ORGANIC GEOCHEMISTRY, PETROGRAPHY AND MINERALOGY OF WUNGKAL-GAMPING MUDSTONE IN BAYAT AREA, KLATEN, CENTRAL JAVA

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PROSIDING PERTEMUAN ILMIAH TAHUNAN IAGI KE-37 HOTEL HORISON BANDUNG, AGUSTUS 2008

ORGANIC GEOCHEMISTRY, PETROGRAPHY AND MINERALOGY OF WUNGKAL-GAMPING MUDSTONE IN BAYAT AREA, KLATEN,

CENTRAL JAVA

Noviar Hendriyanto1 #, Hendra Amijaya2, 1,2

Department of Geological Engineering, Gadjah Mada University, Jalan Grafika 2 Yogyakarta 55281, Telp/Fax. 0274-513668,

Email: nov_hey@yahoo.com

ABSTRACT

The Geology of Bayat Area in Klaten, Central Java, which is regionally part of Southern Mountain, is very interesting to be researched. Near Pendul Hill, Bayat, dark grey mudstone of Wungkal-Gamping Formation can be found. The color of mudstones indicates that this mudstone is probably rich in organic contents and has potential to be a source rocksof hydrocarbon. This research has objectives to know how much organic material contained in Wungkal-Gamping mudstone and its potential to be source rock of hydrocarbon. TOC analysis and organic petrography analysis has been done to attain that objectives.

TOC analysis result shows that 8 samples of mudstone in research area have range of 0.16 % to 0.42 % weight of TOC. Organic petrography analysis also shows that the organic materials occured in each sample are not more than 6 % volume. This results conclude that mudstone in research area does not have potential to be source rock of hydrocarbon. Organic petrography using spore fluorescence color analysis, shows that sporinite in rock sample has moderate orange to red or brown color which is equivalent with Ro of aprroximately 0.6 – 1 % and shows peak mature to late mature in maturity. Furthermore, XRD analysis indicates that clay minerals contents of fresh dark grey mudstone are dominated by chlorite, while clay minerals contents of weathered yellowish to whitish light grey mudstone are dominated by smectite. All of that indicates that dark grey color of mudstone in research area does not show large amount of organic material but shows the occurence of clay mineral, especially chlorite.

Keywords: organic geochemistry, mineralogy, mudstone, Wungkal-Gamping Formation

SARI

Geologi daerah Bayat, Klaten, Jawa Tengah, yang merupakan bagian dari Geologi Regional Pegunungan Selatan sangat menarik untuk diteliti. Di Gunung Pendul, Bayat, ditemukan batulumpur (mudstone) berwarna abu-abu gelap dari Formasi Wungkal-Gamping. Warna batulumpur tersebut mengindikasikan adanya kemungkinan bahwa batuan tersebut kaya akan kandungan material organik dan berpotensi menjadi batuan induk hidrokarbon. Penelitian ini bertujuan untuk mengetahui besarnya kandungan material organik yang terkandung dalam batulumpur Wungkal-Gamping dan potensinya sebagai batuan induk hidrokarbon, serta tingkat kematangan batuannya. Analisis TOC dan analisis petrografi organik dilakukan untuk mencapai tujuan tersebut. Hasil analisis TOC menunjukkan bahwa 8 sampel batulumpur di daerah penelitian memiliki nilai antara 0.16 % - 0.42 % berat TOC. Analisis petrografi organik juga menunjukkan bahwa material organik yang terdapat dalam setiap sampel tidak lebih dari 6 % volume. Hasil tersebut dapat menunjukkan bahwa batulumpur di daerah

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penelitian tidak berpotensi menjadi batuan induk hidrokarbon. Analisis petrografi organik berupa analisis spore fluorescence color menunjukkan bahwa sporinit dalam sample batuan berwarna oranye sedang – merah atau coklat yang setara dengan nilai Ro sekitar 0.6 – 1.% dan menunjukkan tingkat kematangan peak mature sampai late

mature.

Kemudian, hasil analisis XRD menunjukkan bahwa kandungan mineral lempung dari batulumpur berwarna abu-abu gelap dan segar didominasi oleh mineral klorit, sedangkan kandungan mineral lempung dari batulumpur berwarna kekuningan sampai abu-abu terang keputihan dan lapuk didominasi oleh mineral smektit. Keseluruhan hasil tersebut mengindikasikan bahwa warna abu-abu gelap dari batulumpur di daerah penelitian tidak menunjukkan jumlah material organik yang besar tetapi menunjukkan kehadiran mineral lempung, terutama mineral klorit.

Kata kunci: geokima organik, mineralogi, batulumpur, Formasi Wungkal-Gamping

INTRODUCTION

Many geological researches had been conducted in Bayat Area located in south-east of Klaten, Central Java. However, there are still many questions to be answered, especially about its

geological history including the

hydrocarbon play aspect. Geology of Bayat Area, which is regionally included to Geology of Southern Mountain, is very interesting to be researched where dark grey Eosen Wungkal-Gamping mudstone can be found in east side of

Pendul Hill slope. Mudstones in

research area show an indicator to be hydrocarbon source rocks with their color, as the dark grey color in mudstone probably shows significat amount of organic material content. This research will explain how much organic material contained in Wungkal-Gamping mudstone and its potential to be source rock of hydrocarbon which was determined by TOC analysis and

organic petrography analysis.

Furthermore, XRD analysis was

executed to know the mineral contents after the organic material contents were known.

METHODOLOGY

The research location is in east side of Pendul Hill slope, East Jiwo Hills, Bayat Subdistrict, Klaten Regency, Central Java Province (Figure 1). In this research, the executed methods are TOC analysis, organic petrography analysis and XRD analysis. This research uses TOC analysis (following the procedure from Widiarto, 2007) to know how much Total Organic Carbon (% wt TOC) is contained in rock sample

and petroleum potential can be

determined by comparing TOC

percentage from each on Table 1. Organic petrography analysis is used to know type of organic matter (maceral) and determines relative degree of rock

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maturity. Maceral type can be determined by observing polished section using reflected microscope. Relative degree of rock maturity can be determined by determining sporinite maceral color under fluorescence light and converting it to vitrinite reflectance (Ro) value using graphic of relationship between Ro and spore fluorescence color from Geolabnor. XRD analysis is

used to determined mineral

composition, especially clay mineral

composition. This analysis was

executed after petroleum potential of the mudstone was known. The analysisi is based on the methods described by Chen (1977) and Moore & Moore (1997). Eight samples had been analyzed in this research.

RESULTS

Total Organic Carbon (TOC) Analysis

TOC analysis shows percentage of total organic carbon contained in rock sample. The results of TOC analysis

and their hydrocarbon potential

interpretation are shown on Table 2.

From 8 samples, hydrocarbon

potentials shown by this analysis are poor.

Organic Petrography Analysis

Polished section observation under

reflected microscope can identify

maceral contained inside. This contents

are explained with volume percentage. Maceral contained in each 8 samples is not more than 0.6 % (Table 2). It shows that organic material occurence from each samples is very rare. Maceral type shown by each polished section

samples are mostly vitrinite and

inertinite. Vitrinite is very dominant. Liptinites are only found in NHGP0504 sample and their quantity is very small. The type of liptinites are sporinite. Under fluorescence light microscope (spore fluorescence color analysis), the sporinite colors are moderate orange to red or brown. Macerals under reflected microscope can be seen on Figure 2.

X-Ray Diffraction (XRD) Analysis

Based on XRD curve record, minerals contained in the mudstone samples are shown on Table 3. All samples are

composed by quartz, feldspar,

plagioclase, sericite, magnetite, calcite and some clay minerals. The clay minerals are different in some samples. Clay mineral compositions have little variation in different weathering degree; NH01, NH02, NH03 and NH04 samples (relative fresh mudstone samples) are composed by chlorite, illite and little smectite in which chlorite is dominant; NH05, NH06 and NH07 samples (relative weathered mudstone samples) are composed by smectite, illite and little chlorite in which smectite is

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dominant, moreover, in NH08 sample, chlorite is absent.

DISCUSSIONS

Hydrocarbon Source Rock Potential

Megascopic characteristic of the fresh mudstone shows grey to dark grey color. It indicates that the mudstone has possibility to be a hydrocarbon source rock. However, TOC analysis results from 8 samples show that their petroleum potential are poor or they do

not have potential to produce

hydrocarbon.

Based on organic petrography analysis result, organic material content in the mudstone is very low, not more than 0.6 % volume (see Table 3). It can also show that the mudstone does not have

potential to produce hydrocarbon.

Organic materials composition

comprises vitrinite, inertinite and

liptinite. Vitrinite is very dominant, inertinite is a little and liptinite is very rare. If the organic material is abundant, the mudstone is a gas prone source

rock, because it has vitrinite

dominantly, which has tendency to produce gas.

Based on the above paragraph, the dark grey color of mudstone is not caused by large amount of organic material. XRD analysis shows that the fresh mudstone is composed by clay

minerals such as chlorite, illite and small amount of smectite, while the weathered mudstone is composed by clay minerals such as smectite, illite and samll amount of chlorite. This clay

mineral change is followed by

mudstone color. Fresh dark grey

mudstone dominated by chlorite

changes to weathered whitish to

yellowish light grey mudstone

dominated by smectite. It shows that dark grey color in the mudstone is caused by clay mineral composition that is chlorite.

Maturity

Maturity degree of mudstone can be interpreted with spore fluorescence color. From spore fluorescence color analysis result, sporinites found in NH08 sample show moderate orange to red or brown. Their color shows range of vitrinite reflectance (Ro) between 0.65 % to 1.1 %. This Ro range indicates that maturity of the mudstone is peak mature to late mature. It means that the mudstone had been exeeded oil window and reached gas window. The maturity of organic matter in this rock probably also got advantages from the heat of igneous rock intrusion of Pendul Hill.

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CONCLUSIONS

Dark grey color of

Wungkal-Gamping Mudstone is not caused by organic material abundance but caused by composition of clay mineral, that is chlorite.

Wungkal-Gamping mudstone is gas prone but its potency is very low (poor) or it is not potential to be source rock of hydrocarbon even the maturity of Wungkal-Gamping mudstone is peak mature to late mature.

ACKNOWLEDGMENT

We are grateful to Chief of

Geochemistry Group LEMIGAS

Jakarta, FX. Widiarto, and his staffs who had greatly helped and given an opportunity to execute TOC analysis and organic petrography analysis in his laboratory.

REFERENCE

Anonim, _____,

http://www.geolabnor.no (access year 2008)

Chen, P.Y., 1977, Table of Key Lines in

X-ray Powder Diffraction Patterns of Minerals in Clays and Associated Rocks, Authority of The

State of Indiana,

Bloomington, 40pp. Moore, D.M. and R.C. Moore Jr., 1997,

X-Ray Diffraction and The Identification and Analysis of Clay Minerals, 2nd ed., Oxford University Press, New York, 378pp.

Peters, K.E. and M. R. Cassa, 1994,

Applied Source Rock

Geochemistry, The

Petroleum System from Source to Trap : AAPG Memoir 40, p. 93 – 120. Widiarto, F.X., 2007, Analytical Procedures of Organic Geochemistry Analysis, Research and Development Center of Oil and Gas Technology,

ESDM-RI Department,

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A, B, C : vitrinite (ordinary light); D, E : fusinite, inertinite type (ordinary light); F : sporinite, liptinite type (ordinary light); G, H : sporinite (fluorescence light)

Figure 2. Macerals found in the mudstone.

A

B

C

E

D

H

G

F

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Table 1. TOC analysis result and hydrocarbon potential interpretation according to the

Peters & Cassa (1994) classification

No. Sample Code TOC content (%) Petroleum Potential

1 NH01 0.4103 poor 2 NH02 0.4215 poor 3 NH03 0.2943 poor 4 NH04 0.2714 poor 5 NH05 0.1599 poor 6 NH06 0.1966 poor 7 NH07 0.2428 poor 8 NH08 0.1923 poor

Table 2. Maceral compositions from polished section microscopic observation

Vitrinite Liptinite Inertinite Total Maceral Content Mineral Sample Code (%) (%) (%) (%) (%) NH01 95 - 5 < 0.5 > 99.5 NH02 92 - 8 < 0.6 > 99.4 NH03 86 - 14 < 0.4 > 99.6 NH04 80 - 20 < 0.3 > 99.7 NH05 85 - 15 < 0.2 > 99.8 NH06 80 - 20 < 0.3 > 99.7 NH07 79 - 21 < 0.3 > 99.7 NH08 75 1 24 < 0.3 > 99.7

Table 3. Mineral composition of the mudstone based on XRD analysis result

Relative Clay Minerals Weathering Sample Code Degree Q u a rt z S e ri c it e F e ld s p a r (P la g io c la s e ) M a g n e ti te C a lc it e C h lo ri te Il li te S m e c ti te NH01 fresh v v v v v vv v v NH02 fresh v v v v v vv v v NH03 fresh v v v v v vv v v NH04 fresh v v v v v vv v v NH05 weathered v v v v v v v vv NH06 weathered v v v v v v v vv NH07 weathered v v v v v v v vv NH08 weathered v v v v v - v vv