Assessing the Impact of Fault Seal on Hydrocarbon Accumulation: The Result of a Blind Well Test, Onshore Niger Delta

Abstract Faults create one or more lateral trap seals across mukiple Niger Delta oil and gas fields. An intrinsic uncertainty exists over a fault's sealing capacity and leak point elevation over stratigraphy and fault geometry. Recognising that a single deterministic prediction will not allow for the modelling of these uncertainties is essential. To overcome this problem, Stochastic fault and trap analysis can be used to model the parameter uncertainties and thus derive the probability of trapping hydrocarbons and the distributions of hydrocarbon column heights within a reservoir. Our modelling approach provides an estimation of the fault's juxtaposition sealing capacity. A blind well evaluation has been used to assess the effectiveness of our stochastic modelling approach to investigate the error between the predicted and observed column heights and fluid contacts. The evaluation was conducted on a well (Well AA-1) with known fluid contacts and hydrocarbon column heights. The forecasting of hydrocarbon columns was based on structural (fault polygons and regional-scale grids) and stratigraphic data from Well AA-1. This well situated onshore Niger Delta encountered 25 hydrocarbon-bearing reservoirs with an average hydrocarbon column height of 149ft. Of the 25 reservoirs encountered by this well, only 4 penetrated hydrocarbon water contacts (HWC) while 21 penetrated hydrocarbon down-to (HDT). A comparison of the P50 forecast of column heights and the drilled column heights seen by the AA-1 Well suggests that of the four reservoirs which encountered HWC, two were within 12% of the observed depth, while the other two were slightly overpredicted. Juxtaposition analysis outperforms the SGR approach in forecasting 21 reservoirs with HDTs; simulation results suggest that ~86% of the reservoirs have their predicted contacts significantly deeper than the drilled HDTs suggesting thicker columns. The blind well test demonstrates that the stochastic fault seal juxtaposition approach better predicts the hydrocarbon-water contacts while significant departure of the post-drill results from the pre-drill predictions are usually attributable to nonfault related factors such as the effects of stratigraphy.