**1. Introduction**

Decline in oil production is common to mature oilfields where several wells have been drilled and exploited over a period of time. The volume of oil produced and the lifetime of a specific well or reservoir is a function of several variables, which could include the thickness of the oil column, sand-body architecture and geometry, quality of hydrocarbon, and several geologic uncertainties. It is expected that after many years of oil production from a reservoir there should be a time when the water oil ratio (WOR) should be significant. However, early decline in production for a reservoir with high potential is uncommon. The studied reservoir has experienced rapid decline in oil production with a corresponding rapid water breakthrough in just 2 years of the beginning of production.

Stratigraphic features and structural lineaments are geologic elements that will be examined to understand the architecture of the reservoir and the possible effects of lineaments on fluid flow. Fluvial and marine processes have influenced the deposition of sediments in the Niger Delta [1, 2]. Myriads of sub-environments exist within each mega environment either continental or marine. The stratigraphic imprint of these events at various scales can be identified and reconstructed from sedimentary rock bodies in both outcrops and cores [3, 4].

The structural elements that influence fluid flow include fractures, faults and folds, mapped in the subsurface of the Niger Delta [1, 5]. They are significant for the hydrocarbon entrapment and fluid mobility within the reservoir. Faults play an important role in the distribution of fluids in the subsurface and may act as baffles or conduits to flow [6]. In particular, this paper focuses on stratigraphic and structural features and their impact on the hydrocarbon production.

It has been established that facies architecture and reservoir condition influence the movement of fluids within the subsurface [4, 7–9]. However, a few studies have examined the control of geological factors in combination with production management strategy on flow behavior [7, 9, 10]. An integrated approach is adopted in this study to examine the relative impact of a combination of these uncertainties on fluid flow especially water, that can result in a rapid production decline. Rock types and fabrics are more significant than fluid properties in controlling the stratigraphic architecture of the reservoir. This is because, geologic characteristics such as facies properties and architecture, changes in depositional environments, etc., have more control on oil and gas recovery than production methods [11, 12]. In the Niger Delta, most of the sand-bodies hosting hydrocarbon occur as channels, shoreface, beach and barrier sand deposits [2, 13–15].

Studies on reservoir characterization of the Niger Delta have been carried out by several researchers. Osinowo et al. [16] described and characterized the Eni field, located offshore Niger Delta by integrating both seismic and wireline logs. Oyedele et al. [17] discussed structural lineaments and their impact on hydrocarbon accumulation in the EMI field, Niger Delta. The work of [18] focused on the impact of facies on reservoir quality and the application of 3D static model to characterize hydrocarbon potential of the KN field offshore, Niger Delta. The interplay of marine energy fluxes, the creation of accommodation space, sedimentation, facies architecture and hydrocarbon potentials of deep offshore areas have been explained in articles by [4, 19]. Reservoir characterization is essential at every stage in the lifetime of an oilfield for appropriate description of sand-body geometry and architecture [20–23].

This study aims at establishing facies characteristics, lateral continuity of sand bodies and the possible cause(s) of early water breakthrough that result in rapid decline in oil production of the N5.2 sand. This concept will have broad significance in the following areas: risk assessment and drilling optimization, understanding the geometries of sedimentary rocks and changes in their associated porosity and permeability [20].

### **2. Study area**

The study area lies in the shallow offshore area of the Niger Delta, Nigeria (**Figure 1**). It is bounded in the north by Calabar Town, at a distance of about 3 km, and in the east by the Cameroon Volcanic Line about 30 km away. The field is circa 229 km2 , and the structure was described by [24], as an elongate, four-way dip closure in two culminations with a saddle between them. There are over 40 wells drilled in the field some of which include pilot holes, water injectors and horizontal production wells.

**13**

bases (**Figure 3a** and **b**).

*Map showing the study location.*

**Figure 1.**

field (**Figure 3a** and **b**).

**3.1 Environment of deposition**

ray trend shows an upward coarsening sequence.

*Geologic Characteristics and Production Response of the N5.2 Reservoir, Shallow Offshore Niger…*

**3. Reservoir stratigraphic architecture, facies and well log analysis**

The field covered a distance of more than 7 km from the east to the west. The lithologic well correlation indicated that most of the sand-bodies (N5.2 and others) are laterally extensive across the oilfield (**Figures 2** and **3b**). A key stratigraphic marker used to constrain the correlation is the Qua Iboe Shale (QIS) (**Figure 3a**). Gamma ray trends forms the basis for delimiting reservoir tops and

The N5.2 sand is a high quality reservoir characterized by sand with some silt and shale fractions in almost negligible proportions. It is laterally extensive and cuts across the field covering a distance of more than 7 km, and well developed towards the eastern flank. It displays a layer-cake architecture with gross thickness between 33.82 and 105.13 m. The N5 reservoir displays stacked highstand parasequence set and occurred in six sub-units in wells located in the eastern part of the

The well log (gamma ray log) and drill-cutting samples are the major lithologic

tools used to identify lithofacies in the field. Ditch cutting sample description (**Table 1**) [15] indicates that the N5.2 sand contains some glauconite pellets showing sedimentation within a marine environment. The depositional environments inferred using electrofacies and the presence of glauconite within the sample interval are; beach, barrier bar, shoreface and regressive bars [15]. The overall gamma

*DOI: http://dx.doi.org/10.5772/intechopen.85517*

*Geologic Characteristics and Production Response of the N5.2 Reservoir, Shallow Offshore Niger… DOI: http://dx.doi.org/10.5772/intechopen.85517*

**Figure 1.** *Map showing the study location.*

*Sedimentary Processes - Examples from Asia, Turkey and Nigeria*

sedimentary rock bodies in both outcrops and cores [3, 4].

shoreface, beach and barrier sand deposits [2, 13–15].

structural features and their impact on the hydrocarbon production.

Stratigraphic features and structural lineaments are geologic elements that will be examined to understand the architecture of the reservoir and the possible effects of lineaments on fluid flow. Fluvial and marine processes have influenced the deposition of sediments in the Niger Delta [1, 2]. Myriads of sub-environments exist within each mega environment either continental or marine. The stratigraphic imprint of these events at various scales can be identified and reconstructed from

The structural elements that influence fluid flow include fractures, faults and folds, mapped in the subsurface of the Niger Delta [1, 5]. They are significant for the hydrocarbon entrapment and fluid mobility within the reservoir. Faults play an important role in the distribution of fluids in the subsurface and may act as baffles or conduits to flow [6]. In particular, this paper focuses on stratigraphic and

It has been established that facies architecture and reservoir condition influence the movement of fluids within the subsurface [4, 7–9]. However, a few studies have examined the control of geological factors in combination with production management strategy on flow behavior [7, 9, 10]. An integrated approach is adopted in this study to examine the relative impact of a combination of these uncertainties on fluid flow especially water, that can result in a rapid production decline. Rock types and fabrics are more significant than fluid properties in controlling the stratigraphic architecture of the reservoir. This is because, geologic characteristics such as facies properties and architecture, changes in depositional environments, etc., have more control on oil and gas recovery than production methods [11, 12]. In the Niger Delta, most of the sand-bodies hosting hydrocarbon occur as channels,

Studies on reservoir characterization of the Niger Delta have been carried out by several researchers. Osinowo et al. [16] described and characterized the Eni field, located offshore Niger Delta by integrating both seismic and wireline logs. Oyedele et al. [17] discussed structural lineaments and their impact on hydrocarbon accumulation in the EMI field, Niger Delta. The work of [18] focused on the impact of facies on reservoir quality and the application of 3D static model to characterize hydrocarbon potential of the KN field offshore, Niger Delta. The interplay of marine energy fluxes, the creation of accommodation space, sedimentation, facies architecture and hydrocarbon potentials of deep offshore areas have been explained in articles by [4, 19]. Reservoir characterization is essential at every stage in the lifetime of an oilfield for

This study aims at establishing facies characteristics, lateral continuity of sand bodies and the possible cause(s) of early water breakthrough that result in rapid decline in oil production of the N5.2 sand. This concept will have broad significance in the following areas: risk assessment and drilling optimization, understanding the geometries of sedimentary rocks and changes in their associated porosity and

The study area lies in the shallow offshore area of the Niger Delta, Nigeria (**Figure 1**). It is bounded in the north by Calabar Town, at a distance of about 3 km, and in the east by the Cameroon Volcanic Line about 30 km away. The field is circa

, and the structure was described by [24], as an elongate, four-way dip closure in two culminations with a saddle between them. There are over 40 wells drilled in the field some of which include pilot holes, water injectors and horizontal

appropriate description of sand-body geometry and architecture [20–23].

**12**

229 km2

permeability [20].

**2. Study area**

production wells.
