A VENTURE INTO EARLY - MIDDLE MIOCENE CLASTICS: AN EXPLORATION OPPORTUNITY IN THE WESTERN PART OF THE EAST JAVA BASIN

摘要

A recent drilling of the N-1 exploration well by Pertamina EP Cepu Alasdara Kemuning (hereinafter abbreviated as PEPC ADK) in the Alasdara Kemuning Block of the East Java Basin in 2017 penetrated a shallow-marine clastic play in the Tuban (early Miocene Burdigalian) and Tawun Formations (middle Miocene Langhian). Previous workers and authors who interpreted such play as deep-marine depositional environment might have not fully recognized the presence or addressed the importance of the shallow-marine sands. The sands containing shelfal benthic foraminifera with characteristics of coarsening upward parasequence capped by flooding surface can have relatively wide lateral distribution. In the well, the Burdigalian Tuban sands saturated with light oil in overpressure aquifer system have characteristics of coarsening upward parasequence middle - upper shoreface with thickness more than 90m and porosity ranging from 16 - 20% but with tight permeability. Subsequent Langhian Tawun sands saturated with gas-condensate and light oil are also characterized by coarsening upward parasequences of middle - upper shoreface sands with thickness ranging from 8 - 40m, porosity from 16 - 24% and permeability up to 30 mD. Post-drill petrographic and log analysis, showed that the penetrated reservoirs have moderate to good porosity, however in general permeability is low except in Ngrayong sands. Mobility in Ngrayong sands can be more than 600 mD/cP but dropped significantly in Tawun and Tuban sands that could be attributed to diagenetic and compaction effects on fine-grain sands. Shallow-marine clastic plays of Tuban and Tawun Formations in the western part of the East Java Basin are challenging because of their characteristics such as tight permeability, low impedance, and Low Contrast - Low Resistivity (LCLR) with limited to no density-neutron crossover and sometimes high GR. However, proper integrated regional studies from cross disciplines such as structural geology, sequence stratigraphy, pore pressure prediction, reservoir characterization, and utilization of advanced logging tools could help uncover hydrocarbon potential in a relatively new Burdigalian Tuban exploration play as well as in Langhian Tawun, in search of light oil and gas-condensate resources in an otherwise mature area.