IPA93-1.1-115

PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Twenty Second Annual Convention, October 1993

N FORE-ARC ZONE OF SUMATRA: CAINOZOIC BASIN-FORMING TECTONISM AND HYDROCARBON POTENTIAL

D.M. Hall* B.A. D u V

M.C. Courbe** B.W. Seubert** M. Siahaan**

A.D . Wirabudi**

ABSTRACT

In the Bengkulu PSC of onshore and offshore Southwest Sumatra, localized basins containing four distinct seismic megasequences are recognized.

The basal, Paleogene, megasequence was deposited as a syn-rift unit within a series of northeast-trending half graben, probably segmented by northwest-trending transfer faults. A major unconformity separates this unit from a late Paleogene to early Miocene megasequence and appears to mark a change in basin- forming mechanism from orthogonal extension to possible oblique slip. According to this model, the transfer faults of the rift system were rejuvenated by right-lateral oblique slip in the late Paleogene to early Miocene, thereby superposing local pull-apart basins on the underlying graben.

These units are succeeded with strong unconformity by a middle to late Miocene megasequence marking the onset of open marine deposition within a unified forearc basin. Finally, this unit was overlain by a dominantly regressive Pliocene to Recent syn-orogenic megasequence resulting from the main period of uplift and erosion of the Barisan Mountains. The associated basin inversion of the older megasequences increases in intensity from offshore toward this mountain belt.

These results imply that far from accommodating a simple, homogeneous fore-arc basin, the fore-arc is tectonically heterogeneous with considerable potential for localised Paleogene and early Neogene basins.

* Fina Exploration Norway Inc. * * Previously Exploration Members of Fina Bengkulu S.A

Recent exploration of the Bengkulu PSC, targetting the lower two megasequences of Paleogene to early Miocene age, implies that such localized basins within the fore-arc can be prospective for hydrocarbons. Well results indicate the presence of mature source rocks and migrated hydrocarbons, and therefore appear to contradict the widespread assumption that heat flow values in fore-arc areas are insufficient to allow expulsion and migration of hydrocarbons.

INTRODUCTION

Fore-arc basins are commonly assumed to be unrewarding areas for hydrocarbon exploration, a view that appeared to be confirmed by the results of the first phase of exploration activity in the Sumatran fore-arc in the late 1970s to early 1980s. During this period, hydrocarbon indications were limited to uncommercial methane gas discoveries made by Unocal in the northern part of the fore-arc, and a minor oil show in a well drilled by Aminoil in the Bengkulu area of the southern fore-arc. This exploration concentrated almost entirely on shelfal Neogene plays located on the basin margins.

Neogene basin development within the northern Sumatran fore-arc (Figure 1) has also been the subject of a number of non-commercial regional studies (eg. Karig et al., 1980; Beaudry and Moore, 1985; Matson and Moore, 1992). Until recently, however, the southern fore-arc (Figure 2) has not received the same attention, and more significantly for exploration, even less has been known about Paleogene basin history.

An exception to this were the seismic and aeromagnetic data acquired in the Bengkulu area, which indicated the presence of a localised depocentre of presumed

IPA, 2006 - 22nd Annual Convention Proceedings, 1993

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Paleogene age (Howles, 1986). One possibility was that this basin could represent the southward continuation of back arc graben trends known north of the Barisan Mountains. This in turn had obvious implications for hydrocarbon potential.

It was primarily to evaluate this concept that exploration was carried out in the Bengkulu PSC from July 1989 to July 1992 by a group comprising Fina (Operator), Enterprise and British Gas. The exploration work programme included the acquisition of 3480 kilometers of onshore and offshore seismic, gravity and magnetic data (Figure 3). Following this, the Arwana-1 exploration well was drilled to a total depth of 4175m at an offshore location in the southeast of the PSC.

It is the purpose of this paper to discuss the impact that interpretation of this dataset has had on the understanding of basin history, and hydrocarbon potental of the southern Sumatran fore-arc. In this respect, the results of Arwana-1 are particularly significant, as the well represents the first substantive calibration of a basinal Paleogene section anywhere in the Sumatran fore-arc. Furthermore, the presence in this well of mature source rocks and significant oil shows, including indications of migrated oil, challenges some of the conventional views of fore-arc prospectivity .

REGIONAL SETTING

The Bengkulu PSC comprised an offshore-onshore coastal region covering a pre-relinquishment area of 16,800 square kilometers in the southeastern part of the Sumatran fore-arc (Figures 1 and 2). The PSC was situated landward of the shelf-slope break which separates the inner shelfal part of the fore-arc, here termed the Inner Fore-Arc, from the bathymetric deep of the Quter Fore-Arc. Consequently, water depths within the PSC average 50 meters, and only exceed this near the southwest boundary of the contract area. The northeastern part of the PSC included part of the Barisan Mountains, which in turn are bounded to the northeast by the West Sumatra Fault. The Barisan Mountains represent an uplifted and folded complex of sedimentary, igneous and volcanic rocks (Figure 2), and cannot therefore be described solely as a volcanic arc. In the Bengkulu area, the boundary between the Barisan Mountains and the coastal plain is sharply defined by a dextral oblique-slip fault, which appears to be a splay from the main trend of the West Sumatra Fault.

The setting of the PSC suggests two regional factors which may have influenced initial basin development. The first relates to the oblique convergence of the Indian Ocean Plate and Sunda Craton, which may have

commenced prior to the middle Miocene. Evidence for this are the Neogene pull-apart basins in the southernmost part of Sunda Straits (Huchon and Le Pichon, 1984). The formation of these basins has been explained by the northward movement of the Sumatra Sliver Plate (Jarrard, 1986), a term which describes the large region of fore-arc between the West Sumatra and Mentawi dextral strike-slip faults (Figure 1).

Secondly, it has been noted that the southward trend of Paleogene back-arc graben such as the Benakat Gulley (de Costa, 1974) align with the Bengkulu area if it is assumed that subsequent dextral movement along the West Sumatra Fault has been of the order of 100 kilometers (eg. Howles, 1986).

Consequently, the development of Paleogene to early Neogene basins in the Bengkulu area was probably influenced by both extensional and oblique slip tectonics.

PREVIOUS EXPLORATION

Between 1970 and 1972 a total of six offshore wells (Figure 3) were drilled in the Bengkulu area: four by the Jenny Oil Group and Marathon in the Mentawi PSC, and two by Aminoil in the Banten-Lampung PSC. None of these wells reached total depths greater than about 1960 metres, and in each case the proposed objective of Miocene carbonate build-ups, overlying what was interpreted to be volcanic or igneous basement, was water-wet or absent. The only exception to this is the Bengkulu A-lx well, which encountered oil shows in a basal carbonate, originally interpreted as equivalent to the early Miocene Baturaja Limestone of South Sumatra, but now thought to be earliest middle Miocene.

Subsequent interpretation has shown that only the Bengkulu A-lx and A-2x wells were located on valid structural closures. The Mentawi-A1 and Mentawi-C1 wells were drilled on velocity pull-ups created by overlying late Miocene (Parigi Formation) reefs, whereas the Bengkulu X-1 and Bengkulu X-2 wells were located on a gravity high with no clearly-defined structural closure. Furthermore, all of the wells were located outside the main Paleogene depocentres.

It is therefore clear that this first phase of drilling did not fully evaluate the hydrocarbon potential of the area.

DISTRIBUTION OF PALEOGENE- EARLY NEOGENE BASINS

Four Paleogene to early Neogene basins have been identified within the limits of the original Bengkulu

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PSC. Their location is shown by the basement depth structure in Figure 4. Depth to basement within basins located in the offshore area was estimated from the combined interpretation of seismic, gravity and magnetic data. In the onshore area however, seismic definition of basement structure is ambiguous owing to surface-related signal-to-noise problems. Basement interpretation onshore is therefore mostly based on gravity data.

The onshore North Manna Basin and adjacent offshore South Manna Basin are located in the southeastern part of the PSC (Figure 4), and were the prime objectives of data acquisition during the 1989-1992 exploration period. Consequently, these basins are the main subject of this paper. Based on more limited data coverage, two further depocentres are tentatively recognized: one located east of Bengkulu and the other in the northern area of the PSC near Ketahun.

The North Manna and South Manna Basins are broad half-graben, which thicken to the northeast (Figure 5 ) . In addition, the North Manna Basin has been tilted toward the southwest by younger Plio-Pleistocene inversion. The associated uplift of the Barisan Mountains has obscured the northern limit of the North Manna Basin, although the apparent trend of the basin axis suggests that it may have extended northeastward at least as far as the West Sumatra Fault. In contrast. the depositional axis of the South Manna Basin displays a clear northwest trend, offset to the southeast relative to the North Manna Basin. The two basins are separated by a narrow median high which also trends northwest, below the present coastline. Basement depths in the South Manna Basin are interpreted to exceed six kilometers, approximately the same level as the subduction trench in the Outer Fore-Arc.

The nature of basement underlying the Inner Fore-Arc Paleogene basin fill remains uncalibrated by drilling or outcrop exposure. However, in places, a parallel-bedded seismic facies has been recognized (Figures 5 and 8), possibly suggesting that the basement has a sedimentary or metasedimentary rather than crystalline origin. Possible origins include Cretaceous to Paleocene fore arc basins or shelfal platform cover sediments deposited on continental crust. Regardless of origin, it is clear from the contrasting subsidence histories of the Inner (shelfal) and Outer (basinal) Fore-Arc that the boundary between the two area5 coincides with a significant contrast in basement rigidity.

GENERAL STRATIGRAPHY

The lithostratigraphy of the North and South Manna Basins comprises a variety of volcanic-arc derived sediments interbedded with marine claystones and

minor carbonate intervals (Figure 6). Biostratigaphic analysis of the Arwana-1 well within the South Manna Basin indicates a relatively complete Cainozoic section from the early Oligocene or possibly late Eocene, which was deposited in an inner to outer sublittoral environment. The Arwana-1 well provides the only control of Paleogene stratigraphy in the southern fore- arc region, as the previous exploration wells drilled in the early 1970s did not penetrate below the base of the Middle Miocene. In the North Manna Basin, the Paleogene section remains uncalibrated because the onshore outcrop provides no reliable age determinations older than early Miocene.

In both the North and South Manna Basins, an important stratigraphic boundary occurs at the base of the Middle Miocene, representing the downward change from regional to localized basin geometries. This boundary coincides with the base of a widespread carbonate interval informally referred to in this paper as the N9 Limestone after the equivalent Blow (1969) foram zone. The section above the base of the N9 Limestone contains a relatively diverse faunal assemblage, indicating essentially unrestricted access to the open oceanic environment. Below this level however, the Lower Miocene to Paleogene is characterized by a less diverse faunal assemblage, suggesting deposition within a more restricted basin.

SEISMIC STRATIGRAPHY AND LITHOSTRATIGRAPHY

The Recent to Paleogene stratigraphy of the Bengkulu area can be further described in terms of four seismic megasequences (Figures 5 to 8), each characterizing a major tectonostratigraphic phase of basin evolution (ie. sensu Hubbard et al., 1985). Megasequences are bounded by major seismically-defined stratal surfaces which often correlate with important changes in external basin controls such as re-organization of plate movements. Each megasequence is subdivided into component sequences, the boundaries of which also form prominent seismic events interpreted as corresponding to changes in regional relative sea level, basin subsidence or sediment supply.

Megasequence I (? Late Eocene to early Oligocene)

Megasequence I represents the initial fill of the early Neogene - Paleogene basins, which was deposited within a complex mosaic of segmented half graben depocentres. The only direct evidence of Megasequence I lithologies comes from. the basal 60 metres of Arwana-1 , which comprise massive volcanogenic intervals, interbedded with dark brown and grey-green claystones.

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The volcanogenic lithologies are mainly volcanic litharenites, which petrographic studies of sidewall cores indicate comprise welded ignimbrite clasts, lithic volcanic clasts and vitriclasts. Although diagenesis has obscured much of the original rock fabric, there are some reworked intervals with reduced matrix content. This is inferred both from thin sections and log-based interpretation of permeability variations. Faunal and geochemical evidence indicate that the interbedded dark brown claystones are of organic marine origin, whereas the grey-green claystones are probably derived from a volcanic source.

The tentative late Eocene date assigned to the basal part of thewell is based on the recognition of nannoflora taxa Diacoster cf. Saipanensis and Dicoaster cf. barbadiensis and also of palynoflora taxa Proxapertites sp. As these specimens occur in very low numbers, the possibility of reworking into sediments of Oligocene age cannot be excluded. If in situ, the presence of Proxapertites sp., which is thought to be derived from a mangrove habitat, together with the marine nannoflora, indicate a near-shore depositional environment.

Although the base of Megasequence I was not penetrated by Arwana-1, seismic data suggest a section below TD of approximately 2000 metres overlying acoustic basement. Basement is estimated to be at a total depth of approximately six kilometers (two way time 4.50 secs). The internal seismic character of the Megasequence comprises a series of high amplitude events, possibly suggesting a downward continuation of the interbedded volcanoclastic and argillaceous units penetrated by Arwana-1. However, owing to limited seismic resolution at these deeper levels, and absence of well control, it has not been possible to subdivide the Megasequence into component sequences.

The recognition of Megasequence I in the North Manna Basin is less certain owing to the limited deep resolution of the onshore seismic.

Megasequence I is probably, at least in part, equivalent to the Lahat Formation of the South Sumatra Basin. In both cases the sediments represent the initial fill of graben depocentres, although if the late Eocene age of Megasequence 1 in Arwana-1 is correct, deposition in the basins of the Bengkulu region may have commenced earlier than in the South Sumatra Basin. It is also possible that the Kikim volcanics which occur at the base of the Lahat Formation are the time equivalent of the volcanics in Megasequence I.

Megasequence I1 (early Oligocene to early Miocene)

In the South Manna Basin, deposition of Megasequence I1 occurred within an elongate northwest-trending

depocentre, which was superimposed on the underlying system of segmented half graben. Megasequence I1 is also recognized within seismic traversing the North Manna Basin, and at outcrop within the Barisan Mountains (Brown Series of Elber 1938). Detailed depositional relationships within this onshore basin however are not as clearly defined owing to poor seismic resolution and imaging.

Megasequence I1 can be subdivided into the following four sequences:

Sequence 11.1 (early Oligocene): The basal Sequence 11.1 is confined to the deeper parts of the basin, and has a transparent seismic character. In Arwana-1, the sequence comprises regular interbeds of dark brown and grey-green claystone, with juvenile volcanoclastic lithologies. Evidence from sidewall cores and interpretation of logs indicate that the interbedding of these different lithotypes ranges from millimeter scale laminations to beds a few metres thick. The volcanoclastics in Core 3 of Arwana-1 contain a variety of lithologies, including vitric crystal tuffs, tuffaceous sandstones, dark brown mudstones and polymict conglomerates, which based on sedimentological evidence are interpreted as being deposited as submarine mass flow deposits. However, there is no evidence from micropaleontology that deposition of these mass flows took place in a deep environment or that sediments were transported any significant distance. Although globigerine forams were recovered from the core, they were concentrated in discrete horizons and could have been washed into a shallow marine environment. A silled basin with limited open marine access is one possible interpretation of this.

On the basis of age equivalence, Sequence IP.1 can be correlated with the upper part of the Lahat Formation of the South Sumatra Basin (Benakat Member).

Sequence 11.2 (late Oligocene to earliest Miocene): This sequence contains a number of clearly-defined, parallel seismic events corresponding to volcanoclastic interbeds which are thicker than those present in the underlying Sequence 11.1. A further significant contrast between the two Sequences is the absence of dark brown claystones in Sequence 11.2. In Arwana-1, the upward change in lithology across the lower boundary of Sequence 11.2 is abrupt. It is associated with an upward change from a slightly overpressured section into siltier beds which display a characteristic invasion separation on the resistivity logs.

The boundary between Sequences 11.1 and 11.2 in Arwana-1 is also approximately coincident with the top of the early Oligocene which, in turn, is based

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primarily on palynological evidence (last appearance of ? Corrudinium incompositum). The late Oligocene to earliest Miocene age assigned to Sequence I1 is based on the combined evidence of palynology and micropaleontology .

Within Sequence 11.2, reworked early Cretaceous marine palynomorphs were also recognized within a thin calcareous unit. These perhaps suggest the nature of pre-rift basement lithology in the Bengkulu area.

Results from sidewall cores indicate that the interbedded volcanoclastics are comprised of tuffaceous deposits with variable matrix and crystal content. The gamma-ray curve defines probable sediment supply cycles, characterized by an upward-coarsening motif into the main clastic bed, overlain by an upward-fining unit. These cycles probably reflect variations in volcanic activity, and are probably independent of changes in relative sea level.

Sequence 11.2 is likely to be the time equivalent of the Talang Akar Formation of the South Sumatra Basin.

Sequence 11.3 (early Miocene): In Arwana-1, the basal part of Sequence 11.3 is characterized by the re- appearance of dark brown claystones, which within 70 metres pass upward into an argillaceous dolomite. This dolomite can be correlated with the Baturaja Limestone of equivalent age in South Sumatra. Restricted outcrop of the same limestone in a basin margin, skeletal wackestone/packstone facies, also occurs close to the onshore, southeastern boundary of the former contract area (upper part of the Air Saung river). At outcrop, the biofacies of the limestone is distinctive, containing both the key benthonic forams Lepidocyclina and Spiroclypeus .

The upper part of Sequence 11.3 comprises massive dark brown claystone, which in turn passes up into a series of thin (less than 5 metre thick) sandy intervals. Cores 1 and 2 of Arwana-1 suggest that these feldpathic arenites were deposited as storm/flood laminae, or thoroughly mixed by bioturbation with claystones and siltstones. The sandstones are commonly cemented by an early pore-filling calcite cement. Framework grains include bipyramidal beta quartz, indicating derivation from a volcanic provenance, and also unaltered sub-angular to sub-rounded feldspars, suggesting limited transport or exposure to weathering processes. The framework texture is under-compacted, owing primarily to the early calcite cementation.

Sequence 11.4 (latest early Miocene): The unconformity separating Sequences 11.3 and 11.4 is associated with a phase of mild, localized inversion tectonism. In

Arwana- 1, this boundary possibly accounts for missing section between the nannofossl NN2 and "4 zones (in terms of the Blow foram zonation, the missing section would correspond to the N6 to basal N7 zones). The lithostratigraphy comprises a continuation of claystones with occasional interbeds of feldspathic arenites.

Within the South Manna Basin, the seismic facies associated with Sequence 11.4 displays low-angle, progradational clinoforms.

The dark brown claystones of Sequences 11.3 and 11.4 can be correlated on the basis of both biostratigraphy and lithostratigraphy with the Gumai Formation of the South Sumatra Basin. Furthermore, the sandstones in the uppermost part of Sequence 11.3 and within the lower part of Sequence 11.4 can be correlated with similar age sandstones in the South Sumatra Basin.

Megasequence 111 (middle to late Miocene):

Megasequence I11 represents deposition within the regional fore-arc basin, which in the Bengkulu platform area occurred from middle Miocene times onward. The abundance and diversity of middle Miocene forams within Megasequence I11 clearly indicate deposition in an open marine environment.

As it is beyond the scope of this paper to discuss the regional correlation of fore-arc sequence stratigraphy, we have not sub-divided Megasequence I11 into sequences as has been attempted for northern areas of the fore-arc by Beaudry and Moore (1985).

The lower part of Megasequence I11 lithostratigraphy is characterized by a series of high frequent, transgressive-regressive cycles, comprising claystones, siltstones and minor limestones. urthermore, it is clear from Arwana-1 logs that the periodicity of these cycles is irregular. possibly owing to contemporary non-linear subsidence of the Inner Fore-Arc shelf.

In contrast, the upper part of Megasequence I11 is characterized by more massive shelfal limestones, including major reefal build-ups (Parigi Limestone equivalent).

Megasequence IV (early Pliocene to Recent):

Following a major marine transgression in earliest Pliocene, differential subsidence between the Inner and Outer Fore-Arc areas increased. Deposition within the rapidly-subsiding Inner and Outer Fore-Arc areas comprised marine clays, interbedded with massive, prograding siltstone wedges derived from the coeval

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uplift of the Barisan Mountains and associated Plio- Pleistocene volcanic activity.

TECTONIC HISTORY

We interpret the distinctive lithological character of Megasequences I, I1 111 and IV, and the spatial relationships between each of these Megasequences within the Bengkulu Inner Fore-Arc region to reflect their deposition as separate tectonostratigraphic units in four distinct, superposed basin types. At least three and possibly all four of these units is present in both the North and South Manna Basins, which should therefore be regarded as composite basins in the sense of Hubbard et al. (1985). On the platform areas outside the North and South Manna Basins, and outside two other probable Paleogene depocentres tentatively recognized in the Bengkulu area, only the youngest basin-forming units, Megasequences 111 and IV, are present.

The geophysical results and the results from Arwana-1 suggest that Megasequence I was probably deposited during the Paleogene as a syn-rift unit within a system of northeast-trending half graben, which were probably segmented by northwest-trending transfer faults (Figure 5) . Tilted fault blocks bounded by northeast-trending faults are well imaged in some of northwest-southeast oriented seismic lines over the South Manna Basin (Figure 8). These growth faults clearly indicate the syn- tectonic deposition of Megasequence I (Figure 9).

A major unconformity between Megasequences I and I1 is interpreted as marking a change in the basin- forming mechanism from Paleogene extension to possible pull-aparts associated with oblique slip. According to this model, some of the northwest- trending transfer faults segmenting the older rift basin were rejuvenated by right-lateral oblique slip in the late Paleogene to early Miocene, thereby superimposing local pull-apart basins on the underlying Mega- sequence I graben. A transtensional pull-apart origin for the Megasequence I1 basin-fill within the composite South Manna Basin is consistent with its narrow, elongate depocentre (Figures 10 and l l ) , acd the presence of mild, coeval inversion structures along the approximately rectilinear, northwest-trending basin margins. Furthermore, the basal seismic sequence of egasequence I1 (11.1) is clearly offset in places by reactivation of the older northeast-trending, Megasequence I faults (Figure 8), consistent with apull- apart interpretation for the younger, superposed basin.

Megasequence I1 is succeeded with strong unconformity by Megasequence 111, marking deposition in aunified fore-arc basin.

Megasequence IV was deposited during Pliocene to Recent uplift and erosion of the Barisan Mountains, and can therefore be described as syn-orogenic. The associated inversion of Megasequences I and I1 increases in intensity from offshore toward this mountainbelt.

SOURCE ROCK AND RESERVOIR POTENTIAL

Source Rock Potential

Source rock lithofacies are present as dark brown marine claystones in Megasequences I and 11. Within Arwana-1, two main intervals are recognized: an upper unit within Sequence 11.3 and a lower interval, corresponding to Sequence 11.1 and the uppermost part of Megasequence I (Figure 12).

The upper source rock interval displays incipient (threshold) maturity equivalent to a vitrinite reflectance (VR) of 0.5. This maturity level is consistent with the estimated thermal gradient in Arwana-1 of 2.8 degrees celsius/100 metres. Total organic carbon (TOC) values of selected claystone samples are ca. 2%, hydrogen index (HI) values range from 300 to 400, and pyrolysis yields of up to 10 kg/ton were recorded. As most of the interval is lithologically homogeneous, these richnesses also represent bulk rock characteristics.

The lower source rock interval is within the oil window, with a VR of 0.6 estimated at a depth of 3645m. TOC values of selected claystone samples are between 1% and 2%, with hydrogen index values decreasing from 300 to between 100 and 200 (Figure 12). On a bulk rock basis however, these richnesses are significantly reduced by variable interlamination of volcanoclastic lithologies.

Despite this, the reduction in TOC and HI values compared with the upper interval suggests that the lower source rock interval is partially spent. It is therefore reasonable to assume that the TOC, and HI of the lower (mature) interval were originally as good as the upper, incipiently mature interval.

Both the upper and lower intervals can be classified as Type 11, with oil and gas generation capacity.

The distribution of oil shows in Arwana-1 corresponds to the main source rock intervals. Biomarker analysis of an extract from the Baturaja Limestone equivalent (basal Sequence 11.3) suggests low maturity and probable sourcing from the adjacent interbedded, early-mature source rocks. On the other hand, analysis of an oil show in the voIcanic sandstone near the top of Megasequence I (Figure 13) indicates derivation from a

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parent source rock with a maturity of about 0.87% VR. In contrast, the extract from the deepest source rock in Arwana-1 indicates a maturity of 0.67% VRE. This evidence is based on an extract from shows and is ,therefore not conclusive.

However, this contrast in maturities suggests that the hydrocarbons in the volcanic sandstone may have migrated a vertical distance of up to one kilometer from the parent source levels. This in turn suggests a depth to the top of the oil expulsion window of about five kilometers. As the maximum depth to basement is estimated to be greater than six kilometers, it follows that the gross thickness of the oil expulsion window may exceed one kilometer in the basin depocentres, implying the possibility of a substantial hydrocarbon kitchen.

Although the presence of oil shows is encouraging, the hydrocarbon prroducing potential of the basins in the Bengkulu area will depend on bulk rock generative capacity of source rocks, which in turn will be controlled by the extent of heterogeneous interbedding of the source intervals with the non-organic, volcanoclastic lithologies. Other factors such as the effectiveness of migration routes also need to be considered.

Reservoir Potential.

The overall quality of the reservoir lithologies encountered in Arwana-1 is pool'. The volcanoclastic sandstones in Megasequence I exhibited log porosities of ca. 10%, and effective permeability was inferred from a marked invasion profile in the resistivity logs. Unfortunately, however, effective permeabilities could not be confirmed by RFT measurements.

Other clastic intervals exhibited porosities mostly in the range 10 to 15%. Low permeabilities were indicated throughout Megasequence II, with the exception of a crystal-rich tuff bed in Sequence 11.2 and a feldspathic arenite in Sequence 11.4, both of which delivered RFT water samples (Figure 14).

Porosities in Megasequence I and the lower part of Megasequence II in the well were created by an aggressive secondary dissolution process, which appears to be linked to oil migration. Good permeabilities in the volcanoclastics however, depends additionally on original sorting (textural maturity), or the presence of extensive fracture systems.

The reservoir potential of early and middle carbonate build-ups, overlying the Paleogene basin depocentres, remains under-explored.

SUMMARY OF BASIN DEVELOPMENT

Based on the structural and stratigraphic results, basin development can be summarised as follows (Figures 15 to 17):

Megasequence I Time

Within the South Manna Basin, and probably the North Manna Basin deposition of pro-delta marine claystones within the segmented rifts (Figure 15) was periodically interrupted by the input of reworked volcanic sandstones, derived from coeval volcanic activity. It is also possible that the basal sections of some half graben were isolated from marine influenco, and were characterized instead by lacustrine deposition.

The detailed relationship between the North and South Manna graben and the South Sumatra graben of the back-arc area is unknown. The Paleogene Bengkulu and South Sumatra graben may have allowed a continuous depocentre to develop, with a northward transition from marine conditions to the paralic/ lacustrine environments of the back-arc basins. It is however more likely that this trend was segmented by possible transfer or relay fault systems associated with the regional northwest trending structural grain.

Megasequence II Time

Megasequence II time was characterized by arestricted marine environment in which depositional conditions were influenced by variations in subsidence rate and sediment supply, probably within an evolving pull- apart basin. These variations in turn are represented in the contrasting character of the constituent Sequences.

Consequently, Sequence 11.1 represents the initial deepening of the basin, and resulting deposition of rhythmically-interbedded argillaceous and clastic slope deposits. The absence in Arwana-1 of marine claystones in Sequence 11.2 implies that an additional restriction of the marine environment occurred during this time, with deposition of volcanogenic sediments dominating (Figure 16). This is consistent with the regional late Oligocene sea level lowstand recognized in Paleogene basins throughout the Sunda Shield.

Sequences 11.3 and 11.4 represent the final infill of the localized early Neogene basins (Figure 4), and their deposition was associated with the reworking of mature volcanoclastic sandstones in a shallow shelf environment (Figure 17).

The lithological and biostratigraphical similarity of Sequences II.3 and II.4 in Arwana-1 with the Lower

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Miocene outcrop in the Barisan Mountains and the Gumai Shale of the South and Central Sumatra suggests that by the late early Miocene a continuous depositional fairway existed between the Bengkulu area and the South Sumatra Basin.

Megasequences I11 and IV Time (Regional Fore-Arc Basin)

During the middle Miocene, the Inner Fore-Arc shelf was a more-or-less uniformly subsiding surface characterized by the deposition of the transgressive cycles of Megasequence 111. During the Plio-Pleistocene the rate of shelfal subsidence increased significantly coincident with the deposition of the prograding synorogenic sediments of Megasequence IV.

CONCLUSIONS AND IMPLICATIONS FOR STRUCTURE AND PROSPECTIVITY OF THE SUMATRAN' FORE-ARC

We have identified two quite distinct Paleogene to early Neogene basin styles which are superposed within the present Inner Fore-Arc region of the Bengkulu area. An earlier, Paleogene basin type (corresponding to Megasequence I) developed as a result of northeast-trending rifting, and was probably tectonically overprinted by a pull-apart basin (Megasequence 11) when northwest-southeast directed extension changed to northwest-directed oblique slip or transtension.

It therefore follows that the South Manna Basin cannot be described simply as a back-arc basin in a fore-arc setting. Rather, our results highlight the influence of two distinct tectonic systems: a continuation of extensional trends within the Sunda Shield, modified by the onset of right-lateral oblique slip within the Sumatra Sliver Plate. The superposition of the two associated basin types and their Megasequences (I and 11) suggests that zones of structural weakness coincident with the Paleogene graben trends influenced the initial break up of the Sumatra Sliver Plate.

These results imply that far from accommodating a simple, homogeneous fore-arc basin, the Sumatran fore-arc is tectonically heterogeneous, with considerable potential for localized Paleogene and early Neogene depocentres. This in turn has obvious implications for basin development in other fore-arcs where the effects of oblique subduction are apparent.

The results of Arwana-1 have a significant impact on the hydrocarbon potential of fore-arc basins in general. The presence of mature source rock lithofacies and migrated oil in this well contradicts the traditional

assumption that heat flow values in fore-arc basins are insufficient to allow expulsion and migration of hydrocarbons. However, despite this encouragement, the volumetric hydrocarbon-producing potential of these basins remains to be proven. The bulk generative potential of the source rock prism has been identified as a critical factor, and this is determined in turn by the degree of interbedding with non-organic lithologies derived from volcanic sources.

The presence of reservoir clearly also represents a significant risk in the further exploration of fore-arc areas. The reservoir potential of volcanoclastic sediments depends on processes such as secondary dissolution and fracturing, as well as the primary depositional rock fabric. Consequently, these lithofacies types should not be completely dismissed as reservoir targets. There also remains the possibility that qon- volcanogenic lithofacies, not penetrated by Arwana-1, provide good reservoirs elsewhere in these basins.

Our understanding of Sumatran fore-arc basin development and associated hydrocarbon potential is therefore clearly still at an early stage. In addition to the uncertainties in hydrocarbon potential, the distribution of units equivalent to Megasequences I and I1 in other Sumatran fore-arc basins, including other basins in the Bengkulu area, requires attention. This advancement will come from further exploration in what should still be regarded as an under-explored, frontier province.

ACKNOWLEDGEMENTS

We wish to thank the management of Petrofina, Pertamina and Partners British Gas and Enterprise Oil for permission to publish this paper. Pusat Penelitian Dan Pengembangan Geologi (GRDC) provided very helpful assistance and logistical support. We are particularly indebted to Ir. Nana Ratman and Ir. Thamrin Cobrie Amin for their help with field sampling.

We are grateful to all those in Petrofina who assisted with the Bengkulu project. In particular special thanks are extended to Dr. Paul Baumann, who provided valuable technical input during the term of the Bengkulu PSC, Dr. Ralph Burwood for reviewing the results of the geochemical analyses and Serge Froment for his work on the well-site and also for producing the post-well geological report. Biostratigraphical and geochemical analyses are based mostly on the work of P.T. Corelab Indonesia and in particular we would like to thank .Brown, R.E. Hulsbos, J. Harrington and S. Hindmarsh. Finally, we gratefully acknowledge the efforts of Fina Exploration Norway in helping us to

327

produce the manuscript. The interpretations presented in this paper are those of the authors and do not necessarily represent the views of all the co-ventures in the Bengkulu PSC.

REFERENCES

Beaudry, D. and Moore, G.F., 1985. Seismic stratigraphy and Cenozoic evolution of West Sumatra, Bulletin American Association of Petroleum Geologists, 69, 5, p. 742-759.

Blow, W.H., 1969. Late middle Eocence to recent planktonic and foraminifera1 biostratigraphy: Proceedings of the First International Conference on Planktonic Microfossils, Geneva (1967) , p. 199-422.

de Costa, G.L. , 1974. The geology of central and south Sumatra basins, Zndonesian Petroleum Association, 3rd Annual Convention Proceeding, p. 77-110.

Elber, R., 1938. Geologie des Kuestengebietes von Benkoelen zwischen Seblat (NW) und Bintoehan (SE), (Westkueste von Sued-Sumatra): BPM (Shell) Unpub., p. 24.

Howles, A.C., 1986. Structural and Stratigraphic Evolution of the Southwest Sumatran Bengkulu Shelf,

Indonesian Petroleum Association, 15th Annual Convention Proceeding, p. 215-243.

Hubbard, R.J., Pape, J., and Roberts, 1985. Depositional sequence mapping as a technique to establish tectonic and stratigraphic framework and evaluate hydrocarbon potential on a passive continental margin, in O.R. Berg and D. Wolverton eds., seismic stratigraphy 11: an integrated approach to hydrocarbon exploration, AAPG Memoir, 39, p. 79-91.

Huchon, P. and Le Pichon X., 1984. Sunda Strait and Central Sumatra fault, Geology, 12, p. 668-672.

Jarrard, R.D., 1986. Terrace Motion by Strike-Slip Faulting of Forearc Slivers, Geology, 14, p. 780-783.

Karig, D.E., Lawrence, M.B., Moore, G.F. and Curray, J.R., 1980. Structural framework of the fore-arc basin, NW Sumatra, J . Geol. SOC. London, 137, p. 77-91.

Matson, R.G. , Moore, G.F. , 1992. Structural Influences on Neogene Subsidence in the Central Sumatra Fore- Arc Basin, AAPG Memoir, 53, p. 157-181.

Rose, R., 1983. Miocene Carbonate Rocks of Sibolga Basin Northwest Sumatra, Indonesian Petroleum Association, 12th Annual Convention Proceeding, p. 107-125.

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