ABSTRACT: Patterns and Processes of Diagenesis in Cretaceous Rift Sequence of Potiguar Basin, Northeastern Brazil

S. M. C. Anjos, L. F. Sombra, L. F. De Ros, R. S. Souza, E. V. Santos, R. Waick

The Potiguar Basin originated as an oblique rift during the separation of the South American and African plates. The basin was filled mostly with the Lower Cretaceous clastic sequence of the Pendencia Formation. Oil reservoirs in this unit are feldspathic sandstones from fluvial, fan-deltaic, and delta-front turbiditic systems closely associated with organic-rich lacustrine shales.

The lacustrine shales contain an average of 3% total organic carbon, with type II/I mature kerogen. Their clay mineralogy is composed mostly of ordered intrastratified I-S with 80-85% illite layers, which points to an advanced thermal evolution of the basin.

Diagenesis of the reservoirs follows various pathways due to variable interactions among detrital grains, fluids released from shale compaction, clays and organic diagenesis, and meteoric water infiltration.

Different sequences and intensities of diagenetic processes affected the fan-deltaic, fluvial, and deltaic-turbiditic sandstones. Mechanical compaction and kaolinite cementation prevail in the fan-deltaic reservoirs. Carbonate cements (calcite and/or late ferroan dolomite) or chemical compaction are more abundant in the deltaic-turbiditic reservoirs than in the fluvial reservoirs. These differences are directly related to reservoir quality. In fluvial reservoirs porosity and permeability decrease at low rates, whereas in fan-deltaic systems porosity and permeability rapidly decrease with depth.

The diagenetic patterns of the Pendencia reservoirs are being incorporated to the basic criteria of exploration in undrilled areas and to an integrated modeling of porosity evolution within the basin.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990