Identification, Significance And Correlation Of Mass Transport Complexes In Malaysian Deep Water Fields

Abstract

Abstract Due to high well costs, one of the primary risks in developing deepwater fields is the lack of understanding of reservoir compartmentalization at the initial development stage. Observed pressure compartmentalization in a field in deepwater North West Borneo, currently undergoing field development planning, has been interpreted as being due to a combination of faulting and stratigraphy. Direct mapping of the reservoir sands from seismic is not possible as they are below seismic resolution and the sand thickness or character varies along the field. Stratigraphically, the distribution of Mass Transport Complexes (MTCs) in Deepwater Sabah provides a strong control on the distribution of reservoir and seal. The ability to identify MTCs is therefore key to understanding the distribution and correlation of sand bodies. An integrated approach in understanding the distribution of the MTCs will be presented in this paper. This involves sedimentological analysis of core, borehole image, dipmeter and wireline data combined with detailed seismic mapping. As the core coverage is limited to one interval in a single well, the MTCs are identified mainly through borehole image and dipmeter interpretation, with the MTCs typically exhibiting high angle chaotic dips. In intervals with no borehole image/dipmeter data, the MTCs can be identified via the density log, as they are denser than the overlying and underlying sediments. MTCs generate strong seismic markers due to their increased density, and hence acoustic impedance, compared to their surrounding sediments. Therefore the seismic reflectors can be used to accurately tie the MTCs between wells. This has resulted in a more confident well correlation than one attempted through correlation of sand bodies, and the ability to incorporate the risk of compartmentalization into the field development planning. Introduction One of the key subsurface risks in developing deepwater fields is the lack of understanding of structural and stratigraphic compartmentalization at the initial development stage. Structural elements such as faults and stratigraphic elements such as reservoir sand continuity can be interpreted on seismic to explain the cause of compartmentalization provided that they are within the seismic resolution. A recent deepwater discovery in the North West Borneo basin was found to be compartmentalized when wells drilled along its elongated structure encountered different pressure regimes. The interpreted fault patterns are not able to explain all the pressure differences observed between wells as they are mostly parallel to the field orientation and the quality of the available seismic data maybe limiting the ability to image other potential faults or features. Mapping the reservoir sands seismically is not possible as they are below seismic resolution and well data shows the sand thickness or character varies along the field. Instead of focusing on the reservoir intervals to answer the compartmentalization issue, a different approach was taken to understand the distribution of MTCs regionally and locally as they are believed to provide strong control on the distribution of reservoir and seal.