Hydrocarbon Accumulation Uncertainty in Miocene Carbonate Build-Ups, NEJB

Abstract

North East Java Basin (NEJB) is popular with hydrocarbon producer throughout the region. One of prolific reservoirs is Miocene carbonate of JS-1 Ridge Structure, presents as facies of carbonate platform which is inhabited by growth of hundreds localized and amalgamated build-ups. Each closure is characterized by up to 1000’ vertical thickness but often of small areal extent (<1km2). The JS-1 Ridge structure is bounded by two proven hydrocarbon kitchens i.e. East Bawean Trough and Central Depression. The facies geometry of carbonate build-ups has been defined very well using existing 3D seismic data. Highly soluble constituents and repeated exposure process associated with sea level changes are responsible for good reservoir properties among this facies throughout the area. However, main issue in exploring these individual closures is hydrocarbon accumulation variation among them. From drilling results, there are groups of HC-filled reefs (total 38) and wet reefs (total 20), or equivalent to 65% discovery success percentage. For the group of HC-filled reefs, the statistic suggests very high variation with skewed-right shape distribution, ranging from 0.1 to 78 MMBOE. Mode is less than 1 MMBOE, median is 4.8 MMBOE and arithmetic mean is 13.5 MMBOE. The variation is quite extreme in some localities, as dry reef surrounded by HC-filled reefs and vice versa. This might be due to complex geological process which is represented by five petroleum system components, i.e. source rock, timing/migration, reservoir, closure and containment. Based on the assessment, most-likely components responsible for this condition are timing/migration and containment. Several analysis regarding hydrocarbon accumulation prediction have been done but never really managed to explain this condition thoroughly. Trends are observed, but still difficult to be determined. With this condition, prospective resources estimation becomes big issue to be handled, since it will affect further economic analysis and decision. At least there are four estimating method options: deterministic, probabilistic using distribution of entire area, probabilistic using distribution of specific trend and probabilistic using seismic HC prediction. Every option has different advantages and disadvantages, except for deterministic method which is very not recommended to be used. Probabilistic method is strongly suggested for be able to deal with uncertainties. Distribution of entire area method is intended to accommodate all possible outcomes, but variance can be too high. Distribution of specific trend method is intended to reduce variance, but may find pitfall in area with extreme differences. Seismic HC prediction method is also intended to reduce variance, however it really depends on the reliability of prediction accuracy. Input from advance geological and geophysical analysis is absolutely necessary to produce better resources estimation. The variance should be defined carefully so it will not too high and results in an improper decision or too low so that ignores possible outcome that may occurs. However, uncertainty is a nature gift and cannot be avoided. Therefore some efforts can be done to avoid unnecessary possible outcome, such as: by applying correlation between related parameters such as: GRV - NTG, effective porosity - water saturation, Bo – GOR, etc; and by performing reality check for map, parameters and highest/lowest possible outcome.

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