Making Seal Uncertainty the Opportunity: Global Analog-Based Seal Insight

Abstract

Seal is probably the least understood aspect of the petroleum system. Even though in a recent global study it was regarded as the dominant failure mechanism, seal evaluation does not receive the relative investment it deserves compared to the other prospect risk elements. A comprehensive seal investigation has been conducted using nearly 1000 producing oil and gas reservoirs worldwide, which documented the key seal attributes including seal surface geometry, depositional system, lithology, thickness, tectonic setting, structural flank dip, reservoir lithology, hydrocarbon column height and percentage of structural closure height filled by hydrocarbon. The global analog-based seal analysis shows no correlation between seal thickness and the column height nor with the percentage of trap fill. Seal thickness therefore has no bearing on seal capacity. Common lithologies that act as seals are claystone, mudstone, shales, evaporites, fault gouge and mass transport complexes (MTCs). The largest hydrocarbon columns are sustained by evaporite or shale lithologies. Leakage of most underfilled reservoirs was caused by top seal failure, and the most common cause of failure is by tectonic (fault) breaching. Capillary leakage is more common in fields with large productive areas due to lateral facies changes, and in seals formed in coastal depositional systems. It is also revealed that sealing capacity is controlled by both suitable lithology and a suitable 3D surface which impedes movement of hydrocarbon. For updip fault traps, seal capacity is partially controlled by fault throw. Faults with throw < 15% of the reservoir thickness are non-sealing. Reservoirs with fault throw exceeding reservoir thickness are mostly sealed by juxtaposition. There is a moderate to good correlation between fault throw and hydrocarbon column height, and sealing capacity is better in reservoirs with cross-fault juxtaposition against post-tectonic stratigraphic fills. The majority of reservoirs with updip faults are under-filled. All reservoirs with updip faults and structural flank dips >15o are underfilled. The majority of reservoirs with updip faults in rift basins are also underfilled fault reactivation post charge. For lateral stratigraphic traps, the buoyancy pressure increases towards the updip seal. Stratigraphic trapping mechanisms can retain large hydrocarbon columns, but the majority of low-flank stratigraphic traps have a low flank dip where substantial volumes are retained with low hydrocarbon column heights. The largest columns in stratigraphic traps are found firstly in the lateral facies change trapping mechanism. Second are onlap traps which on average retain columns >1000 ft. When a stratigraphic prospect has a trap flank-dip higher than 8o, its seal risk is very high, hence its seal capacity should be investigated in further detail or an alternative interpretation should be considered. Benchmarking against global analogs can improve judgement of uncertainty range and the confidence level / chance of adequacy when assessing column heights. Effective seals need to be present over the entire 3D surface of the trap, hence the seal depositional system has a first order control on areal uniformity of the seal. The ability to cover an area is more important than the strength of the seal. A study of global analog-based seal analysis and benchmarking the seal attributes of an exploration prospect against proven producing reservoirs, aides the calibration of the hydrocarbon column height, validation of the prospect geological model and ultimately gives more confidence in the resource range and risk assessment. A work flow has been developed and employed in several cases studies which demonstrated in improving the chance of drilling success and accuracy of volumetric calculation.

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