An uncertainty analysis study of the gas volumes in the field of Manati was conducted using gOcad™/Jacta™. The main uncertainties are associated to two important lateral limits of the reservoir, both ill-defined due to the poor quality 2D seismic lines available: the geometry of a major regional unconformity, and the Mutá fault. A number of alternative realizations of the conjugated unconformity/Mutá fault surface were generated, using four seismic interpretations. A p-Field-like algorithm was used to sample from a probability function locally taken as a trapezoidal pdf, defined by four sub parallel envelopes built from the four interpretations available. An artifice was conceived in order to overcome a deficiency of the software for dealing with multiple alternative lateral extensions of the model. This artifice consisted in modeling the grid with an extension taken as a maximum, and then computing only the volumes of interest by informing Jacta™ a number of alternative possible realizations of the unconformity/Mutá fault surface, as if they where gas/oil contact scenarios. Accordingly, the real gas/water contact was given to Jacta™ as if it were an oil/water contact. Thus, with the corresponding fluid parameters informed in a coherent way, the gas volumes of interest where reported by Jacta™ as oil volumes. Other sources of uncertainty where taken into account by means of Jacta's regular workflow. Introduction The gas deposit of Manati field is situated in shallow waters of the Camamu basin and was discovered in the year 2000 by the Queiroz-Galvão/Petrobras/Petroserv consortium, through a prospect based on the seismic interpretation of a small number of low quality 2D lines. Fig.1 shows the field's location map. The reservoir is constituted by the tabular fluvial sands of the Sergi Formation, caped by the shales of ItaÍpe Formation and sealed laterally at north and east by the shales of Morro do Barro Formation, which were deposited in the space created by the erosive unconformity of the Tinharé Canyon, of Rio da Serra age. At east and south, the Mutá fault (north-south) and yet another northeast-southwest fault seal the accumulation through the contact with the same shales of Morro do Barro Formation The poor definition of the available 2D seismic lines, visible in Fig.2, induces considerable amount of uncertainty on every geometric elements of the field, particularly on the unconformity and fault's position, which reflects clearly on the different interpretations available.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAn uncertainty analysis study of the gas volumes in the field of Manati was conducted using gOcad™/Jacta™. The main uncertainties are associated to two important lateral limits of the reservoir, both ill-defined due to the poor quality 2D seismic lines available: the geometry of a major regional unconformity, and the Mutá fault.

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