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Org. Geochem. Vol. 17, No. 3, pp. 363--374, 1991 0146-6380/91 $3.00 + 0.00

Printed in Great Britain. All rights reserved Copyright 1991 PergamonPress plc

Petroleum generation and migration from Talang Akar coals and

shales offshore N.W . Java Indonesia

R. A.

N O B L E 1

C. H. Wu 2 and C. D. ATKINSON

'ARCO Oil and Gas Co., 2300 West Plano Parkway, Piano, TX ?50?5, U.S.A.

2Atlantic Richfield Indonesia Inc., Panin Bank Building, Jakarta, Indonesia

( R e c e i v e d 1 0 A u g u s t 1 9 9 0 ; a c c e p t e d 2 5 S e p t e m b e r 1 9 9 0)

bs t r ac t The Oligocene Talang Akar formation is the major source for petroleum (oil and gas) in the

Ardjuna sub-basin. Source rock quality varies within the deltaic to marginal marine complex, with coals

deposited on the lower delta plain being distinctly oil-prone, whereas the surrounding delta plain shales

are largely gas-prone. Shales and drifted coals laid down in marine-influenced interdistributary bay

environments show increasingly oil-prone characteristics, possibly due to preferential transport of

hydrogen-rich plant components to more distal setting. Molecular characteristics of the coals and shales

are similar, with higher plant (angiosperm) biomarkers and their aromatic derivatives occurring widely.

Kinetic studies of hydrocarbon generation, and simulated maturation experiments using hydrous

pyrolysis, indicate that the delta plain coals are the major source for crude oil s in the basin, with a

secondary, but minor contribution from marine-influenced nterdistributary bay shales. Delta plain shales

are interpreted to expel gaseous products only. The stratigraphic and spatial location of oil and gas fields

were examined with respect to their position relative to oil and gas generating source rocks. The field

distribution could not be explained in terms of source rock geochemistry alone, and phase behavior of

petroleum during secondary migration was shown to exert an important influence on the distribution of

oil and gas deposits in the basin.

K e y w o r d ~ - p e t r o l e u m , source rocks, hydrous pyrolysis, kinetics, migration, phase behavior

INTRODUCTION

Non-marine to marginal marine source rocks for

petroleum occur extensively in the Tertiary basins

of Indonesia and other S.E. Asian countries. An

example of such a basin is the Ardjuna depression,

located offshore northwest Java, where commercial

product ion of non-m arine oil has been active for

many years (Fig. 1). The primary source for all

petroleum discovered to date in the sub-basin is

considered to be the deltaic member of the Oligocene

Talang Aka r form ation (Fletcher and Bay, 1975; Roe

and Polito, 1977: Gord on, 1985). The Talang Akar

coals, in particular, have been considered the major

source for crude oil, whereas interbedded shales have

been characterized as being more gas-prone (Gordon,

1985; Horsfield e t a l . , 1988).

Evidence presented to date for crude oil generation

from Talang Akar coals is based on Rock-Eval

pyrolysis, pyrolysis gas chromatography (py-GC),

and detailed organic petrographic studies (Horsfield

e t a l . , 1988; and references therein). The coals are

typically rich in liptinite macerals (15 60%), with

Hydrogen Index (HI) values of 200-400 mg HC/g C.

The expulsion efficiency of these liptinitic coals is

apparently sufficiently high to allow primary mi-

gration of liquid hydrocarbons prior to oil-to-gas

cracking at elevated temperatures. This differs from

vitrinitic (humic) coals occurring in other parts of

Indonesia (e.g. Mahakam Delta and Central Suma-

tra) where lower expulsion efficiencies for oil are due

to the retention of liquid hydrocarbons within the

organic matrix (Monthioux and Landais, 1987; Katz

and Mertani, 1989). Poor expulsion of liquids com-

bined with a high proportion of refractory (gas-

prone) kerogen indicate that vitrinitic coals are more

likely to yield only gas. On the other hand, Horsfield

e t a l .

(1988) have suggested that the high conversion

of exinites to liquids in the Ta lang Akar coals results

in de-activation of adsorptive sites and provides the

critical oil saturation required for primary migration

by continuous monophasic flow. Geochemical and

petrographic studies of other l iptinitic Mesozoic and

Tertiary coals have also provided evidence for

efficient expulsion of crude oil (Hvoslef e t a l . , 1988;

Mukhopadyay, 1989; Zhao

e t a t . ,

1990). A thorough

underst anding of the factors which influence primary

migration from coals is extremely important for

accurate basin evaluation, since incorrect assump-

tions can dramatically affect the outcome of resource

assessment calculations.

In the present study, the relative potential of

Talang Akar coals and shales to generate and expel

oil and gas was examined. Kinetic paremeters for

hydroca rbon generation were determined for samples

from different paleogeographic locations within the

deltaic to marginal marine complex, and tempera-

tures for oil and gas formation were derived using

typical Ardjuna heating rates. Molecular character-

istics of the coals and shales were examined, and

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