--> Abstract: Major Types of Deep-Water Reservoirs from the Eastern Brazilian Rift and Passive Margin Basins, by C. H. L. Bruhn; #90923 (1999)

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BRUHN, CARLOS H.L., Petrobras E&P, Rio de Janeiro, Brazil

Abstract: Major Types of Deep-Water Reservoirs from the Eastern Brazilian Rift and Passive Margin Basins

Turbidites and associated deep-water facies comprise the most important petroleum reservoirs in Brazil.They contain original in place volumes of 57.2 billion bbl of oil, and 27.5 trillion cubic feet of gas, and total reserves of 12.5 billion bbl of oil, and 8.3 trillion cubic feet of gas. Brazilian petroleum-bearing turbidites occur in (1) Carboniferous/Permian, glaciomarine pre-rift (interior cratonic) successions, (2) Neocomian to Aptian, lacustrine rift successions, and (3) Upper Albian to Lower Miocene, marine passive margin successions. Most of the petroleum accumulations are distributed along the eastern Brazilian margin, which tectonic and sedimentary evolution is linked to the Neocomian breakup of Gondwana and the subsequent opening of the South Atlantic Ocean.Turbidites comprise 553 production zones from 171 oil and/or gas fields, mostly concentrated in the Campos, RecĂ´ncavo, Sergipe/Alagoas, and EspĂ­rito Santo basins.This paper presents an overview of the major types of Brazilian deep-water reservoirs, general geological setting of which is illustrated in Figure 1.

The major types of Brazilian deep-water reservoirs are: (1) Gravel/sand-rich, turbidite channel complexes (CC): 10-50 m-thick, 200-2,000 m-wide, and 0.5->10 km-long channel-fills, which can be clustered into 20-100 m-thick, 1-6 km-wide, and 1->10 km-long channel complexes. CC reservoirs have a complex, multi-storied geometry resulting from the amalgamation of many channel-fills and partial preservation of overbank/levee and/or background deposits between channel-fills. CC reservoirs typically fill submarine canyons or troughs incised into the shelf or slope, or fault-bounded troughs in deep lacustrine rifts. (2) Trough-confined, gravel/ sand-rich turbidite lobes (GSLc): 10-140 m-thick, 1-12 km-wide, and 3->20 km-long tabular/lobate reservoirs confined to intra-slope troughs defined by subsidence along listric faults soling out on underlying evaporites. (3) Unconfined, sand-rich turbidite lobes (SLuc): 5-60 m-thick, 2-8 km-wide, and 5-12 km-long lobes filling intra-slope, wide depressions with low bottom gradients, which were developed by downslope gliding of underlying evaporites. Some lobes can be heavily dissected (SLucd) by low- to high-sinuosity, 5-15 m-deep, 200-600 m-wide channels. (4) Sand/mud-rich turbidite lobes (SML): 2-20 m-thick, 1-20 km-wide, and 2->20 km-long lobes, which are composed of turbidite sandstones and frequently interbedded background mudstones and debris flow deposits.They occur in similar settings as those for SLuc reservoirs, or originated from sliding/slumping of sediments previously accumulated on steep-fronted, flexural margin deltas that prograded over deep lacustrine, rift basins. (5) Gravel/sand-rich aprons (AP): up to 2km-thick, 5-20 km-wide, and 5-200 km-long wedges deposited by high-density turbidity currents and debris flows along border fault margins of lacustrine rifts. (6) Sand-rich, lacustrine density underflow deposits (DU): up to 50 m-thick, 600-1,200 m-wide, and 1.5-4.5 km-long sandbodies, which are confined to narrow (< 5 km), fault-bounded troughs subparallel to border fault margins of lacustrine rifts. (7) Sand/mud-rich debris flow deposits (DF): Mounded to channelized, up to 400 m-thick, 200 - 4,000 m-wide, and 0.8-5 km-long muddy sandstone bodies, which are deposited by debris flows derived from the slumping of deltaic systems prograding into deep, lacustrine rifts. (8) Sandy bottom current deposits (BC): Elongated mounds (up to 35 m-thick, up to 20 km-wide, and over 25 km-long) transversely-oriented to the slope. BC mounds are composed of interbedded mudstones and thin-bedded, rippled sandstones, which overlie some of the most proximal SLuc reservoirs; i.e. those accumulated in slope depressions, near the lower reaches of canyons that fed the sand-rich turbidite systems. BC reservoirs seem to have been deposited by tide-related currents that flow up and down some submarine canyons.

Figures 2 and 3 show the distribution of oil and gas original volumes and reserves in Brazilian deep-water reservoirs, according to the major types of deep-water reservoirs described above. SLuc reservoirs are, by far, the most important turbidite reservoirs in Brazil, because they comprise thick, widespread, homogeneous (up to 125 m thick without interbedded mudstones), and highly porous and permeable sandstone bodies. GSLc reservoirs also comprise thick, homogeneous, and highly porous and permeable sandstone bodies, but have a smaller areal distribution (and volume). CC reservoirs tend to be more heterogeneous, and have a more complex geometry than GSLc reservoirs; however, CC reservoirs can be as important as GSLc reservoirs because they occur both in onshore and offshore basins, and comprise a much larger number of oil and/or gas fields. All the other types combined (SML,AP, DU, and DF) contain only 1.9% of the oil reserves, and 8.4% of the gas reserves; this can be explained by their smaller size, more complex geometry, and lower porosities and permeabilities. SML reservoirs are more important as gas reservoirs because they comprise a larger number of gas accumulations in the older and deeper sections from the continental rift megasequence. AP and DF are typical tight gas reservoirs. BC reservoirs contain only minor and still poorly evaluated oil and gas accumulations in present day deep water areas; they are composed of thin-bedded (< 20 cm) sandstones, which are frequently interbedded with mudstones.

Figure 1. - General geologic setting for Brazilian deep- water reservoirs.

Figure 2. - Original oil in place in Brazilian deep-water reservoirs.

Figure 3. - Original gas in place in Brazilian deep-water reservoirs.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England