Mitigating Reservoir Property Uncertainty and Identifying the Boundary between Conventional and Unconventional Reservoirs: An Integrated Case Study from Mature Fields in the South Sumatra Basin

Abstract

It is not uncommon to observe production (test) results that deviate from geological and geophysical reservoir study prognosis. More interestingly, it is even so when the log characters appear similar to offsetting or pre-existing wells. Our experience in the South Sumatra Basin indicated in some wells, reservoirs that were thought to have relatively poor rock quality and low flow potential were reported to yield relatively moderate oil production, while vice versa better-quality reservoir rocks reported oil rate that was below expectation, even lower than its poorer rock quality counterpart. Wells with minimum pay zone (as indicated by petrophysical evaluation results) were able to produce reasonable initial oil rate while wells with relatively thicker pay zones produce only similar or lower oil rate with some or significant water cut. Interestingly this phenomenon is observed not only on a single field but also on neighboring and other fields in the South Sumatra Basin. Over time this is also supported by years of sustained oil production and surveillance data during the development phase of such fields. This phenomenon has been traditionally regarded as complexity or uncertainty in reservoir property description and left without significant further research into investigating the cause and implication of such phenomenon within the context of overall understanding of hydrocarbon presence and distribution. Given this background, we have performed a preliminary study revisiting the properties of reservoir and non-reservoir sections from three oil fields (Jirak/ Tanjung Miring Timur/ Belimbing) in the South Sumatra Basin. The study makes use of actual rock samples and well datasets from different sources; the primary being core and logging datasets, supported by production (test) results as they provide direct measurements of rock (static and dynamic) properties. Geological and petrophysical information resulting from a range of routine and special core analyses measurements are incorporated in this study. Biostratigraphic, petrographic, XRD, and TOC data from special core analysis measurements are used to provide static descriptions of reservoir properties. These core datasets are complemented by a range of logging datasets including that of nuclear magnetic resonance (NMR) and multi-source acoustic logs. Correspondingly, porosity, permeability, capillary pressure and relative permeability data from routine and special core analysis measurements as well as fluid PVT data are examined to provide dynamic descriptions of reservoir properties. Subsequently, the existing geological concept such as regional petroleum system as well as stratigraphic correlations are revisited and integrated in the study. A revisit of regional seismic data is also performed but mainly to validate the stratigraphic correlations made in this study. Further study into this subject aims to incorporate seismic interpretations to a larger scope as well as incorporating data from other fields in the region. The fields being studied are producing fields with production data spanning from 50 to 90 years. This is an interesting fact which also serves as an important calibration point to validate any new geological and petrophysical understanding resulting from this study. At the time this abstract is written, the study is still in progress. However, it is expected to be completed by March/April 2019. Our observations from this study so far reveals a couple of interesting phenomena including the existence of high porosity rocks with significant oil saturation but extremely low movable fluid due to very small pore throats (as supported by short NMR T2 relaxation time), as well as sequences of reservoir and non-reservoir quality rocks with high TOC leading to possibility of having sequences of conventional and unconventional reservoirs in the South Sumatra basin. It is expected that through this study, apart from improved understanding of rock property distribution and mitigating uncertainties associated with it, we shall also be able to improve our understanding of hydrocarbon presence and distribution in this region.

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