oa North West Borneo Deepwater Fold and Thrust Belt: What Controls the Hydrocarbon Column Height?

The deepwater acreage in the active fold and thrust belt of North West Borneo hosts a number of world class hydrocarbon accumulation. One of the critical success factors for exploration in the fold belt play is understanding the key controls on hydrocarbon column height. In the past, mechanical top seal capacity, or 4-way dip closure were seen as the main controls on hydrocarbon column height. This was revisited during the 1st phase of Deepwater Traps and Seals study carried out between 2007 and 2008. This study found evidences that fault-dependent columns are present and a number of structures are not fill-to-spill (geometrically), thus challenging 4-way dip closure as the dominant control. Moreover the study also found that at present day many traps in NWB deepwater still have significant mechanical top seal trap margin at the crest of structure. This required a change of paradigm which led to a 2nd phase of the study to examine the capillary seal capacity. A workflow was derived to investigate the relationship of the capillary seal capacity with depth. The result shows that the buoyancy pressures arising from observed hydrocarbon columns can be modeled by two capillary seal trends: (1) a silty-shale trend and (2) a shale trend, with most of the data clustering around<br>the first trend. This observation suggests that the dominant control on hydrocarbon column height in NWB deepwater is capillary seal capacity, which is in contrast with many other basins in the world. This can be explained by the more silty nature of the top seal, which is supported by core data. An intriguing correlation is also found between capillary seal capacity and seal depositional environment as indicated by the interpreted Depositional System Element (DSE). Capillary seal capacity decreases from drape DSE to slope wedge DSE to fan fringe DSE. Consequently, interpretation of top seal DSE based on seismic facies can be used to determine capillary seal capacity, and used as an input into hydrocarbon column height prediction ranges. In addition, this revised understanding has a significant impact on the NWB deepwater portfolio due to the increased Possibility of Success (POS) associated to fault dependent closures with hydrocarbon column height within the estimated silty-shale capillary seal capacity.<br>Deepwater Traps and Seals study is currently still an ongoing effort, with plans to carry out more mercury injection tests and X-Ray Diffraction aimed at constraining the capillary seal capacity better, as well as additional fault seal analysis.