Page 1 of 21

European Journal of Applied Sciences – Vol. 10, No. 4

Publication Date: August 25, 2022

DOI:10.14738/aivp.104.12766. Iheaturu, T. C., Abrakasa, S., Jones, A. E., & Ideozu, R. U. (2022). Assessment of Fault Sealing in the Gabo Field, Niger Delta, Nigeria.

European Journal of Applied Sciences, 10(4). 570-590.

Services for Science and Education – United Kingdom

Assessment of Fault Sealing in the Gabo Field, Niger Delta, Nigeria

Iheaturu T. C.

Department of Geology, University of Port Harcourt, Rivers State, Nigeria

Abrakasa S.

Department of Geology, University of Port Harcourt, Rivers State, Nigeria

Jones A. E.

Department of Geology, University of Port Harcourt, Rivers State, Nigeria

Ideozu R. U.

Department of Geology, University of Port Harcourt, Rivers State, Nigeria

ABSTRACT

This research assesses the sealing of the fault bounded stratigraphy of the Gabo

Field, Niger Delta, Nigeria. Materials used comprises of 3D seismic volume in seg-y,

ditch cuttings and well logs. The methods applied are standard fault plane

evaluation techniques and they include seismic interpretation, well correlation,

XRD analysis, and application of the Yielding et al., (1997) shale gouge ratio (SGR)

algorithm for fault seal analysis. The tectonic framework was interpreted in terms

of deformational, depositional and post-depositional structures. The order of

magnitude of stress is SHmax > σv > Shmin. The juxtaposed strata relationships were

analyzed by taking cross sections A-A1 and B-B1 and this showed the structural

framework across the field. Cross section A-A1 showed that the depositional and

post depositional structures are pinchouts (channel abandonment or switching)

and shale smears respectively. The cross section B-B1, showed that the

deformational structures are faults F1 and F2 – recognized as closely spaced normal

faults and F3 is a syn-depoitional growth fault. The well correlation showed the

vertical stratigraphy of the wells are comprised of cyclic succession of sands and

shales and the shale thickness increased in intermediate sections. The X-ray

diffraction (XRD) analysis of the shales showed that the predominant minerals are

kaolinite, rutile, gypsum, albite, microcline and quartz. The rock property analysis

showed that the net to gross ranges from 26.28 – 82.0% with an average of 73.7%.

The volume of shale ranges from 18 – 73.72% with an average of 63.59%. The bulk

density of the shales ranges from 2.379 – 2.692g/cc with an average of 2.46g/cc. The

total porosity ranges from 0.159 – 0.317 with an average of 0.167 in the shales and

0.272 in the sand. The effective porosity range from 3.58 – 22.71 with an average of

6.028 in the shale and 20.13 in the reservoir. The drawdown mobility and

temperature range is 2.2 – 4124.4mD/cP and 63.3 – 80.1oF respectively, while the

average is 569mD/cP and 70.63oF respectively. The estimated pore pressure ranges

from 0.4213 – 0.4762psi/ft with an average of 0.47psi/ft in the shales and 0.428

psi/ft in the sand. The fault seal analysis showed that the shale gouge ratio range

from 0.2 – 0.7 and the stratigraphic juxtapositions are predominantly sand to sand,

sand to shale and shale to shale. The results of this research reduces the fault seal

uncertainty in the planning of oilfield development projects and enhanced pressure

Page 2 of 21

571

Iheaturu, T. C., Abrakasa, S., Jones, A. E., & Ideozu, R. U. (2022). Assessment of Fault Sealing in the Gabo Field, Niger Delta, Nigeria. European Journal

of Applied Sciences, 10(4). 570-590.

URL: http://dx.doi.org/10.14738/aivp.104.12766

support for the recovery of bypassed hydrocarbons, most especially in

compartmentalized reservoirs. In addition, the results of this research can be

applied to similar deltaic successions around the world.

Key words: Juxtaposition, phyllosilicate, fault, compaction

INTRODUCTION

Rocks such as sandstone, limestone, granite are stiff and strong, hence deform by brittle failure.

Mudstones, gypsum can be soft and weak, hence easily bend (deform by ductile failure) while

some such as salts, with sufficient time can flow (elastic – ductile deformation) (Haywick, 2009;

Berard and Prioul, 2016). Faulting is a form of brittle deformation in rocks. It is predominant at

the shallow depths of the earth crust where confining pressures, temperatures and stress

magnitudes are relatively low when compared to deeper settings. According to Haywick,

(2008), it is predominant in strong rigid rocks and rocks that are less likely to flex or bend e.g.

sandstones, limestones, granite etc. Rocks that are prone to brittle deformation easily break

when subjected to stress and one of 3 scenarios are possible, they are: fractures, joints and

faults. Fractures or joints occur when a rock deforms with no significant movement on either

side. Faults are formed when the rock deform by brittle failure with movement along the plane

of fracture (Haywick, 2008). According to Cerveny, et al., (2004), a fault may be defined as a

planar discontinuity, or damaged plane, in a rock mass along which there is observable

displacement, or slip. The resultant structure may be capable of transmitting fluid or become a

barrier to the continuous migration of fluid (hydrocarbon). The fault movement or slippage is

usually accompanied by a spontaneous release of energy that may trigger an earthquake. Each

side of the fault plane represents separate fault blocks – the footwall and hanging wall blocks.

The movement of the fault blocks forms stratigraphic juxtapositions. Faults are targets for

hydrocarbon prospecting and are easy to recognize in outcrop while it is interpreted from

seismic volumes in subsurface studies. They have a potential of dividing a wide area of stacked

reservoirs into compartments. Each compartment may have its own fluid and pressure

characteristics and this introduce a new challenge in the planning of enhanced pressure

support for the recovery of bypassed hydrocarbons (Cerveny et al., 2004). During the period of

fault movement there may be dragging, breaking, grinding and mixing of adjacent rock units, as

a result different types of fault rocks develop at the plane of the fault e.g. debris, gouge,

cataclasites, clay smears, cementation and the phyllosilicates (Figure 1) (Allan, 1989; Knipe,

1997; Yielding et al., 1997; Fisher and Knipe, 1998; Yielding, 2002; Koledoye et al., 2003; Vrolijk,

et al., 2016; Konar et al., 2021). The fault movement results to the development of very tiny

pore throats (small isolated pore spaces), very high capillary entry pressures and low

permeability along the fault plane (Yielding et al., 1997). Studies on faulting, its nucleation,

formation and growth is discussed by Anderson, (1905); Anderson (1942/1951); Hubbert,

(1972); Stoyan and Gloaguen, (2011); Healy et al., (2012) among others. According to Healy et

al., (2012), the stress within the brittle upper crust is responsible for the nucleation (origin),

growth and reactivation of faults and fractures, induces seismic activity, affects the transport of

magma and modulates structural permeability, thereby influencing the redistribution of

hydrothermal and hydrocarbon fluids. A review of previous studies in the Gabo Field include

that of Agbasi et al., (2021); Nduaguibe and Ideozu (2019); Didei and Akana (2016) and Etimita

(2015).

Page 3 of 21

572

European Journal of Applied Sciences (EJAS) Vol. 10, Issue 4, August-2022

Services for Science and Education – United Kingdom

Figure 1: Conceptual model for fault sealing across a single shale layer of a fault-bounded trap

(modified from Koledoye et al., 2003)

(a) Initiation of brittle faulting in sand (b) Ductile deformation of shale and segmentation of the

fault across the shale unit. (c) Attenuation of shale along the extensional relay. (d)

Disconnected smeared shale

FAULT SEAL ANALYSIS

Fault seal analysis have been discussed in details by Allan, (1989); Bouvier et al., (1989); Jev et

al.,(1993); Demaison and Huizinga, (1991); Lindsay et al.,(1993); Knipe, (1997); Sales, (1997);

Yielding et al., (1997); Fisher and Knipe, (1998); Yielding (2002); Koledoye et al., (2003);

Cerveny et al., (2004); Vrolijk, et al., (2016); Ideozu, et al., (2018); Konar et al., (2021);

Abrakasa, (2020); Njoku et al., (2021) among others. Yielding et al., (2010), summarized a

unified approach adopted over the years to analyze leak and seal challenges around fault

bounded traps and they are;

1. The measurement of hydraulic properties of a fault zone. This is achieved by analyzing

cored rock samples and calibrating them in the laboratory. The results from the analyzed

samples and fault rock attributes are matched to appropriate depths of the trap- bounded fault zone.

2. Application of a simple algorithm that captures sealing features of the fault plain. Such

algorithms include the clay smear potential (CSP), the shale smear factor (SSF) and the

shale gouge ratio (SGR).

The result of this simply highlights the sealing potentials of the fault plane. In addition, the

strength and capacity of the seal may be estimated. A prediction of the capillary entry pressures

would determine the ability of the seal to hold back excess bouyancy pressures from

hydrocarbon columns. The mercury injection capillary pressure (MICP) analysis is widely

acknowledged to reliably demonstrate the seal capacity and capillary entry pressures

(strength) into a seal (Schowalter, 1979; Milton and Bertram, 1992; Sales, 1997; Yielding et al.,

2010). In addition Sales, (1997) recognized that, results of the leakoff tests (LOT) or minifrac

tests may also be used to determine the pressures (seal strength and capacity) at which a rock

may start to fail through the development of microfractures. The results of these approaches

are not devoid of uncertainties but have proved over the years, to be very valuable in seal

evaluation (Sales, 1997; Yielding et al., 2010). The Niger delta and similar deltas around the

world have a wide variety of structural styles among which are normal faults, syn-sedimentary

growth faults, shale diapairs etc (Evamy, et al., 1978; Elliot, 1986; Doust and Omatsola, 1990).

The reservoir rocks of the Niger delta are predominant in the Agbada Formation comprised of

cyclic succession of sand and shale sequences. These sand and shale sequences are highly

faulted, from the delta plains, continental shelf, slope to the deep marine settings. According to

Milton and Bertram, (1992), the identification of traps and their sealing styles gives a library of

successful analogs to use as a model in search of new hydrocarbon prospects. Vrolijk, et al.,

(2016) described a fault-bounded trap as a subsurface structure formed by fault juxtaposition

of an impermeable bed (e.g., shale) against the reservoir body, or a reservoir that is bounded