The Possibility of Hydrodynamic Trap in Tarakan Sub-Basin as Explanation of the Existence of Water in an Identified Hydrocarbon Zone

Abstract

Hydrocarbon exploration in Tarakan Basin, North Kalimantan, began at the end of nineteenth century. Exploration was focused in some areas in onshore e.g. Tarakan Island, Bunyu Island, and Mandul Island along with several significand discoveries. Almost earlier discoveries are recorded as structural trap, recognized as anticline and Up-thrown block of Growth fault in Deltaic System. Some next discoveries were identified as stratigraphic trap complex in Bunyu Island at the stage of development and production. PHE Nunukan Company, one of operator in Offshore Tarakan Basin, found a case in the identified zone as containing hydrocarbons from wireline log, cutting oil show and mud logging gas data, produces water after a DST test. The hydrocarbons identification is also supported by a plot of wireline formation pressure versus depth which gives a gradient value pressure of 0.306 psi/ft. The value of this gradient indicates that the fluid filling the reservoir is a hydrocarbon with a Gravity API of around 600. One hypothesis proposed earlier is that the hydrocarbons are suspected being in this zone has migrated structurally to another place, so that the DST produces water. Based on the discussions carried out, this is unlikely to occur because if hydrocarbons have migrated, the responses from the wireline log will also change to indicates water, not strongly indicates hydrocarbons. In addition, the concept of hydrocarbons those had migrated is very difficult to explain the value of the pressure gradient of 0.306 psi/ft as described above. The hypothesis is that the zone was completely filled with hydrocarbons, supported by the wireline log response, but as DST test gave the water result is because of the possibility of a 'strong water drive' caused by active hydrodynamic conditions in the reservoir. If this hypothesis is proven, then the contact between hydrocarbons and water in the study area is tilted.

AAPG Datapages/Search and Discovery Article #90354 © 2019 AAPG Asia Pacific Technical Symposium, The Art of Hydrocarbon Prediction: Managing Uncertainties, Bogor, (Greater Jakarta), Indonesia, August 7-8, 2019