Publications

Mobil Cepu Limited has acquired approximately 500 meters of core in the Oligo-Miocene aged Banyu Urip Field, onshore East Java in the Cepu KKS. A full analysis of thin sections and slabbed core reveals systematic patterns of deposition and diagenesis that may be typical of SE Asia Tertiary carbonates. Core and thin section data reveal the detailed sea-level controlled depositional cycles. More importantly, the data reveals details of the overprinted early and late diagenesis that is known to be such a strong control on reservoir quality in Oligo-Miocene aged SE Asia carbonates due to the large sea-level swings during that period. The dataset covers both the interval when the carbonate platform was keeping up with subsidence (aggrading phase) and the interval when the platform was stressed to keep up with the subsidence and finally drowned. Core data indicates that the almost 1000m thick aggrading phase is characterized by repeated 50 meter thick cycles of shallowing-upward facies successions with predictable facies ranging from large benthic foraminifera–rich wackstonepackstone to reefal boundstone at the top. The stressed and drowning phase is up to 300m thick grading upward from normal shallow waterconditions to deeper environments of deposition dominated by red algae. Both successions have clearly identifiable sequence boundaries with exposure confirming the large amplitude of the sealevel variation. Detailed observation from the thin sections shows that the primary depositional facies have been pervasively altered by diagenetic processes. Solution-enhanced molds and vugs, interparticle porosity and microporosity formed by combination of early- and late-diagenesis have increased porosity * Mobil Cepu Ltd. ** ExxonMobil, Upstream Research Company and created a high permeability, well-connected pore system in the reservoir. Early diagenesis associated with exposure to fresh water at sequence boundaries shows predictable patterns of cementation just below the sequence boundary and dissolution over the 50m cycle and more. Permeability is almost constant across depositional facies demonstrating the strength of the diagenesis. This regular cycle pattern of diagenesis allows for a highly predictable reservoir quality model of vertically cyclical and horizontally continuous porosity-permeability zones. Late burial diagenesis is also important and can be demonstrated by evidence of vugular dissolution that cross cuts burial features such as stylolites. Varying amounts of hydrothermal minerals are also deposited in the vugs such as saddle dolomite and kaolinite.