DIAGENETIC HETEROGENEITY AND RESERVOIR QUALITY - FLUVIAL, DELTAIC, AND TURBIDITIC SANDSTONE RESERVOIRS, POTIGUAR AND RECONCAVO RIFT BASINS,BRAZIL

Fluvial and lacustrine (deltaic and turbiditic) sandstones are major h ydrocarbon reservoirs in the onshore (Lower Cretaceous) Potiguar and R econcavo rift basins in northeastern Brazil. Diagenetic elements inclu ding mechanically infiltrated (MI) clay, calcite, dolomite, and chlori te show distinct distribution patterns at interwell and field-wide res ervoir scales. Such distributions, and thereby diagenetic heterogeneit y, have been modeled based on thin-section, core, well-log, and petrop hysical data. At the interwell scale, relationships between sandstone body geometry and the distribution of diagenetic elements are a critic al aspect of reservoir heterogeneity. At this level, (1) MI clay conce ntrations occur in fluvial reservoirs following the orientation of pal eochannels, generating strong compartmentalization of the reservoirs, (2) calcite cement appears either concentrated near sand-shale contact s (peripheral distribution) or dispersed in the interior of the sandst one layers (scattered distribution), (3) dolomite cement may appear co ncentrated along laminations in cross-stratified sandstones, causing a decrease of one order of magnitude in the effective horizontal permea bility, and (4) authigenic chlorite is observed to reduce more signifi cantly the permeability in fine-grained sandstones than in coarse-grai ned sandstones, an effect that may cause important modifications in th e permeability structure of the reservoirs. At the field-wide scale, d iagenetic heterogeneity includes (1) stratigraphic zonation of diagene tic properties in fluvial reservoirs, (2) increasing carbonate cementa tion toward the border of the hydrocarbon accumulations in deltaic res ervoirs, and (3) distinct patterns of carbonate cementation observed i n 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 re servoir representation. Such incorporation is performed by the integra tion of diagenetic properties with other geological attributes, includ ing nature and distribution of depositional facies, structural element s, and stratigraphic framework. This procedure improves reservoir qual ity evaluation and leads to more precise prediction of reservoir perfo rmance.