The variability of turbidite sandbody pinchout and its impact on hydrocarbon recovery in stratigraphically trapped fields
Authors: Gardiner Andy R. 
Published: April 2008

Stratigraphic trapping is an important component of many hydrocarbon fields reservoired in deep-water, turbidite deposits. The trapping may occur at channel margins, onlap surfaces and when turbidite sandbodies exhibit lateral variations in sand quality and/or bed thickness. The range of geometries occurring at these sandbody terminations has been the subject of detailed previous research and a number of classification schemes have been proposed. A single classification scheme, based only on the geometry of the sandbody and individual sandstone beds, is proposed here.

The different geometries of sandbody termination or pinchout will have an impact on both the static and dynamic behaviour of hydrocarbon reservoirs. Dynamic simulation of a range of models of sandbody pinchout by onlap indicates that the recovery factor of stratigraphically trapped fields will be influenced by a range of geological and engineering parameters. Producing wells positioned too far from the sandbody termination run the risk of leaving behind significant volumes of up-dip oil. If wells are moved closer to the sandbody termination, they may intersect the onlap surface, and so not penetrate the lowest sandstone beds. In systems with a low effective vertical permeability, oil from these lower beds may not be efficiently produced. In this case, the optimum well location, in terms of recovery factor, is close to the initiation of onlap. Unfortunately, this position may be difficult to identify in the subsurface without the drilling of many appraisal wells.

Variation in a range of parameters has been modelled, in order to examine their impact on hydrocarbon recovery. For layered sand/shale successions, with low effective vertical permeability, the initiation of onlap, and therefore the optimum well location, moves further from the onlap termination as the angle of onlap decreases. The maximum recovery factor is also lower for the lower onlap dips, as a greater volume of the reservoir lies up-dip of the producer at its optimum location. If individual sandstone beds thin towards the onlap, the volume of oil which might be left up-dip of producing wells is reduced, so that the risk in placing a well away from the sandbody termination is lower.

The degree of trapping of hydrocarbons in the lower layers, as the producing well location moves onto the onlap surface, depends on the effective vertical permeability of the sandbody. If the vertical permeability is zero (as would be the case for a perfectly layered system with continuous sealing shales) no oil will be produced from the lower layers. As the vertical permeability is increased, fluids are able to flow vertically from these beds into higher beds and thence to the producer. The trapping potential is significantly reduced for kv:kh ratios of 2 × 10
−5
or more, which is equivalent to an effective kv of 0.01 mD for a kh of 500 mD. In layered turbidites, this effective kv could be produced by 2 m thick sandstone beds, with a kv of 400 mD, interbedded with thin, non-sealing siltstones or silty shales, with permeabilities of the order of 10
−4
mD.

In practice, the effective kv:kh ratio of interbedded turbidite sandstones and shales is greatly influenced by the local erosion of the shales. Flow simulation through models representing various proportions of shale removal indicates that significant trapping of hydrocarbons in the lower layers may occur for proportions of shale removal below 15%. Above this value of shale removal, little trapping occurs, as fluids are able to move sufficiently easily between the individual sandstone layers. These results suggest that the risk of reduced hydrocarbon recovery, as producing wells are moved closer to the onlap termination, are significant only in the case of well-layered reservoirs with low proportions of shale removal and sand-bed amalgamation. Examination of available core should enable the proportion of bed amalgamation, and therefore the risk of reduced recovery, to be evaluated for a stratigraphically trapped reservoir of this type.