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Abstract: The Influence of Sedimentation and Early Diagenesis on the Petrophysical Attributes of the Carbonate Reservoirs of Macaé Formation, Campos Basin, Brazil

Marçal, R. A.; Spadini, A. R.; and Rodriguez, M. R. - Petrobras/E&P/Cenpes

The Albian carbonates of the Brazilian Continental Margin occur from the Pelotas to the Barreirinhas Basin comprising a 3,000km long gigabank. Despite their similarity in composition (mainly oncolitic/peloidaVoolitic) the petrophysical attributes of these carbonates show significant differences, mostly because of the burial depth and of the intensity of the meteoric diagenesis. Three carbonate fields, from southern Campos Basin, were studied in order to evaluate the early meteoric diagenesis and its relationship with the typical shallowing upward cycles that punctuated the sedimentation. Density, sonic, neutron and gamma ray logs were calibrated with core data in order to obtain log patterns correlated with reservoir characteristics.

A high-resolution stratigraphic analysis of the carbonate reservoirs allowed the identification of three distinct intervals: 1) the basal one comprises a shoaling-up cycle where the grainstones preserved most of the original intergranular porosity. These high energy facies were deposited over deep lagoonal peloidal facies being little affected by meteoric influx; 2) the intermediate interval is formed by small shoaling-up cycles, composed of peloidal and oolitic facies, deposited in a constant shallow water setting. Due to the intermittent exposure (island model) this interval underwent a strong meteoric influx which promoted intense cementation; 3) the upper interval is formed by two thicker shoaling-up cycles with high-energy facies in the upper part of each cycle. In two fields, these grainstones preserved most of the original intergranular porosity. In the third field, the grainstones are strongly cemented by sparry calcite as a result of a strong local meteoric influx. This process also generated a widespread moldic porosity resulting in low-permeability facies.

Core description, detailed petrographic analysis and log evaluation were performed in producing wells. Fifteen lithofacies were defined and were grouped into four clusters according to their textural attributes: 1) fine-grained peloidal packstones; 2) oncolitic/oolftic packstones; 3) microoncolitic/peloidal packstones; 4) oolftic/oncolitic grainstones.

In each of the three intervals, the petrophysical attributes of each cluster could be recognized by their log characteristics. Peloidal facies, characterized by low permeability (around 1mD), is discriminated by their highest gamma-ray values. The grainstones, the better reservoirs with permeabilities up to 3,000mD, are promptly identified by their lowest gamma-ray values. The cemented ones are differentiated by sonic logs, once the porous volume is dramatically reduced. The two other clusters (packstones) required other logs to be identified because of the wide range of permeabilities (1-100mD) that reflect the variation on depositional textures and intensity of cementation.

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