Print this pageAbstract: Meteoric-water Influx and Porosity Evolution in Turbidite Sandstones: Evidence from Late Cretaceous/Early Tertiary Reservoirs of Espírito Santo Basin, Eastern Brazilian Margin
De Ros, L. F.; E. E. Zambonato - Universidade Federal do Rio Grande do Sul; S. Morad - Uppsala University
The Espírito Santo Basin covers 25,000 Km2 (3220 Km2 onshore) of prospectable area in eastern Brazil passive margin. The main reservoirs of the basin are turbidites of the Urucutuca Formation, which contain ~60 million barrels of recoverable oil. In the onshore portion of the basin, turbidites deposition occurred within submarine canyons incised in the border of the platform during sea-level falls that punctuated the overall transgressive setting of the Late Cretaceous and Early Tertiary. Canyon-filling turbidite successions are composed of anastomosed, channelled complexes of coarse-grained, massive sandstones, interbedded with bioturbated mudrocks and fine-grained asymmetrical levee deposits. Two major canyons were excavated in the basin: the Regência Canyon, containing the Lagoa Parda oilfield, and the Fazenda Cedro Canyon, containing the homonymous oilfield. Similar canyons filled by Urucutuca turbidites were incised northwards, in the Mucuri, Jequitinhonha and Almada basins. In the offshore part of the basin, large channelled turbidite lobes were deposited in lows originated by salt tectonics and associated lystric faults.
Canyon-filling turbidites are immature, micaceous arkoses with provenance from Precambrian migmatites, granulites and granites. They show extensive dissolution of feldspars and eodiagenetic calcite and siderite cements, and kaolinization of feldspars, micas and mud intraclasts (Fig. 1). Mesodiagenetic Fe-dolomite/ankerite and calcite engulfed and replaced the kaolinite (Fig. 2).
The stable isotope composition and crystal morphology of the kaolinite indicate that it precipitated from meteoric fluids, which is rather surprising in turbidites covered by thick marine shales. Petrologic and burial-history evidence revealed that the dissolution and kaolinization was related to the influx of meteoric waters during Tertiary major sea-level falls. The abundance of plant debris suggest a warm and humid Early Tertiary climate agreeing with global paleoclimatic reconstructions. Viable conduits for the meteoric waters included canyon margins large fault systems; or, more probably, basal and lateral communication with the Early Cretaceous alluvial sandstones. Kaolinite precipitation resulted in net redistribution and limited enhancement of porosity. Nevertheless, canyon turbidites reservoir quality is high, due to coarse grain size and limited compaction, even though these turbidites have a complex pore system and channelled depositional geometries.
Conversely, offshore turbidites display meteoric leaching limited to the shallower, proximal deposits that show mesodiagenesis replacement of kaolinite by dickite. The deep, distal turbidites show mesodiagenetic illite, Fe-calcite, albite and strong compaction. However, where early oil emplacement limited porosity reduction by compaction, offshore turbidites show good reservoir potential within large, salt-related structures.
AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil