Reservoir rock is an important part of the petroleum system. Reservoir rocks generally has good porosity and permeability, such as sandstones and limestone which have a lot of cavities that can store and drain the fluid. Carbonate rocks are the reservoir rock for oil and gas concerning in recent years the oil and gas industry . Limestone is a term used for a group of rocks that contain at least 90% calcium carbonate or magnesium. The term is also used for the limestone rocks that have a fraction more than the non-carbonate. In limestone, the fraction is composed mainly by mineral calcite.
Knowing the characteristics of the reservoir is very important because it describes the state of the reservoir characteristics or reservoir conditions such as length, width, type of lithology (rock), thickness, porosity, and others that aims to find oil reserved in the reservoir, so that it will be related to applying of suitable exploration and exploitation hydrocarbon.
One that is included into the carbonate rocks are Reef. Reef may be an important reservoir rock. In general, reef consists of a framework of coral, algae and so on of material carbonate growing in clean sea, high-energy waves and having a lot of leaching. So the cavities to be very clean. Also among this framework there are many fragments of corals, foraminifera from other bioclastic granules.
In the oil reservoir, porosity in carbonate rocks (reef) describes the percentage of total available space to be occupied by a liquid or gas . In this case, the porosity is obtained mainly within the framework shaped cavities former filling in cavities creating addition of porosity. In reverse, there is some condition causing the loss of porosity when organism filling in cavities which is usually cemented by spary calcite.
Furthermore, other factors affecting the porosity are dissolution (leaching), which generally will increase the porosity and permeability , dolomitization which will increase the porosity by creating pores bigger, or it can even reduce porosity in the event when growth of interlocking mosaic in dolomite crystals. This process often increases significantly due to the formation permeability dilution hole (solution vug) and post-accumulation cracks (post-burial) greater . Cracks (fracturing) due to brecciation, fault or muscular to be increased permeability , Recrystalization by neomorphism of micrite into a crystal size that would increase the porosity , and cementation will reduce porosity.
Beside that , permeability factor also affects the content of hydrocarbons in the rock. Permeability of the reservoir rock is a trait of rock to pass the liquid through the pores, or forming particles without damaging the rock framework.
The future is a carbonate reservoir. It is said that the volume of carbonate in the world is about 40%. However, the total reserves of carbonate around 60%, this is due to the large pore, so as to keep the hydrocarbons in amounts relatively more.
so what are you waiting for? Carbonate reservoir rocks are ready for you to explore !
Ganjar Asandi P.
Treasurer of AAPG UGM – SC