Diagenetic Heterogeneity and Reservoir Quality: Fluvial, Deltaic, and Turbiditic Sandstone Reservoirs, Potiguar and Reconcavo Rift Basins, Brazil

Abstract

Fluvial and lacustrine (deltaic and turbiditic) sandstones are major hydrocarbon reservoirs in the onshore (Lower Cretaceous) Potiguar and Reconcavo rift basins in northeastern Brazil. Diagenetic elements including mechanically infiltrated (MI) clay, calcite, dolomite, and chlorite show distinct distribution patterns at interwell and field-wide reservoir scales. Such distributions, and thereby diagenetic heterogeneity, have been modeled based on thin-section, core, well-log, and petrophysical data. At the interwell scale, relationships between sandstone body geometry and the distribution of diagenetic elements are a critical aspect of reservoir heterogeneity. At this level, (1) MI clay concentrations occur in fluvial reservoirs following the orientation of paleochannels, generating strong compartmentalization of the reservoirs, (2) calcite cement appears either concentrated near sand-shale contacts (peripheral distribution) or dispersed in the interior of the sandstone layers (scattered distribution), (3) dolomite cement may appear concentrated along laminations in cross-stratified sandstones, causing a decrease of one order of magnitude in the effective horizontal permeability, and (4) authigenic chlorite is observed to reduce more significantly the permeability in fine-grained sandstones than in coarse-grained sandstones, an effect that may cause important modifications in the permeability structure of the reservoirs. At the field-wide scale, diagenetic heterogeneity includes (1) stratigraphic zonation of diagenetic properties in fluvial reservoirs, (2) increasing carbonate cementation toward the border of the hydrocarbon accumulations in deltaic reservoirs, and (3) distinct patterns of carbonate cementation observed in channel fills and lobes in turbiditic reservoirs. Incorporating the distribution and effects of diagenetic elements into the interwell and field-wide geological modeling is essential to achieve a realistic reservoir representation. Such incorporation is performed by the integration of diagenetic properties with other geological attributes, including nature and distribution of depositional facies, structural elements, and stratigraphic framework. This procedure improves reservoir quality evaluation and leads to more precise prediction of reservoir performance.