Gross Rock Volume Estimation And Petrophysical Analysis Of Lower

Transcription

Geodynamics Resea rch International Bulletin (GRIB), Vol. (3), No. 05, SN:13, Spring 2015
All rights reserved for GR IB
Available online at: www.geo-dynamica.com
GRIB
Geodynamics Resea rch International Bulletin
ISSN 2345 - 4997
Vo l. (3), No. 05, SN:13, Spring Issue, 2016
3 rd Article- P. XXVII to XXXV
TCSN 4102 3627
Gross Rock Volume Estimation And Petrophysical Analysis Of
Lower Eocene Sui Main Limestone In Sara West Block Lower
Indus Basin, Pakistan
Furqan Mahmood1, Umair Bin Nisar 2, Sarfraz Khan *3 , Tahir Nagra4,
Syed Amjad Ali Bukhari5 , Aqeel Gohar 6, Muhammad Farooq7
1
Depa rtment of Metrology, COMSATS In stitute o f informa tion Technolog y Isla mabad , Pak istan
Departmen t of Earth Sciences, COMS ATS Institute of in fo rmation Technology Abbottabad , Pakistan .
3
NCE in Geology, University of Peshawar, Pak istan .
4
Qua id-I-Aza m University, Islamabad , Pakistan .
5
Depa rtmen t o f Ea rth Sciences, COMS ATS Institute o f info rmation Technology Abbottabad , Pak istan .
6
Departmen t of Earth Sciences, COMS ATS Institute of in fo rmation Technology Abbottabad , Pakistan .
7
Depa rtmen t o f Ea rth and En vironmen tal Sciences Bahria University Isla mabad, Pakistan.
2
* Corresponding Author ([email protected])
Article History:
Revised: Nov. 12, 2015
Received: May 25, 2015
Accepted: Jan. 05, 2015
Reviewed: Jul. 10, 2015
Published: Mar. 16, 2016
ABSTRACT
In any trap beneath the surface the most influential technique or parameter applied in the determination of estimation and
magnitude of resource volume concentration or potential concentration of the reservoir is the Gross Rock Volume (GRV)
technique. Gross Rock Volume (GRV) is the volume of rock between a top and base reservoir surface and above a known or
postulated hydrocarbon-water contact in a geologic trap. Two Dimensional seismic interpretation of the Sara West
Exploration block, Lower Indus Basin was done to determine the reservoir properties and volume of hydrocarbon contained
within the Lower Eocene Sui Main Limestone (SML). The seismic time sections, time and depth contour maps revealed fault
assisted closures at the center of the field, which corresponded to the horst and graben structures serving as the trapping
medium. The estimated volume of hydrocarbon in place within the interval ranging from 2340 m to almost 2500 m was
calculated as 9.2 x 1013 standard cubic feet (SCF) which is approximately equal to 2.6 x 1013 cubic meter of gas in place.
The study also revealed the feasibility of integrating borehole data and structural m ap in mapping reservoir fluid boundaries
towards calculating the volume of hydrocarbon in place. The B-Sand reservoir of Lower Goru Formation with average
porosity values of 0.13, water saturation of 0.4 and average net to gross ratio value of 0.8 confirms reservoir as a good
reservoir.
Keywords: Gross Rock Volume; B-Sand Reservoir; Volumetric Gas in Place.
1. INTRODUCTION
Seismic waves carry information about the structures,
subsurface faults or valleys, physical characterization
of layers, hydrogeological information and also
sediment or rock type present in the subsurface
(Steeples, 2005). The largest component of
geophysical spending in the exploration and
production business is driven by the need to
characterize the reservoirs (Alao et al., 2013). The
elastic properties, reservoir properties and reservoir
architectural properties are bridged by the science of
rock physics. Rock physics allow for a reliable
prediction and perturbation of seismic response to
changes in reservoir conditions (Saberi, 2014). At the
first phase seismic analyst focuses on the physical
view point with less emphasis on the mathematical
aspects. Then the geophysical parameters which
affect the fidelity of the resulting output from each
process are critically examined via an extensive
series of synthetic and real data examples (Rabbel,
2006; Yilmaz, 2001).
In any trap beneath the surface the most influential
technique or parameter applied in the determination
of estimation and magnitude of resource volume
concentration or potential concentration of the
reservoir is the Gross Rock Volume (GRV)
technique. It is fundamentally important to estimate
XXVII
Geodynamics Research International Bulletin (GRIB), Vol. (3), No. 05, SN:13, Spring 2016
All right s reserved for GRIB
Geodynamics Research Int ernational Bulletin, Vol. (3), No. 04, SN:12, Winter, 2015
both the estimation and the scope of uncertainty for
GRV accuracy and the more possibly close to
approximation in any petroleum analysis. In nature,
complex geometries are exhibited by geological traps
and they are extremely variable. Geological traps
exist in an assortment of ranges in shape, i.e. from
overturned limbs to multiple high points along the
axis of folds, from simple antic lines that resemble the
overturned bowl to all manner of complexly arranged
structural features with variable dips. In these cases
of a diverse suite of trap configurations the reserve
estimation is found by calculating from the method
called Gross Rock Volume (James et al., 2013).
2. THE STUDY AREA
The Sara West Block is located on the leading
northwestern edge of the Indian Plate. The separation
of the Indian Plate from Gondwana is thought to have
occurred in Mid-Late Jurassic times. A major
progradational build-out occurred as the opening of
Tethys spread southwards. From the Late Jurassic
until the Late Cretaceous the sedimentary history of
Pakistan records a passive margin development. With
the separation of Madagascar from the Greater Indian
Plate (~80 MY) the passive margin experienced a
period of extension. The early Paleocene witnessed
another phase of extensional faulting, caused by
mantle p lume impingement, as the Indian Plate
crossed a "hot-spot" on its journey northward; this
led extrusion of the Deccan Trap basalts in northwest
India and southern Pakistan. Foreland basin
development occurred from the Oligocene onward, as
the Indian P late under-thrust the Eurasian Plate. The
resulting loading of the Indian Plate caused flexure,
lead ing to the development of a molasses-filled
foredeep. A major phase of inversion, caused by the
ongoing collis ion, took place in the Late
Pliocene/Early P leistocene (Jamil and McCann,
2009).
In the Figure-1 we see the generalized petroleum
system of the Sara/Suri Area. This figure shows the
major reservoirs and also shows the seal rocks. In
this figure the Sui Upper Limestone, Sui Main
Limestone, Ranikot Formation and the Lower Goru
Formation are the reservoirs. In the Sara West Block
Mari High, a major regional structural feature 40 km
to the west of Sara and Suri, was formed. The Sara
structure was also formed as a result of inversion and
is primarily a four-way d ip-closed structure oriented
along north-south faults. Areal extent of the structure
is approximately 8 sq. km. A thick shale (>200 m) of
Ghazij Formation (Eocene), which is a proven
regional seal, overlies 5-6 m thick sand of the Basal
Ghazij Sand reservoir sand (Figure 1).
Fig. 1. Location of the Sara West Exploratory Block in the Sindh P rovince of P akistan
The reservoir sand is relatively thin but exhibits
excellent reservoir characteristics. Geochemical
analysis indicates that the sections with the greatest
source potential are in the Lower Goru and Sembar
Formation. Migration of the hydrocarbon is believed
to have been through vertical faults connecting
Cretaceous source to Eocene and other reservoirs.
Gas composition in Tertiary reservoirs is different
from Cretaceous and deeper reservoirs (Jamil and
McCann, 2009). A schematic petroleum system of
the Sara West exploration block is shown in Fig. (2).
XXVIII
Geodynamics Resea rch International Bulletin, Vol. (3), No. 04 , SN:12, Winter, 2015
All rights reserved for GRIB
Fig. 2. Generalized petroleum system of Sara/S uri area (Jamil and McCann, 2009)
3. MATERIALS AND METHODS
Eight seismic horizons were identified by using
synthetic seismogram. Among the eight horizons
main focus was on Sui Main Limestone which acts as
a reservoir in the area as shown in the Fig. 2. The two
faults identified on seismic sections are of
extensional origin. The faults specify the tectonic and
structural style that is why special emphasis is made
while making the fault interpretation. Time contour
maps are constructed by taking two way travel times
that were made at different levels which shows the
position of the specific formations in time.
Depth contour maps are generated in order to view
the true image of the subsurface structure. To
generate the depth contours first the velocity is
calculated by taking time from the seismic time
sections and depth from the well data to convert the
time grid into depth grid. Dip of the structure and the
orientation are similar to their time contour maps.
The surface maps represent the lateral and vertical
distribution of the formation and are based on
contour maps. The highlighted portion in the Fig. (3)
shows the polygon area of the Sui Main Limestone
where reserves are estimated. The polygon is made in
this area due to presence of all the wells in this
vicinity. The second reason behind the selection of
this vicinity is because if we see towards the left and
right side of the vicinity there are two faults running.
One fault has the orientation from NW to SE and the
other has the same orientation as the first one. This
shows that the maximum concentration of the
hydrocarbon is present in that area.
XXIX
Fig. 3. The area of the P olygon around which the Gross Rock Volume of the reservoir has been estimated.
Another reason behind the selection of this part is
that all the contours are closing in this area. This
shows that there is surely a structure available in the
subsurface which is supporting the accumulation of
hydrocarbon. The gross volume is defined as the
accumulation of area of the grid cell multiplied by
the Z-Value associated with the grid as follows:
(1)
Gross Volume = Area * Z-value
Where Z-value is the value within the area specified
by the grid or limited by the planimeter polygon. Zvalue can be a depth value or a zone attribute within
the grid cell. Gross volume here is calculated using a
single structure depth grid. From the gross volume
the Net Volume, Pore Volume, Hydrocarbon Pore
Volume, and Hydrocarbons in-place measurements
can be calculated. After that when all these steps are
done then we calculate the gross rock volume in any
area, for this first we have to decide about the
orientation and shape of the structures which are
present in the subsurface. When this is done then the
next step comes in which it is decided about the
contacts of gas, oil or water. In the single structure
grid we calculate the gross volume between one
structure grid and up to two contact values, such as a
flat oil water contact (OWC). In the following
diagram, the volume will be calculated between the
surfaces.
From the single structure grid we can calculate the
pore volume, hydrocarbon pore volume and gas in
place. The formulas for the calculation of the above
mentioned parameters are as follows.
(2)
Pore Volume = Net Volume * Porosity
XXX
(3) Hydrocarbon Pore Volume=
Pore Volume * (1-Water Saturation)
(4)
OGIP =
[UCF×Area×Net×Porosity×(1-Water
Bgi
Saturation)]×
Where UCF = Universal Constant Factor
4. RESULTS AND DISCUSSION
depth contour maps towards the eastern side the time
and depth has less value than on the western side.
This shows that Sui Main Limestone is shallower in
the eastern side as compared to western side and in
the middle portion the formation time and depth
values are in between the values of eastern and
western side and two faults are running within this
formation so this gives the evidence of horst and
graben structure. This horst and graben structures
serves as a perfect trap for hydrocarbons and because
of this reason Sui Main Limestone is a reservoir in
this area.
The Fig. (2) and (3) shows the time and depth
contours of Sui Main Limestone. In the time and
Fig. 3 Time C ontour map of Sui Main Limestone.
The availability of the structure is confirmed from
the location of the faults because the map the Eastern
side of the map the color of the contours is yellow to
reddish green which shows the depth ranging from
1850m-2300m and on the west side of the map the
color of the contour is light blue to dark blue which
shows that the depth is ranging from 2550m-2800m
showing that this part is the deepest.
XXXI
Fig. 4. Depth C ontour map of B-Sand
If we see the middle part of the faults the color of the
contour is reddish green to bluish green and has the
depth ranging from 2310m-2500m which shows that
this part lie in between the deepest and the shallowest
part showing that there is a horst and graben
structure. This faulting confirms that there is a trap in
the subsurface. The results of the vo lumetric
calculations are shown in the following tables (1) and
(2).
Table.1 Information about the Calculation of
Gross Rock Volume
Volumetric Mode l
Single Structure
Grid
B sand Depth
Polygons Used
Basal Sand P olygon
Lower Contact:
-2,475.00
Net/Gross Ratio:
0.8
Average Porosity:
0.13
Average Water Saturation:
0.4
Unit Conversion Constant:
35.31
Gas Volume Factor:
270
Table 2. Information about the Basal Sand
Polygon area and also showing the Gross, Net,
P ore, Hydrocarbon P ore V olume and Gas InPlace.
Polygon Name
Polygon B asal Sand Polygon
P olygon Area
33375000 m2
P olygon Area within the Grid(s)
30964100 m2
Gross Volume
155,645.3538 x 106 m3
Net Volume
124,516.2830 x 106 m3
P ore Volume
16,187.1168 x 106 m3
Hydrocarbon Pore Volume
9,712.2701 x 106 m3
Gas In-P lace
92,593,869.2326 x 106 SCF
The average porosity value is 0.13 whereas average
water saturation is 0.4. Table (2) represents net
volume, pore volume hydrocarbon pore volume and
gas in place. The gas in place is higher than rest
proving it to be gas bearing reservoir
XXXII
4. 1. PETROPHYSICAL INTERPRETATION
The litho logy variation of Sui Main Limestone and
results shows that the formation mainly consists of
Limestone with some sequences of Dolomitic
Limestone and Dolomite. The zone for the Limestone
include from 1180 m to 1250 m approx. and that is
FORMATION
SUI MAIN
LIMESTONE
marked because of the increase in density and
decrease in porosity and the rest of the zone is
Dolomite (tab le 3). In the Fig. (5) and (6) there are
cross plots of RHOB vs PE (PEF) and in this figure
dominance of Dolomitic Limestone throughout the
log is observed.
Table 3 Petrophysical interpretation of Sui Main Limestone in Sara West-1
INTERVAL
THICKNESS
AVG. VOLUME OF
AVG. EFFECTIVE
(m)
(m)
SHALE (VSH)
POROSITY (PHIE)
1180 - 1258
78
11%
10.5%
AVG. WATER
SATURATION (SW)
35%
Fig. 5. Facies analysis of Sara West-1 with density vs neutron porosity log
Fig. 6. Facies analysis of Sara West-1 with density vs P hoto Electric Absorption log.
XXXIII
From the table 1 the results show that Sui Main
Limestone is a good quality reservoir with low
volume of shale and good interconnected porosity. In
Fig. (7) the petrophysical log of the Sui Main
Limestone is shown. We have highlighted the portion
of the logs because here we see that the Gamma Ray
is decreasing and Spontaneous Potential log values
are increasing and similar to this we see that porosity
values are also increasing so by this we can assume
that in this portion hydrocarbons are present.
Fig. 7 P etrophysical Results of S ui Main L imestone of well Sara West-01
5. CONCLUSIONS
In this study we carried out a 2D interpretation of the
Sara West area, characterized and compute it by the
use of well logs and surface seismic sections. The
hydrocarbons in place were within the depth ranging
from 2340 m to almost 2500 m. The estimated
reserve of gas in place was calculated because the
study area is a gas bearing reservoir. From the well
log data we analyzed five hydrocarbon bearing
reservoirs but we were interested in only one that was
B-Sand of Lower Goru Formation. Reservoir areal
extent obtained revealed that the reservoir had an
area estimate of 3.3 x 10 7 m2 and the reserves
calculated were 9.2 x 10 13 standard cubic feet (SCF)
which is equal to 2.6 x 1013 m3 approx. of gas in
place. The structure map and seismic section also
revealed that the principal structure responsible for
the hydrocarbon entrapment in the field was the horst
and graben structure at the center of the field which
shows that the reservoir is trapped within this
structure. The extensive faults which were structure
building faults, support suspected hydrocarbon
prospects which can be explored in the future. For
the B-Sand reservoir sand of Lower Goru Formation
with average porosity values of 0.13, water saturation
of 0.4 and average Net to Gross ratio value of 0.8 are
calculated.
The lower the water saturation, the higher the
hydrocarbon saturation in the reservoir sand also the
higher the net to gross ratio higher the hydrocarbon
saturation. So that is why this area has to be explored
more for further prospects. Special Core Analysis is
the most reliable source of information on average
hydrocarbon saturation and the depth at which
hydrocarbon production is first expected to occur. So
that is why for more accurate information about this
reservoir we should also have the core data. This data
is best obtained from cores cut from the reservoir of
interest, failing this; offset well data may be
satisfactory. Accurate reserves estimation demands
the correct distributions of the parameters that we
have used in the hydrocarbon in place equation and
that dependent petrophysical parameters are linked
where appropriate.
XXXIV
REFERENCES
Alao, P.A. Olabode, S.O. and Opeloye, S.A. (2013)
Integration of Seismic and Petrophysics to Characterize
Reservoirs in “ALA” Oil Field, Niger Delta. The Scientific
World Journal.
James, B. Grundy, A.T. and Sykes, M.A. (2013) The
Depth-Area-Thickness (DAT) Method for Calculating
Gross Rock Volume: A Better Way to Model Hydrocarbon
Contact Uncertainty, AAPG International Conference &
Exhibition. AAPG Search and Discovery Article
#90166©2013, Cartagena, Colombia, 8-11 September.
Jamil, A. McCann, J. (2009) ‘Basal Ghazij Sand (BGS) A
Unique Exploration Play in Eastern Part of Pakistan,
PAPG/SPE Annual Technical Conference, AAPG Search
and Discovery Article #90139©2012, 17-18 November,
Islamabad, Pakistan.
Jamil, A. Waheed, A. and Sheikh, R.A. (2012) Pakistan's
Major Petroleum Plays - An Overview of Dwindling
Reserves. Search and Discovery Article #10399.
Rabbel, W. (2006) ‘Seismic methods’. In: Kirsch R. ed.
Groundwater geophysics – A tool for hydrogeology. Berlin
Heidelberg: Springer.
Saberi, M.H. R abbani, A.R. and Ahmadabadi, K.A. (2014)
The Age and Facies Characteristics of Persian Gulf Source
Rocks. Petroleum Science and Technology, 32(3): 371-378.
Steeples, D.W. (2005) Shallow seismic methods’. In:
Rubin, Y. and Hubbard, S.S. eds. Hydrogeophysics,
Dordrecht, the Netherlands, Springer.
Yilmaz, Özdoğan (2001) Seismic data analysis. 1. Tulsa:
Society of Exploration Geophysicists.
XXXV

Gross Rock Volume Estimation And Petrophysical Analysis Of Lower

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