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Fluvial Sequence Stratigraphy using Thorium & Potassium
Today I post what I believe is the best method for fluvial sequence stratigraphy. Interpreting sequences in the fluvial environment from well logs is difficult due to the many depositional environments present in the fluvial/continental realm. This post explains a good method for correlating fluvial sequence boundaries using the chemical footprint of paleosols that can be read from the typical log suit recorded in most exploration areas nowadays. I hope you find this useful and comments are very welcome!Fluvial Sequence Boundaries - their identification using Th/K spikes formed on paleosols
Kaolinite is a group of common hydrous aluminumsilicate clay minerals with the chemical composition Al2Si2O5(OH)4. Kaolinite is usually white but also occurs in orange or red colors due to iron oxidization. It is commonly produced by the chemical weathering of feldspars in tropical climates. During this weathering process Potassium, which is more mobile, is leached, altering the clay-minerals crystal structure, producing a lower shrink-swell capacity and reducing the cation exchange capacity. Kaolinite forms in humid warm climates after intense weathering and is a chemically mature product of feldspars.
In fluvial environments subaerial exposure and flow of meteoric water enhances diagenetic processes and paleosols are formed. This may occur throughout all system tracts and their formation can be very unpredictable and depends on many environmental influences. However interfluves and areas of nondeposition are the prime areas where these paleosols are preserved, where otherwise channel amalgamation and migration may rework sediment. Interfluves develop primarily due to incision or during times of fluvial bypass which are the result of lowering of the fluvial graded profile. This in turn is mainly caused by base-level fall, discharge increase and/or sediment load decrease. Several allogenic forces influence aggradation and incision. However, in most passive margins tectonic activity is rather low and subsidence gradual, therefore fluvial incision and aggradationis more likely to be influenced by climate and/or eustatic sea-level. Both forces are active on a regional scale and fluctuation thereof may govern formation of regional and time equivalent paleosols.
Increased precipitation or a warmer climate will increase chemical weathering and enhances leaching and chemical maturity of soils is achieved faster in these settings. Also vegetation changes may alter acidity of meteoric water and enhance or reduce the leaching process. Furthermore, historic climatic changes have been successfully deduced through investigating paleosol formation and their chemical character (Ardmas and Weaver, 1958; North and Boering, 1999; Ruffel and Worden, 2000; Schnyder et al., 2006). But also studies on base-level related paleosols formation have been conducted (Hampson et al., 2005). In essence, lower base-level results in lower water tables further inland, which enhances flow of meteoric water and subsequently speeds up the leaching process. Also, because lower base-levels usually force incision or fluvial bypass to occur, interfluves develop where paleosols are better preserved as explained earlier. However, increased discharge due to precipitation may also force flattening of the fluvial graded profile, inducing incision. Both causes for enhanced paleosols formation are equally probable and may actually coincide and strengthen or counteract one another. It is thus particularly difficult to accurately link paleosols formation to a single allogenic force. This, nonetheless, only affects the understanding of the geological history, as the correlative power of paleosols remains, due to the regional control of both forces. On the other hand other indications of climatic character and base-level variations may be incorporated in this analysis and may be able to link the formation of paleosols to a certain allogenic force, which will be done in the Discussion.
Spectral Gamma-ray (SGR
) logging, in which Thorium (Th), Potassium (K) & Uranium (U) gamma-ray wavelength are distinguished, allows the differentiation of these three mayor radio-isotopic sources. Analysis of the sources of the natural gamma radiation can provide information on the composition and likely lithology of a formation. In particular, cross-plots of Thorium (ppm) versus Potassium (%) are used in petrophysical log evaluation as they allow identification of certain clay types. In most studies a Th/K ratio of >12 for kaolinitic clays and a Th/K ratio of >3.5 for illite (a less chemical mature feldspathic clay) is suggested. The Th/K ratio is thus usually included in modern log suites to allow for direct identification of clay types. Hampson et al. (2005) used the Th/K ratio to identify kaolinitic zones in a fluvial sequence of the Bookclifs in Utah and linked them to paleosols formation. His resulting sequence stratigraphic framework corresponded well with the extensively studied deltaic succession of the same delta. The formation of the Potassium depleted kaolinitic zones was related to increased leaching due to low water tables related to base-level fall. A Th/K ratio of >17 was used for the identification of the zones, while a ratio of <3 was used for illite. Furthermore high Thorium and Uranium levels where related to heavy mineral bearing channel lags, which were associated with incision and subsequent backfill of high energy streams.Footnotes
Complication for using the method proposed here arise in the fact that leaching can occur at deep subsurface depths if groundwater is acidic. Potassium leaching below coal seems may therefore induce erroneous interpretation of paleosols. Furthermore migration of hydrocarbons and thermal release of acids from Kerogen may cause kaolinization. This generally occurs at depths <1km as it relies on flow of meteoric water for the removal of Potassium (Ehrenberg, 1991). In addition, it is also important to identify the origin of the Th/K spikes as they should be caused by Potassium depletion rather than a Thorium increase. In general a fining upward sequence should follow the Th/K spike and it should coincide with a sudden density and porosity shift and gamma-ray decrease to fit within the concept of sequence boundaries.
Furthermore, the implications of Kaolonite leaching on secondary porosity are still not fully understood. Several authors have proposed that the dissolution of aluminumsilicate grains can be an important cause of increased porosity and permeability in sandstones. This may be due to the dissolved minerals being removed from the formation in solution rather than precipitated locally as clays (Siebert et al., 1984; Surdam et al., 1984). An alternative theory suggests that secondary porosity produced by aluminumsilicate grain dissolution represents merely a short-range relocation of porosity (Thomas and Stoessell, 1985). Another hypotheses is that porosity variations in leached zones simply reflect variations in primary sand quality, thereby implying that the location of the leached zones are fortuitous (Ehrenberg, 1991). References
Ardmas, J.A.S., and Weaver, C.E., 1958, Thorium-t-Uranium ratios as indicators of sedimentary processes: example of concept of geochemical facies: Bulletin of the American Association of petroleum Geologists, v. 42, p. 43.
Ehrenberg, S.N., 1991, Kaolinized, potassium-leached zones at the contacts of the Garn Formation, Haltenbanken mid-Norwegian continetal shelf: Marine and Petroleum Geology, v. 8, p. 19.
Hampson, G.J., Davies, W., Davies, S.J., Howell, J.A., and Adamson, K.R., 2005, Use of spectral gamma-ray data to refine subsurface fluvial stratigraphy: late Cretaceous strata in the Book Cliffs, Utah, USA: Journal of the Geological Society, London, v. 162, p. 18.
North, C.P., and Boering, M., 1999, Spectral Gamma-Ray Logging for Facies Discrimination in Mixed Fluvial-Eolian Successions: A Cautionary Tale: AAPG Bulletin, v. 83, p. 14.
Ruffel, A., and Worden, R., 2000, Palaeoclimate alaysis using spectral gamma-ray data from Aptian (Cretaceous) of southern England and southern France: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 155, p. 18.
Schnyder, J., Ruffel, A., Deconinck, J.F., and Baudin, F., 2006, Conjunctive use of spectral gamma-ray logs and clay mineralogy in defining late jurassic Cretaceous palaeoclimate change (Dorset, U.K.): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 229, p. 17.
Siebert, R.M., Mackenzie, G.K., and Lahann, R.W., 1984, A theory of framework grain dissolution in sandstones: AAPG Memoire, v. 37, p. 12.
Surdam, R.C., Boese, S.W., and Crossey, L.J., 1984, The chemistry of secondary porosity: AAPG Memoire, v. 37, p. 12.
Thomas, A., and Stoessell, R.K., 1985, Nature of secondary posity created by dissolution of aluminium silicates: AAPG Bulletin, v. 69.