Deposition and diagenesis of Miocene limestones, Senkang Basin, Sulawesi, Indonesia

Abstract

Miocene Knoll-reef limestones form gas reservoirs in the East Senkang Basin of southwest Sulawesi. They are composed predominantly of packstones and wackestones, but grainstones and boundstones also occur. Uplift and exposure of the reef occurred during deposition. This, together with burial, resulted in dissolution of bioclasts, precipitation of meniscus and blocky spar cements, extensive fissuring of the rock and filling of the fissures by internal sediment, predominantly carbonate silts. The meniscus cements are unusual in that they occur mostly within biomouldic pores, particularly after corals. The meniscus form of the cement may be a result of air bubbles trapped within the complex pore-system formed from the dissolution of corals. The fissures, which are probably a product of fracturing associated with local fault movements, form complex networks linking biomouldic pores. Carbonate internal sediments, mostly crystal silt but also pelleted, which form thin graded layers in places, completely fill the fissures and adjacent biomouldic pores. These features, in association with bioclasts dissolution and meniscus cements, suggest a vadose origin. This interpretation is supported by the variation in the ϵ 180 and ϵ 130 stable isotope values of the internal sediment compared with the depositional matrix. Equant sparry calcite cements are locally extensive. On the basis of only moderately depleted ϵ 80 values and elevated strontium values they are tentatively interpreted to be of marine phreatic origin.