ABSTRACT

The Badin Blocks operated by BP Pakistan have provedreserves of around 100MMboe, which are currentlybeing produced from over 50 fields. Over 60% of thereserves and production is from the Lower GoruFormation Upper Sand (Early Cretaceous in age), ofwhich the informally named B Sand (part of the UpperSand, Lower Goru Formation) is the most importantreservoir unit. Traditionally the B Sand has beencorrelated as a lithostratigraphic unit with a sheet likedistribution over the area. In places the B Sand hasbeen subdivided into distinctive lower and upper units,but these are not developed everywhere and the controlson the development of these units was not fullyunderstood.

A reservoir scale sequence stratigraphic review of theB Sand, based on the integration of core, log and reservoirdata, has provided a framework within which thedepositional evolution of the B Sand can be explained. The model further highlights implications for fielddevelopment, particularly reservoir connectivity andthe recognition and prediction of persistent stratigraphicbarriers, and future exploitation of the Badin Area.

INTRODUCTION

BP Pakistan operates four separate concessions in theBadin area of the southern Sindh Province of Pakistan.The concession area covers approximately 2,050 squaremiles. Exploration started in 1977. To the end of 2003,134 exploration wells have been drilled, making 57discoveries (Fig. 1).

Oil and gas accumulations occur in a complex of EarlyCretaceous sandstone reservoirs within the Lower GoruFormation. From top to bottom these are informallytermed the Upper, Middle and Basal Sands and are

Aptian-Albian (possibly earliest Cenomanian) in age(Fig. 2). Of these reservoirs, the Upper Sand iseconomically the most important, hosting 44 discoveries. The Upper Sand itself is further informally dividedinto three distinct sand units, separated by two regionallyextensive mudstones believed to have been depositedduring basin margin flooding events. The middle ofthese sand units is termed the B Sand. It is a high orderprogradational/ retrogradational cycle 4, 6 boundedbelow by the Badin Shale and above by the Turk Shale.It is the most prolific of the Upper Sand reservoirs.

Historically the B Sand has been correlated as alithostratigraphic unit across the area. Whilst it wasinterpreted to have a sheet like distribution, it was alsoclear that there are significant regional variations in thecharacter and thickness of the B Sand (Fig. 3). The BSand, where complete, varies in thickness from around40 to > 300 feet. Local (field wide) stratigraphiccompartmentalization was also observed in a numberof fields. None of these phenomena could besatisfactorily explained by existing geological models. Detailed studies of core material, integration withwireline log, production and pressure data have provideda sequence stratigraphic model to describe the evolutionof the B Sand, explain the regional variations in characterand development and predict the presence of barriersand stratigraphic compartmentalization on a local (fieldwide) scale.

REGIONAL SETTING

The regional tectonic setting for the Badin area is brieflydescribed by [1]. During the Cretaceous Pakistan andIndia was part of a northward drifting passive margin,spreading away from the Arabian/African Plate. In thelatest Jurassic to Early Cretaceous the Badin area waspart of this passive margin (the Thar Platform).

179

SEQUENCE STRATIGRAPHY OF THE B SAND (UPPER SAND, LOWERGORU FORMATION) IN THE BADIN AREA: IMPLICATIONS FORDEVELOPMENT AND EXPLOITATION

Chris Ebdon, M. Wasimuddin, Arif H. Malik and Shakeel Akhter

BP Pakistan Exploration & Production Inc., Islamabad, Pakistan.

Throughout this time a thick, overall shallowing-upwardsclastic sequence was deposited during a series of higherorder transgressive/regressive cycles 4. Towards theend of the Albian, at the time of B Sand deposition, awell-established shallow marine shelf was developed.The shoreline was located to the east and south east ofthe Badin area and deeper water facies towards thenorthwest. In latest Albian to earliest Cenomanian timesa phase of extensional faulting enhanced or imposed thenorthwest/southeast tectonic grain which provides thetilted fault block structures of the Badin Rift, terminateddeposition of shelf sands through rapid deepening andresulted in the development of the regionally extensiveTop Lower Goru Unconformity. The Upper GoruFormation mudstones, deposited in deep water, providea seal to the tilted fault block sandstone plays of theLower Goru Formation.

METHODOLGY

The principles of sequence stratigraphy 2, 5 requirethe recognition of genetically related beds or bed sets(parasequences and parasequence sets) and the keystratigraphic surfaces which bound these geneticallyrelated packages 8, 9, 7, 6. The methodology followedin this study starts with the analysis of available corematerial, which allows the high order identification ofindividual depositional events, sedimentary structures,bioturbation, trace fossils and interpretation ofdepositional facies. From successions of facies,parasequences and parasequence sets are recognizedand vertical trends in the range of water depths presentin successive parasequences are used to identify stackingpatterns (Fig. 4). These can also be calibrated with,and interpreted from, wireline log data.

Key stratigraphic surfaces are also identified in core.Candidate sequence boundaries are identified based onone or more of the following criteria: clearly definederosional truncation, direct evidence of sub aerialexposure, or abrupt basinward shifts of facies. Likewise,potential condensed sections should be recognizedbased on unusual burrowed surfaces, abundantdiagenetic minerals, fossil concentrations, closely spacedbentonite beds, or radioactive shales. Condensed sectionsmay, but do not necessarily, lie along the maximumflooding surface.

During the course of this study, cores from ten B Sandwells have been examined and core photographs froma further eight reviewed. Wireline log data from inexcess of another 100 wells have also been reviewedand incorporated. From the recognition of parasequencesets and key stratigraphic surfaces in cores and onwireline log data the B Sand can broadly be divided

into systems tracts, which have been correlated andmapped over the Badin area.

B SAND SEQUENCE STRATIGRAPHICEVOLUTION: KEY SURFACES AND SYSTEMSTRACTS

The Badin Shale Maximum Flooding Surface

The Badin Shale, where not truncated or removed bythe top Lower Goru unconformity, is well developedover the Badin area, varying in thickness from 50 to>120 feet. The unit is characteristically developed asa mudstone facies. Core material is available from theBadin Shale in only one well, Duphri 2, where it isrepresented by finely laminated, occasionally bioturbatedmudstones interpreted as offshore marine mudstonesdeposited well below storm wave base. The maximumflooding surface itself, however, is not penetrated bycore. On wireline log data a maximum flooding surfaceis interpreted within the Badin Shale at a gammamaximum (e.g. Zaur 9; Fig. 5, Duphri 2; (Fig. 6). Tangri8; Fig. 7) which represents concentration of radioactiveminerals during sediment starvation. More calcareousrich horizons are also associated with this event, whichare consistent with a reduction of clastic depositionalrate. Occasionally it can be difficult to confidentlypick the exact location of the maximum flooding surfacewithin a 10-20 feet thick ‘zone of maximum flooding’. However, on logs an overall fining upwards trend isrecognized below the maximum flooding surface (orzone of maximum flooding), and at a finer scaleretrogradational parasequence sets can be interpreted(e.g. Tangri 8, (Fig. 7), from 7500 feet to MFS). Abovethe maximum flooding surface there is an overallcoarsening upwards profile and stacked progradationalparasequence sets can be interpreted (e.g. Zaur 9,Fig. 5).

The B Sand Highstand Systems Tract (HST)

The HST represents a period of shelf outbuilding andprogradation during a period of relatively stable sea-level following the maximum flooding surface (Fig.4). In the Badin area core is available from the HSTin a number of wells. The best coverage is from wellZaur 9 (Fig. 5), which is selected as the type well.Here one 60 foot core is available, the majority ofwhich is assigned to the HST. The sandstonesencountered are variably argillaceous, locally bioclasticand are typically fine grained and well sorted and forma subtle coarsening upwards succession. Bioclasticmaterial is common in the lower parts of the core andthe amount of bioclastic material and degree ofbioturbation broadly reduce upwards. At the base of

180

the core the sandstones are intensely bioturbated andvariably bioclastic. Bioclasts are dominated by bivalvesand fragmented shell material but also include largecalcareous benthonic foraminifera and rare coralfragments. These units are sharp based and areinterpreted as storm event beds (Fig.5 E). They passupwards into bioclastic cross bedded sandstones andripple laminated sandstones with restricted ichnofabricswhich suggest a shallowing upwards succession withinan outer shelf setting. Within the core at 6576 feet(drilled depth) a candidate parasequence boundary orhigh order flooding surface is identified based on anabrupt increase in bioturbation index 3 and slightlyfiner grained and more argillaceous sediments. Thisparasequence boundary is also expressed on wirelinelogs as a gamma peak (Fig. 5 D).

Above this parasequence boundary similar facies tothose observed at the base of the core are again observed,consistent with the deposition of storm beds within thelower shoreface/offshore transition zone. Theseintensely bioturbated sandstones occasionally showevidence of the development of firmground substratesthrough weakly developed Glossifungites ichnofabrics,and some evidence of erosional truncation at the baseof some of the storm beds. The cored interval continuesto indicate upwards shallowing and progradation of thesand as storm beds pass upward into interbedded crossbedded sandstones and low angle ripple laminatedsandstones, showing an overall decrease in bioturbationupwards. The top of the highstand systems tract ispicked on a distinctive surface in the core, interpretedas a candidate sequence boundary and which isillustrated in Fig. 5 B and is discussed further in thefollowing section.

Wireline log data from the HST in Zaur 9 confirms anoverall coarsening upwards profile for the interval,supporting the shallowing upwards facies trend observedin the core. Both within the cored interval, and below,a series of stacked progradational parasequence setscan be interpreted on wireline logs above the BadinShale maximum flooding surface (Fig. 5).

Elsewhere in the basin core is relatively rare in theHST since these deposits are towards the base of theB Sand succession and therefore, commonly below thehydrocarbon water contact. The HST has, however,been cored in Duphri 2 on the eastern margin of thebasin. Here a thin highstand is preserved beneath whatis interpreted as a sequence boundary. It is developedin finely laminated mudstone facies with evidence ofsome bioturbation (Fig. 6 C). Overall the package hasan upwards-coarsening profile. It is interpreted to havebeen deposited in a relatively deep marine setting wellbelow storm wave base. The top part of the HST isalso cored in the South Mazari 3 well where an overall

coarsening upwards log profile, represented in core byhighly bioturbated offshore mudstones and siltstoneswith occasional storm deposits, is abruptly truncatedby a sequence boundary (Fig. 8 F & E).

Typically, the HST of the B Sand shows a clear upwards-coarsening profile composed of stacked progradationalparasequences as seen in Zaur 9 (Fig. 5). In relativelyproximal settings, the HST is severely truncated by theoverlying sequence boundary (e.g. Duphri 2, Fig. 6),the process of which is discussed further in the nextsection, and only a thin preservational remnant ispreserved. In settings that are more distal, the HST iscondensed, being beyond the position of the HST shelfedge. In some locations, it may also be partly truncatedby the overlying sequence boundary (e.g. Tangri 8; Fig.7).

B Sand Sequence Boundary

The sequence boundary records a period of relativesea-level fall resulting in a basinward shift in faciesand, if the relative sea level fall is large enough, exposureand incision of the highstand shelf which would haveformed a broad coastal plain. A sequence boundary isinterpreted within the B Sand succession over the entireBadin area and is no where more dramatically seenthan at Duphri 2, which occupies a relatively proximalposition in the basin. In Duphri 2 the sequence boundaryis marked by a rapid downward shift in facies, resultingin a sharp-based sandstone directly overlying finelylaminated marine mudstones of the HST. Sandstonesabove the sequence boundary are coarse grained withlow angle planar cross beds (Fig. 6 A & B). Theyoccasionally include laminae rich in shell debris,particularly thick walled bivalves. These sandstonesare interpreted to have been deposited in a very shallowmarine setting in a foreshore, barrier bar or shallowmarine bar setting within the swash zone.

The sequence boundary is also cored in the Zaur 9 wellwhere a downward shift in facies is also observed acrossthe surface. Here, in a more distal setting relative toDuphri 2, the downwards facies shift is more subtlewith clean, cross-bedded sandstones overlyingsignificantly more bioturbated, locally argillaceous andmore poorly sorted sands with little in the form of welldefined primary sedimentary structures. The boundaryis also marked by a sharp erosive surface, inclinedrelative to core normal, which is overlain by a thinbioclastic lag (Fig. 5 B). The sands immediately beneaththe candidate sequence boundary are also tightlycemented for a thickness of at least 5 feet, severelyimpacting reservoir quality and best illustrated in ultraviolet light (Fig. 5 C). A cemented zone immediatelybeneath the sequence boundary is commonly observedin the Badin area. Whilst no direct evidence of sub

181

areal exposure (such as a rooted horizon) is observedin the core it is suggested that the sequence boundarywas sub aerially exposed over wide areas of Badin andthat the widespread cementation immediately belowthe surface represents an early cement, possibly acaliche horizon.

A third core, in South Mazari 3 also records the sequenceboundary. In this well, located in the northern part ofBadin, the surface is also marked by a characteristicdownwards shift in facies. Below the sequenceboundary highly bioturbated mudstones and siltstoneswith occasional storm beds deposited within the offshoretransition zone (Fig. 8 F) are immediately overlain bya sharp based coarse grained sandstone, characterizedby low angle, parallel laminations indicating a veryshallow marine foreshore or backshore setting. Thesurface is also marked by a granule lag at the base ofthe overlying sandstone and boring of the surfaceindicating lithification and providing some support tothe formation of early caliche cements during sub arealexposure (Fig. 8 E).

B Sand Lowstand Systems Tract (LST)

LST deposition is initiated by the phase of relative sea-level fall and becomes established with a period of sea-level stability during relative sea-level lowstand. Inthe Badin area thick LST deposits are recorded in distallocations. The Jagir 2 well has the most complete coredinterval with around 80 feet of core available (Fig. 9). The core provides a record of an overall upwards-shallowing succession of stacked parasequences. Highlybioturbated mudstones and siltstones deposited belowstorm wave base are interbedded with sharp based,fine-grained sands with some primary laminations (Fig.9 C & D). These are storm beds deposited below fair-weather wave base but above storm wave base. Thesuccession remains in a similar facies throughout thecored interval. Towards the top of the LST intenselybioturbated fine grained sands and silts are consistenta low energy environment below fair weather wavebase (Fig. 9 A & B). A shallowing upwards profile is,however, reflected in the wireline log data, indicatingan overall coarsening upwards succession of stackedprogradational parasequences. The base of the LST isnot seen in the core but is expressed on wireline logsas a sharp based, fining upwards sand, which in Jagir2 could represent a mass flow sandstone and theproximal limit of a submarine lowstand fan complex.

The upper part of the B Sand in Jagir 2 is faulted out,but in the nearby well Tangri 8 a complete LST ispreserved with core available from the upper part. Thebase of the LST in Tangri 8 is also marked by a sharp-based sand suggesting a sharp downwards shift infacies. The LST is represented by two large scale

coarsening upwards packages (Fig. 7). The lower ofthese (cored in Jagir 2) is characterised by stackedcoarsening upwards and thickening upwardsprogradational parasequences. Core from the top ofthis unit confirms the upwards shallowing of facieswith the shallowest facies indicating deposition in alower shoreface setting. The top of this unit is terminatedby a more mudstone rich argillaceous unit informallytermed ‘Shaley B Sand’ (Fig. 3) which is representedby intensely bioturbated, very fine grained sands andmudstone facies in core (Fig. 7 C), interpreted to beparasequence boundary (flooding surface). Above theparasequence boundary a second upwards shallowing,coarsening upwards parasequence is present with sandsdeposited in an upper shoreface setting (Fig. 7 B).

The top of the LST is marked by the transgressivesurface, which is described in the following section.

B Sand Transgressive Surface

The transgressive surface marks the end of the LSTand the onset of a period of relative sea-level rise. Indistal settings, where the LST is preserved, the surfaceis difficult to pick in core due to the conformity offacies across the surface. In the Tangri 8 well thesurface occurs within a succession of cross beddedsandstones deposited in an upper shoreface setting (Fig.7 A), and the surface is more reliably picked on wirelinelog data. The transgressive surface is placed at theboundary between the coarsening upwards, stackedparasequences of the underlying LST and theaggradational and retrogradational stacked parasequencesets of the overlying Transgressive Systems Tract (Fig.7). This characteristic change in stacking patterns iswidely recognized as marking the position of thetransgressive surface 8, 7, 9 (Fig. 4).

Over much of the Badin area the transgressive surfaceis coincident with the sequence boundary. Wide areasof the former highstand shelf system were exposedduring lowstand and were not submerged again untilinundated by the diachronous transgressive surface.In these locations the transgressive surface ischaracteristically marked by a thin lag deposit asobserved in Zaur 9 (Fig. 5 B) and South Mazari 3 (Fig.8 E). It marks the return of B Sand deposition in theseareas.

B Sand Transgressive Systems Tract (TST)

The TST records a prolonged period of relative sealevel rise culminating in the Turk Shale flooding event,which terminates deposition of the B Sand. In distallocations (e.g. Tangri), deposition of shelf sanddeposition continues into the TST across thetransgressive surface. The key change noted in distal

182

sections is a change from an upwards coarsening,progradational log profile in the LST (where sedimentsupply exceeds relative sea-level change or creation ofaccommodation space) to an aggradational stackingpattern as sediment supply can only keep pace with theaccommodation space created by relative sea-level rise.

Once the transgression had inundated the previouslyexposed highstand shelf shallow marine sand depositionwas re-established across the majority of the Badinarea. In Zaur 9 the TST is characterised by fine grained,moderately to very well sorted sands with rare smallmudstone intraclasts and rare fragments of robust shellfragments. The most distinctive characteristic of thesesands is the presence of moderately well defined crossbedding. Cross bed sets are typically preserved as 2-6 inch units, but intervals up to 2 feet thick with aconsistent dip orientation have been identified (Fig. 5A). The sandstones are typically unbioturbated butsmall, simple unlined burrows and small vertical tracesof possible Skolithos are occasionally identified.Bioturbation and shell debris, together with crossbedding is consistent with a reasonably high-energy,marine, inner shelf setting as part of a tidal sandwavecomplex.

Shallower marine facies (e.g. evidence of the shoreline)are not preserved in Zaur 9, presumably reworkedshoreward as the transgression proceeded. The typeof section and facies seen in Zaur 9, however, is typicalof many wells and fields in the Badin area as thepeneplained sequence boundary was transgressed toform an extensive shallow marine shelf area. Inproximal settings, however, shoreline deposits arepreserved. At Duphri the TST rest directly on distaloffshore mudstones of the truncated HST and isrepresented by coarse grained sands, characterized bylow angle planar bedding and layers rich in shellsincluding thick walled bivalves (Fig. 6 A). Thesedeposits are typical of very shallow water depositionwithin the swash zone and mark the position of theshoreline, a barrier complex or possibly a shallow shoalfeature on a topographic high. In Duphri 2 theseforeshore deposits are also aggradational with 26 feetof the same facies preserved in stacked aggradationalparasequence sets, which are also clearly manifestedon logs (Fig. 6).

The South Mazari 3 well also records the TST in arelatively proximal setting and displays a log profilethat it is clearly progradational. This is different to thetypically aggradational/retrogradational profilesobserved in the TST in the majority of wells in Badin.Core from the well indicates transgression of thesequence boundary by relatively coarse grained, lowangle parallel laminated sands which pass upwards intoa rooted horizon, mudstones encasing thin cross bedded

sandstones and finally coarse grained sands with highangle unidirectional beds. This section is interpretedas a prograding barrier bar complex with backshoredeposits, passing upwards through a rooted soil horizon,into a lagoon with tidal channels preserved and, at thetop, records deposits close to a fluvial discharge (Fig.8). The record of fluvial sediments at the top of theSouth Mazari 3 section clearly indicates proximity toa local sediment source which provided sediment at agreater rate than accommodation space was created bythe transgression and, therefore, allowed thedevelopment and preservation of a progradational,shallowing upwards succession in the TST.

In Duphri 2 the top of the TST is truncated by the TopLower Goru Unconformity, created by a phase of riftingin the basin, which cuts deeper into more proximalareas, which were uplifted at this time. Elsewhere thetermination of B Sand deposition is very rapid as thetransgression continues, inundates sediment sourceareas and effectively outstrips sediment supply. Thisis marked on logs by a rapidly upwards-finingparasequence set, which culminates in a gammamaximum (e.g. Zaur 9; Fig. 5, Tangri 8; Fig. 7, SouthMazari 3; Fig. 8).

Turk Shale Maximum Flooding Surface

The period of relative sea-level rise, which created theaccommodation space for the deposition of the TST,culminates in a maximum flooding surface within theTurk Shale. This is marked everywhere by a prominentgamma ray maximum, above which the stackedprogradational parasequence sets of the succeedinghighstand can be seen (e.g. Zaur 9, Fig. 5).

Sequence Stratigraphic Summary

Deposition of the B Sand ‘cycle’ was initiated by aperiod of relative sea-level highstand followinginundation of shelf areas by the Badin Shale MaximumFlooding Surface. A highstand systems tract progradedbasinwards, passing upwards from marine shales,through offshore transition zone and lower shorefacesands. The shallowest facies recorded within the HSTare from an upper shoreface setting. (Fig. 10 A). ThisHST shelf system was established in the eastern partof Badin and passes westwards into distal marinemudstone facies.

A period of relative sea-level fall resulted in the exposureand erosion of the highstand shelf system. During thisperiod it is likely that there was also some incision ofthe shelf as river systems re-equilibrated to the newbase level. No evidence of incised valleys has beenseen either in wells or on seismic, though due to theirscale incised valleys are likely to be at the limits of

183

seismic resolution. There is, however, interpretedevidence from wireline log data of lowstand fandeposition, which would have been fed into the basinvia incised valleys (Fig. 10 B). Any incised valleysare likely to be aligned perpendicular to the shelf andwould have been oriented approximately northwest-southeast. This is similar to the current tectonic grainin Badin. Though the major movements on these faultsis generally believed to postdate reservoir deposition,any early development of these faults may haveinfluenced the orientation and position of the incisedvalleys.

As relative sea-level fall culminated, a period ofrelatively stable sea-level at the lowstand ensued.Depositional systems became re-established and alowstand shelf developed basinwards of the previoushighstand shelf system. Two large-scale progradationalparasequences or lobes are developed (Fig. 10 C).Local sediment supply was plentiful from the widelyexposed highstand shelf (in excess of 25 miles wide)where greater than 100 feet of section is estimated tohave been removed in proximal locations (e.g. Duphri2).

As relative sea-level began to rise at the onset of theTransgressive Systems Tract progradational shelfdeposits of the LST give way to largely aggradationaland retrogradational shelf deposits of the TST. As thetransgression developed wide tracts of coastal plain(exposed HST) were inundated and a broad shallowshelf around 25 miles wide was established across theBadin area. Extensive reservoir deposition occurredin an inner shelf setting. Towards the margins of thebasin and on structural highs, shoreline and beachbarrier complexes are preserved (Fig. 10 D). Wheresediment supply was high, progradational (shallowingupwards) barrier island and back barrier complexesdeveloped locally. B Sand deposition was finallyterminated by the Turk Shale Maximum FloodingSurface, which marks a further landward migration ofdepositional systems.

CONCLUSIONS AND IMPLICATIONS FORPETROLEUM GEOLOGY

Correlation

The regional sequence stratigraphic correlation of theB Sand illustrated in Fig. 11 provides an explanationfor the variations in thickness and character of the BSand in the Badin area (Fig. 3). Only the TST is widelydeveloped across the area. At the distal margins of theHST and the proximal margins of the LST cautionshould be exercised correlating sands beneath thetransgressive surface, as they are likely to have beendeposited in different systems tracts.

Reservoir Connectivity

The sequence stratigraphic correlation presented herehas obvious implications for reservoir connectivity ata regional scale. Previous models interpreting the BSand as a sheet like deposit are superceded by themodel presented in this paper and explain the spatialand temporal evolution of the B Sand.

On a local scale there are further implications forreservoir connectivity. One of the most significant,with implications on a local field-wide scale, is thedevelopment of a cemented horizon at the top of theHST, interpreted as an early (?caliche) cement formedduring the lowstand sub areal exposure following theperiod of sea-level fall. This horizon forms a significantbarrier within the B Sand (stratigraphic compart-mentalization). It is present in the Zaur Field (Fig. 5C) and is prominent in many other B Sand Badin Fields,where it can also control the position of the hydrocarbon-water contact (e.g. Ghunghro, Rind and Sonro Fields).

In core data other potential stratigraphic barriers whichmay also have field wide extent, or at least providebarriers locally within a field, must be considered inreservoir zonations and field development. Theseinclude significant parasequence boundaries or floodingsurfaces (e.g. Tangri 8, Fig. 7 B) and less prominentparasequence boundaries (e.g. Zaur 9, Fig. 5 D). Locallycemented horizons, resulting from a concentration ofshell debris such as the storm beds observed in Zaur 9(Fig. 5 E) can be correlated over a distance of over halfa mile and provide pressure barriers.

In back barrier settings such as those seen in SouthMazari 3 some of the sands, particularly the tidalchannel facies encountered within the lagoonal setting,are likely to be of limited lateral extent and may alsobe poorly connected, with implications for reservoirenergy and sweep.

Reservoir Quality

Reservoir quality within the B Sand is primarilycontrolled by depositional facies. The best qualityreservoirs are the upper shoreface cross-beddedsandstones, which are most widely developed duringthe TST. As a result the TST provides the best qualityreservoirs seen within the B Sand.

Alternative Play types

As well as refining correlations and predicting possiblestratigraphic barriers within the reservoir, this sequencestratigraphic model also highlights the potential ofadditional play types within the B Sand, which are yet

184

to be knowingly evaluated. In brief these are:

Lowstand Detached Shoreline: Potential for onlap ofthe lowstand shoreline on to marine mudstones of thehighstand slope (schematically shown in Fig. 10 C).This is a narrow play fairway in a dip sense but isextensive in a dip direction and is approximately definedby the proximal pinch out of the LST.

Lowstand Fan: Tentative evidence of lowstand massflow sands or turbidites deposited at the base of slopeis seen in Tangri 8, and other wells located in distalsettings in Badin. These would be expected to be ofexcellent quality having been reworked from an olderB Sand shelf and would be sealed by the distal toe setsof the succeeding lowstand prograding shelf.

Lowstand Channel Fill: Data from Badin indicatesthat a shelf area in excess of 25 miles wide was emergentduring lowstand and that in excess of 100 feet of sectionwas removed in some locations. It is also highly likelythat the shelf was incised as the drainage system re-equilibrated to the new base level. Therefore, incisedvalleys cut to a depth of 100 feet could exist. Thesedeep incised valleys would have been backfilled duringtransgression with a thick, aggradational succession ofhigh quality shallow marine sands. These channels arelikely to have been oriented approximately northwest-southeast perpendicular to the shoreline and theirposition may have been controlled by existing structuralelements. A key risk with this play fairway would beup-dip and possibly lateral seal.

ACKNOWLEDGEMENTS

The authors acknowledge the management of BPPakistan Exploration and Production Inc. for permissionto publish this data. We are grateful to detailedsedimentological input on the Zaur 9 well from CarlWatkins of Fugro Robertsons Limited and Zahid Mirzain the production of figures.

REFERENCES

[1] Alam, S.M.M, M. Wasimuddin, and S. Ahmad,2002. Zaur structure: A complex trap in a poor seismicdata area. PAPG-SPE Annual Technical Conferenceproceedings special publication, pp. 146-163.[2] Emery, D., and K.J. Myers, 1996, Sequencestratigraphy: Oxford, Blackwell Science, 297 p.[3] Goldring, R., 1995. Organisms and the substrate: response and effect. In: Boscence, D.W.J. & Allison.P.A. (Eds), 1995. Marine Palaeoenvironmental Analysisfrom Fossils, Geological Society Special PublicationNo. 83, pp. 151-180.[4] Haq, B.U., J. Hardenbol, and P.R. Vail, 1987,Chronology of fluctuating sea levels since the Triassic:

Science, v. 235, pp. 1156-1167.[5] Posamentier, H.W., and D.P. James, 1993, Anoverview of sequence-stratigraphic concepts: uses andabuses, in H.W. Posamentier, C.P. Summerhayes, B.U.Haq and G.P. Allen, eds., Sequence stratigraphy andfacies associations: Oxford, Blackwell, pp. 3-18.[6] Vail, P.R., R.M. Mitchum, and S. Thompson, 1977,Seismic stratigraphy and global changes of sea level,part 3: Relative changes of sea level from coastal onlap,in C.E. Clayton, ed., Seismic stratigraphy - applicationsto hydrocarbon exploration: Tulsa, Oklahoma, AmericanAssociation of Petroleum Geologists Memoir 26, pp.63-81.[7] Van Wagoner, J.C., and G.T. Bertram, eds., 1995,Sequence stratigraphy of foreland basin deposits: Tulsa,Oklahoma, AAPG Memoir 64, 490 p.[8] Van Wagoner, J.C., R.M. Mitchum, K.M. Campion,and V.D. Rahmanian, 1990, Siliciclastic sequencestratigraphy in well logs, cores, and outcrops: Tulsa,Oklahoma, American Association of PetroleumGeologists, Methods in Exploration Series, No. 7, 55p.[9] Van Wagoner, J.C., H.W. Posamentier, R.M.Mitchum, P.R. Vail, J.F. Sarg, T.S. Loutit, and J.Hardenbol, 1988, An overview of the fundamentals ofsequence stratigraphy and key definitions. In C.K.Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W.Posamentier, C.A. Ross, J.C. Van Wagoner, eds., Sea-level changes: an integrated approach. Society ofEconomic Paleontologists and Mineralogists SpecialPublication No. 42, pp. 39-45.

185

186

Fig.

1.

Loc

atio

n M

ap B

P O

pera

ted

Bad

in C

once

ssio

ns a

nd O

il &

Gas

Fie

lds

in P

akis

tan.

187

Fig. 2. Badin Area Generalised Cretaceous Stratigraphy.

188

Fig.

3. L

ithos

trat

igra

phic

Cor

rela

tion

of th

e B

San

d Il

lust

ratin

g R

egio

nal V

aria

tion

in T

hick

ness

and

Cha

ract

er.

189

Fig.

4. S

eque

nce

Stra

tigra

phic

Mod

el a

nd M

etho

dolo

gy (

Mod

ifie

d fr

om V

an W

agon

er e

t al.

1990

).

190

Fig.

5. Z

aur

9, S

eque

nce

Stra

tigra

phy

of th

e B

San

d.

191

Fig.

6.

Dup

hri 2

, Seq

uenc

e St

ratig

raph

y of

the

B S

and.

192

Fig.

7. T

angr

i 8, S

eque

nce

Stra

tigra

phy

of th

e B

San

d.

193

Fig.

8.

Sout

h M

azar

i 3, S

eque

nce

Stra

tigra

phy

of th

e B

San

d.

194

Fig.

9. J

agir

2, S

eque

nce

Stra

tigra

phy

of th

e B

San

d.

195

Fig. 10. Sequence Stratigraphic Evolution of the Lower Goru Formation B Sand (for well locations refer Fig-1).

196

Fig.

11.

Reg

iona

l Str

atig

raph

ic E

volu

tion

of th

e L

ower

Gor

u B

San

d.

197

ABOUT THE AUTHOR

Chris Ebdon

Chris Ebdon received his B.Sc.degree in Geology from theUniversity of Southampton andMSc. in palynology from theUniversity of Sheffield. Hestarted his career in thepetroleum industry withRobertson Research in 1982before joining BP in 1985. He has worked extensivelyin the UK, Norwegian and Dutch North Sea, Atlanticmargin basins, South America and the US, principallyas a geologist and sequence stratigrapher. He hasrecently held positions as Geology Team Leader in BP’stechnology organization and Field DevelopmentManager at Wytch Farm, Europe’s largest onshore oilfield. Chris is currently Senior Subsurface Mentor forBP Pakistan based in Islamabad. He is author and co-author of numerous published papers.

Mohammad Wasimuddin

Mohammad Wasimuddin,received B.Sc. (Hons.) andM.Sc. degree in Geology fromuniversity of Karachi in 1979& 1980 and a M.S. Geologydegree with specialization inM i c r o p a l e o n t o l o g y ,Stratigraphy and PetroleumGeology from King Fahd University of Petroleum andMinerals, Dhahran, Saudi Arabia in 1985. Worked asa Cooperative Teacher, Lecturer and Assistant Professorwith University of Karachi during 1980 – 1981 and1986 – 1989. Joined BP Pakistan Exploration andProduction Inc. (Formerly UTPI) exploration departmentin October, 1989. Currently working in BP PakistanSubsurface Team as Badin Geology Team Leader. Activemember of AAPG, PAPG, and SPE. Author and co-authorof several published and unpublished papers.

M. Arif H. Malik

Arif Malik received his B.Sc.(Hons.) and Msc. degree inGeology (specialization inPetroleum and Marine geology)from University of KarachiPakistan. He started his careerin the petroleum industry fromCGG in 1987 and joinedGeoservices mud logging company in 1989 before joiningBPP (former UTP) in 1990. He has 14 years workingexperience in new ventures and exploration mostly asa geologist and open hole log analyst. He started hiscareer with BPP as a junior geologist. Arif is currentlyLead Geologist Exploration for BP Pakistan based inIslamabad.

Shakeel Akhter

Shakeel Akhter received his B.Sc.(Hons.) and M.Sc. degree inGeology from University ofKarachi in 2000 & 2001withGold Medal. He joined BPPakistan in Feburary 2002 as ageologist. He is currentlyworking as a DevelopmentGeologist in Islamabad and is an active member ofAAPG and SPE.