The present invention generally relates to the field of hydrocarbon exploration, and more particularly to a method for identifying hydrocarbon bearing zones in unconventional fractured basement reservoirs based on conventionally recorded well log data associated with well of said basement reservoir.
Background of the Invention
[0002] Basement reservoirs are unconventional and complex in nature. A typical basement reservoir is a metamorphic or igneous rock comprising hydrodynamic fractured network that has resulted from geological processes. Due to extremely low porosity and permeability of basement reservoirs, hydrocarbons are not found in pore spaces but instead are restricted within the extensive fractured network and weathered zones. It has been observed that overall high resistivity of the igneous and metamorphic rocks masks the effect of hydrocarbon presence in the fractures. Therefore, as a result of the aforementioned complexity and unconventional nature of the basement reservoirs, it is extremely difficult to identify the presence of hydrocarbons using well logs in a standard manner.
[0003] Accordingly, one or more techniques have been explored to identify hydrocarbon bearing zones in basement reservoirs. One such technique is barefoot testing of the drilled well column associated with a well of the basement reservoir. However, barefoot testing is expensive, requires extra efforts and is time consuming as the entire drilled basement section is open to production. Moreover, the output of barefoot testing does not provide the exact hydrocarbon bearing zones within the drilled basement section, and therefore cannot be considered accurate. Another technique to identify hydrocarbon bearing zones in basement reservoirs involves the use of acoustic data, where the travel times of the compressional (P-wave) and shear (S-wave) waves are measured. However, as P-wave velocity(Vp) and S-wave velocity (Vs) values are also affected by weathering and fractures, therefore the use of acoustic data based techniques in the fractured basements is inefficient in

indicating the presence of hydrocarbon when used alone. Yet another technique for identifying hydrocarbon in basement reservoirs includes the use of ratio of deep and shallow resistivity. However, it has been observed that the ratio of deep and shallow resistivity when employed alone cannot differentiate whether the high resistivity points are due to massive basement or due to the presence of hydrocarbon.
[0004] In light of the aforementioned drawbacks, there is a need for a method that can identify hydrocarbon bearing zones in unconventional fractured basement reservoirs. There is need for a method which is based on conventionally recorded well log data which is readily available for each of the wells drilled/bored in the basement reservoir. Further, there is need for a method which can eliminate the masking effect of highly resistive basement rocks on well logs. Yet, further, there is a need for a method which is inexpensive and accurate. There is a need for a method which can identify hydrocarbon bearing zones in new as well as old basement reservoirs. Furthermore, there is a need for a method which has wider applicability and is easy to implement.
Summary of the invention
[0005] In accordance with various embodiments of the present invention, a method for identifying hydrocarbon bearing zones in a well of a basement reservoir is provided. The method comprises identifying one or more zones that are weathered or fractured or both and porous within a depth range of the basement reservoir in the well based on P-wave velocity (Vp) / S-wave velocity (Vs) values and compressional sonic wave travel time (DTCO) values throughout the depth range. The Vp/ Vs and DTCO values are derived from conventionally recorded Vp, Vs, and DTCO logs associated with the depth range. The method further comprises ascertaining if the identified one or more zones are weathered or fractured or both and porous based on Poisson's ratio (PR) values and bulk density (RHOB) values of said identified one or more zones, where the PR and RHOB values are extracted from conventionally recorded PR and RHOB logs associated with the depth range. Further, the method comprises determining permeability of the one or more zones that are ascertained as weathered or fractured or both and porous based on a ratio

between deep resistivity (RD) values and shallow resistivity (RS) values of the one or more zones ascertained as weathered or fractured or both and porous, where the RD and RS values are extracted from conventionally recorded deep resistivity logs and shallow resistivity logs associated with the depth range. Furthermore, the method comprises determining if the one or more zones that are determined to be fractured or weathered or both, porous and permeable are hydrocarbon bearing zones based on deep resistivity (RD) values associated with said one or more zones determined to be fractured or weathered or both, porous and permeable. The RD values are extracted from the conventionally recorded deep resistivity logs, and the RD value of any zone determined to be fractured or weathered or both, porous and permeable >= E confirms that the zone is hydrocarbon bearing.
Brief Description of the Drawings
[0006] The present invention is described by way of embodiments illustrated in the accompanying drawings wherein:
[0007] Figure 1 is a flowchart illustrating a method for identifying hydrocarbon bearing zones in basement reservoirs, in accordance with various embodiments of the present invention;
[0008] Figure 2 illustrates an exemplary cross plot between Vp/Vs and DTCO, in accordance with an embodiment of the present invention;
[0009] Figure 3 illustrates an exemplary cross plot between PR and RHOB, in accordance with an embodiment of the present invention;
[0010] Figure 4 illustrates an exemplary cross plot between Vp/Vs and RD/RS, in accordance with an embodiment of the present invention;
[0011] Figure 5 illustrates an exemplary cross plot between RD/RS and RD, in accordance with an embodiment of the present invention; and
[0012] Figure 6 illustrates an exemplary cross plot between RD/RS and RD, in accordance with an embodiment of the present invention.

Detailed Description of the Invention
[0013] The disclosure is provided in order to enable a person having ordinary skill in the art to practice the invention. Exemplary embodiments herein are provided only for illustrative purposes and various modifications will be readily apparent to persons skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. The terminology and phraseology used herein is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed herein. For purposes of clarity, details relating to technical material that is known in the technical fields related to the invention have been briefly described or omitted so as not to unnecessarily obscure the present invention. It is to be noted that, as used in the specification by the term "substantially" it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those skilled in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide. The phrase "selected depth range" as used in the specification refers to a range of depth of basement reservoir selected for hydrocarbon exploration. The term "weathered zones" as used in the specification refers to the zones which show chemical or physical alterations by way of weathering. The term "unaltered basement" as used in the specification refers to the basement which has not been altered physically or chemically and is not fractured.
[0014] The present invention discloses a method for identifying hydrocarbon bearing zones in basement reservoirs. In particular, the present invention discloses a method for identifying hydrocarbon bearing zones in unconventional fractured reservoirs using a combination of readily available well logs, including P-wave velocity (compressional velocity-Vp), S-wave velocity (shear velocity-Vs),

compressional sonic wave travel time (DTCO), Bulk density (RHOB), Poisson's ratio (PR), Deep Resistivity (RD), and Shallow Resistivity (RS). In operation, one or more zones within a selected depth range of basement reservoir that are porous and weathered and/or fractured are identified based on Vp/Vs and DTCO values extracted from respective logs for the respective zones (referred to as point depths) within the selected depth range. In particular, one or more zones within the selected depth range of the basement reservoir having Vp/Vs lower than a predetermined Vp/Vs value (A) and the same zones having DTCO higher than a predetermined DTCO value (C) are identified based on the Vp/Vs values and DTCO values of respective zones within the selected range, where Vp/Vs C is indicative of zones that are fractured and/or weathered as well as porous. Further, a validation is performed to ascertain if the identified one or more zones are porous and weathered and/or fractured based on Poisson's ratio (PR) values and bulk density (RHOB) values extracted from respective logs for the selected depth range. In particular, the identified one or more zones having PR value <=a predetermined PR value(B) and the same one or more zones having RHOB <= a predetermined RHOB value (D) are indicative of zones that are weathered or fractured and have low density, further confirming porous nature of the identified one or more zone. Further, the one or more zones within the selected depth range of the basement reservoir that are identified to be porous and weathered and/or fractured are checked for permeability based on a ratio between deep resistivity (RD) and shallow resistivity (RS) values of the one or more zones. The one or more zones having RD/RS >1 are indicative of zones that are permeable. Finally, a check is performed to determine if the one or more zones within the selected depth range of the basement reservoir that are identified to be porous, permeable, and fractured and/or weathered are hydrocarbon bearing zones based on deep resistivity values associated with said zones. Any zone identified as porous, permeable, and fractured and/or weathered having a deep resistivity (RD) value of greater than or equal to (>=) a predetermined average value of deep resistivity (E) is indicative of a zone that is porous, permeable, fractured and/or weathered and hydrocarbon bearing. Any zone identified as porous, permeable, and fractured and/or weathered having a

deep resistivity (RD) value of less than or equal to E is indicative of dry or water bearing zone.
[0015] The present invention would now be discussed in context of embodiments as illustrated in the accompanying drawings.
[0016] Referring to Figure 1, a flowchart illustrating a method for identifying hydrocarbon bearing zones in fractured basement reservoirs is shown, in accordance with various embodiments of the present invention.
[0017] At step 102, well logs associated with a well of the basement reservoir are recorded. In an embodiment of the present invention, well logs are recorded for a selected depth range of the basement reservoir. In an embodiment of the present invention, the well logs include P-wave velocity (compressional velocity-Vp), S-wave velocity (shear velocity-Vs), compressional sonic wave travel time (DTCO), Bulk density (RHOB), Poisson's ratio (PR), Deep Resistivity (RD), and Shallow Resistivity (RS). In another embodiment of the present invention, the well logs are retrieved from a database. In an example, a well having a depth ranging from 1000-4000m may be drilled or bored in a reservoir. Further, a depth ranging from 3000-4000m of the drilled/bored well may be assumed to be the depth of basement selected for hydrocarbon analysis. In the example, the depth ranging from 3000-4000 is a selected depth range of the basement reservoir. In the example, the well logs are recorded continuously with a sampling rate of 0.15 m throughout the selected depth range of 3000-4000m for individual point depths, such as 3000m, 3000.15m, 3000.30m etc. up to 4000m. In another embodiment of the present invention, the well logs are recorded throughout the selected depth range of 3000-4000m at predetermined intervals. For instance, with a predetermined sampling rate of 5m, well logs may be recorded from 3000-3005m, 3005-3010m etc. up to 4000m. It is to be understood that the depth of basement i.e. from 3000-4000m is for exemplary purposes only, and the depth of the basement will change from well to well.

[0018] At step 104, one or more zones within the selected depth range of basement reservoir that are porous and weathered or fractured are identified. In accordance with various embodiments of the present, the one or more porous zones are indicative of zones capable of holding fluid. In an embodiment of the present invention, the one or more porous and weathered/ fractured zones within the selected depth range of basement reservoir are identified based on Vp/Vs values throughout the selected depth range and DTCO throughout the selected depth range of the basement reservoir. In accordance with various embodiments of the present invention, a low Vp/Vs value at one or more zones within the selected depth range of the basement reservoir is indicative of fractures/weathering within the one or more zones of the selected depth range of basement reservoir. In accordance with various embodiments of the present invention, a relatively high compressional sonic wave travel time (DTCO) at one or more zones in the selected depth range of the basement reservoir is indicative of low density at one or more zones of the selected depth range of basement reservoir due to the porous nature of said one or more zones within the basement reservoir. In accordance with various embodiments of the present invention, the one or more zones in the selected depth range of basement reservoir having Vp/Vs lower than a predetermined Vp/Vs value (hereinafter denoted by constant A) are indicative of zones that are fractured and/or weathered. In accordance with various embodiments of the present invention, the one or more zones that are identified as fractured and/or weathered based on Vp/Vs values and have DTCO higher than a predetermined DTCO value (hereinafter denoted by constant C) are indicative of zones that are fractured and/or weathered as well as porous.
[0019] In an embodiment of the present invention, the predetermined Vp/Vs value (A) below which any zone within the selected depth range of basement reservoir is considered fractured or weathered is the average value of Vp/Vs derived from core data analysis performed on core samples extracted from multiple depths of an unaltered/un fractured basement within the selected depth range of the basement reservoir. In an example, assuming 3500-3700m is the unaltered basement within the selected depth of 3000-4000m, the core samples may be extracted from multiple

point depths, for example 3500.15m, 3500.30m up to 3700 etc. within the selected depth range of 3000-4000m. In another exemplary embodiment of the present invention, the core samples may be extracted from a selected depth range of 3000-4000m at predetermined intervals, for instance 3 500-3505m, 3505-3510m etc. In an embodiment of the present invention, the predetermined DTCO value (C) above which any zone within the selected depth range of basement reservoir is considered to have low density is the average value of DTCO derived from DTCO logs associated with an unaltered/un fractured basement within the selected depth range of the basement reservoir. In an example, assuming 3500-3700m is the unaltered basement within the selected depth of 3000-4000, then the average value of DTCO (C) is evaluated from DTCO log recorded for the unaltered depth range 3500-3700m within the selected depth range of 3000-4000m.
[0020] In an example, where a depth ranging from 3000-4000m is the depth of the basement selected for hydrocarbon analysis, the Vp/Vs values at point depths between 3000m-4000m are calculated based on the retrieved P-wave velocity (compressional velocity-Vp) and S-wave velocity (shear velocity-Vs) logs. Further, the DTCO values at point depths between 3000m-4000m are extracted from the DTCO logs. Yet further, a cross plot is generated between the Vp/Vs values and the DTCO values at point depths between 3000-4000m. Further, the one or more point depths in the selected depth range(3000-4000m) of basement reservoir having Vp/Vs lower than A (a predetermined Vp/Vs value) and DTCO higher than C (a predetermined DTCO value) on the cross plot (Vp/Vs vs DTCO) are indicative of the fractured and/or weathered zones (also referred to as point depths) that are porous. Assuming, that on the cross plot, point depths 3000.15, 3000.30, and 3000.45 are the one or more zones having Vp/Vs lower than A, and zones 3000.30, 3000.45 and 3000.60 are the one or more zones having DTCO higher than C, then the zones 3000.30 and 3000.45 are indicative of the fractured or weathered zones that are also porous.
[0021] At step 106, the one or more zones within the selected depth range of basement reservoir identified at step 104 are ascertained for porosity and

weathering and/or fractures. In an embodiment of the present invention, ascertaining if the one or more zones identified at step 104 are definitely porous and weathered or fractured is performed based on Poisson's ratio (PR) values throughout the selected depth range and bulk density (RHOB) values throughout the selected depth range of the basement reservoir. In accordance with various embodiments of the present invention, the one or more zones in the selected depth range of basement reservoir having Poisson's ratio lower than or equal to a predetermined Poisson's ratio value (hereinafter denoted by constant B) are indicative of zones that are weathered or fractured. In an embodiment of the present invention, the predetermined Poisson's ratio value B is the average value of Poisson's ratio derived via core data analysis performed on core samples extracted from multiple depths of an unaltered/un-fractured basement within the selected depth range of the basement reservoir. In an example, assuming 3500-3700m is the unaltered basement within the selected depth of 3000-4000, the core samples may be extracted from multiple point depths, for example 3500.15m, 3500.30m up to 3700 etc. within the selected depth range of 3000-4000m.
[0022] In accordance with various embodiments of the present invention, the one or more zones in the selected depth range of basement reservoir having RHOB lower than or equal to a predetermined RHOB value (hereinafter referred by constant D) are indicative of zones that have low density. In an embodiment of the present invention, the predetermined RHOB value D is the average value of bulk density derived via core data analysis performed on core samples extracted from multiple depths of an unaltered/un-fractured basement within the selected depth range of the basement reservoir. In an example, assuming 3500-3700m is the unaltered basement within the selected depth of 3000-4000, the core samples may be extracted from multiple point depths, for example 3500.15m, 3500.30m up to 3700 etc. within the selected depth range of 3000-4000m.
[0023] Accordingly, the one or more zones in the selected depth range of basement reservoir having PR value <=B and the same one or more zones having RHOB <= D are indicative of zones that are weathered or fractured and have low density,

further confirming porous nature of the one or more zone within the basement reservoir. In operation, a check is performed to ascertain if Poisson's ratio of one or more zones within the selected depth range of the basement reservoir (identified at step 104) indicating porous nature are less than or equal to B and bulk density(RHOB) of said one or more zones (identified at step 104) is less than or equal to D. In accordance with various embodiments of the present invention, Poisson's ratio of one or more zones (identified at step 104) <= B and RHOB of the same one or more zones (identified at step 104) <= D confirms that the one or more zones identified at step 104 are weathered/fractured and have low density, further confirming porous nature of the one or more zones.
[0024] In an example, where a depth ranging from 3000-4000m is the depth of the basement selected for hydrocarbon analysis, the PR values at point depths between 3000m-4000m are extracted from the retrieved PR logs. Further, the RHOB values at point depths between 3000m-4000m are extracted from the RHOB logs. Yet further, a cross plot is generated between the PR values and the RHOB values at point depths between 3000-4000m. Yet further, the one or more point depths (identified at step 104) within the selected depth range(3000-4000m) of basement reservoir having PR lower than B (a predetermined PR value) and RHOB lower than D (a predetermined RHOB value) on the cross plot (PR vs RHOB) confirms that the one or more zones (identified at step 104) are weathered/fractured and porous. Assuming, that 3000.30 and 3000.45 are identified at step 104 as fractured or weathered zones that are also porous. Further, assuming that on PR vs RHOB cross plot, point depths 3000.15, 3000.30, and 3000.45 are the zones having PR lower than B, and 3000.15, 3000.30, 3000.45 and 3000.60 are the zones having RHOB lower than D, then the zones 3000.30 and 3000.45 are confirmed to be porous and weathered or fractured.
[0025] At step 108, the one or more zones within the selected depth range of the basement reservoir that are ascertained to be porous and weathered or fractured (at step 106) are checked for permeability. In an embodiment of the present invention, the check for permeability is performed based on a ratio between deep resistivity

(RD) values and shallow resistivity (RS) values of the one or more zones within the selected depth range of the basement reservoir (identified at step 106). In accordance with various embodiments of the present invention, the one or more zones within the selected depth range of the basement reservoir having RD/RS >1 is indicative that the one or more zones are permeable. In operation, the RD values and RS values associated with the one or more porous and fractured and/or weathered zones (identified at step 106) are extracted from the Deep Resistivity logs and Shallow Resistivity logs, respectively. Further, an RD/RS value for the one or more zones (identified at step 106) are evaluated, where an RD/RS value of said one or more zones >1 confirms that the zones (identified at step 106) are permeable. In an example, where a depth ranging from 3000-4000m is the depth of the basement selected for hydrocarbon analysis, the RD and RS values at point depths between 3000m-4000m are extracted from the Deep Resistivity logs and Shallow Resistivity logs, respectively. Further, the one or more point depths (identified at step 106) within the selected depth range(3000-4000m) of basement reservoir are checked for permeability based on respective RD/RS values. Assuming, that 3000.30 and 3000.45 are identified at step 106 as fractured or weathered zones that are also porous. Further, assuming that depths 3000.15, 3000.30, 3000.45, 3010.60 and 3020 are the zones having RD/RS value>l, then the zones 3000.30 and 3000.45 are confirmed to be permeable, porous and weathered or fractured.
[0026] At step 110, a check is performed to determine if the one or more zones within the selected depth range of the basement reservoir that are determined to be porous, permeable, and fractured and/or weathered are hydrocarbon bearing zones. In an embodiment of the present invention, the check for hydrocarbon is performed based on deep resistivity(RD) values associated with the one or more zones identified as porous, permeable, and fractured or weathered (at step 108). In an embodiment of the present invention, any of the one or more zones identified as porous, permeable, and fractured and/or weathered (at step 108) having a deep resistivity (RD) value of greater than or equal to (>=) a predetermined average value of deep resistivity (E) is indicative of a zone that is porous, permeable, fractured

and/or weathered and hydrocarbon bearing. In an embodiment of the present invention, the predetermined average value of deep resistivity (E) is evaluated from deep resistivity values at multiple point depths within weathered basement in the selected depth range of basement reservoir. In an example, where a depth ranging from 3000-3700 is a weathered basement within the selected depth range of 3000-4000m, then the RD values of point depths, such as 3000.15m, 3000.30m up to 3700 m are extracted from deep resistivity logs associated with the selected depth of the reservoir. In another example, where depth ranging from 3000-3200, 3300-3700 and 3800-4000m are weathered basements within the selected depth range of 3000-4000m, then the RD values of point depths, such as 3000.15m, 3000.30m up to 3200 m, and 3300.15, 3300.30-3700 and 3800.15, 3800.30-4000m are extracted from deep resistivity logs associated with the selected depth of the reservoir. In operation, the RD values associated with the one or more zones (identified at step 108) are extracted from the Deep Resistivity logs. Further, a check is performed to determine if the RD values of the one or more zones (identified at step 108) is greater than or equal to E, where an RD value of any of the one or more zones greater than or equal to E confirms that said porous, permeable, and fractured and/or weathered zone (identified at step 108) and having RD>=E is hydrocarbon bearing. In an embodiment of the present invention, the RD value of any zone determined to be fractured or weathered or both, porous and permeable <= E is indicative that said zone is water bearing.
[0027] In an example, where a depth ranging from 3000-4000m is the depth of the basement selected for hydrocarbon analysis, the RD values at point depths between 3000m-4000m are extracted from the Deep Resistivity logs. Further, the one or more point depths (identified at step 108) within the selected depth range(3000-4000m) of basement reservoir are checked for hydrocarbon based on deep resistivity(RD) values associated with said depths. Assuming, that 3000.30 and 3000.45 are identified at step 108 as fractured or weathered zones that are also porous and permeable. Further, assuming that depths 3000.30, 3000.45, 3010.60 and 3020 are the zones having RD value >=E, then the zones 3000.30 and 3000.45

are confirmed to be permeable, porous and weathered or fractured zones that have hydrocarbon.
[0028] Advantageously, the method of the present invention facilitates identification of hydrocarbon bearing zones in unconventional fractured basement reservoirs based on conventionally recorded well log data which is readily available for each of the wells drilled/bored in the basement reservoir. Further, the method of the present invention eliminates the masking effect of highly resistive basement rocks on well logs. Furthermore, the method of the present invention is inexpensive, easy to implement, relatively accurate and can be used for identifying hydrocarbon bearing zones in new as well as old basement reservoirs.
[0029] Experimental Analysis:
[0030] An experimental analysis was performed to conclude the method steps of the present invention. In the analysis well logs including P-wave velocity (Vp), S-wave velocity (Vs), compressional sonic wave travel time (DTCO), Bulk density (RHOB), Poisson's ratio (PR), Deep Resistivity (RD), and Shallow Resistivity (RS) were recorded for a basement reservoir ranging from 3000-4000m. Further, the following cross plots were generated: Vp/Vs vs DTCO, Vp/Vs vs RD/RS, PR vs RHOB, and RD/RS vs RD based on the values of Vp, Vs, DTCO, RD, RS, PR, and RHOB (of various points depths within the range of 3000-4000m) extracted from respective well logs.
[0031] During the analysis it was concluded that a cross plot between Vp/Vs and DTCO as shown in Figure 2 indicated the presence of one or more porous and weathered/fractured zones within the basement reservoir. It was observed that one or more zones within a selected depth range of the basement having Vp/Vs values lower than a predetermined Vp/Vs value (denoted by constant A) indicated that the one or more zones were fractured weathered, and DTCO values for the same one or more zones a relatively higher than a predetermined DTCO value (denoted by constant C) indicated low density in the basement possibly due to the presence of fluid within fractured and/or weathered basement, further indicating that the one or

more zones are porous. During the analysis, the Vp/Vs value (A) below which basement was considered fractured and/or weathered was fixed from average Vp and Vs values derived in a lab from core samples extracted from multiple depths of an unaltered/un fractured basement within the selected depth range of the basement reservoir. The predetermined DTCO value (C) above which any zone within the selected depth range of basement reservoir is considered to have low density is the average value of DTCO derived from DTCO logs associated with an unaltered/un fractured basement within the selected depth range of the basement reservoir.
[0032] Further, it was concluded that a cross plot between PR (Poisson's ratio) and RHOB (bulk density) as shown in Figure 3 also indicated the presence of one or more porous and weathered/fractured zones within the basement reservoir. As low PR and low density is indicative of compressible porous fractured and/or weathered zones, which is possible when the hard basement has been fractured or weathered and has fluids in its pore spaces, it was concluded that one or more zones having a low PR as well as low density were porous fractured and/or weathered zones. The value of PR (B) below which any zone in the selected depth of basement was considered weathered or fractured was fixed by evaluating the average of PR values derived in a lab from core samples extracted from multiple depths of an unaltered/un-fractured basement within the selected depth range of the basement reservoir. Further, the predetermined RHOB value D below which any zone was considered to have low density was fixed by deriving an average of bulk density values of core samples extracted from multiple depths of an unaltered/un-fractured basement within the selected depth range of the basement reservoir.
[0033] Yet Further, it was concluded that a cross plot between Vp/Vs and RD/RS (as shown in Figure 4) indicated the presence of one or more permeable zones within a selected depth range of basement reservoir. It was observed that RD/RS values for one or more zones within the selected depth of reservoir higher than 1 showed invasion within the reservoir, which in turn indicated that said one or more zones are permeable and low Vp/Vs values for the one or more zones in the selected

depth of the basement indicated low density due to fractures/weathering, where the fractures and/or weathered zones may contain fluid.
[0034] The above three plots (i.e. Vp/Vs and DTCO, PR and RHOB, and Vp/Vs and RD/RS) indicated the presence of weathered/fractured zones within the basement possibly having fluid in its pore spaces. In view of the three plots, it was concluded that the mutually identified points (i.e. zones) of all the three cross plots, (i.e. the zones having Vp/Vs C in the Vp/Vs and DTCO cross plot, the same zones having PR values <=B and RHOB <= D in the PR and RHOB cross plot, and the same zones having RD/RS>1 and Vp/Vs ) a predetermined DTCO value (C), wherein the one or more zones having Vp/Vs < A are indicative of zones that are fractured or weathered or both, and said one or more zones having DTCO >C are indicative of zones that are fractured or weathered or both as well as porous.
4. The method as claimed in claim 3, wherein the predetermined Vp/Vs value (A) is average value of Vp/Vs derived from core data analysis performed on core samples extracted from multiple depths of an unaltered basement within the depth range of the basement reservoir.
5. The method as claimed in claim 3, wherein the predetermined DTCO value (C) is an average value of DTCO derived from DTCO logs associated with an unaltered basement within the depth range of the basement reservoir.
6. The method as claimed in claim 1, wherein ascertaining if the
identified one or more zones are weathered or fractured or both and porous
based on the Poisson's ratio (PR) values and the bulk density (RHOB) values
of said identified one or more zones comprises:
ascertaining if the PR value of the identified one or more zones is less than or equal (<= ) to a predetermined Poisson's ratio value (B) and bulk density(RHOB) of said identified one or more zones is less than or equal to (<= ) a predetermined RHOB value D, wherein the PR value of the identified one

or more zones <= B and RHOB of the identified one or more zones <= D confirms that the identified one or more zones are weathered or fractured or both and have low density, further confirming porous nature of the identified one or more zones.
7. The method as claimed in claim 6, wherein the predetermined Poisson's ratio value B is average value of Poisson's ratio(PR) derived via core data analysis performed on core samples extracted from multiple depths of an unaltered basement within the depth range of the basement reservoir.
8. The method as claimed in claim 6, wherein the predetermined RHOB value D is average value of bulk density(RHOB) derived via core data analysis performed on core samples extracted from multiple depths of an unaltered basement within the selected depth range of the basement reservoir.
9. The method as claimed in claim 1, wherein determining the permeability of the one or more zones that are ascertained as weathered or fractured or both and porous based on a ratio between the deep resistivity (RD) values and the shallow resistivity (RS) values comprises:
evaluating RD/RS values for the one or more zones ascertained as weathered or fractured or both and porous; and
determining one or more zones from the ascertained one or more zones having an RD/RS value >1, wherein the RD/RS value of any of the ascertained one or more zones >1 confirms that said one or more that are weathered or fractured or both and porous zones are permeable.
10.The method as claimed in claim 1, wherein determining if the one or more zones that are determined to be fractured or weathered or both, porous and permeable are hydrocarbon bearing zones based on the deep resistivity(RD) values comprises:
performing a check to determine if the RD values of any of the one or more zones determined to be fractured or weathered or both, porous and

permeable is greater than or equal to (>=) a predetermined average value of deep resistivity (E), wherein the RD value of any zone determined to be fractured or weathered or both, porous and permeable >= E confirms that said zone is hydrocarbon bearing.
11.The method as claimed in claim 10, wherein the predetermined average value of deep resistivity (E) is evaluated from deep resistivity values at multiple point depths within the depth range of the basement reservoir.
12.The method as claimed in claim 10, wherein the RD value of any zone determined to be fractured or weathered or both, porous and permeable <= E is indicative that said zone is water bearing.