Abstract

The Sanga Sanga PSC in the Kutai Basin of East Kalimantan is host to several gas and oil fields with hydrocarbons principally reservoired in Miocene fluvio-deltaic channel sandstones. Limestones form only a subordinate lithology in this area and carbonate development is restricted both temporally and spatially. The thickest and most extensive limestones are in the uppermost G interval of the Badak Field where several discrete horizons of limestones are interbedded with shales, sandstones and coals. Limestones exhibit an overall thickening upwards trend although individual limestones often display strong lateral variations in thickness and are notably absent from some wells. These limestones do not form reservoirs whereas laterally adjacent sandstones display similar production characteristics to carbonate-free intervals immediately above and below. An extensive blanket of thick shale in the basal F interval overlies this limestone bearing succession.

Previous models explain the lateral contrast in limestone thickness as the product of variable erosion and incision during sea level low stands. An alternative origin is proposed whereby limestones have developed marginal to coeval siliciclastic channel-mouthbar systems. Evidence includes the location of these carbonates within an overall transgressive regime with limestone deposition terminated by deepening or increased admixed fine siliciclastic input rather than exposure. Individual limestones typically pass upwards and laterally into shale mouthbar deposits rather than erosively-based sandstones and there is no evidence of carbonate lags that might reflect local erosion. It is suggested that the lateral variation in limestone thickness reflects their original patchy development. Coeval siliciclastic deposition, although concentrated at mouthbars and channel levees, was also dispersed throughout the carbonate patch reef deposits. The predominantly clay grade fraction within the carbonates strongly influenced the biota present and the development of the patch reefs.

This re-interpretation suggests that the poor reservoir quality of the carbonates reflects their overall transgressive nature and the high percentage of admixed fine grained siliciclastics. Alternative models fail to explain the paucity of secondary porosity that might otherwise develop during exposure in a tropical humid climate. Coeval shales may have provided argillaceous baffles that restricted the exchange of carbonate-bearing pore fluids into the coeval sandstones that have consequently retained reservoir quality.