**3. Methodology**

Geohistory diagrams and similar diagrams have been widely used in geology, particularly in hydrocarbon exploration. These diagrams were adapted to perform numerical modeling of burial, erosion, and thermal histories in sedimentary basins, e.g. [9, 10].

In this study, quantitative one dimensional basin modeling (1-D) is performed for evaluating the thermal histories and timing of hydrocarbon generation and expulsion of the Agwu and Nkporo source rocks in the Lower Benue Trough. The 1-D Basin modeling, was used for the reconstruction of the burial and temperature histories of the two studied wells (**Figure 2**). The reconstruction of the burial, thermal and maturity histories were modeled in order to evaluate the remaining hydrocarbon potential using Schlumberger's PetroMod (1D) modeling software and Lithology data comprising of sandstone, shale and absolute ages.

The modeling approach adopted requires input data which describe the present day geological situation as a result of past events (**Figure 3**). On this basis, the geological history is simulated from the oldest to the most recent. This also involves modeling and calibration of single-point data, whereby geologic and geochemical information are integrated to model the Formations and evolution of a sedimentary basin. As a stand-alone tool, the single point data (well) is constructed and imported from the well editor. The total sedimentary column (lithology) was determined based on well data. Erosion and heat flow changes were established through calibration against maturity data, giving rise to the generation of a conceptual model of the subsidence and thermal history of the region. This model of present day architecture represents the final result of all the processes acting on the basin throughout geologic time. Summarily, the following input data are required for reconstruction of burial history:


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

#### **Figure 2.**

*Plots of Boundary conditions for the studied well (Nzam-1) ; (A) Paleo Water depth Versus geologic Age in million years, notice its fairly constant value of about 240 m between upper Cretaceous and Paleocene and between late Eocene and early Oligocene witnessed to about 50 m and its rapid increase by Oligocene to present day value of about 400 m (B) Sediment Water Interface-Temperature versus geologic Age in Million years, notice the fairly constant value of 28°C (C) Heat flow versus geologic Age in Millions years, notice maximum value of 72 m/Wm<sup>2</sup> between upper Cretaceous and Paleocene and a constant decline from Eocene to a present day value of 48 m/Wm2 .*

#### **Figure 3.**

*Flow chart indicating the steps involved in the construction of the one dimensional burial/thermal history Modeling.*

The geologic model consisting of the depositional, non-depositional and erosional events, was compiled using stratigraphic data which were provided from well reports, bank of Shell Petroleum Development Company (SPDC) of Nigeria and previous

stratigraphic studies (**Figures 4** and **5**). Hydrocarbon generation modeling was based on TOC and HI of the Coniacian Awgu and the upper Campanian Nkporo source rocks in the Lower Benue Trough. The modeling results were also calibrated with computed vitrinite reflectance after [11] and borehole temperatures (BHT) in the study area. Geochemical data (TOC and pyrolysis data), Vitrinite reflectance (**Tables 1**–**3**), Paleo water depth, well log, and geologic data (**Tables 4** and **5**) were used for the construction of the geologic model showing thermal maturity stages of the Agwu and Nkporo Source rocks in the study area. Lithology data comprising of sandstone, shale and absolute ages were put into use (**Figures 4** and **5**). The hydrocarbon generation modeling was based on TOC and HI of the Agwu and Nkporo source rocks in the Lower Benue Trough. The maturity modeling was calculated using the EASY% Ro model of [14, 15]. The TOC and Pyrolysis data of Coniacian Agwu Shales in Nzam-1 were extracted from [12] while those of the upper Campanian Nkporo Shales in Nzam-1 were extracted from [7]. TOC and Pyrolysis data for upper Campanian Nkporo Source Rock in Akukwa-2 were extracted from [8] and the TOC and HI values for Coniacian Awgu Source Rock in Akukwa-2 were extracted from Abubakar [1] (**Figure 6**).

The Modelled Vitrinite Reflectance as calculated after [14] has been related to calculated vitrinite reflectance data after [11] so as to enable the calibration of thermal history (**Figures 7** and **8**).

Paleo-water depth values were used to define the paleogeometry while heat flow and sediment water interface temperatures were the key boundaries conditions that were defined in the course of modeling. Thermal evolution is simulated on the basis of boundary assignments applied to certain time steps. The assigned parameters are heat flow densities in mW/m<sup>2</sup> and surface temperatures in °C. Acquired Total Organic Carbon Content (TOC) and Rock eval pyrolysis of the Agwu and Nkporo Shales from Nzam and Akukwa-2 wells in the Lower Benue Trough are presented here with some additions in respect of some calculated pyrolysis parameters for the purpose of this study. Acquiring these data became necessary because the values of

#### **Figure 4.**

*Plots of Boundary conditions for the studied well (Akukwa-2); (D) Paleo Water depth Versus geologic Age in million years, notice its fairly constant value of about 240 m between Upper Cretaceous and Paleocene and between late Eocene and early Oligocene witnessed decline to about 50 m and its rapid decrease by Oligocene to present day value of about 0 m (E) Sediment Water Interface-Temperature versus geologic Age in Million years, notice the fairly constant value of 28°C (F) Heat flow versus geologic Age in Millions years, notice maximum value of 72 m/Wm<sup>2</sup> between upper Cretaceous and Paleocene and a constant decline from Eocene to a present day value of 48 m/Wm<sup>2</sup> .*

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

#### **Figure 5.**

*Paleo-temperature modeling in Nzam-1 well calibrated using borehole temperature; showing correlation among Burial history with temperature overlay, measured temperature and modeled Temperature for the studied well, notice that the maximum temperature values of 120°C–145°C in the area was attained between mid-Paleocene and mid Miocene (60-15ma) on Coniacian Agwu Source strata, higher temperatures are associated with Santonian tectonic episode*

Hydrogen Index (HI), TOC, Vitrinite reflectance forms vital input parameters in the one dimensional burial/thermal history model construction**.**
