Method for characterizing a geological zone

Abstract

The invention relates to a method for characterizing a geological zone comprising wells, the method comprising: - collecting input data for the geological zone, the input data comprising: • markers for the geological zone, • chronostratigraphic charts of the basin(s) of the geological zone, • lithostratigraphic charts of the basin(s) of the geological zone, - distributing the markers into chronostratigraphic groups and lithostratigraphic groups consisting in affecting some markers into the chronostratigraphic groups and the lithostratigraphic groups depending on the characteristics of said marker, on the chronostratigraphic charts and on the lithostratigraphic charts of the basin(s) of the geological zone, - improving the chronostratigraphic and lithostratigraphic groups consisting in determining representative markers and a composite index for each representative marker, and - characterizing the geological zone on the basis of the chronostratigraphic and the lithostratigraphic groups of markers and of the composite index of each representative marker.

Description

[0001] Method for characterizing a geological zone

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention concerns a method for characterizing a geological zone. The present invention concerns a computer program product and a readable information carrier.

[0004] BACKGROUND OF THE INVENTION

[0005] The characterization of a geological zone implies studying and interpreting the different layers of the geological zone. This interpretation is carried out using markers indicative of a geological or a fluidic discontinuity that can be interpreted on well data. The discontinuities are, for example, stratigraphic limits, lithological changes, sequential stratigraphy boundaries, contact between fluids, faults or volcanics. The interpretation of markers is done using various data such as: well logs, cores, cuttings, hydrocarbon presence in drilling mud, or acoustic impedance changes creating seismic discontinuities.

[0006] Usually, different sources of markers exist for a given geological zone, which could be used as a depart point for the characterization.

[0007] However, the markers from the different sources are difficult to use due to data inhomogeneities, aberrant data or unreliable sources of data, which implies interpretation errors and lack of traceability.

[0008] Therefore, there exists a need for improving the way of characterizing a geological zone using existing markers.

[0009] SUMMARY OF THE INVENTION

[0010] To this end, the invention relates to a method for characterizing a geological zone comprising wells, the method being computer-implemented and comprising the following phases:

[0011] - collecting input data for the geological zone, the input data comprising at least:

[0012] • markers for the geological zone, each marker being indicative of a geological or a fluidic discontinuity, each marker having characteristics among at least a name, an associated well and an associated depth,

[0013] • chronostratigraphic charts of the basin(s) of the geological zone,

[0014] • lithostratigraphic charts of the basin(s) of the geological zone,

[0015] - determining a confidence index for each marker depending on the source of data from which said marker comes from,

[0016] - distributing the markers into groups of markers, the groups of markers comprising chronostratigraphic groups and lithostratigraphic groups, each chronostratigraphic group being representative of a chronostratigraphic unit, each lithostratigraphic group being representative of a lithostratigraphic unit, the distribution phase comprising:

[0017] • affecting some markers into the chronostratigraphic groups and the lithostratigraphic groups depending on at least the characteristics of said marker, on the chronostratigraphic charts and on the lithostratigraphic charts of the basin(s) of the geological zone,

[0018] - improving the chronostratigraphic and lithostratigraphic groups, the improving phase comprising, for each chronostratigraphic and lithostratigraphic group and for each well:

[0019] • identifying the markers, called duplicated markers, corresponding to the well in the considered group,

[0020] • selecting one marker representative of the well in the group among the duplicated markers as a function of: o the confidence index of the duplicated markers, and o the depths of the duplicated markers,

[0021] • detemining a composite index for each representative marker on the basis of at least the confidence index of the representative marker,

[0022] - characterizing the geological zone on the basis of at least the chronostratigraphic and the lithostratigraphic groups of markers and of the composite index of each representative marker.

[0023] The method according to the invention may comprise one or more of the following features considered alone or in any combination that is technically possible:

[0024] - the characteristics of each marker also comprise the type of the marker, at least one type, called unconformity type, indicating an unconformity of a formation, during the improving phase, for each well, when some representative markers which corresponds to the well and which are not of the unconformity type, have the same depth to within a predetermined value, only the representative marker of the youngest group in millions of years is kept;

[0025] - the improving phase also comprises the determination of a depth consistency index for each marker of a chronostratigraphic and lithostratigraphic group consisting in, for each group:

[0026] • computing a means of the depths of the markers,

[0027] • computing a difference between the depth of each marker and the computed means,

[0028] • computing an actualized means on the basis of only the depths of the markers whose computed difference is below a predetermined threshold, • computing an actualized difference between the depth of each marker and the actualized means,

[0029] • affecting a depth consistency index to each marker depending on the actualized difference computed for said marker, and the composite index of each representative marker being also determined as a function of the depth consistency index of said representative marker;

[0030] - the improving phase also comprises the determination of a chrono-coherency index for each marker of a chronostratigraphic and lithostratigraphic group, the chrono-coherency index indicating whether the depth of the marker is coherent with the depth of the marker above said marker and the depth of the marker below said marker, the composite index of each representative marker being also determined as a function of the chrono-coherency index of said representative marker;

[0031] - the input data also comprise a wells dictionary for the geological zone, the wells dictionary comprising at least a name, a position and a total drilled depth for each well of the geological zone, the composite index of each representative marker being also determined as a function of a quality index relative to the corresponding well in the wells dictionary, the quality index indicating the precision of the position of the wells in the wells dictionary;

[0032] - the groups of markers comprise also descriptive groups, each descriptive group being representative of a type of discontinuity, the distribution phase comprising before the affectation of markers into chronostratigraphic and lithostratigraphic groups:

[0033] • suppressing aberrant markers on the basis of the names of the markers,

[0034] • affecting each non-aberrant marker into a descriptive group when the name of said marker corresponds to a descriptive group, the markers affected in the chronostratigraphic and lithostratigraphic groups being the remaining markers;

[0035] - the improving phase also comprises:

[0036] • building a depth map of the geographical zone on the basis of the depths of the representative markers of each well of the geological zone,

[0037] • computing the difference between the depth of each representative marker and the depth of the corresponding position on the depth map,

[0038] • suppressing the representative markers whose difference is above a predetermined threshold,

[0039] • building an actualized depth map on the basis of only the depths of the remaining representative markers, the depth of the suppressed markers being replaced by the depth corresponding to said marker on the actualized depth map, • computing the difference between the depth of each remaining representative marker and the depth of the corresponding position on the actualized depth map, and

[0040] • determining a map index for each remaining representative marker on the basis of the last computed difference for said remaining representative marker;

[0041] - the method also comprises a step of determining markers from the actualized depth map for wells for which no markers are available;

[0042] - the characterizing phase comprises the display on a screen of a cartography of the geological zone on which are superimposed characteristics of the geological zone determined from the chronostratigraphic and the lithostratigraphic groups of markers and from the composite index of each representative marker;

[0043] - the characterizing phase comprises, for at least some wells:

[0044] - the selection, for each well, of a top marker and of a base marker in the chronostratigraphic and lithostratigraphic groups of markers, the base marker having a deeper depth than the top marker, and

[0045] - the calculation of geological properties for such wells in the intervals formed between the top marker and the base marker of each well on the basis of the input data;

[0046] - the input data also comprise interpreted well logs and / or well elevation status and / or well deviations status, the geological properties being calculated on the basis of the interpreted well logs and / or well elevation status and / or well deviations status;

[0047] - the geological properties comprise at least one of the following properties: the gross, the net, the net pay, the net-to-gross ratio, the lithology fraction, the porosity and the water saturation;

[0048] - the representative markers of the chronostratigraphic and lithostratigraphic groups used during the characterizing phase are markers whose composite index is above a predetermined threshold;

[0049] - the input data also comprise a wells dictionary for the geological zone, the wells dictionary comprising at least a name, a position and a total drilled depth for each well of the geological zone, the method comprising, before the distribution phase, a pre-processing phase during which the characteristics of each marker are homogenized taking the wells dictionary as a reference.

[0050] The invention also relates to a computer program product comprising a readable information carrier having stored thereon a computer program comprising program instructions, the computer program being loadable onto a data processing unit and causing a method as previously described to be carried out when the computer program is carried out on the data processing unit.

[0051] The invention also relates to a readable information carrier on which a computer program product as previously described is stored.

[0052] BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The invention will be easier to understand in view of the following description, provided solely as an example and with reference to the appended drawings in which:

[0054] Figure 1 is a schematic view of an example of a calculator allowing the implementation of a method for characterizing a geological zone comprising wells, Figure 2 is a flowchart of an example of implementation of a method for characterizing a geological zone comprising wells, and Figure 3 is an example of a well log with three markers.

[0055] DETAILED DESCRIPTION OF SOME EMBODIMENTS

[0056] A calculator 10 and a computer program product 12 are illustrated on figure 1 .

[0057] The calculator 10 is preferably a computer.

[0058] More generally, the controller 10 is a computer or computing system, or similar electronic computing device adapted to manipulate and / or transform data represented as physical, such as electronic, quantities within the computing system's registers and / or memories into other data similarly represented as physical quantities within the computing system's memories, registers or other such information storage, transmission or display devices.

[0059] The calculator 10 interacts with the computer program product 12.

[0060] As illustrated on figure 1 , the calculator 10 comprises a processor 14 comprising a data processing unit 16, memories 18 and a reader 20 for information media. In the example illustrated on figure 1 , the calculator 10 comprises a human machine interface 22, such as a keyboard, and a display 24.

[0061] The computer program product 12 comprises an information medium 26.

[0062] The information medium 26 is a medium readable by the calculator 10, usually by the data processing unit 16. The readable information medium 26 is a medium suitable for storing electronic instructions and capable of being coupled to a computer system bus.

[0063] By way of example, the information medium 26 is a USB key, a floppy disk or flexible disk (of the English name "Floppy disc"), an optical disk, a CD-ROM, a magneto-optical disk, a ROM memory, a memory RAM, EPROM memory, EEPROM memory, magnetic card or optical card. On the information medium 26 is stored the computer program 12 comprising program instructions.

[0064] The computer program 12 is loadable on the data processing unit 16 and is adapted to entail the implementation of a method for characterizing a geological zone comprising wells, when the computer program 12 is loaded on the processing unit 16 of the calculator 10.

[0065] Operation of the calculator 10 will now be described with reference to figure 2, which diagrammatically illustrates an example of implementation of a method for characterizing a geological zone comprising wells, and to figure 3.

[0066] The geological zone comprises wells. The wells are for example drilling wells (which are wells belonging to one or several hydrocarbon fields), or exploration wells. A field is an area where hydrocarbons were identified and an attempt to extract it can be made by drilling wells. A field typically extends over several kilometers, allowing for several wells in one field.

[0067] The characterization method is implemented by the calculator 10 in interaction with the computer program product 12, that is to say is implemented by a computer.

[0068] The characterization method comprises a phase 100 for collecting input data for the geological zone.

[0069] The input data comprise at least the following data:

[0070] • markers for the geological zone. Each marker is indicative of a geological or a fluidic discontinuity. Each marker has characteristics among at least a name, an associated well and an associated depth. Each marker comes from a source of data having a confidence index. Figure 3 illustrates an example of a well log with three markers M1 , M2 and M3 at different depths.

[0071] • chronostratigraphic charts of the basin(s) of the geological zone. A chronostratigraphic chart is composed of chronostratigraphic units. Chronostratigraphic units are comprised of all rocks formed within certain time spans of Earth history regardless of their compositions or properties. These units everywhere comprise rocks of specific age and, by definition, their boundaries are everywhere synchronous. This is in contrast with lithostratigraphic units that can be objectively recognized wherever there are rocks, and with biostratigraphic, magnetostratigraphic polarity, and unconformity-bounded units that are limited by the occurrence of specific properties or attributes of the rocks. Whereas other kinds of stratigraphic units are distinguished, established, and identified on the basis of observable physical features, chronostratigraphic units are distinguished, established, and identified on the basis of their bounding horizons. • lithostratigraphic charts of the basin(s) of the geological zone. A lithostratigraphic chart is composed of lithostratigraphic units.

[0072] Lithostratigraphic units are the basic units of geologic mapping. Wherever there are rocks, it is possible to develop a lithostratigraphic classification. Lithostratigraphic units are based on the lithologic character of rocks. Lithologic character is influenced more strongly by conditions of formation than by time of origin; nearly identical rock types are repeated in time and again in the stratigraphic sequence, and the boundaries of almost all lithostratigraphic units cut across synchronous surfaces when traced laterally.

[0073] Preferably, the input data comprise also one or several of the following data:

[0074] • a wells dictionary for the geological zone. The wells dictionary comprises at least a name (unique identifier for the well), a position and a total drilled depth for each well of the geological zone. Optionally, the wells dictionary also comprises the drilling date of the well.

[0075] • interpreted well logs (ie petrophysical calculations such as porosity, water saturation, volume of shale...) to perform interval properties calculations.

[0076] • well elevation and deviations status (whether the well is vertical or deviated).

[0077] • if applicable, well deviation survey to be able to compute interval properties in vertical depth and generate depth map with reference to subsea level for interpretation in conjunction to seismic data.

[0078] Preferably, the characterization method comprises a pre-processing phase 200 during which the characteristics of each marker are homogenized taking the wells dictionary as a reference. This phase 200 enables to identify, and classify the markers that come from various sources, follow different formats, and carry inconsistent names using the wells dictionary. For example, the name of the markers may contain prefix that will indicate the year of study and / or the source / interpretation group. This prefix is identified and transformed into a marker source flag to be used in subsequent phase. For example, markers missing any source of possible depth index or coordinates are not kept in further steps of processing.

[0079] The characterization method comprises a phase 300 for determining a confidence index for each marker depending on the source of data from which said marker comes from.

[0080] The sources of data are for example commercial database or database from projects carried out by operators.

[0081] The sources of data are affected with a confidence index depending on the reliability of the database. The confidence index of each source of data is for example affected by an operator and stored in a memory of the calculator 10 or accessible via a server. The confidence index of each source of data is for example a number comprised between zero (lowest confidence) and one (highest confidence).

[0082] For example, the confidence index of each marker is the confidence index of the source of data from which the marker comes from.

[0083] The characterization method comprises a phase 400 for distributing the markers into groups of markers. The groups of markers comprise chronostratigraphic and lithostratigraphic groups. Each chronostratigraphic group is representative of a chronostratigraphic unit. Each lithostratigraphic group is representative of a lithostratigraphic unit.

[0084] The distribution phase 400 comprises:

[0085] • affecting some markers into chronostratigraphic and lithostratigraphic groups depending on at least the characteristics of said marker (in particular the name of the marker) and on the chronostratigraphic charts and lithostratigraphic charts of the basin(s) of the geological zone. o In an example of implementation, the affectation consists in affecting the markers having a name relative to a chronostratigraphic unit into a chronostratigraphic group and the markers having a name relative to a lithostratigraphic unit into a lithostratigraphic group.

[0086] Preferably, the groups of markers also comprise descriptive groups. Each descriptive group is representative of a type of discontinuity. For example, the descriptive groups are representative of at least one of the following discontinuities: fault, fluid, deposit, datums, basement and volcanic element.

[0087] Preferably, the distribution phase comprises before the affectation of markers into chronostratigraphic and lithostratigraphic groups:

[0088] • suppressing aberrant markers on the basis of the names of the markers. For example, the markers which have an invalid name are suppressed. An invalid name comprises for example the term “Test”, “Suppression”, “Cancel”.

[0089] • affecting each non-aberrant marker into a descriptive group when the name of said marker corresponds to a descriptive group. For example, the markers containing in their name the term “fault” (or “fluid”, or “basement”, or “volcanic”) are added in the descriptive group relative to the faults (respectively fluids, basements or volcanics).

[0090] The markers which are affected in the chronostratigraphic and lithostratigraphic groups are the remaining markers.

[0091] The characterization method comprises a phase 500 for improving the chronostratigraphic and lithostratigraphic groups. The improving phase 500 comprises, for each chronostratigraphic and lithostratigraphic group and for each well:

[0092] • identifying the markers, called duplicated markers, corresponding to the well in the considered group,

[0093] • selecting one marker representative of the well in the group among the duplicated markers as a function of: o the confidence index of the duplicated markers, and o the depths of the duplicated markers,

[0094] • determining a composite index for each representative marker on the basis of at least the confidence index of the representative marker. In an example, the composite index is equal to the confidence index. In another example, the composite index is the sum of the confidence index and of other index as will be described in the following optional embodiments.

[0095] For example, the representative marker is the marker having the highest confidence index of the group and if several markers have the same highest confidence index, the representative marker is the marker having the shallowest depth.

[0096] Preferably, the characteristics of each marker also comprise the type of the marker, at least one type, called unconformity type, indicating an unconformity of a formation. During the improving phase 500, for each well, when some representative markers which corresponds to the well and which are not of the unconformity type, have the same depth to within a predetermined value (tolerance for example 10 centimeters), only the representative marker of the youngest group in millions of years is kept.

[0097] Preferably, the improving phase 500 also comprises the determination of a depth consistency index for each marker of a chronostratigraphic and lithostratigraphic group consisting in, for each group:

[0098] • computing a means of the depths of the markers,

[0099] • computing a difference between the depth of each marker and the computed means,

[0100] • computing an actualized means based on only the depths of the markers whose computed difference is below a predetermined threshold,

[0101] • computing an actualized difference between the depth of each marker and the actualized means,

[0102] • affecting a depth consistency index to each marker depending on the actualized difference computed for said marker. For example, the depth consistency index is equal to zero when the actualized difference is above a predetermined threshold and is equal to one when the actualized difference is below the predetermined threshold.

[0103] The composite index of each representative marker is also determined as a function of the depth consistency index of said representative marker. For example, the composite index is the sum of at least the confidence index and the depth consistency index.

[0104] Preferably, the improving phase 500 also comprises the determination of a chronocoherency index for each marker of a chronostratigraphic and lithostratigraphic group. The chrono-coherency index indicates whether the depth of the marker is coherent with the depth of the marker above said marker and the depth of the marker below said marker. For example, the chrono-coherency index is equal to one when the depth of the marker is coherent and is equal to zero otherwise. The composite index of each representative marker is also determined as a function of the chrono-coherency index of said representative marker. For example, the composite index is the sum of at least the confidence index and the chrono-coherency index.

[0105] Preferably, the composite index of each representative marker is also determined as a function of a quality index relative to the corresponding well in the wells dictionary. The quality index indicates the precision of the position of the wells in the wells dictionary. The quality index is for example a number comprised between zero (lowest precision) and one (highest precision). For example, the composite index is the sum of at least the confidence index and the quality index.

[0106] Preferably, the improving phase also comprises:

[0107] • building a depth map of the geographical zone on the basis of the depths of the representative markers of each well of the geological zone,

[0108] • computing the difference between the depth of each representative marker and the depth of the corresponding position on the depth map,

[0109] • suppressing the representative markers whose difference is above a predetermined threshold,

[0110] • building an actualized depth map on the basis of only the depths of the remaining representative markers, the depth of the suppressed markers being replaced by the depth corresponding to said marker on the actualized depth map,

[0111] • computing the difference between the depth of each remaining representative marker and the depth of the corresponding position on the actualized depth map,

[0112] • determining a map index for each remaining representative marker based on the last computed difference for said remaining representative marker. For example, the map index is equal to zero when the last computed difference is above a predetermined threshold and is equal to one when the last computed difference is below the predetermined threshold. The composite index of each representative marker is also determined as a function of the map quality index of said marker. For example, the composite index is the sum of at least the confidence index and the map index.

[0113] Optionally, the method also comprises a step of determining markers from the actualized depth map for wells for which no markers are available. Such markers are for example determined by intersecting the position of the well with the corresponding depth on the actualized depth map.

[0114] The above embodiments can all be combined so that the composite index is the sum of the confidence index, of the depth consistency index, of the chrono-coherency index, of the quality index and of the map index.

[0115] The characterization method comprises a phase 600 for characterizing the geological zone on the basis of at least the chronostratigraphic and lithostratigraphic groups of markers and of the confidence index of each representative marker.

[0116] Preferably, the markers of the chronostratigraphic and the lithostratigraphic groups used during the characterizing phase are markers whose composite index is above a predetermined threshold.

[0117] In an embodiment, the characterizing phase 600 comprises the display, on a screen, of a cartography of the geological zone on which are superimposed characteristics of the geological zone determined from the chronostratigraphic and the lithostratigraphic groups of markers and from the composite index of each representative marker. For example, the representative markers are indicated on such a cartography.

[0118] In a variant or in a complement, the characterizing phase 600 comprises, for at least some wells:

[0119] - the selection, for each well, of a top marker and of a base marker in the chronostratigraphic and lithostratigraphic groups of markers, the base marker having a deeper depth than the top marker. For example, when the markers correspond to groups in millions of years, the top marker is the highest marker of the groups, and if not is the shallowest marker. The base marker is the lowest marker of the groups, and if not is the deepest marker, and

[0120] - the calculation of geological properties for such wells in the intervals formed between the top marker and the base marker of each well on the basis of the input data (interpreted well logs, well deviations...).

[0121] The geological properties comprise at least one of the following properties: the gross, the net, the net pay, the net-to-gross ratio, the lithology fraction, the porosity and the water saturation. The gross interval thickness is the difference between the depth of the top and the depth of the base of considered interval. This calculation is often done using vertical depth index. The net interval thickness is calculated by cumulating the thickness of rocks within considered interval having their petrophysical properties above user defined cut-offs usually on shale volume, porosity.

[0122] In a variant or in a complement, the characterizing phase 600 also comprises the training of a machine learning model on the basis of at least the chronostratigraphic and lithostratigraphic groups of markers and the corresponding calculated geological properties so as to obtain a trained model whose inputs are chronostratigraphic and lithostratigraphic groups of markers and whose outputs are geological properties for some wells having markers in the chronostratigraphic and lithostratigraphic groups.

[0123] Preferably, the markers of the chronostratigraphic and lithostratigraphic groups used during the training are representative markers whose composite index is above a predetermined threshold.

[0124] The different phases of the method can be repeated with updated input data, for example when additional markers are collected.

[0125] Hence, the above characterization method enables to obtain a homogeneous set of geological markers for wells, with a composite index indicating the reliability of the markers. These markers can be arranged in a stratigraphic order and presented on a standard stratigraphic chart. In embodiments, duplicated and erroneous values are eliminated, and the most reliable sources are preferred.

[0126] Hence, the method enables to improve the way of characterizing a geological zone using existing markers. The advantage of this process is that it is more efficient, traceable, scalable, standardized and provides a confidence index for each marker. In particular, the confidence index attached to the markers enables the most reliable ones to be selected for subsequent processing, where they will serve for example as a reference for the creation of propagation ML models using logs, for example.

[0127] In addition, the method can be carried out more quickly and repeatedly when new markers become available or when processing algorithms are improved. This enables very large numbers of wells to be processed.

[0128] The person skilled in the art will understand that the embodiments and variants described above can be combined to form new embodiments provided that they are technically compatible.

[0129] In addition, it should be understood that the different phases of the method can be switched of position if applicable (for example the phase 200 can be put just before phase 400).

Claims

CLAIMS1 A method for characterizing a geological zone comprising wells, the method being computer-implemented and comprising the following phases:- collecting input data for the geological zone, the input data comprising at least:• markers for the geological zone, each marker being indicative of a geological or a fluidic discontinuity, each marker having characteristics among at least a name, an associated well and an associated depth,• chronostratigraphic charts of the basin(s) of the geological zone,• lithostratigraphic charts of the basin(s) of the geological zone,- determining a confidence index for each marker depending on the source of data from which said marker comes from,- distributing the markers into groups of markers, the groups of markers comprising chronostratigraphic groups and lithostratigraphic groups, each chronostratigraphic group being representative of a chronostratigraphic unit, each lithostratigraphic group being representative of a lithostratigraphic unit, the distribution phase comprising:• affecting some markers into the chronostratigraphic groups and the lithostratigraphic groups depending on at least the characteristics of said marker, on the chronostratigraphic charts and on the lithostratigraphic charts of the basin(s) of the geological zone,- improving the chronostratigraphic and lithostratigraphic groups, the improving phase comprising, for each chronostratigraphic and lithostratigraphic group and for each well:• identifying the markers, called duplicated markers, corresponding to the well in the considered group,• selecting one marker representative of the well in the group among the duplicated markers as a function of: o the confidence index of the duplicated markers, and o the depths of the duplicated markers,• detemining a composite index for each representative marker on the basis of at least the confidence index of the representative marker,- characterizing the geological zone on the basis of at least the chronostratigraphic and the lithostratigraphic groups of markers and of the composite index of each representative marker.2.- A method according to claim 1 , wherein the characteristics of each marker also comprise the type of the marker, at least one type, called unconformity type, indicating an unconformity of a formation, during the improving phase, for each well, when some representative markers which corresponds to the well and which are not of the unconformity type, have the same depth to within a predetermined value, only the representative marker of the youngest group in millions of years is kept.3.- A method according to claim 1 or 2, wherein the improving phase also comprises the determination of a depth consistency index for each marker of a chronostratigraphic and lithostratigraphic group consisting in, for each group:• computing a means of the depths of the markers,• computing a difference between the depth of each marker and the computed means,• computing an actualized means on the basis of only the depths of the markers whose computed difference is below a predetermined threshold,• computing an actualized difference between the depth of each marker and the actualized means,• affecting a depth consistency index to each marker depending on the actualized difference computed for said marker, and the composite index of each representative marker being also determined as a function of the depth consistency index of said representative marker.4.- A method according to any one of claims 1 to 3, wherein the improving phase also comprises the determination of a chrono-coherency index for each marker of a chronostratigraphic and lithostratigraphic group, the chrono-coherency index indicating whether the depth of the marker is coherent with the depth of the marker above said marker and the depth of the marker below said marker, the composite index of each representative marker being also determined as a function of the chrono-coherency index of said representative marker.5.- A method according to any one of claims 1 to 4, wherein the input data also comprise a wells dictionary for the geological zone, the wells dictionary comprising at least a name, a position and a total drilled depth for each well of the geological zone, the composite index of each representative marker being also determined as a function of aquality index relative to the corresponding well in the wells dictionary, the quality index indicating the precision of the position of the wells in the wells dictionary.6.- A method according to any one of claims 1 to 5, wherein the groups of markers comprise also descriptive groups, each descriptive group being representative of a type of discontinuity, the distribution phase comprising before the affectation of markers into chronostratigraphic and lithostratigraphic groups:• suppressing aberrant markers on the basis of the names of the markers,• affecting each non-aberrant marker into a descriptive group when the name of said marker corresponds to a descriptive group, the markers affected in the chronostratigraphic and lithostratigraphic groups being the remaining markers.7.- A method according to any one of claims 1 to 6, wherein the improving phase also comprises:• building a depth map of the geographical zone on the basis of the depths of the representative markers of each well of the geological zone,• computing the difference between the depth of each representative marker and the depth of the corresponding position on the depth map,• suppressing the representative markers whose difference is above a predetermined threshold,• building an actualized depth map on the basis of only the depths of the remaining representative markers, the depth of the suppressed markers being replaced by the depth corresponding to said marker on the actualized depth map,• computing the difference between the depth of each remaining representative marker and the depth of the corresponding position on the actualized depth map, and• determining a map index for each remaining representative marker on the basis of the last computed difference for said remaining representative marker.8.- A method according to claim 7, wherein the method also comprises a step of determining markers from the actualized depth map for wells for which no markers are available.9.- A method according to any one of claims 1 to 8, wherein the characterizing phase comprises the display on a screen of a cartography of the geological zone on which aresuperimposed characteristics of the geological zone determined from the chronostratigraphic and the lithostratigraphic groups of markers and from the composite index of each representative marker.10.- A method according to any one of claims 1 to 9, wherein the characterizing phase comprises, for at least some wells:- the selection, for each well, of a top marker and of a base marker in the chronostratigraphic and lithostratigraphic groups of markers, the base marker having a deeper depth than the top marker, and- the calculation of geological properties for such wells in the intervals formed between the top marker and the base marker of each well on the basis of the input data.1 1.- A method according to claim 10, wherein the input data also comprise interpreted well logs and / or well elevation status and / or well deviations status, the geological properties being calculated on the basis of the interpreted well logs and / or well elevation status and / or well deviations status.12.- A method according to claim 10 or 1 1 , wherein the geological properties comprise at least one of the following properties: the gross, the net, the net pay, the net-to- gross ratio, the lithology fraction, the porosity and the water saturation.13.- A method according to any one of claims 1 to 12, wherein the representative markers of the chronostratigraphic and lithostratigraphic groups used during the characterizing phase are markers whose composite index is above a predetermined threshold.14.- A method according to any one of claims 1 to 13, wherein the input data also comprise a wells dictionary for the geological zone, the wells dictionary comprising at least a name, a position and a total drilled depth for each well of the geological zone, the method comprising, before the distribution phase, a pre-processing phase during which the characteristics of each marker are homogenized taking the wells dictionary as a reference.15.- A computer program product comprising a readable information carrier having stored thereon a computer program comprising program instructions, the computer program being loadable onto a data processing unit and causing a method according to any one ofclaims 1 to 14 to be carried out when the computer program is carried out on the data processing unit.

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