Evaluation of shale volume and effective porosity using larionov and archie equations from wire-line logs, Niger delta Nigeria

Received Jun 22, 2019 Revised Aug 20, 2019 Accepted Oct 2, 2019 In Niger Delta region of Nigeria, reservoirs are mostly loose and unstratified sands to hold fluids. In this paper, three different wells in central Niger Delta were assessed for shale volume and actual porosity. The results of the analysis delineate the presence of sand, sand-shale and shale formations. Hydrocarbon prospecting was found to be strong in sand, moderate in sandshale and shallow in shale respectively. However, existent of shale lessens effective porosity and water saturation of the rock formations. The extent of the formation extends from 1300 to 2500 m. Shale volume and actual porosity values extend from 0.00 to 0.302 dec and 0.047 to 0.302 dec which decrease with increasing depth. Comparably, the water saturation and water resistivity extend from 0.432 to 0.779 dec and 0.106 to 2.918 Ohm respectively. These values of actual porosity are strong in sand, moderate in sand-shale and shallow in shale formations. The results from this assessment proof well log a vital and easier tool in assessing of reservoir properties.


INTRODUCTION
Reservoirs in the Niger Delta region of Nigeria indicate a variety of complexities in sandstones, shaly and shale depositions which are loose and unstratified. The presence of shale in the formation within this region has effects on both petrophysical properties and logging tool responses, which reduces effective porosity of the reservoir [1]. Shale are laminated or fissile clastic sedimentary rock with predominance of clay and silt as the detrital components [2,3]. It is classified into effective and passive shales as clayey, silty or sandy shales on the basis of texture. Effective shale contains montmorillionite and bentonite while passive shale contains kaolinite and chlorite with zero cation exchange capacities which can be identified only by neutron tool [4].Shale can be assigned in the formation in three ways: In the form of laminae between layers of sand which does not affect the porosity or permeability of the sand streaks. It can exist as grains or nodules in the formation matrix with similar properties of laminae shale and nearby massive shales. It can also be dispersed throughout the sand, partially filling the intergranular interstices. [5]. Porosity is a fraction of the volume of void over total volume between 0 and 1 or as a percentage between 0 and 100%. It is categorized as actual and total porosity. Actual porosity is portions of total void space that dispatch fluid. Total porosity is the percentage volume occupied by the pore space regardless of the type of fluid contained in the pore space. Information about core analysis technique is important in the assessment of reservoir parameters [6]. Problems such as low productivity of oil, well bore instability, decrease in depth formations and presence of clay particles in void space fount unreliability in depositional settings [7]. The formation evaluation problems are as a result of inadequate knowledge of the shale volume and effective porosity evaluation [8]. However, core analyses of sample from formations have been routinely used by some researcher in this region. This method did not give much areal coverage information and cannot be used in all the rock formations, and this leads to persistence cases of low productivity of oil, well bore instability and decrease in the depth formation due to its coverage limitation outside the coring locations. The current research therefore, based mostly on well logs using Larionov and Archie equations for better evaluation of prospective areas. On the other hand, aid other properties such as lithology, water and hydrogen saturations. Also, information through these well log approaches will aid better identification of reservoir, non-reservoir units and enhance quick decision making in geological setting of the area.

GEOLOGY OF THE STUDY AREA
The area under study is the central Niger Delta region of Nigeria. It is situated on the continental margins of the Gulf of Guinea in equatorial West Africa [9]. The region is about 7500km 2 as the largest delta extending between longitude 3 0 and 9 0 East and latitude 4 0 and 6 0 North [9][10][11][12][13]. The Niger Delta is classified as a tropical rainforest with ecosystems comprising of diverse species of flora and fauna both aquatic and terrestrial species. The region could be classified into four ecological zones; coastal inland zone, freshwater zone, lowland rainforest zone, mangrove swamp zone and this region is considered one of the ten most important wetlands and marine systems in the world [14,15]. Three major stratigraphic units have recognized in the onshore and offshore province, namely: Benin, Agbada and Akata formations. Benin formation which the study is housed comprises sand, gravel and swamp deposit [3]. Agbada formation comprises alternating sandstones [16]. It constitutes the main hydrocarbon habitant in the Niger Delta. Akata formation Eocene to Recent is made up of a sequence of under compacted marine clays with minor study and silty beds [17].

MATERIAL AND METHOD
A total of three wire line logs data were analyzed, in order to evaluate lithology, shale volume and effective porosity respectively [18]. Fundamentally, high formation of gamma ray designates shale while low formation depicts sand [19]. However, lithology interpretation is the first step in well log analysis and very important in reservoir characterization.

Shale Volume (Ѵ )
Shale volume is the most important parameters in formation evaluation. It is expressed as shown in Larionov equations (1) to (3).

Porosity ()
Porosity measures the total amount of void space accessible from the surface. It is expressed as shown in Archie equations (4)

Effective Porosity
Effective porosity is the pore volume in rock or sediments that contributes to permeability in a reservoir. It is expressed as shown in Archie equations (6) to (8).

RESULT AND DISCUSSION
Sand and shale were the prevalent lithologies in Niger Delta region with staunch credibility significance parameters such as permeability, water and hydrocarbon saturation. In order to achieve this goal, three wells namely, wells 1, 2 and 3 were delineated into three zones, namely: 1, 2 and 3 which were modeled appropriately. Sand, sand-shale and shale were identified from the wells as shown in Figure 1 to Figure 3. Table 1 to Table 3 show the evaluation of petrophysical properties within the three wells; 1, 2 and 3 while Table 4 and Table 5 show the ranges of evaluated parameters and characterization of the well formations. In each of the well, three reservoirs where identified. In well 1, the result showed moderate values of the shale volume of 0.347 dec and effective porosity of 0.138 dec, with other parameters such as: mean porosity of 0.211 dec, permeability of 4.324 darc, water saturation and a resistivity value of 0.623 dec and 1.715 Ohmm with moderate hydrocarbon potential from sand-shale formation. In well 2, the result showed high values effective porosity of 0.302 dec and 0.0dec of shale volume with other parameters such as: mean porosity of 0.302 dec, permeability value of 3.847 darc, water saturation and resistivity of 0.432 dec and 2.18 Ohmm respectively. This indicates a high presence of hydrocarbon accumulation from sand formation. In well 3, the result showed high values of shale volume of 0.740 dec and low effective porosity of 0.047 dec with other parameters such as: mean porosity of 0.182 dec, permeability value of 6.454 dec, water saturation and resistivity of 0.799 dec and 2.985 Ohmm respectively. This indicates a low hydrocarbon accumulation from shale formation. However, it is observed that the value of the effective porosity and shale volume ranged from 0.047 to 0.302 dec and 0.182 to 0.302 dec. On the other hand, a higher hydrocarbon potential is allotted to sandy deposition found in well 2 with zero shale volume, bright spot formations. Moderate potential is allotted to sandy-shaly area found in well 1 while moderately potential is allotted to shaly area, less porosity and a higher shale volume found in well 3 depict clay sediments particles. Comparing the evaluated value with core sample analysis delineated much difference in core sample analysis values. This is due to the analytical and conservation sampling approaches compared with the standard values than that of evaluated approaches However, the foregoing comparison of evaluated approaches with core sample values affirms accuracy and applicability approaches using Larionov and Archie Equations in the study.

CONCLUSION
The research and analysis carried out from three well delineate in central Niger Delta using Larionov and Archie equations. The result depicts three reservoirs extents 1300 to 2500m, Shale volumes and effective porosity extents 0.00 to 0.740dec and 0.047 to 0.302 dec. Comparably, water resistivity extent 0.432 to 0.779 dec. The evaluated shale volume and effective porosity were compared to core analysis data which give good agreement of the result. Also, prove the method a useful approach for assessment from wire line log data.