Abstract: Sedimentologic Architecture and Permeability Structure of Braided Fluvial Deposits in the Cretaceous Açu Formation, Potiguar Basin, Northeast Brazil

Becker, M.R. - Petrobras/Cenpes; N. Tyler - Bureau of Economic Geology, The University of Texas at Austin; and L. W. Lake - Dept. of Petroleum and Geosystems Engineering, The University of Texas at Austin

The focus of this paper is the quantification and prediction of depositional variability of reservoir properties in braided fluvial rocks at the oil field interwell scale based on hierarchical boundaries of depositional elements and lithofacies architecture of rock exposures. The sandy, bed load, fluvial facies of the Cretaceous Açu Formation in the Potiguar Basin, northeast Brazil, provide an example of the substantial petrophysical variability involved in this kind of fluvially deposited reservoir.

Depositional architecture of reservoir flow units, baffles, and barriers is primarily determined by the sedimentary processes that controlled the erosion, transport, and deposition of sediments. Braided fluvial reservoir heterogeneity is related to depositional lithobodies. Megascale reservoir heterogeneity encompasses 3 consecutive hierarchical levels of depositional elements (Fig. 1): (1) 1 to 10 m-thick, 380 m-length (outcrops extension), coarse-grained sandstone sheets intercalated with 1 to 3 m-thick and outcrops-extension-length, tabular mudstone layers, limited by 5^th-order boundaries, (2) up to 4 m-thick and to 80 m-length, storeys, i.e., a combination of bar and in-channel deposits, limited by 5^th- and 4^th-order boundaries, and (3) non-differentiated bedsets/beds, up to 2 m-thick and to 50 m-length, limited by lower-order boundaries. At this megascale, the higher the hierarchical position of the boundary surface among these elements, the thicker, more continuous and, consequently, more effective the reservoir flow constraint. At the macroscale, reservoir heterogeneity is formed by both element boundary and internal lithofacies variation.

Arrangement of depositional elements inside the sandstone sheets defines two styles of sedimentologic architecture: (a) an uninterrupted, fining-upward sandstone sheet style and (b) a sandstone sheet style internally truncated by an erosive surface, without defined textural trend. The difference between the two styles is related to a distinct evolution of depositional subenvironments present in the depositional system. The first style corresponds to the migration of a meta-stable island into a first-order channel position. The second style records an abrupt shift in depositional conditions because of channel-belt avulsion.

The close relationship of lithofacies architecture and permeability structure was confirmed by facies-related fluctuations in semivariograms of sandstone sheet and storey-scale elements. Also, the characteristics of the permeability semivariograms at sandstone-sheet, storey, and bed/bedset/lithofacies levels generally indicate the internal presence of smaller scale heterogeneity elements (nested structures) embedded at each level of the sedimentologic elements.

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