Geochemical modelling in rapidly subsiding basins

Advm~s in Organic ~ i s t r y 1967 Org. Geochem. Vol. 13, Nos I-3, pp. 181-186, 1988 Printed in Great Britain. All rights reserved

0146-6380/88 $3.00+ 0.00 Copyright ~ 1988 PergamonPress plc

Geochemical modelling in rapidly subsiding basins M. A. CHIARAMONTE ~, A. TAMBURINI I, T. SALVATORI 2 and A. BERSANI ! ~Agip S.p.A., G E O C 20097 S. Donato Milanese, Milano, Italy 2Eniricerche S.p.A., CHIF 20097 S. Donato Milanese, Milano, Italy Absn'act-----Geochemicalmodelling in rapidly subsiding basins presents some puzzling questions. In trying to clarify them, two African basins related to different domains were studied. The high Tertiary and Quaternary sedimentation rate is a common feature of the two basins. In each of them present heat flow values were computed in different wells on the basis of temperature, stratigraphic and petrophysical data using a one dimensional finite element mathematical model. The resulting distributions show in some cases irregular patterns, which are examined in detail to outline their possible causes. Extrapolation of the present thermal regime to the past generally gives computed maturity values considerably higher than those obtained experimentally. The experimental data include vitrinite reflectance values, Rock-Eval pyrolysis results, sterane and hopane isomerization and sterane aromatization. Hypotheses about thermal history are made in order to explain the areal maturity distribution. They are checked comparing computed maturity values and experimental ones. By combining experimental data and results of the modelling exercise we conclude that the high sedimentation rate, independent of the tectonic domain in which it occurs, can induce noticeable and peculiar thermal anomalies. Key words: modelling, high sedimentation rate, thermal anomalies, maturity, African basins, thermal

history

INTRODUCTION Many Oil Companies now make systematic use of geochemical modelling to evaluate maturity evolution in sedimentary basins; consequently, mathematical models have been applied in many different basins with very unlike geological features. On the basis of these experiences it turned out that each type of basin can present particular problems in the application of geochemical modelling. This paper deals with some aspects of maturity evolution studies in basins characterized by a high Tertiary and Quaternary sedimentation rate. Two African sedimentary basins (hereinafter called basin A and basin B) are here specifically examined in order to analyze the relationships among present heat flow distribution, sedimentation rate and experimental maturity data. These two basins have many common features. For example, overpressured sections are present in both of them, and heat flow values computed in different wells on the basis of temperature data show irregular patterns. Moreover, measured maturity levels are unexpectedly low if the present thermal regime is extrapolated to the past. GEOLOGICALSETTING The two sedimentary basins under examination are related to different tectonic domains: more specifically a passive continental margin basin (A) and a downwarp basin (B) respectively.

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In basin A (see Fig. I) the Upper Cretaceous to Eocene sediments, represented mainly by marls with limestones and shales, were tilted to the West at the end of the Paleogene and unconformably overlain by a regressive sequence, with deep water shales and turbidites, grading upward to nearshore sand and shale (Oligocene-Miocene). On the whole this regressive sequence is characterized by shale with sandy levels, which show good lateral continuity through the basins. An alluvial sand section with minor clay deposits (Pliocene-Pleistocene) follows. In basin B (see Fig. 2) the downwarping took place during Oligocene, with the reactivation of the ancient weakness lines. Clay with interbeds of sand was deposit~, followed, after a major tectonic phase (Lower Miocene), by marls and argillaceous limestone. The Middle-Upper Miocene sequence is characterized by high subsidence and sedimentation rates, with deposition of clay with rare interbeds of sand, grading upward to evaporites. The Pliocene-Pleistocene transgressive sequence is represented by clay with some sand and sand with gravel in the uppermost sector. The following remarks could be made about both basins. Due to the high sedimentation rate and the low vertical permeability of the sediments, the Oligocene-Miocene sequence is always overpressured. Furthermore, the sedimentation rate, increasing towards the deepest part of the basin, could have produced a "lateral pressure gradient" in the same lithostratigraphic units.

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M.A. CHIARAMONTEet al.

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The average geothermal gradients, computed on the basis of temperature data (DSTs and BHTs corrected according to Fertl, (1977) in different wells of basin A, range from 2.5 to 4°C/100 m. In basin B these values have a narrower interval, from 2.5 to 3.2°C/100m. On the basis of temperature data, heat flow values at the base of the sedimentary sequence were also computed for each well. To this aim a finite element mono-dimensional model was used. This model has been extensively described by Novelli et al. (1987) and is based on classical equations describing

the fluid and heat transport in deformable porous media. Stratigraphic and petrophysical data a r e essential input data for the model. Thermal conductivity and permeability values were inferred from experimental measurements performed in AGIP laboratories and, in some cases, from publications. Thermal conductivity analyses were made using a needle probe developed by CISE (Segrate, Milan, Italy). Permeability determinations were performed according to API RP40 (1960). Heat flow values were calibrated in order to obtain a good match between calculated and experimental

Geochemical modelling in rapidly subsiding basins

183

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Fig. 2. Basin B: section showing heat flow, temperature, pressure gradient and sedimentation rate distribution. temperature values. The computed values show an irregular pattern in both basins and Figs 1 and 2 show the heat flow values computed for different wells along two sections in basin A and B respectively. In basin A (see Fig. 1) the geothermal gradient increases from the deepest part of the basin (Well 1) towards the areas of the minor Tertiary sedimentation rate (Wells 2, 3 and 4) as clearly shown by isotherm lines. The heat flow values, computed on the basis of the sedimentary sequence, show a similar trend. In basin B (see Fig. 2) the same geothermal gradient and heat flow pattern is observed. Well 1 represents the thermal situation in the basinal area. Wells 2, 3 and 4, located on the same structure show a significant variation in computed heat flow values. This variation is really surprising when one takes into

account the short distances between these wells and the fact that in many different basins, where the same mathematical model was applied, computed heat flow values vary only by a small percentage over large areas. In both basins the parallelism between geothermal gradient and calculated heat flow is evident; it would therefore seem that thermal conductivity does not play an important role in determining the thermal anomalies found in these basins. It might be added that thermal conductivity values do not vary significantly, vertically or laterally, inside the same formation. Despite these considerations, an hypothetical lateral contrast of thermal conductivity in the deepest and less known sediments cannot be ruled out. In this case, heat could flow laterally along the more conductive sediments causing an irregular tern-

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:ig. 3. Section A (Wells 2-3-4): comparison between observed and computed maturity values (hypothesis of heat flow constant through time).

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perature distribution and an "apparent" uneven distribution of heat flow at sediment/basement interface. Another hypothetical cause of the thermal anomalies could be related to updip lateral water movement from the deepest parts of the basin. This hypothesis will be discussed later. Presently it must be stated that the quantity and the distribution of temperature data are not enough to show, unequivocally, possible local anomalies along the well profile which would confirm this hypothesis. MATURITY

To assess maturity in both the basins under examination some different parameters were used. In addition to Rock.Eval pyrolysis results and vitrinite reflectance, the extent of thermal maturity was measured using the relative concentrations of some specific hydrocarbons. Three well-known reactions were followed: (i) Isomerization of steranes at the C-20 position: Natural product sterols have the 20-R configuration which is retained in the steranes in immature sediments. Upon maturation the 20-R steranes are transformed into the 20-S epimers and an equilibrium 20S/(20R+ 20S) =0.54 is approached. (Mackenzie and McKenzie, 1983). (ii) Isomerization of hopanes at the C-22 position. The hopanoids of the living organism mainly exist with the R stereochemistry inherited by the hopanes in early diagenesis. Upon heating,

185

the 22-R epimers convert into 22-S. The equilibrium value of 228/(22R+22S) is 0.61. (Mackenzie and McKenzie, 1983). (iii) Transformation of monoaromatic steroids into triaromatic steroids. Ring-C monoaromatic compounds are formed during early diagenesis and then change in late diagenesis and catagenesis into ABC-ring triaromatic steroid hydrocarbons, with the loss of the nuclear methyl group on the A/B-ring juncture and of seven hydrogen atoms. With increasing thermal maturity almost all monoaromatics are converted into triaromatics: there is no reverse reaction in this case. The extent of aromatization is measured by T/M + T. Experimental measurements were performed on C~Sa(H), 14a(H), 17=(H) steranes, on C3217=(H), 21#(H) hopanes, on C~C-ring monoaromatic steroids and on CzsABC-ring triaromatic ones according to Mackenzie and McKenzie (1983, and references therein). The degree of maturity shown by the different experimenta] parameters was consistent. Theoretical values of maturity parameters were also computed using the aforementioned model. To obtain vitrinite reflectance values the Tissot and Espitali6 (1975) method was applied using type III kerogen kinetic parameters. The transformation ratio values computed this way, were converted in equivalent R.% values through the relationship proposed by Tissot and Espitali6 (1975), and slightly modified according to our experience. As a first step, maturity parameter computations were arrived at by extrapolating the present thermal regime through the geological past. The relevant results showed a good fit with experimental data in the wells with lower heat flow values (for example Well 1 in Fig. 1 and Well 1 in Fig. 2). This was not so in all the other wells, where computed values overestimate the maturity degree, pointing out a colder thermal regime in the past. A comparison between computed and experimental maturity values for Wells 2, 3 and 4 of section A is displayed in Fig. 3. D~CUSSION

As previously mentioned both the basins under examination are affected by overpressures. The main cause of the anomalous pressure values is the high Tertiary and Quaternary sedimentation rate together with the low vertical permeability of sediments, which may hamper the expulsion of water during compaction. In Figs 1 and 2 pressure gradient profiles are shown for some wells (pressure data were not available for all the wells displayed in the two sections). In the same figures the Tertiary sedimentation rate increases towards the deepest part of basins A and B. On a lower scale, the same can be observed at the

186

M.A. CHIAKAMON'I"~et al.

high shown in Fig. 2. This difference in sedimentation rate, together with the low vertical permeability of sediments, could be responsible for a lateral pressure gradient, capable of driving hot water from the deepest part of the basin updip, along porous and permeable layers, which have good lateral continuity. Head space isotopic analysis profiles of the wells examined show anomalous peaks of thermogenic gas in correspondence in these layers. Fig. 4 shows an example relative to Well No. 4 of section A. This evidence confirms the possibility of fluid migration from the deepest parts of the basins. Hot water flow could result in an increase of temperature values in the areas with lower sedimentation rate. This phenomenon could give rise to an over-estimation of heat flow computed at the base of the sedimentary sequence. In other words the heat flow values at the basement/sediment interface could be uniform in both basins and the apparent local variations could be due to the influence of hot water flow. Apart from overpressure, and irregular distribution of present heat flow values, the two basins are characterized by another peculiar anomaly: the unexpectedly low maturity data, with the exception of the areas with maximum sedimentation rate. In order to explain the discrepancy between experimental and computed values we had to assume that the heating of sediments took place only in recent times. This phenomenon could be related to the hot water flow from the deepest part of each basin. As the water transfer is related to the high Tertiary and Quaternary sedimentation rate, we can reasonably assume that this phenomenon took place only recently as a consequence of a progressive overpressure release. In fact some maturity computations using different thermal histories were performed in each well in order to obtain a good match with experimental values. In particular the lowest heat flow values found in each basin (26 mW/m z in basin A and 36 mW/m ~ in basin B) were used for the past. An increase up to present day values was assumed for the wells with minor tertiary sedimentation rate starting from different ages. In both basins the best results were obtained by increasing the heat flow values during the Pliocene. Figure 5 shows the results relative to the application of this particular thermal history in basin A (maturity trend for section A wells is represented in this figure).

CONCLUSIONS As previously shown the two basins have many analogies. It is therefore logical to assume that the causes of thermal anomalies and maturity behaviour could be the same in both basins; moreover it is reasonable to think that thermal anomalies and maturity behaviour are closely related. On the basis of the observations made, especially about maturity, water circulation seems to be the most probable cause of the irregular pattern of computed heat flow in both basins. However only by using a three-dimensional modelling it is possible to prove the validity of this assumption. Additional detailed experimental data could also be very useful to this aim. The thermal regime in these two basins is complex, and the extrapolation of heat flow values from the wells to the basinai areas can result in significant errors in maturity evaluation. For example, if in basin A only the data relevant to wells 2-3-4 had been available, maturity in the deepest sector of the basin, where the main "kitchens" are located, would have been highly overestimated. Only a better understanding of the thermal regime can avoid this potential source of errors. Finally we can reasonably envisage in these two basins the presence of a close interdependence between the rapid subsidence and the observed thermal anomalies. Acknowledgements--We are grateful to Agip's Management

for permission to publish and to Professor L. Mattavelli and Mr L. Noveili for supporting this work and providing the environment that encourageswork of this kind. We are also indebted to our colleagues for their helpful suggestions and cooperation. REFERENCES API RP40 (1960) Recommended Practicefor Core Analysis Procedure. American Petroleum Institute, Division of Production, New York. Fertl W. H. and Wichmann P. A. (1977) How to determine static BHT from well log data. World Oil 184, 105-106. Mackenzie A. S. and McKenzieD. (1983)Isomerizationand aromatization of hydrocarbons in sedimentary basins formed by extension. Geol. Mag. 120(5), 417-528. Novelli L., Chiaramonte M. A., Mattavelfi L., Pizzi G., Sartori L., Scotti P. (1987) Oil habitat in the northwestern Po basin. In Migration of Hydracarbons in Sedimentary Bas/ns. Editions Technip Paris. Tissot B. P. and Espitali6 J. (1975) L'evolutionthermique de ia matiere organique des sediments: application d'une simulation mathematique. Rev. lnJt. Ft. Pet. 30, 743-777.