ABSTRACT: Lithic Reservoir Diagenesis of Cretaceous Carmopolis Member Sergipe-Alagoas Basin, Brazil

Rogerio Schiffer Souza

The Lower Cretaceous sandstones and conglomerates of the Carmopolis Member are important oil reservoirs in the Sergipe-Alagoas basin, northeastern Brazil. The reservoirs are lithic phyllarenites, rich in schist, phyllite, and slate fragments. This composition is seldom seen in oil reservoirs because the mechanical compaction and deformation of such constituents usually eliminates most of the porosity at the beginning of diagenesis. This apparent anomaly was analyzed in detail.

The diagenetic history comprises an eogenetic stage, influenced by hypersaline lacustrine brines, and a compactional mesogenetic stage. The sequence of diagenetic processes was (1) mechanical infiltration of clays; (2) precipitation of early poikilotopic dolomite or calcite/aragonite rims; (3) mechanical compaction; (4) quartz and feldspar overgrowths; (5) cementation and grain replacement by ferroan calcite; (6) generation of secondary dissolution porosity; (7) reactivation of mechanical compaction; (8) cementation and grain replacement by late ferroan dolomite/ankerite; (9) second stage of porosity generation; and (10) precipitation of kaolinite, quartz, chlorite-smectite clays and sulfides.

The quantitative analysis of reservoirs ranging from 180 to 2200 m depth has shown that mechanical compaction was, as expected, the main process of porosity destruction. Compaction affected mostly the beds

lacking early dolomite cementation, where lithic micaceous fragments were deformed and squeezed into the pore spaces. Early dolomite was essential to support the mechanically unstable lithoclasts against increasing overburden, preserving the potential porosity and permeability. This interpretation was confirmed by experimental analysis of porosity and permeability reduction under confining pressures of 500, 1000, and 2000 psi, developed in samples with variable lithoclasts and dolomite contents.

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