Hydrodynamic Trapping in the Cretaceous Nahr Umr Lower Sand of the North Area, Offshore Qatar (includes associated papers 18521 and 18605 )
Authors: Wells P.R.A. 
Published: December 2007

Wells, P.R.A., Qatar General Petroleum Corp.(Offshore Operations)

Summary.
A hydrodynamic model is described to account for oil and gas occurrences in the Cretaceous of offshore Qatar, in the Arabian Gulf. Variable and inconsistent fluid levels and variable formation waterpotentials and salinities cannot be explained by combinations ofstratigraphic and structural trapping. Indeed, there is no structuralclosure to the southwest of the oil and gas accumulations. Thewater-potential and salinity data and oil distribution are consistent withthis model and indicate that a vigorous hydrodynamic system pervades theCretaceous of the Arabian Gulf region. Extensive upward cross-formationaldischarge is taking place in the North Area. This cross-formational waterflow could be partly responsible for localized leaching and reservoirenhancement in the chalky limestones.

Introduction
During 1970–80, appraisal drilling of the North field Khuff gasaccumulation offshore Qatar unexpectedly led to the discovery ofoil and gas in the overlying Cretaceous reservoirs (Figs. 1 and 2). The general structure of the Cretaceous of the North Area consistsof the broad, gently dipping, north/northeast-plunging nose of theQatar arch. There is no overall structural closure to the south, although small culminations occur within this broad structure. Seismic evidence indicates that minor faults trend roughly east to west. Significant unconformities occur within the Cretaceous and cutprogressively deeper into the underlying strata toward the north. These regional unconformities are commonly overlain by regionalshale seals (Fig. 2). With the exception of the Nahr Umr lowersand, the Cretaceous reservoirs are chalky limestones(packstonesand wackestones) with high porosity but low permeability. The NahrUmr lower sand is thin (10 to 15 ft [3 to 4.6 m]) and ratherheterogeneous and was deposited over the North Area as an extensiveshallow marine sheet sand. Its quality deteriorates toward the westand southeast. In the cleaner parts, porosities and permeabilitiesvary between 20 and 30% and 400 and 1,000 md, respectively. Production tests have yielded in excess of 1,000 B/D [160 m3/d]of medium-gravity (28deg.API [0.89-g/cm3]) oil. The Nahr Umrlower sand is the most economically attractive target of the oil-bearing reservoirs of the North Area Cretaceous. Log correlations, test results, and structural maps showed thatthe oil and gas distribution in the Cretaceous was not compatiblewith simple structural trapping. Lithologic permeability barriershave been assumed to play a major role in the trapping process. However, some unexpected results from the most recent appraisalwells indicated the possibility of hydrodynamic aquifer activity inthe Cretaceous. This paper reviews the evidence for a hydrodynamic trappingmodel from water pressure, salinity data, and hydrocarbon-distributionpatterns. Although this paper is concerned primarily with thedistribution of oil in the Nahr Umr, evidence from the chalkyCretaceous reservoirs is also presented where relevant.

Hydrodynamic Processes: Regional Setting

General.
Water flow in sedimentary basins is driven bygeographic variations in water potential (Fig. 3) that can changeconsiderably in nature and distribution during the development of thebasin. Early in the compaction history of a basin, water flow is upwardand updip to the basin margins because of the relatively higher waterpotential in the basin center created by the expulsion of waterreleased by compaction and clay mineral diagenesis. As the basinmatures, the water-potential system changes if significant topographic relief forms around it. This reverses the earlier potential system because high water potentials are created at the basin margins as a result of elevated water tables. Recharge by rainfall may takeplace, depending on climate. This potential system drives waterflow inward, downdip to the topographic depressions in the basincenter, provided upward discharge is possible there (Fig. 3). Many studies have described this type of basin-wide hydrodynamic regime. The extent to which the inward flow of waterinteracts with any contemporary outward compaction-driven flowdepends on the relative vigor of the flow systems. The inward flowof relatively fresh water through the aquifers in whichhydrocarbons are trapped or migrating has a profound effect onhydrocarbon distribution, reservoir rock diagenesis, and oil chemistry.

Regional Aspects.
Using regional water-potential data, Tothconsidered the Cretaceous of the Arabian Gulf region to exhibitlarge-scale hydrodynamic activity. Extensive water recharge wasproposed to take place in the encircling uplands (Zagros, OmanMountains, and central Arabian Peninsula), where Cretaceous aquiferseither outcrop or subcrop beneath outcropping permeable sediments (Fig. 4). The water potentials in the aquifers exceed the "normal"hydrostatic potential because of the elevation of the water table abovemean sea level in the recharge zones. This water potential driveswater toward the discharge zones in the low-potential area in thecenter of the Arabian Gulf (Fig. 4). An additional or alternativecause of excess internal water potential on the Iranian margin ofthe Arabian Gulf could have been the compaction/dewatering ofCretaceous and Tertiary sediments associated with the developmentof the Arabian Gulf geosyncline during the Tertiary. A keyelement in any flow system is the relatively simple structure andcontinuity of the Cretaceous reservoirs in the Arabian Gulf region.A discharge zone through the Cretaceous, essential foropen-system hydrogeological behavior, was probably established in theNorth Area offshore Qatar because the regional Nahr Umr shaleseal is thinnest there (about 110 ft [34 m]).A dynamic aquifer regime would have had a profound influenceon the migration and trapping of hydrocarbons within theCretaceous of the Arabian Gulf region. Hydrocarbon accumulations instructural or stratigraphic traps, present before the establishment of active aquifer flow, would have been altered in shape-evidenced, for example, by the tilted oil/water contacts (OWC's) in theMishrif Sirri and Fateh fields of Iran, shown in Fig. 4or evenobliterated. The coeval outward and upward migration of buoyanthydrocarbons and the opposing inward and downward flow of waterwould have set up potential hydrodynamic trapping conditions infavorable geologic settings, such as on the gently plunging noseof the Qatar arch (Figs. 1 and 5). Fig. 5 shows schematically some of the effects of hydrodynamicactivity. In addition to these geometric aspects, lateral and verticalformation-water-salinity variations may be expected.
JPT
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