Some Uncertainties in Predicting Overpressure in Indonesia’s Sedimentary Basins
Indonesia’s sedimentary basins have been proven to contain overpressure zones and they have been causing several drilling problems ranging from non-productive drilling time to the worst event, i.e. blowout. For the last ten years, we have been trying to understand the generating mechanism in Indonesia’s sedimentary basins with the ultimate goal to predict the occurrence and magnitude of overpressure accurately. Here, we highlight several uncertainties in predicting overpressure in the area. The first uncertainty that may occur in predicting the magnitude of overpressure in the area is in determining normal compaction trend (NCT). Our investigation found that clay diagenesis in forms of smectite to illite occurs in the area. The smectitic clay has different compaction behavior with illitic clay. The conventional wisdom in estimating overpressure is to use single NCT throughout the section. With respect to different clay type in Indonesia, the use of single NCT may underestimate overpressure magnitude at depth. The second uncertainty in overpressure prediction in the area is caused by the presence of unloading overpressuring at depth. In certain areas (e.g. Lower Kutai Basin), the unloading causes the overpressure magnitude to reach lithostatic stress, commonly termed as lithostatic-scale overpressure. The inability to understand analytically (rather than empirically) the unloading process (such as effective stress history of the shale) may also result in underprediction of overpressure magnitude at depth. The methods of overpressure estimation (e.g. Eaton’s method and Bowers’ method) can only estimate overpressure in shale section. The result of overpressure estimation is usually calibrated by direct pressure measurements in reservoir section (e.g. sandstone and limestone). Theoretically, the reservoir and shale pressure could be different, and we found that this circumstance is ubiquitous in Indonesia’s basin. This condition is the third uncertainty in predicting overpressure in the area. The understanding of geological condition (i.e. reservoir continuity) is required to minimize the uncertainty causing by this factor. The fourth uncertainty of overpressure prediction in the area is the presence of overpressure at shallow depth (e.g. East Java Basin). The wireline log data in the shallow section usually do not available in good quality due to several problems such as hole washout because the materials are poorly consolidated. If the top of overpressure is relatively shallow (as shallow as 200 m in certain areas in East Java Basin), then we are unable to construct NCT to be used to estimate the magnitude of overpressure at depth. In this case, the understanding of overpressure generating mechanism such as disequilibrium compaction may help in determining the maximum likely overpressure in the shallower section. Recently, in the North Sumatra Basin and in the South Sumatra Basin, we found low velocity clay zone at depth, yet the pressure as implied from direct pressure measurements and drilling parameters (e.g gas while drilling and caving) indicated that the pressure in the zone is hydrostatic. We do not know precisely the cause of this low velocity zone (apart from overpressure), but it may be related to the clay mineralogy. The research answering this fifth uncertainty in overpressure prediction is still on going. From the above description, it is clear that plenty overpressure sciences yet to discover in Indonesia’s sedimentary basin in order to predict and estimate overpressure magnitude accurately. The application of empirical methods that are commonly used in the industry may result in some pitfalls. We need to more understand the process behind the generating mechanism of overpressure through analytical analysis such as clay mineralogy vs compaction behavior, effective stress and burial histories, and lateral reservoir drainage.
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