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Abstract: Hydrocarbon Potential of the Upper Mississippian Carbonate Platform, Bechar Basin, Algerian Sahara

MADI, D. ACHOUR, and ERWIN UNGER, Petrel Robertson Ltd., Calgary, Canada; FERIAL BENDERRA, Division Exploration, Sonatrach, Algeria.

The Western Bechar Basin contains a 2000-3000 m thick Carboniferous succession of carbonates consisting of upward shoaling sequences. Wihin the thick Upper Mississippian, several phases of biohermal development are observed. At the bottom of each bioconstructed phase lie Waulsortian-type mounds (up to 120 m-thick) comprised of sponge-fenestellid bafflestone-wackestone passing upward into massive crinoidal-bryozoan wackestone and overlain by well-bedded crinoid and oolitic grainstone. Several seismic anomalies observed in the subsurface of the Bechar Basin indicate the presence of probable Waulsortian-like mounds. These Mississippian carbonates lie unconformably on Upper Devonian shales (Frasnian), a potential oil-prone source rock in the basin, and are sealed by a very thick section of black shales belonging to the Serpukhovian.

During the last decade, Waulsortian bioherms have been petroleum exploration targets since significant oil discoveries have been made within these deposits in North America. This paper presents the hydrocarbon potential of the Mississippian Carbonate Platform as the diagenetic evolution study of these deposits indicates that primary porosity was preserved until late burial.

The diagenetic sequence contains five calcite cements, dolomitization, and silicification that were investigated for isotope geochemistry and fluid inclusion microthermometry. Temperatures of cementation, inferred from stable oxygen isotopes and two-phases fluid inclusion temperature of homogenization, indicate that most pores within the buildups were occluded after only a few hundred meters of burial. In contrast, pores in the ooid and crinoid grainstones stayed open after thousands of meters of burial (900 to 2500 m). It is hypothesized that the ooid and crinoid grainstones have kept open over significant burial depth and served as a regional conduit for hydrocarbon migration from the basin to the carbonate platform. Basin subsidence rate varied from 220 m/My during the VisCan, to 40 m/My during the Serpukhovian, which suggests that 20 My was needed to fill most porosity. As a result, the Western Bechar limestones can be considered a good target for petroleum exploration if the hydrocarbon migration occurred before the end of the Serpukhovian.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah