--> Abstract: Sequence Stratigraphy on a Braided Fluvial System: a Predictive Model on Reservoir Quality, Recõncavo Basin, Brazil, by R. R. Savini, F. M. Oliveira, and A. J. C. Magalhães; #90933 (1998).

Datapages, Inc.Print this page

Abstract: Sequence Stratigraphy on a Braided Fluvial System: a Predictive Model on Reservoir Quality, Recõncavo Basin, Brazil

Savini, R. R.; Oliveira, F. M.; Magalhães, A. J. C. - Petrobras/E&P

The main reservoirs of the Buracica oil field (Recõncavo Basin) are sandstones, mostly fluvial in origin, belonging to the Sergi Formation (Upper Tithonian). These coarse-grained subarkoses were deposited in an intracratonic basin by a braided fluvial system in arid to semi-arid climate conditions. The objective of this study, based on well logs and 223 meters of cores, is to identify the elements that control the porosity and permeability conditions of the reservoirs in the upper part of the Sergi Formation. The studied section is limited by two levels of paleosol, interpreted as sequence boundaries, with basin scale continuity. The internal depositional systems are vertically stacked series of braid-belts, each of them limited at the bottom by an erosive feature and at the top by fine sediments from the abandonment phase of the fluvial system generally enhanced by mechanically infiltrated clays. These floodings, interpreted as parasequence boundaries, would have happened within a 100,000 years periodicity. The stacking pattern of parasequences shows two different sets: a Transgressive System Tract (TST), with a retrogradational pattern, and a Highstand System Tract (HST), with an agradational to progradational pattern (Fig. 1). The parasequences boundaries show reservoir scale continuity. Sand-bodies have a tabular external geometry, with high reservoir horizontal connectivity. Due to the lateral extent of clay horizons above the entire field, reservoir vertical connectivity is low.

Wind reworked fluvial bars present the best reservoir characteristics, but the early diagenetic process of clay infiltration due to water table fluctuations plays an important role on the control of the reservoir quality. Base level fluctuations, coupled with climate variations, is considered to be the main trigger mechanism of water table changes. Rising of base level and of water table during TST acts as an inhibiting barrier to the descending percolation of muddy waters and constitutes an efficient mechanism to preserve wind reworked fluvial bars. These conjugated processes had favored the maintenance of excellent reservoir porosity and permeability conditions. In the HST, the successive falling of the base level and of the fluvial equilibrium profile have permitted the deposition of coarser and heterogeneous sediments, mainly trough cross-bedding sandstones. Progradation with associated erosion subdues the preservation of wind reworked fluvial bars. As the water table is depressed, mechanical clay infiltration is enhanced. As a result, permeabilities at TST are considerable greater than at HST (fig.2). Some predictable facts by the sequence stratigraphy approach were actually observed, corroborating the model: a) the best reservoir-rocks are at TST, due to better conditions to preserve wind reworked fluvial bars during the rise of water table; b) mechanically infiltrated clays are predictable to be less frequent in TST than HST, as a consequence of water table rise leading to best reservoir quality; c) the two levels of paleosol have basin-scale continuity, forming effective permeability barriers; d) the interpreted parasequences can be adopted as flow units or production zones, once their boundaries have field-scale continuity, constituting reservoir-scale heterogeneities.

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