonic setting of the islands. Combined with published data from the adjacent offshore areas, the results provide unique insights into the behaviour of the region during the phases of convergent and divergent tectonic movements that have affected the region since the Permian. Initial cooling of the Falkland Islands in the Late Permian was initially restricted to West Falkland, and was the result of differential uplift and erosion of West Falkland relative to East Falkland, coinciding with D2 Gondwanian deformation. Subsequent Early Jurassic cooling of both East and West Falkland was associated with plume-related thermal uplift preceding the separation of East and West Gondwana and the development of the Falkland Plateau Basin. The Falkland Islands experienced renewed heating during the Mesozoic, probably due to increased burial, suggesting that the offshore basins originally extended onto the present onshore region. Late Cretaceous/Early Tertiary cooling of the Falkland Islands was contemporaneous with the development of a transpressional plate boundary along the southern margin of the Falkland Plateau resulting in erosion of the islands that were situated on a flexural forebulge.

Direct measurement of timing: Underpinning a reliable petroleum system model for the Mid-Continent rift system

The Amoco 1 Eischeid well (Sec. 6, T83N, R35W, Carroll County, Iowa) was the first significant well drilled along the flank of the Iowa horst, providing an opportunity to evaluate the stratigraphic and structural history of the enigmatic Mid-Continent rift system of the United States. New thermal-history data obtained from cutting and core samples show that three paleothermal events affected the drilled section at the Amoco 1 Eischeid well. Results show that three significant cooling events occurred sometime between 300 and 200, 70 and 50, and 35 and 10 Ma. The first of these events was associated with a high paleogeothermal gradient (35C/km), demonstrably greater than the present-day value (16C/km). These new results provide direct evidence of a significantly higher paleogeothermal gradient (and by inference, higher heat flow) during the Paleozoic. In addition, results show that maximum maturity of the rich Precambrian source, the Nonesuch Shale (equivalent), was reached during the 300–200-Ma event and not 600–700 m.y. earlier as previously proposed. Previous work has been significantly influenced by the view that the craton was relatively stable or behaved uniformly throughout the Phanerozoic. Previous models impose a Proterozoic time of generation and entrapment for any Precambrian source intervals. With recognition of significant Phanerozoic heating events, the opportunity for later generation emerges, thus reducing the preservation time and increasing the probability of entrapment and preservation needed for exploration success in the region. Furthermore, these events mapped at the Mid-Continent rift system may be of great regional extent and correlative in time to other well-known events along the North American plate margins (e.g., Alleghanian, Laramide, Ancestral Rockies orogenies), thus offering insights to new possible exploration targets within the rift, perhaps indicative of deep-seated processes in the mantle beneath the North American craton during the Phanerozoic.