Jambi Merang Carbonate Gas Reservoir, Onshore Sumatra: Using 3D Seismic Attributes to Guide Porosity Modelling

Abstract

Abstract Introduction The Jambi Merang gas field of southern Sumatra produces from an organic carbonate build-up reservoir which was deposited as part of a barrier reef complex in the early Miocene. Porosity is relatively well developed in the localized build-up facies and much less well developed in the underlying platform carbonate. Dissolution by meteoric fluids has acted to enhance porosity, which increases systematically towards the top of the build-up. New 3D seismic data were recently acquired over the field, and are interpreted to be quite responsive to internal porosity variability. This is consistent with the observation that acoustic impedance is quite closely related to porosity in a gas-filled carbonate reservoir of high net-to-gross. It was decided, therefore, to use the seismic to guide the porosity modelling stage of the reservoir modelling process. In practice, three contrasting approaches were adopted. Initially, porosity was modelled using the available well log data only. Secondly, a simple 2D amplitude map was used as a secondary trend. Lastly, the seismic data were inverted for Acoustic Impedance (AI), which were then used in a full 3D sense after appropriate depth conversion and resampling. The different porosity distributions that resulted from these approaches were then compared, both qualitatively in terms of overall reservoir character and quantitatively in terms of differing GIIP results. In the case of the Jambi Merang reservoir, the use of the full 3D seismic AI data in the reservoir modelling process is felt to be of significant benefit, not just for the purpose of Volumetrics calculations, but also for the ongoing management of the reservoir throughout the producing life of the field. Workflow Some important data preparation steps are required in order to use the seismic data in the Reservoir Modelling process. Obviously, the seismic must be interpreted in order to generate Top and Base Reservoir surfaces in time, and a Velocity Model is required to convert these surfaces to depth. These surfaces become the external boundaries of the reservoir container. In addition, it is recommended that the seismic data be inverted for Acoustic Impedance (post-stack inversion) or even Elastic Impedance data (pre-stack inversion) in order to generate Rock Property information that can be correlated quantitatively with the well log data. These steps are outlined below. Seismic Interpretation of the Carbonate Reservoir The carbonate reservoir at Jambi Merang is a biohermal (reef) buildup, up to 250m thick, constructed locally on top of a regional carbonate platform. The reef is thought to have built itself in stages, with construction processes strongly affected by sea-level fluctuations over time. The net result is a layered internal geometry, or ‘wedding cake’ morphology that represents the balance between accommodation space being generated by subsidence and/or sea-level rise, versus the erosional effects of sea-level fall and subsequent subaerial exposure. Porosity is seen to generally increase towards the top of the reef structure, due to a combination of both depositional and diagenetic processes. A curious effect of this at Jambi Merang is that as the porosity of the reservoir increases, so the acoustic impedance contrast between it and the overlying shales decreases, such that the crest of the reservoir can be difficult to identify unambiguously on seismic. Figure 1 below shows a seismic section across the reservoir, illustrating both the interpreted layer-cake geometry of the reef build-up, and the changing seismic amplitude response across it as the porosity varies. Figure 2 shows a simple amplitude map of the top reservoir reflector, with warm colours indicating a weaker response and hence greater interpreted porosity.