Publications

The Early Oligocene CD Carbonates of the Ngimbang Formation are an attractive petroleum target and a major reservoir in the North Madura Platform. This paper will discuss the evolution of CD Carbonates and the various controls which affected the facies deposition and subsequent modification of the compositional and textural constituents by episodic diagenetic processes. This evolution is believed to be the primary factor in creating reservoir complexity distribution in the studied area. Furthermore, this complexity has become notoriously difficult to analyse because uncorrected seismic velocity resulted in a push down effect which in turn resulted in inaccurate structure maps. The CD Carbonates started to grow before rifting ended and generally coalesced into a bigger platform area which was mostly controlled by high amplitude sea level changes. In general, carbonate have passed through 4 stages of development based on depositional facies and seismic recognition. Stage 1 (initiation) is an early carbonate development. Stage 2 (keep-up) includes aggradation and rapid growth of shelf margin carbonate. Stage 3 (keep-up and amalgamation) includes progradation of shelf margin and amalgamation of several small patch reefs into a bigger patch reef and stage 4 (demise of carbonate) includes backstepping and drowning of carbonate. Based on detailed analysis of cores and thin sections and seismic mapping, 7 depositional settings have been identified: (1) shelf margin reef, (2) restricted lagoon, (3) patch reef complex, (4) shoal, (5) foreslope, (6) deep water carbonate, and (7) basin/open marine. Depositional cycles, lithofacies composition and clastic influx are the main factors responsible for reservoir formation as well as original reservoir quality development. More importantly, extensive early and late diagenesis has altered the original carbonate fabric by creating solution enlarged vugs and cavities in the carbonate. Early diagenesis is associated with exposure to fresh water/karstification. Late burial diagenesis, which is similarly crucial, was observed in the carbonates by the presence multiple dissolution processes, stylolites, ferroan dolomite, kaolinite and pyrite precipitation. From this work, a diagenetic history chart was created to clearly identify all the process affecting the evolution of reservoir quality. Evaluation of CD Carbonates has provided opportunities for other prospective areas as well as opened new exploration opportunities.