Production Decline and Maturity Assessment of the Rio Del Rey Basin, Cameroon: A Decade-Long Analysis (2012–2022)

Nomenclature

bbl: barrel

Bopd: barrels of oil per day

Bcpd: barrels condensate per day

mD: millidarcy

M bbl : million barrels

Mmscf/d: million standard cubic feet per day

Mtpa: million tonnes per annum

Scf: standard cubic feet

TCF: trillion cubic fee

t Φ: porosity

K: permeability

Abbreviations

API: American Petroleum Institute

AR: Added Reserves

DKC: Douala/Kribi-Campo Basin

EOR: Enhanced Oil Recovery

FLNG: Floating Liquefied Natural Gas

GOR: Gas-Oil Ratio

KPI: Key Performance Indicator

LB: Logone Birni Basin

LPG: Liquefied Petroleum Gas

P&A: Plugged and Abandoned

RDR: Rio Del Rey Basin

R/P: Reserves-to-Production Ratio

RRR: Reserve Replacement Ratio

SNH: Société Nationale des Hydrocarbures (National Hydrocarbons Corporation)

SPE: Society of Petroleum Engineers

USGS: United States Geological Survey

Introduction

For decades, Cameroon's oil sector has been a cornerstone of its economy. However, since the late 2010s, this vital industry has entered a period of sustained structural decline. The numbers tell a clear story: average daily crude production has plummeted from a peak of around 100,000 barrels per day in the 2000s to an estimated 54,000–60,000 barrels in 2024–2025. This downturn is particularly acute in the Rio Del Rey (RDR) Basin, a region in the Gulf of Guinea that has historically supplied over 70% of the nation's oil. This challenge is not Cameroon's alone; it echoes across oil producing nations in Africa like Nigeria and Angola, all grappling with volatile global markets and the worldwide shift towards cleaner energy [1,2]. For Cameroon, the implications are severe. The decline threatens to undermine the country's energy security, reduce crucial state revenues (which still rely on oil for 16 – 20% of its budget), and destabilize macroeconomic growth, all while widening the current account deficit.

To this end, the investigation is guided by three primary objectives:

• To evaluate the maturity level of the Rio Del Rey Basin through a systematic analysis of its production history and reserves data.

• To quantify the observed production decline by employing key industry metrics, specifically the Reserve Replacement Ratio (RRR), the Reserves-to-Production (R/P) ratio, and well flow rates.

• To classify reservoir performance based on production data, thereby distinguishing areas with residual potential from those nearing exhaustion. T

hrough this structured approach, the study seeks to provide a critical diagnostic foundation for strategic planning aimed at mitigating production decline and optimizing the basin's remaining value [3,4].

Context of Petroleum Activities in Cameroon

Cameroon's hydrocarbon resources are geographically and strategically defined by three principal sedimentary basins: the Rio Del Rey (RDR), the Douala/Kribi-Campo (DKC), and the Logone Birni (LB). Together, these formations constitute the core of the nation's oil and gas sector, yet they represent markedly different stages of the resource lifecycle. (Société Nationale des Hydrocarbures n.d.) The Rio Del Rey (RDR) Basin stands as the patriarch of Cameroonian production. Having entered development in 1977, its long history of extraction has led to a state of advanced maturity, which is now characterized by a natural and structural decline in reserves. In contrast, the Douala/Kribi- Campo (DKC) Basin represents a more contemporary chapter. Its development, beginning in 1997, has established it as a strategic hub, distinguished not by oil but by its substantial natural gas reserves, making it the primary focus for 90% of the country's gas resources. The third member of this trio, the onshore Logone Birni (LB) Basin in the Far North, remains an enigma. Its potential is overshadowed by persistent regional insecurity, which has effectively suspended exploration and left its resources untapped [5].

Confronted with the RDR's decline, a strategic national pivot is underway, shifting focus toward the gas-rich DKC basin. This reorientation is not merely theoretical; it is physically embodied by infrastructures like the Floating Liquefied Natural Gas (FLNG) unit at Kribi, which enables the export and monetization of this critical resource. This strategic shift is part of a coherent, multi- axial national strategy. To bolster domestic energy security, gas is being channeled to fuel new power plants, directly addressing chronic electricity deficits. Concurrently, the government seeks to revitalize exploration through new licensing rounds and is modernizing its legal framework under the revised 2019 Petroleum Code to improve governance and attract investment.

The tangible outcome of this gas pivot is most evident in the performance of the Kribi FLNG unit, officially named the Hilli Episeyo [6]. Operated in partnership with Perenco and the state- owned SNH, this facility began exporting in March 2018 with an initial capacity of 1.2 million tonnes per annum (Mtpa). Through an agreement with partners Perenco and SNH, utilization has been steadily increased toward 1.4 Mtpa in 2022 and up to 1.6 Mtpa from 2023 onward. This operation is critical: it not only provides a crucial revenue stream that offsets losses from the RDR's aging oil fields but also supplies condensates and LPG for domestic consumption, while securing Cameroon's place on the global LNG map [7].

Figure 1: Overview Of The Oil And Gas Domain As Of 12/31/2022 (Source: Snh 2022 Report)

Methodology

Synthesis of SNH Reports from 2015–2022

The synthesis of SNH reports was conducted by aggregating and normalizing raw production, drilling, and reserves data to quantify depletion dynamics. This retrospective approach is justified by its ability to identify structural trends over a multi- year period, reducing biases related to annual fluctuations and optimizing extrapolation for future modeling. Data were extracted from official SNH reports and validated by cross-referencing with USGS databases for similar basins.

National Production Dynamics (2012–2022)

To analyze production, annual variations were calculated as follows:

This simple yet robust formula, commonly used in petroleum economics, allows for measuring decline or growth rates, optimizing the detection of peaks (e.g., 2015 for oil) and inflection points (e.g., FLNG launch in 2018 for gas). The choice to include 2012–2014 ensures a pre-peak baseline, avoiding biases from truncated analyses [8-11].

                         Table 1: Evolution of National Hydrocarbon Production (2012–2022)

This method was chosen for its alignment with SPE standards, allowing for international comparison of decline rates [12].

Drilling Activities (2015–2022)

Drilling data were classified by type (development vs. exploration), with the calculation of the average flow rate:

• Average Flow Rate (bopd) = Sum of Flow Rates / Number of Wells Drilled.

This indicator, recommended for assessing unit productivity, optimizes the identification of maturity by quantifying the decreasing efficiency of infill wells.

Exploration/Appraisal Share (%) = (Number of Explo/Appre Wells / Number of Wells Drilled) × 100

It measures the priority given to renewal, a key KPI (Key Performance Indicator) for mature basins [13].

                                              Table 2: Summary of Drilling Activity and Operational Results by Year (2015−2022)

Reserves Evaluation and Sustainability (2015–2022)

The formulas used are:

• Added Reserves (AR) = Reserves_n - Reserves_{n-1} + Production_n

• RRR (%) = (AR / Production_n) × 100

• R/P (years) = Proved Reserves / Production_n

The RRR (Reserve Replacement Ratio) was chosen for its ability to quantify renewal, a critical indicator for mature basins like the RDR, while the R/P (Reserves/Production) ratio assesses temporal sustainability, optimizing depletion projections. The inclusion of 2014 as a baseline ensures calculation continuity [14,15].

Table 3: Dynamics of Proved Oil Reserves, Replacement Rate (RRR) and Life Horizon (R/P) (2015−2022)

               Table 4: Dynamics of Proved Gas Reserves, Replacement Rate (RRR) and Life Horizon (R/P) (2015−2022)

Drilling Data Analysis

Drilling indicators were calculated as follows:

• Average Flow Rate (bopd) = Sum of Flow Rates / Number of Wells Drilled

• Exploration/Appraisal Share (%) = (Number of Explo/Appre Wells / Number of Wells Drilled) × 100

These metrics, standard in productivity analysis, were chosen for their ability to quantify efficiency and strategic priority, optimizing the assessment of RDR maturity [16].

Production Data Analysis

For production dynamics, decline/growth rates were calculated as indicated, with the average post-oil-peak rate being the mean of variations from 2016–2018 and 2021–2022 (excluding the rebound). This choice excludes anomalies for a robust estimate of structural decline. Parallel charts optimize the visualization of the oil/gas decoupling.

For the reserves renewal crisis:

• Negative average RRR (2016–2018) = Absolute mean

• R/P Erosion Rate = [(R/P_2018 - R/P_2015) / R/P_2015] × 100

• Uncompensated Production = Sum 2021–2022

This three-step methodology assesses the asset decommissioning speed: first, the Reserve Replacement Ratio (RRR) measures the rate of non-compensation of reserves, particularly significant before the artificial adjustment in 2019. Second, the R/P Ratio Erosion Rate precisely quantifies the loss of the sustainability horizon that preceded the same rebound. Finally, calculating the total amount of oil extracted from reserves without being replaced during the critical years (2021−2022) certifies the structural depletion of the asset [17].

For gas dynamics:

• R/P plunge = [(R/P_2022 - R/P_2017) / R/P_2017] × 100

• Contribution to RRR (2019) = Gas RRR / Oil RRR

The analysis of gas dynamics is performed in two complementary steps to measure the strategic transition of the sector, a choice optimized for measuring the impact of a major project. First, the Gas Production Curve (M scf) with FLNG Annotation serves to highlight the dependence of exponential gas growth on a single project (Hilli Episeyo) started in 2018. Second, the Gas R/P Curve (Years) allows visualization of the direct impact of this new production on the resource's sustainability horizon [18].

Results & Discussion

 Analysis of Drilling Data

  Figure 2: Curve of Total Wells Drilled Per Year (2015-2022)

This curve highlights the scale of investment required to sustain production levels. The intensive drilling campaigns conducted in 2019 (21 wells) and 2022 (22 wells) reveal a corrective strategy in maturing fields, where increasing the number of wells struggles to compensate for the natural decline of the reservoirs.

Figure 3: Activity Concentration Diagram

This diagram demonstrates the strategic focus. The massive share of Development (or infill) wells confirms that activity is primarily concentrated on maintaining existing reservoirs.

    Figure 4: Average Flow Rate per Well (bopd) Curve

Its purpose is to measure the unit return on investment and demonstrate the declining quality of the targeted reservoirs. Despite increased effort, the average flow rate post-2018 remains stagnant at a low level (300–700 bopd), proving that the new infill zones are structurally less productive.

Figure 5: Exploration/Appraisal Share (%) Curve

The low percentages (10–18% after 2018) signal a neglect of oil Exploration in favor of gas and short-term maintenance. The location of drilling relative to the basins allows for their classification according to their lifecycle.

Table 5: Characterization of Oil and Gas Basins by Drilling Activity (2015–2022)

                  Table 7: Reservoir Classification by Performance and Estimated Petrophysical Properties

                                                                  Table 8: Production Dynamics (Oil vs. Gas)

This juxtaposition confirms the RDR basin's irreversible structural decline in oil production, while simultaneously highlighting the pivotal role of major gas infrastructure in redefining the national energy portfolio.

   Figure 6: Oil Decline and Gas Growth

The graph clearly shows the Oil Peak in 2015 followed by a structural decline (negative average rate), contrasting with the quantum leap in gas in 2018 (+272.28% variation).

Figure 7: Gas Contribution to Total Energy mix

The contribution of gas to total energy produced has skyrocketed since 2018, highlighting the country's de facto energy transition

                                                Table 9: Oil Renewal Crisis

Figure 8: Oil RRR and Oil R/P Curve

This combined graph reveals that the Cameroonian oil industry has been in a structural renewal crisis since 2015, temporarily masked by accounting adjustments.

• Immediate Negative Correlation: Periods of negative RRR (2016−2018) correspond to the erosion of the R/P horizon (which drops from 8.24 to 7.84 years). The lack of exploration reduces sustainability.

• The 2019 (Artificial) Effect: The spectacular RRR of 539.90% in 2019 causes an immediate jump in the R/P to nearly 12 years. This illustrates how major re-evaluations (inclusion of condensates) mask the real renewal crisis.

• Post-2019 Crisis: The R/P begins to decline again after 2019, despite the rebound, because the RRR falls back to 0% in 2021−2022. This confirms that the oil stock continues to be consumed without being renewed.

Figure 9: Combined Gas RRR and Gas R/P Diagram

Analysis of the Combined Gas RRR and Gas R/P Chart reveals the following key points:

• Trigger for Exploitation: The major event is the massive drop in Gas R/P in 2018 (from 435 to 116 years). This plunge is the direct consequence of the FLNG production surge and marks the transition from a dormant reserve to a consumed asset.

• Extreme RRR Volatility: The RRR is either very high (re- evaluation of existing resources) or close to 0% or negative (production without adding new discoveries). The volatility of the RRR is less concerning than for oil, as the R/P horizon remains above 70 years, indicating a very comfortable long- term sustainability.

• Gas Strategy: After 2019, the low RRR suggests the operator is focusing on exploiting already proven reserves rather than drilling for new (costly) gas discoveries.

Figure 10: Combined Gas R/P and Gas Production Curve with FLNG Startup Impact

The commissioning of the Hilli Episeyo FLNG unit in 2018 redefined the dynamics of Cameroon's gas sector along two fundamental axes:

• The transformation of the reserve profile materialized through a significant adjustment of the gas R/P ratio. Although maintained at a comfortable horizon exceeding seventy years, this post-2018 recalibration attests to a strategic transition: gas has shifted from being a strategic reserve to a fully integrated asset within the production portfolio.

• Concurrently, the production architecture underwent a major reconfiguration. The exponential growth recorded since 2018 reveals a near-exclusive dependence on the Hilli Episeyo project. This project has progressively established the DKC basin as the main driver of national hydrocarbon growth, concentrating the majority of investments and sectoral development prospects.

Figure 11: Combined Oil RRR and Gas RRR Curve

Synthesis of Drilling and Production Data

The Rio Del Rey basin now exhibits the characteristic signs of advanced maturity, where reservoir degradation is reflected in a significant drop in flow rates. These rates fell from 4,500 barrels per day in 2019 in the last areas of optimal productivity to just 341 barrels per day in 2022. This erosion of unit productivity forces operators to multiply drilling campaigns (22 wells in 2022) and resort heavily to artificial stimulation techniques, mechanically increasing operational costs and reducing the marginal profitability of projects. Faced with this structural decline in oil production and the simultaneous emergence of exponential gas growth, the basin is undergoing a fundamental energy transition. The very nature of the remaining resources - predominantly gas in the Douala/Kribi- Campo basin or in the form of heavy oils - mandates a strategic reorientation towards liquefied natural gas and enhanced recovery methods. This evolution proves essential to preserving the long- term economic viability of the national hydrocarbon industry.

The current reliance on infill drilling for maintenance is unsustainable, serving only to delay (not prevent) the basin's structural decline. Recent initiatives, however, signal a shift towards revitalization. Cameroon's 2025 licensing round for nine blocks and Tower Resources' planned NJOM-3 appraisal well in 2025 represent critical steps to reinvigorate exploration.

Figure 12: Strategic Correlation between Exploration and Oil Sustainability

Conclusions

The Rio Del Rey Basin is undergoing a structural decline, marked by a zero oil Reserve Replacement Ratio (RRR) and consistently deteriorating well productivity. In response to this well-documented crisis, authorities and operators have initiated measures such as a new nine-block licensing round, the extension of Perenco's operating contract, and targeted drilling campaigns like the NJOM-3 well. While these actions demonstrate a commitment to revitalizing exploration, the current strategy remains overly reliant on conventional methods and marginal field development, failing to implement Enhanced Oil Recovery (EOR) at the necessary scale. A decisive strategic reorientation is therefore imperative, one that integrates an ambitious EOR program, builds on ongoing exploration projects, and ultimately considers repurposing existing infrastructure for COâ?? storage. This integrated pathway represents the most viable approach to maximizing the basin's residual value and ensuring a sustainable energy transition for Cameroon.

Funding

The authors declare that no external funding was received for the preparation of this manuscript.

Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The corresponding author, on behalf of all authors, declares no conflicts of interest.

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