Complex Mixed Carbonate-Siliciclastic Albian Reservoirs from the Espírito Santo Basin, Eastern Brazil
The reservoirs of the Fazenda Santa Luzia and Fazenda São Rafael oilfields, onshore Espírito Santo Basin, eastern Brazil, are constituted by an extremely complex alternation of siliciclastic sandstones and conglomerates, carbonate grainstones and rudstones, and hybrid arenites. The combination of the tri-dimensional intercalation of clastic and carbonate deposits with intense normal faulting and differential diagenesis generated highly compartmentalized reservoirs, with erratic lateral and vertical distribution of pressures and fluids. The best reservoirs correspond to massive to low-angle-stratified, bioturbated, medium to coarse sandstones and sandy conglomerates, essentially composed of angular quartz and feldspar grains, and plutonic rock fragments. These deposits display preservation of intergranular primary porosity, due to limited cementation by K-feldspar overgrowths, dolomite, calcite and kaolinite-dickite, and porosity generation through grain fracturing, dissolution of feldspar grains and carbonate cements. Associated fine-grained sandstones are extremely rich in biotite and strongly compacted. The carbonate grainstones and rudstones are massive, locally normal-graded, and made mainly by reworked, abraded oncoliths and microbial intraclasts, red algae, bivalve and echinoid bioclasts, and peloids. Their primary porosity was extensively obliterated by chemical compaction and calcite cementation. The hybrid arenites, composed of a mixture of these intrabasinal and extrabasinal constituents, show variable porosity. Hybrid arenites with larger amounts of carbonate grains suffered stronger reduction of primary porosity through calcite cementation and pressure dissolution of carbonate grains. The reservoirs are interpreted as the product of re-deposition of alluvial clastic, shallow-marine carbonate and hybrid sediments into deeper water by gravitational flows, possibly with episodic storm reworking. This is suggested by the chaotic alternation of clastic, hybrid and carbonate, commonly massive deposits, and the rarity of oscillatory flow structures. Storms would also provide the high-frequency reworking of carbonate deposits, which constitute the background sedimentation in shallow-marine ramp environments. The definition of the major depositional and diagenetic controls on the quality and heterogeneity of these complex, mixed reservoirs is essential for the construction of realistic models aimed at increasing oil recovery efficiency.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California