DESC:FIELD OF THE INVENTION
The present invention is related to the accurate identification of the hydrocarbon bearing zones in petroleum reservoirs that are contaminated with Low Wax Crude (LWC). More particularly, the invention relates to a method for determining the amount of LWC contamination in Sidewall core (SWC) extracts of Eocene formation or any other geological formations bearing non-biodegraded oil as a result of invasion of LWC into the reservoir andto accurately identify the presence of hydrocarbon bearing intervals and distinguish between LWC and in-situ hydrocarbons present in the reservoir.

BACKGROUND OF THE INVENTION AND PRIOR ART
Low wax crude is used as a lubricant in water based drilling fluids to lubricate borehole sides or clearing stuck pipes during operation in drilling wells. The applied LWC sometimes invades into the reservoir through the borehole sides contaminating the in-situ fluids present in the reservoir. Sidewall cores taken from these zones contain LWC along with the in-situ fluids. It creates difficulty in proper identification of oil bearing zones in the reservoir.
After boring a well many oil companies use a combination of wire-line log data and fluorescence evidence from drill cutting and SWC samples to detect the hydrocarbon bearing intervals in the reservoir. In the Eocene reservoirsin Upper Assam Basin, routine interpretations of the density-neutron logs are mostly misleading. Most of the sand ranges within these reservoirs exhibit appreciable crossover in the density-neutron log, which is an indication of presence of gas, although some of these have been proved to be oil bearing. SWC/drill cutting fluorescence and oil content in SWC from these zones are also not conclusive, as many of the hydrocarbon-bearing zones show characteristics of both oil and gas. Additionally, the resistivity logs also show very high resistivity (up to 800 ohm m) against some possible oil-bearing zones. The application of wire-line formation tester for determination of fluid gradient is also restricted, owing to the thin nature of the sand units and problematic borehole conditions.
A geochemical technique, employing thin layer chromatography with flame ionization detector (TLC-FID) and gas chromatographic (GC) analyses of SWC extracts, has been used successfully to overcome the difficulties and to make formation evaluation more objective and accurate, a supplement to wire-line log evaluation. The geochemical technique is now routinely used for the majority of wells drilled. Using this technique it is possible to predict the type of hydrocarbons, i.e. gas, light oil, normal oil, heavy oil or water present at any depth prior to production testing of the well and producibility thereof.
FIG. 1 depicts the schematic representation of geochemical analysis of sidewall core.The existing technique, geochemical analysis of SWC extract can interpret the oil bearing zones in wells that are drilled with water based mud with almost 100% accuracy. But difficulty arises when LWC is used in drilling wells. Sometimes LWC invades into the reservoir, thuscontaminating the in-situ hydrocarbon. The invasion of LWC increases the oil content of the sidewall core recovered from the zone of interest even though there is very little or noin-situ hydrocarbons present in the reservoirs. This results in wrong interpretation of the presence of hydrocarbon in the reservoir although it may not contain any hydrocarbon at all.
WO2014022794provides a method for prediction of subsurface fluid properties (e.g., phase or API gravity) using gas chromatogram data of a small-volume extract of cores. This method involves in determination of concentrations of a set of n-alkanes or pseudo components (i.e. all compounds occurring between a pair of n-alkanes) to determine the fluid properties. The prerequisite of the method is n-alkanes must be present in both fluid and contaminant. This technique fails to determine fluid properties, if n-alkanes are missing in either fluid or contaminant.
An article entitled “Mixing and biodegradation of hydrocarbons in the Daerqi oilfield, Baiyinchagan Depression, northern China” by Changchun Pan et al discloses about classification of oils from oilfields into three groups based on bulk and molecular geochemistry. The authors have determined the appropriate proportion of contamination by determining the molecular parameters. This method fails to determine the composition of any mixture of two oils if the end member oils composition is known.
Thus there is a requirement of an improved method to determine the amount of saturates, aromatics, resin and asphaltenes (SARA) and particularly the amount of saturates present ina mixture of two distinct types of oils, i.e.low wax crude (LWC) and the native formation oil in the sidewall core or conventional core. The present invention quantifies the amount of LWC mixed with the in-situ hydrocarbons present in the SWC as a result of invasion of LWC into the reservoir, so that accurate determination of the amount of hydrocarbon present in the core is possible and furthermore, accurate identification of the pay zones when LWC, used for drilling operationcontaminates the in-situ hydrocarbons present in the sidewall cores.

OBJECTIVES OF THE PRESENT INVENTION
The objectof present invention is to overcome the drawbacks of the prior art.
Another object of present invention is to provide a method for the accurate quantification of amount of hydrocarbon present in the core which is contaminated with LWC used as lubricant in drilling fluid.
Yet another object of the present invention is to provide a method for accurate identification of pay zones in the reservoir of Eocene formationof Upper Assam Basin or any other geological formations bearing non-biodegraded oil when LWC is used for drilling operation.
Still another object of the present invention is to provide a method to differentiate between light, normal and heavy oils present in the reservoir.
Still another object of the present invention is to provide a method to avoid testing heavy oil-bearing zones, which are difficult to produce.
Another object of the present invention is to provide a method for calculating the percentage mixing of LWC with the native hydrocarbons present in SWC extracts to find out the amount of in-situ hydrocarbon present in LWCcontaminated cores.
Yet another object of the present invention is to provide a method for estimation of amount of LWC mixed with in-situ hydrocarbons present in the sidewall cores using parameters like extract amount, % saturated hydrocarbons, ratios of certain saturated hydrocarbons like Pristane/n-C17 (Pr/n-C17), Phytane/n-C18 (Ph/n-C18), A/n-C19, B/n-C20 andratios of certain aromatic compounds like 1,3,6,7TeMN/xTeMN (xTeMN is an unknown Tetramethylnaphthalene), 1,3,6TMN/1,3,7TMN and CAD/4MDBT.

SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a method for determining the amount of LWC mixed with the native oil present inside sidewall cores taken from reservoirs of Eocene formations or any other geological formations bearing non-biodegraded oil due to invasion of LWC used as a lubricant in water based mud to drill a well into the reservoir through the sides of the borehole, said method comprising:
a) extracting sidewall core (SWC) sample, using Soxhlet extraction technique to obtain extractable organic compounds present in the cores;
b) analyzing the extractable organic compounds obtained in step a) using a Gas Chromatographic analysis to detect contamination of LWC in the extractable organic compounds, wherein
the contamination is detected by comparing the ratios of Pristane/n-C17 and Phytane/n-C18 values in the extractable organic compounds obtained and the ratios of Pristane/n-C17 and Phytane/n-C18 values obtained for Eocene oil;
c) preparing a mixture of Eocene oil and LWC in the ratios selected from 95%:5%, 90%:10%, 85%:15%, 80%:20%, 75%:25%, 70%:30%, 65%:35%, 60%:40%, 55%:45%, 50%:50%, 45%:55%, 40%:60%, 35%:65%, 30%:70%, 25%:75%, 20%:80%, 15%:85%, 10%:90% and 5%:95%,
d) Determining the amount of saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes present in the Eocene oil, LWC and mixtures of Eocene oil and LWC by using a TLC-FID technique,
e) comparing based on Eocene formation, the amount of saturated hydrocarbons present in the Eocene oil, the extractable organic compounds obtained and the mixture of the Eocene oil and LWC;
f) calculating the ratios of 1,3,6,7-TeMN/XTeMN, 1,3,6TMN/1,3,7TMN and CAD/4MDBT using a GCMS analysis of the Eocene oil, the extractable organic compounds obtained and the mixture of the Eocene oil and LWC, and
g) comparing 1,3,6,7-TeMN/XTeMN, 1,3,6TMN/1,3,7TMN and CAD/4MDBT ratios and matching the ratios obtained from the Eocene oil, the extractable organic compounds obtained and the mixture of the Eocene oil and LWC to detect the amount of LWC present in the extractable organic compounds, wherein the percentage of saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes are calculated from the peak areas of saturated hydrocarbons, aromatic hydrocarbons, resin and asphaltenes in TLC-FID chromatogram.

LIST OF ABBREVIATIONS USED IN THE PRESENT INVENTION

LWC : Low Wax Crude
SWC : Sidewall core
OCS : Oil Collecting Stations
TLC-FID : Thin Layer Chromatograph-Flame Ionization Detector
GC : Gas Chromatograph
GC-MS : Gas Chromatograph-Mass Spectrometer
HPLC : High Performance Liquid Chromatograph
SARA : saturates, aromatics, resins and asphaltenes
Pr : Pristane
Ph : Phytane
TMN : Trimethylnaphthalene
TeMN : Tetramethylnaphthalene
DCM : Dichloromethane
CAD : Cadalene
MDBT : Methyldibenzothiophene
SIM : Selected ion monitoring
RTX : Restek Corporation
RIG Time : The time between recovery of sidewall cores after completion of drilling and production testing of the well.

DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 depicts the schematic representation of geochemical analysis of sidewall cores.
FIG. 2 represents a schematic diagram of the invented method to determine amount of LWC contamination in SWC extracts.
FIG. 3 depicts a Soxhlet extraction apparatus.
FIG. 4depicts representative TLC-FID Chromatogram of SWC extract of an oil bearing zone.
FIG.5 depicts the variation of saturates content with change of crude and LWC mixture.
FIG. 6 depicts the graphical representation of the average variation of saturate content with change in LWC content in Eocene crude oils from Upper Assam Basin.
FIG. 7 depicts the graphical representation of the average variation of saturate content with change in crude oil from all formations and LWC Mixture and unknown SWC extracts.
FIG. 8 depicts the graphical representation of the average values of Pr/n-C17 ratios for mixtures of crude oil from Eocene formationfrom Upper Assam Basin and LWC.
FIG. 9 depicts the graphical representation of the average values of Ph/n-C18 ratios for mixtures of crude oil from Eocene formation from Upper Assam Basin and LWC.
FIG.10 depicts the graphical representation of the average values of A/n-C19 ratios for mixtures of crude oil from Eocene formation from Upper Assam Basin and LWC.
FIG. 11 depicts the graphical representation of the average values of B/n-C20 ratios for mixtures of crude oil from Eocene formation from Upper Assam Basin and LWC.
FIG. 12 depicts the graphical representation of Av. Pr/n-C17 values for different concentrations of LWC and Eocene crude mixtures and SWC extracts from X1 with unknown LWC content.
FIG. 13 depicts the graphical representation of Av. Ph/n-C18 values for different concentrations of LWC with Eocene crudes and SWC extracts with unknown LWC content.
FIG. 14 depicts the graphical representation of Av. A/n-C19 values for different concentrations of LWC with Eocene crudes and SWC extracts with unknown LWC content.
FIG.15 depicts the graphical representation of Av. B/n-C20 values for different concentrations of LWC with Eocene crudes and SWC extracts with unknown LWC content.
FIG. 16 depicts the variation of 1, 3, 6,7-TeMN/X-TeMN ratio with change of crude and LWC mixture.
FIG.17 depicts the variation of 1, 3, 6-TMN/1, 3,7-TMN ratio with change of crude and LWC mixture.
FIG.18 depicts the variation of CAD/4-MDBT ratio with change of crude and LWC mixture.
FIG. 19 depicts the amount of LWC contamination present in Eocene SWC determined by 1, 3, 6,7-TeMN/STeMN ratio.
FIG. 20 depicts the amount of LWC contamination present in Eocene SWC determined by 1, 3, 6-TMN/1, 3,7-TMN ratio.
FIG. 21 depicts the amount of LWC contamination present in Eocene SWC determined by CAD/4MDBT ratio.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Low wax crude (LWC) is used as a lubricant in water based drilling fluids to lubricate borehole sides or clearing stuck pipes during operation in drilling wells. The applied LWC sometimes invades into the reservoir through the borehole sides contaminating the in-situ fluids present in the reservoir. Sidewall cores taken from these zones contain LWC along with the in-situ fluids. This creates difficulty in the proper identification of oil bearing zones in the reservoir. The existing method can detect only the presence of LWC in the cores. However, it does not determine the amount of LWC mixed with the in-situ oil. This leads to incorrect interpretation of the presence of hydrocarbon in the reservoir although it may not contain any hydrocarbon at all. However, invasion of LWC into sidewall cores from drilling fluid especially, when LWC is used in the drilling fluid or LWC is used for clearing of stuck pipes during operation in drilling wells, accurate interpretation of the presence of hydrocarbons in the reservoir is not possible unless the percentage of mixing of LWC with reservoir hydrocarbons is known.
Thus the present invention relates to a method for overcoming long existing problem of determining the amount of LWC concentration in SWC extract of Eocene formation or any other geological formations bearing non-biodegraded oil as a result of invasion of LWC into the formation.
The known methods of prior art could identify the oil bearing zones only when the SWC extracts are not contaminated with LWC. Whereas the present method can identify the oil bearing zones even when the SWC extracts are contaminated with LWC. The known methods of prior art cannot quantify the amount of LWC mixed with the in-situ oil present in the SWC extracts. Whereas the present method can quantify the amount of LWC mixed with in situ-oil present in the SWC extracts.
The present method determines mixing of a contaminant (LWC) that is partially depleted in n-alkanes. The method utilizes the differences in the concentration of saturated hydrocarbons present in Eocene oils and LWC and other non n-alkane compounds which are abundant in LWC as well as in the native oil of Eocene formation to determine the degree of mixing. This helps in identifying whether the zoneis oil-bearing or simply contaminated with LWC.
The present invention thus provides a method for determining the amount of LWC contamination in SWC extracts of Eocene formation or any other geological formations bearing non-biodegraded oil as a result of invasion of LWC into the cores. The inventors have surprisingly found that by using the present method it is possible to predict the type of hydrocarbons i.e. gas, light oil, normal oil, heavy oil or water present at any depth prior to production testing of the well. Fig 1 represents a schematic diagram of the technique.
The present invention provides a method for determining the amount of LWC contamination in SWC extracts of Eocene formation or any other geological formations bearing non-biodegraded oil which involves the determination of the variations in the concentration of non n-alkane compounds which are abundant in either LWC or in the native oil to determine the degree of mixing.
The method as described in the present invention differentiates the light, normal and heavy oils present in the reservoir which is not possible using well log data. Thus, testing heavy oil-bearing zones, which are difficult to produce, can be avoided, saving considerable rig time and the cost involved thereof.
‘Sidewall core’ or ‘Sidewall core sample’ as referred herein means a piece of reservoir rock taken out from the side of the drilled well usually by a wire line sidewall coring tool or gun. The sidewall core (SWC) sample is collected from depths 3859.5m and 3888.0m from a drilled well by well logging coring gun from the side of the wall of the borehole.
Depths at 3859.5m and 3888.0m represents SWC samples collected by well logging coring gun from depths 3859.5m and 3888.0m from the side of the wall of the borehole.
The sample depth of 3859.5m and 3888.0m refer to a specific example. The sample depth will be different for different wells.
In an embodiment of the present invention, the method for identification of hydrocarbon bearing zones in petroleum reservoirs that are contaminated with LWC is provided. The present method is able to determine mixing of two distinct types of oils, low wax crude (LWC) and the native oil present in the sidewall core or conventional core of Eocene formations or any other geological formations bearing non-biodegraded oil. The present invention thus quantifies the amount of LWC mixed with in-situ hydrocarbons present in the SWC as a result of invasion of LWC into the reservoir, so that accurate determination of the amount of hydrocarbons present in the core is possible and accurate identification of the pay zones when LWC is used during drilling operation.
In an embodiment of the present invention, there is provided a method for identification of hydrocarbon bearing zones in petroleum reservoirs that are contaminated with Low wax crude (LWC) comprising the steps of:
a) Soxhlet extraction of SWC in an organic solvent by boiling the SWC in the organic solvent for 1 hour and rinsing by the same solvent for 2 hours in order to extract the extractable organic compounds present in the cores;
b) Treating the extract obtained in step a) with said organic solvent at ambient conditions to make approximately 1-5% solution of the extract with organic solvent;
c) Analyzing solution of the SWC extract obtained in step b) using TLC-FIDto determine saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes(SARA) present in the extract;
d) Calculating the percentage of saturated hydrocarbons and aromatic hydrocarbons, resins and asphaltenes present in the extract;
e) Preparing mixtures of Eocene crude oil and LWC at different proportions (95% Eocene crude oil:5% LWC - 5% Eocene crude oil:95% LWC);
f) Analyzing crude oils from Eocene formation, LWC and mixtures of crude oil and LWC at different proportions to determine SARA present in them;
g) Calculating the percentage of saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes present in oils from Eocene formation, LWC and mixtures of crude oil and LWC at different proportions to determine the concentrations of SARA present in them;
h) Gas chromatographic analysis of solution obtained in step b), crude oil of Eocene formations, LWC and mixtures of crude oil of Eocene formations with LWC at different proportions to determine the peak areas of some of the saturated hydrocarbon compounds like n-C17, Pristane (Pr), n-C18, Phytane (Ph), n-C19, n-C20, A and B (A and B are unknown compounds more abundant in LWC than Eocene oils) present in the extract, crude oil, LWC and mixtures of crude oil of Eocene formations with LWC at different proportions;
i) Calculating the ratios of Pr/n-C17, Ph/n-C18, A/n-C19 and B/n-C20 in crude oil, LWC and mixtures of crude oil of Eocene formations with LWC at different proportions;
j) Gas Chromatography Mass Spectrometric (GCMS) analysis of crude oil, LWC and mixtures of crude oil of Eocene formations with LWC at different proportions and LWC contaminated SWC extracts to find out the peak heights of aromatic hydrocarbon compounds like 1,3,6,7TeMN, xTeMN, 1,3,6TMN, 1,3,7TMN, CAD and 4MDBT present in them; and
k) Calculating the ratios of 1,3,6,7TeMN/xTeMN, 1,3,6TMN/1,3,7TMN and CAD/4MDBT.
The present method is able to determine variation in the concentration of saturated hydrocarbons,n-C17, Pristane, n-C18, Phytane, n-C19, n-C20, A, B and aromatic compounds like 1,3,6,7TeMN, xTeMN, 1,3,6TMN, 1,3,7TMN and CAD, 4MDBTwhich are abundant in either LWC or in the native oil to determine the degree of mixing.
The percentage of saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes are calculated from the peak areas of saturated hydrocarbons, aromatic hydrocarbons, resin and asphaltenes in TLC-FID chromatogram. The four peak areas were normalized to give the percent concentrations to the nearest 0.1% (m/m) for saturated hydrocarbons, aromatic hydrocarbons, resin and asphaltenes. The sum of the concentrations of saturated hydrocarbons, aromatic hydrocarbons, resin and asphaltenes equals 100%.
The degree of mixing of LWC in SWC extracts is determined by comparing the values of saturated hydrocarbons present in the SWC extracts with that of the mixtures of LWC and crude oils from Eocene formations at different proportions (5% LWC:95% crude oil – 95% LWC: 5% crude oil). The percentage of mixing is calculated by comparing the values of saturated hydrocarbons present in the SWC extracts with that of the mixtures of LWC and crude oils from Eocene formations at different proportions. When the value of saturated hydrocarbon in SWC extracts falls within the range of % saturated hydrocarbon between two adjacent mixtures say 70% LWC:30% crude oil and 75% LWC:25% crude oil then the amount of LWC present in the SWC extracts is taken as 70-75%. The mixture of Eocene oil and LWC has been prepared in various combinations such as 95%:5%, 90%:10%, 85%:15%, 80%:20%, 75%:25%, 70%:30%, 65%:35%, 60%:40%, 55%:45%, 50%:50%, 45%:55%, 40%:60%, 35%:65%, 30%:70%, 25%:75%, 20%:80%, 15%:85%, 10%:90% and 5%:95% of Eocene oil and LWC. The ratio as mentioned are supported by Tables 2-10, 12-14, 16-18.The preferred ratio ranges from 20:80 to 70:30 of Eocene oil and LWC.
The method uses organic solvent as Dichloromethane. The saturated hydrocarbon compounds like n-alkanes and iso alkanes are selected in the range from n-C13 to n-C36, preferably Pristane/n-C17 (Pr/n-C17), Phytane/n-C18 (Ph/n-C18), A/n-C19, B/n-C20.
The present method provides accurate quantification at an accuracy level of up to 5% variation of amount of hydrocarbon present in the core when the extract is contaminated with LWC. Since quantification of the amount of LWC present in SWC extracts can be done by comparing the amount of saturated hydrocarbons and parameters like Pr/n-C17, Ph/n-C18, A/n-C19, B/n-C20, 1,3,6,7-TeMN/xTeMN, 1,3,6-TMN/1,3,7TMN and CAD/4MDBT of SWC extracts with that of mixtures of LWC and crude oil at ratios of 5% interval the accuracy level of contamination will be of 5% variation. Accurate quantification of the amount of LWC present in extracts of SWC represents the amount of LWC in SWC extracts at an accuracy level of 5% variation.
The present method accurately identifies the presence of LWC in the extract and quantifies the amount of LWC present in SWC extracts from Eocene formationsor any other geological formations bearing non-biodegraded oil. Prior to the present invention it was possible to identify the presence of LWC in SWC extracts of Eocene formationsor any other geological formations bearing non-biodegraded oil but it was not possible to quantify the amount of LWC present in the extracts which creates difficulties in proper identification of oil bearing zones in the reservoir. According to Table 19 if a SWC from an Eocene reservoir is uncontaminated with LWC, and gives > 6mg/g of extract then it is treated as oil bearing zone. However, if a SWC is contaminated with LWC and gives > 6mg/g of extract it cannot be considered as oil bearing. This is due to the reason that if there is suppose 40% LWC present in the extract then the actual in-situ oil present in the extract will be 3.6mg/g. This according to Table 19 is considered to be residual oil or water bearing zone. So, it is very important to know the amount of LWC present in the extract to accurately identify the oil bearing zone in an Eocene formationor any other geological formations bearing non-biodegraded oil.
The present method is quite fast which helps for quick decision making whether to run in casing or not in the drilled well.
1. The present method gives prior information about the quality of oil present in the reservoir before actual production testing of the reservoir takes place.
2. The present method can detect oil in a zone which is interpreted to be gas bearing by Neutron-Density log.

The examples, which are intended to be purely exemplary of the invention, should therefore, not be considered to limit the invention in any way.

EXAMPLE 1:
Three crude oil samples were collected from the well heads namely A1, B1 and C1 respectively from Eocene formations, LWC is collected from oil collecting station (OCS) and LWC contaminated sidewall cores are collected from a drilled well namely X1of an Eocene formation
The saturate, aromatic, resin and asphaltene (SARA) analysis of LWC and crude oil from different wells of Eocene formations was carried out to find out the variations of the saturate content. The variations of saturated content in LWC and crude oil from different wells of Eocene formations are shown in table. 1.
Table1: compositional analysis of selected crude oils and LWC by TLC-FID.

Formation Sample Saturate (%) Aromatic (%) Resin (%) Asphaltenes (%)
LWC 21.6 70.5 5.8 2.1

Eocene A1 53.5 38.0 6.2 2.2
B1 40.2 46.4 8.9 4.5
C1 35.2 55.7 6.1 2.9

From the Table1, it was observed that the saturate content of LWC is significantly lower than the crude oils of Eocene formations. The difference in saturate content of LWC from the crude oils of Eocene formations is used to calculate the percentage of mixing of LWC into the in-situ hydrocarbon present in the SWC.

EXAMPLE 2:
Soxhlet extraction of SWC samples- After removing the mud filtrate and other obvious contaminants from the collected sidewall cores, those were subjected to the soxhlet extraction in presence of Dichloromethane (DCM). SWC extraction was performed by boiling 3 g of SWC in DCM for 1 hour and rinsing for 2 hours in DCMin flowing condition. Then the volume of the solvent was reduced, and the extract was dried under ambient conditions and weighed. Then 1-5%% solution of the extract was made with DCM for bulk composition of the extract for the TLC-FID, GC and GCMS analysis.

Steps of Soxhlet extraction of SWC are given as under:
a) About 3 g of SWC sample (weighed to the nearest 0.0001g), was taken in a previously driedcellulose extraction thimble.
b) The thimble was then plugged with a swab of cotton-wool.
c) Some previously extracted glass beads were put into the extraction cylinder (Fig. 3) to avoid bumping during extraction.
d) The thimble along with the SWCwas then inserted into the extraction cylinder. A thimble holder is fixed to the cylinder at a height of 5.5 cm from the base of the cylinder which is made up of the same glass as that of the cylinder to hold the thimble at a height of 5.5 cm from the base of the cylinder.
e) About 100 ml of dichloromethane (AR grade) was then poured into the extraction cylinder ensuring that the sample is completely immersed in it.
f) The cylinder was then inserted into the heater (Fig. 3) and the condenser is tightly fixed over the cylinder. Both the cylinder and condenser(Fig. 3) were then clamped tightly.
g) Connected the outlet of a re-circulating water bath to the inlet of the condenser and the outlet of the condenser to the inlet of the water bath by nylon tubing.
h) Switched on the water bath to allow water to circulate through the condenser maintaining the temperature of water to approximately 15ºC.
i) A measuring cylinder of 50 ml capacity (Fig. 3) was kept below valve 1 (Fig.3). Valve 1was kept open during the entire extraction period to facilitate escape of any vapors that may not condense in the condenser.
j) Opened valve 2 (Fig. 3) at the right hand side of the condenser.
k) Switched on the heater and the regulator was adjusted in such a manner that dichloromethane just boils.
l) Since the valve 2 was at a height that is slightly lower than that of valve 1, all the condensing vapors were returned to the cylinder through valve 2.
m) Boiling of dichloromethane was continued for one hour.
n) After one hour, valve 2 was closed. The dichloromethane started to drain from valve 1 in to the cylinder kept below. Draining dichloromethane was continued until it comes down to a level of approximately 2 cm below the thimble in the extraction cylinder.
o) Valve 2 was then opened. Since dichloromethane was at a height below the level of the thimble, the condensing dichloromethane rinses the SWC sample.
p) Rinsing was continued for 2 hours.
q) After 2 hours valve 2 was closed and allowed the dichloromethane to drain through valve 1 into the measuring cylinder. Draining was continued till only about 10 ml of dichloromethane was left in the extraction cylinder.
r) Switched off the heater and the water pump.
s) Weighed a 25 ml beaker to 0.0001 g. After the extraction cylinder cooled down to room temperature, the condenser was disconnected from the extraction cylinder and poured dichloromethane containing the extract into this beaker, taking care that glass beads did not fall into the beaker. Rinsed the extraction cylinder with fresh dichloromethane and poured the contents into the beaker.
t) Removed the solvent in the beaker by heating mildly on a water bath.
u) When the beaker has cooled, weighed it to the nearest 0.0001 g.

CALCULATION AND REPORT
The amount of extract, expressed in mg/g is given by the following formula
(m3 – m2)*1000
m1
Where
m1 = mass of rock in g
m2 = mass of empty beaker in g
m3 = mass of beaker + extract in g

EXAMPLE 3:
Gas chromatographic (GC) analysis of SWC extracts crude oil, LWC and mixtures of crude oil and LWC: Gas chromatographic analysis of the SWC extracts, crude oil, LWC and mixtures of crude oil and LWC were carried out using an Agilent 6890N GC. 1µl solution of the SWC extract in DCM was injected into the column in split mode at an injector temperature of 3000C, detector temperature of 3000C, gradient column oven temperature program from 80 to 3000C at the rate of 200C/min, and hold time of 5 min at 3000C. RTX-1 column having dimensions of 30m X 0.25µm X0.25mm is used for the analysis.

EXAMPLE 4:
Gas chromatography-mass spectrometric (GC-MS) analysis of SWC extracts, crude oil, LWC and mixtures of crude oil and LWC:The gas chromatography-mass spectrometric analysis (GC-MS)of SWC extracts, crude oil, LWC and LWC and crude oil mixtures was carried out using Thermo Trace GC which was directly connected to a Thermo DSQ mass spectrometer. 1µl of 1% sample was introduced into Thermo Trace GC which was directly connected to the Thermo DSQ mass spectrometer. The GC column is RTX-1 (60m X 0.25mm i.d, 0.25um film thickness) and the oven temperature program is: 500C (3 min hold) to 1000C at 250C/min (hold 0 min at 1000C) then 1000C to 3100C at 20C/min (hold time 20 min at 3000C).
The chromatographic conditions as maintained were split injector 3000C; split flow 20ml/min; split ratio 20:1; carrier gas Helium at constant flow; transfer line 3000C. The MS operating conditions are; ion source 2000C; SIM scan mode. Total time of analysis: 130 mins.

EXAMPLE 5:
TLC-FID detection of bulk composition of the SWC extracts, crude oil, LWC and mixtures of crude oil and LWC: An Iatroscan TLC-FID Model MK-6 was used for the determination of the bulk composition of the SWC extracts comprising crude oil, LWC and mixtures of crude oil and LWC. A set of 10 silica coated glass rods, chromarods (S-III), were activated by passing through the FID flame. After activation of silica coated glass rods, 2 µl volume of extract was spotted on the rod in 8 equal aliquots containing 0.25 µl volume each by using a 2 µl Hamilton syringe. Sufficient time was given between spotting of each aliquot to allow the drying of the solvent. This step will help to minimize broadening of the spotted point and resulted in better resolution and peak sharpness. The chromatographic elution was carried out in the separating chambers containing sufficient amount of solvent. Chromarods are lined on three sides with filter papers, which are saturated with the solvent.
The process for the elution of spots comprising the steps of,
a) Elution of chromarods (S-III), with n-heptane up to a rod length of 10 cm, to separate saturated hydrocarbons, followed by drying the rods at 600C for 30s;
b) Elution of chromarods (S-III), with toluene and n-Heptane mixture (80:20 v/v) mixture up to a rod length of 5.5 cm, to separate aromatic hydrocarbons, followed by drying at 600C for 30s; and
c) Elution of chromarods (S-III), with dichloromethane and methanol mixture (95:5 v/v) up to a rod length of 2.5 cm, to separate resins, followed by drying at 600C for 90 sec.
In elution procedure, asphaltenes were remained at the spotting point. All the solvents employed in the chromatographic elution are high performance liquid chromatography (HPLC) grade solvents.
After completion of elution, rods were scanned on TLC-FID, and the FID response for each fraction was collected using Iris 32 plus chromatographic software. The baseline correction was carried out on raw data using the interactive graphics module of the software. The corrected area counts for each peak are determined and converted into micrograms by dividing with individual response factors for each fraction, and the composition (i.e., the percentage of saturated hydrocarbons, aromatic hydrocarbons, resins, and asphaltenes) was determined. A representative TLC-FID chromatogram of an extract of a sidewall core from an oil-bearing zone is shown in the Figure 3. The procedure used for the TLC-FID analysis is as given by Karlsen and Larter (1991), and the response factors were determined using the method of Bharati et al. (1994).
Table 2, 3, and 4 presents the SARA compositions of different mixtures of crude oils and LWC from wells A1, B1, C1 respectively.
Table 2: compositional analysis of LWC and A1 mixtures by TLC-FID.

Sample Saturate (%) Aromatic (%) Resin (%) Asphaltenes (%)
100% crude oil 53.5 38.0 6.2 2.2
5% LWC 37.5 57.7 2.8 2.0
10% LWC 36.8 58.6 2.9 1.4
15% LWC 35.5 60.1 3.0 1.4
20% LWC 34.5 61.4 2.8 1.2
25% LWC 33.5 62.1 3.1 1.4
30% LWC 32.6 63.1 3.0 1.3
35% LWC 31.6 64.3 3.0 1.1
40% LWC 30.2 65.7 3.1 1.0
45% LWC 29.2 66.8 3.0 1.0
50% LWC 28.0 68.0 3.1 0.9
55% LWC 27.2 68.8 3.2 0.8
60% LWC 26.1 70.1 3.0 0.8
65% LWC 25.3 70.4 3.2 1.1
70% LWC 24.4 71.6 3.3 0.7
75% LWC 23.3 72.5 3.6 0.6
80% LWC 21.9 74.1 3.4 0.6
85% LWC 21.3 74.6 3.4 0.7
90% LWC 20.4 75.3 3.7 0.6
95% LWC 19.5 76.5 3.5 0.5
100% LWC 21.6 70.5 5.8 2.1

Table 3: compositional analysis of LWC and B1 mixtures by TLC-FID.

Sample Saturate (%) Aromatic (%) Resin (%) Asphaltenes (%)
100% crude oil 40.2 46.4 8.9 4.5
5% LWC 36.8 55.2 4.5 3.5
10% LWC 36.6 55.2 4.5 3.8
15% LWC 36.3 55.9 4.5 3.2
20% LWC 35.8 56.7 4.5 3.0
25% LWC 34.7 57.3 4.6 3.3
30% LWC 34.2 58.2 4.6 3.0
35% LWC 33.5 58.9 4.8 2.8
40% LWC 33.1 60.2 4.5 2.2
45% LWC 32.6 60.9 4.3 2.3
50% LWC 31.2 62.2 4.7 1.9
55% LWC 30.5 62.8 4.9 1.9
60% LWC 29.9 63.7 4.6 1.7
65% LWC 29.0 64.9 4.7 1.4
70% LWC 27.9 66.3 4.5 1.1
75% LWC 27.3 66.8 4.8 1.0
80% LWC 27.0 67.4 4.4 1.2
85% LWC 26.4 68.3 4.3 0.9
90% LWC 26.0 68.4 4.6 1.0
95% LWC 24.7 70.4 4.3 0.6
100% LWC 21.6 70.5 5.8 2.1

Table 4: compositional analysis of LWC and C1 mixtures by TLC-FID.

Sample Saturate (%) Aromatic (%) Resin (%) Asphaltenes (%)
100% crude oil 35.2 55.7 6.1 2.9
5% LWC 32.9 55.9 6.1 5.1
10% LWC 31.1 57.6 6.2 5.1
15% LWC 30.5 59.3 6.1 4.1
20% LWC 30.0 59.7 5.6 4.7
25% LWC 29.6 59.8 6.0 4.6
30% LWC 29.1 60.9 6.2 3.8
35% LWC 28.7 61.5 6.1 3.7
40% LWC 28.4 63.3 5.0 3.3
45% LWC 28.0 64 5.0 3.0
50% LWC 27.6 65.4 4.7 2.3
55% LWC 27.1 65.8 4.8 2.3
60% LWC 26.8 66.8 4.7 2.3
65% LWC 26.5 67.0 4.5 2.0
70% LWC 25.0 68.9 4.6 1.6
75% LWC 24.6 69.4 4.5 1.5
80% LWC 24.0 69.5 4.9 1.6
85% LWC 23.7 70.1 4.9 1.3
90% LWC 23.2 70.7 5.0 1.1
95% LWC 22.9 71.3 5.0 0.8
100% LWC 21.6 70.5 5.8 2.1

It is observed from the Tables 2, 3, and 4 that for all the oils, the saturate contents decrease linearly with increasing proportions of LWC till it reaches a value near the saturate content of LWC. Graphical representations of the variation of saturate contents upon adding 5% to 95% LWC in crude oils are shown in the FIG.5. It is observed from the figure5 that for all the oils, the saturate contents are close to each other for a particular concentration of LWC.
The average variation of the saturate contents in crude oils and their mixtures are presented in Table 5.Average variation of saturate contents in crude oils and their mixtures are graphically represented in Fig.6.
Table 5: average Saturates contents of mixtures of crude oils and LWC.

Sample Saturate contents (%)
Baghjan 12 Dikom 19 Chabua 16 Average
100% crude oil 53.5 40.2 35.2 43.0
5% LWC 37.5 36.8 32.9 35.7
10% LWC 36.8 36.6 31.1 34.8
15% LWC 35.5 36.3 30.5 34.1
20% LWC 34.5 35.8 30.0 33.4
25% LWC 33.5 34.7 29.6 32.6
30% LWC 32.6 34.2 29.1 32.0
35% LWC 31.6 33.5 28.7 31.3
40% LWC 30.2 33.1 28.4 30.6
45% LWC 29.2 32.6 28.0 29.9
50% LWC 28.0 31.2 27.6 28.9
55% LWC 27.2 30.5 27.1 28.3
60% LWC 26.1 29.9 26.8 27.6
65% LWC 25.3 29.0 26.5 26.9
70% LWC 24.4 27.9 25.0 25.8
75% LWC 23.3 27.3 24.6 25.1
80% LWC 21.9 27.0 24.0 24.3
85% LWC 21.3 26.4 23.7 23.8
90% LWC 20.4 26.0 23.2 23.2
95% LWC 19.5 24.7 22.9 22.4
100% LWC 21.6 21.6 21.6 21.6

SARA analysis of SWC extracts is carried out by TLC-FID to identify possible contamination of the extracts with LWC and the amount of LWC in the extracts. Table 6 represents the SARA composition for SWC extracts of X1 at depths 3859.5m and 3888.0m.
Table 6: depicts the SARA Compositions of SWC from X1.

Depth Saturate
(%) Aromatic
( %) Res+Asph
(%) Ext.amount (mg/g)
3859.5m 23.7 61.0 15.5 6.8
3888.0m 28.3 63.9 7.8 11.9

It is observed from Table6 that SWC extracts at depths 3859.5m and 3888.0m contain 23.7% and 28.3% saturates respectively. The values according to Table 7 and Fig. 7 indicate about 80-85% and 55-60% LWC contamination respectively.
The LWC and normal gravity oils from Eocene formation shows a significant difference in parameters like Pristane/n-C17 (Pr/n-C17), Phytane/n-C18 (Ph/n-C18), A/n-C19 and B/n-C20 (where A and B are unknown compounds appearing just before n-C19and n-C20). These parameters like Pristane/n-C17 (Pr/n-C17), Phytane/n-C18 (Ph/n-C18), A/n-C19 and B/n-C20 are selected for the estimation of amount of LWC mixed with in-situ hydrocarbons present in the sidewall cores.
The Pr/n-C17and Ph/n-C18 ratios gradually increases from lower to higher LWC content till they reach values near the value of pure LWC. The Pr/n-C17and Ph/n-C18ratios for a particular mixture of all the Eocene crudes and LWC mixtures are close to each other. Similarly, A/n-C19 and B/n-C20 also increases from lower LWC content to higher LWC content and its ratios for a particular concentration are very close to each other.
Table 7, 8, 9 and 10 represents the Pr/n-C17, Ph/n-C18, A/n-C19 and B/n-C20 ratios and their averages for mixtures of crude oil from Eocene formation and LWC respectively.

Table 7: variation of Pr/n-C17 for Eocene crudes and LWC mixtures.

Sample A1 B1 C1 Average
100% crude oil 1.05 1.89 2.1 1.68
5% LWC 1.25 1.74 1.66 1.55
10% LWC 1.52 2.02 1.88 1.81
15% LWC 1.93 2.67 2.09 2.23
20% LWC 2.35 2.69 2.11 2.38
25% LWC 2.63 2.76 2.59 2.43
30% LWC 2.9 2.85 2.89 2.72
35% LWC 3.85 2.93 3.13 3.19
40% LWC 4.12 3.01 3.62 3.58
45% LWC 5.04 3.36 3.84 4.08
50% LWC 5.25 3.58 4.22 4.35
55% LWC 6.21 5.39 4.75 5.45
60% LWC 6.96 5.76 5.31 6.01
65% LWC 8.11 6.46 5.73 6.77
70% LWC 8.82 7.34 6.5 7.55
75% LWC 7.22 8.9 8.32 8.15
80% LWC 10.76 9.45 8.76 9.66
85% LWC 14.55 10.19 9.83 11.52
90% LWC 19.09 14.55 13.27 15.64
95% LWC 28.27 19.17 16.05 21.16
100% LWC 50.82 50.82 50.82 50.82

Table 8: variation of Ph/n-C18 ratio with change of crude and LWC mixtures.

Sample A1 B1 C1 Average
100% crude oil 0.30 0.49 0.17 0.32
5% LWC 0.32 0.42 0.35 0.36
10% LWC 0.38 0.44 0.44 0.42
15% LWC 0.44 0.46 0.49 0.46
20% LWC 0.51 0.49 0.51 0.50
25% LWC 0.58 0.51 0.62 0.57
30% LWC 0.66 0.60 0.68 0.65
35% LWC 0.79 0.69 0.77 0.75
40% LWC 0.86 0.74 0.88 0.83
45% LWC 0.97 0.83 0.92 0.91
50% LWC 1.11 0.89 1.07 1.02
55% LWC 1.21 1.05 1.14 1.13
60% LWC 1.29 1.40 1.38 1.36
65% LWC 1.43 1.50 1.48 1.47
70% LWC 1.67 1.61 1.71 1.66
75% LWC 1.82 1.72 2.10 1.88
80% LWC 2.14 1.80 2.41 2.12
85% LWC 2.80 2.17 2.97 2.65
90% LWC 3.44 3.84 3.50 3.59
95% LWC 4.63 4.03 4.89 4.52
100% LWC 8.28 8.28 8.28 8.28

Table 9: variation of A/n-C19 ratio with change of crude and LWC mixtures.

Sample A1 B1 C1 Average
100% crude oil 0.04 0.11 0.09 0.08
5% LWC 0.06 0.08 0.08 0.07
10% LWC 0.07 0.09 0.10 0.09
15% LWC 0.09 0.12 0.12 0.11
20% LWC 0.11 0.12 0.14 0.12
25% LWC 0.13 0.15 0.13 0.14
30% LWC 0.15 0.15 0.18 0.16
35% LWC 0.17 0.16 0.20 0.18
40% LWC 0.2 0.22 0.24 0.22
45% LWC 0.25 0.23 0.25 0.24
50% LWC 0.27 0.26 0.29 0.27
55% LWC 0.31 0.27 0.36 0.31
60% LWC 0.35 0.31 0.38 0.35
65% LWC 0.4 0.36 0.44 0.40
70% LWC 0.41 0.43 0.55 0.46
75% LWC 0.43 0.46 0.59 0.49
80% LWC 0.58 0.48 0.68 0.58
85% LWC 0.78 0.59 0.81 0.73
90% LWC 1.05 1.14 1.27 1.15
95% LWC 1.57 1.15 0.21 0.98
100% LWC 2.14 2.14 2.14 2.14

Table 10: variation of B/n-C20 ratio with change of crude and LWC mixtures.

Sample A1 B1 C1 Average
100% crude oil 0.04 0.04 0.06 0.05
5% LWC 0.03 0.05 0.06 0.05
10% LWC 0.04 0.05 0.06 0.05
15% LWC 0.05 0.06 0.07 0.06
20% LWC 0.06 0.06 0.11 0.08
25% LWC 0.07 0.07 0.10 0.08
30% LWC 0.08 0.07 0.11 0.09
35% LWC 0.09 0.08 0.12 0.10
40% LWC 0.1 0.11 0.14 0.12
45% LWC 0.12 0.12 0.15 0.13
50% LWC 0.13 0.13 0.18 0.15
55% LWC 0.14 0.14 0.21 0.16
60% LWC 0.17 0.15 0.21 0.18
65% LWC 0.18 0.20 0.25 0.21
70% LWC 0.19 0.22 0.27 0.23
75% LWC 0.21 0.23 0.65 0.36
80% LWC 0.27 0.24 0.78 0.43
85% LWC 0.39 0.34 0.94 0.56
90% LWC 0.5 0.58 1.16 0.75
95% LWC 0.71 0.65 1.41 0.92
100% LWC 1.11 1.11 1.11 1.11

Graphical representation of the average values of Pr/n-C17, Ph/n-C18, A/n-C19 and B/n-C20 ratios for mixtures of crude oil from Eocene formations and LWC are shown in Figs. 9, 10, 11 and 12 respectively. It is observed from the figures8,9,10, 11 that the ratios for a particular concentration are very close to each other.
The Pr/n-C17 ratio in SWC extracts of most of the Eocene formations is around 1 for cores containing normal gravity oil. Whereas, in LWC contaminated SWC extracts the Pr/n-C17 ratio is much higher than 1. Similarly Ph/n-C18, A/n-C19, B/n-C20 ratios are also much lower in Eocene SWC extracts than LWC.

Table 11: values of Pr/n-C17, Ph/n-C18, A/n-C19 and B/n-C20 for SWC extracts from cores at depths 3859.5m and 3888.0m from X1.

Depth(m) Pr/n-C17 Ph/n-C18 A/n-C19 B/n-C20
3859.5 10.26 2.27 0.66 0.61
3888.0 6.11 1.41 0.34 0.28

These values according to Table 7, 8, 9, and 10 and Figs. 7, 8, 9 and 10 indicate that 60 to 65% and 80 to 85% LWC contamination in 3888.0m and 3859.5m respectively shown in Fig. 11, 12, 13 and 14. It is also supported by the LWC contamination of these two cores as observed from TLC-FID compositional analysis data.
The LWC and normal crude oils from Eocene formation shows a significant difference in aromatic compound parameters like 1,3,6,7TeMN/xTeMN (x is an unknown Tetramethylnaphthalene), 1,3,6TMN/1,3,7TMN and CAD/4MDBT. These aromatic compound parameters are selected for assessing the amount of contamination of LWC in SWC extracts.
Tables 12, 13, and 14 and Figs.16,17 and 18 represents the variations of the aromatic compounds parameters 1,3,6,7TeMN/xTeMN, 1,3,6TMN/1,3,7TMN and CAD/4MDBT with change in the concentrations of Eocene crude oil and LWC mixtures.
Table 12 variation of 1,3,6,7-TeMN/STeMN ratio with change of crude and LWC mixture

Sample A1 B1 C1
100% crude oil 0.86 0.69 0.34
10% LWC 0.92 0.82 0.43
20% LWC 0.96 0.87 0.50
30% LWC 1.12 0.97 0.66
40% LWC 1.31 1.12 0.76
50% LWC 1.41 1.28 0.92
60% LWC 1.68 1.41 1.01
70% LWC 2.21 1.65 1.28
80% LWC 2.64 2.22 1.90
90% LWC 3.60 2.66 2.01
100% LWC 4.64 4.64 4.64

Table 13: variation of 1,3,6-TMN/1,3,7TMN ratio with change of crude and LWCmixture

Sample A1 B1 C1
100% crude oil 0.68 0.63 0.61
10% LWC 0.72 0.66 0.66
20% LWC 0.74 0.70 0.74
30% LWC 0.78 0.76 0.79
40% LWC 0.81 0.82 0.85
50% LWC 0.90 0.86 0.94
60% LWC 0.94 0.91 1.00
70% LWC 0.99 1.05 1.13
80% LWC 1.05 1.14 1.14
90% LWC 1.12 1.18 1.21
100% LWC 1.15 1.28 1.28

Table 14: variation of CAD/4MDBT ratio with change of crude and LWC mixture.

Sample A1 B1 C1
100% crude oil 0.16 0.09 0.03
10% LWC 0.17 0.12 0.06
20% LWC 0.23 0.14 0.08
30% LWC 0.24 0.18 0.12
40% LWC 0.27 0.23 0.14
50% LWC 0.34 0.26 0.21
60% LWC 0.46 0.35 0.24
70% LWC 0.57 0.47 0.34
80% LWC 0.84 0.63 0.51
90% LWC 1.10 1.01 0.70
100% LWC 1.40 1.40 1.40

Table 15: the 1,3,6,7-TeMN/xTeMN, 1,3,6-TMN/1,3,7TMN and CAD/4MDBT values for SWC extracts from X1 3859.5m and 3888.0m.
SWC sample 1,3,6,7-TeMN/XTeMN 1,3,6-TMN/1,3,7TMN CAD/4MDBT
X1 3859.5m 2.36 1.07 0.77
X1 3888.0m 1.81 1.04 0.56

It is observed from the Tables 12, 13, and 14 and Figures 16, 17 and 18 is that the ratios of 1,3,6,7TeMN/xTeMN, 1,3,6TMN/1,3,7TMN and CAD/4MDBT tend to gradually increases with increasing LWC concentrations in all the crude oil and LWC mixtures. The average variations of 1,3,6,7TeMN/xTeMN, 1,3,6TMN/1,3,7TMN and CAD/4MDBT are presented in Tables 16, 17 and 18 respectively.
Table 16: average variation of 1,3,6,7-TeMN/xTeMN ratio of Eocene crude oil and LWC mixtures.

Sample A1 B1 C1 Average
100% crude oil 0.86 0.69 0.34 0.63
10% LWC 0.92 0.82 0.43 0.72
20% LWC 0.96 0.87 0.5 0.78
30% LWC 1.12 0.97 0.66 0.92
40% LWC 1.31 1.12 0.76 1.06
50% LWC 1.41 1.28 0.92 1.20
60% LWC 1.68 1.41 1.01 1.37
70% LWC 2.21 1.65 1.28 1.71
80% LWC 2.64 2.22 1.9 2.25
90% LWC 3.6 2.66 2.01 2.76
100% LWC 4.64 4.64 4.64 4.64

Table 17: average variation of 1,3,6-TMN/1,3,7-TMN ratio of Eocene crude oil and LWC mixtures.

Sample A1 B1 C1 Average
100% crude oil 0.68 0.63 0.61 0.64
10% LWC 0.72 0.66 0.66 0.68
20% LWC 0.74 0.7 0.74 0.73
30% LWC 0.78 0.76 0.79 0.78
40% LWC 0.81 0.82 0.85 0.83
50% LWC 0.9 0.86 0.94 0.9
60% LWC 0.94 0.91 1 0.95
70% LWC 0.99 1.05 1.13 1.06
80% LWC 1.05 1.14 1.14 1.11
90% LWC 1.12 1.18 1.21 1.17
100% LWC 1.28 1.28 1.28 1.24

Table 18: average variation of CAD/4MDBT ratio of Eocene crude oil and LWC mixtures.

Sample A1 B1 C1 Average
100% crude oil 0.16 0.09 0.03 0.09
10% LWC 0.17 0.12 0.06 0.12
20% LWC 0.23 0.14 0.08 0.15
30% LWC 0.24 0.18 0.12 0.18
40% LWC 0.27 0.23 0.14 0.21
50% LWC 0.34 0.26 0.21 0.27
60% LWC 0.46 0.35 0.24 0.35
70% LWC 0.57 0.47 0.34 0.46
80% LWC 0.84 0.63 0.51 0.66
90% LWC 1.1 1.01 0.7 0.94
100% LWC 1.4 1.4 1.4 1.4

The aromatic compound parameters according to Tables 16, 17 and18 and Figs. 19 and 21 indicate that there are 70-80% and 80-90% LWC mixing in the cores at depths 3859.5m and 3888.0m respectively. Fig 20 indicates that there are 60-70% and 70-80 % LWC mixing in the cores at depths 3859.5m and 3888.0m respectively. The actual amount of in-situ hydrocarbons present at depths 3859.5 and 3888.0m are 1.7mg/g and 1.8mg/g respectively.

EXAMPLE 6:
Production testing of a zone contaminated with LWC in Eocene formation: The SWC extract from well Y1 at depth 4366.0m was found to have an extract amount of 9.5mg/g and SARA composition as saturates: 23.9%, aromatics: 65.3% and resin+asphaltenes: 10.8%. The obtained values according to Table 19 indicated that the SWC sample was contaminated with LWC. The amount of LWC contamination was determined from the amount of saturate content and Pr/n-C17, Ph/n-C18, A/n-C19 and B/n-C20 ratios in the extract. The saturate content of 24.0% indicate about 85% of LWC contamination according to Table 6 and Pr/n-C17, Ph/n-C18, A/n-C19 and B/n-C20 ratios of 11.86, 3.43, 1.1 and 0.79 respectively indicate according to Tables 8, 9, 10 and 11 about 90-95% of LWC contamination in the SWC extract at this depth. Perforation is performed at the depth range 4365.0-4367.0m. This sand produced 100% water. This implies that although the extract amount of SWC sample at depth 4366.0m is 9.5, it actually contains only 1.4mg/g of in-situ hydrocarbon. According to Table 19 the values obtained at a depth 4366.0m indicates that the SWC at depth 4366.0m contains either residual oil or water.
Table 19: interpretation guidelines for geochemical analysis of SWC extract.

Reservoir Ext. Amount
(mg/g) Composition GC fingerprint Interpretation
Sat (%) Arom (%) Res+As (%)
Eocene >6 30 – 60 20 –60 0 – 30 Normal distribution of n-alkanes. Pristane and n-C17 peaks approximately equal Normal oil
4 - 6 30 – 60 20 –60 0 – 30 Normal distribution of n-alkanes. Pristane and n-C17 peaks approximately equal Low oil content
>6 0 – 20 20 – 80 30 – 80 Higher n-alkanes predominant. Pristane peak much larger than n-C17 Heavy oil
>6 15 – 40 30 – 70 20 – 60 Regular distribution with slight predominance of higher n-alkanes. Pristane peak larger than n-C17 Intermediate gravity oil
>2 30 – 70 30 – 60 0 – 10 Tapering of intensity of higher n-alkane peaks. Pristane and n-C17 peaks approximately equal Light oil / condensate

0-4 No restriction Irregular distribution of n-alkanes. Predominance of non n-alkane peaks Residual oil/water

>4 20 – 40 40 – 70 0 – 10 Normal distribution of n-alkanes. Pristane peak much larger than n-C17 peak Contaminated with LWC

Production testing of a zone contaminated with LWC in Eocene formation:
The extract amount of a LWC contaminated SWC sample from Y1 at depth 4366.0m was found to be 9.5mg/g. The amount of LWC contamination was determined from the amount of saturate content and Pr/n-C17, Ph/n-C18, A/n-C19 and B/n-C20 ratios in the extract. The saturate content of 24.0% indicate about 85% of LWC contamination according to Table 6 and Pr/n-C17, Ph/n-C18, A/n-C19 and B/n-C20 ratios of 11.86, 3.43, 1.1 and 0.79 respectively indicate according to Tables 8, 9, 10 and 11 about 90-95% of LWC contamination in SWC extract at this depth. When perforation was carried out at the depth range 4365.0-4367.0m 100% water was produced. This implies that although the extract amount of SWC sample at depth 4366.0m is 9.5, it actually contains only 1.4mg/g of in-situ hydrocarbon. According to Table 19 the values obtained at a depth 4366.0m indicates that the SWC at depth 4366.0m contains either residual oil or water.

ADVANTAGES OF PRESENT INVENTION:
1. The invention provides a more efficient method of solvent extraction (3 hours in place of 72 hours as described in prior art). In the context of oilfield operations, this results in considerable savings through quicker decision-making. Implementing the invention in a drilling well results in identification of productive and non-productive zones, and hence, savings in terms of number of rig-days spent in testing various zones. Considering the drilling rig daily cost of INR 24 lakhs (approximately), the savings accrued through the invention could be substantial.
2. The exploration and drilling for hydrocarbons is a very costly endeavor. A typical cost of an onshore well is INR 30.0 crores. A successful well results in an income stream of INR 4.3 lakhs per day (assuming 50 cubic meters of production per day and net realized price of INR 8600 per cubic meter for crude oil) for several years. Therefore, it is important to ascertain the presence or absence of prospective hydrocarbons zones in the well. Often, this information is obtained from wire line logs or visual / microscopic inspection of formation rock (sidewall cores or conventional cores). These methods are not dependable as they are indirect and prone to various errors. The method provided in the present invention can give an accurate idea regarding the nature and quantity of hydrocarbon fluids present in the fluid extracts, and hence, the prospects of hydrocarbons producible from the well.
3. The method described in the present invention can identify the nature of fluids present in the extract (light oil, normal oil, heavy oil, residual oil, LWC, etc.), therefore a decision regarding the completion of the well can be taken to avoid unnecessary costs. For example, if the analysis of fluids through the invention indicates a dry well (no hydrocarbons or residual hydrocarbons), casing pipe need not be run-in, resulting in savings of about INR 23.0 lakhs (considering the cost of casing as INR 1550 per meter and 1500 meters of casing required).
,CLAIMS:1. A method for determining the amount of LWC mixed with the native oil present inside sidewall cores taken from reservoirs of Eocene formations or any other geological formations bearing non-biodegraded oil due to invasion of LWC used as a lubricant in water based mud to drill a well into the reservoir through the sides of the borehole, said method comprising:
a) extracting sidewall core (SWC) sample, using Soxhlet extraction technique to obtain extractable organic compounds present in the cores;
b) analyzing the extractable organic compounds obtained in step a) using a Gas Chromatographic analysis to detect contamination of LWC in the extractable organic compounds, wherein
the contamination is detected by comparing the ratios of Pristane/n-C17 and Phytane/n-C18 values in the extractable organic compounds obtained and the ratios of Pristane/n-C17 and Phytane/n-C18 values obtained for Eocene oil;
c) preparing a mixture of Eocene oil and LWC in the ratios selected from 95%:5%, 90%:10%, 85%:15%, 80%:20%, 75%:25%, 70%:30%, 65%:35%, 60%:40%, 55%:45%, 50%:50%, 45%:55%, 40%:60%, 35%:65%, 30%:70%, 25%:75%, 20%:80%, 15%:85%, 10%:90% and 5%:95%,
d) Determining the amount of saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes present in the Eocene oil by using a TLC-FID technique,
e) comparing based on Eocene formation, the amount of saturated hydrocarbons present in the Eocene oil, the extractable organic compounds obtained and the mixture of the Eocene oil and LWC;
f) calculating the ratios of 1,3,6,7-TeMN/XTeMN, 1,3,6TMN/1,3,7TMN and CAD/4MDBT using a GCMS analysis of the Eocene oil, the extractable organic compounds obtained and the mixture of the Eocene oil and LWC, and
g) comparing 1,3,6,7-TeMN/XTeMN, 1,3,6TMN/1,3,7TMN and CAD/4MDBT ratios and matching the ratios obtained from the Eocene oil, the extractable organic compounds obtained and the mixture of the Eocene oil and LWC to detect the amount of LWC present in the extractable organic compounds, wherein the percentage of saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes are calculated from the peak areas of saturated hydrocarbons, aromatic hydrocarbons, resin and asphaltenes in TLC-FID chromatogram.

2. The method as claimed in claim 1, wherein Soxhlet extraction is carried out in an organic solvent.

3. The method as claimed in claim 2, wherein said organic solvent is Dichloromethane.

4. The method as claimed in claim 1, wherein said sidewall core (SWC) sample is collected from depths 3859.5m and 3888.0m from a drilled well by well logging coring gun from the side of the wall of the borehole.

5. The method as claimed in claim 1, wherein the ratio of Eocene oil and LWC preferably ranges from 20:80 to 70:30 of Eocene oil and LWC.

6. The method as claimed in claim 1, wherein said method is able to determine variation in the concentration of saturated hydrocarbons, n-C17, Pristane, n-C18, Phytane, n-C19, n-C20, A, B and aromatic compounds like 1,3,6,7TeMN, xTeMN, 1,3,6TMN, 1,3,7TMN and CAD, 4MDBT which are abundant in either LWC or in the native oil to determine the degree of mixing.

7. The method as claimed in claim 1, wherein said saturated hydrocarbon compounds like n-alkanes and iso alkanes are selected from n-C13 to n-C36, preferably Pristane/n-C17 (Pr/n-C17), Phytane/n-C18 (Ph/n-C18), and A/n-C19, B/n-C20 .

8. The method as claimed in claim 1, wherein said method provides accurate quantification at an accuracy level of up to 5% variation of amount of hydrocarbon present in the core when the extract is contaminated with LWC.

9. The method as claimed in claim 1, wherein said method enables differentiation between light, normal and heavy oils present in the reservoir.

10. The method as claimed in claim 1, wherein said method enables identification of the oil bearing zone in sand contaminated with LWC.