Page 388 - Petroleum Geology

356

areas” - the sedimentary basin. Orogeny, previously only supplying sediment,
was now advancing and affecting the area directly.
Since Van Bemmelen’s work, petroleum exploration moved offshore and
several large oil and gas fields have been discovered. Four were found in and
off the Mahakam delta: Badak (Gwinn et al., 1975; Huffington and Helmig,
1980) and Handil (Magnier and Samsu, 1976; Verdier et al., 1980) on one
trend, and Attaka (Schwartz et al., 1974) and Bekapai (De Matharel et al.,
1977; De Matharel et al., 1980) on another. These revealed an offshore struc-
tural style that contrasts strongly with that onshore. They are all growth anti-
clines; and Attaka and Bekapai on the seaward trend have growth faults. No
faults are known in Badak, and there is one transverse fault in Handil. It there-
fore appears that the structures onshore are “compressional” while those off-
shore are extensional.
This paradox has not yet been resolved. Abnormal pressures exist in the
massive mudstone formation at depth in the whole area, and it would be of
great interest to know the shape of the top surface of them. The onshore
structures could be the result of sliding following diapirism. Chapman (1974)
postulated that the formation of diapiric anticlines in the pre-orogenic defor-
mation of a regressive sequence would be modified by sliding if orogeny im-
posed an adequate slope on the top of abnormal pressures, and that the spacing
of the resulting thrusts would be determined by the dominant wavelength of
diapirism. He later demonstrated that the mechanics of subaerial sliding was
about twice as efficient as that of submarine sliding, so that contrasting stress
regimes could exist about sea-level (Chapman, 1979). In brief, the compres-
sional tectonics onshore could be a superficial consequence of the stratigraphy.
If the stress field changes from one in which compressional faults were gen-
erated to one in a neighbouring area in which extensional faults were gener-
ated, then mechanical theory would predict that there will be a substantial
zone betdeen the two in which there will be no faults because the magnitude
of the principal stresses will be too similar. The lack of faulting in Badak and
Handil may therefore be highly significant.
It is worth digressing here to note that the petroleum geology of these
fields has other points of interest. Badak contains mainly gas; the others
mainly oil, with some associated gas. The sands are generally lenticular, and
difficult to correlate within the fields. The oil in both Handil and Bekapai
in the south tends to be heavier with increasing depth to the reservoir. Both
are regarded as immature from a petroleum source rock point of view, vitri-
nite reflectance in Bekapai being less than 0.5% and Handil’s between 0.5
and 0.8%. On the grounds of immaturity of the mudstones within the pro-
ductive sequence, Combaz and de Matharel (1978) postulated a source below
2600 m, with considerable vertical migration, possibly up faults. This is in
contrast to the views expressed by Magnier in discussion at the 9th World
Petroleum Congress in 1975, that the haphazard distribution of oil and gas in
the Mahakam delta fields, and lack of faults, suggested source rocks adjacent