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IPA12-G-095

PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION

Thirty-Sixth Annual Convention amp Exhibition May 2012

SUCCESS STORY WITH LOW RESISTIVITY SAND IN AN EXPLORATION BLOCK

WESTERN EDGE OF CENTRAL SUMATRAN BASIN

Heri Setiawan

Panca Widiantoro

Hendarman

Made Primaryanta

ABSTRACT

The exploration area of focus for this study is

located at the western edge of the Central Sumatran

Basin Three exploration wells were drilled to 6500

ft (MD) The wells discovered oil in the Early

Miocene Lower Sihapas Formation estuarine

sandstone The gross reservoir interval was up to

140 ft thick exhibiting low resistivity typically

between 6 ndash 8 Ohm meters Each well tested up to

3000 BOPD of 44 API oil with zero water and

minimal gas The resistivity of the Lower Sihapas

oil-filled sands is lower than the resistivity of

underlying Upper Pematang water-filled sands

This paper will explain the exploration and

development strategy for these low-resistivitysands which can be applied to other similar areas

Keywords Success story low resistivity zone

exploration block Central Sumatran Basin

INTRODUCTION

This exploration block is located at the Western

Edge of the Central Sumatran Basin (Figure 1) The

Central Sumatran Basin is one of a series of rift

basins that occupy a back-arc position along the

leading edge of Sundaland It is the most prolific oil

producing basin in Indonesia

Three exploration wells drilled in the same structure

all discovered oil in the Lower Sihapas Formation

which is a prolific reservoir in the Central Sumatran

Basin The Lower Sihapas Formation comprises

siliciclastic sediments deposited in an estuarine

environment Tidal influence is confirmed from

conventional core sedimentologic structures such as

ripple current shale drapes cross lamination and

wavy lamination in Well-X

Mosesa Petroleum

The resistivity of the Lower Sihapas oil-filled sands

is lower than the resistivity of underlying Upper

Pematang water-filled sands Fluid type in the

sands is confirmed from cuttings MDT samples and

well-tests

This paper will discuss the causes of low and high

resistivities in sands formation pressures fluid

sampling petrophysical estimates and the

exploration and development strategy applicable to

this field and perhaps to similar fields

METHODOLOGY

We analysed data from mud-logs wireline logs

formation pressures fluid samples cores and production tests to understand the cause of low

resistivity in oil sands and high resistivity in water

sands This improved our choice of parameters for

petrophysical calculations and helped to define the

exploration and development strategy in this field

RESULTS

a Drilling Result Review

In May 2011 exploration wells Well-X and Well-Y

were drilled in the Z structure Oil in Zone A was

inferred from cuttings and sidewall cores (poor to

moderate oil shows) and from MDT samples

(100 oil with 440

API) See Figure 2 and Figure

3 Conventional core was cut in Zone A in Well-X

with 87 recovery

Slab core fluorescence photographs (Figure 4) show

that Zone A has poor-moderate oil shows Routine

core analysis revealed porosity in the range of 12-22

permeability in the range of 17-1700 md

Special core analysis (SCAL) was performed to

determine petrophysical parameters relative

permeability and capillary pressure

Water saturation derived from SCAL was in the

range of 20-30 Unusually resistivity in the

Back to Session

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Zone A oil sand was lower than in the underlying

Zone B water sand (water proved from cuttings

sidewall cores and MDT sample with 100 water

saturation)

b Pressure Analysis

Formation pressure tests and fluid samples were

taken in both wells Figure 5 shows the pressure

gradients in Zone A and Zone B in Well X The

Zone A fluid gradient of 0328 psift is consistent

with an oil-bearing zone The Zone B fluid gradient

of 041 psift is consistent with a water-bearing

zone The fluid analyzer results in Figure 6 showed

agreement between the inferred fluid type during

sampling and actual fluid type confirmed when the

sample was opened at surface

c Low and High resistivity values

Low resistivity in hydrocarbon-filled sand can be

caused by dispersed shale andor laminated shale

Shale decreases resistivity because it contains

bound water

XRD data from conventional core in Well-X in

Table 1 shows that Zone A contains clay minerals

such as illite kaolinite and glauconite comprising

over 10 of the total minerals

High resistivity in water-filled sands can be caused by the presence of fresh formation water The

salinity of water samples from Zone B was low at

1000-2000 ppm The water may be meteoric water

flushed from outcrops on the south-western edge of

the Central Sumatran Basin

d Petrophysical analysis

The petrophysical analysis in Figure 7 had to

accommodate the presence of clay minerals in both

dispersed and laminated shale that caused low

resistivity

Petrophysical parameters

Petrophysical parameters were determined from log

and core analysis Water density and matrix density

came from cross plots of bulk density and core

porosity after NOB correction To calculate the total

porosity of shale The wet shale density log and the

neutron log response in shale was cross-plotted

The dry shale density was inferred from the

dominant clay determined from XRD data

Water saturation parameters a m and n came from

SCAL data a and m from a cross plot of core

porosity and formation water and n from a cross

plot of brine saturation and formation resistivity

Vshale

Vshale was calculated from the Gamma Ray log

with the Clavier (1971) method to account for the

mineral response Vshale results from the Clavier

method was compared to Vshale results from the

XRD data

Porosity

Porosity can be calculated from neutron density

and sonic logs with either the single porosity

method and combined porosity method We used

the combined method of neutron and density

porosity and calibrated the result to core porosity

Water Resistivity

Water resistivity came from the Picket plot of RT

and effective porosity as water was not found in

this zone Water is assumed to be bound water

trapped in shale

Water Saturation

As the shale volume is 3-40 and the lithology is

dispersed and laminated shale and sand theModified Simandoux equation was used to

accommodate the presence of clay minerals in both

Water saturation indicated from the Simandoux

modified method was high at 50-70 which for

sand with good porosity and permeability is

inconsistent with the test result of zero water cut

Water saturation from a capillary pressure

measurement from the conventional core from

Well-X is shown in Figure 7 Water saturation from

the capilary pressure show value 30-40 this value

is more reliable for this case

CONCLUSION

Oil was discovered in lower resistivity sand in the

Lower Sihapas Formation as proved by oil shows

in cuttings and sidewall cores MDT fluid samples

fluorescence in core photographs formation

pressure gradients and well tests The anomalously

low resistivity is suspected to be a result of clay

minerals in both dispersed and laminated shale as

seen in the XRD data

The higher resistivity in the underlying Upper

Pematang Formation water sand is suspected to be

due to fresh formation water from outcrop sourced

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meteoric water at the south-western edge of the

Central Sumatran Basin

Petrophysical estimates were based on parameters

from log and core data The water saturation

estimate accommodates the presence of clay

minerals but is still not reliable

The data all supported oil in Zone A so the

development strategy was to perforate only zone A

and produce oil with a single completion

Subsequent production tests showed good AOFrsquos in

Well-X and Well-Y Zone A of 2500 bopd and

20763 bopd respectively as seen in (Figure 8)

Future exploration strategies include drilling look-

alike fault blocks and structural culminations as

well as re-evaluation of old wells using the new

techniques and observations

This paper is a case study that it is possible to find

hydrocarbons in low resistivity sands with the right

data observations and techniques This is good

news for other fields with oil indications and low

resistivity

ACKNOWLEDGEMENTS

The authors acknowledge the Management of PT

Mosesa Petroleum who allowed us to publish the

data and the exploration team UNPAD and

engineering team for their unlimited support and

advance materials and discussion

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TABLE 1- XRD ANALYSIS RESULT

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Figure 1- Research Location

Figure 2 - Composite Log of Well-Y

Oil Show

Sample Points

Sample Points

Research

Location

DST Interval

flow rate test

800-2500 BOPD

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Figure 3 - Oil sample from Zone A and Water Sample from Zone B

Figure 4 - Fluorescent core photograph Zone A with oil shows

Oil Sam le Water Sam le

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Figure 7 - Petrophysics Analysis Using Simandoux Modified Method in Well-X Zone A

Figure 8 - IPR vs VLP Matched Zone A Well X and Well Y

Pwh (psi) Pwf (psi) Rate (bfpd)

300 2105 939

260 2065 1303

140 1937 2525

0 1837 3467

Well-X Well-Y

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meteoric water at the south-western edge of the

Central Sumatran Basin

Petrophysical estimates were based on parameters

from log and core data The water saturation

estimate accommodates the presence of clay

minerals but is still not reliable

The data all supported oil in Zone A so the

development strategy was to perforate only zone A

and produce oil with a single completion

Subsequent production tests showed good AOFrsquos in

Well-X and Well-Y Zone A of 2500 bopd and

20763 bopd respectively as seen in (Figure 8)

Future exploration strategies include drilling look-

alike fault blocks and structural culminations as

well as re-evaluation of old wells using the new

techniques and observations

This paper is a case study that it is possible to find

hydrocarbons in low resistivity sands with the right

data observations and techniques This is good

news for other fields with oil indications and low

resistivity

ACKNOWLEDGEMENTS

The authors acknowledge the Management of PT

Mosesa Petroleum who allowed us to publish the

data and the exploration team UNPAD and

engineering team for their unlimited support and

advance materials and discussion

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TABLE 1- XRD ANALYSIS RESULT

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Figure 1- Research Location

Figure 2 - Composite Log of Well-Y

Oil Show

Sample Points

Sample Points

Research

Location

DST Interval

flow rate test

800-2500 BOPD

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Figure 3 - Oil sample from Zone A and Water Sample from Zone B

Figure 4 - Fluorescent core photograph Zone A with oil shows

Oil Sam le Water Sam le

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Figure 7 - Petrophysics Analysis Using Simandoux Modified Method in Well-X Zone A

Figure 8 - IPR vs VLP Matched Zone A Well X and Well Y

Pwh (psi) Pwf (psi) Rate (bfpd)

300 2105 939

260 2065 1303

140 1937 2525

0 1837 3467

Well-X Well-Y

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TABLE 1- XRD ANALYSIS RESULT

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Figure 1- Research Location

Figure 2 - Composite Log of Well-Y

Oil Show

Sample Points

Sample Points

Research

Location

DST Interval

flow rate test

800-2500 BOPD

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Figure 3 - Oil sample from Zone A and Water Sample from Zone B

Figure 4 - Fluorescent core photograph Zone A with oil shows

Oil Sam le Water Sam le

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Figure 7 - Petrophysics Analysis Using Simandoux Modified Method in Well-X Zone A

Figure 8 - IPR vs VLP Matched Zone A Well X and Well Y

Pwh (psi) Pwf (psi) Rate (bfpd)

300 2105 939

260 2065 1303

140 1937 2525

0 1837 3467

Well-X Well-Y

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Figure 1- Research Location

Figure 2 - Composite Log of Well-Y

Oil Show

Sample Points

Sample Points

Research

Location

DST Interval

flow rate test

800-2500 BOPD

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Figure 3 - Oil sample from Zone A and Water Sample from Zone B

Figure 4 - Fluorescent core photograph Zone A with oil shows

Oil Sam le Water Sam le

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Figure 7 - Petrophysics Analysis Using Simandoux Modified Method in Well-X Zone A

Figure 8 - IPR vs VLP Matched Zone A Well X and Well Y

Pwh (psi) Pwf (psi) Rate (bfpd)

300 2105 939

260 2065 1303

140 1937 2525

0 1837 3467

Well-X Well-Y

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Figure 3 - Oil sample from Zone A and Water Sample from Zone B

Figure 4 - Fluorescent core photograph Zone A with oil shows

Oil Sam le Water Sam le

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Figure 7 - Petrophysics Analysis Using Simandoux Modified Method in Well-X Zone A

Figure 8 - IPR vs VLP Matched Zone A Well X and Well Y

Pwh (psi) Pwf (psi) Rate (bfpd)

300 2105 939

260 2065 1303

140 1937 2525

0 1837 3467

Well-X Well-Y

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Figure 7 - Petrophysics Analysis Using Simandoux Modified Method in Well-X Zone A

Figure 8 - IPR vs VLP Matched Zone A Well X and Well Y

Pwh (psi) Pwf (psi) Rate (bfpd)

300 2105 939

260 2065 1303

140 1937 2525

0 1837 3467

Well-X Well-Y

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Figure 7 - Petrophysics Analysis Using Simandoux Modified Method in Well-X Zone A

Figure 8 - IPR vs VLP Matched Zone A Well X and Well Y

Pwh (psi) Pwf (psi) Rate (bfpd)

300 2105 939

260 2065 1303

140 1937 2525

0 1837 3467

Well-X Well-Y