COMPLETE SPECIFICATION
METHOD OF ASSESSING QUANTITY OF TOTAL ORGANIC CARBON IN A
SOURCE ROCK OF A SUB-SURFACE REGION
Field of the Invention
[OOOI] The present invention relates generally to the field of hydrocarbon
exploration and more particularly to quantitative estimation of Total Organic Carbon
(TOC) of sub-surface formations based on well logs and geochemical data.
Background of the Disclosure
[0002] In the exploration and production of hydrocarbons, it is important to
know properties of rocks in an earth formation that may contain reservoirs of the
hydrocarbons. TOC content present in reservoir rocks is one of the important
.geochemical parameters, which could be used for evaluating residual production
potential as well as the geochemical characterization of hydrocarbon bearing units.
In general, organic rich Shales are characterized by higher sonic transit time, lower
density, higher gamma ray, and higher resistivity compared to other Shales.
[OOOS] A well log is a continuous record (i.e. depth wise) of various physicochemical
properties of the geologic formations penetrated by a borehole. Resistivity
(electrical resistivity) is an intrinsic property of a substance that quantifies how
strongly a given material opposes the flow of 'electric current. R logs are depth-wise
electrical resistivity and DT logs are sonic travel time (inverse of sonic velocity)
property measurements.
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[0004] To find total organic matter present in a geologic rock, Passey (1990)
devised a formula:
TOC = ( A L ~ ~ R1)0 (2 297 - 0 1688 x LOM) ----- (Eq. I)
Where, TOC = total organic carbon content expressed in weight %,
LOM = Level of maturity (thermal).
And ALo~R= loglo (RIR-baseline) + 0.02 (DT - DT-baseline).
Here, R-baseline and DT-baseline are resistivity and sonic travel time values
respectively, of that rock which does not contain any organic matter. The possibility
of presence of organic matter happens only in very fine grained non-porous rock
(known as Shale). The Shale rock which is devoid of organic matter is having lower
resistivity and sonic travel time values compared to organic rich Shale rocks.
[OOOS] The main requirement for using Passey method to determine TOC is
that an authentic fine grained non-source rock section over a significant depth range
should be available above or below the organic rich Shale section. This may not
happen always. Even if the said section is present, chances are that required logs
might not have been recorded or might be affected by bad borehole against this
section.
[0006] To overcome this limitation of Passey Method, Thomas Bowman
(2010), developed a technique known as modified Passey method. This is done by
r
cross plotting Sonic travel time log & logarithm of Resistivity log and then
determining the non-source rock baseline as shown in .iig. 1. This baseline can be . .
used to calculate a Pseudo-Sonic log that is then displayed over existing sonic log to
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identify organic Shale potential for a zone in an individual well.
The detailed procedure of this method is given below:
LogR is first calculated from Deep Resistivity log at every depth point.
DT Vs LogloR values are cross plotted for all depth points as shown in Fig.1
From this cross-plot, determine the shale base line by connecting non-source
rock shale points (having low R & high DT values) through a straight line.
a . This Shalebase line can be represented by mathematical equation:
DTR = b - mLogR (here, b=intercept on x-axis & m= slope of the line)
where DTR is pseudo sonic travel time.
Overlay pseudo-sonic (DTR) over sonic (DT) travel time log and highlight the
cross over to identify the organic rich Shale section, if any.
In water-saturated and organic-lean rocks, these two curves overlay and allow
the Shale calculation line to be determined.
In either hydrocarbon reservoir rocks or organic-rich Shale sections, a
separation between the curves occur. Using Gamma Ray or SP log,
hydrocarbon bearing reservoir intervals can be identified and eliminated from
the analysis.
Finally, organic rich Shale section is interpreted.
[0007] The modified Passey method does not provide any methodology for
quantitative estimation of TOC from logs. The modified Paseey method basically
offers a qualitative way of TOC assessment and is mainly used as quick look
, technique to acknowledge source rock potential.
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[OOOS] Other references are also available for estimation of TOC without well
log information using Passey method. By way of an example, US Patent No.
8,729,903 granted on May 20, 2014 relates to "Method For Remote Identification
And Characterization Of Hydrocarbon Source Rocks Using Seismic, And
Electromagnetic Geophysical Data". This document is based on assessing TOC
(hydrocarbon source rock potential) of a subsurface formation without well log
information. The method uses surface electromagnetic and seismic survey data to
obtain vertical profile (depth-wise) of resistivity and sonic velocity (inversion of travel
time) which are then analysed in the same way as well log data are analysed for
TOC evaluation by Passey method. However, this method again used Passey
method for TOC estimation with different inputs than the inputs disclosed in the
present invention. For ample clarity, this method uses "remote survey data" as
inputs, whereas, the present subject matter uses "well log data" as inputs. In the
present case, two well log data viz. sonic travel time and resistivity data have been
used in evaluating TOC through (1) using the concept of cross-plotting DT & R as
provided in modified Passey method and (2) introducing a new term pseudoresistivity
(to find A LogR).
[OOOS] In yet another example, being US Patent Application No.: 131086,969
published under no. US20'1210095687, Passey method of TOC estimation (Eq. I ) has
been used to evaluate LOM (level of thermal maturity) from logs. For these
purposes, another well log which provides standalone measurement of TOC in
addition of R-DT log combination that provides ALogR has been used. Here also,
limitations of Passey method as envisaged above come into light.
. .
1001 0] Further, in US Patent Application No.: 121818,680 published under no.
US2011/0144913, TOC and other fractional rock volumes like water, Shale,
hydrocarbon etc. of organic rich Shales have been evaluated, based on the concept
of Archie relation after calibrating against non-source rock non reservoir section,
assuming it is 100% water saturated. For TOC estimation, they have adopted a
methodology of cross-plotting of LogR vs. DT for identifying baseline curve
representing 100% water saturated section and comparing LogR & DT values of
other sections w.r.t. baseline values to estimate their organic matter content
potentiality through Passey method. In this method, a hyperbolic curve has been
considered for baseline description on LogR-DT plot as their model is derived from
Archie relation for water saturation estimation. This is contrary to the present subject
matter since in the present case, the equation to fit all non-reservoir (shale) nonsource
rock points on DT (sonic travel time) vs. LogR (logarithm of resistivity) cross 1
plot is considered to be a straight line (linear) and not an hyperbola, as it is based on
the concept of modified Passey method.
[OOI I ] To sum up, the main requirement for using Passey method to
determine TOC is that an authentic fine grained non-source rock section over a
significant depth range should be available above or below the organic rich Shale
section. To overcome this limitation, Thomas Bowman (2010) recently has
developed a new method (known as modified Passey method) by cross plotting
Sonic Vs Logarithm of Resistivity log and then determining the non-source rock
baseline i.e. low resistivity Shale baseline. However, the main drawback of modified
Passey method is that it does not provide quantitative estimation of TOC from logs.
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[OO 1 21 To address these problems, a novel technique has been developed for
quantitative estimation of TOC from logs following the concept of modified Passey
method. The new technique has the dual benefits in the sense that it does not
require, as a prerequisite, any nearby clay rich non-source rock section for Shale
baseline calibration and still provides a TOC log (continuous-depth-wise TOC
values).
Brief Description of the invention
[OOI 31 In one embodiment, the invention is a method of assessing quantity of
Total Organic Carbon in a source rock of a sub-surface region, comprising: (a)
obtaining a continuous record (i.e. depth wise) of resistivity and sonic travel time
properties of the geologic formations penetrated by a borehole, known as resistivity
(R) and sonic (DT); (b) obtaining Pseudo-resistivity (RDT) by using analogy of
equation obtained from modified Passey method using Cross-Plot (DT vs. LogloR)
and drawing a straight line connecting non-source rock shale points; (c) overlaying
logarithmic Pseudo-Resistivity (LogRDT) over logarithmic Resistivity (LogR) log and
highlighting the cross over to identify the organic rich Shale section; (d) obtaining the
positive separation between these two curves in the form of an equation, expressed
as 'ALogR (new) = LogR - LogRDT'; and (e) obtaining TOC (wt%) value by using
positive separation between logarithm of resistivity and pseudo-resistivity logs in the
Passey's equation (in place of ALogR).
Brief Description of the Drawings
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[0014] The present invention and its advantages will be better understood by
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referring to the following detailed description and the attached drawing in which: I
[0015] FIGS. 1 depict cross-plotting sonic travel time log and logarithm of
resistivity log and then determinin'g the non-source rock baseline as suggested in
modified Passey method.
[OO 1 61 FIGS. 2 depict field example showing limitations of Passey method and
applicability of the present invention for T,OC calculation through validation with lab
data on cutting samples.
Detailed description of the invention
[OOI 71 The present invention extends the well known Delta Log R Well-based
method for estimating hydrocarbon source rock TOC, so that it is performed using
well log sonic and resistivity data from wells that has been acquired from the
subsurface hydrocarbon source rock. First, a well log, which is a continuous record
(i.e. depth wise) of various physico-chemical properties of thegeologic
formations penetrated by a borehole, is obtained. These properties are measured by
instruments lowered into the borehole. R logs being the depth-wise electrical
resistivity and DT logs, being the sonic travel time (inverse of sonic velocity) property
measurements are obtained from the well logs. These logs are subsequently
corrected for borehole temperature, borehole mud pressure & salinity etc. for further
use.
[OO 1 81 In modified Passey Cross-Plot (DT vs. LoglOR), the non-source rock
Shale base line is represented in terms of pseudo-sonic (DTR) as:
D TR =. b - m LogR ------ (2)
where b=intercept & m= slope of Shale base line.
At 'this point, a new term Pseudo-resistivity (RDT) has been defined. Pseudoresistivity
(RDT) at any depth of subsurface Shale formation would be that
hypothetical resistivity value of rock had it been completely devoid of organic matter.
Taking analogy from Eq.2, logarithm of Pseudo Resistivity can be defined as:
LogRDT = (b -DT) / m
Where, LogRDT is new Resistivity (Pseudo-Resistivity).
As depicted in Fig. 2, overlay of logarithmic Pseudo-Resistivity (LogRDT) over
logarithmic Resistivity (LogR) log and highlighting the cross over to identify the
organic rich Shale section, if any, is performed as the next step.
Positive separation between these two curves can be expressed as,
ALogR (new) = LogR - LogRDT
Now, this is used in Passey's equation (Eq.1) in place of ALogR, in order to quantify
TOC values.
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Then, TOC can be determined using Eq.1 as,
I TOC (wt%) = (LogR - Log RDT) x 70 (2.297 - 0.1688 x LOM) I
Positive separation between logarithm of resistivity and pseudo-resistivity logs,
expressed as (LogR - LogRDT), has been used in Passey's equation (in place of
ALogR) in order to quantify TOC values. The present inventive method removes the
limitations of Passey method and is applicable to all. types of Shale sections,
especially those of Indian Basins, with. greater degree of authenticity. The present
technique removes limitation of Brown's modified Passey method and provides
quantitative estimation of TOC at each formation depth.
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We Claim:
1. Method of assessing quantity of Total Organic Carbon in a source rock of a
sub-surface region, comprising the steps of:
(a) obtaining a continuous recbrd (i.e. depth wise) of resistivity and sonic
travel time properties of the geologic formations penetrated by a
borehole, known as resistivity (R) and sonic (DT);
(b) obtaining Pseudo-resistivity (RDT) by using analogy of equation obtained
from modified Passey method using Cross-Plot (DT vs. LoglOR) and fitting
non-source rock shale points through a linear equation ;
(c) overlaying logarithmic Pseudo-Resistivity (LogRDT) over logarithmic
Resistivity (LogR) log and highlighting the cross over to identify the organic
rich Shale section;
(d) obtaining the positive separation between these two curves in the form of
an equation, expressed as 'ALogR (new) = LogR - LogRDT'; and
(e) obtaining TOC (wt%) value by using positive separation between logarithm
of resistivity and pseudo-resistivity logs in the Passey's equation (in place of
ALogR).
2. Assessing source rock potential and hence shale gastoil potential in any type
of Shale section through quantitative estimation of Total Organic Carbon by
application of the method as claimed in claim 1.