Multidisciplinary ApproachUnravels Structural Complexities

Of Stampede Reservoir

The “Better Business” Publication Serving the Exploration / Drilling / Production Industry

OCTOBER 2016

By Gopal Mohapatra, Andry Nabasir and Steve Strauss

HOUSTON–First oil from the Hess Corp.-operated Stampede Field is scheduled for

2018 from a new tension leg platform stationed in 3,350 feet of water at Green Canyon

468. Stampede holds total recoverable resources estimated at 300 million-350 million

barrels of oil equivalent in Miocene sands located at subsurface depths of

approximately 30,000 feet, ranking it among the deepest fields ever developed in the

Gulf of Mexico.

The TLP is designed to support two

export risers and multiple production

risers connected to subsea well

centers. The initial plan calls for six

producing wells and four injection

wells, with daily topside processing

throughput capacities of 80,000

barrels of oil, 100,000 barrels of

water injection, and 40 million cubic

feet of produced gas.

Reproduced for Hess Corporation with permission from The American Oil & Gas Reporter www.aogr.com

Three key issues in developing theStampede Field are reservoir compart-mentalization, stratigraphic complexity,and fluid contact uncertainty. These issueshave been studied in detail by integratingstructural and stratigraphic interpretation,and using state-of-the-art dual-coil 3-Dseismic data, fault-seal analysis, andpressure and fluid data from wells, coredata and logs.The products and knowledge from

this integration effort were incorporatedinto a geological model that honors thedata and observations to capture all un-certainties and their ranges. The focusof this effort included trap geometry, thesealing capability of faults, the deposi-tional environment, facies distribution,geochemical fluid analysis, and oil/watercontact (OWC) estimates. The modelthen was applied for flow simulation,resulting in the 10-well “base case” de-velopment plan.This multidisciplinary effort to opti-

mize the Stampede Field’s developmentallowed the project co-owners to:

• Incorporate fault seal-related com-partmentalization into the simulationmodel;

• Properly represent facies types inthe geological model; and

• Better understand the compartmentsand associated OWCs, and understandtheir impact on flow simulation/field de-velopment.

Trap And Fill

The Stampede Field is a four-waystructural closure segmented by faults inGreen Canyon blocks 468, 511 and 512.Figure 1 shows a legacy depth structuremap of the main reservoir (the middleMiocene Gff40s) from wide-azimuth 3-D seismic data. It also denotes the dis-covery and appraisal wells. Note the ab-sence of faults in the western flank ofthe structure.The structure formed as a four-way

turtle in response to salt withdrawal fromthe basin. The evacuated salt formed an

FIGURE 1Legacy Depth Structure Map of Gff40s Main Reservoir Zone

GC468

Pony #2

Pony #1

KH #1

KH #2

GC512

C.I – 200’

FIGURE 2Dual-Coil 3-D Image of Stampede Field

(Gff10s, Gff40s and Sh20s Reservoir Zones)

W E

20,000’

40,000’

30,000’

10,000’

Sh20

Gff10

Gff40

SALT

5,000’

15,000’

20,000’

25,000’

30,000’

35,000’

40,000’

45,000’

0.0 1.00.80.50.2 2.0 miles

SpecialReport: Gulf of Mexico Update

approximately 15,000 foot-thick canopyover the structure. Below the salt canopyis a 9,000 foot-thick section (from thebase of salt to the base of the reservoirsands) that is composed of deepwatershale and turbidite sand packages in theupper/middle Miocene reservoir. As shownin the dual-coil seismic image in Figure2, the three key pay intervals in thefaulted, four-way structure are the middleMiocene (Serravallian) Gff10s, Gff40sand Sh20s.Four wells with several sidetracks

were drilled to discover and appraise thefield, but none of the wells penetratedoil/water contact. Multiple fluid contactswere inferred from pressure data in thewells, implying that reservoir compart-mentalization was difficult to supportwith faults mapped from existing seismicdata.The low-resolution seismic data (in-

cluding both narrow- and wide-azimuth3-D) also made it difficult to discernstratigraphic changes that could createflow barriers or compartmentalization.As a result, engineered faults from previousgeological models were used to explainpressure differences between wells.However, with the latest 3-D dual-

coil seismic data, more small offset faultscould be mapped, which helped explainsome of the pressure differences observed.Fault seal analysis was performed to un-derstand the flow barrier capability offaults, and this information was incorpo-rated into the simulation model.Filling the geological model with reser-

voir facies had its own challenges; thesize and extent of the depositional filland rock types in the fill were problematic.In addition, a process was needed to pop-ulate the geological model representingthe fill, which also proved challenging.The depositional setting for the reser-

voir pay zones varied from unconfinedto weakly confined deepwater channelsystems in a salt withdrawal basin. Withthe help of outcrop analogs, we interro-gated seismic data to define the deposi-

tional environment boundaries. Despiteseveral penetrations in the reservoir zones,there still was uncertainty in facies typingand fluid contacts. Consequently, coredata, logs, fluid parameters and regionalanalogs were used as part of a multidis-ciplinary approach to reduce facies type

and fluid contact uncertainties.This process of integrating data and

information from a variety of sources,tools and methods in the geological andflow simulation model greatly increasedour understanding of the geology andpetrophysics behind the workflow.

FIGURE 3Multidisciplinary Workflow for Building the Simulation Model

Structural Framework

PHIE MPS-Facies

Training Image

EOD Map

Log Data

CompartmentsGrid Design

Analysis

STOIIP +Simulation

ReservoirProperty

Population

FIGURE 4Current Depth Structure Map of Gff40s Reservoir Zone

Using Dual-Coil 3-D Data

0 2,000 10,0008,0006,0004,000 20,000 feet

C.I – 200’

SpecialReport: Gulf of Mexico Update

Improved Trap Definition, Fault Seal

Figure 3 illustrates the multidisciplinaryapproach adopted to build the simulationmodel to address each of the keychallenges involved in field development.Some of the key aspects of the work canbe summarized as building the container(delineating trap and fault sealing), fillingthe container (resolving stratigraphiccomplexities), and filling the reservoir(reducing uncertainties related tocompartmentalization and fluid contact).As the flanks of Stampede Field’s

four-way turtle structural trap subsidedin response to salt withdrawal, crestalfaults were created as a result of structural“flexuring.” The faults were both shortand long, with some long enough to offsetthe base of the salt. To accurately estimatein-place resources and understand fieldcompartmentalization, the structure’sshape had to be interpreted accuratelyand every possible fault had to be mappedcorrectly. In turn, structure shape andfault maps depend on acquiring a goodseismic image.Seismic image quality in subsalt

environments generally is impactedadversely by the overlying salt geometry.In Stampede, while the center of the fieldwas well-imaged, the flanks generallywere not because of steep salt flanks,faulted base salt, intrasalt inclusions, etc.Previous seismic images, including WAZdata, were not clear enough to reliablymap the flanks of the structure and identifysmall offset faults. Because of the low-frequency content (about 12 hertz) of theseismic data, the presence of subseismicfaults could not be ruled out.Both NAZ and WAZ seismic data

were used in the exploration and appraisalphases, which involved limited subsurfaceillumination. As a result, while the centralpart of the trap was imaged relativelywell, the peripheral part of the traps–especially in the north and northwest–were imaged poorly.Dual-coil 3-D seismic data licensed

during the field development phase showedan overall imaging improvement. Weused dual-coil seismic and its derivativeattributes to map the structural flanksmore accurately and to delineate smalloffset faults that could be flow-baffles/barriers causing possiblecompartmentalization.Figure 4 shows the current depth struc-

ture map of the Gff40s main reservoirzone using dual-coil seismic data. Com-pared with the legacy structure map inFigure 1, it shows new faults mapped onthe northwest and central sections of thefield. Discovery and appraisal well locationand trajectories also are shown.The ability of a fault to be a seal or

flow barrier depends on various factors,including its extent, offset, lithology, andthe nature of juxtaposition. Althoughseveral faults were mapped with dual-coil seismic, it also was necessary toperform a numerical modeling test todetermine their ability to seal. Accordingly,3-D multifault seal analysis was conductedfor this purpose.Structural maps, faults, and the

geological model with lithology derivedfrom drilled wells and extrapolated acrossthe fields were input into the seal analysis.Fault properties such as throw distribution,shale gouge ratio (SGR), fault zonethickness, and fault zone permeabilitywere calculated and analyzed.The analysis indicated that faults with

large local throws resulted in a lowfrequency of sand versus sand windows,faults with small local throws had minimalimpact on cross-fault flow, and variationsin cross-fault flow capacity changed alongstrike as the throw and related fault thicknesschanged. Keeping these mixed behaviorsof the faults in mind, fault transmissibilitymultiplier (FTM) distributions werecalculated for each fault and exported intothe flow simulation model. This approachwas a more realistic depiction of flowthrough faults than simple “seal” or “leak”fault categorizations.Figure 5 shows the SGR and FTM

distributions for the fault shown in red inthe inset map at bottom center. Displaysare along the fault plane. High FTMvalues indicate flow across the fault.

Fill And Facies Distribution

The three middle Miocene reservoirintervals (Sh20s, Gff40s and Gff10s) inStampede consist of deepwater turbiditicsands. As noted, the basin started todevelop in response to sediments comingin from the north and pushing the saltout to create space. There was aconsiderable clastic sediment input intothe Stampede basin during the lower andmiddle Miocene period. The depositionalenvironment for these sands changedfrom an unconfined channel setting inSh20s to an increasingly confined channelsetting in Gff10s as the basin itself becamemore confined with developing salt highson the flanks.Seismic data were used to determine

the lateral extent of the reservoir intervalsand their axial, off-axial, and marginalsettings. However, the data did not havehigh enough resolution to determine thedetails of the facies distribution in thereservoir. To compensate, core data fromthe reservoir intervals and logs alongwith outcrop analogs from similardeepwater depositional settings were usedto determine facies geometries anddistribution.Figure 6 illustrates the multipoint

statistics (MPS) facies population approachapplied for each reservoir zone to buildthe geological model. The Gff40s zoneis shown here, with the depositional en-vironment scheme, facies elements, andtheir proportion used for object definition,training image and the final facies model.An object was defined with depositionalfacies elements (e.g., channel axis, medianlobes, etc.), and a training image wasconstructed using that object. The trainingimage was used to control the faciespopulation in the 3-D geologic model.Porosity was calculated from logs and

calibrated by core data, and permeability

SpecialReport: Gulf of Mexico Update

was calculated using porosity/permeabilitytransformation functions derived fromcore data. Facies population controlledporosity and permeability distribution inthe geologic model.

Reservoir Compartmentalization

In addition to none of the Stampedeexploration or appraisal wells penetratingOWC in any of the three reservoirs, alack of pressure data from the “wet”zones in these intervals made estimatingOWC more difficult. To compensate, weused log and core-based saturation dataand modular dynamics tester pressuresin a multidisciplinary tool, Geo2Flow, toidentify reservoir compartments and freewater levels (FWLs). FWL estimation uses J-functions that

couple porosity, permeability and satura-tions. Each well was fit separately tocore-derived J-curves to come up withan FWL. Similar FWLs then were notedbetween the wells to determine possiblecompartments. The work suggested fivecompartments as a best estimate case inthe Gff40s main reservoir.Figure 7 illustrates the outcome of

the integration effort at the StampedeField. It displays the updated depth struc-ture map of the Gff40s reservoir withfive compartments (identified by pinkdashed lines) and their estimated OWCdepths (blue dashed lines). CompartmentC1 is an aquifer.As part of the integration effort, geo-

chemical analysis of fluids in the wellswas incorporated to help better understandcompartmentalization. Alkyl-benzeneanalyses for the reservoirs in the wellsalso supported the reservoir compartmentand connectivity interpretation.

Integrating Higher-Order Details

Dual-coil seismic data enabled us tomap new faults and extract higher-orderreservoir details. A multifault seal analysisprovided information on fault trans -missibility, which was used dynamicallyin the flow simulation model. Integratinginformation from seismic with rock

properties derived from logs, cores andoutcrop analogs yielded a betterunderstanding of variabilities in reservoirparameters and their portrayal in thegeologic model. Tools such as Geo2Flowhelped us combine rock and fluidproperties to estimate fluid contacts.All of these data and tools led to

building a more realistic geologicsimulation model and achieving a betterunderstanding of the uncertaintiesassociated with developing the StampedeField. Based on the results of the

simulation model, the field developmentplan could be finalized with the sixproducing and four injection wells as thebase case.The integrated workflow applied to

optimize the development plandemonstrates that dual-coil seismic datacan provide a better and more detailedpicture of the subsurface, and that com-partmentalization caused by faulting canbe represented more realistically in themodel through FTMs. An MPS approachis an ideal way to incorporate log, core

FIGURE 5Shale Gouge Ratio (Left) and Fault Transmissibility Multipliers (Right)

For Fault Shown in Red in Inset Map (Bottom Center)

FIGURE 6Facies Population Approach for Gff40s Reservoir Zone

User-Defined ObjectEOD Map Lobes Elements

FaciesPopulation

Training Image

SpecialReport: Gulf of Mexico Update

and analog data into facies population.In the absence of penetrated OWCs,

the project showed that compartmentsand OWC estimates were better con-strained by integrating pressure, watersaturation and geochemistry data alongwith seismic interpretation.Going forward, we intend to update

the model with data every time we drill adevelopment well. This should help refinethe drilling and completion plans forfuture wells and optimize the performanceof the Stampede Field. r

Editor’s Note: The authors acknowl-edge WesternGeco for permission to usethe seismic images in this article, as wellas the contributions of subject matter ex-perts on the Stampede project managementteam, and in the technology and excellencegroup at Hess.

Depth Structure Map of Gff40s Reservoir with Compartments And Estimated Oil/Water Contact Depths

FIGURE 7

GOPALMOHAPATRA

Gopal Mohapatra is a geophysicaladviser at Hess Corporation in Houston,where he is involved in generating ex-ploration leads, maturing leads intodrillable prospects, and drilling in thedeepwater Gulf of Mexico. He is thelead geophysicist for the Stampede proj-ect looking after seismic processing,salt/velocity model building, seismic in-terpretation, and well planning andmonitoring. Before joining Hess in 2007,Mohapatra served for 10 years at Exxon-Mobil as a deepwater exploration geo-physicist in the Gulf of Mexico and An-gola. He holds a B.S. and an M.S. ingeoscience/geophysics from IIT Kharag-pur, a Ph.D. in geophysics from theUniversity of Arizona, and an M.B.A.in international finance from the Uni-versity of Houston.

ANDRYNABASIR

Andry Nabasir is a geological adviserat Hess Corporation, focused on deepwaterfield development projects in the Gulf ofMexico. Before joining Hess in 2012, heserved as a geological adviser at Chevronin the United States, Angola and Indonesia.He began his career in 1990 as a devel-opment/well-site geologist at PT CaltexPacific Indonesia, and subsequently servedthe company as senior development ge-ologist.

STEVESTRAUSS

Steve Strauss is senior reservoir en-gineering adviser at Hess Corporation.He joined Hess in 2005 working onBakken Shale and deepwater Gulf ofMexico development projects. After twoyears as senior operations engineer forconventional and unconventional playsat Vitruvian Exploration, Strauss returnedto Hess in 2012. He began his career atMobil, followed by Conoco, where heserved for 21 years as an engineer andsenior reservoir engineer. Strauss holdsa B.S. in petroleum engineering fromTexas A&M University.