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The Terumbu Limestone is the reservoir for 200 tcf of gas (72% CO2) in the Esso D-alpha block, offshore Indonesia. During the middle to late Miocene, 5,000 ft (1,500 m) of platform-reef carbonates were deposited. These limestones have a complex diagenetic history determined from study of 960 ft (290 m) of core from 3 wells.
Partial marine cementation and micritization of grains occurred in platform environments during deposition. Freshwater diagenesis followed, presumably below subaerial unconformities within and at the top of the Terumbu. Aragonitic grains were leached, high-magnesium calcite grains were converted to low-magnesium calcite, and pores were partially cemented by low-magnesium calcite. Pressure solution and further cementation during burial of the Terumbu to 10,000 ft (3,000 m) left only minor amounts of preserved primary and moldic porosity.
During late burial, grains that were originally high-magnesium calcite were leached, forming "interpenetrating" pores and stylolites "floating" within pores. Ferroan-calcite and dolomite cements line these pores and fluorite crystals occlude many pores. Whole-rock stable isotopes are depleted in O18 (-8.0 ^pmil ^dgrO18 PDB, 0.0 ^pmil ^dgrC13 PDB), suggesting high-temperature alteration of carbonate. The isotopic composition of the CO2 in the reservoir is similar (-0.8 ^pmil ^dgrC13 PDB), suggesting this CO2 is derived from dissolved Terumbu Limestone. We envision that fluoride-bearing hydrothermal fluids, derived from the underlying granitic basement, selectively dissolved constituents in the deeply buried Terumbu.
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