Cross Plot Techniques For Unconventional Reservoirs-basalt(trap) 5e1m5e

Cross plotting technique empowers insight into unconventional hydrocarbon reservoirs in igneous rocks, Basalt. -A Case study of Padra field,India R S Chauhan1#, A K Srivastava1, S P Das1, K K Prasad2

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ABSTRACT Igneous rocks like Deccan Trap Basalt in Padra area of Western Onshore Basin are hard and brittle with very low matrix porosity and permeability, consequently reservoir quality depends on the development of secondary porosity. Secondary porosity may be divided into two main kinds by origin; 1. Dissolution porosity (ranging from solution effects in weathering zones or fault zones to effects associated with hydrothermal circulation (Smitt, 1998)). 2. Tectonic porosity (ts, faults, fractures, etc at a range of scales from micro fractures to seismic scale faults and their damage zones). This type of porosity is not discrimination and evaluation friendly unlike clastic rocks like sand, which possess depositional porosity whose evaluation procedure are well understood and are well documented. Present attempt is based on the study carried over well logs and other geological data of four oil producer wells of Padra area of western onshore, where production log depicts hydrocarbon producing zones. It is not always possible to derive the benefit of production logging by running in producer wells, because of inconsistent flow performance, which may several times be below threshold limits of applicability of the sensors. The available reports on Padra trap cores from Dr.B.R.Ambedkar RGL lab Vadodara have been used extensively. The attempt is made to develop an Insight Into these Unconventional Hydrocarbon Reservoirs to the level of identifying them, through powerful Cross plotting Technique as follows:• Production Logs run in Padra wells have served as the backbone of this effort. • Th vs U cross plot brings out dominant weathering effect on Deccan trap Basalts in Padra Area. Weathering/Alteration of basalt appears the source of porosity and reservoir developments. • M*N vs Rhob , M*N vs Phin, AI Vs Rt and AI Vs Phin, TH/U Vs Nphi ,TH/U Vs Rhob cross plots effectively bring out the inter-relationship between the porosity development and weathering/alteration phenomena of basalt. • AI Vs Phin cross plot may supplement seismic in locating basalt reservoir through Phin (porosity) mapping. • Hingle plot corroborates PL based hydrocarbon reservoirs. The Phin matrix derived from the study comes to 8 LPU for Padra basalt. Thus representative reservoir porosity will be equal to the log read φN value in LPU minus 8 LPU.This study has brought out that Weathering/Alteration of basalt is the source of porosity and reservoir developments. The reservoir porosity lies between 6-22 %.The range of porosity is well corroborated with the porosity, range 3-17.72%, of nearby well determined by petrophysical lab of Dr BR Ambedkar RGL,ONGC Vadodara. It has narrowed down the limiting parameters for discriminating hydrocarbon bearing reservoir intervals. Combining more wells covered with production logging in this study is bound to make it more robust and versatile in application to the extent of making cased hole completion a viable practice enabling effective water shutoff and may also supplement seismic in locating basalt reservoir through Phin (porosity) mapping.

INTRODUCTION The Cambay basin is a pericratonic basin which came into existence during Late Cretaceous to Early Paleocene. The Deccan Trap constitutes the basement over which thick sediments ranging in age from Early Paleocene to Pliocene were deposited. The Deccan Trap comprises of dominantly fine grained pyroxene rich basalt, having been deposited by fissure eruption in a continental set up. Among low porosity rocks, Basalt is most susceptible rock type for weathering/alteration phenomenon. Weathering/alteration of rocks is the result of their exposure to water and atmosphere. The degree and depth of weathering/alteration is dependent on the primary lava flow, emplacement structure and environment. The dissolution and precipitation of minerals during weathering/alteration not only change the chemistry of the rocks, but also their physical properties such as porosity, permeability, effective diffusivity, compressive strength and tensile strength. Weathering induced changes in pore network geometry can therefore the change the rates of fluid transport, and thus potentially affect the overall rates of mineral weathering/alteration. The presence of clay minerals and iron oxide is proportionally related to weathering/alteration phenomenon, which also depends on the appearance specifics of Zeolite. In Cambay Basin, hydrocarbon production comes from the pay zones within the Tertiary sediments. However, a number of wells have also been drilled into the Deccan trap Basement, and the hydrocarbon production from these Basaltic igneous rocks comes only from Padra field, which is situated in the eastern margin of southern Cambay basin in Broach-Jambusar block. And so far more than 60 exploratory wells have been drilled here. In the absence of diagnostic log based techniques for effective reservoir characterization in Deccan trap basalt, such unconventional reservoirs remain evaluation and work over unfriendly unlike those in clastic rocks. The prevailing practice is the barefoot completions of Deccan trap

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section, which works effectively, until the well starts cutting water. Since, the selective isolation of the water contributing zone is not possible due to the facts discussed earlier. Only option available remains the water shut off to be carried out in stages always starting isolation through sand dumping from bottom part .Even though water contributing zone may be picked up through production logging , selective isolation of the culprit zone is not possible without casing. Evolution of porosity and diffusivity associated with chemical weathering of a basalt clast across a weathering interface in a weathered basalt clast from Costa Rica was studied 1. The µCT data indicate that below a critical value of ~9%, the porosity is largely unconnected in the basalt clast. The µCT data were further used to construct a numerical pore network model to determine up scaled, effective diffusivities as a function of total porosity (ranging from 3 to 30%) for comparison with diffusivities determined in laboratory tracer experiments. The present attempt is a sequel to the aforementioned realization, which has become a driver thought for developing an insight in to these reservoirs to the level of effectively identifying pay zones. Production log data in four oil producer wells viz. Padra- A, Padra-B, Padra-C and Karjan- A provided the footprints of these reservoirs, while other available logs, testing results and geological data helped in their characterization.

METHODOLOGY •

Four oil producer wells, viz Padra-A Padra-B Padra-C Karjan-A,producing intervals were picked up where production log data was available and these intervals are treated as reservoir, as shown below in fig.1(a).These identified intervals were depicted in open hole log as shown below in fig.1(b)

Figure 1(a):Production log of Karjan-A

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Figure 1(b) :Open hole log of Karjan-A

Conventional lithoporosity plots like Rhob Vs Phin , Rhob Vs DeltaT , DeltaT Vs Phin were employed, along with new look crossplots like M*N Vs Phin , M*N Vs Rhob , Acoustic Impedence (AI) Vs Deep Resistivity , AI Vs Phin for characterising the reservoir. Thorium Vs Uranium plot were generated to appreciate the role of weathering/alteration phenomenon on these rocks, which may be the chief architect of porosity development. The Hingle plot (square root of conductivity Vs Phin is plotted in linear scale) with Sw-100% line Sw50% is plotted using the produced water salinity of 25-30gpl (Rw~0.144 ohm-m @ F.T.) and m=n=2.Hingle plot uses Archies equation to calculate Sw.

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Plotting scheme



The red points in all the cross plots belong to the Production dicated oil producing interval/intervals. The blue to dark blue points cluster pertain to highest resistivity bearing It is designated as Fresh Basalt cluster. The green to dark green points pertain to medium resistivity bearing cluster. It is designated as Weatherd /Altered Basalt.The elongation of the cluster is in the direction of increasing porosity, which tends to be within approximated Clay and Zeolite points derived from the study of four wells composite points cluster. However, the elongation trend sways towards Clay point.This may be indicative of porosity evolution through weathering/alteration. The grey points pertain to lowest resistivity bearing cluster. It is designated as Extremely Weathered/ highly altered Basalt. Both paths of crystallisation, as envisaged in Bowen’s reaction series viz. 1. Continuous one from Ca-plagioclase to potassium plagioclase and 2. Discontinuous one from Olivine to K-feldspar. Apparently designated Fresh Basalt is expected to be the weighted average of these two, representing base rock of Padra area. These two path ends are denoted by Basalt-I(Ca-plagioclase end) and Basalt-II(Olivine-K feldspar end).

OBSERVATIONS/DISCUSSIONS 1.Rhob Vs Phin,Rhob Vs DeltaT,DeltaT Vs Phin cross plots(On linear scale) These cross plots, as shown in figure 2(a),2(b),2(c),2(d),are meant to determine the matrix parameters of basalt rock in the light of Bowen’s reaction series.This series envisages two paths of Basalt rock forming as magma keeps cooling down. One path involves continuous series of crystallisation involving calcium rich Plagioclase Felspar→Na rich Felspar→potassium rich Felspar , whereas the other path involves discontinuous series of crystallisation involving olivine →Pyroxene→ Amphiboles→Biotite Mica→Potassium Felspar . Basalt -1 represents calcium rich Plagioclase feldspar, whereas Basalt-2, ClayPoint and Zeolite Point are derived from the envisaged pull direction on point cluster.

Figure 2(a):Rhob –Phin cross plot of Padra-C

Figure 2(b):Rhob –Phin cross plot of Padra-A,B,C and Karjan-A

Figure 2(c):DeltaT–Phin cross plot of Padra-C

Figure 2(d):DeltaT–Phin cross plot of Padra-A,B,C and Karjan-A

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Figure 2(e):Rhob-DeltaT cross plot of Padra-C

Figure2(f):Rhob-DeltaT cross plot of Padra-A,B,C and Karjan-A

2. Thorium Vs Uranium cross plots(On linear scale) 1

These cross plots are based on the paper wherein all unweathered igneous rock types ,shown in figure 3(a) ,are characterised by Th/U=4 value on Th Vs U cross plot whereas the ratio of Th/U >4 indicates increasing degree of weathering. The Th Vs U cross plots of three wells are shown in fig.3 (b),3(c),3(d).

Figure 3(a):Th-U cross plot for Igneous rock

Figure 3(b):Th-U cross plot of Padra-C

Figure 3(c):Th-U cross plot of Karjan-A

Figure 3(d):Th-U cross plot of Padra-A

3.AI Vs Deep Resistivity cross plots(On linear scale) The cluster of points appears tri-component one.Quasi-parallel to y-axis , showing extreme sensitivity to resistivity and poor sensitivity to AI, on higher resistivity and higher AI end.Quasi-parallel to x-axis , showing extreme sensitivity to AI and poor sensitivity to resistivity, on lower resistivity and lower AI end. A curvilinear (concave) cluster comprises of all the, PL identified producer points, showing moderate sensitivity to resistivity and AI, on intermediate resistivity and AI end. When cluster-1(high resistivity and high AI) ,at its top is ed through cluster-3(Intermediate resistivity and decreasing AI) with cluster-2(low resistivity and lower

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AI) ,at its bottom , a continuous process of porosity/reservoir development appears to be definable between fresh rock and clay through weathering /alteration. The intermediate curvilinear region are likely promising for hydrocarbon reservoir point of view where resistivity and AI indicating weathering phenomena. On the basis of four wells the limit of reservoir is demarcated and the window is placed as shown in fig.4(a),4(b).

Figure 4(a):AI-Deep resistivity cross plot of Padra-B

Figure 4(b): AI-Deep resistivity cross plot of Padra-A,B,C ,Karjan-A

4.AI Vs Phin cross plots(On linear scale) A linear elongation of points cluster is evident with increase in porosity going with decrease in AI value. This elongation trend moves from zero porosity side towards increasing porosity side between zeolite and clay points .The hydrocarbon producer points (PL) are falling in between two porosity ends. Weathering/alteration phenomena again appear as a driver for reservoir development. The limit of reservoir is demarcated and the window is placed where the AI is ranging from 0.037-0.057 as shown in fig.5(a),5(b).

Figure 5(a):AI-Phin cross plot of Karjan-A

Figure 5(b): AI-Phin cross plot of Padra-A,B,C and Karjan-A

5.M*N Vs Phin cross plot(On linear scale) This cross plot is based on conventional cross plot to know the Phin matrix value .The limit of reservoir is demarcated and the window is placed wherein the M*N is ranging from 0.32-0.36 and Phin 14-30 LPU respectively as shown in fig.6(a),6(b).

Figure 6(a):MXN-Phin cross plot of Padra-B

Figure 6(b): MXN-Phin cross plot of Padra-A,B,C and Karjan-A

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The Phin matrix derived from the study comes to 8 LPU for fresh basalt so the representative reservoir porosity will be equal to the log read φN value in LPU minus 8 LPU.Therefore the reservoir porosity range for basalt rock will be 6-22 % which is well corroborated by core derived porosity range with petrophysical lab of Dr BR Ambedkar RGL, ONGC Vadodara.

6.M*N Vs Rhob cross plot(On linear scale) The limit of reservoir is demarcated and the window is placed, the limit of density for viable reservoir is ranging from 2.55-2.85 gm/cm3 as shown in fig.7 (a),7(b). It is observed that the density of fresh basalt is very high and decreases as alteration increases. This shows that the density porosity of fresh basalt is very low and increases towards more altered basalt.

Figure 7(a):MXN-Rhob cross plot of Karjan-A

Figure 7(b): MXN-Rhob cross plot of Padra-A,B,C and Karjan-A

It is observed that the density of fresh basalt is very high and decreases as alteration increases. This shows that the density porosity of fresh basalt is very low and increases towards more altered basalt. The limit of density for viable reservoir is ranging from 2.55-2.85 gm/cm3.

7.SQRT conductivity Vs Phin(On linear scale) This cross plot is Hingle plot with square root of conductivity ( y-axis )and Phin ( x-axis ) on linear scale as shown in fig.8(a),fig.8(b).Water bearing line(Sw-100% ) was plotted on the cross plot through extreme left hand limit of points cluster ,as well as through the matrix point characterised by zero on y-axis & 8LPU on xaxis.It very well corroborates the available salinity data of produced water (25-27 gpl).Subsequently Sw<50% line was drawn on Archie’s equation with m=n=2 and a=1.

Figure 8(a):SQRT Conductivity-Phin cross plot of Padra-C

Figure 8(b): SQRT Conductivity-Phin cross plot of Padra-A,B,C and Karjan-A

Hingle plot clearly brings out all the oil producing intervals ( Production Log based) on the figures 8(a),8(b) above , characterised by their location i.e. on or lower than Sw<50% .

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CONCLUSIONS 1. Production Logs run in Padra wells have served as the backbone of this effort of employing Cross plotting technique wherein PL based hydrocarbon producing intervals plot around or below SW=50% line on Hingle (sqrt conductivity vs. porosity cross plot on linear scale) plot .This undoubtedly holds promise to emerge as an effective tool for the identification of these unconventional hydrocarbon reservoirs in igneous rocks, Basalt. 2. Th vs U, TH/U Vs Phin & TH/U Vs Rhob cross plots elucidate dominant weathering/alteration effect on Deccan trap Basalts in Padra Area. Weathering/Alteration of basalt is the source of porosity and reservoir developments 3. M*N vs Rhob , M*N vs Phin, AI Vs Rt and AI Vs Phin, bring out the inter-relationship between the porosity development and weathering/alteration phenomena of basalt. 4. AI Vs Phin shows the linear relationship which may supplement seismic in locating basalt reservoir Acoustic Impedance (AI) derived Phin( porosity)mapping . 5. On the basis of above plots the range of parameters like M,N and Acoustic Impedance(AI= Rhob/DeltaT) are derived for a viable reservoir and Mineral/Rock parameters for Basalt are given as under: Log parameters for viable reservoir Mineral/Rock parameters for Padra Basalt Log Parameters Rhob Phin DeltaT MXN AI

Rock Para Rhomat Phinmat DTmat M N MXN AI

Range for viable reservoir 2.55-2.85 gm/cc3 14-30 LPU(6-22%) 55-70 µsec/ft 0.32-0.36 0.037-0.057

BasaltI 2.74 -0.02 45 0.83 0.59 0.49 0.06

BasaltII 3.1 0.13 42 0.70 0.41 0.28 0.07

F.Basalt (Padra) 2.94 0.08 43 0.76 0.47 0.36 0.07

Cal.

Clay

Zeolite

2.71 0.0 47.5 0.83 0.58 0.48 0.06

2.25 0.50 120 0.56 0.40 0.22 0.02

2.23 0.30 90 0.81 0.57 0.46 0.02

ACKNOWLEDGEMENTS The authors are highly grateful to Shri Dinesh Chandra,ED-Chief Logging Services for his vision and inspiration, encouragement to carry such study.The authors are highly thankful to Shri P B Pandey,GGM – Basin Manager, WON, Vadodara for providing infrastructure, encouragement and constant during the study.The authors are highly thankful to Dr K K Das,DGM(Geology),Dr BR Ambedkar RGL,ONGC Vadodara for providing geological inputs and suggestions during the course of work.

REFERENCES 1. In-situ gamma-ray spectrometric study of weathered volcanic rocks in Hong Kong-July 2001 by Margie Q.F Chen Lung S.Chan,Department of EarthSciences,The University of Hong Kong, Pokfulam Road,Hong Kong. 2. Alteration effects on petrophysical properties of subaerial flood basalts:site 990, southeast greenland margin in January 1998 by Sverre Planke,2 Brian Cerney,3 Christian J. Bücker,4 and Odd Nilsen2. 3. Establishment of Oleophyllic Facies in Trap section of Padra field-A feasibility study in Nov-1998 by T V N Rao,Dr,V G Rao,K K Prasad,ONGC,Vadodara,India. 4. Depositional dynamics and reservoir attributes of Olpad-Deccan trap sequence in Padra-Kural area redefining Deccan Trap- Olpad boundary in Cambay basin in June-1999 by C L Chatterjee,P K Tirkey,C Prakash,K L Raina, Dr BR Ambedkar Geoscience laboratory ONGC Vadodara,India. 5. Petrological studies and reservoir characteristics of Deccan trap cored section in well Padra-42 in May-1999 by C L Chatterjee,P K Mintz,C Prakash,S S Rana, Dr BR Ambedkar RGL,ONGC Vadodara,India. 6. Evaluation of Deccan trap prospect in Padra field, south Cambay basin in May-1996 by D K Bharktya,Dhruvendra Singh,C Prakash, Dr BR Ambedkar Geoscience laboratory ONGC Vadodara,India. 7. The emplacement of pahoehoe lavas on Kilauea and in the Deccan Traps by H C Sheth ,Department of Earth Sciences, Indian Institute of Technology (IIT) Bombay,India. 8. Reservoir characterization of Olpad and Ankleshwar sands in Padra area in May-2000 by A K Pandey,Ms R S Choudhary,J P Lakhera,Ms S Sen, Dr BR Ambedkar Geoscience laboratory ONGC Vadodara,India. ______________________________________________________________________________________ 1.Well Logging Services,Western Sector,ONGC,Vadodara,India.Email:[email protected] i)K K Prasad,Post graduate in Geophy. from ISM Dhanbad in 1969.ed ONGC in 1970 & retired in 2007.He worked at diff.places in interpretation to find answer to E&P problems. Thereafter worked as consultant in ONGC from 2007-09 at CEWELL Vadodara.He published more than 15 technical papers.

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