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
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 (joints, 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 N 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.
INTRODUCTIONThe 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
1
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 CT data indicate that below a critical value of
~9%, the porosity is largely unconnected in the basalt clast. The
CT 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
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.
2
Plotting schemeThe red points in all the cross plots belong to
the Production Log indicated 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 Bowens 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 Bowens 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 FelsparNa rich
Felsparpotassium rich Felspar , whereas the other path involves
discontinuous series of crystallisation involving olivine Pyroxene
AmphibolesBiotite MicaPotassium 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):DeltaTPhin cross plot of Padra-C
Figure 2(d):DeltaTPhin cross plot of Padra-A,B,C and
Karjan-A
3
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)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).1
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 joined through
cluster-3(Intermediate resistivity and decreasing AI) with
cluster-2(low resistivity and lower
4
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
5
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 N 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