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Comparative Geothermal Gradients and Heat Flows across African Basins derived from deep wells

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Comparative Geothermal Gradients and Heat Flows across African Basins derived from deep wells

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About 1750 geothermal gradient measurements from deep wells have been accessed across the African plate. Thermal conductivities based on North African studies have been used to either directly estimate heat flow from these or calibrate other authors estimates. Converting to such 'comparative heat flow' values reduces many of the lateral contrasts seen in geothermal gradient. The Present Day thermal regime of West Africa is shown to be consistent and featureless, with the exception of high heat flows over Senegal/Mauritania, North Gabon and the Cameroon Line. Other regions show far more variation and generally higher heat flows. Two main controls are observed on Present Day heat flow, namely time elapsed since rifting/break-up and an apparent irregular input of heat from the mantle. Plots of heat flow against age of basin show support for McKenzie models of exponential thermal decline. The Red Sea, as an active spreading and hyperextended magma-poor margin, is therefore a valid analogue for the past thermal regime of many passive margins. Extinct magmatic margins are colder than their amagmatic counterparts. The regions of highest heat flow generally show a correlation with those of Miocene-Recent mantle-derived igneous activity and those of anomalous upper mantle seismic velocities, which indicate a thinned lithosphere.
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Petroleum Exploration Society of Great Britain, Africa is Back : Smarter, Better, Stronger Conference, October 2019, Extended Abstracts
Comparative Geothermal Gradients and Heat Flows across African Basins derived
from deep wells
Duncan S. Macgregor:
MacGeology Ltd, UK
duncan@macgeology.co.uk
ABSTRACT
About 1750 geothermal gradient measurements from deep wells have been accessed across the African plate. Thermal
conductivities based on North African studies have been used to either directly estimate heat flow from these or calibrate
other authors estimates. Converting to such ‘comparative heat flow’ values reduces many of the lateral contrasts seen in
geothermal gradient. The Present Day thermal regime of West Africa is shown to be consistent and featureless, with the
exception of high heat flows over Senegal/Mauritania, North Gabon and the Cameroon Line. Other regions show far more
variation and generally higher heat flows.
Two main controls are observed on Present Day heat flow, namely time elapsed since rifting/break-up and an apparent
irregular input of heat from the mantle. Plots of heat flow against age of basin show support for McKenzie models of
exponential thermal decline. The Red Sea, as an active spreading and hyperextended magma-poor margin, is therefore a
valid analogue for the past thermal regime of many passive margins. Extinct magmatic margins are colder than their
amagmatic counterparts. The regions of highest heat flow generally show a correlation with those of Miocene-Recent
mantle-derived igneous activity and those of anomalous upper mantle seismic velocities, which indicate a thinned
lithosphere.
INTRODUCTION
The first objective of this paper is to map geothermal gradient and heat flow changes across the African plate based on data
from deep wells. No such analysis has previously been published, with most previous regional compilations based on data
from shallow cores and boreholes (e.g. Davies, 2013). As Figure 1 shows, there are many reported geothermal gradients
and heat flows from deep wells across the continent. This paper analyses this diffuse dataset to produce ‘comparative heat
flow estimates between basins,which have calibrated with a simplified common thermal conductivity model. The term is
used to differentiate the figures calculated here from those of previous authors, all of which have used differing thermal
conductivity models. This methodology is designed to reduce systematic errors to a level at which valid regional trends can
be identified, though there still may be miscalibrations to heat flows documented in shallow cores and on other continents.
A second objective of the paper is to analyse the geological causes of the regional variations that can be confidently
identified in these ‘comparative heat flow estimates. Africa is an ideal setting to test ‘Mckenzie’ (Mckenzie, 1978) models of
thermal decay following high heat flow events during rifting or spreading. The age of Africa’s continental margins range from
Triassic to that of the incipient Southern Red Sea ocean, while rifts vary in age from Early Permian to Recent. The Northern
Red Sea provides a crucial analogue of a hyperextended magma-poor rift (Stockli and Bosworth, 2019), reflecting a state
that many passive margins went through in the past. Extinct magmatic and amagmatic margins are also available for
comparison as are cratonic and non-cratonic settings. Finally, the presence of significant young mantle-derived igneous
activity on the eastern and northern parts of the African plate, allow for an analysis of the supply of additional heat from
mantle sources, as does recent advances in mapping S wave velocities in the mantle (Emry et al, 2019). From the
petroleum aspect, it can thus be seen that Africa provides an invaluable wide range of Present Day analogue settings that
can be used to estimate the thermal state of basins through their history and thus calibrate basin models.
DATABASE
Africa is unfortunately a less good choice for study on the strength of public domain datasets. Data here has been compiled
in this study from published papers (circa 70% of the dataset), with the remainder from a variety of oil industry sources,
which range from conference abstracts to personal communications in conference talks. A full listing is available from the
author and will be included in a final published paper. 1748 geothermal gradient estimates (Figure 1) were accessed from
these sources. These are converted to comparative (surface) heat flow through Fouriers Law of heat conduction, i.e.
Heat Flow = Geothermal Gradient * Thermal Conductivity
where Heat flow is in mW/m2, thermal conductivity is in W/m/K and geothermal gradient is in 0C/km.
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Figure 1: Geothermal Gradients (oC/km) in deep wells across the African plate. Individual reported values coloured by value, with highest
gradients in red. Average values for basins are labelled. Plotted versus main tectonic lineaments, occurrence of volcanics and limits of
interpreted cratons. Note the wide range of values for the West African margin, to be compared with a much more limited range for
comparative heat flow (Figure 3). Values seen range from 14 oC/km in the Taoudenni Basin to over 70 oC/km in parts of the southern Red
Sea.
In addition, 1207 surface heat flow estimates from deep wells have also been accessed from the literature. These have a
variable degree of supporting data supplied.
Approximately two thirds of the geothermal gradient and heat flow values, and most of the well documented data, come from
North Africa. A region between Ghana and Angola in West Africa is covered by geothermal gradient estimates of reasonable
quality, as is the Republic of South Africa, but the database in East Africa is generally poor and in some countries such as
Tanzania shows wide questionable variations. Care is therefore taken in this paper that the contrasts in heat flow identified
between basins exceed the error bars estimated for the dataset concerned.
CORRECTIONS, STANDARDISATION AND ERRORS
The estimation of both geothermal gradient (Figure 1) and thermal conductivity (Figure 2) is subject to numerous errors, both
random and systematic. These will be assessed in detail in a full version of this paper. Geothermal gradients are derived
from both literature and personal communications. A set of standardised values are used for the thermal conductivity of
different rock types in different porosity ranges (Rimi and Lucazeau, 1987; Goutorbe et al, 2008) to convert the available
geothermal gradients to comparative heat flows For basins where many previous heat flow points have previously been
published, thermal conductivities have been calculated on a subset of well columns and a correction factor determined
between these and the authors implied values. It can be noted on Figure 2 that the range of average values calculated for
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thermal conductivity is as large as that seen for geothermal gradient, thus it cannot always be assumed that a rise in
geothermal gradient is a rise in heat flow and indeed major changes in trends are seen between Figures 1 and 3.
Error bars for geothermal gradient range from <10% for the best temperature datasets, increasing to >20% for the poorer
datasets. About 80% of the data is either from Horner plot corrected temperatures or drill stem tests. A typical error bar on
thermal conductivity estimation may be 20%, which may reduce with the averaging of large datasets (as random errors are
reduced). Considering the specific errors within each dataset, particularly with respect to the origin of the temperature data,
the accuracy of lithology identification and the number of datapoints averaged to give basin averages, it has been possible
to place each of the basins defined in Figure 1 into three categories, corresponding roughly to 10%, 20% and 30% error
bars. Most North African basins are in the 10% error bar category, most West African basins in the 20% category and most
East African basins in the 30% category. Additional confidence is obtained through reviewing the comparative heat flow
estimates for basins which would be expected to have regionally consistent heat flows, e.g. those that are believed to have
been thermally cooling for a considerable period and which lack current igneous activity. As shown by a range of such
examples (e.g. Angola, Eastern Mediterranean and Nile regions), fairly consistent comparable heat flow values are obtained
across these basins despite considerable variation in lithology and geothermal gradients (Figure 3).
Figure 2: Thermal conductivity (W/m/K) calculations for the well sections assessed in this study, calculated from public domain well
columns. Labelled figures relate to averages for the basins outlined. Note the wide range of values in particular over West Africa, a
function of changing lithologies as one moves offshore from sand and carbonate rich successions to mudstone and salt rich intervals.
Onshore settings have higher average thermal conductivity due to high percentage contents of low porosity sandstones, particularly those
of Paleozoic age.
ANALYSIS OF COMPARATIVE HEAT FLOW ESTIMATES
The average comparative heat flow measured in this study across Africa is 71mW/m2, though this figures has a strong bias
towards North Africa, where the majority of data points lie. The ‘average of averages’ of the basins listed on Table 1 is 68
mW/m2, which represents a more realistic figure for a typical African plate heat flow. The equivalent average for geothermal
gradient is 30oC/km.
Conversion to ‘comparative heat flow’ (Figure 3) has reduced the number of apparent anomalies that would be identified on
geothermal gradient alone. In other words, some contrasts in geothermal gradients are proposed to be the result of lateral
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lithological and thermal conductivity changes, particularly between sand-rich shelfal successions and deep-water muds. This
work suggests that the elevated geothermal gradients of around 35-40oC/km that are frequently reported from relatively
shallow deepwater wells, particularly in West Africa, are the consequence of the low thermal conductivity of the often young
under-compacted mud-rich sections over which these gradients are measured. A wide variation in geothermal gradients
across Angola. for instance, translates to an even pattern in heat flow once compensation is made for wells being
terminated at shallow depths below mudline in deep-water settings, a decrease in sand and carbonate content basinward, a
thickening of undercompacted Neogene sections offshore and variations in thickness of the Aptian salt. This has resulted in
the conclusion that the Present Day comparative heat flow regime of West Africa south of Senegal is rather consistent and
featureless, with the exception of a few anomalies in deepwater North Gabon (Norton et al, 2019) and close to the
Cameroon Line. Nevertheless, this conclusion would ideally require to be checked with a more adequate set of thermal
conductivity data. A further example of such evening out of heat flow in a region of varying geothermal gradient is the Nile
Delta/Israel/Levantine Basin region, where geothermal gradient variations seem to be largely related to the presence or
otherwise of thick Messinian salt.
Figure 3 : Comparative heat flow estimates for the well datapoints assessed in this paper together with averages per basin. Plotted versus
main tectonic lineaments, cratons and regions of interpreted thin lithosphere or anomalously hot upper mantle (Fishwick and Bastow,
2011, Lodhia et al 2016, Lesquer et al, 1990). The northern part of the African plate shows much wider variations and higher values than
the southern part : this can be related to the frequency of Cenozoic volcanics and mantle anomalies. The Red Sea and Algeria now stand
out as the major positive anomalies with Southern Africa, Taoudenni and the Eastern Mediterranean as the main negative anomalies.
COMPARATIVE HEAT FLOW ANOMALIES RELATED TO BASIN FORMATION AND THERMAL COOLING
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When the average heat flows across passive margin and rift basins are compared against the time elapsed since maximum
rifting or the initiation of drift (Figure 4) there is broad support for McKenzie (1978) and Hantschel and Kauerauf (2009)
models of exponential thermal decline from the peaks associated with crustal stretching. Thus the hottest basins seen in this
study are the incipient Red Sea ocean and the East African Rift, while some of the coldest are margins or rifts that have
been inactive for more than 150Ma, e.g. the Levantine Basin, a rift that has been cooling since the Triassic. When the Red
Sea and West African basins are considered together on Figure 4, it seems apparent that the cooling after breakup is
exponential, with basins returning to an equilibrium level of under 70mW/m2 within 100Ma. The rate and shape of the
inferred decline is very similar to that theoretically calculated by Hantschel and Kauerauf (2009), based on various Beta
factors and a declining crustal radioactive input through time. However, many Algerian and Tunisian basins, including some
intracratonic settings that have not experienced significant stretching for 500 My, lie well above the trend, suggesting that
other sources of heat are active in this region.
This relationship to basin age allows us to use Present Day thermal regimes as potential analogues for the past thermal
history of old margins. The Red Sea is of particular importance here, where the Present Day captures a period of heating
either associated with hyperextension (northern Red Sea) or within 4.5Ma of the onset of spreading (southern Red Sea)
(Stockli and Bosworth, 2019). The comparative heat flow averages calculated in these two basins are probably
underestimates, as most wells are located close to the shorelines, in the regions of the lowest interpreted Beta (stretching)
factors. Comparative heat flows of 122mW/m2 at Qusair B1x in the northern Red Sea and between 148-215 mW/m2 in
Eritrean Wells close to oceanic crust are likely to be more representative of regions of higher Beta factor in the far offshore.
These can be compared with an average of 78mW/m2 in the Gulf of Suez, which was originally part of the same rift system,
but was abandoned at around 14Ma. The differences can be attributed to the Red Sea having since advanced to much
higher Beta factors than the Gulf of the Suez, while the latter has now entered a period of post-rift thermal cooling.
Figure 4: Averaged comparative heat flow per basin plus a few peak values for individual wells, plotted versus period elapsed since either
first emplacement of oceanic crust or for failed rift basins, the last main rift episode. With the exception of Tunisian and Algerian values,
exponentially declining trends are seen very similar to those illustrated for a variety of Beta factors by Hantschel and Kauerauf (2009,
Figures 3.29-3.32)
COMPARATIVE HEAT FLOW ANOMALIES RELATED TO CRUSTAL TYPE
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The radioactive component of the underlying crust is thought to contribute a high proportion of heat (Hantschel and
Kauerauf, 2009). We would expect thus regions rich in granites, particularly young granites, to be hot relative to those
underlain by more basic material. Extinct magmatic margins such as Namibia, the Seychelles and Mozambique, which are
predicted to be underlain by seaward dipping reflectors and a gabbroic crust are, as expected, observed to have
comparative average heat flows about 30% less than their amagmatic counterparts in age, a difference well in excess of
estimated error bars in West Africa (Figure 4, compare Namibia vs Angola), through closer to the limit of the error bar in East
Africa (compare Seychelles/Central Mozambique vs other East African margins). Cratonic areas such as the Taoudenni
Basin and interior of South Africa also are observed to be cold as predicted by Allen and Allen (2013), though with Algeria
providing again a noteable exception.
COMPARATIVE HEAT FLOW ANOMALIES RELATED TO DEEP-SEATED IGNEOUS ACTIVITY AND ANOMALOUS
MANTLE
After time of cooling and margin type are taken into account, a series of high heat flow anomalies remain with a contrast to
analogue aged basins that vastly exceeds the error bars. These appear to be concentrated in Algeria and Tunisia, being
particularly apparent for the older basins illustrated on Figure 4. Less areally extensive anomalies are observed elsewhere,
as represented by some of the localised peaks shown for West African basins on Figure 4 and 5.
The Algerian anomaly is the most data-rich and best documented of these (Takherist, 1990 ; Lesquer et al, 1990). The latter
reference emphasises that the thermal anomaly is located over the southern Sahara Basins, including the Ahnet (where the
highest values of over 100mW/m2 occur), Mouydir and Illizi Basins. Comparative heat flows increase in these basins
towards the Ahaggar Massif but the anomaly does not appear to extend over the southern part of the massif on the basis of
heat flow measurements in shallow boreholes (Lucazeau et al, 1991). The anomaly runs at a high angle to the contrasting
NW-SE or N-S trending Basement terranes that characterize this region and cannot therefore be related to variations in
crustal heat production, while no significant extension has occurred since at least the Devonian. Calculations of heat
production from the Basement by Lesquer et al (1990) suggest that only 30-40% of the heat flux at surface is crustally
derived, thus a mantle heat contribution of 60-70mW/m2 is implied. A relationship is implied to a zone of anomalously low S
wave velocities in the upper mantle at around 70-125 km depth, as mapped in a variable fashion by Lesquer et al (1990),
Fishwick and Bastow (2011) and Emry et al (2019), this variation in interpretation being seemingly a consequence of the
lack of receiver stations in this part of Africa. These velocities are thought to indicate partial melting, an interpretation
consistent with extrapolation of the geotherms into the mantle. An active metasomatised mantle is implied by the
composition of Miocene to Recent volcanics in the Illizi Basin and on the northern Ahaggar : some chemical changes could
be exothermic and thus be generating some of this excess heat.
Logan and Duddy (2003) and Makhous and Galushkin (2003) have investigated the thermal history of the Ahnet Basin,
including analysis of maturity data and Apatite Fission Track Analysis. An even higher heat pulse is however interpreted at
around 200Ma, requiring either heat flows of around 200mW/m2 or a lower peak supplemented by hydrothermal movements
of hot waters (Makhous and Galushkin, 2003). It must be questioned as to why such extreme events seem to be repeating
in the same region, hundreds of millions of years apart, a trend which will also be described elsewhere.
Over the scale of the African plate, the mantle S wave anomalies of Fishwick and Bastow (2011) and Emry et al (2019)
broadly correspond to regions of Miocene-Recent uplift, positive interpreted dynamic topography and intrusion or extrusion
of Miocene-Recent alkaline mantle-derived igneous material (Figures 3 and 5) The distribution of such volcanics across
Africa in time and space would suggest that the Miocene was the peak of such activity. Given the degree of definition of the
velocity anomalies, there appears to be a good regional correlation with regions with comparative heat flow exceeding
90mW/m2 on Figures 3 and 5, including the Red Sea and East African Rift. It should also be mentioned however that this
relationship between mantle derived volcanics, low S wave velocities and high heat flow is not perfect. Significant igneous
activity for instance exists on the western flank of the Sirt Basin of Libya, yet calculated comparative heat flows are ‘normal’
at around 50-65mW/m2.
A number examples of smaller anomalies will now be discussed from the West African margin (Figure 5), representing likely
mantle derived anomalies that have had a significant effect on the petroleum systems of the region concerned. The volcanic
chemistries and histories on West African margins (Bellion and Crevola, 1991) are very similar to those of Cape Verde and
the Canary Islands, representing regions that have been proposed to be underlain by mantle convection cells (Lodhia et al.,
2016). A volcanic area extending from northern Senegal to southern Mauritania (often referred to as the ‘Dakar Plume’) may
therefore represent the impingement of one of these mantle swells onto the continental margin. Geothermal gradients
observed in the region of the Cayar volcanic seamount, reach 50o C/km at the Teranga field, this converting to 93mW/m2 in
comparative heat flow. Heat flows fall off to the north and south away from the region of volcanic and intrusive activity to
backgrounds of roughly half these figures. Again, the magmatism is multi-phase, with events in the Late Cretaceous,
Miocene and Quaternary. This clearly has a major effect on source rock maturity in the region, with most petroleum
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discoveries in the region having been made within the limits of the thermal anomaly, and most of these being of relatively
dry gas.
Figure 5: Relationship over NW and W Africa between anomalously high comparative heat flows (>80mW/m2) , known occurrences of
Cenozoic volcanics (polygons exaggerated for clarity) and two interpretations of upper mantle S wave velocity anomalies (Fishwick and
Bastow, 2011 and Lesquer et al, 1990 in brown: Emry et al, 2019 in grey). These differ due to the sparse distribution of receiver stations in
North Africa : it is not known which is most accurate.
To the south of this, along the Gabon-Angola margin, the Present Day heat flow regime is generally consistent and
featureless, with comparative heat flows having arguably returned to regional averages following heat flow peaks associated
with Cretaceous rifting or spreading. In Nigeria, there is a moderate increase in geothermal gradients and comparative heat
flows towards the Cameroon Volcanic Line (which has volcanic chemistries very similar to those in Algeria, again mantle
derived (Kampunzu et al, 1991)). A shear wave velocity anomaly is also mapped in the upper mantle (Figure 5), particularly
in the interpretation by Emry et al. (2019).
The North Gabon deepwater region has three wells showing elevated comparative heat flows between 72-85mW/m2, amidst
a regional background of around 60mWm2, as calculated from informally reported geothermal gradients in these wells of 42-
55 oC/km (e.g. Doran et al, 2016). This region has recently been analysed by Norton et al (2019), who relates the elevated
heat flows to igneous activity. Again, there seems to have been a trend of igneous activity remaining in much the same
location, with Norton differentiating four post-drift igneous phases ranging from Late Albian to Oligo-Miocene. Plume activity
thus again appear to be occurring in pulses in similar locations, as for the other mantle derived anomalies described. Emry
(2019)’s S wave velocity maps suggest that the Cameroon Line mantle anomaly may extend over this region, though again
this is an uncertain interpretation given a shortage of receiver stations in West Africa.
OTHER CAUSES OF HEAT FLOW ANOMALIES
A range of other causes may cause some positive anomalies on a local scale. These include;
Effects of very young uplift of hot aquifers, which could be proposed for the high heat flows in shallow fields in the
northern part of Lake Albert, Uganda.
Hydrothermal effects, i.e. vertical movements of hot waters, also a possible cause in the northern Lake Albert
fields. These may overprint the regional and often mantle imposed trends in regions of igneous activity such as
other parts of the East African Rift System (EARS) and potentially in other hot regions such as Algeria.
Hydrothermal activity has, for instance, led to the formation of oil seeps associated with shallow maturation of
Pleistocene source rocks in EARS basins such as Lake Tanganyika (Simoniet et al, 2000). Because of these
local effects, the comparative heat flow estimate for the EARS may be unrealistically high. Hydrothermal effects
are also evident in the past record, where they can be recognised as vertical or anomalous vitrinite reflectance
trends versus depth : a good example is presented by Baudino et al (2019) for a Late Cretaceous event in the
Kwanza Basin..
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Radioactive heat contributions from thick argillaceous sediment piles, often estimated at about 1Wm2 for each km
of sediment (Hantschel and Kauerauf, 2009). This may cause small relative increases in thick offshore
depocentres
Rapid sedimentation has been proposed to lower surface heat flow (Hantschel and Kauerauf, 2009). However, the
basin with the highest Neogene sedimentation rate in this study is the Niger Delta, and the comparative heat flow
measured here from the deep intervals does not seem to be lower than adjoining basins.
Salt effects, e.g. rising geothermal gradients are documented over salt domes in Senegal (Brun and Lucazeau,
1998). This is the result of the focussing of heat laterally into thermally conductive salt.
CONCLUSIONS
Africa is a dynamic continent, both in terms of crustal and mantle tectonics. This is reflected in wide variations in geothermal
gradient and comparative heat flow. Large positive thermal anomalies are seen most clearly in the most tectonically active
parts of the plate, i.e. in North and East Africa. More stable tectonic regimes such as those of West Africa to the south of
Senegal and Southern Africa appear to show a more consistent thermal regime, with the variations seen in geothermal
gradient being perhaps driven primarily by lithological and thermal conductivity variations.
Most African continental margins appear to have followed a standard ‘McKenzie’ cooling model, with the youngest margins
being the hottest and the older margins generally the coolest. This would suggest that many of the older margins at the time
of their initiation, with heat flows potentially as high as seen today in the Red Sea. Interior rifts show similar age versus heat
flow trends, though perhaps with greater variation. Extinct magmatic margins and most cratonic regions are significantly
colder than otherwise correlative regions.
A series of high heat flow anomalies are seen which cannot be attributed to basin formation processes, particularly in the
northern and eastern parts of the African plate. These seem to be associated with mantle metasomatism and partial
melting, with a relationship observed at a regional scale with the occurrence of mantle-derived volcanics and low S wave
velocity anomalies at 75-150km depth. This suggests an irregular additional supply of heat from the mantle across parts of
the African plate. Related but more areally limited anomalies also occur sporadically on the West African margin, including
regions such as Senegal, the Cameroon Line and north Gabon, where they have a significant petroleum effect. Many such
regions show evidence of repeated igneous and thermal activity over long periods of time.
PETROLEUM IMPLICATIONS
Oil and gas windows are known to vary widely in Africa, with oil generation thresholds for instance in north-east Africa
varying from around 1500 m in the Eritrean Red Sea to 4500 m in the Levantine Basin. These extremes, which are
consequences of the different ages of initiation of these basins, tie to extremes in the petroleum products of these two
basins, which are dominated by thermogenic and biogenic gas respectively. This leads to a certain degree of predictability
in Present Day heat flow, through its relationship with basin age. In some regions, however, this relationship breaks down
due to an overprint of high mantle derived heat flow. This component can be less well confidently predicted through
relationships with mantle seismic velocities and with mantle derived volcanism. Improved seismic definition of the upper
mantle would help considerably in the predictability of heat flow.
Modelling of the past have been a particular uncertainty in basin modelling, with often an under-appreciation of the
significance of past thermal events in Africa having led to an under-prediction of the level of source rock maturity and
predictions of an oil phase rather than the resulting gas. Perhaps the main application of this study will lie in calibrating the
thermal history of data-poor basins, applying Present Day analogues from this study to specific past tectonic settings.
In order to improve the predictability of heat flow histories, both at Present Day and in the past, it is suggested that we need
therefore to:
Consider the plate tectonic setting and igneous history throughout the basin’s history.
Use data on modern analogues to predict the past.
Consider the relationship to volcanics, which involves understanding the chemistry and origin of these
volcanics.
Understand the heat contribution from the Basement and how it may have changed through time.
Improve our imaging of the upper mantle in order to be able to predict anomalies associated with partial
melting in that zone, such as that below Algeria
Apply ranges of conceivable thermal histories, not a single model
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North of 20°N, the northwest part of the stable African shield is characterized by an anomalous high heat flow zone (80-110 mW m-2). This east-west elongated zone is oblique to major Proterozoic structural units and cannot be related to differences in crustal heat production. Maximum heat flow occurs in the southern part of the Sahara basins (100-120 mW m-2). Only a mantle heat flow contribution of 60-70 mW m-2 can account for this very high surface heat flow. The estimated geotherms suggest that the temperature of upper mantle is anomalously high and partial melting may be present at shallow depths. This hypothesis is supported by the existence of a long-wavelength negative component of the gravity field that corresponds with the high heat flow axis and with the very low lithospheric S-wave velocities (4.2-4.4 km/s) down to 160 km obtained from surface wave studies. In addition, the chemistry of the scarce Cenozoic melilitite lavas (Illizi volcanic district), geographically associated with the thermal anomaly, as well as the mineralogy of their inclusions (phlogopitized garnet/spinel peridotite and alkali clinopyroxenite) show that the upper mantle beneath this region is locally highly metasomatised and partially melted. From these preliminary results, it appears that the northwest African upper mantle has been modified by recent thermal rejuvenation processes (including gas transfer) that are likely to be more extensive in the southern part of the Sahara basins.