Abstract: Santos Basin - Merluza Field Turbidite Model Revisited

Enciso, Gonzalo and Tisi, Antonio Luis - Shell E&P International Ventures

The Merluza gas field was discovered in the Santos Basin in 1979 by Pecten/Shell International and its partners with drilling of the well 1-SPS-11 in the center of the structure. In 1982, the second well 1-SPS-21 was drilled in the southern part of the structure and found the shallow marine sands but not the deeper objective turbidite sands present in the first well. In 1984, Pecten/Shell without partners drilled the well 1-SPS-20 in the northern portion of the structure and found at and average depth of about 4500 meters the two gas bearing sands previously encountered in the discovery well. The upper sands are seismically transparent and interpreted as shallow marine, but the lower sand unit presents itself as a single deep marine turbidite lobe. Results of stratigraphic seismic studies along with well data and core studies indicate that the lower gas bearing zone consists of a turbidite channelized fan complex deposited during the Turonian age sea level low stand.

The discovery was developed by Pecten/Shell with six wells. These wells were proposed based on seismic amplitude analysis of the sand response in conjunction with well-calibration. This allowed the successful development of the field (refer to Fig. 1).The Merluza field started producing in 1993. The field, operated by Petrobras, produces today approximately 80 Million CFG/day with associated condensate production that averages 5,000 barrels per day.

The deposition of the deep marine turbidites of Turonian age is apparently confined to a northeast-southwest corridor. These are is bounded on the East by ridges of salt massifs and to the West by the paleo shelf slope (Turonian scarp). The Itajai Turonian turbidites represent the deepwater deposits that reflect a major sea level drop at the beginning of Turonian time and are to date the most important proven pay units in the Santos Basin.

Halokinesis and downward movement of salt began during deposition of the underlying Lower Cretaceous - Albian carbonates forming depressions where salt withdraw occurred. It is in these negative areas that Itajai turbidites are confined in a corridor parallel to the paleo slope. The Merluza ridge was formed under these conditions creating an elongated northwest-southeast trending anticline in the overlying sediments. Shell's original interpretation of the turbidite package was that it represented an isolated, single depositional lobe or at least the deposition of several lobes of variable thickness amalgamated in one main sandstone package. The recent study was done by interpreting seismic data acquired by Pecten/Shell in the past in the central part of the basin in water depths ranging from 100 to 500 meters of water column. The results of the stratigraphic seismic studies concluded that the lower productive sand in the field is not an isolated turbidite fan lobe but the latest turbidite package that thins and onlaps onto the Merluza ridge in a confined basin setting (refer to Fig. 2). The study also allowed the separation of the onlapping turbidite channelized lobes into at least two main turbidite packages: the Merluza amplitude "A" the latest turbidite package and lower productive interval in the field that overlies the Merluza amplitude "B". The map distribution of the amplitudes associated with the turbidite lobes "A" and "B" is presented in this paper. At least two older levels of turbidite deposition were recognized onlapping the Merluza ridge but not shown in this paper.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil