WHY GAS IS TRAPPED STRATIGRAPHICALLY SHALLOWER THAN OIL IN NIGER DELTA

George I. Unomah
Mobil Producing
Nigeria Lagos

The Tertiary Niger delta with more than 12,000 m of miogeosynclinal clastic wedge, which is partitioned into depobelts by east-west major structure building faults, is the habitat of 20-30 billion bbl of oil and more than four times the equivalent in gas (Fig. 1).

Three formations are recognized. They are, from the lowest, the Akata formation, Agbada formation, and Benin formation.

The Akata and Benin formations are composed of mainly marine shale and continental sands, respectively.

The Agbada formation, a lithological monotony of sand-shale interstratification, bears virtually all the known hydrocarbon occurrences.

These accumulations are located in fault-bounded structures either in various anticlinal configurations in the hanging walls or fault-dip closures in the foot wall of growth faults. An increasing number of accumulations in stratigraphic traps have been discovered in recent times.

The apparently erratic distribution of oil and gas within a given field in the delta remains one of the enigmas of the Niger delta. Instances are common where in a field, traps with good closures are wet even though similar ones elsewhere are petroliferous and deep traps are not filled to spill point but shallower ones have larger fills.

Shallow traps can be gas prone, whereas oil filled deeper traps exist, lighter oils can be trapped shallower than their heavier counterparts, and a trap at an intermediate horizon may be wet while identical shallower and deeper homologues are hydrocarbon filled (Fig. 2).

Classically, oil is trapped stratigraphically shallower than gas as the later generated gas normally flushes oil out of the next available successive shallower traps. Reversals in this expected distribution are commonly observed and can be attributed to a number of mechanisms, some of which are considered below.

Interestingly, such reversals are not generally accompanied by a decrease in API gravity of oil with depth in such fields as was observed elsewhere.1 2 It is therefore difficult to rationalize these anomalous oil and gas distributions by such mechanisms as migration segregation. 1

Field examples from the Niger delta illustrating concepts discussed cannot be presented here due to proprietary regulations.

RAPID GENERATION

Within a drainage area, oil and gas can be generated very rapidly and partially synchronously if the authentic source unit resides in the oil generative window (OGW) during a rapid subsidence phase such that the OGW is located at relatively high temperature (Fig. 1).

This is because there is insufficient time for the source rock to attain the prerequisite maturity for oil generation before high temperatures are encountered. This situation is known to be common in parts of the Niger delta from modeling studies and is accentuated by two overbearing factors.

First, the inferred source rock in the delta, namely the Akata and Agbada formations, has enormous thicknesses such that since the commencement of petroleum generation a section of the active source interval has always been buried deeper than the 150 C.

Also, the basin is marked by a very high average sedimentation rate of 500 m/1 million years,3 hence source rocks are buried to great depths (and temperatures) before hydrocarbon generation during the subsidence phase. The temperature of the initial OGW can be quite high in the delta3 4 and ranges from 80-100 C. to more than 150 C. The early product of catagenesis was therefore gas in such parts of the basin where the initial OGW was placed at temperatures in excess of 140 C.3 However, subsequent upward movement of the OGW in the post-subsidence phase could have ultimately placed the evolving OGW at relatively lower temperatures that permitted oil generation in these areas.

The outcome is that gas was the product in the initial or early OGW but substantial liquid hydrocarbon was produced later.

Therefore a larger proportion of gas, because of its higher thermodynamic activity, will successfully bypass deeper traps and accumulate at shallower horizons.

Where the early generation of hydrocarbon occurs, as is known in at least part of the Niger delta, the installation of the deep traps may not even have been completed at the age of this bypassing.

Implicit in Weber5 and his Fig. 7 is the fact that the synthetic and antithetic faults in the macrostructures acquired significant throws after the major petroleum migration phase. This would result in total passing of the deep traps by the petroleum which may however be too slow to migrate beyond some shallow horizons before entrapment infrastructures are formed at that and deeper levels.

Deeper traps would thus be filled mostly during the post-subsidence times when the OGW was cooler. In this post-subsidence stage, temperature and organic matter became the constant parameters in organic matter maturation while time remained the only variant.

Over time, sections of the source rock that hitherto were located at lower temperature, hence immature, would then catagenetically advance to mature status and would generate oil. The deeper traps would thus have higher oil-sand to gas-sand ratios than their shallower counterparts.

TOP SEAL INEFFICIENCIES

Slow but persistent leakage through the top seal, a common but frequently neglected seal characteristic, due to low or variable seal integrity can arise from a number of geologic conditions.

Lithological instability, demonstrated as lateral transition from clay-shale to silt-shale (in a strict sense) or sandy silt-shale, is common in marker shale units (Fig. 3). However, this may be manifest as slightly lowered gamma ray counts, which are too low for clay-shale but too high for sand, but goes commonly unnoticed.

This lithologic variant has relatively low displacement pressure. Furthermore, these shales have low shale smearing and sealing capacities along fault planes.

Also, shales of this integrity have low ductility and are easily fractured during even mild stresses such as the formation of anticlines.

In some parts of the delta, steeply dipping sequences-which are truncated by major unconformities from which tens to hundreds of meters of sediment had been eroded-are well documented. Tectonism in such areas was anything but mild.

FAULTING AND REACTIVATION

Many of the major structure building (MSB) and structure building (SB) faults in the coastal swamp and offshore megastructures in the Niger delta are still active, in fact, possibly as reactivation phase, as depicted by the continuity of the faults to the sea bed.

This implies a disequilibrium tectonic state for the area. These activities and the associated fault block adjustments, cracking of exiting shale smear in the fault plane, possible de-smearing of fault planes and transient sand to sand juxtaposition at successive horizons, would initiate remigration of hydrocarbons.

Weber5 noted, "Shale smear and long wedges of sands, which can be extensively disturbed by numerous subsidiary microfaults and fractures, occupy the fault zones."

These weakest points of trap infrastructure (Fig. 4) are very easily mobilized during even mild fault reactivations, and probability for fluid leakage through these points remains appreciably high all through the non-quiescent tectonic life of the structure. Therefore a predictably perfect sealing condition based on shale smear potential calculations would have leaked hydrocarbon trapped against the fault.

Depending on the caliber of the leaking petroleum (gas-oil ratio), these passage-ways could get clogged by asphaltene precipitates after initial loss of some of the pooled petroleum. Repeated tectonic disturbances and dissipation of part of the hydrocarbon column trapped against the fault would create a mature accumulation in which the spill point is at interception of reservoir top with the fault.5

Furthermore, this reactivation, coupled with substantial fluid pressure at deep horizons, could initiate microfaulting of top seals. This would lead to initial escape of the cap gas, followed by pressure drop and consequent exsolution of the solution gas.

Subsequent escape of oil with a substantial amount of gas such that there would be deasphaltening of the oil is a consequence. The asphaltene precipitates would then clog the microfault zone and terminate further remigration. The deeper trap, not filled to the spill point, would thus be filled mainly with oil while the remigrated gas relocated at a shallower trap.

CONCLUSIONS

Erratic distribution of oil and gas remains a most nagging variable in explorationists' risk analyses of prospects in the Niger delta.

Time-temperature relation of the evolving OGW to trap formation at deep versus shallow horizons, lithologic integrity of top seal and fault plane fills, and reconstruction of structural history can help predict oil and gas occurrence in the basin.

ACKNOWLEDGMENTS

Mike Loudin is acknowledged for helpful review comments.

REFERENCES

1. Silverman, S.R., Migration and segregation of oil and gas in fluids in subsurface environments, AAPG Memoir 4, 1965, p. 53-65,

2. Ross, L.M., and Ames, R.L., Stratification of oils in Columbus basin off Trinidad, OGJ, Sept. 26, 1988, p. 72.

3. Ejedawe, J.E., Lambert-Aikhionbare, D.O., Olofe, K.B., and Adoh, F. O., Evolution of oil generative window and oil and gas occurrence in Tertiary Niger delta basin, AAPG Bull., Vol. 68, 1984, pp. 1,744-51.

4. Unomah, G.I., Kinetic modelling and geohistory of oil and gas generation in Akata formation in Cawthorne Channel, Niger delta, Nigeria, Bull. Nigeria Assoc. Petrol. Explor., 1991.

5. Weber, K.J., Hydrocarbon distribution patterns in Nigerian growth fault structures controlled by structural style and stratigraphy, Journ. Petrol. Sci. and Engineering, Vol. 1, 1987, pp. 91-104.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.