Carbon dioxide that released by burning fossil fuels to atmosphere caused climate change, global warming and very resposible
for greenhouse-gas effects. Therefore creating a system to keep CO2 permanently underground (geosequestration) means directly
reducing the emmisions.
Indonesia commitment based on President‟s message in G20 meeting Pittsburg to reduce CO2 emissions to 26% in 2020 and even
more in further decades. Carbon Capture Storage (CCS) is one of the technologies that can be implemented to achieve the goal
but finding apropriate location must be evaluated more detail.
Gundih field that operated by Pertamina EP in Central Java Indonesia is part of the back arc basin which consists many reservoir
such as clastic and carbonate Kujung Formation and sandstone of Ngrayong Formation. Especially for Ngrayong Formation, it
will be discussed more detail because will be used as first candidate to be reservoir of CO2 injection storege, overlain by sandy
bioclastic limestones of Bulu Formation and claystone of Wonocolo Formation as seal layer.
The properties of reservoir was one of first stage in geological evaluation to select potential storage and to evaluate the CO 2
trapping capacity in subsurface. To get the information, geology mapping and two shallow core drilling in Tuban area has been
done to get representative samples of the reservoir and later to be taken to correlated with surrounds area near Jepon-1 well as
injection well. Detail macro and microscopic observation of Ngrayong Formation combined with labroratory analysis in shallow
drilling samples reveals certain characteristic of the reservoir. As lithology itself, loose clastics quartz sandstone of Ngrayong
Formation from outcrop shows good porosity (20-30%) and sufficient thickness for injection. Geological analogue to correlate
between surface exposure and subsurface approached by integration between surface cross section and 2D seismic that revelaed
diminishing of reservoir thickness and depth elevation of Ngrayong Formation toward Jepon-1 well.