Architecture of deep-water reservoirs

Abstract

The integration of subsurface observations with analog data (outcrop and modern systems) permits three basic architectural types of deep-water reservoirs to be defined; which are: (1) discrete-channel complexes, (2) amalgamated-channel complexes, and (3) distributary-channel and lobe complexes. Discrete-channel complex is the type of deep-water reservoir in which channel fills occur as spatially seg-regated (discrete) elements. Amalgamated-channel complexes are reservoirs formed by laterally-stacked shallow channels. Distributary-channel and lobe complexes are formed by a network of radial distributary channels and associated lateral (spill) and frontal (terminal) lobes. In terms of facies, discrete channel complexes are characterized by the intercalation of thick-bedded, coarse-grained channel-fill deposits, and thin-bedded, fine-grained overbank deposits. Amalgamated channel complexes are characterized by thick sandstone sequences, with relative absence of fine-grained overbank deposits, while lobe complexes present a better preservation the upper parts of the sand beds, including the interbedded fines. A repetitive hierarchy of architectural ele-ments has been recognized, including (1) individual elements, (2) composite elements, and (3) complexes. Although initially defined for channel complexes, such hierarchy has also been observed in lobe complexes. In addition, the hierarchical scheme can also be related to the different orders of high-resolution stratigraphy. In most cases individual elements would commonly be found within 6th-order units, and composite elements within 5th-order units. 4th-order units would then envelope channel and/or lobe complexes. The three reservoir types presented here are common in the basins of the Brazilian continental margin. Even though all types are found at different ages, in the Cretaceous and Lower Tertiary (Paleocene-Eocene), narrow troughs generated by salt-tectonics are predominantly filled by amalga-mated-channel complexes, which form the main reservoirs of fields like Namorado, Carapeba, Roncador, Jubarte, among others. In the upper Tertiary (Oligocene-Miocene), where salt tectonics generated a mini-basin morphology, large lobe com-plexes are found in fields like Marlim, Marlim Sul, and others. The recognition and differentiation of the different reservoir architectural styles is useful to collect and organize information about the reservoir external geometry and internal heterogeneities, to guide the construction of three-dimensional models, and to orient reservoir management.