In marginal fields, STOIIP (Stock Tank Oil Initially In Place) uncertainty often concentrates around two linked issues: where the hydrocarbon contact sits and how pay is defined through cutoffs and petrophysical assumptions. Small shifts in water saturation cutoff, net-to-gross criteria, or assumed contact depth can move STOIIP enough to change development decisions, yet many evaluations report a single contact and a single cutoff set without quantifying sensitivity.
This paper presents a reproducible workflow to stress-test STOIIP against cutoff and petrophysical assumption uncertainty while keeping the analysis transparent and auditable. The method begins by defining a controlled "assumption set library, including alternative cutoff sets such as shale volume, porosity, and water saturation thresholds, alternative saturation models where applicable, and alternative contact interpretations based on available evidence (pressure gradients, test data, resistivity trends, and completion outcomes). Each assumption set is treated as a named scenario with documented rationale, rather than an informal adjustment.
For each scenario, the workflow computes net reservoir, net pay, average properties, and STOIIP using the same calculation engine and data inputs, ensuring changes in outcomes trace directly to the assumption changes. Sensitivity is quantified in three ways: (1) one-at-a-time cutoff sweeps to expose non-linear behavior, (2) paired sweeps that reflect real coupling (e.g., Sw cutoff with porosity cutoff), and (3) Monte Carlo runs where the contact depth and key petrophysical parameters vary within constrained ranges. Outputs include STOIIP response surfaces, tornado ranking of the dominant assumption drivers, and a compact decision risk table showing which assumptions can flip economic viability.
A case study demonstrates how the workflow highlights hidden fragility in a base-case STOIIP estimate and guides targeted data acquisition decisions, such as which pressure points, logs, or tests would most reduce uncertainty per unit cost. The paper concludes with recommended reporting standards for marginal-field evaluations, including minimum sensitivity checks that should accompany any STOIIP number presented for investment decisions.
W. O. Nneli, O. S. Ojo, A. Asaolu· SPE Nigeria Annual Internati...· 0 citations
Some mature basin and marginal field wells are log-poor in the sense that they do not have density-neutron logs, and in some cases, no pressure information at all. Yet, one still needs to derive decision grade formation pressure and fluid contact (FC) estimates in order to derive UGRs for reserves, design completions, and manage risks in development projects. This paper describes a repeatable workflow for deriving FC envelopes in the absence of density-neutron logs, while keeping all assumptions visible and traceable.
A new method of contact depth determination for conventional reservoirs utilizes four common types of data observed even in log-poor wells: resistivity behaviour and invasion indicators, pressure points and gradients (with actual MDT/RFT data used when available and indirect indicators used when no direct data is available), drilling and completion issues (such as losses, kicks, fluid shows and production test responses), and basic cross correlation with the offset wells. Unlike other contact depth determination methods, which yield a single contact depth at the wellbore, this method yields contact depth envelopes defined by P10/P50/P90 quantities that reflect the strength of the various pieces of information and constrained ranges that reflect the quality of the information and the likelihood of the stratigraphic configuration being correct. An assumption set approach is used to qualify the impact of uncertainties in cutoffs and saturation and to ensure that the contact depth does not reflect more than it should any assumption.
Formation pressure is handled similarly. Pressure envelopes are generated from data points and gradient families related to fluid type and depth trends and the pressure/envelopes are constraint checked against drilling break, kick and stable mud weight windows. The resulting pressure and contact envelopes are transmitted into Volumetrics and are therefore used in STOIIP/GIIP estimates to accurately assess the true impact of uncertainty on hydrocarbon resources estimates. A "value of information" guide is generated illustrating which data points (limited pressure data, targeted logs or short test for example) would provide the highest benefit in terms of reducing the uncertainty of the resource estimate for the lowest cost.
D. Ojo, A. Asaolu, W. O. Nneli et al.· SPE Nigeria Annual Internati...· 0 citations
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