________________________________________________________________________________

* ConocoPhillips Indonesia Inc. Ltd.

IPA15-G-033

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION

Thirty-Ninth Annual Convention & Exhibition, May 2015

“COOL” SHALE IN THE UDANG FORMATION, SOUTH BELUT SUB-BASIN

Budi R Permana*

Miftahul Firdaus*

Retno Wijayanti*

Lukman Hidayat*

ABSTRACT

In normal clastic environments, the gamma ray log

is fundamentally used as a sand or shale lithology

indicator. This is due to the fact that shales usually

emit more radiation than other sedimentary rocks –

they are “radioactively hot” compared to

“radioactively cool” sand. However, in certain

circumstance shales will have low radioactivity.

This phenomenon is observed in the Late Oligocene

to Early Miocene Udang Formation at the South

Belut sub-basin and surrounding area in the West

Natuna Sea. Integrated log analysis and test

information from the South Belut and West Belut

fields has shown a prior misinterpretation and

miscalculation of shale volumes in the Udang

resulting in an inaccurate estimation of hydrocarbon

in place.

“Cool” shale may occur when clay minerals in the

shale interval are dominated by constituent minerals

with low radioactivity such as kaolinite or chlorite.

Investigations including petrographic analysis,

XRD, and SEM of side wall cores acquired in the

offset wells show high kaolinite with subordinate

chlorite content within the Udang Formation in the

South Belut Sub-Basin.

Understanding this occurrence, we introduce special

methods that we used to analyze and estimate shale

volume in the Udang Formation when the gamma

ray response due to low radioactivity is not suitable

for lithology indication. Several VShale calculation

equations that could solve this problem are based on

consideration of multiple “clay indicators”

evaluated statistically. These log-based clay

indicators include the single neutron log, density

log, resistivity log and the combination of neutron-

density logs (neutron-density crossplot technique)

where each indicator is calibrated to the borehole

environment.

Further investigations on the “cool” shale were

conducted to analyze the phenomenon in the

regional context, suggesting that the kaolinite clays

are likely coming from sediments in exposed

Sundaland basement rocks. This observation helps

to constrain the provenance drainage system of the

Early Miocene South Belut sub-basin.

INTRODUCTION

South Belut and West Belut fields contain several

stacked hydrocarbon reservoirs from the Miocene

Upper Arang, Lower Arang, and latest Oligocene

Udang sand formation. In the subsurface evaluation

and development of these fields, the key to

identifying the top of the Udang sand formation

during drilling is the presence of a low and

monotonous gamma ray log pattern below a high

gamma ray log pattern from the overlying and

pervasive Barat Shale. The low gamma ray log from

the Udang Formation is not definitive for lithology

interpretation. From wellbore cuttings descriptions

the Udang zone is usually a mixed sand and shale

interval. The shale with low gamma ray in this

Udang formation is called the “cool” shale.

The gamma ray log response in the Udang

formation cannot be used for lithology identification

or shale volume estimation or effective porosity in

the typical petrophysical workflow. Using the

gamma ray response directly as a sand indicator

without understanding the true lithology will lead to

overestimation of the pore volume. In place of the

gamma-ray log response, the neutron-density log

response is used as the primary log suite for

calculating petrophysical inputs for reservoir

characterization.

The first well in the sub-basin was the Belut-1 (in

the current South Belut field), drilled in 1975.

Based on the results of this well, the Udang zone

was seen as a secondary objective in the South

Belut field area. Therefore data acquisition such as

core and detailed evaluation of the mineralogy for

the Udang was not carried out in the South and

West Belut exploration wells. Because of this lack

of data regarding the unique Udang formation clay

mineralogy, geoscientists were unable to prove the

hypothesis that the observed “cool” shale from logs

may be related to an abundance of low radioactivity

clay minerals such as kaolinite or chlorite. In 2014,

during the rotary sidewall core acquisition and

laboratory analysis of the SB-06 development well,

several shale plugs were taken and analyzed for

mineralogy using XRD, SEM, and petrographic

analysis. The data confirmed the presence of an

abundance of low radioactivity clay minerals in the

Udang Formation at South Belut.

After confirming the presence of abundant kaolinite

and chlorite in the Udang Formation at South Belut,

the regional extent of this phenomena and the

provenance for the clays were investigated.

REGIONAL GEOLOGY

Based on Manur and Jacques (2014) nomenclature

and description of the regional geology of the West

Natuna Basin (WNB), initial basin formation due to

rifting began in Eocene-Early Oligocene in N-S

directed extension that utilised existing fabrics in

the basement. Resultant structures in the WNB

formed under transtensional conditions, consisting

of NE-SW trending half grabens with NW-SE

striking sinistral wrench zones. This phase of early

extension resulted in highly dissected half grabens

and grabens with many small fault bounded blocks

and local depocentres, often with an original E-W

trend reflecting the overall regional N-S extensional

direction (Figure 1). The main sub-basins that

ConocoPhillips Indonesia (COPI) operates in the

Block B PSC is the Bawal Graben, Gabus Sub-

Basin, North Belut Sub-Basin, and the South Belut

Sub-Basin (Figure 2). These basins collectively fit

within what is called the Anambas Graben by

Manur and Jacques, 2014.

Following the timing of Manur and Jacques (2014)

the initiation of inversion was diachronous, varying

from ~27 Ma to ~18 Ma across the basin, with peak

inversion timing from ~21 to 15 Ma. This resulted

in the modification of many of the original E-W

trending half grabens by shortening and rotation.

The resultant E-W to NE-SW trending fault

propagation folds (“Sunda Folds”) and the N-S

directed structures provide the traps for the majority

of plays in the basin to date (Manur and Jacques,

2014).

These tectonic events mark four

tectonostratigraphic mega-sequences in the basin:

syn-rift, post-rift deposit, syn-inversion, and post-

inversion. During the syn-rift phase, lacustrine and

fluvial sediment were deposited (Lama, Benua, and

Belut Formations). During the post-rift subsidence

dominated phase, the fluvial to marginal lacustrine

shoreface Gabus Formation was deposited. The

Gabus Formation became the main reservoir target

in the early stages of the Block B development.

During early syn-inversion, a transitional to parallic

system was deposited in the Barat/Udang, and

Arang Formations. The current post-inversion stage

contains dominantly shelfal marine sediments of the

Muda Formation. (Figure 3) shows the illustration

of this tectonostratigraphy and (Figure 4) shows the

comparison between Manur-Jacques and COPI

stratigraphy.

UDANG RESERVOIR IN SOUTH-WEST

BELUT FIELDS

Three exploration wells were drilled in South Belut

field (Belut-1, Belut-5, and Belut-6) and three

exploration wells were drilled in the saddle

separated West Belut field (Belut-2, Belut-3, and

West Belut-2). (Figure 5) shows the location of

South and West Belut fields. The exploration wells

penetrated the Udang Formation with similar low

gamma ray log patterns but with different actual net

sand presence (Figure 6). All exploration wells in

South Belut field found a high net to gross (NTG)

reservoir in the Udang after inspection of the

cuttings. However, two of the West Belut

exploration wells found a low NTG reservoir in the

Udang despite a low gamma ray response, with only

Belut-2 finding a developed sand in the Udang

(cuttings lithology confirmation). Geophysical

attributes and seismic geomorphologic trends

indicate that only Belut-2 is within the trend of the

Udang shoreface at the West Belut sub-culmination

(Figures 7, 8, 9).

During the drilling campaign of the South Belut

development wells, a rotary sidewall core

acquisition program was proposed for the SB-06

well. The data acquisition was directed mainly for

the mineralogic analysis of the low resistivity gas

pay zones of the Arang Formation for porosity-

permeability calibration. Clay mineral identification

using XRD and SEM in the Udang Formation was

an additional objective to clarify the hypothesis

about the effects of low radioactive clay mineralogy

on the gamma ray tool response. There are 6 core

plugs that consist of four sandstone and two clay

stone (Figure 10). Based on petrographic, XRD, and

SEM analysis of four sandstones and two claystone

rotary sidewall cores from the Udang, kaolinite is

the dominant clay mineral (~20% weight), with

chlorite (~10% weight), and illite (2-5% weight).

(Figure 10) shows the analyses.

PETROPHYSICAL ANALYSIS IN COOL

SHALE

A deterministic shaly sand petrophysical model was

used to re-analyze the Udang Formation log data for

this study. This model was derived from analysis

conducted in the analog Udang Formation Belut

exploration wells. The petrophysical analysis for

was performed using the windows-based Interactive

Petrophysics (IP) software application.

Clay Volumes (Vcl) were calculated using multiple

shale indicators, including gamma ray and

neutron/density cross-plots. As discussed, the

gamma ray (GR) is often non-representative of the

sand quality and shale volume in this region

because of the presence of low radioactivity clay

mineral such as kaolinite and chlorite. However,

neutron porosity is unaffected by the common low

radioactive-clay minerals such as kaolinite and

chlorite. Therefore, a petrophysical approach for

calculating Vcl from neutron and density porosity

logs was implemented.

The calculated Vcl is shown in the lithology track

with a green colored shale symbol and sand in a

yellow colored sand symbol (Figure 11). Multiple

shale indicators are dependent on the wireline suite

available and log quality, with the final shale

volume being the most logical shale volume

indicator given the borehole conditions and tool

availability. Combined neutron-density is the most

common shale indicator in Udang Formation, using

a Neutron-Density cross-plot with wet clay and dry

clay point identifiers. This method combined with

the Neutron-Density cross-plot technique usually

offers the best result, although it is altered slightly

by the presence of gas. The gas effect can be

minimized by calculating shale volume from the gas

corrected neutron-density (Figure 11).

A conventional Vcl equation is shown below:

NDVcl = (PhiN-PhiD)/(PhiNsh-PhiDsh)

Where:

NDVcl = volume of clay from Neutron density logs

PhiN = neutron porosity

PhiD = density porosity

PhiNsh = neutron porosity of shale

PhiDsh = density porosity of shale

Vcl for the Udang Formation “cool” shale bearing

interval was calculated using a combination of

density-neutron logs and the resistivity log. This

technique was applied for all West Belut (WB-2,

Belut-2 and Belut-3) and South Belut wells (Belut-

1, 5 and 6) which Udang section. At South Belut,

the Udang Formation shows a monotonous low

gamma ray reading and hydrocarbon-bearing zones

are found at the top of the zone. Resistivity logs in

the hydrocarbon column indicate low readings at

Belut-1 and Belut-6 that are consistent with clay

resistivity. The neutron and density logs are also

showing expected normal cross-over where the

neutron log deflects to the right and density log to

the left. However at some particular intervals, when

the gamma ray log shows a low reading,

anomalously the neutron log deflects to the left and

density log to the right, coinciding with the low

resistivity and low gas chromatograph log reading,

suggesting the logs are responding to a shale-rich

interval.

Porosity was calculated primarily from the neutron-

density cross-plot method when the borehole

condition was adequate and where there was no

significant gas effect to the neutron response. There

was little to no washout observed in the West Belut

wells. The total porosity value was corrected for gas

effect and clay volume resulting in effective

porosity (PHIE) and also verified and confirmed by

core (SCAL) data taken from the WB-2. Effective

porosity was determined by correcting the

appropriate porosity at each sample increment for

the effect of porosity reduction from shale using the

calculated shale volume and the tool’s response in

shale, corrected for any hydrocarbon response. An

example of the Belut-3 Udang Formation interval

petrophysical assessment is shown in (Figure 12).

Water saturation (Sw) was calculated using the

Indonesian equation (modified Archie method) to

compensate the clay effect from any shaly sand

intervals. Therefore, a shaly sand SW equation

(Indonesian equation) should be used to compute

more correctly the estimated water saturation (Sw).

The Indonesian equation is standard practice for

COPI in Block B to calculate Sw as below:

1/Rt0.5

= ((Phim/(a.Rw))

0.5 + Vcl

(1-

(Vcl/2))/Rcl

0.5).Sw

(n/2)

Where:

m = Cementation factor

n = Saturation exponent

a = Tortuosity factor

Vcl = Wet clay volume (clay content)

Sw = Effective water saturation

Rw = Formation water resistivity

Rcl = Resistivity of the clay

Rt = True resistivity

In the Udang zone interval the Rw of 0.49 ohm @

125ºF was used. It was derived from a water sample

analysis from the Belut-6 (~6,989 ppm NaCl)

collected at a water leg below the hydrocarbon

section in the Udang at 4,973 ft. (Figure 13). The

tortuosity index “a” is set to 1, the cementation

exponent m = 1.64 and saturation exponent n =

1.75 were taken from a nearby well.

The log estimated porosity was an important output

of the petrophysical analysis in Udang Formation

and will have a major impact on the well completion

decision and subsequent reservoir productivity

expectation. The permeability was predicted using a

modified Timur equation. The permeability was

controlled and calibrated by permeability (mobility)

from MDT data (Figure 14).

Timur equation K = a. (Phieb/Sw

c)

Where,

K = Permeability, mD

Phie = Effective porosity, pu

Sw = Water saturation (fraction)

‘a’ constant = 8581

‘b’ constant = 4.8

‘c’ constant = 1

COOL SHALE DISTRIBUTION AROUND

SOUTH BELUT SUB-BASIN

During the early Block B exploration period after

Belut-1 was drilled in1974, several exploration

wells were drilled including the Udang-1 (1974).

The Udang-1 well is about 46 km southwest of the

Belut-1 well, (Udang field is no longer being

operated by COPI) and probably has the same

“cool” shale lithofacies in the Udang Formation.

This prediction is based on the adjacent Block B

PSC wells Tiram-1, Udang-5, and Udang-6 wells

which penetrate the Udang Formation.

For this study, additional well correlations focused

on the Udang Formation presence of “cool” shale

were conducted to determine the extent of this

lithofacies. In the southern part of the South and

West Belut fields vicinity, there are two exploration

wells with Udang penetrations (Piranha-1 and

Kodok-1), and to the southwest there are the

mentioned Tiram-1, Udang-5, and Udang-6 wells.

(Figure 15) shows the fields and wells surrounding

the center of the study area (South and West Belut

Fields). (Figure 16) shows the well correlation with

the “cool” Udang shale lithofacies in the South

Belut Sub-Basin and how it decreases in presence

towards the north.

Weaver (1958) concluded that great majority of

clay minerals in sedimentary rocks are detrital in

origin, and that they strongly reflect the character of

their source material and are only slightly modified

in their depositional environment. However, based

on laboratory analysis the clay minerals in the

Udang “cool’ shale were precipitated during

deposition replacing unstable minerals and filling

the pores. Based on Emery et al. (1990) this

eogenetic kaolinite is formed under humid climatic

conditions in continental sediments. These humid

climate conditions are normally related to relative

sea-level lowstands, when large areas of marine

sediment are exposed and subjected to flushing by

meteoric water (Allen and Allen, 2005). (Figure 17)

shows clay mineral formation in a warm wet

eogenetic environment.

It is proposed that the kaolinite clay precursor

minerals and any associated transported clay is most

likely sourced from the southeast exposed highlands

of the Sundaland margin, where weathered granitic

rocks and volcanics would be able to provide

abundant kaolinitic input to the drainage system.

(Figure 18) shows an interpretation of the Udang

“cool” shale lithofacies distribution sub-regionally.

CONCLUSIONS

The gamma ray log pattern in the South Belut sub-

basin Udang Formation shows a low radioactive

response and monotonous gamma ray log pattern.

This response is described as a “cool shale”. XRD

and SEM analysis was used to confirm the clay

mineralogy causing the “cool” shale phenomenon.

For the Udang Formation in the South Belut sub-

basin, abundant kaolinite and chlorite clays are the

main factors causing a lower than expected gamma

ray pattern in the shale sections due to their low

radioactivity.

The gamma-ray log cannot be used to differentiate

sand shale lithologies in a “cool” shale setting.

Therefore, other tool suites utilizing neutron-density

logs and special methods as described in the paper

are necessary to calculate petrophysical parameters

such as volume of clay/shale, porosity, and water

saturation.

The distribution of “cool” shale lithofacies in the

sub-basin indicates it is pervasive to the south and

southeast of this South and West Belut Fields. A

likely source terrain for the abundant kaolinite

precursor minerals and chlorite clays is the

Sundaland Pre-Tertiary basement which was

exposed during Udang deposition, nearby the South

Belut sub-basin.

ACKNOWLEDGEMENTS

We acknowledge ConocoPhillips Indonesia Inc.

Ltd. and the Block B co-venturers (Chevron South

Natuna B Inc. and Inpex Natuna, Ltd.) for

permission to publish this paper. We would like to

acknowledge SKK Migas for their contributions and

support in the Block B area, and also MIGAS for

their permission to publish this paper. The information contained herein is for presentation

and discussion purposes only. ConocoPhillips does

not make any representations or warranties

regarding content accuracy.

REFERENCES

Allen, P.A, and Allen, J.R, 2005, Basin Analysis

Principles and Applications, Second Edition

Blackwell Publishing.

Emery, D., Myers, R.J., Young, R, 1990, Ancient

Subaerial Exposure and Freshwater Leaching in

Sandstone, Geology 18.

Manur, H., Jacques, J.M, 2014, Deformation

Characteristic of West Natuna Basin with regards to

Its Remaining Exploration Potential, Proceedings of

Indonesian Petroleum Association, 38th Annual

Convention.

Morley, R.J., 2003, Sequence Biostratigraphic

Study of Udang and Gabus formation. North Belut

Field, West Natuna Basin, Indonesia, Internal COPI

report.

Weaver, C.E., 1958, A Discussion on The Origin of

Clay Minerals in Sedimentary Rocks, Fifth National

Conference on Clay and Clays Mineral, National

Academy of Science-National Research Council.

Figure 1 - Basin configuration in West Natuna Basin (after Manur and Jacques, 2014)

Figure 2 - Basin and Sub-basin which COPI operates the Block B including South Belut Sub-basin

Figure 3 - Tectonostratigraphical packages illustration (after Morley 2003)

Figure 4 - Stratigraphy comparison between the latest publication and COPI

Figure 5 - Structural map of South-West Belut fields

Figure 6 - Well correlation South-West Belut Fields that shows low monotonous gamma ray log pattern in

Udang Formation

Figure 7 - Rock physics analysis for the AVA response of the Udang Formation

Figure 8 - Amplitude extraction on angle stack cubes

Figure 9 - Seismic attribute in order to interpret stratigraphical pattern of Udang reservoir

Figure 10 - Petrography, XRD, and SEM analysis of Udang Formation in SB-06

Figure 11 - Comparison Vclay calculation in Udang Fm on B-3.

Figure 12 - Gas Chromatograph reading confirms the presence of the gas bearing reservoir in Udang Belut-3

Figure 13 - Water Sample Analysis in Udang Fm

Figure 14 - SB-01 Petrophysical Analysis

Figure 15 - Wells surrounding South-West Belut fields

Figure 16 - Well correlation with surrounding wells that show cool shale distribution in Udang Fm

Figure 17 - Warm, wet eogenetic environment of river channel, crevasse, splays, and interdistributary bay

(after Allen and Allen, 2005)

Figure 18 - Hypothetic interpretation about cool shale distribution in Udang Fm