**6. Conclusions**

Results of the investigation of burial/thermal histories and timing of petroleum generation/expulsion modeling of the Coniacian Agwu and the late Campanian

*Perspective Chapter: Understanding Thermal Maturity Evolution and Hydrocarbon Cracking… DOI: http://dx.doi.org/10.5772/intechopen.106674*

#### **Figure 16.**

*Plot of Production Index against calculated Vitrinite reflectance after Jarvie at al., 2001 showing Kerogen Conversion/Maturity of late Campanian Nkporo Shales at immaturity-early maturity stages and Coniacian Awgu Shales at peak to post maturity stages in the sedimentary section of the Lower Benue Trough, Nzam-1 well indicating that the Nkporo Shale is at immaturity-early maturity stages while Awgu is at peak/post maturity stages and has expelled hydrocarbon.*

Nkporo source rock intervals using Schlumberger's one dimensional modeling software indicates that:

The maximum temperature has been recorded in Agwu Formation where it reached 145°C at depth of 3400m and is responsible for the cracking of the oil to gas. There is also a strong indication that the high temperatures within the Formation were occasioned by deeper burial depth and possibly the Santonian tectonic episode which increased the geothermal gradient in the area. These further confirmed that the end of the Cretaceous era saw uplift, denudation and subsidence in the Lower Benue Trough as well as increased geothermal gradient to have caused complex processes of uplift, denudation, basin heating and cooling.

The hydrocarbon generation and expulsion modeling of the Lower Benue Trough shows that the Agwu Shale reached early phase of oil generation in late Campanian and extended from 75 Ma to 69 Ma in Nzam-1 well while it occurred at early Santonian and extended from 85 Ma to 78 Ma in Akukwa-2 well., Subsequently, main phase of oil generation began during early Paleocene and extended from 65 Ma to 62 Ma in Nzam-1 well and began during early Paleocene and extended from 75 Ma to 70 Ma in Akukwa-2 well. The gas phase began at mid Eocene and extended from 48Ma to present day in Nzam-1 well and began at mid Paleocene and extended from 58Ma to present day in Akukwa-2 well. As for the late Campanian Nkporo Source rock, the Model of Nzam-1 well has shown that the Nkporo Shale reached early phase of oil generation in early Paleocene and extended from 65 Ma to 60 Ma. Subsequently, main phase of oil generation began during late Paleocene and extended from 56 Ma to 42 Ma, whereas hydrocarbon generation and expulsion model of Akukwa-2 well has also shown that the late Campanian Nkporo Source rocks has entered the early phase of oil generation in late Maastrichtian (67Ma) to present day and did not reached the main phase of oil generation and expulsion owing to the fact that it lacks the requisite burial depth, temperature and pressure in favor of oil generation and expulsion.

The above shows that Awgu source rocks are overmature and are at post maturity gas evolution stage, generating gas to the present day whereas the Nkporo Shales are at early to peak maturity stage generating and expelling oil to the present day. Since the temperature continued to decrease exponentially to the present day and such

decrease in temperature have favored the preservation of the gas reservoirs and therefore the survival of hydrocarbons in the deep strata can be guaranteed. More so the geothermics, configuration of the hydrocarbon generation timing and reservoir cap development has favored accumulation and the low geothermal field background after the formation of cracked gas has increased its chances of survival in their respective reservoirs within the Lower Benue Trough due to basin cooling and the resulting present day low heat flow (48 m/Wm2). It can also be said that the hydrocarbon generation period provided favorable conditions for the accumulation of oil and gas from these Coniacian and late Campanian source rocks in the Lower Benue Trough. Given the multiple phases of hydrocarbon generation, the Coniacian Agwu source rocks had more favorable conditions for hydrocarbon accumulation than those of the upper Campanian Nkporo Source rocks. Drift sediments accompanying Coniacian- late Campanian periods possibly formed the major reservoir for the expelled hydrocarbons of the Agwu and Nkporo Shales and could represent potential reservoir units (Agbani and Owelli sandstones member) for hydrocarbons generated in the upper Cretaceous and time equivalent source rocks of Lower Benue Trough. This study suggest that valid petroleum systems exist in the basin with relative differences in the generation and expulsions periods of hydrocarbon generation in the two Formations and Paleogene Shale unit especially Imo Shales is the most important oil and gas cap rocks sealing the Owelli and Agbani sandstones that are being charged by the Coniacian Agwu and the late Campanian Nkporo Shales.

It is recommended that exploration processes be focused on the known deeper location of the Agwu and Nkporo Formations to predict the source kitchen. A 2D and 3D Burial History and Maturity modeling of the upper Cretaceous sediments of the Lower Benue Trough should be carried out and interpreted so as to identify the possible migratory path ways of the generated hydrocarbon by integrating structural information and hydrocarbon timing and generation. This will require the integration of Seismic, Geochemical and stratigraphic information.
