RU2633659C2 - Method of estimation of sedimentation speed - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: measuring the magnetic susceptibility of carbonate rocks at different stratigraphic levels or sections of the section. Graphs or maps of values, the reciprocal of magnetic susceptibility, are used to judge the qualitative variation in sedimentation rates. The rates of sedimentation at an arbitrary level (interval) of the section are calculated taking into account the values of the magnetic susceptibility and sedimentation rates corresponding to the reference intervals of the section and the measured values of the magnetic susceptibility.

EFFECT: accurate determination of the rate of sedimentation for any level or section of the carbonate formation.

3 dwg

Description

The invention relates to the field of geology and is intended to assess the sedimentation rate of carbonate deposits by their magnetic susceptibility.

A known method of studying the geological sections of wells in order to identify disagreements and interruptions in sedimentation (see application for invention RU 92003522, IPC G01V 1/40, G01V 1/00, publ. 06/19/1995). The known method provides for the identification in the context of points of change of geological cycles of sedimentation. To do this, the known method for determining geological disagreements includes geophysical studies, determining the density of rocks from the cuttings continuously selected during drilling, the operation of dividing the section into geological sedimentation cycles is carried out according to the harmonic analysis of GIS curves, converting the GIS curves into a sequence of curves - the main harmonic components with periods equal to the sizes of elementary cyclites in the corresponding intervals of the section, identify the boundaries of intervals with continuous harmonic characteristic, thereby determining the points of change of sedimentation cycles, which are used to judge the location of disagreements.

However, the known method does not allow to determine sedimentation rates other than zero.

Closest to the claimed technical solution is a method for constructing sedimentary attributes, including calculating sedimentation rates (see application for invention US 2014/0278106, IPC G01V 99/00, published on 09/18/2014). The method is based on the well-known dependence of reservoir power on sedimentation rate and includes reconstruction of lateral sedimentation rates and paleogeographic modeling of formation formation.

However, the prototype is applicable only for layers with clearly defined bedding surfaces and allows only a qualitative assessment of sedimentation rate and does not allow to obtain absolute values of sedimentation rate.

The objective of the invention is to enable the quantitative calculation of sedimentation rate for any level or area of the carbonate formation.

The technical result consists in eliminating the disadvantages of the prototype and improving the accuracy of determining the sedimentation rate by using magnetic susceptibility as an indicator of sedimentation rate.

, где K_i – значение магнитной восприимчивости на уровне или участке, для которого вычисляется скорость осадконакопления; a – произвольно выбранное значение при соблюдении условия a < K_min в разрезе;

, где V_э1, V_э2– скорости осадконакопления для первого и второго интервалов разреза с известной длительностью формирования соответственно, K_э1, K_э2 – средние значения магнитной восприимчивости для первого и второго интервалов разреза с известной длительностью формирования соответственно; V_э и K_э' – скорость осадконакопления и среднее значение магнитной восприимчивости, нормированное по формуле:

, соответственно, для любого из двух интервалов разреза с известной длительностью формирования.The specified technical result is achieved in that the method for estimating the sedimentation rate includes measuring the magnetic susceptibility of carbonate rocks at different stratigraphic levels or sections of the section; construction of graphs or maps of values reciprocal of the magnetic susceptibility, which are used to judge the qualitative variation of sedimentation rates; calculation of sedimentation rates for any level or section of a section using the formula

where K _i is the value of magnetic susceptibility at the level or site for which sedimentation rate is calculated; a is an arbitrarily chosen value subject to the condition a <K _min in the context;

where V _e1 , V _e2 are sedimentation rates for the first and second sections of the section with known formation times, respectively, K _e1 , K _e2 are the average values of magnetic susceptibility for the first and second intervals of the section with known formation times, respectively; V _e and K _{e '} - sedimentation rate and the average value of the magnetic susceptibility, normalized by the formula:

, respectively, for any of the two cut intervals with a known formation duration.

The invention is illustrated by drawings, where in FIG. Figure 1 shows a comparison of the paleomagnetic column of the Bolshevik section and the Scale of geomagnetic polarity (Ogg JG, Hinnov LA, Huang C. Cretaceous // Gradstein F., Ogg JG, Schmitz MD, Ogg GM The Geologic Time Scale, 2012, v. 1, p . 793-853) in order to determine the sedimentation rate at two sections of the section, at which magnetic crones are identified; in FIG. 2 is a graph of magnetic susceptibility and a graph of sedimentation rate along the Bolshevik section; in FIG. 3 is a graph of magnetic susceptibility and a graph of sedimentation rate along the Kommunar section. The positions in the drawings indicate:

1 - silt,

2 - phosphorite,

3 - chalk

4 - clay chalk,

5 - glazed limestone ("knotty" chalk),

6 - marl,

7 - clay marl,

8 - a break in sedimentation,

9 - direct geomagnetic polarity,

10 - reverse geomagnetic polarity,

11 - lack of data on geomagnetic polarity,

12 is a reliable line of paleomagnetic correlation.

The inventive method is based on the use of magnetic susceptibility (K) as an indicator of the rate of accumulation of carbonate deposits.

The possibility of this approach is determined by the well-known feedback between the crystallization rate of calcite and the concentration of terrigenous, mainly clay, particles in the sediment. The rock-forming mineral calcite is a diamagnet (characterized by negative magnetic susceptibility), and an increase in magnetic susceptibility to zero and positive values occurs due to foreign impurities in the carbonate matrix having a para- and ferromagnetic effect. Many carbonate formations were formed in the deepwater parts of the paleobasins remote from the coast, where sea level fluctuations had practically no effect on the intensity of terrigenous input. For such objects, the magnetic susceptibility of rocks will be inversely proportional to sedimentation rates.

A qualitative picture of variations in sedimentation rates is obtained by plotting the reciprocal of K over a stratigraphic section.

To obtain quantitative information on sedimentation rates based on magnetic susceptibility data, it is necessary to have reference velocity definitions (V _e ) obtained using traditional methods at least at any two intervals of the studied section.

Considering that carbonate rocks are often diamagnetic, a procedure of normalization of all K values to positive values is necessary before calculations. When recalculating the magnetic susceptibility, the value a <K _min (the minimum value of K in the studied section) is taken as the zero level. If this conditional zero level corresponded to the true value of K authigenic pure (without allogenic impurities) calcite, then the sedimentation rate for any level (interval) of the section (V _i ) was calculated by the formula:

(1),

(one),

where K _e - the arithmetic mean of the values of K _i in the "reference" interval of the section (where the velocity V _e was determined), previously normalized to positive values by adding a:

(2),

(2)

where n is the number of measurements K within the "reference" interval of the section.

The magnetic susceptibility of natural calcium carbonate can vary from (–1.5) to (–0.1) * 10 ^-5 units. SI (Almqvist, BSG, Herwegh M., Schmidt V., Pettke T., Hirt AM Magnetic susceptibility as a tool to study deformed calcite with variable impurity content // Geochemistry Geophysics Geosystems, 2010, v. 11, No. 1, p. 1-15; Kodama KP, Hinnov LA Rock Magnetic Cyclostratigraphy. - NJ: John Wiley & Sons Inc., 2015, 166 p. Et al.). In weakly magnetic carbonate strata, K of which varies, as a rule, from (–1) to 1-2 * 10 ^-5 units. SI, the calculation results of V _i strongly depend on the magnitude of the magnetic susceptibility characterizing calcite without allotigenic components. To calculate the true K of authigenic CaCO ₃ in the studied section, it is necessary to use at least two values of velocities determined for different sections of the section (V _e1 and V _e2 ).

Provided that the autigenic calcite K across the section is a constant value, the values of sedimentation rates obtained for an arbitrary level (interval) of the section according to formula (1) when using both V _e1 and V _e2 should coincide. Therefore, the equation is true:

(3),

(3)

where K _e1 and K _e2 - K _e used to calculate V _e1 and V _e2 , respectively, b is the difference between a and the true value of K of pure calcite.

After calculating b:

(4)

(four)

renormalization of all values of K _{e is} carried out by adding b to them:

(5).

(5).

Sedimentation rates at an arbitrary level (interval) of the section are calculated using the values of the parameters V _e and K _{e '} corresponding to any of two reference sections of the section:

(6).

(6).

The inventive method was tested in sections of the Campanian-Maastricht "Bolshevik" and "Kommunar" (Volsk, Saratov region) (Guzhikov A.Yu., Benyamovsky V.N., Baraboshkin E.Yu., Guzhikova A.A., Kalyakin EA, Kopaevich LF, Pervushov EM, Seltser VB, Yakovishina EV On the lower boundary of the maastricht in the Saratov Volga // Cretaceous system of Russia and the near abroad: problems of stratigraphy and paleogeography. - Collection of scientific works / edited by E.Yu. Baraboshkin, V.S. Markevich, E.V. Bugdaeva, M.A. Afonina, M.V. Cherepanova. - Vladivostok: Dalnauka, 2014, p. . 103–106), presented sediments with _melodic marls, the sedimentation rates were calculated using the paleomagnetic method: V _e1 = 4.8 mm / thousand. years and V _e2 = 10.4 mm / thousand years, for two intervals covered by magnetozones of direct (N) and reverse (R) polarity, respectively (Fig. 1), according to the formulas:

(7),

(7)

(8),

(8),

where M _N and M _R are the powers of N- and R-magnetozones, T _N and T _R are the durations of the chrones C32n1.n and C32n1.r, which are analogues of N- and R-magnetozones in the geomagnetic polarity scale (Ogg JG, Hinnov LA, Huang C. Cretaceous // Gradstein F., Ogg JG, Schmitz MD, Ogg GM The Geologic Time Scale, 2012, v. 1, p. 793–853) (Fig. 1).

Coefficient b was calculated according to formula (4) on the basis of data only according to the Bolshevik, because in Kommunar the boundaries of magnetozones - analogues of magnetopolar crones were not fixed. But, taking into account the lithological similarity of the objects under study and the small distance (4.7 km) between them, the value of a was used to estimate the rates of sedimentation in both sections. Calculations by formula (6) for each of 126 stratigraphic levels in the Bolshevik section (Fig. 2) and 73 levels in Kommunar (Fig. 3), on which magnetic susceptibility measurements were carried out, made it possible to construct detailed graphs of sedimentation rate variations over all investigated intervals of the Bolshevik sections (Fig. 2) and Kommunar (Fig. 3), while the paleomagnetic method for calculating velocities in this case could only be applied to the 3.7-meter interval in the Bolshevik constituting only ~ 5% of the total studied capacity and in two cuts.

The obtained estimates of sedimentation rates are in good agreement with the features of the composition and structure of the sections: the minimum values of V _i are fixed at the border of the Sengileevskaya and Karsunskaya suites, between which there is a large hiatus (Fig. 2, 3); V _i for slowly accumulating laser limestones (Baraboshkin E.Yu., Weimarn AB, Kopaevich LF, Naydin DP Study of stratigraphic breaks during geological surveying. Methodical recommendations. M., Moscow State University publishing house, 2002, 163 pp.) (The tops of the Sengiley Formation) lower than for the rest of the rocks (Figs. 2, 3); higher, in general, the rate of sediment formation in Kommunar is accompanied by large thicknesses of both suites in this section (Fig. 3), compared with the Bolshevik (Fig. 2).

The inventive method for calculating sedimentation rates by magnetic susceptibility is based on a number of assumptions, which nevertheless can be satisfied by many real carbonate formations:

- the magnetic susceptibility of pure (without terrigenous impurities) authigenic calcite is a constant in the studied section,

- the intensity of allogeneic supply for the considered interval of geological time did not significantly change,

- an increase in K of rocks, compared with pure authigenic calcite, is due only to allotigenic impurities.

The proposed method is most effective in deposits that were formed with minimal terrigenous input. Otherwise, to estimate the sedimentation rate from the petromagnetic data, it is necessary to introduce an unlikely additional condition - that the intensity of terrigenous drift remains unchanged over geological time.

Since the magnetic susceptibility inside carbonate strata devoid of terrigenous impurities varies, as a rule, within the first units (10 ^-5 SI units), the results of calculating sedimentation rates strongly depend on the values of K of authigenic calcite, which can vary in a comparable range, in depending on the concentration of certain trace elements (Fe, Mn) in the mineral composition (Almqvist, BSG, Herwegh M., Schmidt V., Pettke T., Hirt AM Magnetic susceptibility as a tool to study deformed calcite with variable impurity content // Geochemistry Geophysics Geosystems , 2010, v. 11, No. 1, p. 1-15). Therefore, the practical use of the proposed method implies the third assumption, which is plausible with respect to paleobasins with pure carbonate sedimentation: K of pure CaCO ₃ in the studied section (or, in other words, the microelement composition of sea water) did not change significantly over the considered time interval.

The undoubted advantage of the petromagnetic method over other known methods for calculating sedimentation rates is the ability to evaluate sedimentation rates in an arbitrary section interval, as a result of which variations in the rate of sediment formation can be established with any required detail. But the disadvantages of the method include the inability to independently determine the rate of sedimentation. A quantitative calculation using magnetic susceptibility data is based on an estimate of the velocity obtained in another way.

Claims (1)

A method for estimating sedimentation rate, characterized in that it includes measuring the magnetic susceptibility of carbonate rocks at different stratigraphic levels or sections of the section; the construction of graphs or maps of values reciprocal of the magnetic susceptibility, which are used to judge the qualitative variation in sedimentation rates; calculation of sedimentation rates for any level or section of a section using the formula

where K _i is the value of magnetic susceptibility at the level or site for which sedimentation rate is calculated; a is an arbitrarily chosen value subject to the condition a <K _min in the context;

where V _e1 , V _e2 are sedimentation rates for the first and second sections of the section with known formation times, respectively, K _e1 , K _e2 are the average values of magnetic susceptibility for the first and second intervals of the section with known formation times, respectively; V _e - sedimentation rate for any of the two sections of the section with a known duration of formation; K _{e '- the} average value of the magnetic susceptibility, normalized by the formula

for any of the two cut intervals with a known formation duration.

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