Nang Nuan Oil Field, Gulf of Thailand_ Karst Reservoirs of Metoric or Deep-burial Origin

Nang Nuan oil field, B6/27, Gulf of Thailand: karst reservoirs ofmeteoric or deep-burial origin?

Alan P. Heward1, Supamittra Chuenbunchom2, Gerard Makel2,3, David Marsland2 andLaurent Spring2,3

1Alan Heward & Assocs, Wallingford OX10 0SD, UKPresent address: Lasmo Plc, 101 Bishopsgate, London EC2M 3XH, UK (email: [email protected])

2Thai Shell Exploration and Production Co. Ltd, Bangkok, Thailand3Present address: Petroleum Development Oman LLC, Muscat, Sultanate of Oman

ABSTRACT: Karst reservoirs in the Chumphon Basin of the Gulf of Thailand haveproduced oil at well rates exceeding 10 000 BBL/d. Meteorically karstified buriedhills were recognized as a potential exploration play. The Nang Nuan discovery wellappeared to confirm such a play, and the concept prevailed despite the accumulationof contrary and unusual data. By the time a subsequent well had produced nearly4106 BBL oil, there was a desire to better understand the prospectivity of theconcession. The accumulated data indicate that the highs are probably syn-rift horstsand inversion features. Karst reservoirs occur in Ratburi carbonates, and Mesozoicand Tertiary clastics, apparently unrelated to subaerial exposure. The karstificationappears to be primarily of deep-burial origin, as indicated by the nature of the karst,substantial pore volumes that are dicult to account for, and temperature and flowanomalies consistent with active geothermal circulation. There are granites and hotsprings in the vicinity, and abundant CO2 in this and neighbouring basins. Suchdeep-burial karst reservoirs have dierent implications for reserves estimation,prospect ranking and well completions.

KEYWORDS: buried hill, karst, porosity, Ratburi, thermal anomaly

INTRODUCTION

This study of the geology of Pre-Tertiary penetrations in theChumphon Basin was undertaken to try to understand thefavourable, but unusual, production performance of the NangNuan-B01 well. As we dug into the records of other wells in thebasin, further conflicting and unusual data emerged. This casestudy is, in our opinion, an example of a good exploration playanalogue being retained too long as anomalous data began toaccumulate. It is also a typical example of how integrating datafrom dierent disciplines can result in a more robust geologicalinterpretation.

The Chumphon Basin is located at the western side ofGulf of Thailand (Fig. 1). The basin is one of a series ofTertiary half-graben whose origins can be linked to a regionaltranstensional regime induced by the Himalayan orogeny(Tapponier et al. 1986; Polachan et al. 1991; Bustin &Chonchawalit 1995). Onshore geology, well penetrations andseismic data indicate a quite complex Mesozoic history. Thedeepest parts of the Chumphon Basin lie in the west, close tothe bounding Klong Marui fault zone. The bulk of the basin fillconsists of Oligo-Miocene lacustrine and floodplain sediments.Thai Shell was awarded the B6/27 concession in 1985, seekinghydrocarbons within folded and faulted Tertiary clastics, and in

Pre-Tertiary karstified carbonates. Only the latter play has beensuccessful, in a modest way, producing to date 0.5106 BBL30API oil from Nang Nuan-A02, and 4.3106 BBL 42APIoil from Nang Nuan-B01 (see Appendix).

The buried hill exploration concept was based on prominentPre-Tertiary highs evident on early seismic, experience of asimilar play from China, and karst towers of Permian Ratburicarbonates, which are common features of Thai scenery (Figs 2& 3). Fracturing, seen in karstified Ratburi outcrops adjacent tothe Chumphon Basin, was thought likely to add to reservoirquality.

The Ratburi carbonates are 200300 m thick in the peninsu-lar Thailand and Chumphon Basin area. They are platformcarbonates in which depositional facies become more restrictedupwards, and dolomitization becomes increasingly common(Baird & Bosence 1993). The carbonates were extensivelystabilized and karstified during the Late Permian to EarlyTriassic, and further cemented, dolomitized and silicified duringMesozoic burial. Remaining matrix porosity is limited (

metres high, tens of metres wide, and often extend from oneside of a tower to the other (Fig. 4). There may be one or twocave levels due to uplift or subsidence, but we have notobserved pervasive multilevel developments occurring through-out a tower (cf. Fei Qi & Wang Xie-Pei 1984; Ford & Williams1989). Speleothems and some degree of sediment fill arenormally evident. Estimates of cave percentage versus the totalmass of a Ratburi tower, are less than 2% of the volume.

NANG NUAN-A01 (NNN-1)The first of Thai Shells wells in the basin, drilled in 1987, testeda prominent Pre-Tertiary high (Fig. 1). The well suered morethan 180 000 BBL of fluid losses into a highly permeable zone.There are wellsite descriptions of bit drops in this interval, butdrill speed and caliper logs, and the recovery of sidewallsamples, suggest a highly porous formation rather than largeopen cavities (Fig. 5). From sidewall samples and the logs, the

Fig. 1. Location map and cross-sections of the Chumphon Basin at the west of the Gulf of Thailand. NNN=Nang Nuan, SLA-1=Salika-1,KTA-1=Katiya-1. Ko Samui, Ko Phangan and Ko Tao are oshore islands north east of Surat Thani, and Khao Muang lies close to the oneof the hot spring locations marked south of Chumphon.

16 A. P. Heward et al.

reservoir was interpreted to be a karstified breccia overlyingthe Ratburi, analogous to an example cored in the onshorePhitsanulok Basin (Legendre et al. 1988). Porosity and resistivitylogs were dicult to evaluate, and only after oil stained sidewallsamples were recovered was the well tested, flowing dry 30APIoil. Permeabilities estimated from well test are 10002000 mD,with a dual or layered permeability character. A possibleoilwater contact was interpreted in the well at 3045 m ah(3016 m ss), towards the base of the porous interval. Thoughnot highlighted at the time, temperatures of 130140C werelogged over the upper part of the porous interval, 2030Chigher than adjacent formation temperatures (Fig. 5).

In view of Shells previous experience of karst reservoirsfrom the Amposta and Tarraco fields, oshore Spain (Seemanet al. 1990), the Nang Nuan discovery was developed initiallyusing a single-well early production system (see Appendix).This was to obtain production information and minimizefinancial risk. Further wells and facilities were planned to beadded once favourable long-term performance had been estab-

lished. The Nang Nuan-A01 well could not be used forproduction for mechanical reasons, and a replacement devel-opment well, Nang Nuan-A02, was drilled to a location some43 m east at reservoir level.

NANG NUAN-A02

Coring of the reservoir interval began as soon as a drilling breakand losses occurred. Three 9 m cores were cut, with recoveriesranging from 87% in the more consolidated parts to 40% in themost porous. The cores consist of bedded conglomerates withtwo oil-bearing intervals, an upper one a few metres thick anda lower one at least ten metres thick (Figs 6 & 7). There wasmuch controversy as to whether these deposits were a residualbreccia resting on karstified Raturi, as per the play concept, orwaterlaid alluvial fan conglomerates. They have, however, thesedimentary characteristics of (syn-rift) alluvial fan conglomer-ates, and recently have yielded PalaeoceneEarly Oligocenepalynomorphs. Clasts are sub-angular to sub-rounded, and

Fig. 2. Prominent irregular Pre-Tertiaryrelief apparent on seismic sections fromthe Chumphon Basin. This migratedsection is from the Nang Nuan area.The resemblance between thePre-Tertiary morphology and moderntower karstified Ratburi scenery isremarkable (cf. Fig. 3).

Fig. 3. Tropical tower karst southeastof Phitsanulok, central Thailand. ThePermian Ratburi platform carbonatesare thickly bedded, with highly irregulartops to the towers and widespreadvertical fluting (karren). The majordevelopment of caves occurs atfloodplain level.

Karst reservoirs, Gulf of Thailand 17

comprise about 70% Ratburi carbonates and 30% ?Mesozoicsandstones, in a calcareous sandstone matrix. The two porousand oil-stained karstified zones occur leached within the con-

glomerates. Controversy again surrounded the origin of thekarstification, Shell Research geologists favouring calcretizationand mixing zone diagenesis of alluvial fan conglomerates, and

Fig. 4. Solutionally enlarged beddingplanes and a typical floodplaincave/temple entrance at Wat ChoengPha Pa Rerai, southeast of Phitsanulok.There are no obvious abandoned higherfloodplain notches and caves aectingthe Ratburi towers in this area. Theremay be further cavernous porositybelow floodplain level, but this is likelyto be sediment filled.

Fig. 5. Composite log display for wellNang Nuan-A01 (also NNN-1). TheEarly Tertiary conglomerates wereoriginally interpreted to be a karstifiedbreccia overlying the Ratburi, theneighbouring Nang Nuan-A02 corerevealed their character (GR=gammaray, SWS=sidewall sample depths,DSL=drill speed log in minutes permetre, calf=caliper, displayed mirroredto depict borehole diameter in inches,FDC/CNL=density and neutronporosity logs, LL9D and LL9S=deepand shallow resistivity logs, DST=drillstem test, 150 refers to a seismicmarker). No open-hole logs wererecorded below 3035 m ah. The bolddashed line in the left-hand track is aportion of a temperature log, run aspart of the cased-hole logging suite, andshows a temperature anomaly oppositethe main oil productive interval. DSTs1 and 2 produced small volumes offresh formation water and sea water,and DST 3, 3400 BBL/d of 30API oil.The density of the formation water isunexpectedly light at 964 kg m3,corresponding to a gradient of 0.418 psift1.


Fig. 6. Early Tertiary conglomerates, Nang Nuan-A02, with part of the upper oil-stained karstified zone at the top (core segments c. 1 m longeach). Matrix and some clasts appear to have been leached within the karstified zone and remaining clasts fractured by compaction. Thetomographic scan at 2998.7 m of a whole core sample shows the leached rock to contain considerable cm-scale pore space (black). The 40bedding dip, evident to the right of the core photo, is a structural one.

Fig. 7. Lower part of the cored Early Tertiary conglomerates, Nang Nuan-A02 (core segments c. 1 m long each). Note the progressive alterationand increase in oil staining downwards towards the main karstified interval. White patches within the leached interval are vugs filled withanhydrite. Tomographic scans at 3016.9 and 3017.82 m of whole core samples show a vuggy porosity (black) on a cm-scale, the lower scan beingof core slightly deeper than that illustrated in the photo to the left. Whole core porosity measurements of leached material range from 9.916.8%.


Thai Shell sta, meteoric diagenesis of a residual breccia and theunderlying Ratburi carbonates. Neither of those origins appearswholly in keeping with the nature of the leaching and thediagenetic mineral assemblage present.

The karstification appears to be by aggressive dissolution ofclasts and matrix, with oil staining, pyritization and colourbleaching diminishing away from the leached intervals (Figs 6 &7). Diagenetic minerals within the porous zones include anunusual assemblage of anhydrite, pyrite, chalcedony, quartzand a kandite mineral, possibly dickite. Interpretations ofthe diagenetic sequence were influenced by the supposeddepositional origin, i.e. anhydrite was considered to be an earlyformed calcrete/playa mineral. Fluid inclusion and isotopeanalyses of the quartz and kandite cements (O and H) indicateprecipitation at around current burial depths and temperatures(135C), from low salinity fluids similar to the present forma-tion water. Hydrocarbon inclusions are also present within thequartz cements. The low salinity of the diagenetic fluids andpresent formation waters was attributed to meteoric rechargeaccompanying karstification.

Nang Nuan-A02 produced oil at rates of 20006000 BBL/dfor three months prior to water breakthrough. Following

breakthrough the well produced at uneconomic rates of1300 BBL oil/d irrespective of gross otake. This has variouslybeen interpreted as a function of matrix imbibition, or that thethinner upper leached interval continued to produce dry oilafter water breakthough in the thicker lower one. The 30APIoil had a gasoil ratio of 80 SCF BBL1, of which 30% of thegas was carbon dioxide. Hydrogen sulphide was present atlevels up to 140 ppm. There were minor equipment problemsduring production due to the oil being hotter than expected(142C reservoir temperature). Once water broke through,carbonate scale formed rapidly in surface pumps and meters,demonstrating a fluid saturated with carbonate at the conditionswithin these facilities. Reservoir pressures declined only slightlyduring the period of production, with indications of substantialaquifer support. Material balance calculations suggest con-nected pore volumes of hundreds of millions of barrels(2001200106 reservoir BBL). For various potential geo-graphical extents (1030 km2) and thicknesses of reservoirrock, this could correspond to porosities of 323%. Suchporosities are in line with those measured on whole coresamples (9.916.8%), and probably the most porous reservoirwas not recovered during coring.

Fig. 8. Penetration chart forPre-Tertiary wells in the ChumphonBasin showing the location of eectiveporous intervals in relation to the BaseTertiary unconformity. Karstificationaects various carbonate and clasticformations from 43 m above, to 420 mbelow, the unconformity. InterestinglyTriassic marls and limestones have notbeen karstified in any of thepenetrations to date. Total losses ofdrilling fluids (large arrows) areinvariably associated with proximity to amajor porous interval. The locations oftemperature anomalies are alsoindicated.


SALIKA-1 (SLA-1), KATIYA-1 (KTA-1) ANDNANG NUAN-A03

Three other wells were drilled on Pre-Tertiary highs in theperiod 19871991, Salika-1, Katiya-1 and Nang Nuan-A03(Fig. 1). Salika-1 penetrated a thick sequence of Triassic marlsand limestones overlying Ratburi carbonates. Losses of drillingfluids began a few metres into the Ratburi and total lossesoccurred in a porous vuggy zone 130 m below top Ratburi and420 m below the Base Tertiary unconformity (Figs 8 & 9).Caliper and dipmeter data indicate that the borehole is washedout on a metre-scale, and that the washouts/vugs are approxi-mately stratiform (i.e. parallel bedding). The Ratburi intervalwas tested and flowed 3900 BBL/d sea and formation water.Spot cores of Ratburi carbonates showed matrix porosities ofless than 4.5% and permeabilities of less than 11 mD. Thepartial losses near the top of the Ratburi may indicate a lesswell-developed leached interval is present, or that limitedfracture permeability occurs.

Katiya-1 encountered thick calcareous, fluvial sandstones, ofprobable Mesozoic age, overlying Triassic marls and limestones(Fig. 8). The well reached total depth in Carboniferous quartz-ites, with Ratburi carbonates apparently absent. Drilling lossesoccurred within the Mesozoic sandstones at the top of a

brecciated decalcified sandstone, and total losses occurred alittle deeper. A series of cavities are present within what appearto have been calcareous sandstones, or interbedded carbonates,carbonate-rich conglomerates or evaporites. These porousintervals were again water bearing, and the main interval flowedhot water during logging (20C hotter than the surroundingformations). Dipmeter data from this well indicate that theseporous intervals are again sponge-like and stratiform. A seriesof short cores of the Mesozoic sandstones yielded porosities of515% and permeabilities of less than 3 mD. Curious vugspresent in a core of the brecciated decalcified sandstones from15 m above the main porous interval, are similar to ones thatoccur occasionally in sandstone clasts in the Nang Nuan-A02core. These vugs are irregular, sinuous, cm-long and mm-high,and the Katiya ones examined in thin section, lack any latercement fill.

Nang Nuan-A03 was drilled as an appraisal well to adownhole location 270 m east and up-dip from watered-outproducer Nang Nuan-A02. It was targeted using 1990,state-of-the-art, 3D seismic amplitude techniques, includingcalibration from the Amposta Field. Even though themost porous formation appeared to lie to the west of NangNuan-A02, the well was located in what appeared to befavourable seismic amplitudes. The well came in deep toprognosis and failed to encounter leached Early Tertiaryconglomerates. It penetrated tight Ratburi carbonates and hasthe unfortunate distinction of being the only Pre-Tertiarypenetration in the Chumphon Basin not to have encounteredeective porosity or suered losses of drilling fluids (Fig. 8).A red silty interval and underlying carbonate breccia some40 m beneath the Base Tertiary unconformity may indicatesediment-filled karst of undetermined age.

Following the disappointments of Nang Nuan-A appraisaland production performance, and the results of other Pre-Tertiary exploration wells, there was considerable pessimism asto the remaining hydrocarbon potential of the ChumphonBasin. Particularly given the typical size of the Pre-Tertiaryhighs still to be drilled and the range of karst porosities appliedin evaluating the volumes they might contain (0.52.5%, Weber& Bakker 1981;

water pressures results in possible oilwater contacts deter-mined from fluid gradients being up to several hundred metresdeeper than the spillpoint.

The well was completed with a pre-perforated, non-cemented liner, in an irregularly washed-out hole. Onproduction test it flowed 42API oil, with a gasoil ratio of200 SCF BBL1. Twenty-five percent of the gas was carbondioxide, and hydrogen sulphide was detected in concentrationsup to 800 ppm. Pressure surveys acquired during the test, andsubsequently, are anomalous in showing increasing flowingtubing head pressures during flowing periods, and decreasingones during shut-in periods. The opposite of what one wouldnormally expect. A production log (PLT) was also interpretedto show an unusual reservoir situation at virgin reservoirconditions. When the well was flowing, almost all the inflowappeared to come from the uppermost porous zones, and whenthe well was shut in, cross-flow was occurring between theporous zones in the well (Fig. 11). Temperature anomalies(+40C) were present opposite the most productive zone andat the well total depth (TD). The reservoir was described ashighly unconventional, but unfortunately the PLT interpret-ation was filed with no proposal to verify its observations orcomment as to its possible implications. Estimates of reservoirproductivity (permeability thickness) from well test data arecomplicated by the diculty of interpreting pressure build-upsin what appears to be a continuously cross-flowing well, incombination with the problem of determining the thickness ofthe productive intervals. Permeability estimates are from a few

tens of millidarcies to a few darcies only. The prevailinganalogue concept was still one of meteoric tower karstifiedRatburi, with floodplain/sea-level caves providing the reservoirvolumes. It was anticipated that Nang Nuan-B01 would waterout in a relatively limited time as with Nang Nuan-A02.

The well was shut-in for a year before a rented earlyproduction system could be organized (see Appendix). Duringthat time, and subsequently when the storage/shuttle tankerwas unloading, it appears that the upper hot zone had cross-flowed many thousands of barrels of oil into the lower,relatively cooler, zones. When producing, the well flowed oil atrates of up to 10 000 BBL/d for several years. By the time ithad produced around 4106 BBL oil and alone was making asignificant contribution to Thai Shells equity production, therewas renewed interest in Nang Nuan, undrilled prospects withinthe concession and quite what these reservoirs were really like.

Just as study work began in earnest, water breakthroughoccurred in January 1997. There were no facilities for handlingwet production and sales volumes had to have a water contentof less than 2%. This period of declining cyclic productionfrom the well, before it became uneconomic, allowed theacquisition of much new data. More PLT data were acquiredwhich repeated earlier anomalous flow patterns. Although thetemperature dierences through the reservoir were by this timea few degrees only, the overall fingerprint was the same. Therewere problems during the main production period of the welldue to the oil being hotter than anticipated, and the somewhatpiecemeal approach to facilities that had been adopted in

Fig. 10. Composite log display for wellNang Nuan-B01 (GR=gamma ray,DSL=drill speed log in minutes permetre, cal=caliper, displayed mirroredto depict borehole diameter in inches,RHOB/NPHI=density and neutronporosity logs, LL9D and LL9S=deepand shallow resistivity logs, DST=drillstem test, 150 refers to a seismicmarker). No coring or sidewall sampling(SWS) was attempted.


expectation of a short lifetime. Anomalous water gradientsmeasured in the well of 0.41 psi ft1 (i.e. lighter than freshwater) went unappreciated, as they had in Nang Nuan-A01.Only after the water was sampled downhole was it found to bealmost saturated with gas, having a gaswater ratio of 23 SCFBBL1 (76% CO2 and 22% CH4). The pressure declineaccompanying production was limited (100400 psi) andmaterial balance calculations indicate connected porevolumes of hundreds of millions to billions of barrels (7002000106 BBL). Such volumes are impossible to accommo-date within the structure applying conventional karst porosities,and there is no other known aquifer. Underlying Carboniferousquartzites are tight, as are adjacent Mesozoic or Tertiary strataat such depths. Earth-tide eects evident on recent pressurebuild-ups using quartz gauges, also indicate substantialconnected pore volumes.

METEORIC TROPICAL TOWER KARST?

These reservoirs were interpreted as meteorically karstifiedburied hills, where typical karst porosity values led to apessimistic view of prospectivity of the concession. But increas-ingly with time, data did not fit a meteorically karst analogue(Fig. 12). The karstification occurs in three dierent formations,two of them essentially clastic! (Ratburi carbonates, ?Mesozoiccalcareous sandstones and syn-rift Early Tertiary con-glomerates). Karstification does not appear to relate to LateCretaceous and Early Tertiary weathering and erosion. Rather,porous zones occur from immediately above or below the baseTertiary unconformity, to at least 420 m deeper, in a basinwhere the buried hills typically have a relief of less than 250 m(Fig. 8). In the cored karst zones of Nang Nuan-A02 and thebrecciated decalcified sandstone of Katiya-1, there is no evi-dence of speleothems, terra rosa, or siliciclastic cave sediments.Rather the cores from Nang Nuan-A02 show leaching andpyritization diminishing away from the karst zones, and amineral assemblage which indicates at least quartz and kandite

cements formed under deep-burial conditions. Carbon dioxideis present dissolved in the oil and the formation waters.Hydrogen sulphide also occurs in low concentrations in the oil.

Fig. 11. Caliper logs indicatingborehole shape, and production loggingtool measurements and interpretationsfrom the production test of August1992. When the well is flowing the bulkof the fluids originate from the top20 m of the Ratburi (75 and 25%),when the well is shut in, cross-flowoccurs between washed out/porouslayers. Borehole-image logs run in thiswell (FMS and FMI) also indicate oilcross-flowing down the well from about3123 m to 3200 m ah. PLT logs run in1997 show a similar 75:25%contribution to flow when only oil isbeing produced at the start of a cycle,and a 55:45% contribution when wetproduction (oil and water) is takingplace. When the well is shut-in,cross-flow continues to occur, in amanner consistent with that illustrated,though details are dicult to resolve.Prominent 40C temperature anomaliesare present associated with the mainproductive interval at the top of theRatburi and tantalizingly towards thewell TD. It is remarkable that thesecontrasts can be maintained over shortvertical distances. A similar temperaturefingerprint, albeit with a smaller rangewas recorded by PLT logs run in 1997.

Fig. 12. Contrasting analogues for the origin of karstification inthe Chumphon Basin. See text for explanation of data that do notfit the original meteoric buried hill analogue.


Logs and sidewall samples from Nang Nuan-A03 may showevidence of sediment-filled karst of Permianearly Tertiary age.

The irregular topography of the Pre-Tertiary on seismic hasfrequently been considered a reflection of its karstifiedcharacter and compared to the appearance of Ratburi outcrops.The resemblance is superficially remarkable (Figs 2 & 3).However, prospect-scale seismic interpretations typically resultin complexly faulted structures, and well data indicate that thehighs can contain a variety of Ratburi, Triassic, Mesozoic andEarly Tertiary strata. Structures mapped from the three 3Dsurveys acquired in the concession, are now interpreted assyn-rift horsts and inversion structures rather than passivelyburied Pre-Tertiary hills.

Petroleum engineering data indicate large active aquifers, andconnected pore volumes of hundreds of millions to billions ofbarrels, volumes that exceed the capacity of the structures ifconventional karst porosities are applied. Such volumes are alsoorders of magnitude greater than those we estimate can becontained in the caves of modern tower karst which we haveexamined in Thailand (for example, the 132.5106 m3 PhaThap Pon outlier near Phitsanulok, in central Thailand, has anestimated cavernous porosity of 1.72.1106 m3 or 1.31.6%,which could contain only some 1015106 BBL of fluids).Permeabilities for the subsurface karst systems are only modest,252000 mD, not those of large open cave systems. Productionlogs from Nang Nuan-B01 show anomalous inflow andcross-flow behaviour between zones even under initial reservoirconditions. Logs run several years later indicate similar flowpatterns. Pressure surveys show unusual trends during flowingand shut-in periods which seem likely to be a response tochanging conditions of inflow and cross-flow. Most of themajor oil- and water-bearing karstified zones encountered inthe Chumphon Basin are associated with positive temperatureanomalies (2040C).

Many of the features described above are not those we wouldtypically expect to be associated with meteoric karst andmeteorically karstified reservoirs (e.g. Esteban & Klappa 1983;Bosak et al. 1989; Palmer 1991; Budd et al. 1995).

DEEP-BURIAL OR HYDROTHERMAL KARST?

All of these features can, however, be accommodated withinthe concept of deep-burial or hydrothermal karstification.

Common criteria for recognizing such karst include:(1) the presence of hot fluids;(2) little apparent genetic relation to exposure surfaces;(3) 3D maze, ramiform or sponge-like form of cavities,

sometimes with convection cupolas;(4) porosities of 2530% over areas of hundreds of m2 to a

few km2;(5) porous, altered or mineralized zones around the main

conduits;(6) exotic high-temperature minerals within cavities;(7) lack of normal speleothems and cave sediments

(Bakalowicz et al. 1987; Ford & Williams 1989; Palmer1995).

In the context of the Chumphon Basin, such an originseems likely to involve hot, low salinity, fluids being pumpedperiodically out of overpressured lacustrine-basin depocentres(Cartwright 1994). Such fluids migrate via major deep-rooted faults probably during periods of tectonism, and causeextensive leaching in carbonate-rich highs and flanking clastics(Fig. 12). The leaching and current presence of hydrocarbonsappear a likely precursor to future mineralization (Dzulynski &Sass-Gustkiewicz 1989; Fowler & Anderson 1991). The

character of the leaching seen in Chumphon Basin cores andthe somewhat unusual mineral assemblage present are entirelycompatible with a hydrothermal karst origin (Bakalowicz et al.1987; Hill 1990; Hulen et al. 1994). The anomalous tempera-tures and fluid movements indicate that these reservoirs arepart of extensive, low temperature, convecting geothermalsystems (Grant et al. 1982). Temperature anomalies of 2040C,in an area with geothermal gradients typically of 45C/100 m,would imply fluids circulating over hundreds of metres verti-cally. The dierent oils present in Nang Nuan A and B (whichpossibly reflect derivation from a similar source rock at dierentlevels of maturity, R. Archer pers. comm; Appendix), anddierent initial pressures and temperatures, indicate that theseaccumulations are part of separate fluid migration systems.

Similar phenomena to those described here have beendocumented from small oil fields in Nevada (Hulen et al. 1994),and from the Yinggehai Basin area of the South China Sea (Haoet al. 1998). A hydrothermal karst overprint is also nowconsidered important in the Casablanca and Amposta fields,oshore Spain (M. Esteban pers. comm.).

There are supportive indications that a deep-burial or hydro-thermal karst analogue is applicable. Regionally there areTriassicCretaceous granites at either margin of the basinforming the oshore islands of Ko Samui, Ko Phangan and KoTao, and extensive outcrops in peninsular Thailand (Fig. 1;Nakinbodee et al. 1985). There are hot springs associated withfaults at the western onshore margin of the basin, accompany-ing thick veins of calcite spar (e.g. at Khao Muang), andabundant CO2 present in gas and oil fields in the nearby Pattaniand NW Malay basins (Fig. 1; Lian & Bradley 1986; Duval &Gouadain 1994). The CO2 in such fields is partly of deep-seatedorigin, from its isotopic character, and occurs preferentially inproximity to major faults.

The Wind and Jewel caves of South Dakota are welldocumented examples of 3D maze caves of probable thermaldissolution origin (Fig. 13). There are many similar cave systemsin the Black Hills area, sometimes occurring in close proximity,yet not connected (e.g. the Jewel and Jasper caves). The Windand Jewel caves have average porosities of 1030% over areasof 110 km2. Jewel Cave presently has 165 km of mappedpassages beneath an area presently of some 310 km2,and Wind Cave 120 km passages in presently some 1.3 km2

(Bakalowicz et al. 1987). Barometric evidence indicates substan-tial connected pore volumes of around 57106 m3 for WindCave and 140106 m3 for Jewel Cave, indicating that only asmall percentage of each system is presently known. Theformation of such 3D networks requires the distribution ofsolutional capacity rather uniformly over areas of several squarekilometres. There is evidence of changing solutional chemistry,with both limestone and dolomite being dissolved at times,dolomite being dissolved preferentially at others, and calcitebeing deposited at others. These maze cave networks arealigned along joint directions and occur at a number of levelswithin the Carboniferous Madison limestones. They formcomplexly interconnected solutional galleries, with a tendencyto descend in stratigraphic elevation in the direction of strataldip. Jewel Cave has extensive wall coatings of calcite spar whilstWind Cave has its characteristic boxwork (protruding networkof veins due to dissolution of carbonate matrix). Calcites fromwall coatings and boxwork have carbon and oxygen isotoperatios typical of burial/hydrothermal conditions (Bakalowiczet al. 1987).

There is great similarity, superficially at least, between theDakota examples and certain types of thermal karst systemdescribed from the Buda Mountains area of Hungary (Jakucs1977; Dublyansky 1995).


IMPLICATIONSA deep-burial or hydrothermal karst interpretation for theorigin of reservoirs in the Chumphon Basin has implications.Porosities can be higher than conventional meteoric karst ones,at least over areas of a few square kilometres. The highestporosities measured from the Nang Nuan A02 cores were 17%,and net/gross values vary between 49% in Nang Nuan-1, 16%in -A02, 12.5% in -B01, and 0% in -A03. Such porosity (5%)and net/gross ranges can accommodate within reasonablereservoir configurations, the substantial connected porevolumes indicated by material balance.

Fluid contacts are often dicult to define in karst reservoirsdue to openhole logs being aected by washouts, lost fluids andtight formation, and fluid gradients may be dicult to obtain orambiguous. With deep-burial karst, fluid contacts may bear norelation to structural spill points at the top of a reservoirformation, and potentially lie much deeper within the forma-tion. Both these factors, increased porosity and a greaterrange of possible hydrocarbon column, positively influence theestimates of hydrocarbons in-place that can be made.

On the counter side, production log evidence indicates thatonly the uppermost zones in Nang Nuan-B01 have produced

hydrocarbons, whilst other zones accept fluids when the well isshut-in. It is not clear what the original fluid content of theselatter zones was. The temperature distribution and cross-flowbehaviour seem most reasonably to indicate circulating fluidsextending some distance from the wellbore. The Wind andJewel cave systems, with their extensive and irregularly con-nected flow conduits, and sometimes rather limited connec-tions between areas of porosity development, may be a usefulanalogue here. However, evidence of poor connectivity in thiscase has negative implications for sweep eciency, which isusually assumed to be high in karst reservoirs.

Due to the inability to cement casing though karst reservoirswith extensive losses, typical well completions consist of slottedor pre-drilled liners or barefoots. With the multizone flow behav-iour exhibited by Nang Nuan-B01, and possibly also by Katiya-1,each interval of a multizone reservoir preferably requires testing,and possibly production, individually. Which intervals have con-tributed to production in Nang Nuan-B01 remains uncertain.External casing packers may be able to provide a greater degreeof zonal isolation for future multizone wells.

Applying a deep-burial karst analogue, remaining highswithin the concession have a dierent ranking and risking.

Fig. 13. Examples of deep-burial or hydrothermal karst systems. (A) 3D rectilinear maze cave showing in plan view a strong control by NWSEorientated joints, and in section a strong stratal influence on the series of interconnected levels that formed towards the middle of theCarboniferous (Madison) limestone. Areas of intense development of cavities tend to be separated by areas with only few connections, often faultzones where gouge has limited fluid flow and karstification (from Bakalowicz et al. 1987; Palmer 1995). Nang Nuan karstification may be of thistype and intensity, though whether the cavities formed are of cave size, or smaller, remains to be established. (B) Convection cupula-form ofSatorkupuszta cave, developed in Triassic carbonates of the Pilis Mountains, Hungary (Jakucs 1977). A chain of spherical cavities about 350 mlong in total, as known, the great hall of which contains thick gypsum accumulations. (C) Dissolution, brecciation and mineralization occurringalong the contact of the Triassic ore-bearing dolomite and limestone, Upper Silesia, Poland (Dzulynski & Sass-Gustkiewicz 1989); (ii) is moremature than (i), with collapse breccias. The lengths of arrows correspond to flow intensity. The brecciated decalcified ?Mesozoic sandstonesoverlying the main karst zone of Katiya-1 may be of such form.


Criteria of importance now become proximity to source rockkitchens and to major deep-rooted faults, likelihood ofcarbonate-rich formations, and the presence of a good top andlateral seals to retain hot fluids and allow them to circulate(Fig. 12). Leaching may not be confined to highs and may occurin any favourably located formation (e.g. syn-rift clastics).

Overall there is a more positive view of value of concession,due to the increased oil-in-place estimates and the dierentprospect risks. If the analogues of the South Dakota orHungarian karst are in any way appropriate, then such karstifi-cation may be widespread throughout the Chumphon Basinarea. The size of the hydrocarbon accumulations discovered aresmall compared to the connected pore volumes indicated bymaterial balance. If one or more such pore volumes have agreater degree of hydrocarbon fill, then the size of possiblehydrocarbon accumulations could be an order of magnitudelarger.

A deep-burial interpretation results in perhaps even greateruncertainty about how these karst reservoirs really work.Unfortunately, even with the latest 1997 high-resolution 3Dseismic, the porous leached intervals cannot be imaged atdepths of 30004000 m. A semiquantitative sparse-spikinginversion of part of the Nang Nuan 3D data, gives hints of lowacoustic impedance zones within seismic facies attributed toRatburi carbonates. The properties of the productive intervalsare poorly known, and more focused diagenetic work could beundertaken on the Chumphon Basin core material. The degreeof connectivity between porous zones is uncertain and may beable to be established by testing in future wells. A majoruncertainty is the origin of the heat fluxes, and where fluids areflowing to? Do they form part of closed-loop convection cells,and does water circulate below the oilwater contact and causeoil to circulate above? Nang Nuan-A01 seems to indicate hotoil moving above relatively cooler water (Fig. 5). Is the hot oila very recent/active charge? If so, what about the hot water ofKatiya? Material balance calculations are based on the expan-sion of fluids under isothermal conditions, and the impactof the hydrothermal nature of the reservoir has not beenquantified. Can fluid displacements in such thermally drivenreservoirs be represented by traditional oilwater displacementprinciples? Are the aquifers just parts of very extensive karstsystems, or is there some other mechanism of pressure supportfrom overpressured source kitchens?

Despite the many uncertainties, there are lessons here incontinuing to apply an analogue in the face of mountingconflicting data, and potentially underestimating (or over-estimating) the value of a prospect, a field, a concession, or aplay fairway. We doubt very much that the karst reservoirs ofNang Nuan and the Chumphon Basin are unique!

We are grateful to Martin Bradshaw and the management of ThaiShell Exploration and Production for their encouragement of thisstudy and for permission to publish its results. We have drawnliberally on the work of many former Thai Shell and ShellResearch sta, Jean-Yves Chatellier, Steve Coutts, MarianneGoesten, Fer Guit and Michael Hauck. They have all providedimportant insights to these intriguing reservoirs, though they maynot share our conclusions. Ray Buchanan, Mateu Esteban andJohn Kantorowicz kindly read a draft of the paper and madehelpful suggestions for its improvement. Lasmo plc. contributed tothe costs of colour printing.

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Appendix Nang Nuan Field data summary

Nang Nuan A Nang Nuan B

TrapType Conglomerates onlapping horst HorstDepth to crest (m ss) 2960 3090Free water level (m ss) c. 3016 3181>3750Oil column (m) c. 56 c. 90 few hundredPay zoneFormation Early Tertiary conglomerates Mid Permian Ratburi carbonatesGross thickness (m) 65 >188Net/gross ratio (%) c. 40 c. 12.5Karst porosity (%) c. 17 c. 17Karst permeability (well test) (mD) 10002000 25500Productivity index (BBL oil/d/psi) c. 60 c. 752000Matrix porosity (%) 5 12Matrix permeability Negligible NegligibleHydrocarbonsOil density (kg m3) [API] 870887 [2931] 817.5 [41.5]GOR (SCF STB1) 6080 200240In situ viscosity (cP) 3.54 0.62Wax content (%) 12 12Pour point (C) 27 21H2S content (ppm) 140 120800CO2 content (mol %) 30 2027Formation waterSalinity (ppm NaCl) c. 5500 25003000Density (kg m3) 964 948GWR (SCF STB1) Not measured 23 (76% CO2, 22% CH4)Reservoir conditionsTemperature (C) 142 165.5Datum (m ss) 3016 3150Pressure (psia) 4246 4638Bubble point (psia) 635665 1250Pressure gradient (psi ft1)

Oil 0.353 0.312Water 0.418 0.407

Field sizeArea (km2) 1.3 7.2STOIIP (106 BBL) 29.5 47.5Formation volume factor 1.112 1.207Drive mechanism Strong aquifer support Strong aquifer supportRecovery factor (%) 21 60Reserves (106 BBL) 6 28.5Remaining reserves (106 BBL) 5.5 24ProductionOil production (106 BBL) 0.48 (Jan.Aug. 1988) 4.31 (Oct. 1993Aug. 1997)Losses of drilling fluids (106 BBL) Nang Nuan-A01: 0.18 Nang Nuan-B01: 0.35

Nang Nuan-A02: 0.06Water depth (m) 32.5 34.5Development scheme Single well, subsea, rented early production system,

consisting of production jack-up,storage tanker and shuttle tankers

Single well, wellhead tripod, rented early production system,consisting of production spread on a

supply boat and a tanker for storage and evacuation

Received 1 March 1999; revised typescript accepted 15 July 1999.


Documents

Nang Nuan Oil Field, Gulf of Thailand_ Karst Reservoirs of Metoric or Deep-burial Origin