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Abstract: São Francisco Basin - Exploration Frontier for Gas in Central Brazil

Romeiro Silva, P. C.; O. P. G. Braun; P. V. Zalán; L. de Paula and M. Mattlns - Petrobras/E&P

The São Francisco Basin (300,000 km2) is an intra-continental sag basin filled by Late Proterozoic sedimentary rocks situated in the central interior portion of Brazil. Seismic analysis carried on 2,815 km of lines tied to 4 petroleum exploratory wells allowed the recognition of two major depositional sequences (hereby informally named Riphean and Vendian Sequences, RS and VS, respectively), separated by an important erosional surface that locally attains the character of an angular unconformity. Shallow crystalline metamorphic rocks and deep Middle Proterozoic aulacogens underlie the sag basin and constitute economic basement.

The RS corresponds to several lithostratigraphic groups: Macaúbas, in the east (conglomerates/diamictites, continental/marine/glacial), Paranoá (quartzites predominant, carbonates/shales subordinate, transitional) and Traíras(?) (carbonates and shales, marine restricted), to the west. The VS (Fig. 1) corresponds to one huge lithostratigraphic supergroup: Bambuí, composed of alternating carbonates and shales (open marine), buried by deltaic progradation of molasse sandstones.

Several seeps of termochemical gas and subcommercial flows of gas in three wells, from carbonate rocks of the Bambuí Sgp, as well as the presence of bitumen in dolomites and the occurrence of organic-rich black shales in the Trafras(?) Gp strongly suggest the presence of a once active petroleum system in the São Francisco Basin.

The suggested petroleum system of the basin would comprise: (i) source rocks: organic-rich black shales of the RS, with TOC's up to 4%-6%, at senile maturity levels, charging gas to (ii) reservoirs: eolian sandstones and fractured quartzites of the Paranoá Group, dolomites and fractured carbonates of the Bambuí Sgp., capped by (iii) seals: organic-lean black shales and tight carbonates of the Bambuí Sgp, all folded into (iv) traps: anticlines associated to different age compressional stresses (Fig. 2), an older event affecting the RS (tectonic transport from SE to NW) and a younger (Brasiliano Orogenic Cycle, from W to E) responsible for a remarkable thin-skinned detached fold-and-thrust belt (Brasilia Belt).

Although the risks associated to the exploration of petroleum in Proterozoic basins are still large it is nowadays well established that commercial accumulations of oil and gas are possible worldwide. Several large fields in the Siberian Platform (where the giant Yurubchen field shines with several billions of barrels of oil and tens of tcf of gas in place), oil production from Oman and gas production from the Sinian in China attest to this reality. The São Francisco Basin has the same rocks as the basins mentioned, the seismic resolution is overwhelmingly good and the evidence for a once active petroleum system is strong. The basin is large, practically unexplored and certainly worth the risk.

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