Carlos H.L. Bruhn
Rio de Janeiro, Brazil
Funded by the AAPG Foundation through the Allen P. Bennison Endowment.
Contrasting Types of Oligocene/Miocene, Giant Turbidite Reservoirs from Deep Water Campos Basin, Brazil
The most prolific Brazilian turbidite reservoirs are included in the Upper Oligocene/Lower Miocene section (32.9-23 Ma) from the present day deep water (400-2500 m) Campos Basin; they contain a total oil-in-place volume of 19.8 billion bbl, and total oil reserves of 5.0 billion bbl, which are mostly concentrated in seven oil fields (Albacora, Barracuda, Caratinga, Marlim, Marlim Leste, Marlim Sul, and Voador). Oligocene/Miocene turbidites form part of a Middle Eocene to Recent regressive succession, which typically displays a progradational pattern throughout the eastern Brazilian margin.
The first discoveries of giant Oligocene/Miocene oil fields in the deep water Campos Basin date from the mid 1980’s. At the beginning, they were considered as homogeneous, widespread turbidite fans. However, the information provided by more than 300 wells, extensive coverage of 3D seismics, hundreds of meters of cores, and cumulative production data have changed this first picture.
More recent studies have found that the Oligocene/Miocene turbidite reservoirs from deep water Campos Basin can be very complex and heterogeneous. This presentation is focused on the stratigraphic framework, sandbody geometry, and reservoir heterogeneities of the most important, contrasting types of Oligocene/Miocene turbidite reservoirs, which include
- trough-confined, gravel/sand-rich channel complexes,
- unconfined, sand-rich lobes heavily dissected by younger, mud-filled channels,
- unconfined, sand-rich lobes,
- trough-confined, sand-rich lobes, and
- sand/mud-rich channel-fills and splays.
Type 1 is illustrated by the Albacora Field, and types 2, 3, 4, and 5 are described from the Barracuda, Marlim and Marlim Sul fields.
The Oligocene/Miocene architectural types of turbidite reservoirs typically comprise the lowstand systems tracts of distinct 3rd- to 4th-order sequences, which can be bounded in the deep water portion of Campos Basin by unconformities and/or correlative, non-erosive surfaces. Some of the sequence boundaries can be correlated to the Haq’s et al. (1988) eustatic, third-order sea-level falls of 30.0 Ma, 28.4 Ma, 26.3 Ma, and 25.5 Ma. However, other sequence boundaries can be recognized, including two undated boundaries between 26.3 Ma and 25.5 Ma, and the sequence boundaries of 25.0 Ma and 24.5 Ma. The transgressive and highstand systems tracts of the sequences mapped in the oilfield areas are composed of cyclically interbedded marls and mudstones containing benthic foraminifera characteristic of upper to lower bathyal settings.
Regional stratigraphic correlations suggest that the Albacora Field gravel/sand-rich channel complexes can be time-equivalent (along basin strike) to lobe successions of Barracuda, Marlim, and Marlim Sul fields. The development of very contrasting turbidite types seems to be related to tectonically-controlled basin gradient/confinement and sediment supply.
Gravel/sand-rich channel complexes occur in areas with slope oversteepening due to upward movement of underlying Aptian evaporites and intense faulting; steep slopes seem to have favoured deep channel incision by turbidity currents, rather than accumulation of turbidite lobes. On the other hand, unconfined lobes (types 2 and 3) fill intra-slope, wide depressions with gentle bottom gradients, which are also related to withdrawal of underlying, Aptian evaporites. In the area of the Barracuda and Marlim Sul fields, the stacking of types 2 (Barracuda), 3 and 4 (Marlim Sul) gave rise to a progradational, offlapping succession.
Type 2 reservoirs include more proximal, unconfined sand-rich lobes, which were heavily dissected by low-sinuosity, mud-filled channels, probably during the relative sea level fall that gave rise to the progradation of the turbidite system to southeast and the accumulation of Type 3 reservoirs. Type 4 reservoirs comprise elongated, sand-rich lobes, which filled fault-bounded, strike-oriented troughs located farther into the basin; these reservoirs were mostly fed by channels that managed to divert or partially erode Type 3 lobes. Types 2, 3, 4 are overlain by a marl-rich condensed section (marker bed red) that can be widely correlated in deep water Campos Basin.
Following another relative sea level fall, it took place the development of the thick (up to 125 m), sand-rich succession of Marlim Field (mostly Type 3), located to the north of Barracuda and Marlim Sul fields. Sand/mud-rich channel-fills and splays (Type 5) filled a depression in between two major depocenters of Type 3 reservoirs at the Marlim Sul Field, following a sea level rise that led to the end of the turbidite sedimentation in the Oligocene/ Miocene Campos Basin.
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