Modeling the Distribution of Diagenetic Elements within Deltaic Reservoirs of NE Brazil

MORAES, MARCO A. S., University of Wyoming, Laramie, WY

Lacustrine deltaic sandstones are important reservoirs in Potiguar and Reconcavo rift basins of northeastern Brazil. They commonly occur as delta front deposits accumulated in lows associated with growth faults. Diagenetic elements, including calcite, secondary porosity, dolomite, and chlorite, present specific distribution patterns at different reservoir levels. Such distributions have been modeled based on thin section, core, and petrophysical data.

At the microscopic level, the most significant heterogeneity is the association between reduced primary (RP) porosity and carbonate dissolution (CD) porosity. Reduced primary porosity commonly occurs

as islands of small pores surrounded by zones presenting larger CD pores. This type of pore structure causes high irreducible water saturation to appear in many reservoirs. At a larger scale level, dolomite cement is observed associated with micas and mud clasts concentrated in discrete stratification planes, a feature that increases permeability anisotropy, lowering the effective permeability of cross-bedded zones. Chlorite rims cause permeability reduction in fine-grained sandstones but have little influence in coarser sandstones. This grain size dependent effect produces significant alterations in the permeability structure of the reservoirs. At the field wide level, the more important diagenetic heterogeneity is the increase of carbonate cementation toward the border of the accumu ations, either faults or sandstone pinch-outs.

Relating the distribution of diagenetic elements to the facies architecture of deltaic sandstones permits generating reservoir models that integrate depositional and diagenetic heterogeneities at different levels, an approach that leads to more accurate determination of effective properties used for performance prediction and numerical simulation.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)