Six vertical and directional drillings in Tugu Barat (TGB) field have been proven to be oil and gas production wells. They are infiltrating three hydrocarbon reservoirs, consecutively from the oldest, the Lower Miocene Baturaja Formation, the Middle Miocene Zone 14 and Zone 16 of Upper Cibulakan Formation. Baturaja Formation is composed by limestone intercalated with shale. While the thin Zone 14 and Zone 16 are composed mainly by limestone.

High Carbon Dioxide (CO2) concentration examined in wells results various concentrations without any pattern vertically or horizontally. This gas concentration has been being an interesting phenomenon. Intensive geochemical and petrophysical experiments are needed to predict the sources and process of the CO2 and the effects of CO2 generation in migration and production rates.

Two possibilities have been concluded to be answer of the problem : CO2 is generated from Decarboxylation of coals in the Upper Oligocene-Early Miocene Talangakar Formation or is accounted for by dissolution of the Baturaja carbonate reservoir by formation water, although questioned, has caused in enhancement of CO2 generation.

CO2 generation plays role, although minor, in secondary porosity (fractures). Gas generation, usually causes an over- pressuring inside the formation due to the increasing pore volume. The Decarboxylation is a rapid and ephermal process and can be associated with dissolution of carbonate material.

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