Petroleum evaluation of the Aure thrust belt, Gulf of Papua, Papua

Transcription

PETROLEUM EVALUATION OF THE
AURE THRUST BELT, GULF OF PAPUA
PAPUA NEW GUINEA
Tim Buddin
Simon Petroleum Technology Limited
December 1993
SOPAC Technical Report 183
Prepared for South Pacific Applied Geoscience Commission (SOPAC)
Hydrocarbon Program
[3]
CONTENTS
Page
............................................................................. 7
SUMMARY (by SOPAC)...............
ACKNOWLEDGEMENTS........................................................................................................................ 6
OBJECTIVES........................................................................................................................................... 8
INTRODUCTION
...............................................................................................................................................................................................
8
STRATIGRAPHIC SUMMARY..................................................................................
PLATE TECTONIC SETTING
......................................................................................................14
Introduction.................
Plate Tectonic Activity.............................
............................................................................... 14
STRUCTURAL DEVELOPMENT
Seismic Interpretation..
......................................................................................................... 17
Structural Style ...............................
.................................................................................... 18
HYDROCARBON POTENTIAL
......................................................... 20
Potential Source Rocks................................................
Summary of Source Rock Intervals Penetrated in Adjacent Wells ...
.................................................................. 22
Maturation and Migration.....................................
Potential Reservoirs.......................
Traps.........................
............................................................... 30
.................................................................................................. 35
Summary of Hydrocarbon Potential.....
CONCLUSIONS (by SOPAC)....................
............................... 35
........................................................................ 37
RECOMMENDATIONS (by SOPAC)...........................................
REFERENCES ............
................................ 21
............................................ 38
..................................................................................................39
[TR183 - Buddin]
[4]
LIST OF FIGURES
Figure
1
Major tectonic elements..................................................................................................... 9
....
2
General chronostratigraphy of region.
3
Distribution and stratigraphy of wells adjacent to study area (from Slater and Yokker, 1993) .... 11
4
Hydrocarbon occurrences.
5
Plate tectonic evolution (modified from Smith, 1990), for approximate location, see Figure 1....15
6
Decompacted burial and maturity history, location K97-09 sp1 (foreland)......................
7
Conversion history at base of layer 6 (Top Moogli Mudstone), location K87-09 sp1........... 24
8
Generation history at base of layer 6 (Top Moogli Mudstone), location K87-09 sp1.............
9
Generation history at base of layer 7 (Base of Moogli Mudstone), location K87-09 sp 1
..........................................
13
25
....26
10 Conversion history at base of layer 7 (Base of Moogli Mudstone), location K87-09 sp1 ......... 27
11 Decompacted burial and maturation history, location K87-09 sp370 (hinterland) ............... 28
12
Current VRE and VRM, location K87-09 sp370...........................
...
13 Conversion history at base of layer 7 (Top Moogli Mudstone), location K87-09 sp370........... 31
14 Conversion history at base of layer 8 (Base Mudstone), location K87-09 sp370
.32
15 Generation history at base layer 7 (Top Moogli Mudstone), location K87-09 sp370................ 33
16 Generation history at base of layer 8 (Base Moogli Mudstone), location K87-09 sp370........... 34
[TR183-Buddin]
[5]
LIST OF ENCLOSURES
1
Intra-mid Miocene time structure (green horizon) including kitchen areas
2
Near top Miocene, Talama Formation time structure (red horizon)
3
Sequential restoration of a depth converted geoseismic section from seismic line K87-09
4
Interpreted seismic line K87-01
5
6
7
8
9
Interpreted seismic line K87-06/06A
10 Interpreted seismic line K87-07
11
14 Interpreted seismic line K87-12/12A
[TR183 - Buddin]
[6]
ACKNOWLEDGEMENTS
This work was supported by the Commission of the European Community (EC). The project was
coordinated by J.A. Rodd, SOPAC Petroleum Coordinator who is funded by the Commonwealth Fund for
Technical Co-operation (CFTC). Substantial assistance was also provided by the Government of Papua
New Guinea.
Thanks are extended to the staff of Simon Petroleum Technology Ltd (SPT) for assistance in the timely
execution of the project; and to SPT's consultant Dr Tim Buddin who carried out the evaluation
simultaneously with a training exercise for PNG geophysicist Francis Advent.
[TR183 - Buddin]
[7]
SUMMARY
by
J.A. Rodd, Petroleum Co-ordinator, SOPAC
The Aure Thrust Belt (ATB) is a southeastern extension of the oil and gas producing Papuan Fold Belt.
Although structural styles of these thrust and fold belts are broadly similar, owing to the structural
complexity and poorly understood hydrocarbon habitat of the ATB, drilling to date has been unsuccessful.
This report provides a reassessment of the structural development using balanced and restored sections to
constrain seismic interpretation and structural mapping, and a review of the hydrocarbon potential of the
region.
The ATB was initiated around 10 Ma as a result of collision of the Melanesian Arc with the northern margin
of the Australian-India Plate. The collision resulted in substantial tectonic shortening of the hinterland by
about 25%. Structural deformation occurred as thin-skinned thrusting of the Cainozoic sequence with
detachment at a depth of 7-8 km. Thrusts and associated hanging wall anticlines generally propagated insequence towards the foreland. Out-of-sequence thrusting is limited to late reactivation of some larger
faults which may have adversely affected the integrity of some traps due to seal breaching. The top of the
Late Miocene Talama Formation (6-7 Ma) marks the top of the pre-deformational sequence whilst the
overlying Pliocene Orubadi Beds exhibit marked growth sequences, thickening into the footwalls of adjacent
thrust and thinning over the culminations.
The principal source rocks in the ATB occur in the Miocene Aure Formation. They contain kerogen type lll
and are thus likely to be gas prone. The older Palaeogene Moogli Mudstone is thought to have oil and gas
potential, whilst deeper Jurassic and Cretaceous source rocks may be speculated, but are likely to be gas
generative. Burial and maturity modelling indicates that the Aure Formation and Moogli Mudstone reached
peak generation at 3-4 Ma, consequently the early-formed traps towards the hinterland are more likely to
have been charged. The oil window is predicted to be between about 3.5 to 6.0 km depth below sea floor.
Potential reservoirs occur in the Talama Formation pyroclastics/volcaniclastics, Aure Formation sandstones
and Pliocene Kairuku Limestone shelf carbonates. More speculative reservoir objectives include deeper
Eocene Nebilyer Limestone fractured micrites, and Cretaceous Pale Sandstone and Barune Sandstone
either where the detachment level deepens in towards the northeast or beneath the detachment. However,
the latter cannot be resolved on seismic data at present.
Structure maps have been produced for the principal reservoir, the Talama Formation, and source rock, the
Aure/Chiria Formation. Based on the new interpretation, three large structural traps and several smaller
traps have been identified at top Talama level each with closures in the range 40-70 km'. The traps occur in
hanging wall ramps bounded by major thrusts and comprise two faulted dip closures and one four-way dip
[TR183 - Buddin]
[8]
closure. In order to further evaluate the ATB and gain a better insight into the hydrocarbon habitat it is
recommended (i) that the seismic data be reprocessed to enhance steep dips and structural interpretation;
and (ii) that full evaluations be made of the structures already drilled in order to improve predictions for
future exploration.
OBJECTIVES
The main aim of this study is to illustrate the structure of part of the Aure Thrust Belt (ATB) of Papua New
Guinea (PNG) from seismic and well data, and to establish the hydrocarbon potential of the area in a more
quantitative way than has previously been attempted.
INTRODUCTION
The study area (PPL66) is located offshore PNG in the eastern Gulf of Papua some 50 km NW of Port
Moresby. The following report is based on the interpretation of some 350 km of seismic data from part of
the Aure Thrust Belt (ATB) (see Figure 1). The seismic data was recorded over shelf and deeper water
areas. There are no wells located in the PPL66 seismic grid and extrapolation from adjacent on and
offshore areas enabled interpretation of the seismic data (Figure 3).
There was limited supplementary data from the onshore part of the ATB; available onshore geological maps
(1978) were of limited use in aiding the construction of regional cross sections since they contained no
topographic information and did not indicate the positions of thrusts at the surface and seismic data (one
line) adjacent to the offshore grid is of very poor quality. The lack of any well data from deeper than the
upper Miocene level in the region makes the conclusions drawn by the interpretation tentative. However, the
application of established thrust fault principles to what is a thin skinned thrust belt has enabled the
interpretation to be as constrained as possible with the available data.
STRATIGRAPHIC SUMMARY
A summary of the main stratigraphic units, seismic horizons and main source and reservoir horizons for the
region is shown on Figure 2.
The Neogene stratigraphy of the study area is known from well penetrations to the upper Miocene
(Figure 3) with information on older rocks being gleaned from adjacent onshore exposures.
[TR183 -Buddin]
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The nearest detailed data on the pre-Neogene stratigraphy derive from the Aure Scarp where there is a
4.5 km section of Neocomian to Miocene rocks including more than 1 km of Palaeogene carbonates and
200m of Campanian sandstone. The Palaeogene sequence at the Aure Scarp is thought to correspond in
the area of PPL66 and PPL60 (Figure 4) to the Eocene Port Moresby Beds (cherty limestones, calcarenites
and siliceous mudstones), Paleocene - Eocene Burns Peak Formation (calcareous mudstones equivalent to
Moogli Mudstone?) and Paleocene Bogoro Limestone (argillaceous mudstone) and therefore the sequence
has both source and reservoir potential. The Barune calcarenite is the equivalent of the Campanian Pale
sandstone at the Aure Scarp and may have significant reservoir potential, although it is very deeply buried in
the area (generally greater than 7 km).
The Neogene succession comprises a thick sequence of clastics and local volcanics and reef limestones.
The Aure Beds (lower Talama Formation) comprise interbedded lithic sandstones and mudstones which
have been interpreted to be supra-fan and distal fan turbidites deposited in mid-upper bathyal
environments. The beds have been penetrated in Au Tabua-1 (Figure 3) where they are characterised by
fining upwards packages 25-75m thick which may result from hinterland tectonic activity providing uplifted
source areas. The lithology of the Aure Beds varies considerably with respect to palaeogeographic location;
locally the beds are sand rich and were deposited in a supra fan environment and may represent potential
reservoirs. No regional markers have been identified within the Aure Beds and their recognition is generally
based on biostratigraphic data and general lithology. They are estimated as being at least 2.5 km thick.
In the study area the Aure Beds are overlain by, and interdigitate with, a sequence of volcaniclastic
sandstones and pyroclastic flows of the Talama Formation (upper Talama Formation). This ranges from 86 Ma in age and has good reservoir potential as well as providing a clear seismic horizon across the study
area. On Yule Island and Iokea-1 (Figures 3 and 4) the Kairuku and Kapuri reef limestone of uppermost
Miocene to Pliocene age represent a potential reservoir.
The Aure Beds and Talama volcanics are overlain by the first truly syn-orogenic (foreland ll) sequence of
sediments namely the Orubadi Beds. These consist of upper bathyal mudstones with neritic sands in the
upper parts which crop out to the north of the area in the cores of synclines. The thickness of the Orubadi
beds is seen to be very variable on seismic lines. Growth packages are well developed with marked
thinning and onlap of the Orubadi beds onto the developing hanging wall anticlines, and locally very thick
sequences in the footwalls of the thrusts (e.g. lines K-87-04, 07, 09, Enclosures 7, 10 and 12).
The Orubadi beds reflect a pronounced increase in water depth from the late Miocene sands of the upper
Aure and Talama volcaniclastics, which probably represents the initiation of the foreland basin ahead of the
forelandward propagating thrust front.
Overlying the Orubadi Beds are the Plio-Pleistocene Era Beds which are composed mostly of a sandstone
and mudstone sequence with subordinate conglomerates. The sequence was deposited in shallow marine,
[TR183 - Buddin]
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[TR183-Buddin]
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transitional, and nonmarine environments. Some of the anticlinal crests seen in the offshore on the seismic
data appear to have been subaerially exposed with the Era Beds locally erosionally truncated.
PLATE TECTONIC SETTING
Introduction
The Aure thrust belt lies to the southeast of the main Papuan thrust belt and forms a narrow band ahead of
the Owen-Stanley ophiolite complex to the north. The thrust belt trends in an NW-SE direction (Figure 1)
with the main transport direction being to the SW.
Compressional deformation in the region during the Neogene has resulted from the obduction of the OwenStanley ophiolite at around 15 Ma, followed by the oblique collision of the Melanesian arc resulting in
compression from around 8 Ma to the present day. The main deformation seen in the Aure thrust is related
to the later collisional event.
The sequence of events applicable to the development of the Aure Thrust Belt in terms of their direct
relationships to the contemporaneous (Cainozoic) plate margin will be reviewed in the following section (see
Smith, 1990 for further details).
Plate Tectonic Evolution
During the Paleocene (Figure 5a) northerly motion of Australia commenced, with spreading occurring in the
Coral Sea area between 60 and 50 Ma with extensive regional uplift and erosion taking place at the end of
the Palaeocene. This is seen in the central Gulf of Papua where Tertiary sediments rest directly on
Palaeozoic basement. This event is interpreted to represent the thermal uplift of the Coral Sea region
associated with the onset of sea-floor spreading.
Southerly subduction beneath the Melanesian arc continued to form the oceanic Solomon Sea Plate during
the Middle Eocene. Eventually carbonate deposition replaced clastics over much of PNG at this time as a
result of the migration of the region into warmer climates (Figure 5b).
During the late Oligocene to Middle Miocene continental back-arc extension is postulated for much of the
PNG. This would have controlled the accumulation of the pre-thrust sequences in the study area and may
have led to substantial thickness variations within the pre-thrust sequences. Alternatively in the Aure Thrust
Belt there may have been only minor normal faulting with subsidence controlled thermally.
[TR183-Buddin]
[16]
By the Middle Miocene ophiolite obduction of the southern edge of the Solomon Sea Plate occurred along
the northern and northeastern margin of the Australian Plate in PNG (Figure 5c). This resulted in the
formation of the Owen-Stanley Obduction complex in the Papuan Peninsula and the Sepik Obduction
complex in mainland PNG. The development of these obduction complexes appears to have had little effect
on the sequences developing on the passive margin/proto-foreland, however deformation in the form of
subtle inversion occurred in the Papuan foreland area (present sense). Flexure of the foreland region as a
result of loading due to this obduction did, however, result in the development of a significant foreland basin
(foreland basin l, Figure 5c) from this time onwards, and led to the first northerly provenance foreland basin
sequences.
To the north by early Miocene times subduction had switched to the southern edge of the Melanesian Arc
with resulting consumption of the Solomon Sea Plate by the northerly dipping subduction zone to the south
of the arc. This may have been the result of the collision of the Ontong-Java plateau with the eastern end of
the Melanesian Arc effectively jamming the subduction system (Figure 5c).
Following the early - middle Miocene obduction event the undeformed part of the northern margin of the
Australian Plate continued to subside, primarily as a result of the thrust load related flexure of the crust. This
is in contrast to the post rift thermal subsidence phase that appears to have controlled sedimentation for
much of the Palaeogene (Coral Sea rifting related).
From the Middle Miocene onwards the northern margin of the Australian Plate was controlled by the
southwards subduction of the Solomon Sea Plate with development of the Marumani Arc related to this
subduction at the site of the Owen-Stanley and Sepik obduction complexes. To the north the Solomon Sea
Plate (SSP) was being subducted beneath the Melanesian Arc and this north and southward subduction
resulted in a rapid reduction in size of the SSP during the late Miocene. Ultimately this resulted, at around
10 Ma, in the collision of the northern plate margin with the Melanesian Arc which is today preserved as the
Adelbert-Finisterre Block. This collision initiated development of the Aure Thrust Belt and the formation of a
true foreland basin (Figure 5d).
The onset of deformation associated with the collision of the Melanesian Arc is interpreted to young from
west to east, occurring first in Irian Jaya in the Late Miocene. As a result of this the intensity of strike-slip
faulting and amount of displacement decreases to the east with the Owen-Stanley Obduction Complex only
partly bound to the north by the extensive strike-slip faults seen further west (Figure 1). The collision
resulted in substantial tectonic shortening and uplift which has propagated southwards to form the Papuan
and Aure Thrust Belts. The development of these thrust belt has applied an increasing load to the northern
edge of the Australian Plate and has resulted in the continued deepening and southerly migration of the
foreland basin to the southwest.
In the Aure Thrust Belt the deformation in the frontal part of the belt is comparable in age to the adjacent
[TR183 - Buddin]
[17]
Papuan Thrust Belt although the hinterland area in the Aure Thrust Belt is characterised by slightly earlier
deformation (Middle Miocene, Rogerson et al. 1987). This may be related to the earlier obduction of the
Owen-Stanley Complex which has given rise to a different structural grain in those hinterland areas affected
by this deformation phase. This grain trends northwestwards into the Sepik Obduction complex to the east
of the Kubor Anticline (Figure 1). Southerly propagation of the thrust belts onto the northern Australian Plate
continues to the present day.
The sequence of events shown in Figure 5 applies to the central part of PNG although the same events
apparently controlled the development of the Aure Thrust Belt. The main difference between the areas in
the large thickness of clastic sediments, both pre- and syn-orogenic that are involved in the deformation
compared to the relatively thin carbonate dominated sequence to the NW of the Aure Thrust Belt.
STRUCTURAL DEVELOPMENT
Seismic Interpretation
The seismic database for the study comprised a set of 1:26 666 60 fold stack, migrated lines with a total
length of around 350 km, which were recorded to a two way travel time (TWT) of 8 seconds. The quality of
the data was generally moderate with over migration being a problem on many of the lines. The best
processing sequence involved wave equation migration of a 'moves stack' or dip compensated stack,
although this was unfortunately done on only one of the lines (K-87-09). The remainder of the lines were
migrated CDP stacks and had significantly lower resolution, particularly on the steeper limbs of some of the
folds.
Ties to the seismic data were achieved via regional lines with wells which penetrated the upper Miocene
sequence (deepest stratigraphic level). The only regionally correlatable reflector (red horizon) tied to wells to
the north and northwest was the top of the Talama Formation volcanics which have a strong acoustic
impedance contrast with the overlying Orubadi clastics. The younger sequences which were resolved to the
north and northwest could not be reliably correlated to the study area due to rapid thickness, and probable
facies changes in this syn-thrusting sequence. Ties were also made to an adjacent onshore line of very
poor quality.
This allowed a tentative interpretation of an intra-Miocene horizon (green horizon), with the base of the
Tertiary sequence (blue horizon) picked on the basis of estimated thicknesses from onshore and a regional
tie to the west along a BMR regional line for which an interpretation was available. The level of the base of
the Tertiary sequence as picked across the area generally corresponds to the detachment level for the
overlying imbricate fan. Resolution of the pre-Tertiary succession was very poor and no attempt was made
to interpret this deep structure. There is some evidence for re-activation (positive inversion) of pre-existing
[TR183-Buddin]
[18]
Mesozoic extensional structures (line K87-18) (Enclosure 16). The detachment level generally appears to
be constant however and is dipping gently towards the hinterland on most of the remaining lines.
The seismic was interpreted using established rules of thin-skinned thrust systems and the dip lines were
tied with several good quality strike-lines ensuring the internal consistency of the interpretation.
Structural Style
Interpretation of the seismic data suggests that deformation of the Aure Thrust Belt in PPL66 involved
(Enclosure 1 -seismic database) the thin-skinned thrusting of a thick Cainozoic sequence of mixed clastic
and carbonates with a basal detachment at around 7-8 km depth. The thickness of the Cainozoic sequence
was estimated from the thickness of the onshore and offshore sections in adjacent areas and, as such, is
an approximation. The effects of underestimating Cainozoic thicknesses would be to involve the Mesozoic
sequences in the deformation, with over-estimation of Cainozoic thicknesses suggesting no involvement of
the Mesozoic sequences. Without well data in the seismic grid it is difficult to establish thickness accurately,
although this will have no effect on the geometry of the interpretation, only on the age of the rocks involved.
Other workers in the area (Smith, 1990) have also suggested that the detachment level in the Aure thrust
Belt is at or near the base of the Tertiary sequence. This contrasts with deformation in the Papuan Thrust
belt to the northwest where the Tertiary sequence is much thinner and the thrusts detach within the
Mesozoic sequence.
The top of the pre-deformational sequence in this part of the thrust belt, near to the foreland, lies at the level
of the Top Talama Formation, corresponding to an age of approximately 6-7 Ma. Above this level are well
developed growth sequences thickening into the footwalls of the major thrust, with marked thinning onto the
culminations. The correlation of chronostratigraphically defined units in this growth interval is not possible
although several general features with regard to the sequence of development of the structures are
apparent. Firstly the deformation sequence was in a forelandward propagating sense so the linked thrust
system can be termed a leading imbricate fan (Boyer and Elliot, 1982). This basically means that new
thrusts propagated in the footwalls of pre-existing thrusts, towards the foreland. No unusual out-ofsequence geometries are thus observed and the thrusts follow the in-sequence geometrical constraints well
established in thrust belts throughout the world.
The exception to this generally in-sequence development is the local re-activation of some of the larger
faults in more recent times to give imbrication of the growth sequence and local displacement of the seafloor, indicating that re-activation was very recent. This is seen clearly on line K-87-09 (Enclosure 12) with
an out-of-sequence fault restoring to a steep angle on the restored section. Some of the out-of-sequence
thrusts are in a back-thrust (hinterlandward) sense and can give rise to a pop-up geometry, which on time
[TR183-Buddin]
[19]
sections can look similar to a flower structure due to the falsely steep dip of the faults and poor resolution of
more steeply dipping beds on the seismic data.
The best quality seismic section was depth converted and a balanced and restored interpretation was
made. The restoration exercise tests the geometrical validity of the initial time interpretation in that:
.
all the thrusts drawn on the initial interpretation should restore to reasonable angles (ideally less
than 4')
.
they should cut up stratigraphic section towards the foreland and rocks should return to their regional
elevation above the basal detachment 'flat'; and
.
line lengths and areas should be conserved.
This can only be tested by the construction of a deformed state section in depth, produced from the seismic
interpretation, although when interpreting the seismic a close approximation to a balanced interpretation can
be made.
A balanced, partially and fully restored section presented in Enclosure 3 illustrates the structural style
interpreted for the area to be a valid, and geometrically feasible one. The partially restored section shows an
intermediate stage of deformation sometime after 7 Ma and before the very late re-activation occurred. Only
dating of the growth sequences would allow more accurate determination of the approximate age of this
partially deformed state. The amount of shortening involved in the deformation was approximately 25%
which is relatively low, primarily due to the presence of long relatively steep ramps and small amounts of
horizontal displacements along the flats. This also results in the 'snakes head' geometries seen for many of
the hanging wall anticlines.
A more complete picture of the development of the Aure Thrust Belt would be obtained by taking a regional
perspective and constructing a regional section from the Owen-Stanley ophiolite to the undeformed
foreland, but on the evidence of the available geological maps, some detailed re-mapping would have to be
carried out and any onshore seismic reprocessed.
Although the balanced and restored section shown in Enclosure 3 (balanced and restored K-87-09)
illustrates that a detachment at around 7-8 km (probably at the base of the Tertiary) is compatible with the
presented interpretation, other lines in the area indicate that the assumptions used in constructing the
section in Enclosure 3 may not wholly reflect the nature of the structural style. There is evidence for
thickness variation within the Tertiary succession which may have resulted from pre-existing normal faults
or palaeohighs which may have developed during Palaeogene extension. Also, there is locally evidence
(line K87-18) that the pre-existing normal faults and half graben in the Pre-Tertiary succession may have
[TR183-Buddin]
[20]
been reactivated during Neogene compression. In the absence of any well and onshore data it is not
possible to quantify these pre-thrust geometries, and therefore to construct the balanced section shown the
thicknesses were kept constant i.e. a 'layer cake' template was used.
The general structure of the area is illustrated in Enclosures 1 and 2 (time-structure maps) which show a
series of large NW-SE trending thrusts to be controlling the deformation of the area. Large hanging wall
anticlines are developed, some extending across most of the seismic grid. Numerous smaller splays from
the major faults are also developed and locally these form oblique or lateral ramps.
At the shelf edge, a number of seismic sections show apparent slumping which locally (K87-01-03) appears
to have associated normal faulting. Seismic quality in these areas is very poor, possibly due to poor static
corrections. On some of the seismic sections (02-05, 07-09, 13, 14 and 16) a blue (dark) horizon has been
picked and may represent the base of the Era beds. Elsewhere this pick was not discernible.
HYDROCARBON POTENTIAL
Potential Source Rocks
Cainozoic
The source rocks penetrated by wells in the study area have generally low organic contents as the data
below suggests. However, high cuttings gas readings (methane) have been encountered in shaley intervals
in Brolga-1 and Au Tabua-1 (Figure 3). Locally the Neogene may also be oil prone giving rise to the oil
shows seen in lokea-1. Regionally source rocks from the Tertiary have been favoured by geochemical
studies as the source for the hydrocarbons of the Pasca and Pandora gas and gas/condensate discoveries
(Figure 1). Significant amounts of migrant oil have been trapped in Palaeogene carbonates at the Aure
Scarp and these are thought to derive from marine Tertiary source rocks (Carman et al., 1992). Reiman and
Dielwart (1975) have described carbonaceous mudstones in the Talama Formation as being fair to good
but immature oil sources. The Palaeogene Moogli Mudstone (Figure 2) may represent a good oil and gas
source. It crops out mainly to the northwest of the study area but if present in the area it is likely to be more
distal in facies and may represent a good source where buried to sufficient depths in the footwalls of the
major thrusts seen on the seismic data.
Mesozoic
The source rock potential of Jurassic shales in the area is unknown due to lack of well penetration or
exposure. Regionally, Jurassic source rocks have been developed in a terrestrial organic facies.
[TR183 - Buddin]
[21]
Cretaceous rocks exposed at the Aure Scarp include up to 400 m of marine mudstones and siltstones from
Early to Late Cretaceous age. Geochemical analyses (Petro-Canada, 1986) give a typical TOC's of 1.32%.
The thickness of the Tertiary sequence identified on the seismic data and recognised onshore in the study
area, probably means Jurassic and Cretaceous shales are likely to be sources for gas only and may well be
overmature.
Summary of Source Rock Intervals Penetrated in Adjacent Wells
(for location see Figures 3 and 4)
Au Tabua-1
Potential source rocks within the Aure Formation - samples at:
1.
1625 m - 1650 m: Argillaceous sandstone (carbonaceous flakes); kerogen type lll, TOC 1.09.
2.
2340 m - 2360 m: kerogen type ll, TOC 0.50.
3.
1850 m - 1875 m: Shale, grey locally silty; kerogen type lll, TOC 0.54.
Samples analysed are predominantly terrestrially derived kerogen type III bearing source rocks with the
sample from 2340 m - 2360 m being kerogen type II but of little potential as a source rock due to its low
TOC value. The gas potential of the remaining source rocks is limited again due to their low TOC's.
Maiva-1
The interval analysed is thermally immature to around 7100' and mature below this depth. No source rocks
positively identified, with locally high TOC values attributed to drilling mud additives. Possible deeper oil
prone source rocks (Reiman and Dielwart, 1975).
Orokolo-1
There are no source rocks in the analysed interval although oil stain is present at 11,630' -1 1,640'. Oil
identified as migrant hydrocarbon derived from marine algal organic matter at an early to middle stage of
thermal maturity. Source not identified.
Iokea-1
Oil shows analysed from 2704' (Orubadi Formation) and 4555' (Talama Formation) suggest that the former
is a biodegraded oil and the latter is a live oil generated from predominantly terrestrially derived organic
matter, The source has not been identified.
[TR183 - Buddin]
[22]
Maturation and Migration
The maturation of potential source rocks in the study area was modelled using a burial history software
package which allows decompaction, complex lithological combinations and varied source rock types,
thermal histories etc. to be modelled. The data used to calculate the burial histories was gleaned from a
depth converted seismic interpretation in the south of the area with vitrinite reflectance data and BHT's
compiled from nearby wells. The sections chosen were within unfaulted segments of the section in the
footwalls to major thrusts and thus contain a complete stratigraphic section with minimal uplift or repetition
of sequences.
The absolute ages of the respective horizons is approximate, with better constraint for the younger horizons
which have well penetration. The top of the oil window was taken from a Petro Canada study for
kerogen type lll, the main source rock type for the area. This corresponds to a vitrinite reflectance value of
approximately 0.6, which in the model corresponds to an approximate depth (present day) of around 3.5 -
4 km. This figure will vary according to the heat flow used in the model and how closely this corresponds to
the heat flow as established from measured vitrinite reflectance results. Heat flow in thrust belts is not well
understood and is generally low although the figure most appropriate for the measured vitrinite reflectance
data for the area corresponds to a heat flow of 40 mW/m' which is very low. This may be real or there may
have been contamination through caving.
The modelled sections were taken from a depth conversion of seismic line No. K-87-09 (Enclosure 12). Two
sections were constructed, one from the footwall of a hinterlandward thrust (Figures 11 and 12) and one
from the local foreland on the section (Figures 6-10). There was no uplift or repetition on either section, with
the main difference being the 1 km water depth in the foreland section.
Both profiles (Figures 6 and 11) show gentle initial subsidence for most of the Palaeogene with a substantial
increase in subsidence rate from approximately 16 Ma (foreland basin 1-obduction of Owen-Stanley
ophiolite?) and a further rapid increase in subsidence of a higher magnitude at around 7 Ma (foreland basin
ll-Melanesian Arc collision and development of true foreland basin). Any Palaeogene source rocks present
will have entered the oil window at around this time with potential generation from older Mesozoic source
rocks before this period being difficult to quantify as there may be the additional effects of a Coral Sea
related rifting phase in these sediments.
Section 1 (local foreland) (Figure 6)
Generation from the top of the likely Paleocene source rock horizon (Moogli Mudstone equivalent)
according to the modelled section (Figure 8) began at about 4 Ma with only approximately 30% conversion
having been achieved at the present day (Figure 7). However at the base of this likely source rock interval
(Figure 9), generation began at around 5 Ma and peaked at 3 Ma with 100% conversion achieved by 1.5 Ma
(Figure 8). Any hydrocarbons generated from this source rock interval are likely to have been migrating in
[TR183-Buddin]
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[TR183-Buddin]
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[TR183-Buddin]
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[TR183-Buddin]
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[TR183-Buddin]
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[TR183-Buddin]
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[TR183-Buddin]
K87 -09 sp370.
[30]
significant volumes from approximately 3-4 Ma onwards. Thus the early formed more hinterlandward
structures - if they are sourced from this interval, are the more prospective in the area, as they are likely to
have been in existence prior to migration. Deeper sources than the Palaeogene may have charged these
early formed structures although timing is difficult to establish from this interval.
Section 2 (footwall section) (Figure 11)
Generation and conversion in this more hinterlandward section are significantly more advanced for the
same source rock levels probably as a result of the relatively shallow water depths compared to the deep
water foreland section which keeps temperature gradients low for the same depth BSL. Generation at the
top of the Palaeogene source rock interval begins at 4-4.5 Ma and peaks at around 2 Ma (Figure 15) with
conversion being 95% (cf. 30% for Section 1) at present day (Figure 13). The base of this potential source
rock horizon began generating hydrocarbons at around 7 Ma with peak generation at 4-4.3 Ma (Figure 16),
100% conversion being achieved by 2 Ma (Figure 14). Once again deeper source rocks may have charged
early formed structures in the hinterlandward part of the area.
Potential Reservoirs
Due to the nature of the deformation in this part of the Aure Thrust Belt, with the detachment estimated to
be at the base of the Cainozoic section, the potential reservoirs are considered to lie within the Cainozoic
sequence, with little chance of exploitable deeper reservoirs existing. Where the detachment level is deeper
and the amount of shortening greater (i.e. towards the northeast) there is a significant likelihood of
encountering the Mesozoic reservoirs such as the Pale Sandstone and Barune Sandstone at relatively
shallow levels.
In the Cainozoic, there are several potential reservoir units in the Aure Thrust Belt area. In the Palaeogene
Nebilyer Limestone sequence, micritic limestones may provide reservoir potential if significantly fractured
(anticlinal crests) and locally the Eocene has been identified as a potential reservoir with limestone and
more distal chert horizon, if fractured, being of significant potential (Carman 1990).
Neogene reservoirs, which are the most accessible in terms of structural setting, include clastics of the Aure
Group, with local porosities up to 22% and permeabilities of up to 120 mD (Tavala-1A). The Talama
Formation (Talama volcanics) which is present throughout the study area and whose top is one of the better
resolved seismic horizons, has proven potential as a reservoir unit. The unit is composed of pyroclastics
and volcaniclastic sandstones. In lokea-1 the Talama flowed the equivalent of 8300 BWPD over the interval
1384 m - 1475 m. Other reservoirs within the Neogene include the Kairuku Limestone which crops out on
Yule Island (Francis et al. 1986). Shelf limestones of this age have been encountered in Kapuri where a
DST flowed 2640 BWPD equivalent between 1557 m and 1575 m and 4800 BWPD equivalent between
1680 m and 1685 m. This limestone is likely to be present at shallow levels on the crests of the major
[TR183-Buddin]
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[TR183-Buddin]
[32]
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Figure 14. Conversion history at base of layer 8 (Base Moogli Mudstone), location K87-09 sp370.
[TR183-Buddin]
[33]
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[TR183-Buddin]
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[TR183-Buddin]
[35]
hanging wall anticlines. The Chiria Formation conglomerates, sandstones and limestones (intra-Aure Beds)
which also crop out in Yule island were penetrated in Oroi-1 and have recorded porosities of between 10%
and 22% over a 600 m interval.
No other significant reservoirs have been documented in the study area.
Traps
The main trap type in the study area is the hanging wall anticline, generally without 4-way dip closure and
requiring lateral fault seal. The traps are defined in Enclosure 2 which is a map at top Talama level, and
represents the top of the main potential reservoir unit of the area.
There are several potential traps in the area with the largest (Trap 2, Enclosure 2) (Top Talama time
structure map) having an area of approximately 65 km' and occurring in the southern and central part of the
study area. The trap requires lateral fault seal, with top seal likely to be provided by the shales of the
Orubadi Beds as with all the traps at this level.
Four way dip closure does exist in trap No.1 (Enclosure 2) (Top Talama (red) T-S map) with a minimum
area of closure of 32 km'. This trap requires better definition from further seismic data as it is constrained
by only 2 seismic lines. The potential area of closure may therefore be substantially larger.
Trap 3 is a medium size (approximately 45 km') fault seal dependent hanging wall anticline in the central
part of the area (Enclosure 2, Top Talama) and is the most hinterlandward of the potential traps, and
therefore, for reasons discussed earlier (5.1, Section 1) it represents a good potential target as it is likely to
have been charged by early formed hydrocarbons.
Recent movement on several of the thrusts defining the traps may have resulted in some leakage and/or
remigration of trapped hydrocarbons.
Summary of Hydrocarbon Potential
The most likely source horizons in the Cainozoic have become mature for generation of hydrocarbons,
probably mainly gas, during the last 6-7 Ma. As the main traps in the area were forming from 7 Ma onwards
there is a critical relationship between generation of the hydrocarbons and their migration into fully formed
traps. As has been outlined in Section 4.2 the structures seen in PPL66 have developed in a hinterland to
foreland sequence from around 6-7 Ma onwards, and therefore the earliest formed traps, those towards the
hinterland, should have been fully developed when hydrocarbon generation was at its peak. This depends
[TR183-Buddin]
[36]
on the rate of formation of the structures and the rate of the propagation of the thrust front towards the
foreland. More constraint on the timing of development of the traps could be achieved by accurate dating of
the growth sequences in PPL66 as has been achieved further to the north (Kugler, 1993). Correlation of the
dated sequences from the north along the regional seismic data was not possible due to the rapid variation
in the thickness and facies within the growth interval.
Balanced section construction has, however, revealed that the structures did form largely in sequence from
the hinterland to the foreland and as such the hinterlandward structures should have developed and not
have been substantially modified after approximately 7-5 Ma. The later more forelandward structures to the
west are less likely to have substantial amounts of hydrocarbons as the structures were only partly formed
when hydrocarbons were migrating and subsequent folding and faulting probably adversely effected the
likely development of significant sealed traps. Late reactivation of a number of faults may have corrupted
existing traps in the recent past as is seen on lines.
Reservoir horizons are numerous within the stratigraphic section with the main targets lying in the Talama
volcanics and locally the overlying Kairuku and Kapuri type reef limestones. The Orubadi beds will provide a
good top seal to the reservoirs and lateral seal will depend on amount of closure and lateral seal provided
by the thrusts. Generally the main traps are hanging wall anticlines adjacent to major thrust ramps.
The displacement on the thrusts puts the reservoir interval in lateral contact with the likely sealing horizons
of the lower Orubadi Beds thus providing lateral seal if the faults in the region are non-sealing.
[TR183-Buddin]
[37]
CONCLUSIONS
by
1.
The Aure Thrust Belt (ATB) formed as a result of thin-skinned thrusting of a Cainozoic hinterland
sequence due to collision of the Melanesian Arc with the northern margin of the Australian-India
Plate, which commenced around 10 Ma.
2.
Interpretation of seismic data using balanced and restored sections, indicates that the detachment
level is at a depth of approximately 7-8 km and that crustal shortening of about 25% has occurred,
Deformation is largely in-sequence with the oldest thrusts and hanging wall anticlines being to the
northeast, or most hinterlandward.
3.
The ATB is likely to be a gas province, charged predominantly from source rocks of the Miocene Aure
Formation which contain type lll kerogen. Gas charge is further indicated by numerous gas seeps
onshore and in exploration wells. The Palaeogene Moogli Mudstone may have potential to generate
oil and gas and is thought to be a source for oil shows encountered in wells. Maturity modelling
shows that these source rocks reached peak oil generation at 3-4 Ma and thus the early formed traps
towards the hinterland are more likely to be charged.
4.
Structural mapping at top Talama (Late Miocene) reservoir level has identified three large structural
traps situated in hanging wall ramps, each with closures between 40 and 70 km'. Trapping
geometries are faulted dip closures and four-way dip closures. Several other smaller structural traps
have also been identified. Other potential reservoirs in the Aure Formation, Pliocene Kairuku
Limestone, Eocene Nebilyer Limestone and Cretaceous Pale and Barune sandstones constitute
secondary objectives.
[TR183-Buddin]
[38]
RECOMMENDATIONS
by
1.
At least ten wells have been drilled in the ATB and its vicinity, all without success. It is essential that full
evaluations of these drilling results be carried out, integrating well data with seismic data to determine
the reason for these dry tests, gain more knowledge of the hydrocarbon habitat and so improve future
predictions.
2.
One of the key factors resulting in the high geological risk of exploration in the ATB is structural
uncertainty arising from poor quality seismic data. Reprocessing of the data paying attention to
resolution of steep dips and attenuation of multiples would improve the structural interpretation
considerably.
3.
The timing of individual structural ramps within the ATB has been beyond the scope of the present
study. Following seismic reprocessing it may be possible to make a more detailed seismostratigraphic interpretation of the syn- and post-deformational sequence in order to make a more
detailed assessment of the timing of trap formation and hydrocarbon charge.
[TR183-Buddin]
[39]
REFERENCES
Boyer, S.E. and Elliot, D., 1982. Thrust systems. AAPG Bulletin 66, pp. 1196-230.
Carman, G.J., 1990. Excursion Guide to the Aure Scarp. First PNG Petroleum Convention.
Carman, G.J., Robertson Research, Martin, B.A. and Cawley, 1992. Geochemistry of oil from the Aure
Scarp and Seepages from the Vailala - Parari area, unpublished. Austin Oil report. Open File report
F1/R/92-318.
Francis, G., Rogerson, R., Haig, D.W., Sari, J., 1986. Neogene stratigraphy, sedimentation and petroleum
potential of the Oiapu-Yule Island - Oroi Region, PNG. Geosea v Proceedings, v. 1, Geol. Soc.
Malaysia Bulletin 19, p. 123-152.
Hill, K.C., 1991. Structure of the Papuan Fold Belt, PNG AAPG Bulletin v. 75, no. 5, p. 857-872.
Hobson, D.M., 1986. A thin skinned modal for the Papuan Thrust Belt and some implications for
hydrocarbon exploration. APEA Journal, v. 26, p. 2154-224.
Kugler, K.A., 1993. Detailed analysis from seismic data of the structure within the Aure Fold and Thrust Belt,
Gulf of Papua, PNG. Proceedings of 2nd PNG Petroleum Convention, Port Moresby.
PetroCanada lnc., 1986. An appraisal of the hydrocarbon potential of PPL30, Papua New Guinea; Open
File Report F1/R/87-57.
Reiman, K. and Dielwart, J.E., 1975. Source rock and carbonisation evaluation of well Maiva-1: Gulf of
Papua, unpublished Shell (Australia) Pty. Ltd. open file report.
Robertson Research Australia Ltd. and Flower Doery Buchan Ltd., 1984. Petroleum Potential of the Papuan
Basin, PNG report for Geol. Survey of PNG.
Rogerson, R.J. et al., 1987. The geology and mineral resources of the Sepik headwaters region, Papua
New Guinea. PNG Geological Survey Memoir 12.
Slater, A. and Dokker, F., 1993. An overview of the Petroleum Geology of the Eastern Papuan Fold Belt,
Based on Recent Exploration. Proceedings of 2nd PNG Petroleum Convention, Port Moresby, 499516.
Smith, R.I., 1990. Tertiary Plate Tectonic Setting and Evolution of Papua New Guinea. Proceedings of 1st
PNG Petroleum Convention, Port Moresby. 229-244.
[TR183-Buddin]
[40]
ENCLOSURES
1
Intra-mid Miocene time structure (green horizon) including kitchen areas
2
Near top Miocene, Talama Formation time structure (red horizon)
3
Sequential restoration of a depth converted geoseismic section from seismic line K87-09
4
5
6
7
8
9
11
18
20
[TR183 - Buddin]

Petroleum evaluation of the Aure thrust belt, Gulf of Papua, Papua

Transcription

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