The Abadi Gas Field: IPA03-G-141

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© IPA, 2006 - 29th Annual Convention Proceedings, 2003 IPA03-G-141

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PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Twenty-Ninth Annual Convention & Exhibition, October 2003 THE ABADI GAS FIELD H. Nagura* Nagura* I. Suzuki* T. Teramoto* Teramoto* Y. Hayashi* T. Yoshida*

H. MP Bandjarnahor* Bandjarnahor* K. Kihara* Kihara* T. Swiecicki** R. Bird***

ABSTRACT

The Abadi field is Indonesia’s first discovery in the Middle Middle Jurassi Jur assicc Plover Plove r Formation. ormation. This potentially potentially giant gas discovery is located some 350 km east ea st of Timor Island and 350 km north of Darwin, Australia. It lies lie s just north no rth of the international nternati onal boundary with Australia, Australia, in 400-800 400-800 m water depth. The Masela Production Sharing Contract (PSC) PSC) was awarded to Inpex as Operator with 100% interest in November 1998. 1998. The discovery well Abadi-1 Abadi-1 was drilled and completed completed in late 2000. 2000. Following discovery, a 2,060 2,060 2 km  3D seismic survey was acquired in 2001 and two successful appraisal wells were drilled in 2002. Geologically, the field comprises relatively undeformed Australian continental margin that extends into Indonesian waters. It lies lies on the the eastern eastern extremity of the Sahul Platform and occupies a large tilted fault block bounded to the east and south by the Calder-Malita Grabens. The accumulation contains a significant gas column, reservoired within shallow marine, highly mature, quartzose sandstone of the Middle Middle Jurassic Plover Formation. Close analogues analogues are to be found in the giant Greater Sunrise and BayuUndan fields. Reservoir quality, at a depth of ~3,900 ~3,900 m, varies from surprisingly good to poor, reflecting a complex interaction of primary depositional controls and later diagenetic influences. A preliminary preliminary estimate of reserves size is about 5 TCF. Inpex has undertaken a variety of exploration projects and studies, including high-resolution sequence stratigraphy stratigraphy and seismic seismic -driven paleofacies reconstructions to model the reservoir architecture. * Inpex Masela, Ltd. ** Cerberus Consultants *** P.T. Paradigm Geophysical Indonesia

Utilizing this framework, a full-field 3D model is currently being developed to better characterize the likely range of reserves and to provide the basis for a more accurate understanding of the economic value of Abadi. INTRODUCTION

The Abadi gas field was discovered in late 2000 with the successful drilling of the Abadi-1 Abadi-1 well. The field field is located within the Masela PSC block in the eastern  part  part of the Timor Timor Sea, Eastern Eastern Indone Indonesia, sia, along along the IndonesiaIndonesia-Australia Australia international boundary (Figure 1). Approximately 100 km north of the block, the Babar Islands and Tanimbar Islands follow an east-west trend along the outer ridge of the Banda Arc. The  block  block name name of Masela Masela origin originates ates from from one of these these islands. islands. The deep Timor Trough with more than 1,500 m water depth lies between betwee n the island arc and the block block.. The Masela Block is on the the upper upper slope area of the Australian Continental Shelf with associated associa ted water depths ranging from 300 m to 1,000 m. Active exploration work was conducted on the Australian side of the Timor Sea from the mid 1960s, 1960s, and the discovery discovery wells of the Sunrise-Troubadour Sunrise-T roubadour gas fie field ld were drilled in the early stages (Troubadour1 in 1974, Sunrise-1 Sunrise-1 in 1975). 1975). Although exploration efforts were halted in the Timor Gap area in the late 1970s 1970s and 1980s by the boundary dispute between Indonesia and Australia, the discoveries of the Evans Shoal gas field (Evans Shoal-1 in 1988) and the Bayu Undan gas field (Bayu-1 in 1995) were eventually added to the area’s commercial reserve s. Some of these fields are now on on track for development. development. On the other hand, no exploration work was conducted on the the Indonesian side of the Timor Sea until Geco-Prakla

shot a 2,594 km 2D spec seismic survey in 1996 under an arrangement with MIGAS. Following Geco-Prakla’s spec survey, Pertamina opened the Masela Block PSC tender on October 19, 1997, together with the adjacent Leti Block. Inpex conducted a regional evaluation using the GecoPrakla data together with open file seismic and well data from the Australian side. The prospectivity was assessed, and a bid was submitted to Pertamina for the Masela PSC on April 3, 1998. The Masela PSC was awarded to Inpex and the PSC was signed on  November 16, 1998, between Inpex and Pertamina with the witness and approval of MIGAS. Inpex Masela, Ltd. was established as the project company to execute the exploration program with financial support from the Japan National Oil Corporation. This paper summarizes the exploration activities conducted in the Masela Block to date.

The lithostratigraphy of the Abadi field area is shown in Figure 3. Fluvio-deltaic to shallow marine deposits of the Middle Jurassic Plover Formation accumulated in a pre-rift to early syn-rift tectonic regime. This formation is the main reservoir objective in the area. Marine claystone of the lower Cretaceous Echuca Shoals Formation unconformably overlies the Plover Formation. Overlying the Echuca Shoals Formation is the predominantly carbonate Jamieson Formation, followed by thick prograding shelf and slope sediments of the Cretaceous Wangarlu Formation. The Tertiary section consists of drift phase deposits,  predominantly thick shelf carbonates. The late Miocene collision between the Indo-Australian and Sunda plates initiated the final tectonic phase in this area and continues to the present day. Left-lateral transtension and plate flexure rejuvenated Jurassic normal faults and produced a new set of normal faults with a dominant north-easterly strike. EXPLORATION HISTORY

REGIONAL SETTING Pre Bid Evaluation

The Abadi field lies within the Northern Bonaparte Basin, on the Sahul Platform at the eastern end of the Sunrise-Troubadour High (Figure 2). It is bounded to the east by the Masela Deep, which is the northern extension of the Calder Graben. The Malita Graben lies further to the southwest and accommodates thick Cretaceous-Tertiary sediments. The northwest trending Goulburn Graben, with thick Paleozoic sediments, is located to the southeast. The Timor Trough lies to the north. Approximately 250 km west of Abadi, the SunriseTroubadour gas field (proved & probable recoverable reserves: 8.4 TCF; public information from Northern Territory Government of Australia) occupies the axis of the Sunrise-Troubadour High. The Evans Shoal gas field (proved & probable recoverable reserves: 6.6 TCF; public information from Northern Territory Government of Australia) is approximately 150 km southwest of Abadi between the Sunrise-Troubadour High and Malita Graben. Development of the Northern Bonaparte Basin was influenced by rifting and continental separation in middle Jurassic to early Cretaceous times along the northwestern Australian margin, and later modified  by the collision between the Indo-Australian and Sunda plates from the Miocene to the present (Whittam et al. 1996).

In 1997/98, the Masela Block was evaluated based on approximately 2,600 km of Geco-Prakla spec seismic data (GPARI-96 series) shot during 1996 in Indonesian waters. Available Australian open file 2D seismic (3,500 km) and well data (13 wells) were also incorporated for the evaluation. These well data included Troubadour-1, Sunrise-1, Loxton Shoals-1, Evans Shoal-1, Lynedoch-1, Tuatara-1 and others. The primary reservoir rocks of these discoveries were Jurassic sandstone similar to many other North West Shelf discoveries. No exploratory wells had been drilled in Indonesian waters of this area up to that time. Line spacing of the available 2D seismic data within the Masela Block was roughly 10 km x 25 km. The Albian carbonate marker above the Jurassic sandstone level provided a readily mappable horizon on seismic data throughout the area, and this became the key horizon for our interpretation. Time mapping and simple depth conversion of the Albian marker identified two structural leads. One of these structures, located just west of the Masela Deep, was in a favourable position to trap hydrocarbons migrating from the Masela Deep with a closed area of 2 more than 1,000 km . Basin modeling suggested the generation and migration of hydrocarbons from the Masela Deep to this adjacent area.

This primary lead, named Abadi, was located in the central part of the block in water depths ranging from 400 m to 800 m. It was initially expected to hold hydrocarbon reserves of more than 2 TCF with an upside potential of 5 TCF in the Jurassic reservoir. Inpex submitted a bid to Pertamina to obtain the Masela Block on April 3, 1998, and the Masela PSC was awarded to Inpex on November 16, 1998.

Additional Evaluation after Signature of PSC

After signing the PSC, a 2,948 km 2D seismic survey (IM99 dataset) was conducted utilizing M/V Geco Rho (PT Geco-Prakla) in February to March 1999 to evaluate in detail the structure and stratigraphy of identified leads and to continue exploration over the entire block. The seismic program was designed to infill the existing GPARI-96 grid, reducing the blockwide line spacing to 2 km x 4 km. The GPARI-96 lines in the Masela Block area were also reprocessed to have seismic images consistent with the new acquisition. To develop a more detailed regional understanding, seismic data were exchanged with Shell, operator of the adjacent NT/P49 block in Australian waters just south of the Masela Block. A  pre-drilling well data trade was also agreed with Shell, and well data from Lynedoch-2 and Tyche -1 were added to our database. These data trades were conducted with MIGAS approval. Regional maps and prospect interpretations were updated and refined utilizing these data . The seismic time interpretation was converted to a depth map using a two-layer model to remove the water depth effect. The Abadi structure was defined as a drillable  prospect and the location of an exploration well was selected to test it. The Abadi prospect has three-way dip closure,  bounded to the east by the north-south trending eastward dipping normal fault that marks the western  boundary of the Masela Deep. The prospect was interpreted to be a paleo-high structure during the Mesozoic syn-rift phase, and structural closure was enhanced by compression in the Miocene collision  phase. The major risks were presence of effective reservoir rock within the Plover Formation and the  presence of source rocks in the Masela Deep. The risk related to the trap and the seal were considered to  be low.

Abadi-1

Abadi-1, the first exploration well in the Masela Block, was located 760 km east of Kupang, or 350 km north from Darwin, Australia. The drill ship Energy Searcher arrived on location on September 29, 2000, and Abadi-1 spudded in 457 m of water on October 1, 2000. The objective of the well was to test the hydrocarbon potential of sandstones in the Middle Jurassic Plover Formation. The well was drilled without a riser using seawater until reaching the 133/8” casing point at 2,139 m, in the uppermost part of the Wangarlu Formation. After Setting 9-5/8” casing at 3,795 m in the Echuca Shoals Formation, the objective Jurassic Plover sandstone was intersected and 18 m of conventional core was cut with 100% recovery. TD was called at 4,230 m, after ‘hot-shot’  palynological analyses confirmed a Bajocian age (Middle Jurassic) for the bottom sample and a VSP showed that Abadi-1 had penetrated the objective seismic reflections. Comprehensive wireline logging confirmed the presence of gas-filled sandstone in the upper part of the Plover Formation, with 73 m of gross hydrocarbon column including 21 m of extremely good quality reservoir . Though not readily recognizable from log data, the gas water contact was established at 3,900 mSS from MDT pressure measurements (Figure 4). One production test was conducted in the Plover Formation over the interval 3,867 m - 3,887 m. This flowed gas, condensate and water at the sustained rate o of 25 mmscfgpd, 260 bcpd (52.5  API) and 178 bwpd through a 40/64” choke. The water condensed from water vapour within the gas. The gas contained 9.6 % CO2 as determined by laboratory test. After completing the testing program, the well was plugged and abandoned as a gas and condensate discovery, and the rig was released on December 11, 2000. This success was the first Plover discovery in Indonesian waters. 3D Seismic Acquisition and Interpretation

The available map and drilling results indicated that the Abadi structure could hold significant gas reserves, hence exploration continued with plans for 3D seismic acquisition and processing. A 3D seismic survey of 2,060 km2  was conducted from July to September 2001, utilizing the PGS M/V Ramform Challenger with 8 streamers and a 4,200 m cable 2 length. A 700 km   “fast track area” was selected to

expedite interpretation over the main part of the Abadi structure and allow delineation drilling from st the 1 quarter of 2002. On-board processing was completed for this “fast track area,” and the migration cube was available for interpretation by the middle of September 2001. Processing for the full cube continued in the onshore processing center and was completed by the end of 2001. 3D seismic interpretation of the “fast track area” focused on determining optimal delineation well locations. A clearer image of the Abadi structure emerged as time interpretation maps were converted to depth, showing the structure to consist of northern and southern blocks separated by an east-west trending normal fault (Figure 5). Seismic facies analyses and paleogeographic modeling were conducted at the reservoir level, integrating the 3D seismic and Abadi-1 well data. Based on these interpretation results, two delineation well locations were selected. One was in the northern fault block, 13.5 km northeast of Abadi-1, and the other was in the southern fault block, 16.5 km southwest of Abadi1. The predicted depths of the Top Plover level at  both wells were estimated to be approximately 20 m deeper than that of Abadi-1 and approximately 50 m shallower than the indicated gas water contact. Abadi-2/2ST

Inpex began delineation drilling at Abadi-2, situated in 580 m of water, using the drill ship Energy Searcher. The rig arrived at the location on March 19, 2002, and spudded Abadi-2 on March 20, 2002.

sidetracked hole, a 6” hole was drilled to 3,986 m and then wireline logging was conducted. As no shows were detected in the bottom part of the section and  penetration into the lower reservoir section of the Plover Formation was confirmed by the PEX log suite, this depth (3,986 m) was selected as TD of Abadi2ST. The wireline logging data and MDT pressure plot confirmed the presence of the gas reservoir in the upper reservoir section of the Plover Formation (Figure 4). The gas pressure data of Abadi-2ST fall almost on the same trend line as Abadi-1 suggesting the gas to be in pressure equilibrium between Abadi-1 and Abadi-2ST. The pressure data of the possible water sandstone was obtained below the shale zone of the verrucosa  maximum flooding surface, which is described in the reservoir section below. The  pressure of this zone is slightly higher than the water  pressure in the Abadi-1, and if we assume this sandstone is in the same reservoir unit with the gas reservoir above the verrucosa  zone, the GWC is estimated to be approximately 3,890 mSS in Abadi2ST. After wireline logging was completed, one production test was carried out in the Plover Formation over the intervals of 3,829 m - 3,839 m and 3,845.5 m 3,850.5 m. The test flowed gas, condensate and water at the rate of 18.6 mmscfgpd, 150 bcpd (51 o  API) and 127 bwpd through a 44/64” choke. The gas contained 9.5 % CO2   by laboratory test, and 5 ppm H2S were detected in wellsite measurements. Abadi-3

The well reached the principle reservoir objective at 3,756 m, 113 m shallower than prognosis. Two conventional cores were cut from 3,783 m to 3,837 m (54 m) within the Plover Formation, though total recovery was only 9.6 m (17.8%). Intermediate check shot data revealed that interval velocities in the Abadi-2 area are much slower than estimated from seismic velocity analysis. This velocity difference contributed to the significant error in the depth  prediction. The well encountered a problem with 9-5/8” casing  just after cutting #2 core, which caused us to plug  back the original hole and sidetrack from 3,556 m as Abadi-2ST. Because of this casing trouble, no wireline logs were run for the coring interval of Abadi-2. After setting a 7” liner at 3,756 m in the

After completion of Abadi-2/2ST drilling, the rig was towed to the Abadi-3 location and the well was spudded on July 29, 2002. The well was situated in 423 m of water. Abadi-3 intersected the objective Jurassic Plover sandstone at 3,832 m, 19 m shallower than prognosis. Four conventional cores were cut within the upper  part of the Plover Formation, resulting in the recovery of 58.16 m of core. The 8-1/2” hole was drilled to 4,032 m, which was considered deep enough to evaluate the upper Plover reservoir section. The wireline logging data and MDT pressure plot confirmed the presence of gas in the upper reservoir section of the Plover Formation (Figure 4). The gas

 pressure data of Abadi-3 lies on exactly the same line as the gas pressure data of Abadi-1. These data confirm that the gas is in pressure equilibrium  between the two wells. The GWC was not observed on the wireline logs. No reliable formation pressure data was acquired in the water bearing zone below the gas column because of super-charging and/or sealfailure of the MDT tool. However, the highest interpreted water based on PEX log response is at 3,896 mSS. Given these results from Abadi-3 and pressure data from Abadi-1 and Abadi-2ST, the GWC appears to be different for each of these three wells. This difference may be due to hydrodynamic flow across the basin from the Timor Trough area as discussed by  Nowell (1999). Two production tests were conducted in the Plover Formation. The interval of 3,899 m - 3,906 m was  perforated for DST #1, which resulted in no flow due to a tight reservoir. DST #2 was conducted over the interval 3,855 m - 3,867 m and flowed gas and condensate through 44/64” choke at a rate of 13.8 mmscfgpd, 266 bcpd (51-56 o  API) and 57 bwpd. The gas contained 9.2 % CO2  by laboratory test, and 6.66  ppm H2S were detected in wellsite measurements.

southern block, by an east-west trending normal fault (Figure 5). Multiple conjugate fault sets cut the field,  but to date there is no clear evidence of fault compartmentalization. A structural spill point to the west separates Abadi field from the SunriseTroubadour accumulations updip. Movement along the bounding faults to the south and east juxtaposed the Plover reservoir succession against early Cretaceous shales of the Echuca Shoals Formation, providing the primary side-seal. Topseal is also provided by the regional Echuca Shoals Formation. Reservoir Stratigraphy

A high resolution, reservoir-scale stratigraphic framework has been developed drawing on evidence from wireline log data, conventional core, CST (sidewall core) and MSCT (rotary sidewall core) samples, biostratigraphy and ichnofacies. A total of 86 m of conventional core was cut in the 3 wells. Palynological zonation with an age resolution of  between one and four million years (Helby et al. 1987, and Laurie and Foster, 2001) provides the main chronostratigraphic framework for the correlation. Stratigraphic correlation of the 3 Abadi wells is shown in Figure 6.

PETROLEUM GEOLOGY Source and Migration

The source for the Abadi gas is postulated to be laterally equivalent marine shales deposited contemporaneous with the Plover Formation. Thermal maturity studies indicate such source rocks should be mature for gas in the Calder-Malita Grabens, Masela Deep, and directly down-dip from the Abadi field towards the Timor Trough. The TM Grains containing Oil Inclusion (GOI ) technique of Eadington et al (1996) did not detect oil inclusions on quartz grains in the gas filled sandstone (GOI values are less than 0.2%), indicating no liquid hydrocarbon migration before the gas trap. Trap and Seal

The Abadi structure is a paleo high that has been reactivated and modified by subsequent rifting both in the latest Jurassic/earliest Cretaceous and in the  Neogene. The Abadi field is fault bounded to the east, and divided into two parts, a northern block and a

The stratigraphic succession can be readily divided into a series of genetically related zones based on the  presence of bounding surfaces related to periods of rapid marine flooding. These have been named by reference to the palynological interval containing them. The deepest sediments penetrated in Abadi-1 appear to occur just above the   caddaense flooding event recognized within the Sunrise-Troubadour field (Seggie et al. 2000). The Zone 4 sediments overlying this comprise a 220 m thick succession of stacked,  progradational to aggradational deltaic sediments with limited reservoir potential encountered to date. This deltaic succession is capped by a thin shale associated with a flooding event, the verrucosa   maximum flooding surface (mfs). The Abadi gas reservoir confirmed so far lies above this maximum flooding surface. This lower deltaic unit, however, lies above the GWC in higher structural positions that remain undrilled. The verrucosa mfs is well defined on log, palynology and seismic data and is a key surface for mapping. Overlying this is an 80-100 m thick, laterally

 persistent coarsening upward succession comprising Zone 3. This zone passes from a lower interval of offshore shales with thin storm sands, into lower shoreface silts and fine sands and eventually into fine to medium grained, planar and cross-bedded, extensively bioturbated upper shoreface sands. On seismic data, this interval shows well-developed  progradational geometries that, together with the well data, are indicative of shoreline progradation (Figure 7). No evidence of emergence has been seen to date, though sedimentary structures and ichnofabrics indicate a strong tidal influence. Zone 3 sands were tested in the Abadi-2ST well and flowed at over 18 mmscfgpd. This zone is capped by a thin flooding event, the base  indotata mfs. The base   indotata  mfs is well defined by log,  palynological and seismic data and is another key surface for mapping. Overlying this is a 50 m thick, laterally persistent coarsening upward succession comprising Zone 2. This zone passes from a lower interval of offshore shales with thin storm sands, into lower shoreface silts and fine sands and eventually into fine to medium grained, planar and cross-bedded, extensively bioturbated upper shoreface sands. No evidence of emergence has been seen to date. Capping this zone are massive, extensively  bioturbated, predominantly medium grained sands that display a characteristically blocky log profile and excellent reservoir properties. Relict sedimentary structures and ichnofabrics indicate a strong tidal influence, and these sands are interpreted to have  been deposited in a tidal delta setting. These sands were tested in the Abadi-1 and 3 wells and flowed at rates of up to 25 mmscfgpd. The sediments of Zone 2 are capped by a thin flooding event, the base  aemula mfs. The base aemula mfs can be defined on log and  palynological data. Sediments of Zone 1, above the  base aemula mfs, comprise a thin succession of nonreservoir quality sands possibly representing deposits of an offshore bar in Abadi-1, whilst in Abadi-3 the interval comprises offshore marine shales. In Abadi2ST, Zone 1 sediments are absent due to erosion. Reservoir Quality

Reservoir quality within quartzarenite sandstones of the Plover Formation is dependant on a complex interaction of primary depositional setting and diagenesis. In the Abadi field the cleanest sands

 preserve excellent reservoir quality despite burial depths in excess of 3,400 m below the seabed. Although the burial depth of the Plover sandstone is nearly 1,500 m deeper than in the Sunrise-Troubadour field, the sandstones in the Abadi field still possess reservoir properties capable of commercial flow rates. A crossplot of porosity vs. permeability from core  plug data shows three clear trends indicative of different litho-facies within the Plover reservoir section of the Abadi field (F igure 8). Facies 1 shows high porosity and permeability values and corresponds to tidal delta deposits. The data  points for the sandstones of core #1 of Abadi-1 and core #1 of Abadi-3 fall in this facies group. The sandstones of core #1 in Abadi-1 comprise medium to coarse-grained quartzarenite with excellent visible  porosity (Figure 9). The framework grains are composed mostly of quartz (72-75 %), with very minor amounts of K-feldspar (